Aluminum-controlled local lattice distortion and structural rearrangement mechanisms in BCC HfNbTiZrAl x refractory high-entropy alloys

Local lattice distortion (LLD) is often invoked as a strengthening mechanism in body-centered-cubic (BCC) refractory high-entropy alloys, but its direct connection to deformation pathways remains difficult to isolate experimentally. Here, we use molecular dynamics simulations based on a second-nearest-neighbor modified embedded-atom method potential to examine single-crystal HfNbTiZrAlx alloys (x = 0, 0.25, 0.50, 0.75, and 1.00) under [100] tension. Al addition contracts the BCC lattice and broadens the first two coordination shells in the radial distribution function, indicating a progressive increase in LLD without destroying long-range BCC order. This structural change produces a clear separation between elastic and plastic trends: the apparent longitudinal modulus under the imposed uniaxial-strain condition decreases by approximately 15%, whereas the yield strength increases by approximately 68% at 300 K. Common-neighbor analysis shows that plasticity proceeds mainly through loss of BCC local order into common-neighbor analysis-classified disordered environments, while FCC and HCP motifs are transient intermediates rather than stable product phases. Temperature- and strain-rate-dependent simulations reveal a crossover from a thermally activated pathway governed by transient, fault-like local structural rearrangements in Al-lean alloys to a more local, LLD-dominated disordering pathway in Al-rich alloys. The results clarify how Al-induced lattice distortion can raise the barrier for yielding while reducing sensitivity to temperature and loading rate, and they also delimit the interpretation of the calculated stresses to the single-crystal, high-strain-rate molecular dynamics regime.