Abstract: Face-centered cubic (FCC) equi-atomic multi-principal element alloys (MPEAs) exhibit excellent mechanical properties over a broad temperature range from cryogenic temperatures (CTs) to room temperature (RT). Specifically, while the deformation mechanism is dominated solely by dislocation slip at RT, the reduction in stacking fault energy (SFE) at CTs leads to enhanced strain hardening with deformation twinning. This study employs in-situ neutron diffraction to reveal the temperature-dependent deformation behavior of the FCC/body-centered cubic (BCC) dual-phase (DP) Al₇(CoNiV)₉₃ medium-entropy alloy (MEA), which possesses a matrix exhibiting deformation behavior analogous to that of representative equi-atomic MPEAs. Alongside the increased lattice friction stress associated with reduced temperature as a thermal component, deformation twinning at liquid nitrogen temperature (LNT) facilitates dislocation activity in the FCC matrix, leading to additional strain hardening induced by the dynamic Hall-Petch effect. This would give the appearance that the improved strengthening/hardening behaviors at LNT, compared to RT, are primarily attributable to the FCC phase. In contrast, the BCC precipitates are governed solely by dislocation slip for plastic deformation at both 77 K and 298 K, exhibiting a similar trend in dislocation density evolution. Nevertheless, empirical and quantitative findings indicate that the intrinsically high Peierls-Nabarro barriers in the BCC precipitates exhibit pronounced temperature-dependent lattice friction stress, suggesting that the BCC precipitates play a more significant role in the temperature-dependent strengthening/hardening behaviors for the DP-MEA. This study provides a comprehensive understanding of deformation behavior by thoroughly analyzing temperature-dependent strengthening/hardening mechanisms across various DP-MPEA systems, offering valuable guidelines for future alloy design.

Key words: In-situ neutron diffraction, Medium-entropy alloy, Dual-phase microstructure, Deformation behavior, Mechanical properties