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Peculiarities of deformation-induced amorphization in CoZrNi alloys
Scripta Mttallrtrgica et Idateaialia, Vol. 33, No. 6, pp. 8%855,1995 Elsevia Science Ltd ckpyi&to1995ActaMet.4llurgicaIuc. F’rh!edintheUSAAUri&ts-d 09%716x195 $9.50 + .oo
Pergamon
0956-716x(95)00259-6
PECULIARITIES OF DEFORMATION-INDUCED AMORPHIZATION IN CoZrNi ALLOYS E.E. Novikova, Ye. V. Tatyanin and V.G. Kurdjumov Institute of High Pressure Physics, Russian Academy of Sciences, Troitsk - 142092, Russia (Received July 19,1994) (Revised February 1,1995)
In previous studies it was shown that an amorphous state in the intermetalhc compound NiTi can be produced by shear under pressure, hydroextrusion or cold-rolling [l-5]. A volume fraction of amorphous phase was demonstrated to be dependent on deformation value. The amorphous phase nucleation was observed at the twin boundaries in NiTi. Then it was shown that further growth of the amorphous component takes place in this alloy simultaneously with the fragmentation process at which the fragmentation size decreases with increasing strain. Similar structural changes were revealed for amorphization in the intermetallic compound Ni 3 Al [6] and in Si [7] by mechanical milling. The latter permitted one to assume that the mechanism of defbrmation-induced amorphization (DIA) is co~ected with the fragmentation up to the critical size. Other approaches for the explanation of deformation-induced amorphization were proposed in [5,8,9]. It was &resting to examine as far as the observable structural changes are characteristic for DIA in other materials. This paper is basically concerned with the initial stages of DIA in CoZrNi alloys. ExDerimentd
Deformatian-induced amorphization was investigated in alloys Co,&.,,Ni,, (Al) and Co&&& (A2) subjected to shear under pressure (SUP) in a Bridgman anvil (shear angle z = 45 - 7* 103deg., P = 4 GPa) or to axial compression (AC) using the supporting steel rings (true strain E up to 3.8; where E = m&,/h), h, and h-heights of qxcimen before and a&r the deformation). Ni was supplemented in order to increase the plasticity of alloy.Befare the deformation both the alloys consisted of two phases: ordered B2 intermetallic CoZr (long-range order parameter S = 0.98) and C 15 intermetalhc Co&. The particles of Co&r with a diameter of l-2 l.tmwere situated on CoZr grain boundaries. The volume traction of Co& was equal to nearly 50% in alloy Al and 70% in ahoy A2. The snuctum evohrtion during defbrmation was studied by TEM (JEM- 1OOC,1OOkV)and X-ray difhaction. The determination of long-range order parameter was performed in SUP-specimens by using the tnmsmissiongeomenyofx-raydi&tction measmements due to the specimen texture which was formed after AC-deformation. The objects for TRM investigation were prepared from the slices cut parallel and perpendicular to the detbrmation axis, thinned mechanically and then electrolytically. The electropolishing of slices was performed with the ele&olyte of 88 vol.% glacial acetic acid and 12 vol.% perchloric acid.
