Influence of heat treatment modes on the structure and mechanical properties of wrought VTI-4 alloy based on orthorhombic titanium aluminide

Problem. Currently, in the field of research on alloys based on orthorhombic Ti ₂AlNb aluminide, there are no data on the relationship between heat treatment modes, structural parameters, and mechanical characteristics to ensure a balance of strength and ductility. Aim. To establish the relationship between heat treatment modes, microstructural characteristics (O-phase sizes, α₂-phase volume fraction) and mechanical characteristics at room temperature, as well as to determine the heat treatment mode that provides the best balance of strength and ductility for the VTI-4 alloy. Methods. In this work, the method of isothermal multidirectional forging was applied to form a fine-grained globular structure in order to improve the ductility characteristics of the alloy. Subsequent two –stage heat treatment was carried out in the α₂+β+O-phase region. Mechanical characteristics were determined by tensile testing. Results. The influence of quenching and aging temperature on the size and volume fraction of α₂ and O-phase particles was investigated. The choice of heat treatment modes aimed at forming a structure that ensures high strength and ductility properties of the VTI-4 alloy is substantiated. It was established that when heating for quenching to T=900 °C, O-phase precipitates are retained at triple junctions along β-grain boundaries; during subsequent aging, O-phase particles and interlayers are formed, the thickness of which increases with rising temperature, leading to a decrease in strength and ductility. It was also established that heating for quenching from the upper part of the α₂+β+O-phase region (T=960 °C) preserves the globular microstructure, leading to the dissolution of O-phase particles and, accordingly, saturating the β-phase with alloying elements. Increasing the quenching temperature also reduces the volume fraction of α₂-phase particles and prevents the formation of O-phase interlayers along β-grain boundaries during aging in the temperature range of 760–800 °C. Conclusions. The study of the influence of aging temperature in the range of 760–840 °C revealed that the lower the temperature, the smaller the thickness of O-phase particles, the higher the strength, and the lower the ductility. The optimal heat treatment mode was determined: quenching at T=960 °C for τ=2 h and aging at T=800 °C for τ=6 h. After such heat treatment, the wrought VTI-4 alloy demonstrates high strength and ductility (σ_0.2=1180 MPa, σ_в=1300 MPa, δ=6.2 %).