Effect of pre-strain on transformation behavior and recovery stress stability under cyclic loading in Fe-Mn-Si-Cr-Ni-VC shape memory alloys

The present study investigates the influence of pre-strain on transformation behavior, recovery stress formation, and its stability under cyclic loading in Fe-17Mn-5Si-10Cr-4Ni-1(V,C) shape memory alloys. Pre-strain levels between 1.6 and 19.7 % and activation temperatures ranging from 160 to 400 °C were examined. At low pre-strain levels, recovery stress remains limited to low values independent of activation temperature. In contrast, higher pre-strain levels enable a steady increase in recovery stress, rising from 350 MPa to 624 MPa. Moreover, under strain-controlled cyclic loading, stress relaxation is less pronounced with increasing pre-strain, despite higher initial recovery stresses. Irrespective of the extent of stress relaxation, the initial recovery stress was fully restored upon repeated thermal activation. Complementary electron backscatter diffraction analyses revealed an increased fraction of ε-martensite and higher kernel average misorientation values with increasing pre-strain, indicating increased local dislocation density. While recovery strain is governed by the extent of reversible phase transformation during heating, recovery stress formation under constrained conditions is limited by austenite stability against stress-induced martensitic forward transformation and dislocation activity during cooling. The increased local phase stability associated with dislocations is thus identified as a key factor enabling higher recovery stresses while simultaneously enhancing their stability. This multi-factorial improvement in functional performance is directly relevant for high-performance applications in construction, such as prestressed strengthening systems or couplers using Fe-based shape memory alloys.