A New Maraging Stainless Steel with Excellent Strength–Toughness–Corrosion Synergy

materials

Article A New Maraging Stainless Steel with Excellent Strength–Toughness–Corrosion Synergy

Jialong Tian 1,2, Wei Wang 2,*, M. Babar Shahzad 2, Wei Yan 2, Yiyin Shan 2, Zhouhua Jiang 3 and Ke Yang 2 1 School of Materials Science and Engineering, Northeastern University, Shenyang 110004, China; [email protected] 2 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China; [email protected] (M.B.S.); [email protected] (W.Y.); [email protected] (Y.S.); [email protected] (K.Y.) 3 School of Metallurgy, Northeastern University, Shenyang 110004, China; [email protected] * Correspondence: [email protected]; Tel.: +86-24-8397-8745

Received: 17 October 2017; Accepted: 8 November 2017; Published: 10 November 2017

Abstract: A new maraging stainless steel with superior strength–toughness–corrosion synergy has been developed based on an innovative concept of alloy design. The high strength–toughness combination is achieved by forming dispersive nano-sized intermetallic compounds in the soft lath martensitic matrix with a slight amount of residual austenite. The good corrosion resistance is guaranteed by exactly controlling the Co content based on understanding the synergistic effect between Co and Cr. The ﬁne structure characteristics of two dominant strengthening precipitations including Ni3Ti and Mo-rich phases were ﬁnely characterized associated with transmission electron microscope (TEM) and atom probe tomography (APT) analyses. The relationship among microstructure, strength and toughness is discussed. The precipitation mechanism of different precipitates in the new maraging stainless steel is revealed based on the APT analysis.

Keywords: maraging stainless steel; alloy design; strength and toughness; corrosion resistance; atomic probe tomography; precipitation mechanism

1. Introduction Maraging stainless steels (MSS) are a class of high strength stainless steels with excellent comprehensive performances including high strength, superior corrosion resistance and good weldability, etc. [1–5]. Since the first maraging stainless steel was developed by Carnegie Illionois Steel Corporation in 1946 to meet military demands, many efforts have been successfully realized to improve its performance [6–14]. Nowadays, new maraging stainless steel, as an important candidate material to ensure the lower energy consumptions and pollution reduction owing to its unique combination of properties including high strength, high toughness and superior corrosion resistance, is attracting much greater attention. However, a significant bottleneck that hinders the future applications of maraging stainless steels as potential structural materials is the trade-off dilemma among their strength, fracture toughness and corrosion resistance. As far as the strength of MSS is considered, different methods have been tried to increase their strength, such as multiple-phase strengthening [1–3], co-precipitation of nano-scale particles [7,8], and minimal lattice misfit between matrix and precipitate [13]. Meanwhile, attentions have been concentrated on characterizing the relationship between strength and precipitate, for instances, the lattice structure, orientation relationship and evolution process during aging treatment. Besides the strength of MSS, corrosion resistance is another important and particularly critical property for future applications, because more and more materials will be subjected to extreme mechanical loads and harsh environmental conditions where corrosion is an important issue. Unfortunately, MSS with

Materials 2017, 10, 1293; doi:10.3390/ma10111293 www.mdpi.com/journal/materials Materials 2017, 10, 1293 2 of 11 high strength is always accompanied with a loss in toughness or corrosion resistance. In general, two methods are usually considered to increase the strength of MSS. One is to add carbon (C) to strengthen the lath martensitic matrix. However, higher content of C could present a gradual incremental trend to form chromium carbides, which are harmful for both toughness and corrosion resistance [15,16]. Another method is to add more strengthening elements to increase the strength by precipitation hardening. However, most strengthening elements such as Mo, Ti and Al are also ferrite formation elements and they will promote the formation of delta-ferrite, which is harmful to the toughness. Different from those strengthening elements, Co is often added to increase the strength of maraging stainless steel by its synergistic effect with strengthening elements such as Ti and Mo [11–14]. However, Co also accelerates the spinodal decomposition of Cr atoms during the aging process, which could deteriorate the corrosion resistance [17]. In this study, a new maraging stainless steel, deﬁned as “IMR steel” (Steel from the Institute of Metal Research), was developed based on an innovative concept of synergistic effect of multiple elements in maraging stainless steels. The microstructure of quenched IMR steel was comprised of soft lath martensitic and residual austenite. In the subsequent aging process, large amounts of nano-sized intermetallic compounds precipitate in the matrix. The IMR steel exerts a high strength in combination with high toughness and good corrosion resistance, showing outstanding performance compared with other commercial maraging stainless steels. It is expected that the new maraging steel will have potential applications in the aircraft, power generation, tools and automotive industries.

