Serrated Flow in Powder Metallurgy Al–5%Mg–1.2%Cr Alloy Processed by Equal Channel Angular Pressing

Institutional Repository

This document is published in:

Materials Characterization, 73 (2012) 16-30 DOI:http://dx.doi.org/10.1016/j.matchar.2012.06.013

M. Eddahbi⁎, M.A. Monge, A. Muñoz, R. Pareja

Departamento de Física, Universidad Carlos III de Madrid, 28911 Leganés, Madrid, Spain

Abstract: The microstructure, texture and mechanical behavior of the powder metallurgy Al 5wt.%Mg 1.2 wt.%Cr alloy subjected to equal channel angular pressing (ECAP) has been investigated. The material processed by ECAP, as well as in the homogenized condition, exhibited room temperature serrated flow (SF) up to fracture. The critical stress for the serration onset decreased with increasing strain rate or ECAP temperature. The results indicated that this SF was induced by shear banding. The stress oscillations were attributed to the interaction between shear bands (SBs) and obstacles like second phase particles, and dislocation locks produced by strain hardening. The early stages of the stress strain σ ε curves of the ECAP processed samples showed a transition from type B serrations with continuous strain hardening to type B serrations superimposed on a succession of constant stress plateaus when the tensile strain rate was increased from 10 4 s 1 to 10 3s 1. The plateaus in σ ε curves obtained at a strain rate of 10

3s 1 were ascribed to the nucleation of a band at one end of the sample gauge region and subsequent propagation towards the opposite end. At a low strain rate of 10 4s 1 the sites for band nucleation should be randomly distributed along the sample gauge region. The disappearance of the plateaus in the σ ε curves are attributed to the activation of a new moving band before the completion of the deformation banding cycle of the preceding band.

Keywords: Aluminum alloys ECAP, Serrated flow Mechanical properties Shear banding

1. Introduction

deformation bands. Type A serrations are associated with the Serrated flow (SF), or the Portevin Le Chatelier (PLC) effect, periodic nucleation of deformation bands at one of the ends has been reported in several materials such as Fe, Ni, Cu and of the tensile sample, and their continuous propagation Al base alloys tested under different conditions [1 9].This toward the other end along the gauge region of the sample; effect is a manifestation of discontinuous plastic flow, or this kind of serration occurs at low temperatures or high plastic instabilities, from microstructural origin inducing strain rates. Type B serrations are also associated with bands either zero or negative strain rate sensitivity [10].It iswidely nucleated at one end but propagate discontinuously to the recognized that the interaction of solute atoms with mobile other end; they are characterized by a rapid succession of dislocations, i.e. dynamic strain aging (DSA), is the main rather regular stress oscillations that arise from DSA of the source of SF [11,12]. Several types of serrations, labeled as mobile dislocations in the band. This serration type may be types A trough D have been identified and their characteris the result of a transformation of type A serrations into type B tics reported [12]. These serration types, which depend on the with increasing strain, or occurs at the onset of the SF for low test conditions and the state of the material, are found strain rates and high deformation temperatures. Type C associated to the formation and propagation of localized serrations are differentiated by regular saw tooth like stress drops with a rather high frequency below the track of the flow ⁎ Corresponding author: Tel.: +34 91 624 8734; fax: +34 91 624 8749. E mail address: [email protected] (M. Eddahbi).

