Bending Deformation and Indentation Hardness of Electro Chemically Deposited Nano Crystalline Nickel-Iron Alloys

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MECHANICAL TESTING 1041 Bending deformation and indentation hardness of electro chemically deposited nano crystalline nickel-iron alloys

Joachim E. Hoffmann, Martin-T. Nanocrystalline nickel-iron microstructures, manufactured by means of Schmitt, Dietmar Eifler, Tilmann an electrochemical deposition process via electrolyte solutions, were Beck, Patrick Klär and Monika investigated to collect relevant information for the use of nickel-iron for Saumer, Kaiserslautern, Germany micro-components. By varying the current density, nickel-iron coatings can be set to show specific grain sizes, iron content, lattice strains and textures. Uniform microstructures exist in each of the deposited nickel- iron coatings. The grain sizes determined using x-ray analysis (XRD) cover a range of 6 to 17 nm. XRD texture analyses parallel to the deposition plane resulted in {111} and {200} orientations. To characterize the material’s mechanical properties indentation hardness measurements and micro-bending tests were performed. For a 0.01 %-offset bending

Article Information yield strength (Rp0.01*), grain sizes of 6 to 17 nm clearly demonstrate Correspondence Address Hall-Petch behavior. In addition, the investigations show lower work Prof. Dr.-Ing. Joachim E. Hoffmann hardening and lower values for remaining edge strain at fracture for Werkstoffkunde und Werkstoffprüfung Morlauterer Str. 31 decreasing grain size. In contrast to Rp0.01*, the Young’s modulus, 67657 Kaiserslautern, Germany indentation modulus, indentation hardness values and the bending E-mail: [email protected] strength, within their scatter bands, all remain largely unaffected by the Keywords different microstructures. Overall, all measured strength and hardness Nanocrystalline nickel-iron, electrochemical deposition, bending deformation, indentation values of the considered nanocrystalline microstructures are very high hardness, Hall-Petch behavior in comparison to microcrystalline microstructures.

Nanocrystalline (nc) and ultrafine-crystal- ture strain, and also manifest lower ther- size D > 1 μm) into ufg or nc microstruc- line (ufg) materials are defined as materi- modynamic stability [2, 4, 5]. In addition, tures by “severe plastic deformation” (SPD) als possessing grain sizes smaller than lower Young’s moduli of up to 30 % result [8, 9]. During “bottom up” processes, mi- 100 nm and between 100 and 1000 nm, for nanocrystalline metals when the speci- crostructures are generated by assembling respectively [1]. Owing to their excellent mens are manufactured via powder metal- individual atoms. These include electro- properties, nanocrystalline materials are a lurgy [6]. This behavior is attributed to chemical deposition and physical vapor topic of many scientific publications. In nano-pores. By contrast, electrochemically deposition processes. Deposition technolo- particular, the mechanical properties of na- deposited nc metals contain nearly no gies are limited to fabricating coatings and nocrystalline materials are a matter of in- pores so that their Young’s moduli remain micro-components. creasing interest [2-4]. They exhibit ex- unaffected by the grain size [7]. LIGA technology [10] (Lithographie, Gal- tremely high strength and hardness values Manufacturing processes for producing vanoformung und Abformung: Lithogra- [2-5]. Hardness and strength values are up nanocrystalline microstructures can be di- phy, Electroplating and Molding) is a well- to 2 to 10 times higher than that of conven- vided into two groups: so-called “top down” known process line in micro-system engi- tional materials which have grain sizes in and “bottom up” processes [2, 3, 8, 9]. “Top neering in which a micro-structured the µm-range. However, nano-grain sizes down“ processes transform conventional molding tool manufactured by electro- often reduce ductility to less than 2 % frac- microcrystalline (mc) microstructures (grain chemical metal deposition, is used for fab-

60 (2018) 11 © Carl Hanser Verlag, München Materials Testing 1042 MECHANICAL TESTING

ricating polymer components. Using the Furthermore, grain size, grain struc- can usually be well described using the Hall-

so-called direct LIGA process, metallic mi- ture, twin density and textures are influ- Petch relationship [2, 31, 33]. Here, σ0 and

cro-components are lapped and used di- enced by manufacturing parameters [2, 3, kσ, also H0 and kH, are empirically fitted ma- rectly as components [11]. This process 17, 18, 20, 21]. For example, the grain size terial-dependent constants for yield stress provides extremely precise micro-compo- in NiFe coatings decreases with increas- and indentation hardness, respectively. How- nents with high aspect ratios (component ing iron content because a higher number ever, for very small grain sizes, in some ma- height to width ratio) for fabricating micro- of nuclei exists [18]. For pure nickel depo- terials even below 20 nm [31, 32, 34], devia- electro-mechanical systems (MEMS) [11, sition, increasing current density causes tions occur due to Hall-Petch behavior. 12]. Typical examples include micro-drives the grain size to fall to 25 nm at 5 A × dm-2 Strength and hardness values increase di- made from nc NiFe alloys [11]. [21]. Conversely, for nickel-iron alloys, a minishingly with decreasing grain size and Electrochemical metal deposition offers lesser increase in grain size from 13 to reach a maximum of between 30 to 20 nm many opportunities for influencing mate- 17 nm is measured when current density [4]. Still smaller grains can cause strain sof- rial properties; such as bath process pa- increases from 6 to 15 A × dm-2 [18]. This tening of the material by means of an inverse rameters (e. g. temperature, electrolyte can be explained by a decrease in iron Hall-Petch effect [5, 34, 35]. Measured inden- flow), bath composition (e. g. type of metal content with elevated current density tation hardness results using nickel and salt, pH value, metal-ion concentration, ad- which results in a lower number of nucle- nickel-iron are depicted in Figure 1 [25, 36]. ditives), adjustable current parameters ation sites. During pulse-current deposi- Here, material strain softening occurs for a (e. g. current density, current-time function, smaller grains than those formed grain size of about 10 nm. The straight Hall- tion) and structural geometry (e. g. aspect during direct current deposition result Petch line in Figure 1 is computed from the

