Multimodality of critical strength for incipient plasticity in L12- precipitated (CoCrNi)94Al3Ti3 medium-entropy alloy: Coherent interface-facilitated dislocation nucleation

Abstract

The stochastic nature of the incipient plasticity in (CoCrNi)94Al3Ti3 medium-entropy alloy (MEA) precipitation-hardened by L12 phase of various sizes is explored systematically by nanoindentation. As the L12 phase continuously coarsens during aging (with an average radius varying from 3 to 82 nm), the critical strength for incipient plasticity, τy, measured with a small radius tip unexpectedly decreases first and then increases, attaining its minimum when the aging time t = 60 mins. The distributions of τy for all samples are intrinsically multimodal, indicating the co-existence of different dislocation nucleation mechanisms. Specifically, a trimodal distribution is found for the samples in the homogenized state and aged up to 18 mins, whereas the strength distributions in the samples aged for 60 mins and more is bimodal. Considering the size of the stressed volume during the indentation and the inter-spacing between various crystalline defects (dislocations, monovacancies, divacancies, and coherent phase interfaces), the potential deformation mechanisms in all aged samples were ascertained to include monovacancy-induced heterogeneous dislocation nucleation in the matrix near and far from the precipitate/matrix interface, interface-assisted heterogeneous dislocation nucleation, as well as homogeneous dislocation nucleation (only for large L12 particles). Results suggest that the coherent phase interface can reduce the critical stress for dislocation nucleation and thus can act as a unique generator of dislocations, which depends on the inter-particle spacing and the size of L12 phase.