851
AMORPHIZATIONlN CoZrNi
852
Vol. 33, No. 6
Results
The transition from crystalline to the amorphous state was observed in CoZr phase at first by shear to z = 3* IO&g. under pressme. The amotphous phase was distinctly revealed by EM microdi&action iiom heavily deformed SUP specimens [Fig. 11. However, its volume tmction was very low in the case of AC even at E > 3. The volume fraction of the amorphous phase was higher for Al ahoy at the same deformation. The changes of the structure before the amorphization and the primary stage of amorphization were investigated mainly in specimens subjected to AC-deformation. At the initial stage of deformation, glide dislocations with Burgers vector a-400>, laying along { 1101 planes, were observed in Cozr phase. Furthermore, a local martensitic transformation B2 + B 19 was detected in alloy Al which is likely as an additional mechanism of deformation. The dislocation density in CoZr phase incmaxs rapidly to 10” cme2at E > 0.5 as follows horn the determination of the mean distance between dislocations. A very homogeneous dislocation structure was typical without any remarkable tendency to a cell structure formation. According to TEM data the mean distance between the matrix dislocations changes insigniscantly with increasing strain at least up to 2. At AC strain E = 0.36, on the background of randomly distributed dislocations there were detected the bands parallel to { 110) and consisted of microband bundles. The microband boundaries were of low-angle and mixed type. Two systems of intersecting bands were often observed, one being parallel to the specimen surface, i.e. perpendicular to the compression axis. The typical micmmucmm of ahoy at E = 2,3 is shown in Fig. 2. The microbands of the average thickness 7Onm,approximately parallel to the specimen surface were observed in B2-phase at this strain Very distinct boundaries represent a &am&xi&c featum of these microbands. The density of microbands was not the same in diffiznt parts of the specimen. The average size of Co& particles did not change in comparison with the initial stage of the alloy. Thin amorphous interlayers between the microbands were observed by dark field method (Fig. 2). The halo Tom the amorphous phase was not visible on microdigraction patterns at such deformation. To realize dark field observations, the objective aperture was situated at the place where the tirst halo f?om the amorphous CoZr is supposed to be presented, i.e. at the radius k = 0.272 nm-‘. According to TEM-results the B2 crystalline phase remains chemically ordered up to the appearance of the volume amorphous phase. As it was shown by X-ray difbactometry, the intensity of supemtructural
(a)
08
Fii 1. strumae of alloy (Al) subjectedto shear under high prmsure, shear angle 7* 1O’deg.(a) TEM bright-fieldimage, amorphous matrixandcqZrnanocystalls.(b)TypicalelectrondifFractionpattem,sharpacattering~correspondtoCqZrphase.
Vol. 33, No. 6
853
AMORPHIZATIONIN CoZrNi
(a)
(b)
Figure 2. Amorphous intulay~ behveeuthe mk&a& imag with objective aperture at k = 0,272 nm”.
at shia 23; axial compresion.
(a) Bright-field and @) cmreqmdinS
dark-field
renections hm the B2 phase as well as that of the basic ones decreases with increasing strain. Fig. 3 shows a strain dependence of long-range order parameter S determined as iu [lo] by relation of the intensities of (100) superstruc~al and (200) basic reflections hm B2 phase in alloy A2,
0.80 Q 0
s
0.60
rotation
angle,
T/m
Figure 3. Variation of long-range order parameter. S, with the rotation angle, T.
854
Vol. 33, No. 6
AMORPHEATION IN CoZrNi
As can be seen, S decreases with r during shear deformation under pressure and reaches a magnitude lower than 0.6 at t = 2*lO’deg., i.e. before the amorphization origin. The above facts obtained by TEM show that at E > 1 the microbands become the basic variable element of defect structure. The thickness of microbands decreases simuhaneously with their density increasing. At E = 3.8, the average thickness of microbands was 20 mn. At E > 3 the volume amorphous microchannels could be detected on the background of the above mentioned stmctum as elongated areas with homogeneous gray contrast. The thickness of amorphous microchannels is equal to the microbands’ size or exceeds the latter pig. 41. The invariable bright-field contrast of gray areas obtained by specimen rotation up to large angles and characteristic amorphous dark-field contrast observed using the first halo are evidence for the amorphous state of these areas. Two types of the amorphous microchannels were revealed. One type represents the channels situated along microbands. The other one crosses them without a siguificautchange in the propagation direction. The amorphous microchannels nonparallel to microbands are incliued by 30” to 45 Qwith respect to the specimen surface. In the investigated alloys the volume amorphous microcharmels were observed for an average dislocation density which is not suthcient for crystal-to-amorphoustransformation according to the energy estimation. The energy contribution from chemical disordering at S = 0.6 may be approximately estimated as 7 kJ/mol using the data of [ 111.This value is compamble with the crysta&ation heat of amorphous CoZr. (The crystallization heat of 8 kJ/mol was obtained for amorphous Al alloy in our calorimetry measurements) Thus, by cold deformatin of CoZr the chemical disordering can determine significantly a necessary energy increase of alloy However, it may be noted that the energy criterion of amorphization will change under applied stress, i.e. the necessary increase of internal energy due to structure defects or chemical disordering may be lowered. The microstructure of alloys near the amorphous microchannels is not nanocrystalline, as it would be expected in the case of continuous refinement of crystalline structure below some critical value. Occasionally, the latter mechanism of amorphization is proposed [6]. The peculiarities of microstructure when the amorphization appears, the orientation of some amorphous micmchannels with respect to the compression direction and especially their ability to cross the microbands may suggest that the amorphous microchatmels can arise due to a local structural instability. The propagation of this instability results in the volume amorphous microchanneling. Thus, the present observations show that two ways of DIA development are possible: 1. Through the deformation processes at the microbands’ boundaries and then at amorphous-crystalline boundaries. Their evolution leads to the amorphous microchannels formation along microbauds. 2. Through the structural instability which extends without a noticeable crystallographic relation to the deformation structure of the material.
