Microstructure, creep, and tensile behaviour of a Ti-15Al-33Nb (at.%) beta plus orthorhombic alloy

The microstructural evolution, creep and tensile deformation behaviour of a Ti-15Al-33Nb (at.%) alloy was studied. Monolithic sheet material was produced through conventional thermomechanical processing techniques comprising non-isothermal forging and pack rolling. Electron microscopy studies showed that depending on the heat-treatment schedule, this alloy may contain three constituent phases including: beta ( disordered body-centred cubic), alpha(2) (ordered hexagonal close-packed based on Ti3Al) and O ( ordered orthorhombic based on Ti2AlNb). Heat treatments at all temperatures above 990 degrees C, followed by water quenching, resulted in fully-beta microstructures. Below 990 degrees C, Widmanstatten O-phase or alpha(2)- phase precipitated within the b grains. The fine-grained as-processed microstructure, which exhibited 90vol.% beta-phase, exhibited excellent strength (UTS 916 MPa) and ductility (epsilon(f)> 12%). After heat treatment, greater volume fractions of the orthorhombic phase precipitated and resulted in lower epsilon(f) values with UTS values ranging between 836-920 MPa. However, RT elongations of more than 2% were recorded for microstructures containing up to 63vol.% O-phase. Specimens subjected to 650 degrees C tensile experiments tended to exhibit lower strength values while maintaining higher elongation-to-failure. Tensile creep tests were conducted in the temperature range 650-710 degrees C and stress range 49 - 275 MPa. The measured creep exponents and activation energies suggested that grain boundary sliding operates at intermediate stress levels and dislocation climb is active at high stresses. Microstructural effects on the tensile properties and creep behaviour are discussed in comparison to a Ti-12Al-38Nb O + beta alloy.