Fundamental Study On Microstructure And Mechanical Properties In Multi-phase Al-Cr-Fe-Ni-M High Entropy Alloys

Differring from the design concept of traditional alloys based on one or two principal elements, multi-principal high entropy alloys generally include multi-principal elements. Because of the high entropy effect, the sluggish diffusion effect, the severe lattice-distortion effect and the cocktail effect, high entropy alloys (HEAs) have excellent physical, chemical and mechanical properties, such as high hardness, high strength, high wear resistance, excellent corrosion resistance, and high low temperature toughness. Therefore, the high entropy alloys possess broad prospects of application and research value.In this paper, the Al-Cr-Fe-Ni-M (M= Co, Mo, V, W) alloy systems were taken as research object. The effect of different elements on microstructure in Al-Cr-Fe-Ni-M alloy systems were studied systematically. And the relationship between phase composition and mechanical properties was discussed. Based on d-orbit energy level parameter, a model was built to predict the stability of Topologically Close-Packed (TCP) phase in high entropy alloys. And combined with muti-phases solid solution formation rules, some high strength and high ductility alloys which has the compressive strength larger than 3GPa or the largest tensile yield strength and fracture strength were designed and prepared. In this paper, main conclusions are as follows:1) Based on the atomic radius difference (?R), the mixing enthalpy (?H), the valence electron concentration (VEC) and the electronegativity difference (?x) parameters, multi-phases solid solution formation rules were studied systematically. The results showed that the multi-phases FCC HEAs formed when ?R< 4.9%,-5.9 kJ·mol-1? ?H? 3.2 kJ·mol-1, VEC?8.2, ??Pauling< 12.2%, and ?Allen?10.6%; the multi-phases FCC+ BCC HEAs required that 4.7% ? ?R? 6.2%,-13.1 kJ·mol-1??H?-2.7 kJ·mol-1,7.1? VEC? 8.2, ??pauling?14.6%, and ??Allen?16%; the multi-phases BCC HEAs should satisfy the rules that 5.5%??R?7.1%,-22.8 kJ·mol-1??H?0.3 kJ·mol-1, VEC?7.5, and ??Allen?16.6%.2) The model of d-orbit energy level in HEAs was built and can successfully predict the stability of TCP phases. For some HEAs containing mainly Co, Fe, and Ni elements, when the matrix has FCC structure and (?)> 0.9, the Topological Close-Packed (TCP) phases tend to form; in BCC matrix the TCP phases are stable when (?)> 1.09.3) In AlCrFeNiMox alloy system, AlCrFeNiMoo.2 alloy was a typical regular lamellar eutectic alloy which consisted of the AlNi-type B2 intermetallic compound and the FeCr-type BCC solid solution phases. The compressive yield strength, fracture strength and plastic strain of the alloy were as high as 1487.4MPa,3222MPa, and 28.7%, respectively, which was due to solid solution strengthening, the second-phase strengthening and the ultra-fine eutectic microstructures.4) In AlCoCrFeNiVx alloy system, with the increase of V content, the morphology of BCC and B2 phases changed. The spinodal microstructure disappeared and the BCC nano-phase particles distributed in B2 matrix in homogeneous dispersion state. Due to the solid solution strengthening, the hardness and compressive yield strength increased with the increased of V element. For the AlCrFeNiMoo.2 alloy, the compressive yield strength, fracture strength and plastic strain were as high as 1492.4MPa,3297.8MPa, and 26.8%, respectively.5) The AlCoCrFeNi2Wx alloys consisted of the primary BCC phase plus eutectic FCC/BCC phases. The addition of W elements increased the volume fraction of BCC phases and changed the morphology of FCCn. The AlCoCrFeNi2 alloy had good casting fluidity and excellent tensile properties due to the solid solution strengthening, the second phases strengthening and eutectic FCC/BCC composite strengthening. The fracture strength and elongation were 853MPa and 15.3%, respectively.6) According to the guidance of high entropy alloys design theory and the alternative methods of similar chemical elements, the AlCrFe2Ni2 alloy consisted of noodle-like FCC phases, particles-like BCC phases, and continuous net B2 phases were designed and prepared. Due to the ultra-fine microstructure and FCC+BCC/B2 composite strengthening, the alloy had yield strength of 780MPa, ultimate strength of 1228MPa and elongation of 17%, respectively. The AlCrFe2Ni2 alloy had the largest yield strength and fracture strength in the reported as-cast crystal HEAs and had the potential industrial application value.