Atom-probe study of Cu and NiAl nanoscale precipitation and interfacial segregation in ananoparticle-strengthened steel

Jiao, Z. B.; Luan, J. H.; Guo, W.; Poplawsky, J. D.; Liu, C. T.

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Publication details:Jiao, Z. B., Luan, J. H., Guo, W., Poplawsky, J. D., & Liu, C. T. (2017). Atom-probe study of Cu and NiAlnanoscale precipitation and interfacial segregation in a nanoparticle-strengthened steel. Materials ResearchLetters, 5(8), 562-568. https://doi.org/10.1080/21663831.2017.1364675

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Atom-probe study of Cu and NiAl nanoscaleprecipitation and interfacial segregation in ananoparticle-strengthened steel

Z. B. Jiao, J. H. Luan, W. Guo, J. D. Poplawsky & C. T. Liu

To cite this article: Z. B. Jiao, J. H. Luan, W. Guo, J. D. Poplawsky & C. T. Liu (2017)Atom-probe study of Cu and NiAl nanoscale precipitation and interfacial segregationin a nanoparticle-strengthened steel, Materials Research Letters, 5:8, 562-568, DOI:10.1080/21663831.2017.1364675

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MATER. RES. LETT., 2017VOL. 5, NO. 8, 562–568https://doi.org/10.1080/21663831.2017.1364675

ORIGINAL REPORT

Atom-probe study of Cu and NiAl nanoscale precipitation and interfacialsegregation in a nanoparticle-strengthened steel

Z. B. Jiaoa,b, J. H. Luana, W. Guoc, J. D. Poplawskyc and C. T. Liua

aCenter for Advanced Structural Materials, Dept. of Mechanical and Biomedical Engineering, College of Science and Engineering, City Universityof Hong Kong, Hong Kong, People’s Republic of China; bDept. of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong,People’s Republic of China; cCenter for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

ABSTRACTThe Cu and NiAl nanoscale precipitation and interfacial segregation in the martensite and austenitephases within a high-strength steel were studied by atom-probe tomography (APT). In the marten-site phase, APT reveals the precipitation of isolated NiAl nanoparticles and NiAl/Cu co-precipitates,indicating thatNiAl nanoparticles form first in the precipitation sequence. In comparison, the austen-ite phase contains only CunanoparticleswithNi segregation at theparticle/matrix interface, inwhichthe Ni segregation reduces the Cu nanoparticle interfacial energy. In addition, Mn and C exhibit anenrichment at themartensite/austenite interface, and themechanism for the interfacial segregationwas also discussed.

IMPACT STATEMENTThe understanding of nanoscale precipitation and interfacial segregation mechanism provides thebasis for the control of precipitate microstructures and mechanical properties of nanoparticle-strengthened steels.

ARTICLE HISTORYReceived 11 May 2017

KEYWORDSCu and NiAl precipitate;nanoscale precipitation;interfacial segregation;high-strength steel;atom-probe tomography

Introduction

Precipitation of nanoparticles has been recognized as oneof the most effective methods to increase the strengthof steels, and precipitation hardening has become thefoundation in the development of many grades of high-strength steels [1–8]. It is known that the degree ofstrengthening so obtained is highly dependent upon theprecipitate microstructure, including the structure, mor-phology, size, and interparticle spacing of the nanopar-ticles [9]. Fundamentally, the precipitate microstructureis determined by the thermodynamic properties, such asthe elastic and interfacial energies as well as the chemi-cal driving force for precipitation, and the kinetics of thesystem, which, in turn, are strongly affected by the com-position and crystal structure of the matrix [10]. In otherwords, the precipitation behavior of specific nanopar-ticles can be significantly different in different matrix

CONTACT C. T. Liu chainliu@cityu.edu.hk

phases. Practically, for many high-strength steels, thematrix is tailored to have amixture of two ormore phases,rather than simply a single phase, to obtain an optimumcombination of high strength and high ductility [11–14].Among them, Cu and NiAl nanoparticle-strengthenedsteels represent an important class of advanced high-strength steels, which have attracted significant attentionin recent years due to their good combination of highstrength, good ductility, and good weldability [15–22]. Itwas found that there are two types of pathways for the for-mation of the NiAl/Cu co-precipitates in the martensitephase (i.e. ‘solid solution → Cu particles → Cu parti-cles+NiAl particles’ and ‘solid solution → NiAl parti-cles → NiAl particles+Cu particles’), depending uponthe Cu, Ni, and Al concentrations [21,22]. In addi-tion, the welding study revealed that the NiAl/Cu co-precipitates which are dissolved in the matrix during

© 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,distribution, and reproduction in any medium, provided the original work is properly cited.

