Deformability of TiNiHf shape memory alloy under rolling with pulsed current

Abstract

Introduction. The deformation capacity of materials is one of the main mechanical characteristics that determine the possibility of its production using various technological processes for metal forming. Among intermetallic compounds, a special role belongs to alloys with a high-temperature shape memory effect (SME) based on TiNi with the addition hafnium. Most of these alloys are not only difficult to deform, but also quite brittle. Therefore, the development of any technological schemes to increase the deformation capacity of these alloys is relevant. The purpose of the work: to study the deformation capacity and the possibility of using electric pulsed current during cold rolling of the TiNiHf alloy. This processing method has not previously been applied to these alloys. In this work, the deformation capacity during cold rolling of a strip 2 mm thick made of a hard-to-deform high-temperature TiNi-based shape memory alloy with the addition of hafnium is studied. To increase the deformability, an external action in the form of a high-density pulsed current of more than 200 A/mm2 is investigated. The research methods are: X-ray analysis to assess the initial phase state; analysis of the evolution of true and engineering deformation to failure (appearance of visible macrocracks in the deformation zone); optical microscopy with magnification from 50 to 100 and measurement of Vickers hardness at room temperature. Results and discussion. An increase in the deformability under the influence of a pulsed current compared to rolling without current and the achievement of a maximum strain of 1.7 (true) and 85% (engineering) are established. The initial coarse-grained equiaxed martensitic microstructure (50 μm) is transformed into a microstructure elongated along the rolling direction, while the hardness increases by 50%. The absence of noticeable structural changes and the observed hardening may indicate a nonthermal effect of the current in increasing the deformability. Thus, the results of the conducted studies indicate the prospects of the method of rolling with a current of a hard-to-deform TiNiHf shape memory alloy as a method of metal forming.