Effects of volume fraction of tempered martensite on dynamic deformation properties of a Ti-6Al-4V alloy having a bimoda
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I. INTRODUCTION
THE Ti-6Al-4V alloy has a high specific strength and stiffness, outstanding corrosion resistance and high-temperature properties, and also has a low density,[1–6] which provides a great potential for structural armor plates. In general, armor plates are under dynamic loading such as high-speed compressive or torsional loading and ballistic impact, instead of quasi-static loading. In this case, adiabatic shear bands are often formed, as plastic flow is highly localized before the thermal energy arising from impact is emitted.[7–11] The ballistic performance of armor plates, which can be obtained from high-speed ballistic impact tests, is known to correlate well with dynamic properties as well as with the formation of adiabatic shear bands, but only limited information is available on the dynamic deformation and fracture behavior of the Ti-6Al-4V alloy. Mechanical properties of the Ti-6Al-4V alloy are largely affected by its microstructure, which is classified into Widmanstätten, equiaxed, and bimodal microstructures, according to heat-treatment conditions. Among them, the bimodal microstructure shows a better combination of strength and ductility than the other two microstructures.[12] Recently, Lee et al.[13] investigated the possibility of adiabatic shearband formation in these microstructures by analyzing how deformation energy was distributed and consumed by either void initiation or adiabatic shear banding. Their results indicated that the bimodal microstructure could have excellent DONG-GEUN LEE, Postdoctoral Research Associate, and YOU HWAN LEE, Research Assistant, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784 Korea. CHONG SOO LEE and SUNGHAK LEE, Professors, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, are jointly appointed with the Materials Science and Engineering Department, Pohang University of Science and Technology. Contact e-mail: shlee@ postech.ac.kr Manuscript submitted April 14, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
ballistic performance, because the possibility of adiabatic shear-band formation was the lowest of the three microstructures. It was also confirmed that the ballistic performance of the bimodal microstructure was better than that of the equiaxed microstructure after high-speed ballistic impact tests and exploding-bridge wire tests.[14,15] However, detailed studies of the microstructures and heat-treatment conditions under which adiabatic shear banding is suppressed while excellent mechanical properties are promoted, are required. For example, critical microstructural factors such as the size and volume fraction of phase and tempered martensite affect the quasi-static and dynamic deformation behavior,[13,16–18] but studies on these have not been made yet. In the present study, bimodal microstructures were obtained by varying the heat-treatment conditions of the Ti6Al-4V alloy, and the effects of microstructural factors such as the volume fraction of
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