2014-Science-A fracture-resistant high-entropy alloy for cryogenic applications

DOI: 10.1126/science.1254581

, 1153 (2014);

345 Science et al.Bernd Gludovatz A fracture-resistant high-entropy alloy for cryogenic applications

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cites 45 articles This article

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w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o m

did not observe the formation of any well-defined structures in the absence of an applied magnetic field (see,e.g.,fig.S8J).24.A.Dong et al .,Nano Lett.11,841–846(2011).

25.S.Brooks,A.Gelman,G.Jones,X.-L.Meng,Handbook of Markov Chain Monte Carlo (Chapman &Hall,London,2011).26.Z.Kakol,R.N.Pribble,J.M.Honig,Solid State Commun.69,793–796(1989).

27.ü.?zgür,Y.Alivov,H.Morko?,J.Mater.Sci.Mater.Electron.20,789–834(2009).

28.

The formation of helices,and the self-assembly of NCs in our system in general,is likely facilitated by entropic forces;OA used in large excess during self-assembly may act as a

depletion agent,inducing crystallization of NCs during hexane evaporation as reported previously (29).

29.D.Baranov et al .,Nano Lett.10,743–749(2010).

30.

On the basis of measurements of electrophoretic mobility [see (34)]and the lack of literature reports on electric dipole moments of magnetite nanoparticles,we did not consider

electrostatic and electric dipole-dipole interactions in our analysis of interparticle interactions.At the same time,we cannot exclude 31.S.Srivastava et al .,Science 327,1355–1359(2010).32.S.Das et al .,Adv.Mater.25,422–426(2013).

33.J.V.I.Timonen,https://www.wendangku.net/doc/a73444079.html,tikka,L.Leibler,R.H.A.Ras,O.Ikkala,

Science 341,253–257(2013).

34.Previous self-assembly experiments performed in nonpolar

solvents excluded a significant role played by electrostatic interactions [e.g.,(35,36)].Although the degree of

dissociation of OA in hexane (dielectric constant =1.84)is negligible,the large excess of OA as well as the nature of our experimental setup (self-assembly at the liquid-air interface)might potentially promote dissociation of OA;to verify this possibility,we used a Malvern Zetasizer Nano ZS to perform electrophoretic mobility (m e )measurements of our nanocubes in hexane both in the absence and in the presence of additional OA (5%v/v).The results [0.00706(T 0.00104)×10?4cm 2V –1s –1and 0.0218(T 0.00710)×10?4cm 2V –1s –1,respectively]indicate that in both cases,the nanocubes are essentially neutral [compare with (37)].35.Z.Chen,J.Moore,G.Radtke,H.Sirringhaus,S.O ’Brien,

J.Am.Chem.Soc.129,15702–15709(2007).37.S.A.Hasan,D.W.Kavich,J.H.Dickerson,https://www.wendangku.net/doc/a73444079.html,mun.

2009,3723–3725(2009).

ACKNOWLEDGMENTS

Supported by Israel Science Foundation grant 1463/11,the

G.M.J.Schmidt-Minerva Center for Supramolecular Architectures,and the Minerva Foundation with funding from the Federal German Ministry for Education and Research (R.K.)and by

NSF Division of Materials Research grant 1309765and American Chemical Society Petroleum Research Fund grant 53062-ND6(P.K.).

SUPPLEMENTARY MATERIALS

https://www.wendangku.net/doc/a73444079.html,/content/345/6201/1149/suppl/DC1Materials and Methods Figs.S1to S28References (38–92)

31March 2014;accepted 14July 2014Published online 24July 2014;METAL ALLOYS

properties required for structural applica-tions.Consequently,alloying elements are added to achieve a desired microstructure or combination of mechanical properties,such as strength and toughness,although the re-sulting alloys invariably still involve a single dom-inant constituent,such as iron in steels or nickel in superalloys.Additionally,many such alloys,such as precipitation-hardened aluminum alloys,rely on the presence of a second phase for me-chanical performance.High-entropy alloys (1–3)represent a radical departure from these notions.

they contain high concentrations (20to 25atomic percent)of multiple elements with different crystal structures but can crystallize as a single phase (4–7).In many respects,these alloys rep-resent a new field of metallurgy that focuses attention away from the corners of alloy phase diagrams toward their centers;we believe that as this evolving field matures,a number of fas-cinating new materials may emerge.

