Influence of Heat Treatment Temperature on Microstructure and Mechanical Properties of TiB2@Ti/AlCoCrFeNi2.1 Eutectic High-Entropy Alloy Matrix Composites

This study systematically investigates the effects of heat treatment at 800–1000 °C on the microstructure and mechanical properties of 10 wt.% TiB2@Ti/AlCoCrFeNi2.1 eutectic high-entropy alloy matrix composites (EHEAMCs) prepared by vacuum hot-pressing sintering. The results show that the materials consist of FCC, BCC, TiB2, and Ti phases, with a preferred orientation of the (111) crystal plane of the FCC phase. As the temperature increases, the diffraction peak of the BCC phase separates from the main FCC peak and its intensity increases, while the diffraction peak positions of the FCC and BCC phases shift at small angles. This is attributed to the diffusion of TiB2@Ti from the grain boundaries into the matrix, where the Ti solid solution increases the lattice constant of the FCC phase. Microstructural observations reveal that the eutectic region transforms from lamellar to island-like structures, and the solid solution zone narrows. With increasing temperature, the Ti concentration in the solid solution zone increases, while the contents of elements such as Ni decrease. Element diffusion is influenced by binary mixing enthalpy, with Ti and B tending to solidify in the FCC and BCC phase regions, respectively. The mechanical properties improve with increasing temperature. At 1000 °C, the average hardness is 579.2 HV, the yield strength is 1294 MPa, the fracture strength is 2385 MPa, and the fracture strain is 19.4%, representing improvements of 35.5% and 24.9% compared to the as-sintered state, respectively, without loss of plasticity. The strengthening mechanisms include enhanced solid solution strengthening due to the diffusion of Ti and TiB2, improved grain boundary strength due to the diffusion of alloy elements to the grain boundaries, and synergistic optimization of strength and plasticity.