1 curve; they are attributed to dislocation unlocking and occur at higher temperatures and lower strain rates than the other 2. Experimental Procedure two serration types. The serrations consisting in a succession of stress plateaus in the stress strain curve, referred to as The Al 5%Mg 1.2%Cr (wt.%) alloy was produced by sintering Type D, are attributed to propagation of bands without strain of rapidly solidified powder followed by extrusion. The details hardening. Type B serrations are often found superposed to of the production and composition of the alloy are given in type A or type D serrations. Ref. [31]. After a homogenization treatment at 500 °C for 48 h Also, it has been found that the SF onset in substitutional followed by furnace cooling, 10 mm× 10 mm× 50 mm billets ε alloys appears after attaining a threshold or critical strain, c, were ECAP processed at 200 °C, 250 °C and 300 °C applying a −1 which may depend on temperature, strain rate, grain size pressing rate of 20 mm min . The ECAP die had an internal Φ Ψ and solute content [6,11,13,14]. Furthermore, the initial angle of 90° and an outer angle of 0°, which produced an amount of cumulative deformation and the strain path effective strain of ~1 by pass. The billets were subjected to a have an effect on the SF characteristics [8,15].Althoughthe single pass, and two passes via routes B (2pB) and C (2pC), i.e. effectofthesevereplasticdeformation on the SF character after the first pass, they were respectively rotated 90° or 180° istics of the pure metals and alloys has been scarcely around the extrusion axis direction (ED), Fig. 1e and f. The investigated, the PLC effect has been reported in Al Mg and billet geometry and shear deformation underwent by a cubic Al Cu alloys processed by equal channel angular pressing element of material during the ECAP processing are schemat (ECAP) [16 20]. Some of these results indicate that the ically represented in Fig. 1d. Initially, the billet was introduced submicron structures developed in Al Mg alloys by the in the entrance channel in such a way that the plane normal ECAP processing exhibit less susceptibility to the PLC effect, to the transversal direction (TD) is the flow plane in exist and higher strain rate sensitivity, than the coarser ones channel, as shown in Fig. 1b. The billet, appropriately [17 19]. However, other results suggesting the opposite has lubricated, was inserted in the die heated at the selected also been reported [16]. temperature using built in cartridge heaters. After each pass, On the other hand, there exist models attributing the the billets were water quenched. stress drops in the SF to the passage of the shear bands (SBs) Flat tensile samples with gauge dimensions 1.5 mm× through the grains [4]. Moreover, the SF behavior in some Al 3 mm×20 mm were spark machined from the ECAP deformed alloys has in part attributed to shearing of precipitates by billets with their longitudinal axis parallel to the extrusion dislocations and other precipitation effects [21 25].In direction as shown in Fig. 1c. Tensile tests at RT were carried −4 −1 particular, the precipitation treated samples deformed at a out in an AG I Shimadzu machine at strain rates of 10 s −3 −1 given strain rate exhibited lower critical strains and higher and 10 s . At least two ECAP treated samples, as well as two stress oscillations in comparison with the alloys in the homogenized samples, were tested at each of the strain rates as cast condition [22]. However, it should be noted that to ensure the reproducibility of the measurement and avoid there are reports indicating that shearing of precipitates is misleading effects. The strain rate sensitivity of the samples not a sufficient requisite for the appearance of SF in Al alloys was evaluated from the relationship: [26]. lnðÞσ 1=σ 2 Al Mg alloys, which exhibit SF and poor creep resistance, m ð1Þ lnðÞε_ 1=ε_ 2 might be suitable for automotive and aerospace applications σ σ if their formability is enhanced by preventing SF and where 1 and 2 are the stress mean values at a strain of 0.06 improving the mechanical properties at elevated tempera in the true stress true strain curves for the strain rates ε1 −4 1 _ −3 1 tures. The SF inhibition would avoid the development of the 10 s and ε2 10 s , respectively. detrimental localization of plastic flow in the parts during Metallographical analyses by optical microscopy (OM) and their forming processes. The enhancement of the creep scanning electron microscopy (SEM) were performed on ECAP resistance would allow fabricating drive train parts for treated samples, as well as on homogenized samples, after operating temperatures above 120 °C. The addition of tran being tensile deformed at RT. Graff Sargent and Keller re sition metals such as Cr or Fe enhances the microstructure agents were used for revealing the microstructure. stability and mechanical properties of the Al Mg alloys at In order to correlate the deformed microstructure with the elevated temperature via dispersion strengthening [27 29]. SF characteristics of σ ε curves, specimens for transmission Among these alloys, rapidly solidified Al 5Mg 1.2Cr (wt.%) electron microscopy (TEM) were prepared from the samples consolidated by extrusion is an attractive material for tensile tested at RT. The specimens were electropolished at superplastic forming since it exhibits superplastic behavior 15 V and 35 °C in a solution of 20% perchloric acid in ethanol. − − at strain rate as high as 10 2 s 1 [30]. Although the processing The TEM observations were carried out in a JEOL JEM 2032 effects on the microstructure of this alloy have been microscope operated at 200 kV. reported, its plastic flow characteristics are still little Texture measurements were performed on the longitudi known [31]. nal plane of samples tensile tested up to fracture. The The main objective of the present work was to investigate measurements were carried out in a Siemens diffractometer the effect of the ECAP processing on the Al 5Mg 1.2Cr alloy. In equipped with a D5000 goniometer. Incomplete (111) pole particular, an attempt has been made to describe the figures (PF) were recorded over a range of azimuthal angles characteristics of the SF observed in stress strain (σ ε) curves 0°≤χ≤80° in steps of Δχ ΔΦ 5°. A textureless standard during room temperature (RT) deformation at strain rates of sample of pure Al was used for defocusing correction of the − − − − 10 4 s 1 and 10 3 s 1. polar coordinates χ and Φ. 2 Pressing

TD X (ED) (a) (b) (c)

flow plane

rotation 2 passes via route B (2pB) (e) (d)

1 pass 2 passes via route C (1p) (2pC) (f)

Fig. 1 – Sketch showing the ECAP process: (a) starting ECAP billet, (b) 90° ECAP die, and (c) ECAP deformed billet and sample cut for tensile tests. Shearing of a cube element induced by (d) a single ECAP pass (1p) and two passes via (e) route B (2pB) or (f) route C (2pC).

{531}〈112〉 towards the Bs orientation {110}〈112〉 [10]. This 3. Results texture is commonly observed in Al and Al alloys deformed at moderate and high temperatures. 3.1. Initial Microstructure 3.2. Flow Characteristics The microstructure and texture of the extruded PM Al 5Mg 1.2Cr alloy have been described in detail elsewhere [10]. For Fig. 3 shows the σ ε curves of the tensile tests for the comparison, OM and TEM images of the grain structure and homogenized and ECAP deformed samples performed at RT − − − − distribution of second phase particles in the homogenized and strain rates of 10 4 s 1 and 10 3 s 1. All these curves after alloy are shown in Fig. 2. The grain structure is coarse and yielding reveal a remarkable SF up to fracture. The ECAP − − elongated along the extrusion direction but fine grained deformed samples tensile tested at 10 4 s 1, as well as the zones, indicated by arrows, also appear as Fig. 2a and b homogenized one, exhibit a single short serrated plateau reveals. These fine grained areas should have been developed preceded by the initial hardening after yielding, as it is by recrystallization during the extrusion process. It is impor indicated by arrows on the σ ε curves of Fig. 3a. This serrated tant noticing that the reagent causes preferential etching of plateau has been observed in the Al 5Mg alloy, and identified the areas with higher stored energy. Then, the gray level in in the deformation range where a Lüders band forms and the OM images reveals the place where the plastic deforma propagates [8], which corresponds to the Lüders strain. In tion and finer microstructure is located. These images show contrast, the σ ε curves of the ECAP deformed samples tested − − that the deformed zones are associated with a fine grained at 10 3 s 1 reveal a characteristic succession of serrated microstructure and a high density of the second phase plateaus, or stepwise stress jumps with regular stress particles. TEM images confirm that this initial microstructure oscillations superimposed as shown in Fig. 3b, d and f. This is partially recrystallized as the large grains still contain succession of plateaus does not appear in the σ ε curve of the − − deformation substructures composed by small subgrains homogenized alloy tested at 10 4 s 1. During the initial associated with a fine dispersion of particles. hardening stage after the plastic flow onset, stress oscillations The texture of the homogenized microstructure is de of very small amplitude appear in all the σ ε curves. These scribed by a β fiber texture, i.e. an orientation tube running stress oscillations, formerly reported in the Al 5Mg alloy from the Cu orientation {112}〈111〉 through the S orientation [8,13], have been attributed to type A serrations produced by 3 of the first plateau detected in the σ ε curves just at the end of the initial hardening stage. Attention to these type B a ε serrations is paid in the present work. The c values extracted from the σ ε curves are shown against ECAP temperature in ε Fig. 4.Itisevidentthat c decreases with increasing ECAP temperature or tensile strain rate. Also, the same strain rate ε effect on c is observed for the homogenized alloy. It should be ε mentioned that this inverse dependence of c on strain rate has also been reported for some Al 5Mg alloys [32,33]. Fig. 5 shows the ECAP temperature and strain rate effect on σ the stress at the onset of type B like serrations c. A drastic σ decrease of c with increasing ECAP temperature is evident, but the effect of the strain rate is rather ambiguous. 100 μm 3.3. Deformation Texture

