Effect of TiAlNbCrSi Alloy Microstructure Produced by Mould Casting (MC) or Electron Beam Powder Bed Fusion (EB-PBF) on Scale Grown Under Dry Air and Steam

Abstract

TiAl parts fabricated with additive manufacturing have begun to find applications in car and aviation industry. Optimization of their properties concentrates on introduction of new alloying additions improving the TiAl ductility and oxidation resistance. Their effect on the mechanical properties was worked out quite well, but the knowledge how they affect the oxidation processes is still limited. Therefore the present experiment was aimed at investigation of scale grown over mould cast (MC) and electron beam powder bed fusion (EB-PBF) Ti–48Al–2Nb–0.7Cr–0.3Si (at. pct) alloys. It was conducted at 650 °C for 1000 hours both in dry air and steam covering early stages of this process. The scale microstructure, chemical and phase composition was examined with the transmission electron microscopy (TEM). Applied treatment caused development of three-layer scale, i.e. with major portions of R-TiO2 + α-Al2O3/R-TiO2/α-Al2O3. The one formed during dry-air oxidation of the EB-PBF alloy was most compact. Steam oxidizing changed morphology of rutile present at its surface from rod/plate-like into whiskers. The Si turned out to be especially active during scale growth diffusing up to its surface. The presence of steam further increased mobility of both Si and Cr rising their presence in the upper part of the scale. The Nb was found to accumulate within the substrate area adjoining to the scale. Refinement of EB-PBF microstructure as compared with the MC alloy resulted in promoting reaction at the scale/substrate front contributing significantly to development of thicker oxide coating and nitrides bearing oxidation affected zone.