MICROSTRUCTURE AND CORRESPONDING TENSILE PROPERTIES OF AS-CAST, β-SOLIDIFYING, γ-TIAL BASED TNM ALLOYS

摘要

VAR ingots of γ-TiAl based TNM alloys, which solidify completely via the β-phase, exhibit a fine grained microstructure consisting of lamellar γ/α_2 colonies and globular γ and β/B2 grains. The volumetric ratio between the three phases depends mainly on the aluminum content and the content of the major β-stabilizing alloying elements molybdenum and niobium. The thermodynamic stability of the microstructure at operating temperature is characterized by the investigation of the equilibrium phase formation at elevated temperatures, using high temperature XRD measurements. The excellent tensile properties, at room temperature and 700°C, of thermally stress relieved as-cast VAR materials are related to the microstructure, and thus, to the chemical composition of the TiAl alloy. It is expected that the composition of β- solidifying γ-TiAl based alloys can be adjusted to the specific needs of both additional processing steps and final application.