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Scripta Materialia 68 (2013) 130–133

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Deformation-induced x phase in nanocrystalline Mo

G.M. Cheng,a H. Yuan,a W.W. Jian,a W.Z. Xu,a P.C. Millettb and Y.T. Zhua,⇑aDepartment of Material Science and Engineering, North Carolina State University, Raleigh, NC 27695, USA

bFuels Modeling and Simulations, Idaho National Lab., Idaho Falls, ID 83415, USA

Received 7 August 2012; revised 26 September 2012; accepted 29 September 2012Available online 5 October 2012

A deformation-induced hexagonal x phase was first observed in pure nanocrystalline body-centered cubic (bcc) Mo using high-resolution transmission electron microscopy. As the grains were refined into nanometer sizes by deformation using high-pressuretorsion under a pressure of �4 GPa at room temperature, the x phase formed at the grain boundaries of bcc Mo with a crystallo-graphic relationship close to the {112}h111i twin orientation. Its formation was mainly attributed to the shear deformation on{112} planes in bcc Mo.Published by Elsevier Ltd. on behalf of Acta Materialia Inc.

Keywords: Phase transformation; Nanocrystalline; Body-centered cubic; Deformation mechanism; High-pressure torsion

The hexagonal x phase (space group: P6/mmm) isa widely studied metastable phase in numerous group IVand V transition metals (Ti, Zr, Hf and Ta) and theiralloys [1–12]. It is formed from a hexagonal a phase ora body-centered cubic (bcc) b phase through phase trans-formation during high-temperature aging or under static/dynamic pressure. This phase transformation is of greatinterest because it could provide an additional strength-ening mechanism for these materials [4,6]. Previous stud-ies have revealed that the x phase transformation is verycommon in b-Ti alloys during heat treatment or conven-tional mechanical deformation [9–11]. Its existence wouldgreatly improve the alloy’s mechanical properties, such asultralow elastic modulus and ultrahigh strength. In addi-tion, shock-induced x phase transformation has beenreported in polycrystalline Ta and its alloys that wereshock-deformed at a very high pressure (�45 GPa) [12].It is suggested that the shear deformation on {112}planes is the main mechanism for the formation of xphase.

Refining the grains of materials into nanometer sizescan significantly alter the physical, chemical andmechanical behavior of the materials [13–22]. For exam-ple, nanocrystalline (nc) Cu and Ni have displayed supe-rior strength with good ductility when their deformationis significantly controlled by twins and partial disloca-tion emission at the grain boundaries [13–15]. Nanoscale

1359-6462/$ - see front matter Published by Elsevier Ltd. on behalf of Actahttp://dx.doi.org/10.1016/j.scriptamat.2012.09.033

⇑Corresponding author; e-mail: ytzhu@ncsu.edu

grain size also has a significant effect on the phase trans-formation in metals and alloys [23–25]. Wang et al. [25]have investigated the grain size effects on reversible b-to-x phase transformation in a Ti-based alloy processed byhigh-pressure torsion (HPT). The x phase occurred atthe initial stage but disappeared when the grain sizedecreased to �50 nm.

In this paper, we report a deformation-induced xphase transformation in pure nc Mo processed byHPT under a pressure of �4 GPa at room temperature.Mo has a bcc structure (space group: Im3m; a =0.3147 nm [26]), and exhibits no equilibrium phasetransformation up to its melting temperature at ambientpressure. To date, no reports have been published on thex phase in pure Mo. Here we investigate for the firsttime the morphology, formation mechanism and interfa-cial structure of the x phase in nc Mo.

Sample disks, 0.76 mm thick and 10 mm in diameter,were cut from an Mo sheet and deformed using HPT forsix turns under a pressure of 4 GPa. A detailed descrip-tion of HPT procedure can be found elsewhere [27,28].Wedge-shaped thin foils for transmission electronmicroscopy (TEM) observations were sampled fromthe edge of HPT-processed Mo disks. TEM and high-resolution TEM (HRTEM) observations were carriedout using a JEOL 2010F microscope operating at200 kV. HRTEM image simulations were performedusing the EMS software package.

TEM investigations revealed an average grain size of�90 nm in nc Mo, based on at least 500 randomly selected

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Figure 2. HRTEM investigations of x phase in nc Mo, taken from the½2�1�10�x zone axis. (a and b) Experimental and simulated (defo-cus = 4 nm, thickness = 8 nm) images of x phase, respectively. Theatomic projection of an x unit cell is given in (b).

