Microstructural evolution of a nanocrystalline Ti-47Al-3Cr alloy on annealing at 1200°C

Abstract

Because of their attractive properties including high strength-to-density ratio, excellent creep resistance at elevated temperatures and good oxidation resistance, ordered intermetallic titanium aluminides bases on Ti{sub x}Al (x = 1 or 3) are attractive candidates for applications in aerospace and airframe components. However, the major drawbacks to their possible utilization are their low ductility and toughness at ambient temperatures and poor workability. In recent years, far-from-equilibrium processing techniques with ternary alloying additions have been employed to improve these characteristics. It is possible to produce homogeneous and high-quality nanometer-size-grained powders by combining the two far-from-equilibrium techniques: gas atomization (GA) and mechanical alloying (MA). High-energy ball milling of the gas-atomized (GA) powder causes the breakdown of long-range-ordered superlattices resulting in disordered solid solutions or amorphous phases. The hot isostatic pressing (HIP`ing) process can be efficiently used to compact these powders while retaining the nanosized grains and achieving full density. A question however arises about the high temperature stability of the nanometer-sized structure. In the present work, a fully dense nanocrystalline TiAl-base alloy was prepared by HIP`ing of MA/GA pre-alloyed powder. The microstructural evolution of the nano-grained structure during annealing at T = 1200 C was studied using TEM, SEM, EDS and XRDmore » techniques. A particular attention was paid to an analysis of grain growth and twin formation.« less