ratio, structure width) [2, 3, 13-19]. without pores and without texture [2]. constants of nickel H0 = 1100 MPa and 1/2 Here, owing to their better mechanical Otherwise, relatively narrow grain size kH = 21000 MPa × nm [37]. As can be and soft-magnetic properties, nickel-iron distributions are produced using the elec- seen, the Hall-Petch relationship, Equation alloys are a frequently selected material trochemical deposition process [2]. (2), is no longer valid for very small grain group for fabricating the smallest struc- Nickel coating with fibrous texture oc- sizes [36, 37]. tures [15-19]. Their iron content is sub- curring in the growth direction result Due to the very high strength of na- stantially influenced by bath composition from direct-current deposition, that is nocrystalline materials, their ductility is and current density. For example, in coat- type <100>, <110>, <210> or <211> [22]. usually low with fracture strains frequently ing depositions operating in a direct cur- By contrast, nickel-iron coating frequently below 2 % [4, 5]. An exception is artifact rent of up to about 2 or 3 A × dm-2, one ob- forms dual fiber textures <100> and free nanocrystalline or multi-modal micro- serves a clear increase in Fe content, but <111> which increase in size according to structures exhibiting high strength and on further increasing the current density, the electrochemical deposition direction relatively good ductility [32, 34]. the Fe content falls [15, 16, 19]. By con- [16, 22-25]. The Hall-Petch behavior of metallic mate- trast, during the manufacturing process of Residual stress formation also depends rials is due to dislocations. These are carri- LIGA micro-components, additional, locally on the process parameters [3, 15, 16, 26- ers of plastic deformation in grain sizes non-uniform current densities, electrolyte 28]. Tensile residual stress and possible down to about 20 nm. Grain boundaries compositions and therefore Fe content are cracks are the result of, among other represent barriers to dislocation glide and observed. The micro-component’s corners things, increasing current density [3, 26]. exert a strengthening effect by means of and edges grow the fastest because, at Changed electrolyte composition results in dislocation pile-ups. For very small grains, these locations, the highest current densi- varying compressive residual stress for Ni the grain boundaries – and thus the barri- ties predominate and this, in turn, leads to coatings [27]. However, residual stress, ers – increase so sharply that dislocation a lower Fe content in the deposited coat- owing to the distortion during the electro- motion and dislocation strengthening is no ings. Moreover, differing Fe contents – also chemical deposition, is undesirable [16]. longer possible. Other deformation mecha- current density-dependent – may occur By adding saccharin to the electrolyte, low nisms then develop. For grain sizes below above the growth level [15]. residual stress values and, moreover, 20 nm to 10 nm, dislocation glide passes to smaller grain sizes can be achieved [15]. grain-boundary slip with a reduction in Compared to mc microstructures, mate- strength [2, 4, 38]. As a consequence, only rials with nanocrystals exhibit highly im- low strain hardening or possibly strain sof- proved yield stress, strength and micro- tening results. hardness values [2, 15-17, 22, 25, 29-31]. In the present work, nanocrystalline The values can be in an order of magnitude nickel-iron alloys were electrochemically higher than those having conventional mc deposited using direct current, and the grain sizes [32]. The relationship between generated microstructures and mechanical grain size D and yield stress properties were characterized [39, 40]. The objective of the investigations was to de- k σ scribe the correlation between the different ReS = σ0 + (1) D microstructures and the resulting proper- Figure 1: Indentation hardness HIT as a ties by varying the current density, for function of grain size D and the straight or frequently also the indentation hardness which purpose basic knowledge about the Hall-Petch line for nickel, computed from 1/2 relationships between manufacturing, mi- kH = 21000 MPa × nm with H0 = 1100 MPa k [37], measured points for Ni and NiFe, respec- H crostructure and properties was obtained. HIT = H0 + (2) tively [25, 36] D Moreover, the work is pertinent for deter-

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mining mechanical properties for design- and lattice strains by means of modified ers. The testing load employed was ing micro-components. Williamson-Hall plots [41] from the meas- 98.1 mN. The load was applied for 20 s, ured {311}, {222} and {400} interference held for 10 s and unloaded in 20 s. The in- Experimental method line widths. To verify the grain sizes deter- dentation hardness values evaluation re- mined by the XRD, the results in Table 1 sulted from the load-indentation curves ac- The specimen production was carried out are compared with the TEM images. The cording to the DIN EN ISO 14577. The in- by a manufacturer of micro-drives. An in- grain sizes determined using both methods dentation hardness was measured using dustrial electrolyte (pH = 3) was used for are well in agreement with those micro- the 5 × 5 mm² square specimens from the the electrochemical deposition. Efforts specimens deposited using direct-current same discs as those of the micro-bending were made to generate the most uniform and a current density 3 A × dm-2 (DC-0- specimens. microstructures on larger surface areas. 3-RT) as well as those deposited using Micro-bending tests with plain specimen For this reason, nickel-iron coatings were pulse-reverse current at 200 Hz and a cur- geometry (see Figure 4) were used to deter- deposited on 100 mm diameter copper rent density 1 A × dm-2 (PR-200-1-RT). mine key parameters for the quasi-static discs (see Figure 2). The current density XRD texture analyses, possessing rota- material behavior. The parameters are was varied. It was possible to take tionally symmetric pole figures in all cases, 0.01-% offset yield strength Rp0.01*, bend- 5 × 5 mm² square specimens and strips can be evaluated by the relatively simple ing strength Rm*, Young’s modulus E and from the deposited coatings using a preci- Harris-Texture index remaining edge strain at fracture εE,r. sion cutting machine. To determine these parameters, a micro- Imes The machining of the micro-bending {hkl} tensile testing machine (produced by the rel specimens from the coating strips was car- I company: Messphysik, type: µStrain) was T = {hkl} (3) ried out first by attaching the coating strips {hkl} mes mes mes used. For the micro-bending tests an accu- 1 ⎛ I I I ⎞ to a specimen carrier using hot glue (see ⎜ {111} + {200} + {220} ⎟ rate three-point bending tool was produced. 3 ⎜ Irel Irel Irel ⎟ Figure 3) and surface grinding the copper ⎝ {111} {200} {200} ⎠ and nickel-iron faces. Subsequently, the coating strips were further machined to for the tilt angle ψ = 0° [42]. The lattice Electrochemically XRD TEM deposited specimen 0.3 mm thickness using finish-grinding planes {111}, {200} and {220} were investi- Grain size D (nm) and polishing equipment. This resulted in gated. I mes and I rel are the measured and {hkl} {hkl} DC-0-3-RT 11 8 the desired specimen surface finish with- relative [43] intensities of interference out grooves having a low roughness value from an NiFe micro-specimen. If an aniso- PR-200-1-RT 17 11 of Ra < 0.1 μm. An Al2O3 cutting disc was tropic microstructure is present, then T{hkl} Table 1: Grain sizes D of electrochemically employed to separate the micro-bending assumes a value greater than 1, and even deposited nickel-iron alloys determined by specimens (see Figure 3b). Finally, the mi- up to a maximum of 3 for a strongly pro- different methods cro-bending specimens were detached nounced texture. For isotropic microstruc- from the specimen carrier by heating and tures, T{111} = 1, T{200} = 1 and T{220} = 1. melting the hot glue. Figure 4 shows the Moreover, a scanning electron micro- geometry of the micro-bending specimens scope (SEM) (produced by the company: and a finished micro-bending specimen. Zeiss, type: SUPRA 40) having EDX (pro- Thus, all the specimens exist in the manu- duced by the company: Bruker, type: factured state (as-coated) having polished QUANTAX 5010) was employed for micro- surfaces and without heat treatment (see structural and element analyses. RT: room temperature, Table 1). In addition, it was possible to render the The characterizing of the available na- nanocrystalline microstructures visible nocrystalline microstructures was per- with the aid of a transmission electron mi- formed by means of x-ray diffraction (XRD). croscope (TEM made by the company: Joel, Here, a ψ-diffractometer (manufactured by type: JM2010), and to compare the nano- the company: GE Sensing & Inspection grain sizes with those determined by the Technologies GmbH, type: XRD 3003 PTS XRD measurements. µB) was employed on the specimen surface An instrumented indentation-hardness using a Cu Kα-beam and a 0.2 mm diame- tester (produced by the company: Fischer, ter primary beam at the center of the dif- type: Fischerscope) having a diamond pyr- Figure 2: Positions of coating strips containing the sketched micro-bending specimens and fractometer. amid indenter, was employed to carry out 5 × 5 mm² square specimens for nickel-iron It was possible to determine grain sizes micro-hardness testing according to Vick- coatings on a 100 mm diameter copper disc