Fig. 6 shows the measured (111) PF for samples ECAP treated at b 200 °C and 300 °C after being tensile tested at RT and strain − − − − rates of 10 4 s 1 and 10 3 s 1. It is expected that the tensile tests in the ECAP treated samples, irrespective of the strain rate, not produce any significant change in the crystallo graphic texture, since the strain to fracture in all the samples is quite smaller than the strain produced by the ECAP treatments, i.e. less of ~0.17 against ~1 produced by a single ECAP pass. However, some apparent changes are noted for samples processed at two ECAP passes. A single ECAP pass at 200ÂÃ °C (1p/200 °C) appears to induce the formation of the ðÞ111 253 orientation, as Fig. 6a and b μ 100 m reveal. In contrast, the texture induced in the samples treated at 2 ECAP passes via route B and 200 °C (2pB/200ÂÃ °C treatedÂÃ ðÞ114 423 þ ðÞ114 45 4 samples) exhibits twoÂÃ components:ÂÃ − − − − for 10 4 s 1, and ðÞ122 254 þ ðÞ122 212 for 10 3 s 1, as shown c in Fig. 6e and f, respectively. The texture of the 2pC/200 °C −4 −1 treatedÂÃ sample strained at 10 s can be interpreted as ðÞ254 443 orientation despite the apparent spreading of orientations toward the Y direction (Fig. 6i). In contrast, the − − texture of the 2pC/200 °C treated sample strained at 10 3 s 1 is ÂÃ similar to that developed in the 1p/200 °C sample, ðÞ111 253 , (Fig. 6j). The texture characteristics developed in the samples ECAP treated at 300 °C are somewhat different. The textures found in theÂÃ 1p/300 °C samples,ÂÃ shown in Fig. 6c and d, are ðÞ254 443 and ðÞ111 341 depending on if the strain rate −4 −1 −3 −1 was 10 s or 10 s , respectively. In the 2pB/300ÂÃ °C sample −4 −1 ðÞ111 25 3 strained atÂÃ 10 s the texture found is (Fig. 6g) but ðÞ132 135 – – – the component appears in the sample strained at Fig. 2 Microstructure of PM Al 5Mg 1.2Cr in the − − 10 3 s 1 as Fig. 6h reveals. The last orientation is related with homogenized condition: (a) OM image on a longitudinal the texture of 2pB/200 °C sample by a 35° rotation around a plane, (b) OM image on a transversal plane and (c) TEM image ÂÃ 〈111〉 axis. It should be mentioned that a weak ðÞ235 111 showing the grain structure and the dispersion of the second component is also observed in the 2pB/300 °C sample strained phase particles. −3 −1 at 10 s . The measured PF in the samplesÂÃ 2pC/300 °CÂÃ are shown in Fig. 6k and l. The ðÞ423 332 and ðÞ112 531 − − variations in the propagation rate of deformation bands that components are found in the sample strained at 10 4 s 1 −3 −1 may be induced by imperfections in the sample surface. In while the texture inÂÃ the sample strainedÂÃ at 10 s is the hardening stage following the initial one, the stress composed of the ðÞ243 302 and ðÞ122 201 components. oscillations exhibit a behavior corresponding to type B serrations in agreement to the results reported for the 3.4. Microstructure after RT Tensile Deformation Al 5Mg alloy tensile deformed in the strain rate range of −4 −1 −3 −1 ε 10 s 10 s [13]. Then, the c for the development of ECAP deformation induces the refinement of the initial type B serrations appears to correspond to the Lüders strain microstructure by grain fragmentation and the second phase 4 a 500 b 500

2 1 2 1 400 3 4 400 3 4

300 300

. ε=10-3 s-1 200 200

. -4 -1 True stress (MPa) ε=10 s True stress (MPa)

o (111)[253] o 100 1: 1p/200 C 100 1: 1p/200 C (111)[253] 2: 1p/250oC 2: 1p/250oC o o 3: 1p/300 C (254)[443] 3: 1p/300 C (111)[341] 4: Homogenized 4: Homogenized

0 0 0 0,05 0,1 0,15 0,2 0 0,05 0,1 0,15 0,2 True strain True strain c 500 d 500

1 1 2 2 400 400

300 300

200 200 . ε=10-4 s-1 ε. -3 -1 True stress (MPa) =10 s True stress (MPa)

o o 100 1: 2pB/200 C (144)[423]+(144)[454] 100 1: 2pB/200 C (122)[254]+(122)[212] 2: 2pB/250oC 2: 2pB/250oC o o 3: 2pB/300 C (111)[253] 3: 2pB/300 C (132)[135]+(235)[111]