The CrMnFeCoNi alloy under study here is a case in point.Although first identified a decade ago (1),the alloy had never been investigated mechanically until recently (5,6,8),yet is clearly scientifically interesting from several perspec-tives.It is not obvious why an equiatomic five-element alloy —where two of the elements (Cr and Fe)crystallize with the body-centered cubic (bcc)structure,one (Ni)as face-centered cubic (fcc),one (Co)as hexagonal close-packed (hcp),and one (Mn)with the complex A 12structure —should form a single-phase fcc structure.Fur-thermore,several of its properties are quite unlike those of pure fcc metals.Recent studies indicate

that the alloy exhibits a strong temperature de-of the yield strength between ambient cryogenic temperatures,reminiscent of bcc and certain fcc solid-solution alloys (6).any temperature-dependent effect of rate on strength appears to be marginal (6).the marked temperature-dependent in strength is accompanied by a substan-increase in tensile ductility with decreasing between 293K and 77K (6),which counter to most other materials where an dependence of ductility and strength is seen (9).Preliminary indications sug-that this may be principally a result of the ’s high work-hardening capability,possi-associated with deformation-induced nano-which acts to delay the onset of any instability (i.e.,localized plastic deforma-that can lead to premature failure)to higher (5).

We prepared the CrMnFeCoNi alloy with high-elemental starting materials by arc melting drop casting into rectangular-cross-section copper molds,followed by cold forging and cross rolling at room temperature into sheets roughly 10mm thick.After recrystallization,the alloy had an equiaxed grain structure.Uniaxial tensile spec-imens and compact-tension fracture toughness specimens in general accordance with ASTM standard E1820(10)were machined from these sheets by electrical discharge machining.[See (11)for details of the processing procedures,sam-ple sizes,and testing methods.]

Figure 1A shows a backscattered electron (BSE)micrograph of the fully recrystallized micro-structure with ~6-m m grains containing numer-ous recrystallization twins.Energy-dispersive x-ray (EDX)spectroscopy and x-ray diffraction (XRD)indicate the equiatomic elemental dis-tribution and single-phase character of the al-loy,respectively.Measured uniaxial stress-strain curves at room temperature (293K),in a dry ice –alcohol mixture (200K),and in liquid nitrogen (77K)are plotted in Fig.1B.With a decrease in temperature from 293K to 77K,the yield strength s y and ultimate tensile strength s uts

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1

Materials Sciences Division,Lawrence Berkeley National Laboratory,Berkeley,CA 94720,USA.2Department of Materials Physics,Montanuniversit?t Leoben and Erich Schmid Institute of Materials Science,Austrian Academy of Sciences,Leoben 8700,Austria.3Materials Sciences and Technology Division,Oak Ridge National Laboratory,Oak Ridge,TN 37831,USA.4Materials Sciences and Engineering Department,University of Tennessee,Knoxville,TN 37996,USA.5Department of Materials Science and Engineering,University of California,Berkeley,CA 94720,USA.

*Corresponding author.E-mail:georgeep@https://www.wendangku.net/doc/a73444079.html, (E.P.G.);roritchie@https://www.wendangku.net/doc/a73444079.html, (R.O.R.)

increased by ~85%and ~70%,to 759and 1280MPa,respectively.Similarly,the tensile ductility (strain to failure,e f )increased by ~25%to >0.7;the strain-hardening exponent n remained high at ~0.4,such that there was an enhancement in the frac-ture energy (12)by more than a factor of 2.Table S1provides a detailed summary of the stresses and strains at the three different temperatures,as well as the corresponding strain-hardening exponents.

