Study On Microstructure And Properties Of FeCoNiCuAl_x High Entropy Alloys Prepared By Microwave-assisted Combustion Synthesis

High entropy alloy with multi-principal elements is a novel alloy. It has broad application prospects. It is extremely significative to investigate its preparation technology, phase formation and transformation rules, properties and heat treatment theory, etc. FeCoNiCuAlx high entropy alloys (x=0,0.5,1,1.5 and 2, named Alx) were prepared by microwave-assisted combustion synthesis, took the oxides as raw materials, based on design concept of high-entropy alloys and aluminum thermal reaction mechanism, and studied a new preparative technique for high entropy alloy. The adiabatic temperatures were calculated theoretically, and the Gibbs free energy of solid solution and intermetallic compounds which may be formed were calculated respectively according to Mi edema model. The microstructure, properties as well as transformation rules of heat treatment were investigated. The results showed that:The adiabatic temperatures were very high, and they decreased witn the increasing of Al content. The lowest adiabatic temperature was 3201K. According to the thermodynamic analysis, the system tended to form solid solution because of high entropy, and Ni-Al intermetallic compounds may occur in the system.FeCoNiCuAlx high entropy alloys were prepared by microwave-assisted combustion synthesis. They formed simple solid solution structure (BCC+FCC or BCC). With the increasing of Al content, BCC phase increased, when x=2, the structure was BCC completely. When x≥1, the ordered BCC phase appeared. After heat treatment, the stable phases at high temperature of five-element Al0.5, Al1, Al1.5, Al2 alloys were both BCC, line with the general law, but for the four-element Alo alloy, FCC was the stable phase at high temperature, similar to the steel. During heat treatment process, the phase transition temperatures increased with the raise of Al content. Al1 alloy exhibited good thermal stability. An orderly transition occurred in the Al0.5, Al1.5, Al2 alloys, and an orthorhombic phase of Al3Ni type occurred in the Al1.5, Al2 alloys.FeCoNiCuAlx high entropy alloys had typically dendritic structure. Cu tended to segregate at the interdendrite. When the x value reached 1, needle-like precipitates were found in the dendrite, and tiny precipitates were found in the interdendrite. With more addition of Al content, the dendrites became to thick, the segregation of Cu increased, and the needle-like precipitates became to round and tiny. After heat treatment, the uniformity of composition was improved, but the alloys still retained dendritic morphology.FeCoNiCuAlx high entropy alloys had excellent mechanical properties. With the increasing of Al content, the plastic strain decreased, while the hardness values rose, the compressive yield stress and fracture strength of Al1 alloy reached the highest, then they declined. Al2 alloy had the largest hardness, it was 571HV. Al1 alloy exhibited good comprehensive properties, the yield stress, fracture strength and plastic strain reached 1231Mpa,1455Mpa and 9.7%, respectively. After heat treatment of 700℃, the hardness of Alo and Al0.5 alloys were enhanced greatly, then they declined with the increasing of temperature. After heat treatment of different temperatures, the hardness of Al1, Al1.5, Al2 alloys were lower than cast, the alloys showed good resistance to temper softening. Compared with the cast, heat treatment could improve the comprehensive compression properties of Al0, Al0.5, Al1.5 alloys.