EFFECT OF HEAT TREATMENT ON THE STRUCTURE AND PROPERTIES OF TITANIUM ALUMINIDE ALLOY TI-AL-V-NB-CR-GD PRODUCED BY SELECTIVE ELECTRON BEAM MELTING

Abstract

This study examines the influence of hot isostatic pressing and heat treatment on the microstructure and mechanical properties of samples manufactured by selective electron beam melting (SEBM) of metal powder composition (MPC fraction 40-100 pm) of a new six-component intermetallic beta-solidifying TiAl alloy Ti-44.5Al-2V-1Nb-2Cr-0.1Gd, at % (Ti-31.0Al-2.5V-2.5Nb-2.5Cr-0.4Gd, wt %). It is shown that SEBM with a high line energy input (El = 285 J/m) produces a fine-grained microstructure in the as-built material with a grain size of 5-14 pm and residual porosity of less than 0.5 vol %. An increase in the electron beam current (I) from 9.5 to 19.0 mA causes Al evaporation, and as a result the fraction of large columnar grains (d = 30-100 pm in width, h = 150-400 pm in height) formed mainly in Al-depleted regions (layers) increases. Heat treatment of the as-built SEBM samples by two-stage annealing in the (a + y) and (a2 + у + в) phase fields or by thermal cycling in the (a + y) phase field leads to complete or partial fragmentation of columnar grains. For the samples produced at lower values of I, a combined post-processing treatment by hot isostatic pressing in the а phase field and two-stage annealing completely eliminates residual porosity and transforms the columnar structure into a fine-grained one with a grain size of less than 150 pm. As a result, the achieved short-term mechanical characteristics at 20 °С (аВ = 525 ± 5 MPa, 5 = 1.1%) and 750 °С (аВ = 405 ± 10 MPa, 5 = 3.8%) are comparable to those of the studied TiAl alloy in the as-cast state.