In light of the extensive plasticity involved in the deformation of this alloy,we evaluated the fracture toughness of CrMnFeCoNi with non-linear elastic fracture mechanics,specifically with crack-resistance curve (R curve)measurements in terms of

the J integral.Analogous to the stress intensity K for linear elastic analysis,provided that specific validity criteria are met,J unique-ly characterizes the stress and displacement fields in the vicinity of the crack tip for a non-linear elastic solid;as such,it is able to capture both the elastic and plastic contributions to the fracture process.J is also equivalent to the strain energy release rate G under linear elastic conditions;consequently,K values can be back-calculated from J measurements assuming a mode I equivalence between K and J :specifically,J =K 2/E ′,with E ′=E (Young ’s modulus)in plane stress and E /(1–n 2)(where n is Poisson ’s ratio)in plane strain.E and n values were determined by resonance ultrasound spectroscopy at each tem-perature (13).Our toughness results for the CrMnFeCoNi alloy at 293K,200K,and 77K are plotted in Fig.1C,in terms of J R (D a )–based resistance curves showing crack extension D a in precracked and side-grooved compact-tension specimens as a function of the applied J .Using these R curves to evaluate the fracture toughness for both the initiation and growth of a crack,we measured a crack initiation fracture toughness J Ic ,deter-mined essentially at D a →0,of 250kJ/m 2at 293K,which in terms of a stress intensity gives K JIc =217MPa·m 1/2.Despite a markedly increased strength at lower temperature,K JIc values at 200K and 77K remained relatively constant at K JIc =221MPa·m 1/2(J Ic =260kJ/m 2)and K JIc =219MPa·m 1/2(J Ic =255kJ/m 2),respectively.After

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Fig.1.Microstructure and mechanical properties of the CrMnFeCoNi high-entropy alloy.(A )Fully recrystallized microstructure with an equiaxed grain structure and grain size of ~6m m;the composition is approximately equiatomic,and the alloy is single-phase,as shown from the EDX spectroscopy and XRD insets.(B )Yield strength s y ,ultimate tensile strength s uts ,and ductility (strain to failure,e f )all increase with decreasing temperature.The curves are typical tests at the individual temperatures,whereas the data points are means T SD of multiple tests;see table S1for exact values.(C )Fracture toughness measure-ments show K JIc values of 217MPa·m 1/2,221MPa·m 1/2,and 219MPa·m 1/2at 293K,200K,and 77K,respectively,and an increasing fracture resistance in terms of the J integral as a function of crack extension D a [i.e.,resistance curve (R curve)behavior].(D )Similar to austenitic stainless steels (e.g.,304,316,or cryogenic Ni steels),the strength of the high-entropy alloy (solid lines)increases with decreasing temperature;although the toughness of the other materials decreases with decreasing temperature,the toughness of the high-entropy alloy remains unchanged,and by some measures it actually increases at lower temperatures.(The dashed lines in the plots mark the upper and lower limits of data found in the literature.)

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initiation,the fracture resistance further increased with extensive subcritical crack growth;after just over 2mm of such crack extension,a crack growth toughness exceeding K =300MPa·m 1/2(J =500kJ/m 2)was recorded [representing,in terms of ASTM standards,the maximum (valid)crack extension capacity of our samples].Such toughness values compare favorably to those of highly alloyed,austenitic stainless steels such as 304L and 316L,which have reported tough-nesses in the range of K Q =175to 400MPa·m 1/2at room temperature (14–16),and the best cryogenic steels such as 5Ni or 9Ni steels,with K Q =100to 325MPa·m 1/2at 77K (17–19).Similar to the high-entropy alloy,these materials show an expected increase in strength with decreasing temper-ature to 77K;however,unlike the high-entropy alloy,their reported fracture toughness values are invariably reduced with decreasing temperature (20)(Fig.1D)and furthermore are rarely valid (i.e.,they are size-and geometry-dependent and thus not strictly material parameters).

The high fracture toughness values of the CrMnFeCoNi alloy were associated with a 100%ductile fracture by microvoid coalescence,with the extent of deformation and necking behavior being progressively less

apparent at the lower temperatures (Fig.2,A and B).EDX analysis of the particles,which were found inside the voids of the fracture surface and acted as initiation sites for their formation,indicated either Cr-rich or Mn-rich compounds (Fig.2B,inset).These particles are likely oxides associated with the Mn additions;preliminary indications are that they are absent in the Mn-free (CoCrFeNi)alloy (6).Both microvoid size and particle size varied markedly;the microvoids ranged in size from ~1m m to tens of micrometers,with particle sizes ranging from <1m m to ~5m m (Fig.2B,inset)with an average size of 1.6m m and average spacing d p ≈49.6m m,respectively.

