Stress-Induced Shape Changes and Shape Memory in the R and Martensite Transformations in Equiatomic NiTi
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THE work presented here is part of a study on near equiatomic NiTi, undertaken because of the potential biomedical applications of the shape-memory effect exhibited by these alloys. In particular, the work has been directed towards the development of a vena cava filter implant described by Simon) In that application the device is inserted as a straight wire and assumes its functional filter shape when in place. The high temperture phase in near-equiatomic NiTi alloys is considered to be the CsC1 (B2) type, 2,3 and the martensite a monoclinic distortion of the B 19 structure. 4,s There is also a separate phase transformation of the B 2-phase occurring below a critical temperature, TR),6-~ Among many effects, the latter transition is characterized by a rapid increase in electrical resistance, 2,3,6-~2extra diffraction spots at 1/3 positions of the B2 reciprocal lattice 3:2 and a splitting in the {011 } and {211 ) B2 X-ray reflections while the {002} peaks are unchanged. 2,3,7,ml The peak splittings have been interpreted as resulting from a rhombohedral distortion of the B2 unit cell, 2,3,7although Salamon, et aP also suggested it might be due to more complex crystallographic changes. Continuous variation of the new 1/3 diffraction intensities 3:2 and of the peak splitting, TM and specific heat data 3 suggest that this is a higher order transition. Khachin and co-workers called it "a continuous martensitic transformation. ''8,9 This transition is preempted by the martensitic transformation when M, HUNG C. LING is Member of Research Staff, Western Electric EngineeringResearchCenter, P.O. Box 900, Princeton, NJ 08540 a n d ROY KAPLOWis Professor,Department of Materials S c i e n c e a n d Engineering, Massachusetts Institute of Technology,77 Massachusetts Avenue,Cambridge, MA 02139. Manuscript submitted January 21, 1981.
>> T R; application of factors which lower M, relative to T R may be necessary to see both transformations. For example, thermal cycling lowers M, without affecting TR; increasing the Ni content to more than 50 at. pet or substituting Ni by a few pet of Co or Fe lowers both M, and TR,6,~~ but results in a larger temperature interval between T R and M : We refer to the structure below T R as the R-phase, which is a designation also used in the Russian literature, 8-~~and refer to the structure encompassing both B2 and R as the fl phase. Martensite may form from the B2 or the R-phase, depending on the relative temperatures, TR and M r The primary mechanism for the shape memory effect is generally considered to be the interaction of stress with the martensitic transformation) 4-~9Khachin, et al 7 first described a practically hysteresis-free, reversible form change effect connected with the B2 ~ R transition after prior plastic deformation in TiNi4si3 Fe,.7 alloy. Later a shape memory effect was also described ~~ in the same alloy where T R = - 50 ~ and M , = - 196 ~ The various effects were attributed to mechanisms involving R-phase domains. Our previous work on equiatomic NiTi (Ref. 11) showed also that
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