Effects of grain size on the precipitation pathway and mechanical properties of (CoCrFeNi)94Ti2Al4 high entropy alloy

Abstract

(CoCrFeNi)94Ti2Al4 (Co23.5Cr23.5Fe23.5Ni23.5Ti2Al4, at. %) high entropy alloy (HEA) was fabricated through arc-melting and conventional thermomechanical process. After cold-rolling with 90% thickness reduction and varied annealing treatment (900 °C/30 min, 1000 °C/30 min), (CoCrFeNi)94Ti2Al4 specimens with varied grain size (d) were obtained. Subsequently, the fine-grained (FG, d∼10.5 μm) and coarse-grained (CG, d∼57.9 μm) specimens exhibit the obviously different microstructure after aging heat treatments. The CG specimen mainly shows the homogeneous γ′-precipitates in the γ-matrix. In contrast, FG specimen displays the more complicated microstructure consisting of γ-matrix, intra-granular γ′ precipitates, inter-granular blocky-Heulser and discontinuous γ′-precipitates. After 650 °C/30 h aging treatment, the room-temperature (RT) yielding strength and tensile strength of FG specimen are enhanced by 97% and 42%, compared with CG specimen counterpart, along with a reasonable uniform elongation ∼20.8%. At the higher tensile temperature of 600 °C, the flow stress of FG specimen rises to a maximum value, followed by flow softening with increasing strain, whereas the flow softening does not occur for CG specimen. Considering the thermodynamic and kinetic factors, the effects of temperature and grain size on precipitation pathway and mechanical properties were discussed.