Elastic constants of Ti₅₀Ni₃₀Cu₂₀alloy prior to martensitic transformation

Abstract

Elastic constants of Ti 50 Ni 30 Cu 20 alloy prior to the martensitic transformation B2 B19 was measured for the ® rst time by using the rectangular parallelepiped resonance method. The softening in both shear modulus c and c 44 was found with approaching transformation temperature. As a result of soft c 44 , the anisotropy factor shows a low value of 2.8 in the vicinity of the transformation temperature. c 11 , c 12 , and bulk modulus were found to show little temperature dependence in the vicinity of the transformation temperature. Compared with the binary Ti 50 Ni 50 alloy, Ti 50 Ni 30 Cu 20 alloy exhibits a 40% higher and increasing anisotropy with approaching transformation temperature. Based on the experimental results, we explained why Ti 50 Ni 30 Cu 20 transforms into B19 martensite, while Ti 50 Ni 50 transforms into a strange monoclinic B19 martensite. §1. Introduction Despite the fact that Ti± Ni is the most important shape memory alloy for various technological applications (for example, Otsuka and Shimizu (1986)), its martensitic transformation (MT) mechanism, which governs its properties, has remained poorly understood for many years compared with other shape memory alloys. The central problem is how to explain the abnormal structure of TiNi martensite B19 .In contrast to other martensites of b (B2) phase alloys which are invariably formed by di erent ways of basal plane stacking (see Nishiyama (1978) for a review), B19 martensite exhibits a large and strange non-basal distortion ({001} 110 B2 shear) to the well-known basal plane structure B19 (Heheman and Sandrock 1971, Kudoh et al. 1985). This non-basal shear gives a monoclinic distortion to B19 structure and lowers the orthorhombic symmetry of B19 to monoclinic, thus the distorted martensite is named as B19 . The reason for such a non-basal distortion is unclear. On the other hand, the addition of 20 at.% Cu into Ti 50 Ni 50 alloy changes the transformation product into common B19 martensite for Ti 50 Ni 30 Cu 20 alloy (Nam et al. 1990b), although the parent phase for both alloys has the same B2 (CsCl) structure. Accompanying such a change of transformation product, the

Physical metallurgy of Ti–Ni-based shape memory alloys

“…The results are shown in Fig. 38 [170][171][172]72]. We can see a common feature among all Ti-Ni-based alloys: for all the tested alloys, c 44 exhibits significant softening besides a softening in c 0 .…”

“…For first order transformation softening in c 0 is only partial, so the temperature dependence of c 0 is small. On the other hand, elastic constant is strongly dependent on composition in martensitic alloys [170] due to the fact that alloying alters lattice dynamic properties significantly (this can be seen from Fig. 39 where alloying leads to significant change in phonon dispersion curve).…”

“…37(c)). For a long time there had been no explanation for the origin of the non-basal shear in forming the B19 0 structure, but very recently important results from elastic constant measurement enabled a simple understanding of the monoclinicity of the B19 0 martensite [169][170][171][172]72]. The details will be given in the following section.…”

“…Elastic constant measurements [166][167][168][169][170][171][172]72] always show a softening of elastic constant c 0 prior to martensitic transformations; Neutron scattering experiment in most cases [173][174][175][176][177][178] confirm a low-lying TA 2 acoustic phonon which further softens with approaching transformation temperature. Elastic neutron scattering in most cases reveals a central peak [174,175], and X-ray diffraction reveals diffuse scattering and extra diffuse spots [179,104,97,107].…”

Ultrasonic investigation of the B2 ↔ B19 martensitic transition in a Ni40Ti50Cu10 alloy

“…In a series of recent ultrasonic investigations carried out in single crystals of the alloys Ni 30 Ti 50 Cu 20 [3,4], Ni 40 Ti 50 Cu 10 [5], Ni 48 Ti 50 Fe 2 [6] and Ni 50. 8 Ti 49.2 [7], Otsuka and co-workers have accounted for the main transformation features of Ni-Ti(Me) (Me = Cu or Fe) alloys in terms of softening processes occurring in these systems on both the elastic constants C 44 and C .…”