Effects of microstructural morphology on quasi-static and dynamic deformation behavior of Ti-6Al-4V alloy
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I. INTRODUCTION
Ti-6Al-4V alloy is a typical ␣ ⫹  Ti alloy having low density, high specific strength, outstanding resistance to corrosion, and excellent high-temperature properties.[1–5] This alloy is characterized to be sensitive to microstructural variations such as volume fraction of ␣ and  phases, phase morphology, and crystallographic arrangement. Its microstructure is largely divided into Widmansta¨tten, equiaxed, and bimodal microstructures according to heat treatment conditions. The Widmansta¨tten microstructure is known for high creep strength and fracture toughness, and excellent resistance to crack propagation, while the equiaxed microstructure yields high tensile strength and elongation, and excellent resistance to fatigue crack initiation.[6,7] The bimodal microstructure is considered to be a mixture of the two other microstructures. Major microstructural factors affecting mechanical properties of these microstructures are prior  grain size, colony size, thickness of boundary ␣ phase, and volume fractions of ␣, , and tempered martensite. Many researchers have worked on obtaining desired mechanical properties by controlling these microstructural factors through heat treatments or thermomechanical treatments.[8–12] However, most studies to improve mechanical properties of the Ti-6Al-4V alloy have been mainly evaluated under static or quasi-static loading, and very few studies have been done on the deformation and fracture behavior occurring under hostile conditions, such as dynamic loading. Since the Ti-6Al-4V alloy practically undergoes highspeed deformation processing, such as forging and superplastic forming, in order to be applied to aerospace, automotive, and precision machining parts, the dynamic
deformation and fracture behavior should be taken into critical consideration in terms of selection, development, and designing of materials. Particularly, resistance to deformation and fracture under dynamic loading is lower in general than that under quasi-static loading,[13,14] and thus, the dynamic deformation and fracture behavior should be closely examined and evaluated before materials are applied to deformation processing of parts. For instance, plastic deformation is often localized under dynamic loading, which is called an adiabatic shear band. In this localized area, hardness increases greatly, and the capability to carry load seriously deteriorates, thereby causing defects or eventual failures during high-speed processing.[14] Therefore, studies on microstructural modification and high-speed process controlling essentially are required in order to improve dynamic properties of the Ti6Al-4V alloy. In the present study, three microstructures of Widmansta¨tten, equiaxed, and bimodal were obtained by heat treating the Ti-6Al-4V alloy, and their quasi-static and dynamic deformation behavior was investigated. Quasi-static and dynamic torsional tests were conducted on them by using a torsional Kolsky bar, and deformed microstructures and fracture surfaces were observed to investigate various fact
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