2. Alloy Design According to the conventional “stainless theory”, it is generally believed that Cr must be added to above 11 wt % to impart “stainlessness” to steels. At this Cr level, an adherent and self-healing chromium oxide can be formed on the steel surface in the relatively benign corrosive environment, especially with 13 wt % Cr addition in steel, and superior corrosion resistance could be achieved [18]. Thus, the Cr content has been set as 13 wt % to guarantee a superior corrosion resistance. Co is a key element in maraging stainless steel since it controls the balance between strength and corrosion resistance. According to previous research [17], when Co content was 13 wt %, the MSS showed a poor corrosion resistance even with 13 wt % Cr addition. When Co content was decreased to 7 wt %, the MSS showed a good corrosion resistance in 3.5% NaCl solution. Thus, the Co content has been set as 7 wt % to achieve the balance between strength and corrosion resistance. Ti is the most effective strengthening element in maraging stainless steels. In order to reach a strength target of 1900 MPa, Ti content has been set as 1.7 wt %. Considering that the co-addition of Mo and Ti could form a core-shell structure [2] that is good for both strength and toughness, Mo content is set as 3 wt %. Although Mo is good for both strength and corrosion resistance, more Mo addition is infeasible considering the occurrence of delta-ferrite. The role of Ni in maraging stainless steel is crucial. On the one hand, Ni could enlarge the austenite region and avoid delta-ferrite formation during cooling via austenite phase ﬁeld extension. On the other hand, Ni could form the intermetallic phase, especially form η-Ni3Ti which is the dominant strengthening precipitate in the Ti-alloyed maraging stainless steels. Also, Ni content should be designed precisely to control the volume fraction of residual austenite that is important for both strength and toughness. Ni content in the IMR steel is designed to be 7.5 wt %. Finally, in order to guarantee the high fracture toughness, impurities such as C, O and N should be kept less than 30 ppm, respectively. Based on the above alloy design analyses, the nominal composition of the IMR steel is designed to be (wt %) Cr 13.0%, Ni 7.5%, Co 7.0%, Mo 3.0% and Ti 1.7%.

3. Experimental Details The IMR steel was ﬁrst melted in a 25 kg vacuum induction melting furnace and then remelted by vacuum arc melting. Fe, Cr, Ni, Co, Mo and Ti were added in the form of high purity metals (>99.99%) to obtain a super-clean steel. Chemical composition of the steel analyzed by ICP (inductively coupled Materials 2017, 10, 1293 3 of 11 plasma) is shown in Table1. Casting ingot with a diameter of 100 mm was homogenized at 1200 ◦C for 48 h and then forged into square bars (20 mm × 20 mm). Specimens for tests and microstructure characterizations were subjected to solution annealing at 1050 ◦C for 1 h followed by cryogenic treatment (CT) in liquid nitrogen for 8 h. Finally, aging treatment (AT) with different holding time (10 min, 0.5 h, 4 h, 16 h, 40 h, 100 h) was performed at 480 ◦C. Other commercial maraging stainless steels were taken from the steel enterprise and were peak aging (PA) treated to reach its highest strength. Afterwards, corrosion current density of all steels has been tested. Mechanical property (fracture toughness and ultimate tensile strength) of three steels (IMR steel, 1RK91 and Custom 475) were tested practically and other steels’ mechanical property was taken from the references. In order to guarantee the reliability, three samples have been tested to get the average.

Table 1. Chemical composition of Institute of Metal Research (IMR) steel (wt %).

Cr Ni Co Mo Ti C O N Fe 12.53 7.45 7.16 3.14 1.75 0.0024 0.0028 0.0026 Bal.

TEM characterization was conducted using a JEM 2100 transmission microscope (Japan Electron Optical Laboratory, Tokyo, Japan) operated at 200 kV. Thin TEM foils were prepared with care for a large electron transparent area and minimum magnetic inﬂuence. 3 mm diameter discs were ﬁrst mechanically polished to 50 µm and then jet electropolished in a bath containing 10 vol % ◦ HClO4 + 90 vol % C2H5OH at −20 C. Needle-shaped specimens required for APT were prepared by a standard two-step electro-polishing technique. Local electrode atom probe (LEAP) acquisitions were accomplished using a LEAP™ 3000HR under an ultrahigh vacuum of ~1 × 10−8 Pa at a base temperature of 60 K with a pulse frequency of 200 kHz and a 20% pulse fraction. Imago Visualization and Analysis Software (IVAS) version 3.6.2 was used for data reconstruction, composition analysis and the creation of iso-concentration surface. Tensile tests were conducted according to ASTM E8 on an AG-100KG testing machine (Shimadzu, Tokyo, Japan) at a strain rate of 2.0 × 10−3 s−1 at room temperature. Fracture toughness measurements were performed on a SCHENCK-100KN fatigue test machine (Schenck Process GmbH, Darmstadt, Germany) according to ASTM E399 at a crosshead speed of 0.5 mm/min. Potentiodynamic polarization (PDP) tests in 3.5 wt % NaCl solution were conducted using a PARSTAT4000 electrochemical workstation (Princeton Applied Research, Oak Ridge, TN, USA) with a scan rate of 0.1667 mV/s. The surface area of the working electrodes was reduced to 1.0 cm2 using epoxy resin, in order to avoid the edge effect. A conventional three-electrode (reference, counter and working electrodes) electrochemical cell was used in this study.

4. Results and Discussion

4.1. Microstructure and Properties Figure1 shows the fracture toughness, corrosion resistance and ultimate tensile strength of the IMR steel in comparison with some typical commercial maraging stainless steels. Compared with the commercial maraging stainless steels, IMR steel shows a higher ultimate tensile strength (1920 MPa) which is slightly lower than that of Custom 475 (2000 MPa). In addition to strength, two other critical properties of maraging stainless steels are fracture toughness and corrosion resistance. The corrosion current density obtained from PDP scans was used as an indicator of corrosion resistance. It can be seen clearly that 17-4 PH shows the most outstanding corrosion resistance, which could be attributed to its higher chromium content (~17 wt %). In comparison, IMR steel shows an equivalent corrosion resistance to that of 15-5 PH. Moreover, fracture toughness of IMR steel shows notable advantage compared to those maraging stainless steels. In general, strength and toughness show an inverse Materials 2017, 10, 1293 4 of 11 variation trend and it is difﬁcult to improve both simultaneously. However, IMR steel exhibited both high toughnessMaterials 2017, and10, 1293 superior strength, which could be attributed to its unique microstructure.4 of 11

