High-strength superelastic as-cast Ni50.9Ti49.1-TiB2 in-situ composites

Abstract

In this study, as-cast Ni50.9Ti49.1−TiB2 in-situ composites were prepared by adding boron in the vacuum induction melting (VIM) process. With boron content of 1 at.%, TiB2 phases appear in the form of flakes at grain boundaries of the NiTi alloy matrix. These TiB2 flakes are two dimensional in geometry, featured with an ultrafine thickness less than 200 nm and two other dimensions up to ~20–60 μm. Increasing boron content to 2.4 at.% alters the morphology of TiB2 from discrete flakes to agglomerative clusters. This morphological dissimilarity has noticeable impact on resulting mechanical properties, superelasticity and shape memory effect. Overall, the NiTi-1at%B composite outperforms both the monolithic Ni50.9Ti49.1 alloy and the NiTi-2.4 at%B composite. More specifically, it exhibits complete superelasticity for pre-strains below 4% and up to 4.3% recoverable strain for 6% pre-strain, which is comparable to the pure as-cast Ni50.9Ti49.1 alloy. Meanwhile, the corresponding strength under 6% pre-strain reaches 1800 MPa, 20% higher than that of Ni50.9Ti49.1. In compression, the yield strength is 65% higher than that of pure Ni50.9Ti49.1. Moreover, a shape memory recovery ratio of 45.5%, slightly below 48.6% of the monolithic NiTi, was also achieved in NiTi-1at%B. This indicates that the addition of 1 at.% B can effectively improve the strength of NiTi while maintaining its superelasticity and shape memory effect.