0 0 0 0,05 0,1 0,15 0,2 00,050,10,150,2 True strain True strain e 500 f 500

1 1 2 3 2 400 400 3

300 300

200 200 . ε=10-3 s-1 True stress (MPa) . True stress (MPa) ε=10-4 s-1

o 100 o 100 1: 2pC/200 C (111)[253] 1: 2pC/200 C (254)[443] o o 2: 2pC/250 C 2: 2pC/250 C o o 3: 2pC/300 C (243)[302]+ (122)[201] 3: 2pC/300 C (423)[332]+ (112)[531]

0 0 0 0,05 0,1 0,15 0,2 00,050,10,150,2 True strain True strain

5 0,05 500 1p 1p . -4 -1 . -4 -1 ε=10 s { 2pB ε=10 s { 2pB 2pC 2pC 450 . -3 -1 1p . -3 -1 1p ε=10 s { 2pB ε=10 s { 2pB 0,04 2pC 2pC

400

0,03 c ε (MPa) 350 c σ

300 0,02 Critical strain strain Critical

Critical stress stress Critical 250

0,01

200

0 150 150 200 250 300 350 150 200 250 300 350 o ECAP temperature ( C) ECAP temperature (oC)

– Fig. 4 Effect of the ECAP temperature and strain rate on the Fig. 5 – Effect of the ECAP processing on the critical stress for critical strain for the onset of type B serrated flow in the onset of type B serrations in the Al–5Mg–1.2Cr alloy. Al–5Mg–1.2Cr alloy.

effect of SB propagation on recrystallized areas surrounding particles redistribution. The ECAP treatment at 200 °C pro the grain boundaries, especially, those boundaries parallel to duced a microstructure finer than the ones observed in the the short axis of the grains as indicated by arrows in Figs. −4 −1 homogenized material or after ECAP deformation at 300 °C. 7eand9b. The 2pC/200 °C sample tensile strained at 10 s The slight coarsening of the microstructure observed at 300 °C exhibits a microstructure slightly coarser than the is due to the recovery enhancement at this temperature, corresponding one developed in the 1p/200 °C sample, compare which decreases the stored energy preventing therefore grain Fig. 7gandc.In contrast, when a 2pC/200 °C sample is −3 −1 refinement. In order to assess the effect ECAP deformation on deformed at 10 s the opposite occurs, besides exhibiting a the flow characteristics of the Al 5Mg 1.2Cr alloy after RT severely deformed micro structure and perceptible SB traces −4 tensile deformation, the microstructure was examined in the alike to those observed in the 1p/200 °C sample strained at 10 s −1 gauge and grip regions of the tensile strained samples. The (Fig. 7h). microstructures observed in strained regions were only Fig. 8 shows the microstructure of the material ECAP slightly finer than those in unstrained areas were. processed at 300 °C. Now, the microstructure appears to be A general view of the microstructures for the samples ECAP less differentiated each other, and apparently coarser than deformed at 200 °C after being tensile strained at RT, along the corresponding to the samples ECAP treated at 200 °C, with those corresponding to the homogenized material, are compare Fig. 8a and e to Fig. 7c and g, respectively. A similar shown in Fig. 7. The microstructure of the homogenized effect of SB propagation on the microstructure is observed as −4 −1 −3 −1 material strained up to fracture at 10 s and 10 s is shown shown in Fig. 9c and d. for comparison in Fig. 7a and b, respectively. The grain Fig. 10 shows SEM images that reveal the interaction of SBs structure is clearly elongated along the tensile axis. The first with the second phase particles in the homogenized material ECAP pass produces a fairly equiaxial grain structure −4 −1 tensile tested at RT at 10 s . These second phase particles homogeneously deformed, see Fig. 7c and d. OM close images have been identified elsewhere as Al18Mg3Cr2 [34]. Itwas showing traces of SBs and the interaction of deformation with observed that the initial bands nucleated at the curved ends of grain boundaries and particles are shown in Fig. 9a. The grain the sample gauge region, where a high stress concentration structure in the 2pB/200 °C is elongated along a direction ~15° should be developed, and propagated toward the opposite end. inclined respect to the tensile axis. A general characteristic of These bands during their propagation can be either temporally these microstructures is the retained or completely arrested by obstacles. In this last case, a

− − − − Fig. 3 – σ–ε curves for the ECAP deformed Al–5Mg–1.2Cr alloy tensile tested at RT and strain rates of 10 4 s 1 and 10 3 s 1: (a, b) a single ECAP pass (1p), (c, d) two ECAP passes via route B (2pB); (e, f) two ECAP passes via route C (2pC). The σ–ε curves for the homogenized alloy are included for comparison. The arrows indicate the onset of the type B serrations. The texture is also included for comparison. 6 ECAP 002/PACE o tsetelisnetTR+C 003/PACE o tsetelisnetTR+C process

p1 a b c d

(m=-0.009) (m= 0.000) Bp2 e f g h

(m=-0.011) (m=-0.014) Cp2 i j k l

(m= 0.000) (m=-0.010)

Fig. 6 – (111) PF showing the crystallographic texture, and the strain rate sensitivity value m, for ECAP deformed Al–5Mg–1.2Cr − − − − after being tensile strained up to fracture at 10 4 s 1 and 10 3 s 1. The m value is determined from Eq. (1).