Figure 4: a) Geometry of micro-bending specimens, b) micro-bending specimen Figure 3: a) Coating strip attached to specimen carrier by hot glue process, b) micro-bending specimens manufactured by electrochemical deposition, separated by cutting (schematic) grinding and polishing

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2 The distance between the lower bearings of b h . Fe content: an initial increase and then a Wb = the three-point bending tool was l = 4 mm 6 decrease. with a central upper bearing for load appli- All specimen series exhibit {111} and cation. All bearings, i. e. the two lower and Furthermore, the edge strain was deter- {200} textures parallel to the analysed depo- the upper bearing, had cylindrical geome- mined, in fact, from the measured deflec- sition plane. This is caused by values greater try, each with a 0.5 mm bearing radius. The tion f [44] than one for the Harris Texture Index. bearings were rigidly mounted in press fits For all previously considered results, without any mobility in the tool. Micro- 6 f h each of the specimens was taken from the εE = 2 (6). bending specimens were always freely l center (see Figure 2, position 000) of the placed on the lower bearings. The bearing copper discs. The influence of different distances, the bearings in height, the paral- Thus the Young’s modulus from the linear specimen positions in the copper disc (ex- lelism and flatness of the bearings were gradient cluding the edge region) was investigated quite accurate in the µm range. To carry out by employing a specimen series deposited σ -2 the measurement, a force load cell of up to E = E (7) using a DC at 5 A × dm . Here, XRD micro- 0.5 N and a laser speckle extensometer εE structural analyses showed no dependen- (LSE) with a resolution of 20 nm for measur- cies on grain size or lattice strain. Further ing the maximum deflection were available. and the 0.01-% offset bending yield strength investigations of the specimen position

Verifying the LSE measurements by steel Rp0.01* as material resistance against ex- were carried out employing copper discs specimens was well in agreement with the ceeding a remaining edge strain of following DC deposition using varied cur-

results using strain gauges and had suitable εE = 0.01 % could be determined from the rent densities. Within the scope of the scat- values for the Young’s modulus. bending hardening curve [44]. ter bands, the profiles of these results are Almost all micro-bending tests were car- similar to those depicted in Figure 5 for the ried out using a constant edge strain rate Results values of the center position. For this rea- · -3 -1 εE= 10 s . However, in some tests the son, the position of the specimens for the used strain rate was different. The microstructural analyses of each coat- nickel-iron coatings can be neglected. Before the evaluation of material param- ing deposited, using different DC densities The indentation hardness values for var- eters, it was necessary to determine the from 0.2 to 10 A × dm-2 , were carried out ious current densities are shown in Fig- bending hardening curve from the force- on the 5 × 5 mm² square specimens at the ure 6. The highest value of about 8500 MPa deflection curve (F-f curve). For this, the disc’s centre (see Figure 2, position 000). was achieved for the smallest and the larg- edge stress in elastic loading The results for the iron content depicted in est current densities of 0.2 A × dm-2 and Figure 5a show an initial increase at low 10 A × dm-2, respectively. The minimum M b current densities and then a decrease with hardness value of 7600 MPa for 5 A × dm-2 σE = (4) Wb increasing current density. As described in is still relatively high. the literature, increasing current densities Specimens for the three-point bending and the fictitious edge stress in elastic- initially lead to higher electrode potential tests were taken from the 2nd, 3rd and 4th plastic loading and to an elevated uptake of iron [15, 16]. quadrants of the copper discs onto which the By contrast, a further increase in current nickel-iron coatings were deposited using M b density causes a deficiency in Fe ions in different current densities (see Figure 2). σE* = > ReS (5) Wb the electrolyte in front of the cathode and The Young’s modulus determined in the therefore decreases the iron content in the micro-bending test is hardly influenced by had to be calculated from the bending mo- coating. The grain size increases with cur- the different current densities. The values ment rent density (see Figure 5b). This agrees obtained, shown in Figure 7, lie between well with the results of other authors and is F l related to decreasing Fe content [18]. Mb = 4 The measured lattice strain depicted in and section modulus Figure 5c shows a similar trend to that for