To verify the high measured fracture tough-ness values,we used three-dimensional (3D)ster-eophotogrammetry of the morphology of these fracture surfaces to estimate local crack initia-tion toughness (K i )values for comparison with the global,ASTM-based K JIc measurements.This technique is an alternative means to characterize the onset of cracking,particularly under large-scale yielding conditions.Under mode I (tensile)loading,the crack surfaces completely separate from each other,with the regions of first sepa-ration moving the farthest apart and progres-sively less separation occurring in regions that crack later.Accordingly,the formation and coa-lescence of microvoids and their linkage with the crack tip allow for the precise reconstruction of the point of initial crack advance from the juxta-position of the stereo images of each fracture sur-face.This enables an evaluation of the crack tip opening displacement at crack initiation,CTOD i ,which then can be used to estimate the local stress intensity K i at the midsection of the sample at the onset of physical crack extension,where D a =0(21).Specifically,we used an automatic fracture surface analysis system that creates 3D digital surface models from stereo-image pairs of the corresponding fracture surfaces taken in the scan-ning electron microscope (Fig.2C);digitally re-constructing the crack profiles by superimposing the stereo-image pairs allows for a precise mea-surement of the CTOD i s of arbitrarily chosen crack paths (which must be identical on both fracture surfaces).Figure 2D indicates two examples of the approximately 10crack paths taken on both fracture surfaces of samples tested at 293K and 77K.The two corresponding profiles show the point at which the first void,formed ahead of the fatigue precrack,coalesced with this pre-crack to mark the initial crack extension,there-by locally defining the crack initiation event and

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Fig.2.Images of fractured CrMnFeCoNi samples.(A )Stereomicroscopic photographs of the fracture surfaces after testing indicate less lateral defor-mation and necking-like behavior with decreasing temperature.(B )SEM image of the fracture surface of a sample tested at room temperature shows ductile dimpled fracture where the void initiation sites are mainly Mn-rich or Cr-rich par-ticles,as shown by the EDX data (insets).(C )Three-dimensional digital fracture

surface models were derived from SEM stereo-image pairs,which indicate the transition from fatigue precrack to ductile dimpled fracture and the presence of the stretch zone.(D )Profiles of identical crack paths from both fracture halves of the fracture surface models were extracted to evaluate the crack tip opening displacement at the first physical crack extension,CTOD i ,which was then converted to J i using the relationship of the equivalence of J and CTOD (50).

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the fracture toughness (22).Using these pro-cedures,the initial crack tip opening displace-ments at crack initiation were found to be CTOD i =57T 19m m at 293K and 49T 13m m at https://www.wendangku.net/doc/a73444079.html,ing the standard J-CTOD equivalence relationship of J i os o CTOD i =K i 2/E ′gives es-timates of the crack initiation fracture toughness:K i =191MPa·m 1/2and 203MPa·m 1/2at 293K and 77K,respectively.These values are slightly con-servative with respect to the global R curve –based values in Fig.1C;however,this is to be expected,as they

are estimated at the initial point of physical contact of the first nucleated void with the precrack,whereas the ASTM-based measurements use an operational definition of crack initiation involving subcritical crack ex-tension of D a =200m m.

To discern the micromechanisms underlying the excellent fracture toughness behavior,we fur-ther analyzed the fracture surfaces of samples tested at 293K and 77K by means of stereomi-croscopy and scanning electron microcopy (SEM).Some samples were additionally sliced in two halves,embedded,and metallographically pol-ished for BSE microscopy and electron back-scatter diffraction (EBSD)analysis of the region in the immediate vicinity of the crack tip and in the wake of the crack,close to the crack flanks,specifically “inside ”the sample where fully plane-strain conditions prevail.

SEM images of the crack tip region of sam-ples tested at ambient and liquid nitrogen tem-peratures show the formation of voids and their coalescence characteristic of the microvoid co-alescence fracture process (Fig.3A).A large population of the particles that act as the void

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Fig.3.Deformation mechanisms in the vicinity of the crack tip in the center (plane-strain)section of CrMnFeCoNi high-entropy alloy samples.(A )Low-magnification SEM images of samples tested at 293K and 77K show ductile fracture by microvoid coalescence,with a somewhat more distorted crack path at the lower temperature.(B )EBSD images show numerous annealing twins and pronounced grain misorientations due to dislocations —the primary defor-mation mechanism at 293K.(C )At 77K,BSE images taken in the wake of the propagated crack show the formation of pronounced cell structures resulting from dislocation activity.Both BSE and EBSD images show deformation-induced nanotwinning as an additional mechanism at 77K.[The EBSD image is an overlay to an image quality (IQ)map,which is a measure of the quality of the collected EBSD pattern used to visualize certain microstructural features.]