(4
@I
Figm 4. Bright-field image of amorphous rm‘crochmels at strain 3.8: (a) lying along and (b) crossing the microbands. The specimen sectionisparalleltothecompm3si~axis.
Vol. 33, No. 6
AMORPHIZATIONIN CoZrNi
855
Both the chemical disordering and the evolving defect structure play an important role in DIA, preparing the conditions (or a stress concentration or a critical volume dilation of crystal material) for the structure instability which is realized in some regions of material at corresponding internal stress. Acknowledrment This work was supportedby the Russian Foundation of Fundamental Investigations, grant No. 94-02-06007-a. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
V.B. Fedorov, V.G. Kurdjumov et al., Dokl. Akad. Nauk SSSR, 269,885 (1983). Ye.V. Taiyanh, V.G. Kurdjumov and V.B. Fedorov, Fiz. Metal. Metalloved, 62,133 (1986). Ye.V. Tatyanin and V.G. Kurdjumov, Phys. Stat. Sol. (a), 121,455 (1990). J. Koike, D.M. Parkin and M. Nastasi, J. Mater. Res., $1414 (1990). J. Koike, D.M. Parlcinand M. Nastasi, Phil. Ma& L&t., 62,257 (1990). J.S.C. Jang and C.C. Koch, J. Mater. Res., 5,498 (1990). E. Ga&tandM. Hamwilia,J. LcsC!ommonMet_, 157,201(1990). D. Kuhhatm-Wilsdorfaud MS. Bednar, ScriFtaMetall. Mater., 28,371(1993). VA Pavlov, Fii Metal. Metalloved., 67,924 (1989). D.E. Mikkola and J.B. Cohen, Acta Met., 14,105 (1966). D.L.Be.ke,H.BakkerandP.I.I_c&ActaM&,39, 1267(1991).
Pergamon
0956-716x(95)00259-6
PECULIARITIES OF DEFORMATION-INDUCED AMORPHIZATION IN CoZrNi ALLOYS E.E. Novikova, Ye. V. Tatyanin and V.G. Kurdjumov Institute of High Pressure Physics, Russian Academy of Sciences, Troitsk - 142092, Russia (Received July 19,1994) (Revised February 1,1995)
In previous studies it was shown that an amorphous state in the intermetalhc compound NiTi can be produced by shear under pressure, hydroextrusion or cold-rolling [l-5]. A volume fraction of amorphous phase was demonstrated to be dependent on deformation value. The amorphous phase nucleation was observed at the twin boundaries in NiTi. Then it was shown that further growth of the amorphous component takes place in this alloy simultaneously with the fragmentation process at which the fragmentation size decreases with increasing strain. Similar structural changes were revealed for amorphization in the intermetallic compound Ni 3 Al [6] and in Si [7] by mechanical milling. The latter permitted one to assume that the mechanism of defbrmation-induced amorphization (DIA) is co~ected with the fragmentation up to the critical size. Other approaches for the explanation of deformation-induced amorphization were proposed in [5,8,9]. It was &resting to examine as far as the observable structural changes are characteristic for DIA in other materials. This paper is basically concerned with the initial stages of DIA in CoZrNi alloys. ExDerimentd
Deformatian-induced amorphization was investigated in alloys Co,&.,,Ni,, (Al) and Co&&& (A2) subjected to shear under pressure (SUP) in a Bridgman anvil (shear angle z = 45 - 7* 103deg., P = 4 GPa) or to axial compression (AC) using the supporting steel rings (true strain E up to 3.8; where E = m&,/h), h, and h-heights of qxcimen before and a&r the deformation). Ni was supplemented in order to increase the plasticity of alloy.Befare the deformation both the alloys consisted of two phases: ordered B2 intermetallic CoZr (long-range order parameter S = 0.98) and C 15 intermetalhc Co&. The particles of Co&r with a diameter of l-2 l.tmwere situated on CoZr grain boundaries. The volume traction of Co& was equal to nearly 50% in alloy Al and 70% in ahoy A2. The snuctum evohrtion during defbrmation was studied by TEM (JEM- 1OOC,1OOkV)and X-ray difhaction. The determination of long-range order parameter was performed in SUP-specimens by using the tnmsmissiongeomenyofx-raydi&tction measmements due to the specimen texture which was formed after AC-deformation. The objects for TRM investigation were prepared from the slices cut parallel and perpendicular to the detbrmation axis, thinned mechanically and then electrolytically. The electropolishing of slices was performed with the ele&olyte of 88 vol.% glacial acetic acid and 12 vol.% perchloric acid.