Figure 6: Indentation hardness HIT as a function Figure 5: a) Iron content cFe, b) grain size D, c) lattice strain η as a function of current density j of DC of current density j of DC deposited nickel-iron deposited nickel-iron coatings coatings

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164 and 170 GPa. The indentation moduli Figure 9a depicts the effects of iron con- shows a plot of indentation hardness val- are also plotted in Figure 7a. With values of tent on indentation hardness. Varied iron ues in contrast to current density. As can 166 to 191 GPa, the indentation moduli content from 6 to 17 wt.-% can be produced be seen, very high values for indentation demonstrate similar elastic behavior as the in nickel-iron coatings by modulating cur- hardness have a scatter band range of 7500 Young’s modulus determined in the micro- rent density (see Figure 5a). Figure 6 to 8500 MPa (see Figures 6 and 9a). In ad- bending test. For these tests, the selected edge strain rate was 10-3 s-1. By varying the edge strain rate from 10-5 to 10-1 s-1, it was also possible to establish that the Young’s modulus hardly changes for the specimen series deposited using different current densities (e. g. DC-0-0.2-RT), as Figure 7b illustrates. Again, the values lie between 164 to 170 GPa. The effects of both the current density during DC deposition (DC, pulse fre- quency = 0 Hz) and the edge strain rate on the 0.01-% offset bending yield strength and bending strength are given in Figure 8. As expected, strength values increase with the edge strain rate for all coatings or material microstructures. With regard to the current density, a more obvious correlation Figure 7: a) Young’s modulus E and indentation modulus EIT as a function of the current density j, b) Young’s modulus E as a function of the edge strain rate ε· of DC deposited nickel-iron coatings is detectable for offset bending yield E strength (see Figure 8a). The highest values were obtained for the lowest current density 0.2 A × dm-2. A decrease in the offset bending yield strength was associated with an increase in current density. The nickel-iron coating deposited using 10 A × dm-2 had the lowest 0.01-% offset bending yield strength. However, it was not possible to establish this correlation for the current density of the bending strength, as Figure 8b confirms. Moreover, the range of bending strength determined is nar- rower than that for offset bending yield strength. In addition, it should be stated that all strength values depicted in Fig- ure 8 are remarkably high. By contrast, the remaining edge strain at fracture of about Figure 8: a) 0.01-% offset bending yield strength R *, b) bending strength R * as a function of the 2 % is quite low. · p0.01 m edge strain rate εE of micro-bending specimens from DC deposited nickel-iron coatings Discussion

Microstructures having a relatively narrow range of grain sizes of a few nanometers are at the basis of the following observations concerning bending deformation behavior and measured indentation hardness values. This corresponds to the usual grain sizes in electrochemical deposition technology for which current density is an important influencing parameter [2, 18, 21]. Very small grain sizes produce considerably high hardness values and strength for nanocrystalline metals [2, 15-17, 20, 25, 29, 30]. Other influences on mechanical behavior arise from alloying elements, textures and impu- rities of the microstructure’s electrochemi- Figure 9: Indentation hardness HIT of DC deposited nickel-iron alloys as a function of a) iron content cFe, cal deposition [17, 20, 29]. b) grain size D compared with the calculated Hall-Petch line for nickel

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dition to these values determined for ure 10). Both figures contain a Hall-Petch line smaller grains, the stress around the dis- nickel-iron coating (open circles), micro- according to Equation (1), where locations is so high that dislocation mo- 1/2 tensile specimen values are depicted (open kσ = 7000 MPa × nm and σ0 = 370 MPa tion is no longer possible in the grains. diamonds) in Figure 9a for Direct-LIGA [37]. The 0.01-% offset bending yield Other mechanisms such as grain-bound-

nickel-iron. The micro-tensile specimens strength Rp0.01* approximates very well the ary slip are then activated, which leads to were produced using the Direct-LIGA pro- Hall-Petch line for the investigated grain inverse Hall-Petch behavior. By contrast, cess. Their hardness values conform to size range of 6 to 17 nm or at decreasing indentation hardness and bending those of nickel-iron coating. Based on these edge strain rates of 0.1, 0.001 and 0.00001 s-1, strength deviate too far from yield measurements and within the scope of the lie parallel to the Hall-Petch line (see Fig- strength owing to larger plastic deforma- scatter band, practically no influence of the ure 10a). Here, lower edge strain rates pro- tions. For this reason, these parameters

varied iron content can be established on duce lower values for Rp0.01*. Only for the are less suitable for assessing Hall-Petch the hardness values. micro-bending specimens possessing the behavior. Nevertheless, in some publica- If the corresponding Hall-Petch relation- smallest grain sizes of about 6 nm is a cer- tions hardness is described as a function ship is plotted against indentation hard- tain deviation from Hall-Petch behavior ob- of grain size using the Hall-Petch relation- ness via D-1/2, the diagram in Figure 9b will served. ship [2, 25, 30, 31, 35-37]. However, the be the result. This pertains to the same If one considers the bending strength current results demonstrate that the Hall-

specimens and hardness values as those in Rm*, similar to indentation hardness, no Petch relationship is valid for 0.01-% off- Figure 9a. If, as already discussed, it can be dependency on grain sizes between 6 and set bending yield strength for a grain size assumed that iron content exerts no influ- 17 nm can be observed (see Figure 10b). range of 17 to about 6 nm and not for in- ence on indentation hardness (see Fig- Therefore, yet again no Hall-Petch behavior dentation hardness values and bending ure 9a) and grain size within the range of 6 exists. Similar to those for indentation strengths (see Figure 10).

