Mechanical Twinning in Duplex Ti-48Al-2Nb-2Cr at 1038K
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MECHANICAL TWINNING IN DUPLEX Ti-48A1-2Nb-2Cr AT 1038K Zhe Jin and Thomas R. Bieler Department of Materials Science and Mechanics Michigan State University, East Lansing, Michigan 48824-1226 ABSTRACT As a part of an ongoing study of creep deformation mechanisms, mechanical twinning behavior was investigated in the lamellar region of a creep specimen. A multistress jump creep test was performed and the microstructures before and after deformation were investigated using optical and transmission electron microscopy. The identification of mechanical twins in a lamellar microstructure is discussed. Extensive mechanical twinning was observed in lamellar regions, in addition to slip and subgrain formation. The occurrence of mechanical twinning depended on the lamellar orientation with respect to the tensile axis. The mechanical twins are analyzed and discussed in terms of possible crystallographic twinning systems. In this case, a maximum resolved shear stress criterion for mechanical twinning is proposed to account for the observed orientations. INTRODUCTION Gamma Titanium aluminide (TiAl) has long been considered as a potential candidate material in high temperature and high performance applications. In the past decade, much work has been done in order to understand its deformation mechanisms both at room temperature and high temperatures and to improve its ductility at low temperature. Most experiments have been conducted in short times, such as tensile strength measurements [I3]. Much less has been done with long term measurements such as creep experiments. Since one goal of TiAI components will be to replace nickel and cobalt base superalloys in aircraft applications, understanding creep deformation at high temperature is necessary. Some microstructure investigations in creep deformed TiAl [4-9] have shown that creep deformation occurred by a combination of dislocation slip, mechanical twinning, subgrain formation, grain boundary sliding, and recrystallization. It has been clear from the these previous studies that the contribution of these deformation mechanisms to creep deformation will be different for the different testing conditions. For example, creep tests in an investment cast Ti-48Al-2Nb-2Cr show power-law creep with stress exponent n=3 at low stresses and n=7 at higher stresses [4], but the rate limiting mechanisms of creep deformation have not yet been identified. For an individual deformation behavior, there are some differing observations and interpretations. For instance, the mechanical twinning was observed in some specimens [5,8,9] but not in others [6]; pseudo-twinning was found in a creep deformed TiAl specimen [8]. The objective of this paper is to present some preliminary observation and analysis of mechanical twinning in creep deformed investment cast Ti-48A1-2Nb-2Cr.
Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Society
776
EXPERIMENTAL PROCEDURES The material used in this study was investment cast y TiAl made by Vacuum Arc Remelting (VAR) and casting at Howmet Corp., Whiteh
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