A Novel Training-Free Processed Fe-Mn-Si-Cr-Ni Shape Memory Alloy Undergoing δ → γ Phase Transformation

Abstract

We not only suppress the formation of twin boundaries but also introduce a high density of stacking faults by taking advantage of d fi c phase transformation in a processed Fe-19.38Mn-5.29Si-8.98Cr-4.83Ni shape memory alloy. As a result, its shape memory effect is remarkably improved after heating at 1533 K (1260°C) (single-phase region of d ferrite) and air cooling due to d fi c phase transformation.Low-cost Fe-Mn-Si-based shape memory alloys (SMAs), possessing one-way shape memory effect, have attracted much attention for several decades as a possible alternative to the expensive Ti-Ni-based SMAs. [1][2][3][4][5][6] A large recovery strain of 9 pct has been reported in a single-crystalline Fe-30Mn-1Si alloy. [1] However, only 2 to 3 pct recovery strain is attained in ordinary polycrystalline-processed Fe-Mn-Si-based alloys without special treatment. [4][5][6] There are three kinds of special treatment-training, [7][8][9] thermomechanical, [5,10,11] and ausforming [12,13] -that improve the recovery strain of polycrystalline-processed Fe-Mn-Si SMAs to 4 to 5 pct. The preceding treatments not only increase the production cost, but also make it difficult to fabricate the components with complicated shape. Recently, we manufactured a novel training-free cast Fe-20.2Mn-5.6Si-8.9Cr-5.0Ni alloy by simple synthesis processing, consisting only of casting plus annealing, and made a breakthrough to attain a tensile recovery strain of 7.6 pct in this cast alloy. [14] Nevertheless, there are still shortcomings for cast alloys as compared with processed alloys, i.e., lower recovery stress and worse mechanical properties. Thus, we raise an issue regarding how to obtain training-free processed Fe-Mn-Si-based SMAs.Recently, we investigated the interaction between twin boundaries and stress-induced e martensite in Fe-MnSi-based SMAs and found that this interaction not only suppresses the stress-induced e martensitic transformation, but also severely distorts the twin interfaces. [14] As a result, a high density of twin boundaries results in a poor shape memory effect in processed Fe-Mn-Si-based SMAs, and a good shape memory effect could be obtained by inhibiting the formation of twin boundaries. Furthermore, Kajiwara indicated that a high density of stacking faults is beneficial to obtaining a good shape memory effect for processed Fe-Mn-Si-based SMAs. [5] Indeed, the previously mentioned special treatments that effectively improve the shape memory effect introduce the high density of stacking faults besides reducing the twin boundaries for processed Fe-Mn-Si-based SMAs. [5,8,[13][14][15] Here, we also achieve the purpose of both reducing the twin boundaries and introducing the high density of stacking faults using d fi c phase transformation in a Fe-19.38Mn-5.29Si-8.98Cr-4.83Ni alloy. Therefore, in the present article, a novel training-free processed Fe-Mn-Si-Cr-Ni alloy is developed based on d fi c phase transformation.The ingot of Fe-19.38Mn-5.29Si-8.98Cr-4.83Ni SMA was prepared by induction melting in an argon atmosphere. Thi...

Key criterion for achieving giant recovery strains in polycrystalline Fe-Mn-Si based shape memory alloys

“…Fourthly, the formation of annealing twins should be suppressed in Fe-Mn-Si based SMAs [34,[68][69].…”