Exploring room-temperature deformation mechanisms of a B2-strengthened refractory compositionally complex alloy

Abstract

The room-temperature deformation mechanisms in a B2-precipitation-strengthened 27.3Ta–27.3Mo–27.3Ti–8Cr–10Al (at.%) body-centered cubic (BCC) refractory compositionally complex alloy (RCCA) was examined using nanoindentation under two aging conditions. Aging at 900 °C for 1 h resulted in nearly spherical B2 precipitates with an average diameter of (5 ± 1) nm. Prolonged aging for 1000 h resulted in coarsened, elongated precipitates aligned along three orthogonal <001> directions, averaging (80 ± 16) nm in length, (26 ± 4) nm in width, and a volume fraction of 0.25 ± 0.05. The precipitates maintained a coherent interface with the matrix under both aging conditions. After 1000 h of aging, a small lattice parameter mismatch of + (0.6 ± 0.4) % was estimated, demonstrating good microstructural stability. Deformation at room temperature involved cooperative shearing of B2 precipitates by paired a/2⟨111⟩ dislocations, with an antiphase boundary energy of (150 ± 39) mJ/m². The dislocation-precipitate interaction evolved from weakly to strongly coupled pairwise cutting between the 1 h and 1000 h aged samples, leading to an approximately 6–12 % hardness reduction. The estimated critical radius and precipitation strengthening contribution further validate this transition, showing reasonable agreement with TEM observations. No Orowan looping, twinning, or phase transformation was observed.