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MATER. RES. LETT. 563

welding can be re-precipitated upon proper post-weldheat-treatments, leading to a recovery of the strength[22]. It should be pointed out that the matrix of mostCu and NiAl nanoparticle-strengthened steels is primar-ily martensite, but contains a small amount of austenite,the amount of which depends on the alloy compositionand heat-treatment histories. While most of the previ-ous studies were focused mainly on the understandingof precipitation behavior of Cu and NiAl nanoparti-cles in the martensite and ferrite phases [15–22], lim-ited detailed information is available on the precipitationbehavior and related mechanism in the austenite phase.Recently, in a welding study of a newly developedCu/NiAl co-precipitation strengthened steel [22], weobserved an interesting microstructure containing bothmartensite and austenite phases together with complexnanoscale precipitation behavior in the heat-affectedzone. Although the thermal history of the heat-affectedzone is complicated in nature, while for an understand-ing of the welding microstructure, it is interesting andimportant to study the precipitation behavior and inter-facial segregation in the heat-affected zone. In this Letter,we intend to investigate this interesting microstructurein detail and comparatively study the precipitation andsegregation behavior in the martensite, austenite, andtheir interfaces, aiming at elucidating the basic mecha-nism involved in the nanoscale precipitation, interfacialsegregation, and solute partitioning in different matrixphases, which has not been well understood yet. Theprecipitation characteristics, including the size,morphol-ogy, and composition of Cu and NiAl nanoparticles inthe alloy, were investigated by atom-probe tomography(APT). Particular attentions were paid to understandingtheCu andNiAl nanoscale precipitation in themartensiteand austenite phases as well as the interfacial segregationbehavior at the martensite/austenite interface.

Experimental

The chemical composition of the steel was Fe-5Ni-2Al-3Mn-1.5Cu-1.5Mo-1.5W-0.07Nb-0.05C-0.01B (wt.%).Alloy ingots were solution-treated at 900°C for 30min,followed by water quenching and aging at 550°C for2 h. The plates were welded using the arc welding tech-nique [22], and needle-shaped specimens for APT stud-ies were fabricated by lift-outs from the heat-affectedzone and annular milled in a FEI Nova 200 focusedion beam/scanning electronmicroscope (FIB/SEM). TheAPT characterizations were performed in a local elec-trode atom probe (CAMEACA LEAP 4000X HR). TheAPT specimens were analyzed in voltage mode with aspecimen temperature of 50K, a pulse repetition rate of200 kHz, a pulse fraction of 20%, and an evaporation

detection rate of 0.5% atom per pulse. Imago Visualiza-tion andAnalysis Software version 3.6.12was used for the3D reconstructions and data analysis.

Results and discussion

Atommaps of Ni (red), Mn (blue), Al (cyan), Cu (green),and C (wine) taken from the APT analysis of the steel areshown in Figure 1(a–e), respectively, and their overlay isdisplayed in Figure 1(f). The atom maps reveal a com-plex but interesting nanostructure containing three typesof compositional features: (i) a Ni-, Mn-, Cu-, and C-enriched phase at the upper middle portion of the maps,(ii) the rest of the matrix enriched in Al and depletedin Ni, Mn, Cu, and C, and (iii) high number densitiesof spherical nanoparticles with sizes of 1–5 nm, most ofwhich are enriched in Ni, Al, Cu, and Mn. It is wellknown that Ni, Mn, Cu and C are all austenite stabi-lizers and preferred to partition in the austenite phase,whereas Al, as a ferrite stabilizer, tends to concentrated inthe ferrite/martensite phase [23]. In addition, the previ-ous study revealed that the specimen contains martensitetogether with a small amount of austenite [22]. There-fore, it is reasonable to presume that the Ni/Mn/Cu/C-enriched phase is austenite, while the rest of the matrixenriched with Al is martensite.

To quantitatively measure the solute partitioningbetween the austenite and martensite phases and thesolute segregation at the interphase boundary, a one-dimensional concentration profile was performedthrough the martensite/austenite interface shown inFigure 2(a). The region the concentration profile wastaken has dimensions of 25 nm× 25 nm× 11 nm and isshown by yellow in Figure 1(f). The Ni, Mn, Cu, and Ccontents decrease from austenite to martensite, whereasthe Al and Fe contents behave in an opposite way. Inaddition, Mn and C exhibit a strong segregation at theinterface between the austenite and martensite phases,achieving peak concentrations of approximately 9 and 3at.%, respectively. The composition of the austenite andmartensite phases derived from the plateau data points onthe concentration profiles are summarized in Figure 2(b).The concentrations of Ni, Mn, Cu, and C in the austen-ite phase (∼6.6± 0.4, 7.3± 0.4, 1.8± 0.5, and 2.0± 0.1at.%, respectively) are much higher than those in themartensite phase (∼4.6± 0.3, 2.7± 0.2, 0.9± 0.1, and0.2± 0.1 at.%, respectively). In contrast, the Al level inthe austenite phase (∼3.1± 0.2 at.%) is lower than thatin the martensite phase (∼3.7± 0.3 at.%).

