Martensite transformation temperatures and mechanical properties of ternary NiTi alloys with offstoichiometric compositions

Abstract

Effects of ternary additions, Co and Cr, on martensite transformation temperatures and mechanical properties of offstoichiometric NiTi alloys were investigated. Martensite transformation start temperature (Ms) and austenite transformation start temperature (As) were determined by means of differential thermal analysis (DTA) in the temperature range between 77 and 423 K. Mechanical properties of yield stress and work-hardening coefficient were investigated using B2-stabilized NiTi alloys, whose alloy compositions were based on Ni-49 mol% Ti. It has been shown that (1) Ms and As considerably decrease with decreasing Ti concentration on the Ti poor side of stoichiometry (the concentration of Ti is less than 50 mol%); (2) Ms and As change slightly when Ti concentration is more than 52 mol%, where the alloys are in the two phase region; and (3) Ms and As decrease with increasing Cr and Co content with a constant Ti concentration. In the present analysis for evaluating the effect of ternary additions on Ms, we use Ms change by adding 1 mol% of ternary elements. Ms changes by Cr addition are −65 K mol%−1 on the Ti rich side and −46 K mol%−1 on the Ti poor side. Ms changes by Co addition are −15 K mol%−1 on the Ti rich side and −30 K mol%−1 on the Ti poor side. Both Cr and Co atoms are suggested to locate Ni sites preferably if only Ms changes are considered. Characteristic stress-strain curves have indicated that the stress induced martensitic transformation (SIMT) occurred at 77 K. Above room temperature, work hardening coefficient at a permanent strain of 1% was found to be between 2 and 11 GPa. It should be emphasized that yield stress and work hardening coefficient increase with increasing test temperature between room temperature and about 650 K in most alloys. This strength anomaly is not related to SIMT but to precipitation hardening and/or anomalous dislocation motion, probably in a similar manner to B2-type CoTi.