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X-ray evidence for stacking faults in the d.h.c.p. phase of the nickel-titanium system
~ot~~~zalof the Less-Commota dlrtals Elscx~ier Sequoia S..\., I,ausanne
Short
Communication
X-ray
evidence
for stacking
nickel-titanium
Although
203
I’rintctl in The Netherlantls
faults
in the d.h.c.p.
phase of the
system
many face-centred
cubic (f.c.c.)
and hexagonal
close-packed
(h.c.p.)
metals and alloys have been subjected in the last two decades to X-ray and electronmicroscopic examination for the incidence of stacking faults on plastic deformation, the structurally
similar, double hexagonal close-packed (d.h.c.p.) metallic phases with the ABACABAC-stacking sequence or the DO, 04 structure have, so far, received very little attention in this respect. Among metals, s-lanthanum, /!I-cerium, a-praseodymium,
and a-neodymium
are known to exhibit
the DO24 structure,
but they are
not particularly reactivity. broadening
suited for study in the form of powder or filings because of their high X-ray lineHARRIS AND RAYNOR 1 have, however, noted anomalous effects,
suggestive
of faulting,
in the basal
planes
in deformed
a-neo-
dymium as well as in annealed ,%praseodymium. Among the many alloy phases known to crystallize in the d.h.c.p. structure, a few are not only quite stable and convenient to work with, but also have axial ratios close to the ideal value (3.267), a factor that may be considered as conducive to easy faulting on cold work. We report here the results of an X-ray line-breadth analysis carried oul on one such d.h.c.p. alloy phase, 7tiz., NizTi (a=2.55o5 Li; c=8.3067 11; c/n=3.257), in both the cold-worked and annealed conditions. As a four-layer
structure,
any d.h.c.p.
metal
or alloy nzay be expected
to
display greater complexity in faulting than the two-layer h.c.p. or the three-layer f.c.c. phases. LELE, PRASA~I AXI) RAO” have shown that seven different types of stacking faults,
six of them intrinsic
and the other extrinsic,,
are possible in d.h.c.p.
crystals.
Among them, the so-called intrinsic ch faults, and the intrinsic c as well as the intrinsic h faults, correspond to deformation and growth faults, respectively, as per earlier notations for f.c.c. and h.c.p. structures, and are perhaps the more likely to form on plastic deformation and phase transformation, respectively. The X-ray diffraction effects of intrinsic ch or deformation faults in d.h.c.p. crystals have been worked out by LELE, PRASAU ASD ASASTHARAMAN~, while similar computations for the intrinsic c and the intrinsic h (growth) faults have been made by LELE ASD PRXSAD’. The d.h. c.p. X-ray reflections (hkil) with (h-k) divisible by three are not affected by stacking faults of either type, exactly as in case of the f.c.c. and the h.c.p. structures. The other reflections broaden to different extents as in the cases of the other two closepacked structures, but the dependence of the fault broadening on I is different for the * On leave from Banaras Hindu University, Varanasi, Intlia. ** On leave from the Indian Tnstitute of Technology, Madras, Intlia.