FigureFigure 1. The 1. The strength–toughness–corrosion strength–toughness–corrosion property property profile proﬁless of of the the IMR IMR (Institute (Institute of Metal of Metal Research) Research) steelsteel and comparativeand comparative maraging maraging stainless stainless steels steels underunder peak-aged conditions. conditions. Sphere Sphere denotes denotes the the propertyproperty of comparative of comparative maraging maraging stainless stainless steelsteel an andd pentacle pentacle denotes denotes the the property property of IMR of IMRsteel. steel. The mechanicalThe mechanical property property data data of of ﬁve five commercial commercial maraging stainless stainless were were taken taken from from references: references: 15-5 PH15-5 [ 19PH], [19], 17-4 17-4 PH PH [20 ],[20], PH PH 13-8 13-8 Mo Mo [21 [21],], Custom Custom 465465 [22], [22], Ferrium Ferrium S53 S53 [23]. [23 ].

Figure 2a is a bright field TEM image of the specimen after cryogenic treatment (CT). Typical Figuremartensitic2a is lath a is bright clearly fieldvisible TEM while imagehigh density of the dislocations specimen can afterbe found cryogenic as shown treatment in Figure 2b. (CT). TypicalThe martensitic high-density lath dislocations is clearly in visiblethe matrix while can high offer density preferred dislocations nucleation sites can beand found lead to as the shown in Figureformation2b. The of dispersive high-density precipitates dislocations in the following in the matrixaging process can offer [24,25]. preferred Two types nucleation of strengthening sites and lead toprecipitates the formation were found of dispersive in the peak-aged precipitates specimen in the as following shown in Figure aging process2c. According [24,25 to]. the Two high- types of strengtheningresolution precipitatesTEM image were(Figure found 2(d1,d2)) in the and peak-aged the corresponding specimen FFT as shown pattern in (Figure Figure 2(d3,d4)),2c. According the to the high-resolutionsphere-like precipitates TEM image and rod-like (Figure 2precipit(d1,d2))ates and are the identified corresponding as Mo-rich FFT phase pattern and (Figure Ni3Ti phase,2(d3,d4)), respectively. the sphere-like precipitates and rod-like precipitates are identified as Mo-rich phase and Ni3Ti phase, respectively.As a powerful technique to characterize the nano-sized precipitates, atom probe tomography (APT) is characteristic for its near-atomic spatial resolution and can offer the information of the As a powerful technique to characterize the nano-sized precipitates, atom probe tomography precipitations such as composition, spatial morphology, size, number density, etc., which is beyond (APT)the is scope characteristic of TEM analysis. for its In near-atomic this study, regions spatial that resolution contain more and than can 10 offer at % Mo the and information 35 at % Ni of+ the precipitationsTi are defined such as as Mo-rich composition, phase and spatial Ni3Ti morphology, phase, respectively. size, number Figure 3a density, shows the etc., 3-D which reconstruction is beyond the scopeof of the TEM atomic analysis. positions In thisof Fe study, (pink points), regions isocon that containcentration more surface than for 10 regions at % Mo containing and 35 at more % Ni than + Ti are defined10 at as % Mo-rich Mo (red phase surfaces) and and Ni 335Ti at phase, % Ni + respectively. Ti (green surfaces). Figure A3a careful shows observation the 3-D reconstruction of two types of of the atomicprecipitates positions revealed of Fe (pink that points), each precipitate isoconcentration has two different surface shapes, for regions sphere-like containing Mo-rich more phase than and 10 at % Mo (redbar-like surfaces) Ni3Ti andphase, 35 atas %shown Ni + Tiin (greenFigure 3b,c, surfaces). which A were careful also observation observed by ofTEM. two Meanwhile, types of precipitates Mo- rich phase and Ni3Ti phase have another two refined structures which were not observed by TEM. revealed that each precipitate has two different shapes, sphere-like Mo-rich phase and bar-like Ni3Ti phase,As as shown shown in Figure in Figure 3d,e,3b,c, a ring-like which Mo-rich were also phase observed and sphere-like by TEM. Ni Meanwhile,3Ti phase can be Mo-rich observed. phase It is and believed that small sphere-like Ni3Ti phases were formed at the beginning stage of aging treatment Ni Ti phase have another two refined structures which were not observed by TEM. As shown in 3 and, due to lack of time or hindrance of phase interface, they could not grow to big rod-like Figure3d,e, a ring-like Mo-rich phase and sphere-like Ni Ti phase can be observed. It is believed that precipitates. On the contrary, those with the minimal interface3 energy between Ni3Ti and matrix grew smalland sphere-like finally formed Ni3Ti rod-like phases wereNi3Ti formedprecipitates. at the In beginningcase of Mo-rich stage phase, of aging except treatment for sphere-like and, due to lack ofprecipitates, time or hindrance ring-like precipit of phaseates interface, were also they observed could loca notted grow at tothe big interface rod-like between precipitates. Ni3Ti and On the contrary,matrix. those The with corresponding the minimal 1-D interface concentration energy profile between and Ni atomic3Ti and maps matrix within grew a selected and finally region formed rod-likecrossing Ni3Ti the precipitates. Ni3Ti/matrix interface In case ofare Mo-rich shown in phase, Figure except4. The region for sphere-like between the precipitates, two dashed lines ring-like corresponds to the Mo-rich phase with thickness of about 5 nm. The results indicate that Mo-rich precipitates were also observed located at the interface between Ni3Ti and matrix. The corresponding 1-D concentration profile and atomic maps within a selected region crossing the Ni3Ti/matrix interface