new front of a new band, initiated at any of the ends of the frequently observed as the images of Fig. 12 reveal. These TEM sample gauge region, would move toward the opposite one. images evidence the interaction between the SBs and Then, a halting band propagation yields the serrations observed the particles, i.e. band pinning and release by particles as the in σ ε curves. A remarkable feature of the band structure is the SEM image in Fig. 11b shows. It should be mentioned that zigzag band fronts developed on the sample surfaces due to the neither development of a subgrain structure nor particle interaction of the bands with the second phase particles as shearing and void formation at the particle/matrix interface Fig. 10b reveals. Also, the interaction between SBs gives rise to a are found in the tensile deformed samples. characteristic cross banded structure with intersection angles of ~70° as the images in Fig. 10aandcshow. The SB structures induced by tensile deformation at RT in 4. Discussion the samples ECAP processed at 200 °C and 300 °C are shown in Fig. 11. They are comparable to the ones developed in the 4.1. Strain Rate Effect homogenized alloy irrespective of the ECAP conditions and tensile strain rate. The bands are found oriented about 45° in The microstructure of the extruded PM Al 5Mg 1.2Cr alloy in the respect to the tensile axis, and spread all over the grains in the homogenized condition is composed by coarse elongated grains whole gauge region as observed for the case of the homoge containing substructure and small recrystallized grains (Fig. 2). nized material. Neither dissolution nor precipitation of the second phase particles is expected to occur during the ECAP treatments or tensile tests as 3.5. TEM Observations has been confirmed by differential scanning calorimetry [34]. The σ ε curvefor thealloyinthehomogenized The microstructure of the tensile tested samples revealed by condition shows serrations starting at around the onset of the TEM was alike for all the samples irrespective of the treatment plastic deformation (Fig. 3). Then, no critical plastic strain appears underwent before testing. Fig. 12 shows the microstructure of −4 −1 to be needed to trigger the SF in this case. Apparently, these the alloy tensile deformed at 10 s , in the homogenized serrations are similar to the type B serrations frequently reported condition and after being ECAP treated. In all the cases, plastic in Al Mg alloys undergoing the PLC effect. It was attributed to deformation was found confined in extensive areas parallel bands going across the entire gauge region of the sample containing a very high density of dislocations. Deformation resulting in a surface relief without formation of micro shear zones associated with the second phase particles present in bands [13]. However, the surface of the homogenized material the alloy are 7 after tensile deformation at RT exhibits quite different structural interaction of SBs with the second phase particles. Then, the SF features, i.e. groups of SBs passing across the grains along behavior in this alloy cannot be exclusively explained in terms of different directions, and microbands in some isolated zones. the solute dislocation interaction suggesting that shear banding Moreover, the SB fronts show a zigzag path due to their and particles play a fundamental role in the SF behavior of the σ ε interaction with the second phase particles, as shown in Fig. curve. 11b and c. This indicates that the flow instabilities in the alloy The interaction of SBs with particles in the Al 5Mg 1.2Cr under the present testing conditions may be affected by the alloy points out that both the particle size and the inter particle

10−−−4 s−1 10−−−3 s−1 a b

Homogenized

tensile axis c d

1p/200oC

e f

2pB/200oC

g h

2pC/200oC

100 μm

Fig. 7 – OM images of the etched microstructures of Al–5Mg–1.2Cr ECAP treated at 200 °C and in the homogenized condition, − − − − tensile strained up to fracture at 10 4 s 1 and 10 3 s 1. The discontinuous lines indicate SBs traces. 8 spacing may control the SF. If it is tentatively assumed that The obstacle strength for the bands and the local stress plastic flow is produced by thermally activated propagation of acting at the band front would determine the value of ΔE, and the band, the strain rate would be given by the equation [14]: the waiting time tw for a given temperature would decrease with increasing strain rate according to the above equations. Ω ε_ ð2Þ Thus, for a given temperature, ΔE value at a strain rate of tw − − 10 3 s 1 should be smaller than the one for a strain rate of − − where Ω is the plastic strain produced by a band moving to the 10 4 s 1, i.e. the strength of the obstacles decreases with in next obstacle, and tw the waiting time of the band for creasing strain rate and lowering the amplitude of the stress surmounting the obstacles that would be given by oscillations in the flow stress curve. This explains the present

−1 results for the homogenized and ECAP processed material in tw v expðÞΔE=kT ð3Þ o relation with the strain rate effect on the serration amplitude. where vo is the attempt frequency, ΔE the average energy barrier Now, the severe plastic deformation effect on the features to be surmounted by the moving band at the obstacles, k the of the SF will be considered. The main features resulting from Boltzmann constant and T the sample temperature. Then, the the analysis of the σ ε curves are summarized as follows: strain rate can be expressed by: i. Two distinguishable features in the σ ε curves have been ε_ v Ω expðÞΔE=kT : ð4Þ o found depending on the strain rate: the initial hardening

− − 10−−4 s 1 10−−−3 s−1 a b

1p/300oC

c d

2pB/300oC

e f

2pC/300oC

100 μm

Fig. 8 – OM close view images of the etched microstructures induced in ECAP treated Al–5Mg–1.2Cr after being tensile strained − − − − up to fracture at 10 4 s 1 and 10 3 s 1. 9 10−−−4 s−1 10−−−3 s−1 a b

1p/200oC 2pB/200oC

tensile axis c d

o 1p/300 C 2pC/300oC

20 μm

Fig. 9 – OM images of the etched microstructures in Al–5Mg–1.2Cr ECAP treated at 300 °C and tensile strained up to fracture at − − − − 10 4 s 1 and 10 3 s 1.

