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 (ϵ_f>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.

Acknowledgments

This work was supported by the National Science Foundation through grants DMR-0533954 and DMR-0320992 and the New York State Office of Science and Technology for Academic Research (NYSTAR) Grant No. C020080. The authors are grateful to Dr Dingqiang Li of Alfred University for performing electron microscopy analysis on selected samples.

Notes

All alloy compositions are given in atomic percent.