Insights on niobium micro-alloyed laser in situ synthesised gamma titanium aluminide alloys

Abstract

The effects of micro-alloying gamma titanium aluminide ( -TiAl) with niobium (Nb) using a laser melt pool as a melting pot are reported. The Optomec LENS machine was used to carry out the laser in situ alloying experiments where Nb, ranging from 6 to 10 (at. %), was added to the stable binary -TiAl alloy. The results of this study concluded that when a stable binary -TiAl alloy is micro-alloyed with Nb, there is a definite microstructural transformation, anneal twinning, promotion, and retardation of aluminium solubility in the dual and pure phases, respectively. Twinning in the as-built in situ alloyed ternary Ti–48Al–xNb was for the first time reported in this study. It was observed that 6 at. % Nb promoted twinning in the as-built sample, which inferred that the sample might have room temperature ductility. In fact, it was shown that the twins formed in the as-built sample dissipated with the addition of Nb. A heat treatment temperature of 1200C promoted anneal twinning only in the binary alloy, as confirmed by XRD data. Meanwhile, this twinning was absent in all the ternary alloys when they were heat treated to a temperature of 1200 C. Anneal twinning was confirmed only for the alloy containing 8 Nb (at. %) at 1400 C. Stalk faults, dislocations, and dislocation pile-ups were observed in the a2 phase of the alloys. Aluminium solubility was seen to increase in the a2 + (±49 at. %) phase alloy and sharply decrease in the pure (>49 at. %) phase alloys. Most importantly, this study determined that the laser in situ alloying process was highly exothermic. The heat gained by the reaction was found to increase with the increase in niobium content.