Pre-martensitic phenomena of thermoelastic martensitic transformation of NiTiCu alloys studied with positron annihilation lifetime spectroscopy

Abstract

Many anomalous phenomena have been observed in NiTi alloys above martensite start temperature, such as softening of the shear constants, increase of internal friction and diffuse scattering of TEM. However, little information has been obtained about the change in the electron system of the parent phase prior to the martensitic transformation, which should be the origin of all pre-martensitic phenomena. In this work, the temperature change of positron annihilation lifetime, which is quite sensitive to electronic-structural changes of matter, were carried out for NiTiCu alloys. The alloys show martensitic transformation from a B2 (cubic) phase to a B19 0 (monoclinic) phase via a B19 (orthorhombic) phase as the temperature is lowered. We have found anomalous positron lifetime changes in Ni 10 Ti 50 Cu 40 and Ni 15 Ti 50 Cu 35 alloys which show a B2-B19-B19 0 phase transformation. Positron lifetime increases anomalously with decreasing temperature at temperatures higher than the transformation temperature into a B19 0 phase. On the other hand, positron lifetime does not show any anomaly in the B2 phase of Ni 30 Ti 50 Cu 20 alloy which shows B2-B19 phase transformation. The positron lifetime of the parent B2 phase shows good agreement with the theoretically-calculated value. Those of the martensite phases of B19 and B19 0 , although, are about 30 ps longer than calculated ones. This big difference between experimental and calculated positron lifetimes cannot be explained by any existing theory.

Untitled

“…Vacuum induction melting (VIM) with graphitic crucible is used extensively [28]. The alloy fabricated by VIM need to homogenize for 1 or 2 h of homogenization heat treatment at 850-1100 °C after producing NiTi shape memory [11,12,[29][30][31].…”

Caracterização da transformação martensítica em temperaturas criogênicas.

“…Existem vários métodos descritos na literatura para o calculo e a determinação da resistividade elétrica com corrente contínua (direct current DC) ou corrente alternada (alternating current AC) o método mais usado é o método de quatro pontas para a determinação da resistividade elétrica de condutores metálicos e semicondutores, devido a sua aplicabilidade é muito utilizado para acompanhar a transformação martensítica entre eles: ligas Fe-Ni-C (HSU; JIAN; ZHENPENG, 1986), Fe-Ni-B (GUO et al, 1988), ligas de Fe-Ni (KAUFMAN; COHEN, 1958), Fe-Mn-Cr (TROIANI et al, 2009), etc. Na transformação martensítica se apresentou anomalias na resistividade elétrica como é mostrado por Makita; Kobukata e Nagasawa (1986) em ligas Cu-Al-Zn como é mostrado na Figura 19, também foram mostrados em Co-Ni-Ga (CHEN et al, 2008), ligas de Ni-Ti-Cu(KATSUYAMA et al, 2004), ligas Ti-Ni-C (YIN et al, 2008.…”