Isoconcentration surfaces at 10 at.% Cu and 20 at.%(Ni+Al) were used to visualize and identify the Cu- andNiAl-enriched nanoparticles, respectively. The spatialdistribution of the two types of nanoparticles, together

564 Z. B. JIAO ET AL.

Figure 1. Atommaps of Ni, Mn, Cu, Al, and C in the Fe-Ni-Mn-Al-Cu-based steel.

with the solute atoms showing the location of the austen-ite and martensite phases, is displayed in Figure 3. In theaustenite phase, only Cu nanoparticles were observed,and no NiAl nanoparticles were detected. The aver-age radius of the Cu nanoparticles is estimated to be∼2.0± 0.1 nm. In comparison, high number densities ofboth Cu and NiAl nanoparticles can be observed in themartensite phase. A statistical analysis of the particle con-figuration reveals that the majority (77± 5%) of the NiAlnanoparticles are NiAl/Cu co-precipitates, in which Cunanoparticles are located on the outside surface of NiAlparticles, and only a small fraction (23± 5%) of NiAlnanoparticles are isolated particles without any Cu par-ticles associated with them. It is worthwhile to point outthat no isolated Cu nanoparticles were detected in themartensite phase. The average radius of the Cu nanopar-ticles in the martensite phase was determined to be∼1.1± 0.3 nm, smaller than that in the austenite phase(∼2.0± 0.1 nm), whereas that of the NiAl nanoparticleswas found to be ∼1.8± 0.9 nm.

To understand the partitioning behavior of soluteatoms in the Cu and NiAl nanoparticles, 1-nm-thickatom maps through the centers of a Cu nanoparticlesin the austenite phase, a NiAl/Cu co-precipitate in themartensite phase, and an isolated NiAl nanoparticle inthe martensite phase are shown in Figure 4(a–c), respec-tively, in which the relative positions and extents of theNi (red), Mn (blue), Cu (green), and Al (cyan) atoms arealso indicated. TheCunanoparticle in the austenite phase

(Figure 4(a)) is embedded in a Ni- and Mn-enrichedaustenite phase. The center of the Cu particle is almostfree of Ni and Mn, whereas a considerable amount ofNi and Mn can be observed at the Cu particle/austeniteinterface. In comparison, a Cu particle is located onthe outside surface of a NiAl particle in the martensitephase (Figure 4(b)), and the two particles overlap largelywith each other. For the isolated NiAl nanoparticle inmartensite (Figure 4(c)), Ni and Al together with a smallamount ofMn andCu are partitioned to the particles, andthere is no solute segregation at the NiAl particle/matrixinterface. The quantification of the solute partitioningbetween the nanoparticles and matrix was determinedfrom proximity histograms. The proximity histogramsof the Cu nanoparticles in the austenite phase, and Cuand NiAl nanoparticles in the martensite phase are dis-played in Figure 4(d–f), respectively, and the quantitativecomposition analysis of the nanoparticles is summarizedin Figure 4(g). For the Cu particles in austenite (Figure4(d)), the partitioning of Cu and Al to the particles andthe partitioning of Fe andMn to the matrix is clearly evi-dent, whereas Ni exhibits a local segregation (∼10 at.%)at the Cu particle/matrix interface and a strong depletionin the Cu nanoparticles. The Cu nanoparticles consistmainly of Cu (∼88.5± 6.3 at.%) together with a smallamount of Fe (∼7.7± 5.2 at.%) and Al (∼3.9± 3.7at.%). The Cu particles in martensite are enriched inCu (∼44.6± 5.8 at.%) but contain a significant amountof Ni (∼12.2± 3.8 at.%), Al (∼20.3± 4.7 at.%), Mn

MATER. RES. LETT. 565

Figure 2. (a) Concentration profile across an austenite-martensite interface and (b) compositions of the austenite andmartensite phases.

Figure 3. Cu and NiAl nanoparticles together with solute atommaps showing the austenite and martensite phases in a 3Dreconstruction.

