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

(CoCrFeNi)(94)Ti2Al4 (Co23.5Cr23.5Fe23.5Ni23.5Ti2Al4, at. %) high entropy alloy (HEA) was fabricated through arcmelting and conventional thermomechanical process. After cold-rolling with 90% thickness reduction and varied annealing treatment (900 degrees C/30 min, 1000 degrees C/30 min), (CoCrFeNi)(94)Ti2Al4 specimens with varied grain size (d) were obtained. Subsequently, the fine-grained (FG, d similar to 10.5 mu m) and coarse-grained (CG, d similar to 57.9 mu m) specimens exhibit the obviously different microstructure after aging heat treatments. The CG specimen mainly shows the homogeneous gamma'-precipitates in the gamma-matrix. In contrast, FG specimen displays the more complicated microstructure consisting of gamma-matrix, intra-granular gamma' precipitates, inter-granular blocky-Heulser and discontinuous gamma'-precipitates. After 650 degrees 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 similar to 20.8%. At the higher tensile temperature of 600 degrees 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.

Automated summary

In this study, (CoCrFeNi)(94)Ti2Al4 high entropy alloy was fabricated and samples with different grain sizes were obtained through annealing treatments. The results showed that the fine-grained specimens had higher yielding strength and tensile strength after aging treatments, along with good elongation.