Novel Precipitate Strengthening Mechanism in a Medium-Entropy Alloy

Precipitates can confine dislocations, limit their mean free paths, and induce strengthening. As a result, high strengths can be obtained in precipitate-strengthened alloys—but usually at the expense of strength. Here we show that, under the right conditions, nanoprecipitates in a model Fe-Ni-Al-Ti medium-entropy alloy can serve an additional function, also <i>via</i> spatial confinement. They can hinder an otherwise favorable FCC to BCC martensitic phase transformation. If controlled well, precipitates can suppress the formation of thermal martensite during processing and heat treatment but allow martensite to form during subsequent deformation. Such a deformation-induced phase transformation can lead to high work hardening, strength and ductility. The governing factors include: (1) the chemical driving force for martensite formation, which was kept constant in our study to eliminate it as a confounding variable, (2) volume fraction of precipitates, which was also kept constant in our study, and (3) size and spacing of precipitates, which were systematically varied. When the nanoprecipitates are tiny (closely spaced), they physically restrain the martensitic transformation from occurring, resulting in low strength, low work hardening, and high ductility (i.e., only dislocation plasticity occurs). When they are coarse (spaced far apart), they offer scant constraint to the formation of even thermal martensite, which results in a fully martensitic matrix that cannot undergo further transformation during deformation and is, therefore, relatively weak and brittle. At intermediate precipitate sizes (spacings) the degree of spatial confinement is ‘just right’: thermal martensite is suppressed but deformation-induced martensite is allowed to occur. Consequently, in this Goldilocks regime, an optimal combination of strength and ductility is obtained. It is worth noting that this trend runs counter to classical precipitation strengthening where strength increases as the precipitates become finer and more closely spaced. Here, it is the reverse, with the coarser precipitates resulting in significantly higher strength and work hardening. This can only be explained by invoking deformation-induced phase transformation as the governing mechanism. Our results show that, not only is it possible to activate multiple deformation mechanisms in HEAs/MEAs, but that by carefully controlling the precipitate characteristics (size and spacing, which define the spatial confinement that suppresses transformation) and matrix composition (which provides the chemical driving force for transformation), it is possible to call upon specific deformation mechanisms precisely when needed. An additional knob that can be turned is the volume fraction of precipitates, which can be tweaked to further enhance strength.