(∼9.5± 3.4 at.%), and Fe (∼13.5± 4.0 at.%), indicatingthat the composition of the Cu particles in martensite isfar from equilibrium. In addition, a pronounced segre-gation of Ni, Al, and Mn at the Cu particle/martensiteinterface can be observed, consistent with previous stud-ies on Cu and NiAl nanoparticle-strengthened marten-sitic steels [15–22]. For the NiAl particles in martensite(Figure 4(f)), the enrichment of not only Ni and Al, butalso Mn and Cu change monotonically toward the centerof the NiAl nanoparticles, and the concentration of Ni,Al, Mn, Cu, and Fe of the particles are estimated to be39.1± 2.3, 26.8± 2.1, 9.7± 1.4, 15.4± 1.7, and 8.6± 1.3at %, respectively.

The APT results presented above indicate that theCu and NiAl nanoparticles exhibit different precipita-tion behavior in the martensite and austenite phasestogether with interesting solute segregation at themartensite/austenite interface. For an understanding toeventually control the complex precipitation and segre-gation behavior, it is of fundamental importance to eluci-date the basic mechanism involved in the precipitationof Cu and NiAl nanoparticles in the martensite andaustenite phases as well as the interfacial segregation atthe interface between the two phases.

First, the co-precipitation mechanism of Cu and NiAlnanoparticles in martensite will be briefly summarizedfor a comparison with that in austenite. It has beengenerally recognized that there are two types of pre-cipitation pathways for the formation of the NiAl/Cuco-precipitates, depending mainly upon the Cu/Ni andCu/Al ratios [24]. In the current steel, NiAl nanoparticles,enriched in Ni, Al and Cu, nucleate first from the super-saturated solid solution, andCu solutes are rejected to theNiAl particle surface at later stages of precipitation, lead-ing to the precipitation of Cu nanoparticles on the outersurface of NiAl nanoparticles [22].

Second, the precipitation mechanism in the austenitephase will be discussed. APT reveals the precipitation of4-nm-diameter Cu nanoparticles in the austenite phase.From the nucleation point of view, the interfacial energyand strain energy play an important role in the forma-tion of nanoparticles in a supersaturated solid solution[10]. The lattice constants of fcc-Cu and fcc-Fe are 3.61and 3.65 nm [25], respectively. The very small differencebetween their lattice constant leads to a very small strainenergy and, hence, favors a coherent interface betweenthe fcc Cu nanoparticles and austenite matrix. There-fore, the Cu nanoparticles in the fcc austenite phase areexpected to directly form with a fcc equilibrium struc-ture of Cu, the formation of which is different from theinitial structure of bcc Cu nanoparticles in the bcc fer-rite/martensite phase [26,27]. In addition, the segrega-tion of Ni has been observed at the interface between

566 Z. B. JIAO ET AL.

Figure 4. 1-nm-thick atommaps of (a) a Cu nanoparticle in austenite, (b) a NiAl/Cu co-precipitate in martensite, and (c) an isolated NiAlnanoparticle in martensite, proximity histograms of (d) Cu nanoparticles in austenite and (e) Cu and (f ) NiAl nanoparticles in martensite,and (g) compositions of the three types of nanoparticles.

the Cu nanoparticles and the austenite matrix, which issimilar to but less pronounced than that as observed forthe Cu precipitation in ferrite/martensite [15–22]. It isknown that the heat of mixing between Fe and Cu is largeand positive (+13 kJmol−1), whereas that for both Fe–Niand Cu–Ni pairs is very small (−4 and +2 kJ mol−1,respectively) [28]. As a result, the interfacial segregationof Ni can act as a buffer layer to further reduce the inter-facial free energy between the Cu-enriched nanoparti-cles and the Fe-enriched matrix, thereby promoting theprecipitation of Cu nanoparticles. Furthermore, it is wor-thy to point out that the austenite phase containing Cu

nanoparticles in the as-welded condition is probably ina non-equilibrium state due to the complicated ther-mal history of welding. After a post-weld heat-treatmentfor 30min at 550°C, the microstructure is much moreclose to an equilibrium state, and no Cu particles can beobserved in the austenite phase [22]. Lastly, it is inter-esting to note that, unlike that in martensite, no NiAlnanoparticles were formed in austenite. The reasons forthe absence of NiAl nanoparticles could be twofold. Oneis the depletion of Al in the austenite phase (Figure 2), asAl is known as a ferrite stabilizer and tends to partitionto ferrite/martensite and deplete in austenite; the other

MATER. RES. LETT. 567

reason is the relative higher solubility of Ni in austen-ite (fully miscible) as compared with that in martensite(

568 Z. B. JIAO ET AL.

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