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Nucleation of hcp and fcc phases in bcc iron underuniform compression: classical moleculardynamics simulationsTo cite this article: B T Wang et al 2010 J. Phys.: Condens. Matter 22 435404

 

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IOP PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 22 (2010) 435404 (8pp) doi:10.1088/0953-8984/22/43/435404

Nucleation of hcp and fcc phases in bcciron under uniform compression: classicalmolecular dynamics simulationsB T Wang1,2, J L Shao2, G C Zhang2, W D Li1 and P Zhang2

1 Institute of Theoretical Physics and Department of Physics, Shanxi University,Taiyuan 030006, People’s Republic of China2 LCP, Institute of Applied Physics and Computational Mathematics, Beijing 100088,People’s Republic of China

E-mail: zhang ping@iapcm.ac.cn

Received 9 August 2010, in final form 2 September 2010Published 12 October 2010Online at stacks.iop.org/JPhysCM/22/435404

AbstractBy classical molecular dynamics simulations employing an embedded atom method potential,we have simulated the bcc to hcp/fcc structural transition in single-crystal iron under uniformcompression. Results showed that the transition pressure is different from uniaxial compressionand shock loading. The transformation occurs on a picosecond timescale and the transition timedecreases along with the increase of pressure. The nucleation and growth of the hcp and fccphases under constant pressure and temperature are analyzed in detail. The nucleation planes,all belonging to the {110}bcc family and parallel to the three compression directions [100],[010], and [001], have been observed. About 20% bcc atoms have transformed to fcc phaseunder pressure just over the critical point, and under higher pressure the fraction of the fccphase increases steadily to exceed that of the hcp phase. We have investigated the transitionmechanism of iron from initial bcc to hcp/fcc and found that the transition mainly consists ofcompression, rotation, and shuffle.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Iron, the ultimate element of thermonuclear burning in bigstars, is abundant in our planet earth. Undergoing five thousandyears of development, iron has become the most well knownmetal and is widely used in every walk of life. Following theobservation of the solid to solid phase transformation occurringat a pressure of 13 GPa reported by Bancroft et al [1] in 1956under shock wave compression, a large number of theoreticaland experimental studies [2–7] have been performed toinvestigate the bcc to hcp transition in iron because of itscritical importance in condensed-matter physics, materialsscience, and geophysics [8, 2, 9]. Recently, using in situ x-ray diffraction, Kalantar and co-workers [10–12] confirmedthat single-crystal iron indeed transforms to the high-pressurehcp phase in shock loading conditions when shocked alongthe [100] direction. At the same time, the extended x-rayabsorption fine structure (EXAFS) measurements [13] also

presented a direct, atomic-level, and in situ proof of shock-induced transformation of iron.

In static experiments [14–16], the bcc to hcp transitionwas confirmed to occur at a similar pressure and the c/aratios at some constant pressures were analyzed in detail.Reference [16] proposed three possible transition mechanismsbased on lattice shearing movements and suggested that atransition mechanism involving an intermediate fcc phase isconsistent with their experimental results. More recently,Mathon et al [17] investigated the dynamics of the magneticand structural bcc to hcp phase transition in iron bycombining the simultaneous x-ray magnetic circular dichroismtechnique and the x-ray absorption spectroscopy technique.The magnetic and structural transitions were observedsimultaneously and were found to be first order transitions.In addition, the use of the diamond anvil cell in radial x-raydiffraction geometry [18] enabled us to study the rheology andelasticity of iron in situ at ultrahigh pressures.

