The effect of inhomogeneously distributed β-Nb on confined martensitic transformation

The confined martensitic transformation mechanisms in spatial constraint strategies can provide different combinations of mechanical properties and unprecedented functional properties. In this study, we construct TiNiNb alloys with different β-Nb phase morphologies and distributions in the cross-section and longitudinal section by rolling deformation and analyze the confined martensitic transformation behavior and mechanical properties. It is shown that the inhomogeneous distribution of β-Nb dispersoids produces heterogeneous microstructures, where the NiTi matrix inside the dense β-Nb exhibits nanometer-sized grains, while the matrix around the dispersed β-Nb has micrometer-sized grains. This inhomogeneous distribution characteristic mitigates the obstructive effect of β-Nb on the thermally induced martensitic transformation during the cooling process, causing an increase in martensitic transformation temperature. Similarly, the restraining effect of β-Nb on the stress-induced martensitic transformation during deformation is affected, leading to a decline in the critical stress, which has the possibility to raise the upper limit of the superelastic temperature range. Furthermore, the combined role of β-Nb with dislocation interactions and grain size effects provides a degree of synergistic strength-ductility enhancement. This study offers an available avenue for the development of controllable TiNiNb alloys by modulating the microzone with β-Nb.