EFFECTS OF NB ADDITION ON THE MICROSTRUCTURE OF TI-NI ALLOYS

The microstructure and the M(s) temperature were systematically investigated for three types of Ti-Ni-Nb shape memory alloys, Ti50-x/2Ni50-x/2Nbx, Ti50Ni50-xNbx and Ti50-xNi50Nbx covering a wide composition range, by utilizing optical microscopy, SEM, EPMA, X-ray diffraction and DSC. Consequently, the following results were obtained. Addition of Nb promotes phase separation. In Ti50-x/2Ni50-x/2Nbx and Ti50Ni50-xNbx alloys, the microstructure changes with increasing Nb addition in the following way: B2 phase --> primary B2 phase + eutectic --> eutectic --> primary bcc phase + eutectic. A pseudo-binary "TiNi-Nb" phase diagram was proposed to explain the above behavior. In Ti50-xNi50Nbx alloys, the microstructure was the primary B2 phase only within the observed composition range, but slight micro-segregation of Nb was observed. The M(s) temperatures of Ti-Ni-Nb ternary alloys were measured for the three types of alloys as a function of Nb addition, and the result was rationalized, based on the observations that the M(s) temperatures in the alloys are mainly dependent on the Ni/Ti ratio in the B2 phase.