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initiation sites can be seen on the fracture surfaces (Fig.2B);these particles have a substan-tial influence on material ductility and likely contribute to the measured scatter in the failure strains (Fig.1B).Macroscopically,fracture sur-faces at 77K appear significantly more deviated from a mode I (K II =0)crack path than at 293K (Fig.3A).Although such deflected crack paths act to reduce the local crack-driving force at the crack tip (23)and hence contribute to the rising R curve behavior (i.e.,crack growth toughness),this mech-anism cannot be responsible for the exceptional crack initiation toughness of this alloy.Such high K i values are conversely derived from the large CTOD s at crack initiation and are associated with the intrinsic process of microvoid coalescence;as such,they are highly dependent on the formation and size of voids,the prevailing deformation and flow conditions,and the presence of steady strain hardening to suppress local necking.

Using simple micromechanical models for fracture (24),we can take advantage of a stress state –modified critical strain criterion for ductile fracture to derive estimates for these high tough-ness values (25–27).This yields expressions for

the fracture toughness in the form J Ic ≈s o e f l *o

,where s o is the flow stress,e f is the fracture strain in the highly constrained stress state in the vicinity of the crack tip [which is roughly an order of magnitude smaller than the un-iaxial tensile ductility (28)],and l *o

is the char-acteristic distance

ahead of the tip over which this critical strain must be met for fracture (which can be equated to the particle spacing d p ).Assum-ing Hutchinson-Rice-Rosengren (HRR)stress-strain distributions ahead of a crack tip in plane strain for a nonlinear elastic,power-law hard-ening solid (strain-hardening coefficient of n )(29,30),and the measured properties,specifically E ,s o ,e f ,n ,n ,and d p ,for this alloy (11),estimates of the fracture toughness of K JIc =(J Ic E ′)1/2of ~150to 215MPa·m 1/2can be obtained for the measured particle spacing of d p ~50m m.Although approx-imate,these toughness predictions from the critical fracture strain model are completely consistent with a fracture toughness on the order of 200MPa·m 1/2,as measured for the CrMnFeCoNi alloy in this study (Fig.1C).

In addition to crack initiation toughnesses of 200MPa·m 1/2or more,this alloy develops even higher crack growth toughness with stable crack growth at “valid ”stress intensities above 300MPa·m 1/2.These are astonishing toughness levels by any standard,particularly because they are retained at cryogenic temperatures.A primary factor here is the mode of plastic deformation,which induces a steady degree of strain hardening to suppress plastic instabilities;expressly,the mea-sured strain-hardening exponents of n ~0.4are very high relative to the vast majority of metals,particularly at this strength level.Recent studies have shown that,similar to mechanisms known for binary fcc solid solutions (31,32),plastic de-formation in the CrMnFeCoNi alloy at ambient

