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PLASTIC ANISOTROPY OF Ti 3AI SINGLE CRYSTALS Y. UMAKOSHI, T.NAKANO, K.SUMIMOTO AND Y.MAEDA Department of Materials Science and Engineering, Faculty of Engineering, University, 2-1, Yamada-Oka, Suita, Osaka 565, Japan

Osaka

ABSTRACT The orientation and temperature dependence of the operative slip systems and critical resolved shear stress (CRSS) were investigated in Ti 3AI single crystals containing 24.4at%Al and 33.Oat%Al. Prism {1i0} slip occurs preferentially in comparison with basal (0001) and pyramidal {1121} slip. The CRSS for prism and basal slip decreases with increasing temperature, while that for pyramidal slip exhibits positive temperature dependence having an anomalous peak around 500*C. The temperature dependence of the CRSS for these slip systems does not strongly depend on Al content. The ductility of Ti3AI single crystals is influenced by operative slip systems. Coarse slip bands on the basal plane often act as a trigger for crack nucleation. The compositional deviation to the Al-rich side is harmful for ductility of Ti3AI alloys. INTRODUCTION Ti3AI is a very attractive material with potential application for high-temperature structural use because of its high strength at high temperatures and low density. It has the hexagonal-based D0 19 structure (c.2 phase) at low temleratures and exhibits strong plastic anisotropy. The prism {1010}, basal (0001) and pyramidal {1121} slip systems can be activated in ordered Ti3AI with a near stoichiometric composition depending on crystal orientation [1-3]. The critical resolved shear stress (CRSS) for pyramidal slip is much higher than that for prism slip and activation of pyramidal slip is limited only for an orientation very close to [0001]. Basal slip is also seldom observed due to a high CRSS [1-3]. Such a finite number of available activated slip systems curtails the material's use in structural components. It is well known that TiAI alloys exhibit a superior strength-to-weight ratio and excellent oxidation resistance. In particular, a good balance of higher strength and better ductility is found in two phase TiAI alloys composed of a y matrix and a small amount of c.2 in a lamellar structure [4-91. Most recently, Umakoshi et al. found that the anisotropy of deformation mode of the c.2 phase strongly affects the plastic behavior of lamellar TiAl alloys and they suggested that the control of plastic anisotropy of the cE2 phase holds the key to improvement of ductility of these alloys [10, 11]. In this paper, the temperature and orientation dependence of the activated slip systems and mechanical properties of Ti3AI were investigated using single crystals of a stoichiometric and an Al-rich off-stoichiometric Ti 3AI having a composition similar to that which is in equilibrium with the y phase. EXPERIMENTAL PROCEDURE Master ingots of Ti3AI containing 24.4at%Al and 33.Oat%Al were prepared by melting high purity Ti and Al in a plasma arc furnace. Single crystals with dimensions of 10mm in diameter and 100mm long were grown by the floating zone method at a rate of