Shape memory alloys are a unique class of materials that can recover their original shape upon heating after a large deformation. Ti-Ni alloys with a large recovery strain are expensive, while low-cost conventional processed Fe-Mn-Si-based steels suffer from a low recovery strain (o3%). Here we show that the low recovery strain results from interactions between stress-induced martensite and a high density of annealing twin boundaries. Reducing the density of twin boundaries is thus a critical factor for obtaining a large recovery strain in these steels. By significantly suppressing the formation of twin boundaries, we attain a tensile recovery strain of 7.6% in an annealed cast polycrystalline Fe-20.2Mn-5.6Si-8.9Cr-5.0Ni steel (weight%). Further attractiveness of this material lies in its low-cost alloying components and simple synthesis-processing cycle consisting only of casting plus annealing. This enables these steels to be used at a large scale as structural materials with advanced functional properties.
The evolution of α′ martensite with different thermo‐mechanical treatment and its effect on the shape memory effect were studied in an Fe‐14Mn‐5Si‐8Cr‐4Ni alloy. The α′ martensite was introduced by only 5% pre‐deformation, and its amount increased with increasing pre‐deformation up to 20%. The α′ martensite started to transform into austenite when the annealing temperature was 773 K. As the annealing temperature increased to 1 073 K, the α′ martensite almost transformed fully into austenite. The α′ martensite introduced by the thermo‐mechanical treatment could prevent collisions between different ε martensite bands during deformation. The intrusion of α′ martensite was another key reason that the stress‐induced ε martensite bands in Fe‐Mn‐Si based shape memory alloys formed in a domain‐specific manner in addition to that of uniformly distributed stacking faults after thermo‐mechanical treatment.
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