region is followed by either SF with continuous strain On the other hand, the strain rate sensitivity m, evaluated − − − − − − hardening until failure at a strain rate of 10 4 s 1 or a from the σ ε curves at 10 4 s 1 and 10 3 s 1 using Eq. (1),isclose − − succession of plateaus with SF superimposed at 10 3 s 1. to zero or negative depending on the ECAP processing condi ε σ ii. A critical strain c (or stress c) for the onset of type B tion. These m values are shown, along with the texture results − − − − serrations is observed at strain rates of 10 4s 1 and 10 3s 1, in Fig. 6 in order to find some plausible correlation between which decreases with increasing ECAP temperature or texture and strain rate sensitivity. The 1p/200 °C, 2pC/200 °C strain rate. and 1p/300 °C samples exhibit values of m 0, and the rest m<0. iii. The serration amplitude increases with increasing These results suggest that the texture induced by ECAP, as well strain, and attains a saturation value near the fracture as the microstructure, could have capability to lower the m strain. value to negative values. It should be mentioned that a decrease of the strain rate sensitivity with increasing strain induced SF occurs under the effect of the applied and the internal by cold work has been reported for the 5086 AA alloy [36]. stresses [7,35]. Therefore, the internal stresses produced by the severe plastic deformation may have an effect on the 4.2. Texture Effects mode of nucleation and propagation of the bands. The ε σ stress value for c is the threshold stress c beyond which Now, let us check the relationship between the ECAP SBs form during tensile deformation, and move along the conditions and the mechanical characteristics of SF in σ ε shear plane through the gauge region of the sample. For a curves. For the material ECAP treated at 200 °C, the 2pC/200 °C σ −4 −1 given ECAP route and number of passes, the decrease of c sample tensile strained at 10 s exhibits a high SF ampli ε σ with increasing ECAP temperature (5) is attributable to the tude, and c and c values higher than the corresponding dynamic recovery and rearrangement of dislocations during values for the 1p/200 °C and 2pB/200 °C samples (see Figs. 4 ε the ECAP deformation, which would be stronger at higher and 5). In contrast, the elongation to failure f is low. This processing temperature. As a result, in the samples ECAP discrepancy is, in principle, associated with the deformed treated at 200 °C, the contribution of the internal stresses to microstructure and texture. The grain structure of the 2pC/ the local effective stress for band nucleation and propaga 200 °CÂÃ sample is quite fine and its preferential orientation ðÞ254 443 tion should in general be smaller all over the samples , which has only two active slip systems. OnÂÃ the ðÞ111 25 3 requiring a higher applied stress. However, the internal contrary, the samples 1p/200 °C with orientation ÂÃ ðÞ114 423 stresses in the samples ECAP deformed at higher temper and theÂÃ 2pB/200 °C samples with orientations and atures would be localized at the dislocation walls so that ðÞ114 454 would have four and five slip systems, respective the threshold stress for band nucleation would attained ly. Also, the samples ECAP treated at 300 °C and strained at − − under smaller applied stresses at the regions with disloca 10 4 s 1 exhibit a similar variety of preferential orientations tion rearrangement. and active slip systems, along with similar SF characteristics. 10 a the samples ECAP treated at a given temperature, i.e. the lower the number of slip systems the higher the critical strain for the SF and the lower the strain to failure. However, the result comparison between the samples ECAP treated at the same temperature does not confirm such correlation irrespective of the strain rate. When the results for samples ECAP treated at the different temperatures are compared, those exhibiting identical textures, or nearly the same, do not ε σ ε yield comparable c, c and f values, nor SF characteristics, as the corresponding σ ε curves shown in Fig. 3 reveal. The above indicates that the effect of texture on the SF of the ECAP treated Al 5Mg 1.2Cr alloy results to be ambiguous, and therefore, the grain structure in conjunction with the characteristics of the second particles particle would be the b factors controlling the SF of the alloy tensile strained under the present conditions.

4.3. Effects of the Grain Structure and Second-Phase Particles

The σ ε curves of the ECAP processed samples tested at a − − strain rate of 10 3 s 1 exhibit a succession of serrated plateaus (Fig. 3b, d and f). Similar plateaus have also been observed in other Al alloys although their origin has not been clearly established. For instance, the σ ε curves of the pre deformed Al 5Mg 0.8Mn alloy exhibited SF on a sequence of plateaus. These plateaus did not appear for non pre deformed samples that showed SF accompanied by continuous strain hardening [37]. The development of plateaus in σ ε curves for Al Mg alloys c has also been associated with generation of bands, and each tensile axis oscillation superimposed on the plateaus would corre spond to formation of a moving band ahead of a prior one that has been pinned by obstacles [38]. Fig. 13 illustrates schematically a mechanism proposed to accountfortheSFcharacteristics of theECAPtreatedAl 5Mg 1.2Cr alloy in conjunction with the evolution of the microstruc ture. It is assumed that the most plausible sites for the nucleation of thefirst bandaretheends ofthesamplegaugeregion dueto high concentration of stresses that would be developed at these curved ends [39].In fact,theappearanceofthefirstmovingSB during the tensile tests was observed at one of the ends of the sample gauge region. The hard particles in the matrix represent obstacles for the band propagation. The formation of the first plateau observed in all σ ε curves is attributed to the nucleation – Fig. 10 SEM images showing the structure of SBs in the of this first SB and its propagation toward the opposite one – – homogenized Al 5Mg 1.2Cr alloy tensile tested at RT and (Fig. 13a). This band during its propagation would interact with −4 −1 strain rate of 10 s : (a) band fronts propagated along particles and shear the grains favorably oriented respect to the different directions; the inset shows details of parallel bands, tensile axis, i.e. those grains with a high Schmid factor for which (b) interaction of SBs with the second phase particles and the resolved shear stress would be high enough for band (c) cross-banded structure inside a grain. propagation [7]. The band moves on the glide planes of a grain until it runs into a grain boundary. If the adjacent grains are favorably oriented to activate dislocation slip on a glide plane, then the band For instance,ÂÃ the 1p/300 °C sample with preferential orienta ðÞ254 443 ε σ ε tion exhibit high c and c values and low f like the propagation would continue. On the contrary, if this condition is 2pC/200 °C sample. Moreover, the 1p/300 °C samples having not satisfied, the band remains hindered by the obstacles until the two active slip systems developed a grain structure elongated effective stress, or applied strain, gives rise to a favorable along the tensile axis, but the samples 2pB/300 °C and 2pC/ dislocation arrangement to surpass the obstacle. A new band 300 °C with four or five slip systems show grain elongation would not form during the expansion of the plateau although stress along an axis clearly rotated respect to the tensile axis (Fig. 8c oscillations superimposed on the plateau appear and e). All the above suggests a correlation between the duetotheactionof theeffectivestresson apriorband hampered crystallographic texture and the characteristics of the SF of by the grain boundaries and second phase particles. 11 a b

c d

Fig. 11 – SEM images showing the structure of SBs in the ECAP processed Al–5Mg–1.2Cr alloy tensile tested at RT: − − − − (a, b) 2pB/200 °C samples tensile tested at strain rates of 10 4 s 1 and 10 3 s 1, respectively. (c, d) 2pC/300 °C samples tensile − − − − tested at 10 4 s 1 and 10 3 s 1, respectively.