0953-8984/10/435404+08$30.00 © 2010 IOP Publishing Ltd Printed in the UK & the USA1

J. Phys.: Condens. Matter 22 (2010) 435404 B T Wang et al

However, the experimentally established fact of thebcc to hcp transition in iron was challenged by severalrecent molecular dynamics (MD) simulations both in shockloading studies [19, 20] and in a uniaxial compressionstudy [21]. After successfully simulating the shock-inducedphase transition of bcc iron with an embedded atom method(EAM) potential [22, 23] shocked along the [001] direction,Kadau et al [24, 19] further investigated the structuraltransition of single-crystal iron by shock loading along [011]and [111] directions and found that a mixed high-pressurestage of hcp and fcc with smaller grains may be formed. Ina study of polycrystalline iron, depending on shock strengthand grain orientation, some fraction of the fcc structure wasobserved [20]. Our previous simulations [21] also observedthe bcc to hcp/fcc transition of iron under uniaxial compressionfor loading along [011] and [111] directions. We stated thatthe fraction of the fcc structure increases steadily along withthe increase of the pressure and may exceed that of the hcpstructure under higher pressure. In this paper, we describethe nucleation and growth of the close packed phases (bothhcp and fcc) in single-crystal iron compressed under uniformcompression conditions by use of MD simulations. Thecorresponding transition mechanism is also discussed.

2. Methodology of the simulation

The scheme of our MD simulations recently has been usedto study the dynamic properties [25] and the orientationdependence [21] of structural transition in iron. Both of theseworks were performed under uniaxial compression. For thiswork, we employed the EAM potential of iron developed byVoter–Chen (VC) [26] to study the structural transition ofsingle-crystal iron under uniform compression. All samplesconsist of 50x,[100] × 50y,[010] × 100z,[001] atomic cells withdifferent lattice constants for different strains. Periodicalboundary conditions were adopted in all the directions. Theoriginal temperature was set at 60 K by the speed calibrationmethod [27] and in our present simulations the temperature wasfixed at 60 K either in dynamical compression or in uniformcompression. The compressive stress tensor was calculatedaccording to the virial formula [28, 29]:

pαβ = 1

V

(∑i

miviαviβ +∑

i

∑i> j

ri jα fi jβ

), (1)

where the summation is over all atoms and α(β) represent x , y,or z axes. The first term in the stress tensor is the momentumflow of atom i and the second term is the microscopic virialpotential stress. The Verlet algorithm [30] was used to integratethe equation of motion and all simulations were performedin the NVT ensemble. After such MD simulations, the localstructure around each atom was resolved by using the commonneighbor analysis (CNA) [31] technique.

3. Results and discussion

3.1. Pressure analysis

Before investigating the transition of iron under uniformcompression, we compress the initial bcc sample at zero

Figure 1. Variation of (a) pressure and (b) fcc and hcp phases massfractions λ with the strain ε in loading and unloading processes.Pictorial representations of the sample at the beginning of thetransition and at a strain of 0.25 in the loading process are also shownin the insets where the light gray (green) stands for bcc structure,dark gray (red) for hcp, white for fcc, and off-white (yellow) forgrain boundaries.

pressure dynamically to high pressure through varying thevolume isotropically at every step with a time step of 2 fs.After loading up to a strain of 0.386 with a high strainrate of ∼108, we unload the sample with the same strainrate. This kind of dynamical process is different from aquasistatic process. The calculated pressure–strain curvesfor both loading and unloading processes are presented infigure 1(a), where ε = 1−V/V0 and V0 is the initial volume ofthe sample. Note that the data above strain 0.25 are not shown.Obviously, the pressure relaxes at around 31 GPa in the loadingprocess, which implies the transition of bcc iron to closepacked phases, while in the unloading process the pressurerelaxes at ∼10 GPa, which implies the reversal transformation.Employing the CNA technique, we plot in figure 1(b) the massfraction λ for the hcp and fcc phases during their evolutionwith the loading/unloading strain. The large hysteresis andphase coexistence range can be seen. The starting andfinishing transition critical points in the strain–pressure spaceare Ts,load(0.129, 31.11) and Tf,load(0.145, 31.50), respectively,for the loading process, while in the unloading process they areTs,unload(0.079, 13.12) and Tf,unload(0.044, 8.58), respectively.Here, the criteria used to determine the transition finish pointin the loading process and the transition starting point in theunloading process is the same as in our previous study [21].