to 17 nm will have no influence on indenta- hardness values, Rm* results are scattered In addition, nickel-iron strain hardening, tion hardness (see Figure 9b). The Hall- around a high mean value of 3860 MPa and hence, dislocation motion for grain Petch line according to Equation (2) is also (see Figure 10b). For these tests, the edge sizes between 17 and 6 nm, continuously plotted in Figure 9b using the constants strain rate was 0.001 s-1. The results in Fig- decreases. Thus, as Figure 10b clearly 1/2 kH = 21000 MPa × nm and H0 = 1100 MPa ure 8b likewise conform to these, where no shows, the difference between 0.01-% offset [37]. A comparison with the values meas- relationship between current density and bending yield strength and bending ured confirms that no Hall-Petch behavior bending strength can be detected within strength progressively diminishes with de- can be detected. The mean of the measured the scope of the scatter bands. creasing grain size because dislocation values lies on the Hall-Petch line but the val- A 0.01-% offset bending yield strength glide and plastic deformation must occur ues are not parallel to the line. Corroberat- can be used as an alternative value for with increasing resistance. Moreover, a de- ing results from the literature on grain sizes yield strength. Yield strength is a materi- crease in plastic deformation confirms the below 20 nm also demonstrate no Hall- al’s resistance to incipient plastic defor- remaining edge strain at fracture, which, Petch behavior for indentation hardness mation by means of dislocation glide. for example, recedes from 2 % to 0.5 % for (see Figure 1) [25, 36]. Moreover, owing to Here, the influence of grain size according an edge strain rate of 10-5 s-1 and grain these results, practically no influence of the to the Hall-Petch relationship is taken into sizes between 17 and 6 nm. lattice strain (see Figure 5c) can be detected account using Equation (1) [45]. In some To evaluate the micro-bending test, the

on the indentation hardness. publications, this relationship is used to bending strength Rm* in Figure 11 is com- With respect to the micro-bending test, determine grain sizes down to about pared to the tensile strength Rm. Tensile the circumstances can be represented differ- 20 nm in nanocrystalline metals [2, 4, 6, strength data values were estimated from ently in the Hall-Petch diagram (see Fig- 17, 25, 29, 31, 33, 34, 37]. For even the indentation hardness values [46]

Figure 11: Ratio of bending strength Rm* to tensile strength Rm, estimated from indentation Figure 10: a) 0.01-% offset bending yield strength R * as a function of grain size D determined for hardness, as a function of the micro-specimens’ · p0.01 · different edge strain rates εE, b) 0.01-% offset bending yield strength Rp0.01* and bending strength Rm* edge strain rate εE of DC deposited nickel-iron of micro-bending specimens as a function of grain size D with plotted Hall-Petch lines coatings

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H IT • By carefully setting current density, Acknowledgement Rm = (8) 3 electrochemical deposition enabled nickel-iron alloys to be produced having The authors would like to thank the BMBF,

The ratio values Rm*/Rm depicted apply to microstructures possessing specific iron grant number 1713X08, and the “Stiftung the nickel-iron coatings considered whose content, lattice strains, grain sizes and Rheinland-Pfalz für Innovation, grant num- DC deposition was carried out using cur- textures. Nano-crystals were obtained ber 961-386261/997” for the project’s fi- rent densities between 0.2 and 10 A × dm-2. having a grain size in a relatively narrow nancial support. Our thanks go to the com- As already illustrated in Figures 8 and 10a range of 6 to 17 nm. This corresponds to pany Micromotion GmbH in Mainz Gon- for offset bending yield strength and bend- the usual values given in the literature senheim, Germany for manufacturing the ing strength, Rm*/Rm also increases with regarding this technology. micro-specimens. the edge strain rate in Figure 11. However, • By varying current density, the Young’s References the very high values for Rm*/Rm = 1.27 to modulus and the indentation modulus 1.77 are remarkable. Low edge strain rates remain practically unaffected. But iron 1 H. Gleiter: Nanocrystalline materials, Pro- exhibit ratio values below 1.5. According to content, grain size and lattice strain pro- gress in Materials Science 33 (1989), computations, Rm*/Rm should not exceed a duced in this way are influenced by var- pp. 223-315 value of 1.5, even for full plastic behavior, ying current density. DOI:10.1016/0079-6425(89)90001-7 which is not the case here. In the literature, • Within the scope of their scatter, the in- 2 M. Dao, L. Lu, R. J. Asaro, J. T. M. De Hosson, a maximum value of 1.5 [44] is also as- dentation hardness values and bending E. Ma: Toward a quantitative understanding of mechanical behavior of nanocrystalline sumed. One reason for the high ratio value strength seem not to be influenced by a metals, Acta Materialia 55 (2007), pp. 4041- obtained may be due to a very high bend- variation in current density and the mi- 4065 ing strength of up to 4450 MPa (see Fig- crostructures produced. Hall-Petch be- DOI:10.1016/j.actamat.2007.01.038 ure 8b and 10b). This may be associated havior is not exhibited. 