Microstructure and phase evolution in Ti60-Ti₂AlNb gradient material prepared by laser solid forming

Ti60 titanium alloy and Ti₂AlNb alloy have been developed to serve at 600 and 650-850°C, respectively. Because only some regions of hot-end components encounter extreme high temperature environments, it is appropriate to use functionally gradient materials (FGMs) according to the distribution of the working temperature. In this paper, a thin-wall Ti60-Ti₂AlNb alloy with composition gradient was fabricated by laser solid forming (LSF). The phase morphological evolution and microstructure evolution along the gradient direction were investigated. With the increase of Al and Nb contents, a series of phase evolutions along the compositional gradient occurred: α + β → α + α ′ → α ′ → α + β → α + β/B2 + α₂ → β/B2 + α₂ → β/B2 + α₂ + O → B2 + O → B2. α-phase can exist in a wide composition range from Ti60 to Ti60-60%Ti₂AlNb (mass fraction). The hardness of the material increases with the increase of Al and Nb contents, and reaches the maximum when B2 + O phases appear, and then decreases sharply as obtaining the whole B2 phase at the top position of Ti₂AlNb part. Based on the non-equilibrium phase diagram of the Ti-rich corner, the phase morphological evolution during forming of the gradient materials was explained on combining with the analysis of the influence of the Al and Nb on the stabilities of α, α₂, β/B2 and O phases in titanium alloys and the effects of recurrent tempering/annealing and heat accumulation in laser solid forming.