Effect of Boron Concentration on Martensitic Transformation Temperatures, Stress for Inducing Martensite and Slip Stress of Ti-24 mol%Nb-3 mol%Al Superelastic Alloy

Abstract

Effects of boron (B) addition on martensitic transformation temperature, stress for inducing martensite and slip stress of a Ti-24 mol%Nb-3mol%Al (TiNbAl) superelastic alloy were investigated in a composition range from 0 to 0.10mass%B. It was found that a second phase is formed by the B addition being higher than 0.05 mass%B. This second phase was estimated to be TiB. The averaged grain size of TiNbAl was decreased by the B addition being higher than 0.05 mass%B. This decrease must be explained by the suppression of grain growth by the second phase. The second phase plays a role of pining sites of grain boundary movement during the solution treatment. The martensitic transformation temperatures (M s ) measured by differential scanning calorimetry were decreased by the B addition. Superelastic behavior was evaluated by a cyclic loading-unloading tensile test at room temperature with a constant strain increment of 1%, and it was found that superelasticity appeared regardless of the amount of B addition. It was also found that stress for inducing martensitic transformation (σ SIM ) increased with increasing B concentration up to 0.05 mass%B. This increase of σ SIΜ by the B addition can be explained by the lowering of M s by the B addition. The critical stress for slip (σ SLIP ) increased with increasing the B concentration being up to 0.05 mass%B. The increase of σ SLIP by B addition was evaluated to be 3GPa/mass%B, and it was significantly higher than that of σ SIM (500MPa/mass%B) when the B concentration was less than 0.05 mass%B. These results indicate that B addition is effective to reduce the permanent unrecoverable strain introduced during deformation.