Materials 2017, 10, 1293 5 of 11 are shownMaterials in2017 Figure, 10, 12934. The region between the two dashed lines corresponds to the Mo-rich5 of 11 phase Materials 2017, 10, 1293 5 of 11 with thickness of about 5 nm. The results indicate that Mo-rich phase is more likely to nucleate at the phase is more likely to nucleate at the Ni3Ti/matrix interface, and its forming mechanism will be Ni3Ti/matrixphasediscussed is more interface,later. likely andto nucleate its forming at the mechanism Ni3Ti/matrix will interface, be discussed and its later.forming mechanism will be discussed later.

FigureFigure 2. Microstructure 2. Microstructure characterization characterizationby by high-resolutionhigh-resolution TEM TEM (transmission (transmission electron electron microscope). microscope). (a) TypicalFigure(a) Typical martensitic2. Microstructure martensitic lath lath characterization in in the the specimen specimen by after high-resolafter CTCT (cryogenic ution TEM treatment) (transmission treatment) treatment, treatment, electron lath microscope). lathboundary boundary is outlined(isa )outlined Typical by bymartensitic red red dashed dashed lath line;line; in the ( (bb) )specimenDislocations Dislocations after observation CT observation (cryogenic in the treatment) in region the regiontaken treatment, from taken the lath square from boundary the in (a square); is(c )outlined Morphology by red ofdashed Ni3Ti line; and ( bMo-rich) Dislocations precipitates observation in the in PA-treatedthe region taken specimen; from the(d1 square) shows in (thea); in (a); (c) Morphology of Ni3Ti and Mo-rich precipitates in the PA-treated specimen; (d1) shows the (high-resolutionc) Morphology imageof Ni 3ofTi Mo-richand Mo-rich precipitate precipitates and (d2 in) showsthe PA-treated the corresponding specimen; FFT (d1 )(fast shows Fourier the high-resolution image of Mo-rich precipitate and (d2) shows the corresponding FFT (fast Fourier high-resolutiontransform) pattern image in theof Mo-richinset. (d3 precipitate) shows the and high-resolution (d2) shows the image corresponding of Ni3Ti precipitate FFT (fast and Fourier (d4) transform) pattern in the inset. (d3) shows the high-resolution image of Ni3Ti precipitate and (d4) transform)shows the correspondingpattern in the FFTinset. pattern (d3) shows in the theinset. high-resolution image of Ni3Ti precipitate and (d4) showsshows the correspondingthe corresponding FFT FFT pattern pattern in in the the inset.inset.

Figure 3. Morphology of precipitates observed by atom probe tomography (APT) analysis in PA- FigureFiguretreated 3. Morphology 3.specimen. Morphology (a of) 3-D precipitates of reconstructionprecipitates observed observed of the by atomic by atom at ompositions probe probe tomography oftomography Fe (pink (APT)points (APT)), analysis isoconcentrationanalysis in in PA-treated PA- specimen.treatedsurface ( a specimen.for) 3-D regions reconstruction ( containinga) 3-D reconstruction more of the than atomic 10of theat % positionsatomic Mo (red positions surfaces of Fe ( ofpink) and Fe ( pointspink35 at %points ),Ni isoconcentration + Ti), isoconcentration(green surfaces); surface for regionssurface(b) Sphere-like for containing regions Mo-rich containing more phase than more outlined 10 than at %10by Moat 10 % at (Mored % ( redMo surfaces surfacesisoconcentration) and) and 35 35 at surface; % % Ni Ni + Ti +(c (green) Ti Rod-like (green surfaces); Ni surfaces);3Ti (phaseb) Sphere-like outlined Mo-richby Ni (green phase) and outlined Ti (grey by) 10atoms; at % (dMo) Flake-like isoconcentration Mo-rich surface;phase outlined (c) Rod-like by 10 Ni at3 Ti% (b) Sphere-like Mo-rich phase outlined by 10 at % Mo isoconcentration surface; (c) Rod-like Ni3Ti phasephaseMo outlined isoconcentration outlined byNi by (Nigreen surface; (green) and) (ande) Ti Sphere-like (Tigrey (grey) atoms;) atoms; Ni3Ti ( dphase ()d Flake-like) Flake-like outlined Mo-rich Mo-richby Ni (green phasephase) and outlined Ti(grey by by) atoms.10 10 at at % % Mo Mo isoconcentration surface; (e) Sphere-like Ni3Ti phase outlined by Ni (green) and Ti(grey) atoms. isoconcentration surface; (e) Sphere-like Ni3Ti phase outlined by Ni (green) and Ti(grey) atoms.

Materials 2017, 10, 1293 6 of 11 Materials 2017, 10, 1293 6 of 11

Figure 4. 1-D reconstruction of of the atomic positions ( d–f), and the corresponding one-dimensional concentrationconcentration profile proﬁle ( a–c), across the Ni3Ti/matrix interface interface for for peak-aged peak-aged specimen. specimen.

