Re-examination of the effect of reducing annealing twin boundary density on the shape memory effect in Fe-Mn-Si-based alloys

Reducing annealing twin boundary (ATB) density was proposed to substantially improve the shape memory effect (SME) of Fe–Mn–Si-based alloys, irrespective of their composition, deformation temperature and thermomechanical treatments. Here, we re-examined the effect of reducing ATB density on the SME of a Fe[sbnd]20Mn[sbnd]5.5Si[sbnd]9Cr[sbnd]5Ni (wt%) alloy. We set off with three starting states, i.e., the as-cast state, the hot forged and solution-treated (ST) state as well as the subsequently δ-annealed (DA) state. Our results reveal that the as-cast sample with the lowest ATB density exhibits the lowest shape recovery ratios, while the shape recovery ratios of the DA sample are close to those of the ST sample although the ATB density in the former is only ~1/3 of that in the latter. In other words, reducing ATB density does not necessarily lead to the improvement of SME. Compared to the ST sample, the lower ATB densities in the as-cast and DA samples are actually concomitant with higher densities of low angle grain boundaries (LAGBs), larger γ grain sizes and a limited amount of χ precipitates (volume fraction < 1.6%). The increase of LAGB density and the precipitation of elongated χ precipitates are deleterious to the SME, while the increase of γ grain size improves the SME. Thus, reducing ATB density cannot result in a monotonic change to the SME, given that the decrease of ATB density is concomitant with other microstructural changes.