Enhanced corrosion resistance of Ti-Al-Mo alloy through solid state transformation driven by rapid solidification

Rapid solidification of undercooled liquid Ti50Al44Mo6 alloy was realized by electromagnetic levitation. Primary (βTi) dendrite grew rapidly from melt with decreasing temperature. As undercooling increased, nucleation rate and growth driving force of primary (βTi) dendrites increased. Growth velocity of primary (βTi) dendrite increased significantly, reaching 13.5 m·s-1 at the maximum undercooling (233 K). After solidification, primary (βTi) dendrite decomposed simultaneously through β→α→α2 transformation and martensite transformation β→γ. Homogeneity of solute distribution in primary (βTi) phase affects the solid-state phase transformation mode. Solid-state phase transition was mainly dominated by diffusion-controlled β→α→α2 transformation at small undercooling. Solid-state phase transition gradually was dominated by displacive martensite transformation at deep undercoolings, and corresponding microstructure was mainly characterized by more refined martensite needles. The refined microstructure and martensite transformation domination contributed to the formation of passivation films with improved corrosion resistance. Moreover, this weakens micro-galvanic effect, significantly reduces size of pits, maintains corrosion scales over pits to effectively alleviate the corrosion process, and consequently enhances corrosion resistance.