G. M. Cheng et al. / Scripta Materialia 68 (2013) 130–133 131

grains. A typical deformation microstructure of nc Mo isshown in Figure 1a. From the corresponding selected areadiffraction (SAD) pattern in Figure 1b, the two markedlattices (A and B, both in ½1�10� zone axes from bcc Mo)are from the grains marked A and B in Figure 1a, respec-tively. They can be clearly distinguished in the two dark-field TEM images in Figure 1d and e, respectively. Fromthe composite SAD pattern in Figure 1b, the angle be-tween the two (110) planes of lattices A and B are�67.3�, which is very close to the 70.5� angle of a twinrelationship in bcc systems.

In addition to grains A and B, there is also an inter-granular phase with a width of �30 nm between them,which is framed by the white solid line (zone C) in anenlarged image in Figure 1f based on HRTEM observa-tions. It is identified to be a x phase according to the fastFourier transform (FFT) diffraction pattern in Figure 1c,which is directly taken from an HRTEM image corre-sponding to the framed rectangular zone I in Figure 1f.Here the ½2�1�10�x zone diffraction patterns of the x phaseare superimposed with the ½1�10�a zone diffraction patternof the parent a phase (we call the bcc Mo phase the aphase). The orientation relationship between the x anda phases is determined to be: ½2�1�10�x==½1�10�a andð01�10Þx==ð11�2Þa. The lattice parameters of the x phaseare ax ¼

ffiffiffi

2p

aa ¼ 0:445 nm and cx ¼ffiffiffiffiffi

3ap

a=2 ¼0:273 nm (c/a = 0.613). The x phase is distributed contin-uously at the intergranular zone (zone C), with an irregu-lar shape between grains A and B, as shown in Figure 1d–f. This is completely different from the observations inprevious studies [4,12,25], which reported that the xphase always occurs in the bcc phase in a plate-like orcuboidal shape.

The existence of x phase in nc Mo is further verifiedby HRTEM micrographs in Figure 2. Figure 2a displaysan experimentally observed HRTEM image of x phasetaken from the ½2�1�10�x zone axis, in which the dots show

Figure 1. Bright-field (a and f) and dark-field (d and e) TEM imagesshowing the typical morphology of x phase in nc Mo. The relatedgrains are marked by A, B and C, respectively. (b) A composite SADpattern corresponding to (a); (c) an FFT diffraction pattern of a and xphases taken from an HRTEM image corresponding to the framedzone I in (f). Both (110) spots of lattices A and B in (b) were used totake the two dark-field images (d and e, respectively).

periodic bright and weak configurations along the½0�110�x direction. Figure 2b shows a simulated HRTEMimage of x phase in the same zone axis obtained at adefocus of �4 nm and a thickness of �8 nm. From theatomic projection of an x unit cell in Figure 2b, thetwo atomic columns in the x structure can only displayone weak spot in the simulated image under the currentimage conditions. Thus, the simulated result agrees wellwith the experimental image.

In order to investigate the formation mechanism ofthe x phase, Figure 3a presents a schematic illustrationof the crystallographic relationship between the x and aphases. The marked hexagonal x lattice can be com-pletely contained in the 2� 3� 2 a superlattice. Theoccupancies of the atoms in the x lattice points are onthe sites of the atoms in the a superlattice. However,the sites of the two marked atoms A and B must bemoved a distance of 1=12 ½111� along the [111] and½�1�1�1� directions (A! A0 and B! B0), respectively, toform a hexagonal x structure. The transformation pro-cess is clearly illustrated in Figure 3b, which displays atwo-dimensional atomic projection of the x (upper)and a (bottom) phases taken from zone axes of½2�1�10�x==½1�10�a. Both x and a phases have six atomiclayers (marked in Fig. 3b) along the ½0�110�x and ½�1�12�adirections, and two layers along the projected directions.The marked atoms A and B at the bottom of Figure 3bcorrespond to the two atoms A and B in Figure 3a. Thex phase can be formed when the atoms (A and B) in lay-ers 2 and 3 of the a phase shuffle 1=12 ½111� along the

Figure 3. Illustration of the formation mechanism of x phase in ncMo. (a) Schematic showing the crystallographic relationship betweenthe x and a phases. (b) Composite atomic model illustrating how toform hexagonal x phase (upper) from bcc a phase (bottom), viewedfrom the zone axes of ½2�1�10�x==½1�10�a.

Figure 4. HRTEM investigations of the interfacial structure between aand x phases, taken from the zone axes of ½1�10�a==½2�1�10�x. (a and b)Marked zones I and II, respectively, in Figure 1f. Zone I in (a) displaysthe interfacial structure between a and x phases, while zone II in (b)shows the intergranular x phase between grains A and B.

132 G. M. Cheng et al. / Scripta Materialia 68 (2013) 130–133

[111] and ½�1�1�1� directions, i.e. reaching the locations ofA0 and B0 in layers 20 and 30 of the x phase.

