Evolution of shape memory effect with aging time during aging after pre-strain in Fe–Mn–Si–Cr–Ni–C shape memory alloys

Abstract

The shape memory effect of Fe–Mn–Si–Cr–Ni–C shape memory alloys could be significantly improved by aging after pre-strain, because a high density of directional Cr23C6 particles is precipitated. However, the variation of shape memory effect under a large deformation-strain with aging time is not evident in the Fe–Mn–Si–Cr–Ni–C alloys subjected to aging after pre-strain. In order to clarify the above issue, the effect of aging time on the microstructures and the recovery-strain under a large deformation-strain of 7.5% for the Fe–Mn–Si–Cr–Ni–C alloy subjected to aging for 10–300 min after 10% pre-strain at room temperature was systematically investigated by XRD, SEM, EBSD, and EPMA. When the aging time was 30 min, the maximum recovery-strain of 5.4% was achieved. The recovery-strain increased from 4.9% to 5.4% when the aging time was extended from 10 min to 30 min. It is attributed to that the fraction of twin boundaries, inhibiting stress-induced ε martensitic transformation, decreased from 19.1% to 14.2%, as the precondition that the density of stacking faults, promoting stress-induced ε martensitic transformation, remained nearly constant. When the aging time was extended from 30 min to 180 min, the recovery-strain decreased from 5.4% to 4.6% due to both the decrement of stacking faults and the increment of twin boundaries.