851
AMORPHIZATIONlN CoZrNi
852
Vol. 33, No. 6
Results
The transition from crystalline to the amorphous state was observed in CoZr phase at first by shear to z = 3* IO&g. under pressme. The amotphous phase was distinctly revealed by EM microdi&action iiom heavily deformed SUP specimens [Fig. 11. However, its volume tmction was very low in the case of AC even at E > 3. The volume fraction of the amorphous phase was higher for Al ahoy at the same deformation. The changes of the structure before the amorphization and the primary stage of amorphization were investigated mainly in specimens subjected to AC-deformation. At the initial stage of deformation, glide dislocations with Burgers vector a-400>, laying along { 1101 planes, were observed in Cozr phase. Furthermore, a local martensitic transformation B2 + B 19 was detected in alloy Al which is likely as an additional mechanism of deformation. The dislocation density in CoZr phase incmaxs rapidly to 10” cme2at E > 0.5 as follows horn the determination of the mean distance between dislocations. A very homogeneous dislocation structure was typical without any remarkable tendency to a cell structure formation. According to TEM data the mean distance between the matrix dislocations changes insigniscantly with increasing strain at least up to 2. At AC strain E = 0.36, on the background of randomly distributed dislocations there were detected the bands parallel to { 110) and consisted of microband bundles. The microband boundaries were of low-angle and mixed type. Two systems of intersecting bands were often observed, one being parallel to the specimen surface, i.e. perpendicular to the compression axis. The typical micmmucmm of ahoy at E = 2,3 is shown in Fig. 2. The microbands of the average thickness 7Onm,approximately parallel to the specimen surface were observed in B2-phase at this strain Very distinct boundaries represent a &am&xi&c featum of these microbands. The density of microbands was not the same in diffiznt parts of the specimen. The average size of Co& particles did not change in comparison with the initial stage of the alloy. Thin amorphous interlayers between the microbands were observed by dark field method (Fig. 2). The halo Tom the amorphous phase was not visible on microdigraction patterns at such deformation. To realize dark field observations, the objective aperture was situated at the place where the tirst halo f?om the amorphous CoZr is supposed to be presented, i.e. at the radius k = 0.272 nm-‘. According to TEM-results the B2 crystalline phase remains chemically ordered up to the appearance of the volume amorphous phase. As it was shown by X-ray difbactometry, the intensity of supemtructural
(a)
08
Fii 1. strumae of alloy (Al) subjectedto shear under high prmsure, shear angle 7* 1O’deg.(a) TEM bright-fieldimage, amorphous matrixandcqZrnanocystalls.(b)TypicalelectrondifFractionpattem,sharpacattering~correspondtoCqZrphase.
Vol. 33, No. 6
853
AMORPHIZATIONIN CoZrNi
(a)
(b)
Figure 2. Amorphous intulay~ behveeuthe mk&a& imag with objective aperture at k = 0,272 nm”.
at shia 23; axial compresion.
(a) Bright-field and @) cmreqmdinS
dark-field
renections hm the B2 phase as well as that of the basic ones decreases with increasing strain. Fig. 3 shows a strain dependence of long-range order parameter S determined as iu [lo] by relation of the intensities of (100) superstruc~al and (200) basic reflections hm B2 phase in alloy A2,
0.80 Q 0
s
0.60
rotation
angle,
T/m
Figure 3. Variation of long-range order parameter. S, with the rotation angle, T.
