Anomalous Temperature Dependence of Yield Stress and Work Hardening Coefficient of B2-Stabilized NiTi Alloys

Abstract

ABSTRACTYield stress and work hardening coefficient of B2-stabilized NiTi alloys are investigated using compression tests. Compositions of NiTi alloys are based on Ni-49moJ.%Ti, to which Cr, Co and Al are chosen as ternary elements which reduce martensitic transformation temperatures of the B2 phase. Mechanical tests are carried out in liquid nitrogen at 77K, air at room temperature (R.T.) and in an argon atmosphere between 473K and 873K. Only at 77K, some alloys show characteristic stress-strain curves which indicate stress induced martensitic transformation (SIMT), but the others do not. Work hardening coefficient is found to be between 2 and 1 lGPa in all the test temperature range. The values are extremely high compared with Young's modulus of B2 NiTi. Yield stress and work hardening coefficient increase with test temperature between R.T. and about 650K in most alloys. The anomalous temperature dependence of mechanical properties is not related to SIMT but to precipitation hardening and/or anomalous dislocation motion similar to B2-type CoTi. Solution hardening by adding ternary elements is evaluated to be small for Cr and Co additions, and large for Al addition, depending on difference in atomic size of the ternary element with respect to Ni or Ti.