Temperature dependence of the amorphization of NiTi irradiated with Ni ions
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J. P. Simmons Metallurgical Engineering and Materials Science Department, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
N. H. Packan Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6376 (Received 30 May 1989; accepted 2 February 1990)
NiTi was irradiated with Ni ions at various temperatures in order to study the temperature dependence of the irradiation-induced crystalline-to-amorphous transition. The irradiations were conducted above the Af temperature, and thus the specimens contained only the ordered B2 (CsCl) phase. The irradiations to similar doses at 150, 200, and 250 °C showed that the amorphization kinetics slow down appreciably as the temperature is increased in this range. No amorphization was detected at irradiation temperatures of 350 °C or higher, even after doses of 4 dpa. The small volume fraction of amorphous material observed after irradiation to 0.67 dpa at 250 °C indicates that the cutoff temperature for amorphization is in the vicinity of this temperature. The amorphous regions of partly amorphous samples are distributed in a nonuniform manner and exhibit a morphology similar to the martensitic microstructure that existed in the specimens before heating to the irradiation temperature. Large amorphous regions in these samples exhibit some very fine crystalline debris which tends to disappear with increasing irradiation dose. Post-irradiation annealing experiments indicated that no thermally activated crystallization occurred during irradiation at temperatures up to 250 °C. I. INTRODUCTION
Intermetallic Ni-Ti alloys of near equiatomic composition are well known for their shape memory property which depends on the occurrence of a thermoelastic martensitic transformation. This property has motivated intensive research on the phase transformations and phase stability of these alloys. The high temperature phase is an ordered compound with a B2 (CsCl) structure up to the melting point.1 Upon cooling, the B2 structure transforms martensitically into an orthorhombic phase with a slight monoclinic distortion, B19'.2'3 The temperature (Ms) at which the martensitic transformation begins on cooling appears to be strongly dependent upon composition for nickel-rich alloys,4"7 decreasing rapidly as the titanium content decreases. Ms is sensitive not only to composition4"6 but also to impurity content8 and thermal history.910 In equiatomic and in nickel-rich alloys a phase of rhombohedral symmetry, known as the R-phase, can form on cooling.711"13 The type of phase transformation that occurs appears to be strongly dependent upon the cooling rate.14 The coupling between cooling rate and composition is also well demonstrated in rapid solidification experiments. Buschow15 obtained amorphous alloys of the Ni-Ti system in the range 24 to 40 at. % Ni and 58 to 932
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64 at. % Ni by melt spinning, while alloys with nickel content between 40 and 58 at. % solidified in the crystalline state. Buschow studied the crystallization behavior of
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