Phase equilibria and transformations in the Ti-Al-Nb system

The phase equilibria and transformations in the Nb-Ti-Al system in the vicinity of the Sigma phase field have been examined with respect to the Liquidus Surface (Phase 1), the Phase Equilibria and Equilibrium Transformations (Phase II), and the Phase Transformations, Mechanisms and Kinetics (Phase III). Eight alloy compositions were produced by arc melting. The alloys were characterized by differential thermal analysis, metallography, X-ray diffraction, scanning electron microscopy (BSEI), electron probe microanalysis and transmission electron microscopy.; The liquidus examination shed new light on previous microstructural interpretations, opening up new possibilities for microstructural development and control of multiphase alloys. Differential thermal analysis has identified the existence of a β to σ + γ transformation in an alloy where it was not previously thought to exist. The results differed from the calculated diagram by higher titanium solubility in the σ and δ phases than predicted at lower temperatures and a lower solubility of α2 and γ.; The high temperature βo transforms to γ + σ in a eutectoid fashion resulting in a desirable lamellar structure of σ and γ. The existence of a new body centered tetragonal crystal structure ao = 5.11Å and co 28.12Å with a point group symmetry of P4/mmm, at 700°C was discovered.; A plethel section was found for the σ + β two phase alloys. A βo + O + σ three phase field passes through the alloys between 981 and 1000°C. The plethel section at the transformation has an eutectoid characteristic, however, the nature of the transformation changes to a peritectoid.; At temperatures below 970°C, the first transformation to occur is the decomposition of the metastable βo phase to an intermediate metastable phase O′. Reasonable values for Q were obtained, applicable to the diffusion limited region of the TTT-curve.; The microstructure evolution of the σ + β morphologies established at 1100°C, was determined for various cooling rates (water quenched, air cooled, vermiculite cooled, and furnace cooled). The results indicated a gradual increase in the volume fraction of the β to O phase transformation with slower cooling rates. There was no apparent effect to the σ morphology or distribution.; With very high quench rates, the β to βo ordering transformation was observed during TERM analysis to occur at approximately 1138°C in alloy 1. Differential thermal analysis of alloy 1 suggests the β to ω ″ transformation occurs at 930°C on heating (15°C/min) and is a reversible transformation.; The 890°C series heat treatments confirmed the formation of the ω ″ and suggest the new tetragonal phase observed at 700°C may be an intermediate transitional phase formed due to the difficulty of nucleation and growth of the σ phase at low temperatures. (Abstract shortened by UMI.)...