An Investigation on Dynamic Tensile Properties of TiAl Intermetallic Alloy
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ABSTRACT Room-temperature tensile properties of polycrystalline Ti-47A1-2Mn-2Nb alloy with nearly lamellar (NL) microstructures were investigated at the strain rates between i1 5 and 1000 s-1 using a self-designed Split-Hopkinson tensile bar setup with a rotating disk and conventional testing machine. It was found that tensile ductility varies within a narrow range with the strain rate while dynamic strengths (yd) of the alloy are obviously higher than static strengths (as). There exists a linear relationship between cas and the logarithm of the strain rate (lnt), and between ed and the strain rate itself (t). Fractography analysis indicated that the alloy fractured in a mixed mode of predominant transgranular cleavage and minor intergranular cracking under static and dynamic strain rates. Environmental effect was excluded from the main cause for the room-temperature brittleness of the investigated alloy. INTRODUCTION TiAl based alloys, usually composed of a major phase of --TiAl and a minor phase of ct2-Ti 3Al, have received increasing attention recently because of their potential as advanced high-temperature structural materials, which arises from their high specific strength and stiffness, excellent oxidation resistance, and low density [1]. Thanks to extensive efforts, rapid advance has been made in understanding the mechanical properties of TiAl alloys. Most of such works were carried out under static loading conditions, with corresponding strain rates usually lower than Is-1. The behavior of the materials under dynamic or impact loading, however, is still not well understood. Unfortunately, some potential utilization of TiAl alloys, such as compressor blades in jet engines or turbo-charger in automobile engines, will subject the alloys to dynamic or shock impulse loads, which necessitates the knowledge of mechanical properties at high strain rates. Chin et al [2] and Maloy et al [3-7] investigated mechanical response to dynamic compression in Ti-48A1-(5-15)vol%TiB and some intermetallic alloys including Ti-48A12Nb-2Cr under the strain rates of 500 and 2000s-, respectively. Sun et al [8], using a servohydraulic high velocity testing machine, carried out tensile test and fracture toughness tests at loading velocities up to 12m/s or strain rates up to 320s-'. Wang et al [9], using a self-designed Split-Hopkinson tensile bar (SHTB) setup, investigated Ti-47AI-1.5Cr-0.5Mn-2.8Nb alloy under the strain rates between 5x10-4 and 800s-1. In this paper, the same SHTB setup was used to study tensile properties of polycrystalline Ti47A1-2Mn-2Nb alloy with nearly lamellar microstructures under the strain rates between 20 and 103s-1. Dynamic properties were compared with static properties obtained from conventional tensile testing. EXPERIMENTAL The investigated Ti-47at%A1-2at%Mn-2at%Nb (atomic percent) alloy was produced by ingot metallurgy [10]. Its initial microstructure was etched by a mixture of 2.5vol%HF+ 2.5 vol%HNO 3 +95vol%H 20 and observed by a Neophot-II optical microscope. KK5.5.t Mat. Res. Soc. Symp. Proc. Vol.
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