The good combination of of strength and and toughness for for the the IMR IMR steel can be attributed to the followingfollowing two two aspects. aspects. First, First, since since Ti precipitates Ti precipitates most most rapidly, rapidly, strongly strongly and completely and completely during during aging in the maraging stainless steel [26], Ni3Ti precipitates make a dominant contribution to the strength. aging in the maraging stainless steel [26], Ni3Ti precipitates make a dominant contribution to the Meanwhile, Mo-rich phase retards the growth of Ni3Ti precipitates through its accumulation at the strength. Meanwhile, Mo-rich phase retards the growth of Ni3Ti precipitates through its accumulation interface [1]. Thus, Mo-rich phase could promote the maintenance of highly dispersive Ni3Ti nano- at the interface [1]. Thus, Mo-rich phase could promote the maintenance of highly dispersive Ni3Ti precipitates and enhance the strengthening effect of Ni3Ti precipitates. Second, the highly uniform nano-precipitates and enhance the strengthening effect of Ni3Ti precipitates. Second, the highly distribution of Ni3Ti precipitates effectively reduces stress concentration. In addition, Mo-rich phase uniform distribution of Ni3Ti precipitates effectively reduces stress concentration. In addition, Mo-rich at the interface results in a small lattice misfit between Ni3Ti and matrix [1]. Thus, the associated phase at the interface results in a small lattice misﬁt between Ni3Ti and matrix [1]. Thus, the associated elastic interaction between precipitate and dislocat dislocationion should be lowered. Therefore, the core-shell structurestructure effectively prevents prevents the crack initiation at the precipitate-matrix interface owing owing to the negligible strain accumulation and thus guarante guaranteeses a good combination of strength and toughness.

4.2. Precipitation Precipitation Mechanism Mechanism of Ni 33TiTi and and Mo-Rich Mo-Rich Precipitates Precipitates APT was usedused toto further further characterize characterize the the evolution evolution of threeof three elements elements (Ni, (Ni, Ti, Mo) Ti, duringMo) during the aging the agingtreatment, treatment, and Figure and Figure5 shows 5 shows the visual the visual atomic atomic position position reconstruction. reconstruction. It can It be can seen be clearlyseen clearly that thatNi and Ni Tiand show Ti show similar similar distribution distribution positions, positions, which which indicates indicates that Ni that and Ni Ti formand clustersTi form togetherclusters togetherwhereas whereas Mo forms Mo cluster forms separately. cluster separately. Maximum Maximum separation separation envelope envelope methodology methodology (MSEM)was (MSEM) used wasto further used to analyze further the analyze precipitate the precipitate kinetics of kinetics different of different clusters. clusters. In this method, In this method, the spherical the spherical volume volumeregion around region thearound selected the atoms,selected as definedatoms, as by defined a maximum by a separationmaximum distance, separation dmax distance,, is searched dmax, for is searchedany other for selected any other solute selected atoms. solute The minimum atoms. The solute minimum atoms number solute atoms (Nmin) number threshold (N parametermin) threshold was parameterused to limit was the used size to of limit the solute the size clusters of the to solute those cl moreusters than to those the threshold more than value. the threshold The cluster value. numbers The clusterwere counted numbers based were on counted specific parametersbased on specific (dmax andparameters Nmin) and (dmax the and cluster Nmin density) and the was cluster defined density as the wasratio defined of cluster as numberthe ratio over of cluster the volume number of analyzed over the body.volume Table of 2analyzed shows the body. statistical Table results 2 shows about the statisticalthe clusters results density about under the differentclusters density aging condition under different based on aging the MSEM.condition based on the MSEM. Both cluster analysis results in Table 2 and atomic positions reconstruction in Figure 5a indicate that no cluster exists in the steel matrix after CT. When aging time was 10 min, Ni and Ti clusters

Materials 2017, 10, 1293 7 of 11

Both cluster analysis results in Table2 and atomic positions reconstruction in Figure5a indicate Materials 2017, 10, 1293 7 of 11 that no cluster exists in the steel matrix after CT. When aging time was 10 min, Ni and Ti clusters −5 −3 occurredoccurred as shownas shown in in Figure Figure5(b1,b2), 5(b1,b2), and and the the cluster cluster density density of of NiNi ++ TiTi reachedreached 59.92 ×× 1010−5 nmnm−3 as as shownshown inin TableTable2 2.. However, However, nono MoMo clustercluster formedformed afterafter 10 minmin according to to the the cluster cluster analysis analysis −5 −3 results.results. Cluster Cluster density density of of Ni Ni + + Ti Ti cluster cluster reached reached toto maximummaximum (116.78 (116.78 ×× 1010−5 nmnm−3) after) after aging aging for 0.5 for 0.5 h, and with increase of aging time, time, cluster cluster density density decreased decreased slightly slightly as as a aresult result of of the the aggregation aggregation andand growth growth of Niof 3NiTi3Ti precipitates. precipitates. According According to to thethe clustercluster analysis results results of of Mo Mo in in Table Table 2,2 ,it it can can be be foundfound that that Mo Mo cluster cluster density density only only reached reached to to 0.26 0.26× ×10 10−−55 nmnm−33 eveneven after after aging aging for for 4 4h. h. Meanwhile, Meanwhile, no obviousno obvious cluster cluster was was observed observed in in the the atomic atomic reconstruction reconstruction (Figure(Figure5 5(d3))(d3)).. The maximum value value of of MoMo cluster cluster density density was was 51.52 51.52× ×10 10−−55 nm−3 3afterafter aging aging for for 1616 h. h.

FigureFigure 5. Three-dimensional5. Three-dimensional reconstruction reconstruction ofof thethe atomicatomic positions of of Ni Ni (green (green points points), ),Ti Ti (grey (grey pointspoints) and) and Mo Mo (red (red points points) for) for specimens specimens aged aged at at 753 753 K K forfor differentdifferent time. All All the the analyzed analyzed volumes volumes 3 areare with with the the dimension dimension of of 30 30× ×30 30× × 8080 nmnm3..