Two mechanisms have been suggested for the forma-tion of x phase [4,12]. One is the shuffling mechanism,which is a thermally activated x phase transformationthat has been observed in group IV transition metalsand alloys during heat treatment [4]. The other is the dis-location mechanism for x phase transformation duringshear deformation [12]. For the latter, the atoms in dif-ferent a layers must move a distance of 1=12 ½111� (layer2), 1=3 ½111� (layer 3) and 1=12 ½111� (layer 4) in thetransformation process, respectively. Therefore, the xphase could be generated by the glide of partial disloca-tions of types 1=3 ½111�, 1=6 ½111� and 1=12 ½111�, whichare dissociated from 1=2 ½111� perfect dislocation in thebcc structure according to the discussion in Ref. [12].In our case, a high density of dislocations is introducedinto the grains during the HPT process at room temper-ature and the density of dislocations in the grain with asize of �100 nm is of the order of �1016 m�2, based onour recent HRTEM investigations [29]. Therefore, a-to-x phase transformation is most likely formed by thegliding of partial dislocations during the severe sheardeformation in HPT nc Mo.

From the above observations, the growth of grain C(x phase) is associated with grain B (a phase), and theinterfaces between grains C and B are closely parallelto the ð002Þa and ð110Þa planes (from right to left) butnot ð11�2Þa in Figure 1. Therefore, HRTEM investiga-tions of the interfacial structure between the x and aphases could provide further details about the formation

mechanism of the x phase. Here, two typical HRTEMimages from the framed zones I and II in Figure 1f arechosen to elucidate how the x phase is formed fromthe a phase, as shown in Figure 4a and b, respectively.It is clear in Figure 4a that there are steps on the inter-face between the a and x phases. One of the facets isparallel to the atomic planes ð11�2Þa orð01�10Þx, whilethe other is close to ð22�3Þa orð02�21Þx. The height of eachstep is almost integral multiples of the length of1=2 ½01�10�x. Clearly, the growth of x phase is primarilyvia the shear deformation on ð11�2Þa plane of grain B.This suggests that a consecutive shear of two1=12 ½111� and one 1=3 ½111� partials in the ð11�2Þa planesforms a three-layer x phase. However, the shear defor-mation on ð11�2Þa plane is inhomogeneous, which causesthe steps on the interface. Furthermore, the distortion inthe x phase is more severe than that in the a phase, andthe atomic misalignments on the ð01�10Þx plane aremainly caused by the 1=3 ½0�112�x dislocation, markedby b

*

in Figure 4a.Figure 4b also shows that there is a very thin layer of

x phase (5–10 nm) between grains A and B. The inter-granular x phase is severely distorted since it shouldbear the misfit (�3.2�) of the two grains A and B. More-over, there are many defects in both a grains in Figure4b, especially [001] mixed or edge sessile dislocations(showing as 1/2h110i edge components, marked by\), which would hinder the growth of x phase in thisarea. From our TEM and HRTEM observations, thex phase mostly appears at the a grain boundaries, witha crystallographic relationship similar to a {11 2}h111itwin relationship. It is suggested that the intergranularx phase must be in a transition state to adapt the misfitof the a grains in nc Mo.

From the above results, there is no doubt that xphase transformation can take place in pure Mo viastrong shear deformation during the HPT process at arelatively low applied pressure (�4 GPa). However, thex phase transformation in pure Ta is activated at a veryhigh shocked pressure of �45 GPa [12], and previous re-sults have revealed that the alloying elements could pro-mote the formation of x phase in b-Ti alloys and Ta–Walloys [1,12]. Note that there are several phase transfor-mations in both Ti and Ta metals and alloys accordingto their phase diagrams, especially Ti alloys, whichmakes one wonder if the x phase is a metastable phasethat is related to one of the phase transformations. Incontrast, there is no reported equilibrium phase trans-formation in Mo. Our observation of the x phase inpure Mo suggests that it could be a common phenome-non in most bcc metals processed via severe plasticdeformation.

In summary, a deformation-induced x phase wasfirst observed by TEM in HPT-processed nc Mo. Thex phase occurred at the grain boundaries of bccMo, with a crystallographic relationship close to the{112}h11 1i twin orientation. It was distributedcontinuously at the intergranular zone, with a widthof less than 30 nm. Its formation was mainly attributedto the shear deformation on {112} planes of bccMo. The occurrence of x phase in nc Mo helps us

G. M. Cheng et al. / Scripta Materialia 68 (2013) 130–133 133

understand of phase transformation in group VI bccmetals via severe plastic deformation.

This work was supported by the Laboratory Di-rected Research and Development Program Office of theIdaho National Laboratory. The authors wish to thankDr. Dieter Wolf, whose insight and discussions withthe authors inspired and initiated the current study ofbcc metals.

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