In this study, it is proposed that coarsening austenitic grains is a key criterion for achieving giant recovery strains in polycrystalline Fe-Mn-Si based shape memory alloys. In order to verify the hypothesis, the relationship between recovery strains and austenitic grain-sizes in cast and processed Fe-Mn-Si based shape memory alloys was investigated. The recovery strain of cast Fe-19Mn-5.5Si-9Cr-4.5Ni alloy with the coarse austenitic grains of 652 μm reached 7.7% while the recovery strain of one with the relatively small austenitic grains of 382 μm was only 5.4%. Moreover, a recovery strain of 5.9%, which is the highest previously published value for solution-treated processed Fe-Mn-Si based shape memory alloys, was obtained by coarsening the austenitic grains through only solution treatment at 1483 K for 360 min in a processed Fe-17Mn-5.5Si-9Cr-5.5Ni-0.12C alloy. However, its recovery strain was still 5.9% after thermo-mechanical treatment consisting of 10% tensile strain at room temperature and annealing at 1073 K for 30 min. This happens because annealing twins play a negative role, refining the austenitic grains, limiting the recovery strains to below 6%. In summary, coarse austenitic grains enable the achievement large recovery strains by two mechanisms. Firstly, the grains are bigger, and consequently there are fewer grain boundaries, and thus their suppressive effects of grain boundaries on stress-induced ε martensitic transformation is reduced. Secondly, coarse austenitic grains are advantageous to introduce  martensite with single orientation and reduce the collisions of different martensite colonies, especially when the deformation strain is large. As such, the ceiling of recovery strains is dependent on the austenitic grain-sizes.

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In order to clarify the relationship among grain size, annealing twins and the shape memory effect in Fe-Mn-Si based shape memory alloys, the Fe-21.63Mn-5.60Si-9.32Cr-5.38Ni (weight %) alloy with a grain size ranging from 48.9 μm-253.6 μm was obtained by adjusting the heating temperature or heating time after 20% cold-rolling. The densities of grain boundaries and annealing twins increase with a decrease in grain size, whereas the volume fraction and width of stress-induced ε martensite after 9% deformation at M s + 10 K decrease. This result indicates that grain refinement raises the constraint effects of grain boundaries and annealing twins upon martensitic transformation. In this case, the ability to suppress the plastic deformation and facilitate the stress-induced ε martensite transformation deteriorates after grain refinement owing to the enhancement of the constraint effects. It is demonstrated by the result that the difference at M s + 10 K between the critical stress for plastic yielding and that for inducing martensitic transformation is smaller for the specimen with a grain size of 48.9 μm than for the specimen with a grain size of 253.6 μm. Therefore, the shape memory effect declined by decreasing the grain size.

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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...