SHORT COMMUNICATIONS
264
d.h.c.p. structure. As a first approximation, and for fault densities lower than 0.2 (i.e., not more than one fault on the average for every five basal planes), the integral breadth (/I*) of the broadening in the 1 direction in reciprocal space for d.h.c.p. reflections may be simply expressed in terms of the deformation fault density (ad) or the growth fault density (LYE) as 3&i for all values of 1 or, 3aB, BLX~,and Q for 1 equal to 4~2, (492) I), and (4nf2), respectively. /Y* may be computed additively when both types of stacking faults are present. In the case of both cold-worked and annealed d.h.c.p. phases, the fault-unaffected reflections (e.g., 0004, 1120, 1124 and 0008) may be used, to arrive at the density of faulting, as standard reflections to correct for instrumental and other effects in an X-ray line-breadth analysis. In the present work, the alloy NisTi was prepared by melting the two metals (each of >gg.q% purity) in the required proportion in an argon arc melting unit. After homogenization at 1200°C, the alloy was filed and sieved to < 300 mesh. The resulting fine powder was examined immediately after filing, after stabilization at 2o°C, and after annealing at 800°C for 2 h. The X-ray examination was conducted in a Siemens X-ray diffractometer using filtered CuKor radiation as well as in a NoniusGuinier camera that employs X-radiation from a bent-crystal monochromator. In the latter case, the Debye-Scherrer patterns were photometered in a Joyce microdensitometer. The X-ray line broadening was found to be very large in NiaTi filings and only the low-angle reflections, viz., IOIO, rofr, 0004, and 1oY2, were considered reliable for quantitative analysis, even though these also suffered to some extent from mutual overlap (Fig. I). Apart from the usual broadening due to plastic deformation, the reflections with (h-k) not divisible by three displayed additional broadening that was obviously due to appreciable faulting on the basal planes. This fault broadening was found to persist to some extent even after annealing at 800°C for 2 h, as is evident even from a visual inspection, Fig. I, of the annealed 0004 profile in relation to its neighbours. As reflections like IOPO have a special shape due to the abrupt drop
-Annealed -Cold-worked
-
44 2 8 (in degrees)
46
48
5
Fig. 1. Intensity distribution in the first four Debye-Scherrer reflections from NinTi in the annealed and cold-worked conditions. (CuKn radiation; Nonius-Guinier camera.) J. Less-Common
Metals, ao (1970) 263-265
SHORT COMMI_J&‘ICATIONS
liresh filings Filings stabilized ;tt zo”C for I week
Nonius--Guinier camera
Filings stabilized at ZOT for I week
Siemens diffractometer
Filings annealed 8oo”C for 2 h
Xonius--Guinicr camera
at
of intensity at 1=o and, hence, are not suited for X-ray line-breadth analysis, the broadening of only ~oSr and ~oiz was subjected to further detailed study with 0004 as standard to correct for instrumental, micro-strain, and particle size effects, in accordance with the procedure adopted earlier by RAO x?jn AXANTHARAMAN~for cold-worked h.c.p. and f.c.c. phases. The so-called parabolic relation, due to ASAKTHARAMANAK‘T~CHRISTJAS~, was used to arrive at the fault broadening (B) from the observed (R) and standard (b) breadths. The broadening in reciprocal space @*) could be evaluated from p in the usual way 5, taking into account deviation angle (ze), the X-ray wave length (a), the lattice constant (c) and the I value for each reflection. Table I summarizes the results of the X-ray line-broadening analysis on cnldworked and annealed NisTi. The stacking fault densities ,Q and &d in the last two columns have been arrived at on the assumption that aEl the observed fault broadening is caused e~clzksivel~ by growth faults 01 deformation faults, respectively. A careful examination of the computed fault densities leads to the conclusion that this d.h.c.p. alloy phase develops numerous stacking faults, $vrdorninantly of the deformation type (&d?o.o37), on filing at room temperature. These faults are quite stable at room temperature, but start disappearing on heating to relatively high temperatures. After an anneal for z h at Boo%, about one fifth of the faults appear to persist, even though all normal cold-work broadening seems to have disappeared. The agreement between the ad values for filed Ni,Ti in the last column of Table I is considered quite satisfactory in view of the large broadening, and appreciable overlap, of the concerned reflections in the cold-worked state, and also in the light of the limitations in the experiment and the assumptions in theory underlying such )i’-ray analysis. The authors are indebted to Professor V. GEROLI) for the experimental facilities and for his continued interest in this work. One of us (K.V.) would like to thank the Humboldt Foundation for the award of a Fellowship. HARRIS AND G. V. RAYNOR, ,J. Less-Commolz ,ZfPtaZs.zr (IC#O) 286. LELE, R. PRASAD AND P. Ii. RAO, Matcv.Sci. Elzg.,J (1969) 262. j S.LELE,I~. PRASAD AND R.T..~NANTHARAMAN, ActnC:?,ysf..:? 25 (1909) 471 4 S. LELH AND B. PRASAD, /DictaCvyst.,in press. j f'.R RAO AND T. R. .~NANTHARAAIAN,~. Metnllk.. 5.8 (1903) 65X. 0 T. R .\NANTHAKAIIAN AND J. 'vv.CHKISTIAX, .gCtraCsysf.,!,(r9=$) 479.