a b c

Al18Mg3Cr2

d e f

Fig. 12 – TEM images showing areas of plastic deformation zones associated with the second phase particles in the − − Al–5Mg–1.2Cr alloy tensile tested at RT at 10 4 s 1: (a)–(c) samples in the homogenized condition and (d)–(f) 2pC/250 °C samples. 12 Plateaus with serrated flow Serrated flow with continuous hardening

curvature Volumen, vo

gauge length

grip

first moving band

previous SBs

moving band True strain

b vo

necking

Fig. 13 – Outline describing the propagation mechanism of the SBs in the Al–5Mg–1.2Cr alloy tensile tested at RT that accounts for the variation of the SF features induced by a change in the strain rate: (a) interaction of the moving SBs with the grains along the sample gauge length and (b) representation of the volume swept (Vo) by the moving SBs during the deformation cycle of a band.

When the plateau in the σ ε curve is over, the moving band by the successive SBs becomes gradually smaller. This produces there will sweep a determined volume of sample, Vo in Fig. 13b, the disappearance of visible plateaus, and thus the saturation of which means that a deformation banding cycle of grains has just the stress oscillations and failure of the sample. All of the above − − been accomplished. The stress jump from a plateau to the next account for the SF behavior of the samples tested at 10 3s 1. − − one indicates that a threshold stress is required to activate a new For the samples tested at 10 4 s 1,the σ ε curves (Fig. 3a, c band at one of the ends of the sample gauge region. Then, the and e) exhibit SF with continuous hardening just after the new plateau appears in the σ ε curve giving rise to other completion of the Lüders stage. No plateaus are observed in this deformation banding cycle. This would induce cross banding in regime, apparently. The SF behavior in this case is similar to that − − grains initially sheared and plastic deformation in unsheared observed for the samples tested at 10 3 s 1 after completion of grains. Later on, as the applied strain increases, the nucleation the plateaus stage. Thus, the corresponding mechanism pro − − probability of SBs also increases all over the gauge region of the posed for the samples tested at 10 3 s 1 can be extrapolated to − − sample; i.e. the threshold stress for band nucleation lowers and explain the SF with continuous hardening at 10 4 s 1.Atthis the new SBs form faster. Now, beyond certain tensile strain a new strain rate, the velocity of mobile dislocations is expected to be band forms and moves before the prior one completes its low enough to allow a more homogeneous distribution of − − propagation. This reduces the volume swept by each moving deformation in comparison to the strain rate of 10 3 s 1. Then, band during its deformation cycle and makes less appreciable the the plastic flow should be practically distributed in the entire plateaus in the σ ε curves as schematically outlined in Fig. 13b. As volume of the deformed sample, and much more potential sites the applied strain still increases more, the sample volume swept for the band nucleation there would be all along the gauge region 13 of the sample. Although the nucleation of the first band is [2] Pink E, Kumar S. Patterns of serrated flow in low carbon steel. expected at the ends of the sample gauge region, the following Mater Sci Eng A 1995;201:58 64. [3] Hayes RW. On a proposed theory for the bands may be nucleated all over the gauge region. However, the disappearance of serrated flow in fcc Ni alloys. Acta Metall nucleation of a new band in this case occurs before the preceding 1983;31:365 71. band finishes its deformation cycle. Increasing applied strain, [4] Dziadon A. The effect of grain size on serrated flow of nickel. the successive bands would sweep less volume and the cycle of Scr Mater 1996;34:375 80. deformation banding would become shorter. At this stage, [5] Mohamed FA, Murty KL, Langdon TG. The Portevin Le plastic flow instability may be attained in some areas of the Châtelier effect in CuAu. Acta Metall 1974;22:325 32. gauge region resulting in necking and failure of the sample. [6] Hayes RW. The disappearance of serrated flow in two copper alloys. Mater Sci Eng 1986;82:85 92. [7] Fujita H, Tabata T. Discontinuous deformation in Al Mg alloys under various conditions. Acta Metall 1977;25:793 800. 5. Conclusions [8] Robinson JM, Shaw MP. Observations on deformation characteristics and microstructure in an Al Mg alloy during The microstructure and mechanical behaviors at room temper serrated flow. Mater Sci Eng A 1994;174:1 7. ature of extruded PM Al 5Mg 1.2Cr processed by ECAP have been [9] Robinson JM. Serrated flow in aluminium based alloys. Int investigated. The main conclusions are summarized as follows: Mater 1994;39:217 27. [10] Louchet F. Flow stress anomalies, mobile dislocation exhaustion and strain rate sensitivity. Phil Mag 1995;72: 1. The alloy ECAP processed in the temperature range of 200 905 12. 