Comparative study of the densification kinetics of the FCC phase Al0.3CoCrFeNi and BCC phase AlCoCrFeNi high-entropy alloys during spark plasma sintering

Abstract

The densification kinetics of the FCC-matrix Al0.3CoCrFeNi and BCC-matrix AlCoCrFeNi HEAs at sintering temperatures ranging from 950 ℃ to 1050 ℃ are comparatively studied utilizing the steady-state creep model. The densification activation energy of the Al0.3CoCrFeNi and AlCoCrFeNi powder in their low-stress exponent stage is linearly fitted to be 264.91 and 220.99 kJ/mol, respectively. The densification mechanism of both the two HEAs in the low-stress exponent stage is proved to be atomic diffusion. However, the densification activation of the Al0.3CoCrFeNi in the high-stress exponent stage increases to 554.77 kJ/mol and the densification mechanism changes to creep. The densification activation energy of the AlCoCrFeNi in its high-stress exponent stage cannot be obtained by linear fitting because of the abnormal increase of the stress exponent at the sintering temperature of 1050 ℃. The BCC-FCC transformation in the AlCoCrFeNi reduces its creep resistance because of different deformation behavior in BCC (dislocation cell) and FCC phase (cross-glide and twinning). As a result, dislocation density is higher in the FCC phase (4.3 ×1013/m2) compared with the BCC phase (3.8 ×1013/m2), which benefits the densification in the creep-controlled high-stress exponent stage. Furthermore, the stronger electromigration effect induced by a higher heating rate is demonstrated to further reduce the densification activation energy from 124.20 to 72.53 kJ/mol and accelerate densification in the diffusion-controlled heating period.