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J. Phys.: Condens. Matter 22 (2010) 435404 B T Wang et al

Figure 2. Time evolutions of pressure under constant strains. Redand black arrows point to the pressure transition points for strains at0.1227 and 0.1236, respectively.

The transition critical value of pressure (about 31 GPa) is muchlarger than our previous studies [25, 21] where we find that thecritical transition pressure of uniaxial compression is around13–15 GPa. From figure 1(b), we find that more fcc atoms arenucleated than hcp atoms at the beginning of the bcc to hcp/fcctransition. In the high-pressure domain, the fraction of the hcpphase increases up to 0.51 which is evidently larger than thatof the fcc phase. Upon unloading, the fcc phase also beginstransition prior to the hcp phase. The close packed phasespersist until below 8.58 GPa. This is consistent with a previousexperimental study [6]. In addition, the microscopic views ofthe compressed sample at the beginning of the bcc to hcp/fcctransition and at the end of the loading are also shown in theinset of figure 1. At the beginning of the transition, an initialhomogeneous nucleation of the hcp/fcc phase is observed.And at the end of the loading, most atoms are nucleated toclose packed phases and the grain boundaries in the (110)bcc

planes can divide the sample into a sandwich structure. Thiskind of sandwich structure has also been observed in a MDstudy of iron driven by uniaxial compression along [011] and[111] directions [21]. In the present triaxial compression, thenucleation planes are (011)bcc and (101)bcc.

To study the bcc to hcp/fcc transition under uniformcompression, we construct a series of bcc samples withdifferent volumes and relax them within our MD formalismup to 70 ps with a time step of 2 fs. Note that this time thesamples are relaxed at constant volumes to approach differentpressures. The obtained pressure–time curves at some constantstrains are presented in figure 2. Here, the strain ε = 1− V/V0

and V0 is the volume of the sample at zero pressure where thelattice constant a0 = 0.287 25 nm. Apparently, in strain up to0.1208 the pressure still behaves calmly and we have not foundany hcp or fcc atoms in the whole relaxation process throughthe CNA technique. With the strain increasing, the samplebecomes destabilized. Up to 0.1227, the pressure begins anabrupt change between 3.4 and 11.2 ps, as shown in figure 2.And under this strain, we do find hcp and fcc atoms, whichcan be seen from the phase mass fraction curves in figure 3(a).

Figure 3. Time evolution of the fcc and hcp phases mass fraction λunder strains equal to (a) 0.1227 and (b) 0.1236. Arrows indicatesome critical points for careful analysis in the text.

All these analyses illustrate that the transition critical pressureunder uniform compression is at ∼33 GPa.

For larger strains, as can be seen from figure 2,abrupt changes of the pressures are also observed and theconvergent pressures increase with the increase of strains. Theabrupt change process of pressures indicates that the samplesovercome the Gibbs free-energy barrier when they obtainenough pressure and fluctuations [32]. Note that the pressureof strain at 0.1236 behaves slightly differently in that at around32 ps it decreases to lower than the pressure of strain at 0.1227.Besides, the pressure–time curve of strain at 0.1236 has fourtransition points. The first transition point of pressure at 1.9 psis due to the emergence of the fcc phase atoms which are moreclearly shown in figures 3(b) and 4(a) at 2.4 ps. The secondtransition point at 3.3 ps originates from the nucleation of thehcp atoms, as shown in figures 3(b) and 4(b) at 3 ps. Thethird transition point at 12.7 ps is the same as other strainsand the fourth transition point at 32.1 ps can be attributed tothe nucleation of more fcc and hcp atoms from bcc or grainboundary atoms. The nucleation of close packed phases at32 ps is clearly indicated by figure 3(b). In addition, we findthat with the increase of strain the transition relaxation time,from the beginning to the first transition point, decreases stepby step. Specifically, the relaxation time for a strain of 0.1227is approximately 3.4 ps and for a strain of 0.1236 is around1.9 ps. This fact also can be seen from the phase mass fractionanalysis in figure 3. Moreover, for all relaxations at constantstrains, the fcc phase atoms emerge earlier than the hcp phase

