Torsional behavior of Ni-rich NiTi alloys obtained by powder metallurgy and hot deformation

The effects of severe plastic deformation on NiTi alloys' structure and properties have been extensively studied over the past decades. However, there is a notable lack of systematic data regarding the impact of industrial hot deformation techniques on these alloys. This gap arises from challenges in manufacturing processes related to the unevenness of ingots produced by casting technologies. This study investigates the effects of hot rotary swaging, extrusion, and radial shear rolling on the martensitic transformation, shape memory effect, superelasticity, and damping capacity of NiTi Ni-rich alloys fabricated through powder metallurgy. The properties were investigated under torsional load on wires prepared by spark eroding from deformed rods. Our findings indicate that samples after rolling and extrusion exhibit a superelastic strain of 14 ± 0.5% attributed to a high yield stress of approximately 600-800 MPa and torsional testing providing the material to be fully involved in recovery process. Samples after rolling and swaging demonstrate a high level of reversible strain with a one-way shape memory effect ranging from 5 to 7%. Conversely, extrusion, due to the inhomogeneity of resulting workpieces, induces a complex, multi-stage martensitic transformation that undermines the shape memory effect. Furthermore, all deformation methods except extrusion contribute to increased alloy homogeneity, resulting in a narrower temperature range for martensitic transformations. Rotary swaging notably increases the height of an internal friction peak from 0.015 to 0.045 compared to the undeformed material, whereas rolling gives the lowest value of 0.012 among others. This study provides valuable insights into how hot thermomechanical processing influences the properties of NiTi alloys and shows that powder metallurgy combined with hot deformation can be considered an alternative approach for achieving high functional properties of these alloys.