Strength-plasticity regulation via nanoscale precipitation and coprecipitation in cobalt-free medium-entropy alloys

Nanoprecipitate-strengthened Co-free medium- and high-entropy alloys represent a new candidate for nuclear applications because of their low costs and excellent radiation resistance. However, the premature stress concentration around the nanoprecipitates during deformation tends to greatly reduce the tensile ductility. In this work, L1 2 and D0 22 nanoprecipitates were introduced for the primary purpose of strengthening Co-free Cr 25 Fe 32 Ni 35 Mo 3 Al 5- x Nb x alloys ( x = 1, 2, or 3). The Al/Nb content can influence both the secondary phase formation and the type of nanoprecipitates. Calculations show that order and coherent strengthening play a major role in the D0 22 nanoprecipitation strengthening. In contrast, the strengthening effect of the L1 2 nanoprecipitates was dominated by an order strengthening mechanism. Precipitation not only increases the strength, but also controls the deformation mechanism, which can activate an abnormal deformation mechanism under the condition of low stacking fault energies. This abnormal deformation mechanism originates from the regulation of nanoprecipitates, which determines whether to induce stacking faults or suppress stacking faults to induce microbands, thereby contributing a large ductility. These discoveries provide new insights into the development of alloys with excellent mechanical properties. • D0 22 /L1 2 nanoprecipitates-strengthened Co-free MEAs were designed. • The Al/Nb content affects the phase composition and the type of the nanoprecipitates. • Precipitation not only increases the strength, but also controls the deformation mechanism. • SFs and microbands improve the ductility and strain hardening rate to overcome the strength-ductility tradeoff.