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J. Phys.: Condens. Matter 22 (2010) 435404 B T Wang et al

(a) (b) (c)

(f)(e)(d)

Figure 4. Morphology evolution of the sample under ε = 0.1236. The panels show snapshots at different times: (a) 2.4 ps, (b) 3 ps, (c) 6 ps,(d) 12 ps, (e) 60 ps, and (f) 60 ps. The coordinates are shown in panel (a). Note that only hcp and fcc atoms are shown in (a)–(c). To indicatethe nucleation planes more clearly, only fcc atoms are presented in panel (f). The color criteria are the same as in figure 1.

atoms. But the number of hcp phase atoms always exceeds thenumber of fcc phase atoms after 6 ps.

3.2. Nucleation and growth of hcp and fcc phases

Our previous simulations [25, 21] found that the fcc phaseatoms appear in hcp atoms in stacking faults style when thesamples were compressed uniaxially along the [001] direction.And with the increase of the pressure, the fcc atoms wouldtransit to hcp phase or grain boundary atoms. As for shockloading studies [19], the nucleated fcc atoms are also very fewcompared with hcp atoms for loading along [100]. However,in the present simulations, the ratio of the fcc atoms reachesto approximately 20% under strains of around 0.123 (seefigure 3).

To investigate the nucleation and growth of the hcp andfcc phases in bcc iron, the morphology evolution of the sampleunder strain at 0.1236 is presented in figure 4. Obviously, thefcc atoms emerge firstly at a relaxation time of 2.4 ps (seefigure 4(a)). Then, the hcp atoms begin emerging at 3 ps andthe fcc atoms grow in the whole sample (figure 4(b)). At 6 ps(figure 4 (c)), the close packed phases nucleate to a relativelyinerratic small bulk structure. Meanwhile, the fraction of thehcp phase exceeds that of the fcc phase (see figure 3(b)).In this study, the criterion to determine the transition finishpoints is that the mass fraction of the close packed phasesis over 0.7 [21]. Here, we find that the transition finishes ataround 10 ps for both strains at 0.1227 and 0.1236. Therefore,the transformation is prompt and occurring on a picosecondtimescale, which is similar to previous studies [19, 33] undershock compression. Up to 12 ps, the fcc phase mass fraction

reaches its maximum point. And the close packed phasesare mostly formed under this strain. So, the transition is notsluggish in our present study. The formed relatively thinnersandwich structures, perpendicular to the [101] direction, areapparent in panel (d) of figure 4. Panels (e) and (f) showthe sample at the same time. While panel (e) gives out thewhole atoms including bcc, grain boundaries, fcc, and hcpstructures, panel (f) only presents the fcc phase atoms toinvestigate the nucleation planes. Compared with panel (d),the sandwich structures in panel (e) become thicker and thesandwich structures perpendicular to the [110] direction growto become more clear. In addition, the fcc atoms in the stackingfaults structure at 60 ps vanish to over half a per cent ofthose at 12 ps. All these evolutions result in bulk hcp andfcc structures. From panel (f), we find that the nucleationplanes under uniform compression are (101)bcc, (110)bcc, and(011)bcc. All these planes belong to the {110}bcc family. Thisis different from uniaxial compression [25, 21] where only(110)bcc and (110)bcc nucleation planes have been observedwhen loading along the [001] direction. Therefore, we canconclude that the nucleation planes relate tightly to the loadingorientations. The nucleation planes are always parallel to theloading directions and belong to the {110}bcc family. Thus weconfirm the fact that bcc to hcp transformation occurs via theNishiyama–Wassermann mechanism [34]: {011}bcc‖{0001}hcp.