3 H. A. Padilla II, B. L. Boyce: A review of fa- with extremely high stress gradients and • 0.01-% offset bending yield strength tigue behaviour in nanocrystalline metals, Experimental Mechanics 50 (2010), pp. 5-23 with frictional effects during the three- clearly increases and grain size clearly DOI:10.1007/s11340-009-9301-2 point micro-bending test. Moreover, very decreasese by decreasing current den- 4 S. H. Whang: Nanostructured Metals and high strength during the three-point micro- sity from 10 to 0.2 A × dm-2. Hall-Petch Alloys, Woodhead Publishing Limited, bending test were determined for a gold al- behavior is observed in grain sizes down Cambridge, UK (2011), pp. XXI-XXXV loy [47]. This behavior was attributed to to about 6 nm. Moreover, strain harden- DOI:10.1016/B978-1-84569-670-2.50028-9 the large stress gradients and assessed as a ing and the remaining edge strain at 5 C. C. Koch, K. M. Youssef, R. O. Scattergood, K. L. Murty: Breakthroughs in optimization size-effect. A further reason for the high fracture or ductility diminish by reduc- of mechanical properties of nanostructured Rm*/Rm values may be associated with the ing grain size owing to increasing im- metals and alloys, Advanced Engineering fact that tensile strength is estimated using pediments to dislocation glide. Materials 7 (2005), pp. 787-794 indentation hardness. Moreover, indenta- • As expected, strength values increase DOI:10.1002/adem.200500094 tion hardness was measured using a con- with edge strain rate. 6 P. G. Sanders, J. A. Eastman, J. R. Weertman: stant rate of force application and not a Overall, the strength determined, hardness Elastic and tensile behavior of nanocrystalline copper and palladium, Acta Mater. 45 constant deformation rate as during the and Rm*/Rm values of the nanocrystalline (1997), pp. 4019-4025 micro-bending test. nickel-iron coatings are very high in com- DOI:10.1016/S1359-6454(97)00092-X parison to conventional microcrystalline 7 S. C. Tjong, Haydn Chen: Nanocrystalline Conclusions materials. The basic correlations between materials and coatings, Materials Science manufacturing parameter current density, and Engineering R45 (2004), pp. 1-88 DOI:10.1016/j.mser.2004.07.001 In this study, fundamental knowledge re- the microstructures produced and mechan- 8 C. C. Koch: Nanostructured materials: An garding the correlation between manufac- ical behavior are used in the manufacture overview, J. Zehetbauer, Y. T. Zhu (Eds.): Bulk turing-microstructure-properties of electro- and design of micro-components made of Nanostructured Materials, Wiley-VCH, chemically deposited nickel-iron alloys is nanocrystalline nickel-iron alloys. How- Weinhein, Germany Ch.1 (2009), pp. 3-20 described. The manufacturing parameter ever, micro-components for micro-systems DOI:10.1002/9783527626892.ch1 used was current density during DC deposi- are manufactured using Direct-LIGA tech- 9 R. Z. Valiev: Producing bulk nanostructured metals and alloys by severe plastic deformation. To generate uniform microstructures, a nology. Here, in contrast to the NiFe coat- tion, S. H. Whang (Ed.): Nanostructured Met- nickel-iron coating was deposited on copper ings considered, inhomogeneous deposi- als and Alloys, Woodhead Publishing Lim- discs. The micro-bending specimens were tion conditions exist and therefore inhomo- ited, Cambridge, UK (2011), Ch. 1, pp. 3-39 cut from the coatings, ground and polished. geneous microstructural states as well as DOI:10.1533/9780857091123.1.3 The influence of roughness on mechanical inhomogeneous mechanical properties can 10 E. W. Becker, W. Ehrfeld, P. Hagmann, A. Maner, D. Münchmeyer: Fabrication of micro- behavior was skirted as far as possible by be observed. For this reason, it is necessary structures with high aspect ratios and great means of a final polishing. Analyses of the to assess the strength of micro-components structural heights by synchrotron radiation microstructures, grain sizes, lattice strains from indentation hardness measurements lithography, galvanoforming and plastic and textures were carried out using x-ray locally according to Equation (8). Owing to moulding (LIGA process), Microelectronic En- diffraction and were verified by means of yet to be explained very high strength val- gineering 4 (1986), pp. 35-56 transmission electron microscopy. To char- ues, the results of the micro-bending test DOI:10.1016/0167-9317(86)90004-3 11 R. Slatter, R. Degen: The micro harmonic acterize the mechanical properties, indenta- only permit an observation of trends but drive: A high precision gear system miniatur- tion hardness measurements and micro- indicate clear Hall-Petch behavior for the ized by direct-LIG, in Proc. of the 4th Euspen bending tests were used. The following re- 0.01-% offset bending yield strength for International Conference, Glasgow, Scotland sults were obtained: grain sizes down to about 6 nm. (2004), pp. 13-14

