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

Abstract

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.

3D and 4D Printing of Complex Structures of FeMnSi‐Based Shape Memory Alloy Using Laser Powder Bed Fusion

“…Fe-SMAs have attracted a considerable attention as a cost-effective alternative to the NiTi alloys over the past two decades [11][12][13]23] Fe-SMAs exhibit high stiffness and yield strength and high recovery stress [24] and induce a strong SME. [25] It has been known that the SME of the Fe-SMAs can be further enhanced when fine precipitations such as NbC or VC are distributed in the microstructures. [12,13] Very recently, we successfully fabricated a Fe-17Mn-5Si-10Cr-4Ni (wt.%) alloy via L-PBF for the first time, which showed a strong SME and high mechanical strength even though the alloy did not contain any carbide forming element for enhancing the SME by the fine precipitations.…”

Structural Changes in a Powder Metallurgy 66fe-14mn-6si-9cr-5ni (Mass. %) Shape Memory Alloy Subjected to Bending Creep

“…It was pointed out that the alloy can be hot rolled with total reduction degrees up to 80 %, without cracking, which enabled an ultimate strain up to 72 %, during tensile failure tests [1]. Thus, FeMnSiCrNi SMAs have been recognized as corrosion resistant, with the capacity to accommodate high recovery strains, reaching 7.7 % in the case of an as-cast Fe-19Mn-5.5Si-9Cr-4.5Ni alloy with an average grain size above 0.6 mm [3]. Based on these characteristics, practical applications were devised.…”

On the Free Recovery Bending Shape Memory Effect in Powder Metallurgy FeMnSiCrNi

“…The development of Fe-Mn-Si based SMAs led to Fe-Mn-Si-Cr (1990) [10] and Fe-Mn-Si-Cr-Ni (1991) [11]. Fe-Mn-Si-Cr-Ni SMAs are corrosion resistant and they are able to recover strains as large as 7.7 % [12].…”