Effect of Second Phase on Martensitic Transformation Temperature in Cu-Al-Ni Shape Memory Alloys

A Cu-14Al-4Ni ternary alloy and three Cu-12Al-5Ni-2Mn-1Ti(mass%) alloys, were quenched from temperatures between 973 K and 1173 K, in order to investigate the effect of quenching temperature on the martensitic transformation temperature of the alloys. Dependence of the martensitic transformation temperature on quenching temperature was observed to exist in the alloys containing Ti, and hence containing a second phase called “x-phase” enriched with Ti and Ni. The martensitic transformation temperatures were lowered by 10-15 K, when the quenching temperatures were increased by 100 K. The phenomenon was explained by the solution of the x-phase particles, especially by partial dissolution of the smallest particles of xS-phase having an average radius of 50 nm.The concentration factor C. F.i(=Cix/Cim) of an alloying element i, where Cix and Cim represent the concentration of element i in xS-phase and in the matrix, respectively, was measured and compared to each other. It was found that the value of C. F.i was almost equal to unity in the case of Al and Mn, whereas it was 7.5 and 239 in the case of Ni and Ti.Transmission electron microscopy revealed that the size of the small xS-particles was reduced, when the specimens were heated at a high temperature such as 1173 K. It is concluded that the observed lowering of the martensitic transformation temperatures is due to redistribution of Ni and Ti atoms from the xS-phase particles to the matrix.