In recent years, additive manufacturing of metal composites has been studied extensively, and nanoscale ceramic particles have been introduced into the metal matrix as a reinforcing phase, which can improve the microstructure of the alloy and the comprehensive properties of a material. The additive-manufactured composite specimen, which has great potential for industrial production and application, is found to have a large residual stress. Consequently, further heat treatment is required. However, most of the studies directly use the heat treatment system of forgings, and the microstructural characteristics of additive manufacturing are quite different from those of the forgings. Therefore, studying the corresponding heat treatment system based on the characteristics of additive manufacturing is necessary. This study considers TiB₂/In718 composites as the research object to explore the effects of different heat treatment methods on the microstructure and precipitated phase characteristics of the composites to provide a certain reference for subsequent research.

The TiB₂/In718 composites were prepared by laser additive manufacturing equipment and the resulting TiB₂/In718 composites were subjected to high-temperature homogenization heat treatment. The morphologies of the precipitated phases were observed using a field emission scanning electron microscope (SEM), which was combined with the energy dispersive spectrometer (EDS) to analyze the distribution of elements in the alloys.

The TiB₂/In718 composites were subjected to high-temperature heat treatment. The microstructures of TiB₂/In718 composites exhibit dendrites following heat treatment at 1175 ℃, and no recrystallization grains are observed, indicating that the recrystallization temperature of TiB₂/In718 is above 1175 ℃. When the high-temperature homogenization temperature is 1200 ℃, the dendrite substructure disappears, grain morphology is equiaxed, and large-angle grain boundaries exhibit jagged arch traces. Moreover, some recrystallized grains can be observed near the original grains (Fig. 1), which is characteristic of the early stage of recrystallization. It provides a favorable location for the generation of recrystallized grains, which nucleate through the grain boundary arching mechanism. When the high-temperature homogenization temperature is 1225 ℃, the TiB₂/In718 composite material undergoes obvious recrystallization, a large number of fine equiaxed grains are formed, grain refinement occurs, and the grain morphology is fully equiaxed, indicating that the TiB₂/In718 composite is completely recrystallized. The In718 alloy can be completely recrystallized following heat treatment at 1100 ℃ for 1 h, indicating that the recrystallization temperature of In718 increases following the addition of TiB₂ nanoparticles. This is because the TiB₂ strengthening phase particles hinder the slippage of dislocations and the migration of grain boundaries, which is inconducive to the nucleation and growth of recrystallization and hinders the recrystallization process, requiring higher crystallization temperatures. When the high-temperature homogenization temperature is 1250 ℃, the recrystallized grains grow further. Additionally, during the high-temperature homogenization process, the TiB₂ nanoparticles decompose, and owing to the low solid solubility of B atoms in the nickel matrix, the decomposed B atoms segregate at grain boundaries to form borides. Following direct aging (DA) heat treatment, a large number of Laves phases are present between the dendrites of TiB₂/In718 (Fig.5), and this formation of the Laves phase is related to the microscopic segregation of Nb elements in the dendrite part during the solidification of the alloy. Meanwhile, TiB₂ dissolves in the matrix during solidification, resulting in the precipitation of Nb and other elements from the supersaturated matrix and diffusion to the dendrites. This leads to the segregation of Nb and other elements between the dendrites and the formation of the Laves phase. Following solution treatment and aging (STA) heat treatment, the Laves phase partially dissolves, and its morphology changes from chain to block (Fig. 6).

When the high-temperature homogenization treatment is performed at 1175 ℃, the laser-solid-formed TiB₂/In718 composites do not undergo an obvious recrystallization process. When the temperature exceeds 1200 ℃, obvious recrystallization occurs, recrystallization grains are formed, and with the increase of solution treatment temperature, many columnar crystals are transformed into equiaxed crystals, and the recrystallization phenomenon becomes prominent. When the temperature increases to 1225 ℃, the recrystallized grains of TiB₂/In718 composites grow. Additionally, when the samples are treated with a solid solution above 1200 ℃, the TiB₂ nanoparticles react with the matrix to form boride. With the increase in the number of TiB₂ nanoparticles and the number of cell crystals, the distribution morphology of the Laves phase also changes from chain to network distribution, and the shape of the Laves phase changes from a long chain to a block. Following STA heat treatment of TiB₂/In718 composites, the distribution and structure of alloying elements become more uniform, and the layer structure is basically eliminated. In addition, during the heat treatment process, the Laves phase partially dissolves, and the Laves phase morphology changes from a long chain to a block.