Influence of thermal annealing on the martensitic transitions in Ni-Ti shape memory alloys

We report mainly on resistance R(T) measurements between 4.2 and 1300 K on NixTi100-x alloys, with 51 < x < 54.5at%, quenched from different annealing temperatures T-A to room temperature. When quenched from the B2-phase stability range (T-A = 1273 K), alloys with 51 < x < 54 show an increase in R(T) with decreasing T below 300 K. Subsequent annealing at T-A = 653 K (1 h) and quenching leads to a reduction of the R(T) anomaly below 320 K and the occurence of a martensitic transition (MT) from the B2- to the R-phase, with T-R = 310 K, rather independent of x. After annealing at 723 K (1 h) and 823 K (1 h), respectively, two-step MTs occur from B2 to R and subsequently to B19', with M-S(B19') depending on x and T-A. After annealing at 923 K or higher T-A, MTs cannot be found anymore, and the R(T) behaviour is similar to that after quenching from 1273 K. Studies of R(T) at high T on samples quenched from 1273 K reveal the occurence of mainly two annealing stages. The first one at around 500 K marks structural changes inducing the martensitic phases at lower temperatures. The second one at about 900 K marks the formation of the B2-phase and the disappearance of other phases triggering the MT. The R(T) results are compared with the thermal expansion a(T) and X-ray investigations. The structural phase diagram of Ni-Ti around NiTi is discussed. (C) 1999 Elsevier Science S.A. All rights reserved.