Figure 5 shows the cross section of the compressedsample relaxed to 60 ps. Here, the gray (cyan) lines clearlydraw the outline of the grain boundaries. These lines canplot out five relatively large zones Z1–Z5. Atoms in Z1and Z3 have the same nucleation plane (011)bcc which isparallel to the compression orientation [100]. And atoms in

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J. Phys.: Condens. Matter 22 (2010) 435404 B T Wang et al

Figure 5. Cross section of the compressed sample relaxed to 60 psshown in figure 4(e). The gray (cyan) lines show the grainboundaries clearly and can plot out five relatively large zones Z1–Z5.In Z1, Z2, and Z3, the fcc stacking faults along the [100] and [010]directions are observed. In Z5, the fcc atoms nucleate to grainmorphology. Color criteria the same as in figure 1.

Z2 and Z5 nucleate on the (101)bcc plane which is parallelto the [010] compression direction. Atoms in Z4 form anabsolute hcp structure and nucleate on the (110)bcc plane.Fcc stacking faults along the [100] and [010] directionshave been observed in Z1, Z2, and Z3. In Z5, the fccatoms nucleate to grain morphology. Thus, the morphologystructures in our simulations are different from previous studiesno matter whether in uniaxial compression along the [001]direction [25, 21] or in shock loading [19] conditions.

To study the nucleation morphology under higherpressure, we relax samples under some bigger constant strains.The fractions of the fcc and hcp phases under different strainsrelaxed for the same time 60 ps, as well as the variation ofpressure with the strain, are shown in figure 6. Obviously,the transition is not sluggish. This fact is consistent with onerecent report [17] where the pressure domain of the transitionwas observed to be 2.4 ± 0.2 GPa. This value is different fromanother previous experimental study [6] where the transitionwas stated to extend over 8 GPa at ambient temperature.Moreover, from figure 6, one obvious behavior is the expectedoscillating behavior just over the transition critical points andanother is that the fraction of the fcc phase increases to over47% along with the decrease of the fraction for the hcp phase.Under strain at 0.1694, the mass fraction of the fcc phaseexceeds that of the hcp phase and under higher pressure thefraction of the fcc over that of the hcp becomes more evident.This indicates local slip of atoms between neighboring layersto transit the nucleated hcp structure to the fcc phase. Themorphology picture of the compressed sample under strain at0.1694 relaxed to 60 ps is presented in figure 7. The grainboundaries in the (011)bcc planes can divide the sample intoa sandwich structure. The fcc and hcp nucleation planes are(101)bcc and (110)bcc. Here, the hcp and fcc atoms form somerelatively small grains and also mutual stack faults in manyzones. Therefore, combining this with former discussions, wecan establish the conclusion that both strain and relaxation

Figure 6. Mass fractions λ for the fcc and hcp phases change withthe strain, ε, under which the samples are relaxed up to 60 ps to reachequilibrium states. The variation of pressure with the strain is alsopresented.

Figure 7. Morphology picture of the compressed sample(ε = 0.1694) relaxed to 60 ps. The grain boundaries along (011)bcc

planes can divide the sample into a sandwich structure. The fcc andhcp nucleation planes are (101)bcc and (110)bcc. Color criteria thesame as in figure 1.

time affect the morphology structures of pressure-inducednucleation of single-crystal iron under uniform compression.

3.3. Transition mechanisms analysis

The microview of the phase transformation of iron has beenstudied both in hydrostatic compression [15, 6] and in shockloading [11, 33]. For hydrostatic compression experiments,Wang and Ingalls [6] put forward three different possiblemechanisms (models I, II, and III) for the transition. ModelI contains two steps: first, the bcc lattice contracts alongthe [001] direction and expands along the [110] direction toform a hexagon, and second, the adjacent (110) planes shufflerelatively along the [110] direction to create the hcp structure.Model II also contains two steps: first, the atoms in the (112)

planes shear in the [111] direction and compress in the [112]direction to produce a hexagon, and second, the alternate (110)planes shuffle relatively along the [110] direction to create the