300 °C, as well as in the homogenized condition, exhibited [11] McCormick PG. Dynamic strain aging. Trans Ind Inst Met SF when it was tensile deformed at room temperature. The 1986;39:98 106. threshold stress and strain for the serration onset de [12] Rodriguez P. Serrated plastic flow. Bull Mater Sci 1984;6: creased with increasing ECAP temperature or strain rate. 653 63. [13] Chihab K, Estrin Y, Bubin LP, Vergnol J. The kinetics of the 2. All the samples ECAP processed and tensile tested under the Portevin Le Châtelier bands in an Al 5Mg alloy. Scr Metall present conditions developed shear banding. Interaction of 1987;21:203 8. σ ε the SBs with the second phase particles was observed. The [14] Kubin LP, Estrin Y. Evolution of dislocation densities and the curves for the ECAP processed samples show a transition critical conditions for the Portevin Lechâtelier effect. Acta from SF with continuous strain hardening to SF superimposed Metall 1990;38:697 708. to a sequence of constant stress plateaus, when the strain rate [15] An YG, Wilson DV, Bate PS. The effect of strain path on the − − − − is changed from 10 4s 1 to 10 3s 1. critical strain for serrated flow in solution treated AA6082. Scr − − Mater 1996;34:1641 6. 3. The plateaus in σ ε curves developed at a strain rate of 10 3s 1 [16] Hayes JS, Keyte R, Plangnell PB. Effect of grain size on tensile were ascribed to the nucleation of a band at one end of the behavior of a submicron grained Al 3Mg alloy produced by sample gauge region and its subsequent propagation towards severe deformation. Mater Sci Tech 2000;16:1259 63. the opposite one, i.e. to a cycle of deformation banding. The [17] Muñoz Morris MA, Oca CG, Morris DG. Mechanical behavior following plateau was associated with the nucleation of a of dilute Al Mg alloy processed by equal channel angular new band in one end of the sample gauge region and its pressing. Scr Metall 2003;48:213 8. [18] Markushev MV, Murashkin MY. Structure and mechanical propagation. − − properties of commercial Al Mg 1560 alloy after equal 4. At a low strain rate of 10 4 s 1 the sites for band nucleation channel angular extrusion and annealing. Mater Sci Eng A should be randomly distributed along the gauge region of the 2004;367:234 42. sample. The disappearance of the plateaus and continuous [19] Kapoor R, Gupta C, Sharma G, Chakravartty JK. Deformation strain hardening in the σ ε curves are attributed to the behavior of Al 1.5Mg processed using the equal channel activation of a new moving band before the completion of angular pressing technique. Scr Metall 2005;53:1389 93. thedeformationbandingcycleoftheprecedingband. [20] Prados E, Sordi V, Ferrante M. Tensile behaviour of an Al 4Cu alloy deformed by equal channel angular pressing. Mater Sci 5. No obvious relationship between the texture and the SF Eng A 2009;503:68 70. characteristics has been found. [21] Saha GG, McCormick PG, Rao PR. Portevin Le Châtelier effect in an Al Mn alloy II: yield transition and strain rate sensitivity measurements. Mater Sci Eng A 1984;62:187 96. Acknowledgments [22] Pink E. The effect of precipitation on characteristics of serrated flow in Al5Zn1Mg. Acta Metall 1989;37:1773 81. The experimental work has been carried out at the LMNM [23] Kumar S, McShane HB. Serrated yielding in Al Li alloys. Scr Mater 1993;28:1149 54. (LM 290) laboratory supported by Madrid Community through [24] Kumar S, Pink E. Serrated flow in aluminium alloys the project TECHNOFUSION (S2009/ENE 1679) and Spanish Min containing lithium. Acta Mater 1997;45:5295 301. istry of Science and Innovation (contract ENE2008 06403 C06 04). [25] Shen YZ, Oh KH, Lee DN. Serrated flow behavior in 2090 Al Li alloy influenced by texture and microstructure. Mater Sci Eng A 2006;435:343 54. REFERENCES [26] Chmelík F, Pink E, Król J, Balík J, Pesicka J, Lukác P. Mechanisms of serrated flow in Al alloys with precipitates investigated by acoustic emission. Acta Mater 1998;46: 4435 42. [1] Hayes RW, Hayes WC. A proposed model for the [27] Loannidis EK, Marshall GJ, Sheppard T. Microstructure and disappearance of serrated flow in two Fe alloys. Acta Metall properties of extruded Al 6Mg 3Cr alloy prepared from 1984;31:259 67. rapidly solidified powder. Mater Sci Tech 1989;5:56 64. 14 [28] Abramov VO, Sommer F. Structure and mechanical [34] Eddahbi M, PhD dissertation, Facultad de Ciencias Físicas \properties of rapidly solidified Al (Fe, Cr) and Al Mg (Fe, Cr) (1998). Universidad Complutense, Madrid. alloys. Mater Lett 1994;20:251 5. [35] Kim IS, Chaturvedi MC. Serrated flow in Al 5wt.%Mg alloy. [29] Maeng DY, Lee JH, Hong SI, Chun BS. Microstructure Mater Sci Eng A 1979;37:165 72. and mechanical properties of rapidly solidified [36] Roopchand BJ, Morris JG. The effect of cold work on the strain Al 7wt.%Mg X (X=Cr, Zr or Mn) alloys. Mater Sci Eng A rate sensitivity of 5086 aluminium alloy. Scr Metall 1973;7: 2001;311:128 34. 839 46. [30] Shin DH. Superplastic behavior of rapidly solidified [37] Korbel A, Dybiec H. The problem of the negative strain rate Al 5Mg 1.2Cr alloy. J Mater Sci Lett 1989;8:1412 3. sensitivity of metals under the Portevin Lechâtelier [31] Eddahbi M, Carsí M, Ruano OA. Characterization of a deformation conditions. Acta Metall 1981;29:89 93. thermomechanically powder metallurgy Al 5Mg 1.2Cr alloy. [38] Ait Amokhtar H, Boudrahem S, Fressengeas C. Mater Sci Eng A 2003;361:36 44. Spatiotemporal aspects of jerky flow in Al Mg alloys, in [32] Král R, Lukác P. Modelling of strain rate hardening and its relation with the Mg content. Scr Mater 2006;54:2113 8. relation to the onset of Portevin Le Châtelier effect in Al Mg [39] Thuillier S, Rauch EF. Interaction of microbands with grain alloy. Mater Sci Eng A 1997;234 236:786 9. boundaries in mild steel. Scr Mater 1995;32:541 6. [33] Balík J, Lukác P, Kubin LP. Inverse critical strains for jerky flow in AlMg alloys. Scr Mater 2000;42:465 71.