Phase evolution in laser solid formed compositionally graded TI60-TI ₂ALNB alloys

A thin-wall Ti60-Ti₂AlNb alloy, with continuous composition gradient along the deposited direction, was fabricated by laser solid forming (LSF). The phase morphological evolution and microstructure evolution along the deposited direction were investigated. With the increase of aluminum and niobium 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. There is a large composition range from Ti60 to Ti60-60wt.%Ti ₂AlNb for the existence of α phase. The hardness of the graded material increases with the increase of aluminum and niobium contents, and reaches the maximum with the formation of B2+O phases, then decreases sharply as obtaining the whole B2 phase at the top position of Ti₂AlNb parts. Based on the non-equilibrium phase diagram of the Ti-rich corner, the phase morphological evolution during laser solid forming of the graded materials is 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.