Effects of Structure on Creep of Ti-53.4mol%Al Intermetallics
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EFFECTS OF STRUCTURE ON CREEP OF Ti-53.4mol%AI INTERMETALLICS Tohru TAKAHASHI and Hiroshi OIKAWA Tohoku University, Dept. of Materials Science, Faculty of Engineering, Aramaki Aoba, 980 Sendai, JAPAN ABSTRACT Constant stress compression creep behavior at elevated temperature was investigated on Ti-53.4mol%AI polycrystalline intermetallics of gamma single phase in different grain morphologies: coarse columnar grains, fine The shape of creep curves depended recrystallized grains and equiaxed grains. on the applied stress: normal primary transient followed by steady creep was found at higher stresses, but at lower stresses steep acceleration was This behavior was common to all observed after taking minimum creep rates. the grain morphology. The creep rate itself was not greatly different among the three types of structure, and the stress exponent of about 4 and the apparent activation energy of about 340kJ/mol at 0.3 true strain were common to all three types. The shape and size of grains do not affect the essential creep features of this material. INTRODUCTION Titanium aluminide intermetallics, TiAI, is widely received as a very promising material because of its light weight and high specific strength at Its lack of room-temperature ductility has been the elevated temperature[I]. crucial problem which prevents TiAI intermetallics from being applied to Recent attempts to improve the room-temperature ductility practical use[2,31. When we are to make by macro- and micro-alloying achieved 1.5% ductility[4]. at elevated temperature, the fundamental good use of the excellent strength mechanism is and its underlying temperature strength about high understanding essential. Yield/proof stress or flow stress of TiAI intermetallics at elevated temperature has been measured at relatively high strain rates which are common in "hot-working"[2,5,6]. Experimental information about the mechanical behavior under low strain rates, however, are scarce, which are essential when considering performance as heat-resisting materials in long-term service [4,7,8]. In this study, creep behavior under constant compressive stress was investigated in polycrystalline materials of titanium aluminide intermetallics comprised by TiAI single phase for a basic research, with a special reference to the effects of metallographic structure exerted on the creep behavior. EXPERIMENTALS Titanium sponge of 99.8% purity and aluminum shots of 99.9% purity were melted together in a calcia crucible by high frequency induction heating under reduced argon atmosphere and cast into metal mould which was 30mm in diameter. Chemical composition of this ingot was Ti-53.4mol%AI for its main constituents (hereafter this material is referred to Ti-53.4AI), and the major impurity was about 1000ppm of oxygen. Three types of specimens which received different thermo-mechanical treatments were prepared from this ingot: cast and vacuum-annealed material (CA), isothermally-forged material (F) and isothermally-forged and vacuumFig.1 shows the optical micrographs of CA, F and FA ann
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