Influence of iridium on the martensitic transformation in Ni–Ti shape memory alloys

Abstract

In the binary system Ni–Ir it is known that Ni and Ir form a complete solid solution over the entire composition range. Starting from this, Ni48.5−xTi51.5Irx alloys with x=0, 2.5, 5 and 10 at.% and a Ni49.5Ti48Ir2.5 alloy have been prepared by arc melting in an argon atmosphere in order to investigate the effect of the substitution on the martensitic transformation. The microstructural characterisation of the samples was mainly done by DSC measurements and transmission electron microscopy (TEM). In accordance with the phase diagram, the binary Ni48.5Ti51.5 alloy contains Ti2Ni precipitates and the matrix exhibits a martensitic phase transformation from the B2-phase to the monoclinic B19′-phase. TEM investigations on the x=2.5 and 5 at.% Ir samples show that precipitates of type Ti2(Ni+Ir)1 are present. Upon cooling these samples, a two-step martensitic transformation occurs from the B2-phase to R-phase to B19′-phase. The martensite start temperature of the B19′-phase decreases with increasing Ir content and, in the vicinity of x=10 at.% Ir, the martensitic phase transformation disappears completely. In a Ni49.5Ti48Ir2.5 alloy quenched from T=1000 °C, only the B2-phase is present. Aging this alloy at T=500 °C for t=100 h leads mainly to the growth of (Ni+Ir)4Ti3 precipitates in a B2-type matrix. In addition, a few (Ni+Ir)3Ti2 precipitates can be found.