Role of Annealing in Improving Shape Memory Effect of As-Cast Fe-Mn-Si-Cr-Ni Shape Memory Alloys

Abstract

The impact of annealing temperature and annealing time on the shape memory effect of an as-cast Fe-18.54Mn-5.70Si-8.91Cr-4.45Ni alloy was investigated by optical metallography, XRD, SEM, TEM, and bending techniques. The recovery strain of the as-cast alloy was only 5.6 pct because a high density of stacking faults was not obtained. The shape memory effect of the as-cast alloy was improved by annealing for 30 minutes at 573 K, 873 K, or 1073 K (300 °C, 600 °C, or 800 °C) as well as at 873 K (600 °C) for 5, 30, or 720 minutes, whereas it deteriorated after annealing at 1373 K (1100 °C) for 30 or 750 minutes. The maximum improvement of the shape memory effect was achieved after annealing at 873 K (600 °C) for 30 minutes because dislocations, derived from low-angle grain boundaries, decomposed into stacking faults. It was concluded that the variation of the shape memory effect with different annealing processes is associated with the evolution of dislocations derived from low-angle grain boundaries for cast Fe-Mn-Si-based shape memory alloys.