Phase identification and effect of W on the microstructure and micro-hardness of Ti 2AlNb-based intermetallic alloys

Abstract

The O-phase (orthorhombic Ti 2AlNb phase) intermetallic alloy has been considered one of the strongest materials for high temperature application. The primary compositions of the test alloys in this paper are Ti–22Al–27Nb (at.%) and Ti–22Al–20Nb–2W (at.%). By in-situ X-ray diffraction (XRD) tests, each phase is identified and the lattice parameters of each phase are calculated for different temperatures. XRD tests show that a Ti 2AlNb-β phase, partially ordered b.c.c. phase, exists besides O, α 2 and B2 that are usually expected in the Ti–Al–Nb alloy system. In this paper, the effects of tungsten on the microstructure and micro-hardness of a Ti 2AlNb-based (O+b.c.c.) alloy are investigated. The effects of W on Ti 2AlNb-based intermetallic alloys are widely known to cause a refinement of microstructure and improvement of mechanical strength, however, the reason has not yet been reported. In this paper, the role of W in the improvement of mechanical strength is clarified. The main effect of W addition is not the formation of a solid solution or the formation of W-precipitates but the refinement of the Widmanstätten structure.