TableTable 2. Cluster 2. Cluster analysis analysis of specimensof specimens under under different different aging aging conditions. conditions. (Ni(Ni + Ti cluster: d dmaxmax = =0.5, 0.5, NminNmin= 100;= 100; Mo Mo cluster: cluster: d dmaxmax = 0.5, N Nminmin = =10). 10). Items CT AT 10 min AT 0.5 h AT 4 h AT 16 h AT 40 h AT 100 h Volume ofItems analyzed body (nm3) 638,160 CT AT745,996 10 min 554,895 AT 0.5 h757,120 AT 4 h772,475 AT 16 h874,380 AT 40 h 799,779 AT 100 h VolumeNi + Ti of cluster analyzed density body (10 (nm−5 nm3) −3) 638,1600 59.92 745,996 116.78 554,895 114.51 757,120 79.74 772,475 56.04 874,380 15.63 799,779 Ni + TiMo cluster cluster density density (10 (10−5−5nm nm−−33) 00 59.92 0 116.78 0 0.26 114.51 51.52 79.74 3.09 56.04 9.38 15.63 Mo cluster density (10−5 nm−3) 0 0 0 0.26 51.52 3.09 9.38 The results indicate that Ni and Ti showed much quicker aging response than Mo at the early agingThe resultsstage. Considering indicate that the Ni diffusion and Ti showedcoefficients much of the quicker portioning aging elements response (Ni, than Ti, MoMo,at Fe) the in earlythe agingα-Fe stage. matrix Considering [27–29], the thediffusivity diffusion of Mo coefﬁcients is much lower of the than portioning that of the elements other elements. (Ni, Ti, This Mo, indicates Fe) in the α-Fethat matrix Mo acts [27– as29 a], kinetically the diffusivity controlling of Mo iselement much lowerfor the than precipitation that of the process, other elements.as revealed This by indicatesthe fact thatthat Mo Mo acts is as rejected a kinetically from controllingthe precipitate element into forthe the matrix, precipitation accumulating process, before as revealed the Ni3Ti/matrix by the fact interface. that Mo is rejected from the precipitate into the matrix, accumulating before the Ni3Ti/matrix interface. Figure 6 shows the distribution characteristics of Ni3Ti phase outlined by green Ni + Ti 35 at % isoconcentration surface and Mo-rich phase outlined by red Mo 5 at % isoconcentration surface at different aging stages. When the aging time was 0.5 h, sphere-like Ni3Ti particles are evident, as

Materials 2017, 10, 1293 8 of 11

Figure6 shows the distribution characteristics of Ni 3Ti phase outlined by green Ni + Ti 35 at % MaterialsisoconcentrationMaterials 2017 2017, ,10 10, ,1293 1293 surface and Mo-rich phase outlined by red Mo 5 at % isoconcentration surface88 of of 11 at 11 different aging stages. When the aging time was 0.5 h, sphere-like Ni3Ti particles are evident, as shown showninshown Figure in in 6Figure b.Figure When 6b. 6b. theWhen When aging the the timeaging aging extended, time time extended, extended, a further a a further further growth growth growth of particles of of particles particles seems seems seems to occur, to to occur, occur, and their and and theiraveragetheir average average diameter diameter diameter also increases.also also increa increa Butses.ses. theBut But evolution the the evolution evolution of Mo-rich of of Mo-rich Mo-rich phase phase phase was different,waswas different, different, as no as as Mo no no 5Mo Mo at %5 5 atisoconcentrationat % % isoconcentration isoconcentration surface surface surface was was was detected detected detected in thein in the the specimen sp specimenecimen aged aged aged for for for 4 4 h, 4 h, h, as as as shown shown shown in in in FigureFigure Figure6 6c,c, 6c, whichwhich which couldcould be be attributed attributed toto a a low low MoMo Mo concentrationconcentration concentration atat at NiNi Ni33Ti/matrix3Ti/matrix interface. interface.interface. According AccordingAccording to toto the thethe proximity proximityproximity histogramhistogram analysis analysis shown shown in in Figure Figure7 7a, a,7a,the the theMo Mo Moconcentration concentration concentration at at at Ni Ni Ni33Ti/matrix3Ti/matrix interface interfaceinterface approximates approximatesapproximates tototo thethe averageaverage concentrationconcentration of ofof MoMo inin thethe matrixmatrix (1.58(1.58 atat %).%). When WhenWhen aging agingaging time timetime prolonged prolongedprolonged to toto 40 4040 h h (Figure(Figure7 7b) b)7b)and and and 100 100 100 h h (Figureh (Figure (Figure7c), 7c), 7c), the the the Mo Mo Mo concentration concentration concentration at Niat at Ni 3NiTi/matrix3Ti/matrix3Ti/matrix interface interface interface increases increases increases to to 2.09to 2.09 2.09 at at %at %and% and and 3.06 3.06 3.06 at at %,at %, respectively.%, respectively. respectively.

