Structure and Microstructure of Al 2 Ti Intermetallic Alloy
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STRUCTURE AND MICROSTRUCTURE OF AI 2 Ti INTERMETALLIC ALLOY JOHN E. BENCI*, JOHNScience C. MA*,and AND THOMASWayne P. FEIST** *Department of Materials Engineering, State University, Detroit, MI 48202. "**Central Research and Development, E.I. DuPont de Nemours & Company, Inc., Wilmington, DE 19880. ABSTRACT Alloys based on intermetallic compounds in the Ti-Al binary system have been extensively investigated as candidate light weight structural materials for elevated temperature applications. However, one compound in this system, AI 2 Ti, has been virtually ignored. AI 2Ti, like A13Ti, is expected to possess lower density and better oxidation resistance compared to Ti3 AI and TiAl based alloys. Since the structure and stability of AI 2Ti are poorly understood, this work was undertaken to determine the compound's equilibrium crystal structure and the microstructures in the as-cast, melt-spun, and annealed conditions. Powder X-ray diffraction shows that annealed melt-spun AI 2Ti has the tetragonal Ga 2 Hf crystal structure containing 24 atoms per unit cell with a = 3.9705 A, c = 24.322 A and a calculated density of 3.530 g/cm 3 . Optical microscopy and scanning electron microscopy with energy dispersive spectroscopy were used to investigate the material's microstructures. Cast AI2Ti has a small volume fraction of TiAI second phase particles. Rapidly solidified and annealed AI 2Ti is single phase. Powder processing (melt spinning and hot isostatic pressing) A12 Ti yields material with a finer microstructure, a higher hardness, and increased resistance to room temperature cracking than as-cast material. Additionally, microhardness results indicate that AI 2 Ti has an increased resistance to cracking at room temperature than identically processed AI 3Ti with a similar hardness. INTRODUCTION Low densities, high moduli, and high melting temperatures make intermetallic compounds in the Ti-Al binary system attractive for high temperature structural applications. 1 The majority of research and development work has concentrated on Ti3Al-based alloys, ,2 with a substantial amount of attention also focused on TiAI 3 ,4 and AI 3 Ti alloys. 1 ,5 -7 By contrast AI 2Ti is a virtually unexplored intermetallic compound in this system. The higher aluminum content of A12Ti would be expected to impart oxidation resistance superior to, and result in a lower density than, Ti3 AI and TiAI. AI 2 Ti is also likely to possess a higher fracture toughness than AI3Ti. The AI 2Ti phase was first identified by Potzschke and Schubert 8 who determined it has a crystal structure isotypic with Ga2Hf. The binary Ti - Al phase diagram published by Murray 9 included an AI2Ti phase with the Ga 2 Hf structure, fig. 1. However, the AI 2Ti portion of the phase diagram was bounded by dashed lines, indicating uncertainty associated with its composition and stability. According to Murray's diagram, upon heating, AI 2 Ti first decomposes into two phases, y-TiAI and 8-Ti 2Al5/Ti 9Al 23 , at approximately 1240'C, and the first liquid phase starts to form at a tempe
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