1 I. l<.
2 S.
Short
Communication
X-ray
evidence
for stacking
nickel-titanium
Although
203
I’rintctl in The Netherlantls
faults
in the d.h.c.p.
phase of the
system
many face-centred
cubic (f.c.c.)
and hexagonal
close-packed
(h.c.p.)
metals and alloys have been subjected in the last two decades to X-ray and electronmicroscopic examination for the incidence of stacking faults on plastic deformation, the structurally
similar, double hexagonal close-packed (d.h.c.p.) metallic phases with the ABACABAC-stacking sequence or the DO, 04 structure have, so far, received very little attention in this respect. Among metals, s-lanthanum, /!I-cerium, a-praseodymium,
and a-neodymium
are known to exhibit
the DO24 structure,
but they are
not particularly reactivity. broadening
suited for study in the form of powder or filings because of their high X-ray lineHARRIS AND RAYNOR 1 have, however, noted anomalous effects,
suggestive
of faulting,
in the basal
planes
in deformed
a-neo-
dymium as well as in annealed ,%praseodymium. Among the many alloy phases known to crystallize in the d.h.c.p. structure, a few are not only quite stable and convenient to work with, but also have axial ratios close to the ideal value (3.267), a factor that may be considered as conducive to easy faulting on cold work. We report here the results of an X-ray line-breadth analysis carried oul on one such d.h.c.p. alloy phase, 7tiz., NizTi (a=2.55o5 Li; c=8.3067 11; c/n=3.257), in both the cold-worked and annealed conditions. As a four-layer
structure,
any d.h.c.p.
metal
or alloy nzay be expected
to
display greater complexity in faulting than the two-layer h.c.p. or the three-layer f.c.c. phases. LELE, PRASA~I AXI) RAO” have shown that seven different types of stacking faults,
six of them intrinsic
and the other extrinsic,,
are possible in d.h.c.p.
crystals.
Among them, the so-called intrinsic ch faults, and the intrinsic c as well as the intrinsic h faults, correspond to deformation and growth faults, respectively, as per earlier notations for f.c.c. and h.c.p. structures, and are perhaps the more likely to form on plastic deformation and phase transformation, respectively. The X-ray diffraction effects of intrinsic ch or deformation faults in d.h.c.p. crystals have been worked out by LELE, PRASAU ASD ASASTHARAMAN~, while similar computations for the intrinsic c and the intrinsic h (growth) faults have been made by LELE ASD PRXSAD’. The d.h. c.p. X-ray reflections (hkil) with (h-k) divisible by three are not affected by stacking faults of either type, exactly as in case of the f.c.c. and the h.c.p. structures. The other reflections broaden to different extents as in the cases of the other two closepacked structures, but the dependence of the fault broadening on I is different for the * On leave from Banaras Hindu University, Varanasi, Intlia. ** On leave from the Indian Tnstitute of Technology, Madras, Intlia.
