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

Abstract

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: β (disordered body-centred cubic), α2 (ordered hexagonal close-packed based on Ti3Al) and O (ordered orthorhombic based on Ti2AlNb). Heat treatments at all temperatures above 990°C, followed by water quenching, resulted in fully-β microstructures. Below 990°C, Widmanstätten O-phase or α2-phase precipitated within the β grains. The fine-grained as-processed microstructure, which exhibited 90 vol.% β-phase, exhibited excellent strength (UTS = 916 MPa) and ductility (ϵf>12%). After heat treatment, greater volume fractions of the orthorhombic phase precipitated and resulted in lower ϵ f values with UTS values ranging between 836–920 MPa. However, RT elongations of more than 2% were recorded for microstructures containing up to 63 vol.% O-phase. Specimens subjected to 650°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°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 + β alloy.