Microstructural heterogeneity and texture of as-received, vacuum arc-cast, extruded, and re-extruded NiTi shape memory alloy

Abstract

As-received and vacuum arc-cast near equiatomic Ti-50.6 at.% Ni billets were extruded and re-extruded at 815 °C at Case Western Reserve University (CWRU) with an extrusion ratio of 4 to 1. The effects of arc-casting, extrusion and re-extrusion on the grain size, volume fraction of recrystallized grains, grain boundary misorientation and texture were analyzed via the high-resolution electron backscatter diffraction (EBSD). The results show that extruding and re-extruding the as-received NiTi shape memory alloy produces a 〈111〉texture//ED and {110}〈uvw〉texture//TD. The size of austenite grains and the volume fraction of recrystallized grains significantly decrease but the fraction of LABs slightly increases and the grain size distribution becomes more homogeneous for the re-extruded NiTi in comparison to the first extrusion. For the vacuum arc-cast extruded NiTi shape memory alloy, the microstructure predominantly consists of elongated austenite grains. There is a strong {001}〈001〉texture//TD at the Butt (i.e. as-cast NiTi). Re-extrusion of the vacuum arc-cast NiTi shape memory alloy produces a material that exhibits two strong textures: (110)[11¯0]//CD(ED) and {001}〈100〉//TD in the Extrudate ①. The re-extruded material exhibits few elongated grains at this location. Instead, the second extrusion of the as-cast material produces fine and equiaxed recrystallized grains.