Comparative study on mechanical and shape memory properties of hot forged NiTiNb in axial and radial direction

The NiTiNb shape memory alloys (SMAs) used in practice usually exist in a polycrystalline form with a strong texture. However, the performance differences in forged or rolled rods in the axial and radial directions caused by microstructures such as texture are often ignored. This study compared the mechanical and shape memory properties of the hot-forged NiTiNb alloy in the axial and radial directions. This investigation shows an increase in elongation of tensile specimens in the axial direction when compared with that in the radial direction (the maximum difference is 22.6%). However, the shape memory properties of hot-forged NiTiNb rods in the radial direction are more excellent than those in the axial direction. The types of stress-induced martensite variants are different in the axial and radial directions, but the recoverable transformation strains are the same. The primary reason is that the contribution of {2 0 1‾} compound twins to transformation strain in the radial direction is equivalent to the plastic deformation of the β-Nb phase caused by the increase in flow stress. The maximum recovery stress of the radial sample with pre-strain 10% is 20.7% higher than that of the axial sample.