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J. Phys.: Condens. Matter 22 (2010) 435404 B T Wang et al

Figure 8. The traces of the atoms under ε = 0.1227 show the dynamic phase transformation course from the bcc to hcp ((a)–(e)) and from thebcc to fcc ((f)–(k)) structures. The times for the snapshots are (a) 0.2 ps, (b) 6 ps, (c) 7 ps, (d) 8 ps, (e) 20 ps, (f) 0.2 ps, (g) 6 ps, (h) 7 ps,(i) 8 ps, (j) 11 ps, and (k) 20 ps. Color criteria the same as in figure 1.

hcp structure. Model III is a three-step process. Here, theinitial step and the final structure are the same as in modelII. An intermediate fcc structure was proposed by Wang andIngalls to occur before the formation of the hcp structure.They concluded that model III is preferred under their EXAFSmeasurements because the energetics are more favorable forthe series of shear and slip systems and that temperaturedependent effects suggest a metastable fcc structure. However,for shock loading investigations, Hawreliak et al [11] and Cuiet al [33] observed that the mechanism of bcc to hcp transitionin iron follows model I, but with no expansion of the latticealong the [110] direction.

In our simulations, the transition mechanism is basicallythe same as in model II raised by Wang and Ingalls [6].Pictures (a)–(e) ((f)–(k)) in figure 8 show snapshots of theatoms in the hcp (fcc) phase nucleation zone from the initialbcc structure to the final hcp (fcc) structure under strain equalto 0.1227. Clearly, the course of the transition containsthree main motions: compression, rotation, and shuffle. Thecompression along the [100] direction and elongation alongthe [011] direction can be clearly seen from the dhcp2(dfcc2)

and dhcp3(dfcc3) curves in figure 9, respectively, for bcc tohcp (fcc) transition. Note that the elongation along thedirection perpendicular to the compression direction has notbeen observed in shock loading studies [11, 33]. The rotationis the result of shear along the [111] direction and compressionalong the [211] direction. As shown in figure 8, the latticeis rotated by approximately 4.4◦ from 0.2 to 20 ps withcompression along [211] equal to 5.9%. Previously, Hawreliaket al [11] proposed that the hexagon is rotated by 5.3◦ withoverall compression of 8% under hydrostatic conditions. Theshuffle process can be clearly seen from figures 8 and 9 whichshow that alternate (011) planes shuffle toward the [011] ([011]or [011]) direction to create the hcp (fcc) structure. Thedifference between our transition mechanism and model IIraised by Wang and Ingalls [6] rests with the sequence of theabove motions of the lattice. While model II consists of twosteps with the first step including compression and rotationand the second step shuffle, in our case, the three motionsprogress simultaneously from 6 to 8 ps in one step for bcc tohcp transition. This difference probably originates from the

Figure 9. Distance evolution of the atoms under ε = 0.1227. Here,dhcp1 (dfcc1) shows the distance between atoms 4 and 8 in figure 8 forthe hcp (fcc) transition zone; dhcp2 (dfcc2) shows the distance betweenatoms 3 and 5; dhcp3 (dfcc3) shows the distance between atoms 2 and7; dfcc4 shows the distance between atoms 4 and 13. While theevolution of dhcp2 (dfcc2) and dhcp3 (dfcc3) shows the compressionalong the [100] direction and the elongation along the [011]direction, respectively, the evolution of dhcp1 (dfcc1 and dfcc4) showsthe shuffle procedure.

different compression styles. Besides, model II and model IIIcannot be differentiated using x-ray diffraction [11]. In oursimulations, this difference can be distinguished by the CNAtechnique. However, we have not found the intermediate fccstructure in the whole sample no matter whether for strainat 0.1227 or other strains. In addition, the transition of bcc

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J. Phys.: Condens. Matter 22 (2010) 435404 B T Wang et al

