Effects of Strain Rate and Prestraining on Tensile Behavior of Duplex Gamma Titanium Aluminides
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EFFECTS OF STRAIN RATE AND PRESTRAINING ON TENSILE BEHAVIOR OF DUPLEX GAMMA TITANIUM ALUMINIDES
D. S. SHIH, D. S. SCHWARTZ, and J. E. O'NEAL McDonnell Douglas Aerospace, MC 111 1041, P. O. Box 516, St. Louis, MO 63166-0516
ABSTRACT The effects of strain rate and prestraining on tensile behavior of two-phase (y+a2) titanium aluminides at 20 and 730'C have been investigated. At 20*C the elongation remains at about 1.4% as the strain rate increases from 5x10- 5 to 5x10-2 s-1 and it drops to nearly zero at 5x10-1 s-1. At 730°C (i.e. above DBTIT) the plastic strain is about 13% when tested at 5x10- 5 s-1., while it reduces significantly to less than 3% at 5x10-4 and 5x10-2 s-1. Again, the elongation is about zero at the highest strain rate tested, 5x10- 1 s-1. Regardless of the strain rate, fracture by an intergranular mode of the primary equiaxed 'y appears to increasingly dominate as temperature changes from 20 to0 730'C. Introduction of prior plastic deformation by prestraining beyond yielding at 945 C obviously increases the 20°C yield stress, however, with little influence on ductility. Transmission electron microscopy reveals that a number of dislocation loops are produced during prestraining. These loops are generally immobile resulting in the observed increase of flow stress and unchanged ductility. INTRODUCTION For the past 5-6 years, research and development efforts on two-phase (Y-+a2) titanium aluminides have made a significant progress in advancing the technology. The progress is mostly reflected on alloy development, microstructure-property relationship, and deformation behavior [1-4]. While the mechanical behavior of two-phase (y+cL2) titanium aluminides is believed to be very sensitive to strain rate, limited work has been conducted and the understanding is far from being complete. It is well established that a ductile-brittle transition temperature (DBTT) exists in tensile behavior of two-phase (y+a2) titanium aluminides; the DBTI typically ranges between 550 between 750°C depending strongly on alloy chemistry and microstructure [1]. For a duplex microstructure, the DBTI is generally about 650'C [2]. Furthermore among other factors, strain rate is considered to have an impact on DBTI. An increased strain rate tends to increase the friction stress, which should raise the DBTT. For steels the DBT" increases by about 15°C for each order magnitude of increase in strain rate [5]. Therefore, one purpose of this study is to investigate the effects of strain rate on tensile behavior at room temperature and at 730°C (which is above the DBIT under a normal strain rate of 1-5x10 4 s-1 ) of a binary near y -TiAl alloy having a duplex microstructure. Introduction of prior plastic deformation at 945'C could possibly result in two beneficial effects in y-TiAl based aluminides. Firstly, it may increase room temperature ductility by activating normally immobile slip systems, e.g. on non-{ 111) planes. Secondly, prestraining may promote strain hardening by increasing specific fracture energy, thereby lowering DBT-I. Th
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