FigureFigure 6.6. Three-dimensionalThree-dimensional reconstructionreconstruction reconstruction ofof of thethe atomic atomic positions positions of of Fe FeFe ( (purple(purplepurple pints pintspints)) ) and and isoconcentrationisoconcentrationisoconcentration surface surfacesurface for for for regions regions regions cont cont containingainingaining more more more than than than 35 35 35at at % at% Ni %Ni + Ni+ Ti Ti +( green( Tigreen (green surfaces surfaces surfaces) )and and) 5 and5 at at % % 5 MoatMo % ( red( Mored surfaces (surfacesred surfaces) )for for )specimens specimens for specimens ( a(a) )aged aged (a) aged for for 10 10 for min; min; 10 min;( b(b) )aged (agedb) aged for for 0.5 for0.5 h; 0.5h; ( c(c h;) )aged aged (c) aged for for 4 for4 h; h; 4( d( h;d) )aged (agedd) aged for for 40for40 h; h; 40 ( e( h;e) )aged aged (e) aged for for 100 100 for h. h. 100 Bounding Bounding h. Bounding box box size: size: box ( a( size:a) )58 58 × (× a59 59) 58× × 218 ×218 59nm nm×3;3 ;(218 b(b) )55 nm55 × × 355 ;(55 b× ×) 144 144 55 ×nm nm553;3 ;( ×c(c) )64144 64 × × nm65 65 3× ×; 182(c182) 64nm nm×3;365 ;( d(d)× )59 59182 × × 60 nm60 × ×3 247 ;(247d )nm nm 593×;3 ;( e(60e) )67 ×67 ×247 × 69 69 nm× × 173 1733;( nme nm) 673.3 . × 69 × 173 nm3.

Figure 7. Proximity histograms of Ni Ti precipitate in different specimens (a) aged for 4 h; (b) aged for FigureFigure 7. 7. Proximity Proximity histograms histograms of of Ni Ni33Ti3Ti precipitate precipitate in in different different specimens specimens ( a(a) )aged aged for for 4 4 h; h; ( b(b) )aged aged for16for h; 1616 (c ) h;h; aged (c(c) ) foragedaged 100 forfor h. The100100 insertsh.h. TheThe show insertsinserts the showshow 1 nm thickthethe 11 atom nmnm thick mapsthick atom throughatom mapsmaps the centers throughthrough of the representativethe centerscenters ofof Ni Ti precipitate in specimens under different aging conditions (Ni for green sphere, Ti for grey sphere, representativerepresentative3 Ni Ni3Ti3Ti precipitate precipitate in in specimens specimens under under different different aging aging conditions conditions (Ni (Ni for for green green sphere, sphere, TiMoTi for for for grey grey red sphere, sphere).sphere, Mo Mo for for red red sphere). sphere).

SinceSince the the diffusivity diffusivity of of Mo Mo isis muchmuch lowerlower thanthan thosethose ofof NiNi andand Ti,Ti, thisthis indicatesindicates thatthat NiNi3Ti3TiTi nucleatesnucleates headmost headmost and and will will seize seize mostmost priorprior nucleationnucl nucleationeation sitessites suchsuch such asas as graingrain grain boundary,boundary, boundary, blocky/lathblocky/lath blocky/lath boundaryboundary and and dislocation. dislocation. Consider ConsideringConsideringing that thatthat these thesethese defects defectsdefects are areare also alsoalso favorable favorablefavorable diffusion diffusiondiffusion paths, paths,paths, a aa low low nucleationnucleation raterate ofof Mo-richMo-rich phasephase cancan bebe predicted.predicted. ThisThis wellwell explainsexplains thethe largerlarger sizesize andand lowerlower numbernumber density density Mo-rich Mo-rich phase. phase. As As shown shown in in Figure Figure 3a, 3a, the the dimension dimension of of sphere-like sphere-like Mo-rich Mo-rich phase phase isis almost almost similar similar to to the the cross cross section section (~50 (~50 × × 50 50 nm nm2)2 )of of the the APT APT analytical analytical body. body. Attributed Attributed to to the the size size limitationlimitation of of APT APT analytical analytical body, body, sphere-like sphere-like Mo-r Mo-richich phase phase was was not not included included in in the the analytical analytical body, body,

Materials 2017, 10, 1293 9 of 11 nucleation rate of Mo-rich phase can be predicted. This well explains the larger size and lower number density Mo-rich phase. As shown in Figure3a, the dimension of sphere-like Mo-rich phase is almost similar to the cross section (~50 × 50 nm2) of the APT analytical body. Attributed to the size limitation ofMaterials APT analytical 2017, 10, 1293 body, sphere-like Mo-rich phase was not included in the analytical body, as shown9 of 11 in Figure6d,e. This also results in an anomaly phenomenon in Table2 that Mo cluster density had a as shown in Figure 6d,e. This also results in an anomaly phenomenon in Table 2 that Mo cluster sharp decrease after aging time reached to 40 h and 100 h. density had a sharp decrease after aging time reached to 40 h and 100 h. Based on the above characterization on the formation and evolution of the Ni3Ti and Mo-rich Based on the above characterization on the formation and evolution of the Ni3Ti and Mo-rich phases, the precipitation mechanism in the new maraging stainless steel can be revealed as follows phases, the precipitation mechanism in the new maraging stainless steel can be revealed as follows and schematically illustrated in Figure8. At the initial stage of aging, Ni and Ti firstly precipitate out and schematically illustrated in Figure 8. At the initial stage of aging, Ni and Ti firstly precipitate out of the supersaturated solid solution to form Ni + Ti cluster together. During the rapid initial growth of the supersaturated solid solution to form Ni + Ti cluster together. During the rapid initial growth process of Ni Ti, Mo solutes are rejected from Ni Ti and finally segregate at the Ni Ti/matrix interface. process of Ni3 3Ti, Mo solutes are rejected from Ni33Ti and finally segregate at the Ni33Ti/matrix interface. Since the precipitation of Ni Ti has seized most prior nucleation sites and favorable diffusion paths, Since the precipitation of Ni33Ti has seized most prior nucleation sites and favorable diffusion paths, suchsuch as as grain grain boundary, boundary, blocky/lath blocky/lath boundary boundary and and dislocation, dislocation, Mo-rich Mo-rich phase phase is is inclined inclined to to nucleate nucleate at theat Nithe3Ti/matrix Ni3Ti/matrix interface interface as a resultas a ofresult Mo atomsof Mo accumulation.atoms accumulation. After stable After Mo-rich stable phaseMo-rich nucleation phase occurs,nucleation considering occurs, considering that the diffusion that the of diffusion Mo is difficult, of Mo is most difficult, nuclei most grow nuclei up slightlygrow up to slightly form final to flake-likeform final Mo-rich flake-like phase Mo-rich adjacent phase to Niadjacent3Ti phase. to Ni Thus,3Ti phase. the flake-like Thus, the Mo-rich flake-like phase Mo-rich and Niphase3Ti phaseand formNi3Ti a core-shellphase form structure a core-shell together. structure In different together. sites, In portion different of nucleussites, portion grow upof nucleus drastically grow to formup largedrastically size sphere-like to form large Mo-rich size sphere-like phase and evenMo-rich build phase a crosslink and even to build each other.a crosslink to each other.

