In situ evaluation of the transformation behaviour of NiTi-based high temperature shape memory alloys

Abstract

Fine grained polycrystalline NiTi shape memory alloys containing 15 at.% Hf and Zr and zero or 3 at.% Cu fabricated by ingot metallurgy were investigated using in situ synchrotron X-ray diffraction in order to examine the viability of producing stable and affordable high temperature shape memory alloys. The alloys produced had a high thermal hysteresis, in excess of 70 • C but A f temperatures of over 250 • C were obtained for Ni 50 Ti 35 Hf 15. 3 at.% Cu additions did not significantly reduce the per-cycle degradation of transformation temperatures but did reduce the transformation temperatures. The evolution of the lattice parameters during the first five thermal cycles was observed. Negative thermal expansion was found in the b B19 cell direction in all the alloys examined and significant deviations in the lattice parameters in the region of transformation were found. A per-cycle evolution in the end-point B19 lattice parameters was observed, but no such evolution was found for the B2 phase, which is rationalised by appealing to the increase in population of interface dislocations.

Lattice instability during the martensitic transformation in the high temperature shape memory alloy Zr(Cu0.5Co0.25Ni0.25)

“…The peak transformation temperatures, A p and M p corresponding to 50% martensite on heating and cooling are observed to be 498 and 337°C respectively. Therefore the peak transformation hysteresis (A p -M p ) in the current alloy is 161°C, about 3.5 to 6Â larger than in NiTi [3] and %2.7Â larger than in the Ni 50 Ti 35 Hf 15 [27][28][29]. The forward B19 0 to B2 transformation occurs over a wider temperature range than the reverse transformation during cooling.…”

“…This presence of retained austenite in between growing martensite variant groups is expected to be due to the stress stabilisation of the austenite by the transformation strains [2]. In the current alloy the amount of retained martensite is slightly higher than that observed in Zr-and Hf-containing NiTi based HTSMAs [27,28]. Although the martensite finish (M f ) temperature in the current alloy is very high, the overall transformation occurs over a wider temperature range, Fig.…”

“…This is probably best understood 1%) on cooling at the end of the first thermal cycle, but subsequently stabilises. Such per-cycle evolution was observed to be much more prominent in NiTi based HTSMAs [27,28]. It is instructive to compare the B2 and B19 0 lattice parameters.…”

Recent Development of TiNi‐Based Shape Memory Alloys with High Cycle Stability and High Transformation Temperature

“…The well-accepted product is B19 0 martensite, which has a monoclinic structure and space group of P2 1 /m. [63,64] For the monoclinic crystal structure, a is the shortest axis, b is the intermediate axis, and c is the longest axis. The monoclinic angle β is the angle between the a and c axes.…”

“…Upon cooling, the parent phase may change into various kinds of martensite, depending on the composition and heat treatment. The well‐accepted product is B19′ martensite, which has a monoclinic structure and space group of P2 1 /m . For the monoclinic crystal structure, a is the shortest axis, b is the intermediate axis, and c is the longest axis.…”