Figure 10. Radial distribution functions of the different strain (ε)materials under uniform compression. The samples under strains of0.1236 and 0.1800 have been relaxed to the equilibrium state (60 ps).As a comparison, the radial distribution functions of the ideal bcc,fcc, and hcp structures as well as the sample compressed uniaxiallyup to ε = 0.1500 along the [001] direction [21] are also presented.

iron to fcc, as can be seen in figures 8 and 9, is differentfrom our bcc to hcp transition in the sequence of the motions,i.e. compression, rotation, and shuffle of the lattice. Thetransition of bcc to fcc is a two-step process with the first stepbeing rotation and shuffle and the second step compression.The intermediate hcp, as shown in figure 8(i), and fcc structuresare both possible transition states in the fcc nucleated zones.The difference between the transition mechanisms of bcc tohcp and bcc to fcc need more investigation.

To understand our results in detail, structural analysisof the bcc to hcp/fcc transition is analyzed by the radialdistribution function (RDF). Figure 10 displays the RDFsunder different strains at 0.1236 and 0.1800. Both thesimulation snapshots have been relaxed to the equilibrium state(60 ps). As a reference, the RDFs of the ideal bcc, fcc, andhcp structures as well as the sample compressed uniaxiallyup to ε = 0.1500 along the [001] direction [21] are alsopresented. Note that we have applied configurational averagingto several spherical cutouts of the simulation snapshots as wellas the ideal bcc, hcp, and fcc structures containing 1000 atoms.Clearly, the RDF of the ideal bcc has six peaks in the wholerange from 2.0 to 5.7 A, the ideal hcp has seven, and the idealfcc five peaks. Therefore the third and fifth peaks of the idealhcp are the two additional peaks compared with those of theideal fcc. The first and second peaks of the ideal hcp and fccare absolutely identical. The third peak of the ideal hcp is thesmallest peak and in all our simulations it cannot be identifiedseparately. The most important difference in RDFs betweenthe ideal hcp and fcc is that the RDF of the ideal fcc does nothave the fifth peak of the ideal hcp. In our simulations, theRDF of strain at 0.1800 has a bigger peak in the position ofthe fourth peak of the ideal hcp than that of strain at 0.1236and the RDF of strain at 0.1500 has a little shoulder whichis a feature of the third peak of the ideal hcp. The shoulder

of the peak in the position of the fifth and sixth peaks of theideal hcp for strain at 0.1800 is much weaker compared withthat of strains at 0.1694 and 0.1500. These results illustratethat with the increase of strain the relaxed equilibrium statesample will nucleate more fcc atoms, as shown in figure 6.Besides, the most obvious feature to distinguish the bcc andthe close packed phases is located at the second peak of theideal bcc structure. The RDFs of all our simulation snapshotsdo not have this peak (see figure 10). This indicates that almostall the atoms in the samples have transformed into the closepacked phases or grain boundaries with pressure overcomingthe transition critical points.

4. Conclusions

In conclusion, we have studied the nucleation and growth ofthe hcp and fcc phases in bcc single-crystal iron under uniformcompression by the MD method. The transition pressurewas found to occur at around 31–33 GPa either in dynamicalcompression or in uniform compression. The fcc phase wasobserved to nucleate at first and the structural phase transitionof bcc iron to close packed phases is prompt and occurs ona picosecond timescale. The nucleation planes are parallel tothe compression directions and belong to the {110}bcc family.The morphologies have been described in detail. Meanwhile,we have found that the fraction of the fcc phase increases withthe increase of pressure and under high pressure it will exceedthat of the hcp phase. In addition, the transition mechanismhas been investigated in our MD simulations. We suggest thatthree motions (compression, rotation, and shuffle) of the latticeare responsible for the transition.

Acknowledgments

This study was supported by the Foundations for theDevelopment of Science and Technology of the ChinaAcademy of Engineering Physics (grant Nos 2008B0101008and 2009A0102005) and NCET of the Ministry of Educationof China (NCET-08-0883).

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