Effect of B addition on the microstructure and superelastic properties of a Ti-26Nb alloy

The mechanical properties and shape memory behavior of Ti-26Nb-(0-1.0)B and Ti-27Nb alloys were investigated. The stress for inducing martensite and the critical stress for slip deformation were greatly affected by increasing B content up to 0.1 at%, and less affected by further increase in B content up to 0.7 at%. The martensitic transformation start temperature decreased suddenly with a slope of 350 K/at% B when B content was less than 0.1 at% and decreased with a smaller slope of 15 K/at% B with further increase in B content up to 1.0 at%. In all samples tested, only superelasticity was observed at room temperature. TEM investigations revealed that the solubility of B in Ti-26Nb alloy was around 0.1 at%. In spite of the low solubility limit, B was effective to stabilize the superelastic behavior by effectively increasing the critical stress for slip deformation. On the other hand, while the stress for inducing martensite increased effectively by the addition of 1.0 at% Nb to the Ti-26Nb alloy, the critical stress for slip deformation remained almost constant and hence superelasticity was deteriorated.