854
Vol. 33, No. 6
AMORPHEATION IN CoZrNi
As can be seen, S decreases with r during shear deformation under pressure and reaches a magnitude lower than 0.6 at t = 2*lO’deg., i.e. before the amorphization origin. The above facts obtained by TEM show that at E > 1 the microbands become the basic variable element of defect structure. The thickness of microbands decreases simuhaneously with their density increasing. At E = 3.8, the average thickness of microbands was 20 mn. At E > 3 the volume amorphous microchannels could be detected on the background of the above mentioned stmctum as elongated areas with homogeneous gray contrast. The thickness of amorphous microchannels is equal to the microbands’ size or exceeds the latter pig. 41. The invariable bright-field contrast of gray areas obtained by specimen rotation up to large angles and characteristic amorphous dark-field contrast observed using the first halo are evidence for the amorphous state of these areas. Two types of the amorphous microchannels were revealed. One type represents the channels situated along microbands. The other one crosses them without a siguificautchange in the propagation direction. The amorphous microchannels nonparallel to microbands are incliued by 30” to 45 Qwith respect to the specimen surface. In the investigated alloys the volume amorphous microcharmels were observed for an average dislocation density which is not suthcient for crystal-to-amorphoustransformation according to the energy estimation. The energy contribution from chemical disordering at S = 0.6 may be approximately estimated as 7 kJ/mol using the data of [ 111.This value is compamble with the crysta&ation heat of amorphous CoZr. (The crystallization heat of 8 kJ/mol was obtained for amorphous Al alloy in our calorimetry measurements) Thus, by cold deformatin of CoZr the chemical disordering can determine significantly a necessary energy increase of alloy However, it may be noted that the energy criterion of amorphization will change under applied stress, i.e. the necessary increase of internal energy due to structure defects or chemical disordering may be lowered. The microstructure of alloys near the amorphous microchannels is not nanocrystalline, as it would be expected in the case of continuous refinement of crystalline structure below some critical value. Occasionally, the latter mechanism of amorphization is proposed [6]. The peculiarities of microstructure when the amorphization appears, the orientation of some amorphous micmchannels with respect to the compression direction and especially their ability to cross the microbands may suggest that the amorphous microchatmels can arise due to a local structural instability. The propagation of this instability results in the volume amorphous microchanneling. Thus, the present observations show that two ways of DIA development are possible: 1. Through the deformation processes at the microbands’ boundaries and then at amorphous-crystalline boundaries. Their evolution leads to the amorphous microchannels formation along microbauds. 2. Through the structural instability which extends without a noticeable crystallographic relation to the deformation structure of the material.
(4
@I
Figm 4. Bright-field image of amorphous rm‘crochmels at strain 3.8: (a) lying along and (b) crossing the microbands. The specimen sectionisparalleltothecompm3si~axis.
Vol. 33, No. 6
AMORPHIZATIONIN CoZrNi
855
Both the chemical disordering and the evolving defect structure play an important role in DIA, preparing the conditions (or a stress concentration or a critical volume dilation of crystal material) for the structure instability which is realized in some regions of material at corresponding internal stress. Acknowledrment This work was supportedby the Russian Foundation of Fundamental Investigations, grant No. 94-02-06007-a. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
V.B. Fedorov, V.G. Kurdjumov et al., Dokl. Akad. Nauk SSSR, 269,885 (1983). Ye.V. Taiyanh, V.G. Kurdjumov and V.B. Fedorov, Fiz. Metal. Metalloved, 62,133 (1986). Ye.V. Tatyanin and V.G. Kurdjumov, Phys. Stat. Sol. (a), 121,455 (1990). J. Koike, D.M. Parkin and M. Nastasi, J. Mater. Res., $1414 (1990). J. Koike, D.M. Parlcinand M. Nastasi, Phil. Ma& L&t., 62,257 (1990). J.S.C. Jang and C.C. Koch, J. Mater. Res., 5,498 (1990). E. Ga&tandM. Hamwilia,J. LcsC!ommonMet_, 157,201(1990). D. Kuhhatm-Wilsdorfaud MS. Bednar, ScriFtaMetall. Mater., 28,371(1993). VA Pavlov, Fii Metal. Metalloved., 67,924 (1989). D.E. Mikkola and J.B. Cohen, Acta Met., 14,105 (1966). D.L.Be.ke,H.BakkerandP.I.I_c&ActaM&,39, 1267(1991).