SHORT COMMUNICATIONS
264
d.h.c.p. structure. As a first approximation, and for fault densities lower than 0.2 (i.e., not more than one fault on the average for every five basal planes), the integral breadth (/I*) of the broadening in the 1 direction in reciprocal space for d.h.c.p. reflections may be simply expressed in terms of the deformation fault density (ad) or the growth fault density (LYE) as 3&i for all values of 1 or, 3aB, BLX~,and Q for 1 equal to 4~2, (492) I), and (4nf2), respectively. /Y* may be computed additively when both types of stacking faults are present. In the case of both cold-worked and annealed d.h.c.p. phases, the fault-unaffected reflections (e.g., 0004, 1120, 1124 and 0008) may be used, to arrive at the density of faulting, as standard reflections to correct for instrumental and other effects in an X-ray line-breadth analysis. In the present work, the alloy NisTi was prepared by melting the two metals (each of >gg.q% purity) in the required proportion in an argon arc melting unit. After homogenization at 1200°C, the alloy was filed and sieved to < 300 mesh. The resulting fine powder was examined immediately after filing, after stabilization at 2o°C, and after annealing at 800°C for 2 h. The X-ray examination was conducted in a Siemens X-ray diffractometer using filtered CuKor radiation as well as in a NoniusGuinier camera that employs X-radiation from a bent-crystal monochromator. In the latter case, the Debye-Scherrer patterns were photometered in a Joyce microdensitometer. The X-ray line broadening was found to be very large in NiaTi filings and only the low-angle reflections, viz., IOIO, rofr, 0004, and 1oY2, were considered reliable for quantitative analysis, even though these also suffered to some extent from mutual overlap (Fig. I). Apart from the usual broadening due to plastic deformation, the reflections with (h-k) not divisible by three displayed additional broadening that was obviously due to appreciable faulting on the basal planes. This fault broadening was found to persist to some extent even after annealing at 800°C for 2 h, as is evident even from a visual inspection, Fig. I, of the annealed 0004 profile in relation to its neighbours. As reflections like IOPO have a special shape due to the abrupt drop
-Annealed -Cold-worked
-
44 2 8 (in degrees)
46
48
5
Fig. 1. Intensity distribution in the first four Debye-Scherrer reflections from NinTi in the annealed and cold-worked conditions. (CuKn radiation; Nonius-Guinier camera.) J. Less-Common
Metals, ao (1970) 263-265
SHORT COMMI_J&‘ICATIONS
liresh filings Filings stabilized ;tt zo”C for I week
Nonius--Guinier camera
Filings stabilized at ZOT for I week
Siemens diffractometer
Filings annealed 8oo”C for 2 h
Xonius--Guinicr camera
at
of intensity at 1=o and, hence, are not suited for X-ray line-breadth analysis, the broadening of only ~oSr and ~oiz was subjected to further detailed study with 0004 as standard to correct for instrumental, micro-strain, and particle size effects, in accordance with the procedure adopted earlier by RAO x?jn AXANTHARAMAN~for cold-worked h.c.p. and f.c.c. phases. The so-called parabolic relation, due to ASAKTHARAMANAK‘T~CHRISTJAS~, was used to arrive at the fault broadening (B) from the observed (R) and standard (b) breadths. The broadening in reciprocal space @*) could be evaluated from p in the usual way 5, taking into account deviation angle (ze), the X-ray wave length (a), the lattice constant (c) and the I value for each reflection. Table I summarizes the results of the X-ray line-broadening analysis on cnldworked and annealed NisTi. The stacking fault densities ,Q and &d in the last two columns have been arrived at on the assumption that aEl the observed fault broadening is caused e~clzksivel~ by growth faults 01 deformation faults, respectively. A careful examination of the computed fault densities leads to the conclusion that this d.h.c.p. alloy phase develops numerous stacking faults, $vrdorninantly of the deformation type (&d?o.o37), on filing at room temperature. These faults are quite stable at room temperature, but start disappearing on heating to relatively high temperatures. After an anneal for z h at Boo%, about one fifth of the faults appear to persist, even though all normal cold-work broadening seems to have disappeared. The agreement between the ad values for filed Ni,Ti in the last column of Table I is considered quite satisfactory in view of the large broadening, and appreciable overlap, of the concerned reflections in the cold-worked state, and also in the light of the limitations in the experiment and the assumptions in theory underlying such )i’-ray analysis. The authors are indebted to Professor V. GEROLI) for the experimental facilities and for his continued interest in this work. One of us (K.V.) would like to thank the Humboldt Foundation for the award of a Fellowship. HARRIS AND G. V. RAYNOR, ,J. Less-Commolz ,ZfPtaZs.zr (IC#O) 286. LELE, R. PRASAD AND P. Ii. RAO, Matcv.Sci. Elzg.,J (1969) 262. j S.LELE,I~. PRASAD AND R.T..~NANTHARAMAN, ActnC:?,ysf..:? 25 (1909) 471 4 S. LELH AND B. PRASAD, /DictaCvyst.,in press. j f'.R RAO AND T. R. .~NANTHARAAIAN,~. Metnllk.. 5.8 (1903) 65X. 0 T. R .\NANTHAKAIIAN AND J. 'vv.CHKISTIAX, .gCtraCsysf.,!,(r9=$) 479.
1 I. l<.
2 S.