NiTi and NiTi-TiC composites: Part 1. transformation and thermal cycling behavior
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I.
INTRODUCTION
NEAR-stoichiometric NiTi alloys exhibit a thermoelastic phase transformation between a high-temperature cubic phase 132 (austenite) and a low-temperature phase B19' (martensite).t~] This transformation is responsible for the shape-memory and pseudoelastic effects in deformed NiTi alloys. The martensitic transformation in NiTi is made more complex by the appearance, after thermomechanical treatment, of an intermediate rhombohedral phase (R phase). The transformations sequence is then B2 --->R --->B 19' during cooling. During heating, either the reverse sequence B19' --->R ---> B2 [2,3] or a direct transformation B19' --->B2 is obtained, depending on the thermal treatment.t4] The transformation to the R phase is reported after cold work [5,6] or annealing between 400 ~ and 700 ~ of nickel-rich NiTi.tT,8] Since the transformation in NiTi shape-memory alloy is thermoelastic, stress has a significant influence on the transformation thermodynamics and kinetics. For thermoelastic transformations, the phase volume fraction at a given temperature depends on the balance between the chemical driving force and the elastic strain energy.tg,~0,H] While the effect of externally applied stresses on the transformation has been investigated,t2,~21the effect of internal stresses is not well understood, due to the difficulty of separating the mechanical effects, affecting the strain energy, from the chemical and microstructural effects, affecting the chemical driving force. Sources of internal stress in NiTi are multiple: mismatch between grains, dislocations, second phases, and precipitates. Furthermore, growing martensite plates induce internal stresses due to the geometric misfit among different variants: the equilibrium triangular arrangement of variants often observed in NiTi is a direct consequence of
D. MAR1, formerly Postdoctoral Fellow, Department of Materials Science and Engineering, Massachusetts Institute of Technology, is now C.E.O. at Advanced Composite and Microwave Engineering, CH-1015 Lausanne, Switzerland. D.C. DUNAND, AMAX Assistant Professor, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted January 8, 1994. METALLURGICALAND MATERIALSTRANSACTIONS A
the minimization of that strain energy.[~3] Cold work and thermal cycling have similar effects on the transformation temperature of NiTi: an increase of the R-phase transformation temperature and a decrease of the martensite start temperature (Ms). It is generally admitted that such effects are related to an increase of dislocation density. Dislocations have been observed as nucleation sites for the R phase t14]but clearly depress the Ms temperature,[5,6,~5,16Jlimiting the propagation of martensite plates. BT] The effect of particles and precipitates is of particular interest. Extensive studiest3,v,s.~8]have been devoted to the effect of nickel-rich precipitates (Ni4Ti3 and Nil4Ti.) formed after annealing at 400 ~ to 500 ~ of NiTi alloys with nickel content hig
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