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Abstract annealed nanostructured NiFe alloys, Scripta Materialia 65 (2011), pp. 1-4 DOI:10.1016/j.scriptamat.2011.03.029 Biegeverformung und Eindringhärte von Mikroproben aus elektroche- 25 J. D. Giallonardo, U. Erb, K. T. Aust, G. Palumbo: misch abgeschiedenen nanokristallinen Nickel-Eisen-Legierungen. Bei The influence of grain size and texture on the Young’s modulus of nanocrystalline nickel der Erzeugung definierter nanokristalliner Gefüge aus Nickel-Eisen kommt and nickel-iron alloys, Philosophical Magazine das elektrochemische Abscheideverfahren aus wässrigen Elektrolytlösun- 91 (2011), pp. 4594-4605 gen zur Anwendung. Durch Variation der Stromdichte lassen sich Nickel- DOI:10.1080/14786435.2011.615350 26 J. D. Giallonardo, U. Erb, G. Palumbo, G. A. Botton, Eisen-Schichten mit definierten Korngrößen, Eisengehalten, Gitterverzer- C. Andrei: Internal stresses in nanocrystalline rungen und Texturen einstellen. In den abgeschiedenen Nickel-Eisen- nickel & nickel-iron alloys, Materials Science Forum 706-709 (2012), pp. 1607-1611 Schichten liegen jeweils gleichmäßige Gefüge vor. Die röntgenographisch DOI:10.4028/www.scientific.net/ (XRD) ermittelten Korngrößen schließen den Bereich von 6 bis 17 nm ein. MSF.706-709.1607 XRD-Texturanalysen parallel zur Abscheideebene ergeben {111}- und 27 A. M. El-Sherik, J. Shirokoff, U. Erb: Stress measurement in nanocrystalline Ni electro {200}-Orientierungen. Zur Charakterisierung der mechanischen Eigen- deposits, Journal of Alloys and Components schaften werden Eindringhärtemessungen und Mikrobiegeversuche 389 (2005), pp. 140-143 DOI:10.1016/j.jallcom.2004.08.010 durchgeführt. Für die 0.01-%-Biegedehngrenze (Rp0.01*) ergeben die Korn- 28 D. M. Allen, N. Duclos, I. Garbutt, M. Saumer, größen von 6 bis 17 nm eindeutiges Hall-Petch-Verhalten. Zudem zeigen C. Dhum, M. Schmitt, J. E. Hoffmann: The die Untersuchungen geringere Verfestigung und geringere Werte für die effect of additives on the physical properties of electroformed nickel and on the stretch of rückbleibende Randdehnung bei Bruch, wenn die Korngröße abnimmt. Im photoelectroformed nickel components, TIMA Gegensatz zu R * bleiben der Elastizitätsmodul, der Eindringmodul, die Editions/DTIP 2006, pp. 316-321 p0.01 29 H. Li, F. Ebrahimi: Synthesis and characteriza- Eindringhärte und die Biegefestigkeit im Rahmen der Streuungen durch tion of electrodeposited nanocrystalline die unterschiedlichen Gefüge weitgehend unbeeinflusst. Insgesamt sind nickel-iron alloys, Materials Science and Engineering A347 (2003), pp. 93-101 alle ermittelten Festigkeits- und Härtewerte der betrachteten nanokristal- DOI:10.1016/S0921-5093(02)00586-5 linen Gefüge sehr hoch im Vergleich zu mikrokristallinen Gefügen. 30 R. Schwaiger, J.-T. Reszat, K. Bade, J. Aktaa, O. Kraft: A combined microtensile testing and nanoindentation study of nanocrystalline LIGA Ni-Fe, International Journal of Materials Research 100 (2009), pp. 68-75 12 P. Meyer, J. Schulz, L. Hahn, V. Saile: Why you pp. 2430-2433 DOI:10.3139/146.101785 will use the deep x-ray LIG(A) technology to DOI:10.4028/www.scientific.net/ 31 J. Chen, L. Lu, K. Lu: Hardness and strain produce MEMS?, Microsystems Technologies MSF.654-656.2430 rate sensitivity of nanocrystalline Cu, Scripta 14 (2008), pp. 1491-1497 19 F. Giro, K. Bedner, C. Dhum, J. E. Hoffmann, Materialia 54 (2006), pp. 1913-1918 DOI:10.1007/s00542-007-0503-1 S. P. Heussler, L. Jian, U. Kirsch, H. O. Moser, DOI:10.1016/j.scriptamat.2006.02.022 13 N. Kanani: Galvanotechnik, Hanser, Munich, M. Saumer: Pulsed electrodeposition of high 32 K. M. Youssef, R. O. Scattergood, K. L. Murty: Germany (2009) aspect-ratio NiFe assemblies and its influence Ultrahigh strength and high ductility of bulk 14 J. Horkans: Effect of plating parameters on on spatial alloy composition, Microsystem nanocrystalline copper, Applied Physics electrodeposited NiFe, J. Electrochem. Soc. 128 Technologies 14 (2008), pp. 1111-1115 Letters 87 (2005), pp. 091904-1-091904-3 (1981), pp. 45-49 DOI:10.1007/s00542-008-0614-3 DOI:10.1063/1.2034122 DOI:10.1149/1.2127385 20 A. Vincenco, P. L. Cavalotti: Mechanical prop- 33 N. Hansen, B. Ralph: The strain and the 15 A. Thommes, W. Stark, W. Bacher: The Electro erties of electrodeposited nickel-iron alloys, grain size dependence of the flow stress deposition of Iron-Nickel in LIGA Microstruc- Proc. of the Interfinish World Congress and of copper, Acta Metallurgica 30 (1982), tures (Die galvanische Abscheidung von Exhibition 15 (2000), pp. 1-15 pp. 411-417 Eisen-Nickel in LIGA-Mikrostrukturen), 21 A. M. Rashidi, A. Amadeh: The effect of current DOI:10.1016/0001-6160(82)90221-8 Wissenschaftliche Berichte FZKA 5586 (1995) density on the grain size of electrodeposited na- 34 S. Cheng, E. Ma,. Y. M. Wang, L. J. Kecskes, (in German) nocrystalline nickel coatings, Surface and Coat- K. M. Youssef, C. C. Koch, U. P. Trociewitz, 16 U. Kirsch: Electrochemical deposition of ings Technology 202 (2008), pp. 3772-3776 K. Han: Tensile properties of in situ consoli- low-stress nickel-iron alloy layers and their DOI:10.1016/j.surfcoat.2008.01.018 dated nanocrystalline copper, Acta Materialia properties for components of microsystem 22 A. Godon, J. Creus, S. Cohendoz, E. Conforto, 53 (2005), pp. 1521-1533 technology, (Elektrochemische Abscheidung X. Feaugas, P. Girault, C. Savall: Effects of DOI:10.1016/j.actamat.200412.005 von spannungsarmen Nickel-Eisen-Legierungs grain orientation on the Hall-Petch relationship 35 A. H. Chokshi, A. Rosen. J. Karch, H. Gleiter: schichten und ihre Eigenschaften für Bauteile in electrodeposited nickel with nanocrystalline On the validity of the Hall-Petch relationship der Mikrosystemtechnik), Dissertation, Albert- grains, Scripta Materialia 62 (2010), pp. 403-406 in nanocrystalline materials, Scripta Metallur- Ludwig-Universität, Freiburg, Germany (2000) DOI:10.1016/j.scriptamat.2009.11.038 gica 23 (1989), pp. 1679-1684 (in German) 23 T. E. Buchheit, S. H. Goods, P. G. Kotula, DOI:10.1016/0036-9748(89)90342-6 17 H. Li, F. Ebrahimi: Synthesis and characterisa- P. F. Hlava: Electrodeposited 80Ni-20Fe 36 A. M. El-Sherik, U. Erb, G. Palumbo, K. T. Aust: tion of electrodeposited nanocrystalline (Permalloy) as a structural material for high Deviations from Hall-Petch behaviour in as-pre- nickel-iron alloys, Materials Science and aspect ratio microfabrication, Materials Sci- pared nanocrystalline nickel, Scripta Metallur- Engineering A 347 (2003), pp. 93-101 ence and Engineering A 432 (2006), pp. 149-157 gica et Materialia 27 (1992), pp. 1185-1188 DOI:10.1016/S0921-5093(02)00586-5 DOI:10.1016/j.msea.2006.05.149 DOI:10.1016/0956-716X(92)90596-7 18 Y. Marita, I. I. Yaacob: Structural characteriza- 24 H. Li, F. Jiang, S. Ni, L. Li, G. Sha, X. Liao, 37 J. E. Carsley, J. Ning, W. W. Milligan, S. A. Hackney, tion of electrodeposited nickel-iron alloy films, S. P. Ringer, H. Choo, P. K. Liaw, A. Misra: E. C. Aifantis: A simple mixtures-based model Materials Science Forum 654-656 (2010), Mechanical behaviors of as-deposited and for the grain size dependence of strength in

