METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 29A, MARCH 1998—943 Effect of Deformation Temperature on Fatigue and Fracture Behavior in TiAl Polysynthetically Twinned Crystals Y. UMAKOSHI, H.Y. YASUDA, T. NAKANO, and K. IKEDA The temperature and orientation dependence of cyclic deformation, fatigue life, and fracture behavior in TiAl polysynthetically twinned (PST) crystals were investigated, focusing on the change of plastic strain energy and deformation mode in the  domains. Stress-controlled fatigue tests were performed at 1 or 10 Hz using the same stress amplitude in tension and compression (R =-1) over a tem- perature range from -196 °C to 700 °C. The fatigue strength at  = 45 deg ( being the angle between the loading axis and lamellar planes) decreased monotonically with increasing temperature. At  = 0 deg, the fatigue strength was high up to 500 °C, but the fatigue life decreased rapidly above 600 °C because of dynamic recovery and interlamellar separation. The plastic strain energy– stress amplitude curves in specimens fatigued with  = 45 deg increased monotonically with stress amplitude for all temperatures and for higher temperatures with  = 0 deg. At 25 °C and -196 °C with  = 0 deg, three regions in the plastic strain energy–stress amplitude curves were observed. This anomalous change in the plastic strain energy at lower temperatures was due to a transition in primary deformation mode between twinning and slip by ordinary dislocations in some domain orientations. I. INTRODUCTION SINCE the discovery of a good combination of high strength and good ductility in two-phase Ti-rich TiAl al- loys, a great deal of attention has been paid to gamma TiAl as a potentially attractive material to satisfy the strong de- mand from the aerospace and automobile industries for new lightweight, heat-resistant materials. Significant progress has recently been made in improving the mechanical prop- erties of the gamma TiAl alloys through alloy development, processing, and microstructure control. [1–5] Structural appli- cations of these alloys require knowledge not only of mon- otonic deformation but also of plastic behavior under cyclic loading conditions. A unique deformation substructure is known to form during the to-and-fro motion of dislocations under cyclic loading. A fundamental understanding of the deformation substructure and fatigue behavior in metals has been obtained from the work conducted on single crystals of fcc metals. [6,7] For example, veins, persistent slip bands (PSBs), and labyrinth and wall structures were all formed in Cu single crystals during fatigue loading, depending on the stress and/or strain amplitudes. [7] The PSBs were ac- companied by parallel arrays of edge dislocation ladders or walls. These bands often produced heavy extrusion/ intrusion steps on the specimen surface, which caused crack initiation leading to failure. In intermetallics of Ni 3 Al single crystals, FeCo-V polycrystals, and TiAl polycrystals, a large number of dislocation dipoles and loops were ob- served instead of ladder structures. [8,9,10] Several investiga- Y. UMAKOSHI, Professor, H.Y. YASUDA and T. NAKANO, Research Associates, and K. IKEDA, Graduate Student, are with the Department of Materials Science and Engineering, Faculty of Engineering, Osaka University, Osaka 565-0871, Japan. This article is based on a presentation made in the symposium ‘‘Fundamentals of Gamma Titanium Aluminides,’’ presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees. tions of fatigue deformation and fracture behavior showed that polycrystalline TiAl alloys have exceptionally high ra- tios of fatigue strength to ultimate tensile strength in fully lamellar microstructured conditions. [11–19] TiAl polysynthet- ically twinned (PST) crystals have been used to obtain fun- damental information on the effect of the lamellar structure on plastic deformation and fracture behavior. [20–28] Recent investigations of the effects of loading axis and lamellar structure on cyclic deformation in TiAl PST crystals found strong anisotropy in cyclic hardening, deformation sub- structure, and fatigue life. [13–19] In monotonic loading con- ditions, plastic behavior such as operative slip and twinning systems, ductility, and yield stress depended strongly on deformation temperature. [20–25] In cyclic loading conditions, polycrystalline TiAl alloys also show a strong dependence of deformation and fracture behavior on microstructure; [12] however, the microstructure effect at the transmission elec- tron microscope (TEM) level has not been investigated in detail. In this article, we report on the temperature dependence of cyclic deformation behavior and fatigue life in TiAl PST crystals, focusing on the effect of lamellae and deformation mode in  domains, under constant stress amplitudes. II. EXPERIMENTAL PROCEDURE Oriented fatigue specimens were cut from TiAl PST crystals containing 49.1 at. pct Al by spark machining into plate fatigue specimens 15-mm long, 5-mm wide, and 2-mm thick, with gauge dimensions of 2  2 mm 2  5 mm. Curved grips of the specimens were tightly fastened by spe- cial metal holders, as shown in Figure 1. Two orientations of  = 0 and 45 deg were selected, where  was the angle between the loading axis and the lamellar planes along a 10 zone in the  phase. The loading axes for each  1 domain and  2 phase in the specimens are shown in Figure 2. The specimen surfaces were cut parallel and perpendic- ular to the lamellar planes. Specimens were first polished