temperatures is associated with planar glide of 1/2?110?dislocations on {111}planes leading to the formation of pronounced cell structures at higher strains (5).However,at 77K,in addition to planar slip,deformation-induced nanoscale twinning has been observed both previously (5)and in the present study (Fig.3C)and contributes to the increased ductility and strain hardening at lower temperatures.Both the planar slip and nanotwinning mechanisms are highly active in the vicinity of the crack tip during fracture,as illustrated in Fig.3.EBSD images taken ahead of the crack tip inside the sample of a fracture toughness test performed at room temperature show grain misorientations resulting from dis-location activity as the only deformation mech-anism (Fig.3B).Aside from numerous annealing twins resulting from the recrystallization step during processing,twinning does not play a role at ambient temperatures,with only a few single nanotwins in evidence.With decrease in tem-perature,cell structure formation is more appar-ent,as shown by the BSE image in Fig.3C,taken in the wake of a crack propagating at 77K.Here,however,excessive deformation-induced nano-scale twinning occurs simultaneously with planar dislocation slip,leading to a highly distorted grain structure,which can be seen in both the BSE and IQ +EBSD images in the vicinity of the growing crack.[The EBSD image is shown as an overlay of an image quality (IQ)map to enhance visual-ization of structural deformations of the grains.]Note that several other classes of materials show good combinations of strength and ductility when twinning is the dominant deformation mecha-nism.These include copper thin films (33–36)and the recently developed twinning-induced plas-ticity (TWIP)steels (37–40),which are of great interest to the car industry as high-Mn steels (41–44).We believe that the additional plasticity mechanism of nanotwinning in CrMnFeCoNi is critical to sustaining a high level of strain hard-ening at decreasing temperatures;this in turn acts to enhance the tensile ductility,which,to-gether with the higher strength at low tem-peratures,preserves the exceptional fracture toughness of this alloy down to 77K.

We conclude that the high-entropy CrMnFeCoNi alloy displays remarkable fracture toughness properties at tensile strengths of 730to 1280GPa,which exceed 200MPa·m 1/2at crack initiation and rise to >300MPa·m 1/2for stable crack growth at cryogenic temperatures down to 77K.The alloy has toughness levels that are comparable to the very best cryogenic steels,specifically cer-tain austenitic stainless steels (15,16)and high-Ni steels (17–19,45–48),which also have outstanding combinations of strength and ductility.

With respect to the alloy ’s damage tolerance,a comparison with the other major material classes is shown on the Ashby plot of fracture toughness versus yield strength (49)in Fig.4.There are clearly stronger materials,which is understand-able given that CrMnFeCoNi is a single-phase material,but the toughness of this high-entropy alloy exceeds that of virtually all pure metals and metallic alloys (9,49).

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Fig.4.Ashby map showing fracture toughness as a function of yield strength for high-entropy alloys in relation to a wide range of material systems.The excellent damage tolerance (toughness combined with strength)of the CrMnFeCoNi alloy is evident in that the high-entropy alloy exceeds the toughness of most pure metals and most metallic alloys (9,49)and has a strength comparable to that of structural ceramics (49)and close to that of some bulk-metallic glasses (51–55).

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ACKNOWLEDGMENTS

Sponsored by the U.S.Department of Energy,Office of

Science,Office of Basic Energy Sciences,Materials Sciences and Engineering Division.All data presented in this article can additionally be found in the supplementary materials.Author contributions:E.P.G.and R.O.R.had full access to the

experimental results in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.The alloys were processed by D.C.and mechanically characterized by B.G.,A.H.,and D.C.Study design,interpretation and analysis of data,and preparation of the manuscript were performed jointly by B.G.,A.H.,D.C.,E.H.C.,E.P.G.,and R.O.R.The authors declare no conflict of interest.

SUPPLEMENTARY MATERIALS

https://www.wendangku.net/doc/a73444079.html,/content/345/6201/1153/suppl/DC1Materials and Methods Supplementary Text Fig.S1Table S1

9April 2014;accepted 18July 2014process,representing the initial transfor-mation of a disordered phase into an or-dered one.It is also the most difficult part of the process to observe because it hap-pens on very short time and length scales.In the

bate as to whether classical nucleation theory (CNT),as initially developed by Gibbs (1),is a suitable framework within which to describe the process,or whether nonclassical elements such as dense liquid phases (2–4)or (meta)stable clusters (5)play important roles.Furthermore,uncertainty exists as to whether a final,stable phase can nucleate directly from solution or whether it forms through a multistep,multi-phase evolution (6,7).In the case of multistep nucleation pathways,whether transformation from one phase to another occurs through nu-cleation of the more stable phase within the

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1

Department of Materials Science and Engineering,University of California,Berkeley,CA 94720,USA.2Molecular Foundry,Lawrence Berkeley National Laboratory,Berkeley,CA 94720,USA.3Physical Sciences Division,Pacific Northwest National Laboratory,Richland,WA 99352,USA.4Department of

Materials Science and Engineering,University of Washington,Seattle,WA 98195,USA.

*Corresponding author.E-mail:james.deyoreo@https://www.wendangku.net/doc/a73444079.html,

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