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nanophase metals, NanoStructured Materials Universität (TH) Karlsruhe, Germany (2000) Prof. Dr.-Ing. Tilmann Beck, born 1967, 5 (1995), pp. 441-448 (in German) studied Mechanical Engineering at the University DOI:10.1016/0965-9773(95)00257-F of Karlsruhe (now KIT), Germany, from 1989 to 38 H. S. Kim, Y. Estrin, M. B. Bush: Plastic defor- Bibliography 1995. He received his Doctoral degree in 1999 mation behaviour of fine-grained materials, writing on isothermal thermomechanical fatigue of Acta Materialia 48 (2000), pp. 493-504 DOI 10.3139/120.111259 fiber-reinforced aluminum alloys. From 1999 to DOI:10.1016/S1359-6454(99)00353-5 Materials Testing 2006 he was head of the Near Service Loadings 39 M.-T. Schmitt: Characterization of electro- 60 (2018) 11, pages 1041-1049 Lab at the Institute of Materials Science and Engi- chemically produced nanocrystalline nickel- © Carl Hanser Verlag GmbH & Co. KG neering at the University of Karlsruhe. In 2007, iron alloys (Charakterisierung galvanisch ISSN 0025-5300 he joined the Institute of Energy and Climate hergestellter nanokristalliner Nickel-Eisen- Research IEK-2 at Forschungszentrum Jülich, Ger- Legierungen), Dissertation TU Kaiserslautern, The authors of this contribution many, as head of the department “Metallic Struc- Germany (2014) (in German) tural Materials”. From 2008 to 2014 he was profes- 40 M.-T. Schmitt, J. E. Hoffmann, D. Eifler: Micro- Prof. Dr.-Ing. Joachim Ernst Hoffmann, born in sor of High Temperature Materials Mechanics at the structural parameters and their effect on the 1953, studied Mechanical Engineering at the RWTH Aachen University, Germany, and from indentation hardness of electrodeposited and University of Karlsruhe, Germany, from 1975 to 2011-2014 head of the Materials Mechanics section annealed nickel-iron micro-specimens, 1979. From 1979 to 1984 he studied and received of IEK-2. In 2014 he became full professor and Advanced Engineering Materials 15 (2013), his doctorate at the Institute for Materials Science chair of Materials Science and Engineering at the pp. 442-448 I. He worked as a group leader on the develop- University Kaiserslautern, Germany. DOI:10.1002/adem.201200253 ment of electronic fuel pumps at Robert Bosch Prof. Dr.-Ing. Patrick Klär, born in 1965, studied 41 T. Ungar, I. Dragomir, A. Revesz, A. Borbely: GmbH in Stuttgart, Germany. In 1988 he was Material Sciences at the Saarland University, Saar- The contrast factors of dislocations in cubic appointed to a chair at the University of Applied brücken, Germany from 1986 to 1991. Thereafter, crystals: the dislocation model of strain anisot- Sciences Kaiserslautern, Germany in the Depart- he worked as a scientific assistant for nonlinear ropy in practice, J. Applied Cryst. 32 (1999), ment of Materials Science. modelling and finite element simulations at the pp. 992-1002 Dr.-Ing. Martin-Tobias Schmitt, born in 1971, chair of Technical Mechanics at the University of DOI:10.1107/S0021889899009334 studied Mechanical Engineering at the University Saarland and he achieved his PhD degree in 1995. 42 V. Valvoda, M. Järvinen: On the Harris texture of Applied Science in Kaiserslautern, Germany, Subsequently, he worked as deputy head of the de- index, Powder Diffraction 5 (1990), pp. 200-203 where he completed his Dipl.-Ing. (FH) degree in velopment and quality assurance department of a DOI:10.1017/S0885715600015797 1999. After several years in industry, he started subsidiary company of the Saarstahl AG, Germany. 43 PCPDFWIN, JCPDS International Centre for basic research at the University of Applied Sci- From 1999 to 2000 he was operating chief for hard Diffraction Data 2003, V 2.6 ences in Kaiserslautern. In 2014 he received his materials manufacturing at a subsidiary company 44 E. Macherauch, H.-W. Zoch: Praktikum in doctoral degree from the University Kaisers of the Saarstahl AG. Since February 2000, he has Werkstoffkunde, Springer Vieweg, Berlin, lautern. Since 2014 he has been working at the been Professor for design and simulation of mi- Germany (2014) Voestalpine Böhler Welding Fontargen GmbH in crosystems at the University of Applied Sciences in 45 N. J. Petch: The Cleavage Strengh of Polycrystals, Eisenberg as Manager Global R&D Brazing. Kaiserslautern. His working group researches in Journal of the Iron and Steel Institute 174 Prof. Dr.-Ing. habil. Dietmar Eifler, born in the field of design and finite element simulation of (1953), pp. 25-28 1949, received his PhD from the University of microsystems and in the field of micro-cutting of 46 I. Brooks, P. Lin, G. Palumbo, G. D. Hibbard, Karlsruhe, Germany. From 1991 to 1994 he was microsystem materials and components. U. Erb: Analysis of hardness-tensile strength Professor at the University of Essen, Germany. Prof. Dr. rer. nat. Monika Saumer was born in relationships for electroformed nanocrystal- From 1994 until 2015 he was Professor and from 1961. She graduated in chemistry at the Univer- line materials, Materials Science and Engi- 2015 to 2017 Senior Research Professor at the sity in Bonn, Germany. After receiving her PhD neering A 491 (2008), pp. 412-419 Institute of Materials Science and Engineering at in Chemistry at the University of Karlsruhe, DOI:10.1016/j.msea.2008.02.015 the University of Kaiserslautern, Germany. Re- Germany she worked at the Forschungszentrum 47 M. U. Auhorn: Mechanical properties of search activities are focused on the characteriza- Karlsruhe (now: KIT) at the Institute of Microstruc- primary shaped microbending specimen from tion of the fatigue behavior of metallic materials turing Technology. In 1998 she obtained a profes-

AU58Ag23Cu12Pd5 and ZrO2 (Mechanische using mechanical, electrical, magnetic, acoustic sorship at the University of Applied Sciences, Kaiser- Eigenschaften urgeformter Mikrobiegeproben as well as thermal measuring techniques in the slautern/Zweibrücken, Germany for Chemistry in aus AU58Ag23Cu12Pd5 und ZrO2), Dissertation LCF-, HCF- and VHCF regime. Microtechnology.

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