Figure 8. Schematics showing precipitation mechanism of Ni3Ti and Mo-rich phases in the new Figure 8. Schematics showing precipitation mechanism of Ni3Ti and Mo-rich phases in the new maraging stainless steel. maraging stainless steel.

5. Conclusions 5. Conclusions This study presents an understanding of the mechanism of the evolution of precipitation in a This study presents an understanding of the mechanism of the evolution of precipitation in a new new maraging stainless steel. The features of different kind of precipitations are carefully maragingcharacterized stainless at atomic steel. The scale features based ofon different combination kind ofof precipitations APT and TEM are analyses. carefully characterizedThe following at atomicconclusions scale based are drawn on combination from the study: of APT and TEM analyses. The following conclusions are drawn from theA study:new maraging stainless steel with high strength (above 1920 MPa), high toughness (~80 MPa·mA new1/2) and maraging superior stainless corrosion steel resistance with high(compa strengthrable to (above15-5 PH) 1920 was MPa), developed. high toughnessThe new 1/2 (~80maraging MPa·m stainless) and superiorsteel is strengthened corrosion resistance by two (comparabledominant strengthening to 15-5 PH) wasprecipitations developed. including The new maragingNi3Ti and stainless Mo-rich steel phases. is strengthened Different morphologies by two dominant of these strengthening two strengthening precipitations precipitates including (rod-like Ni3Ti andand Mo-rich sphere-like phases. Ni3Ti Different phase, flake-like morphologies and sphere-like of these twoMo-rich strengthening phase) were precipitates observed by (rod-like APT. and sphere-likeIn the Ni aging3Ti phase, process, ﬂake-like Ni and and Ti solutes sphere-like show Mo-rich a much phase)quicker were precipitation observed response by APT. than Mo solute.In the Associated aging process, with morphology Ni and Ti observation solutes show and a muchcluster quicker analysis, precipitation the precipitation response mechanism than Mois solute.succinctly Associated proposed with as: morphology supersaturated observation solid solution and cluster → Ni analysis,+ Ti cluster the → precipitation Ni3Ti nucleus mechanism → Ni3Ti isnucleus succinctly and proposed Mo-rich segregation as: supersaturated → core-shell solid structure solution (Ni→3TiNi phase + Ti cluster and flake-like→ Ni3Ti Mo-rich nucleus phase)→ Ni or3Ti nucleuslarge sphere-like and Mo-rich Mo-rich segregation phase →dependingcore-shell on structure the grow (Ni up3 processTi phase of and Mo-rich ﬂake-like nucleus. Mo-rich phase) or large sphere-like Mo-rich phase depending on the grow up process of Mo-rich nucleus. Acknowledgments: This research is sponsored by Youth Innovation Promotion Association of Chinese Acknowledgments:Academy of SciencesThis (2017233), research isInnovation sponsored Project by Youth of In Innovationstitute of PromotionMetal Research Association (2015-ZD04) of Chinese and National Academy ofNatural Sciences Science (2017233), Foundation Innovation of ProjectChina ofResearch Institute Fund of Metal for Internationa Research (2015-ZD04)l Young Scientists and National (No. Natural51750110515). Science FoundationWe acknowledge of China Wenqing Research Liu Fundfor APT for experiment International support Young and Scientists relevant (No.discussion 51750110515). on the data We analysis. acknowledge Wenqing Liu for APT experiment support and relevant discussion on the data analysis. Author Contributions: Ke Yang, Zhouhua Jiang and Yiyin Shan designed the project. Wei Yan, M. Babar Shahzad and Wei Wang produced the samples used in this study. Jialong Tian peformed the microstructural analyses and the properties test. All authors contributed to the interpretation of the results and the writing of the manuscript.

Conflicts of Interest: The authors declare no conflict of interest.

Materials 2017, 10, 1293 10 of 11

Author Contributions: Ke Yang, Zhouhua Jiang and Yiyin Shan designed the project. Wei Yan, M. Babar Shahzad and Wei Wang produced the samples used in this study. Jialong Tian peformed the microstructural analyses and the properties test. All authors contributed to the interpretation of the results and the writing of the manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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