Impact Response and Microstructural Evolution of Biomedical Titanium Alloy under Various Temperatures
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TITANIUM alloys are characterized by a high strength to weight ratio, excellent corrosion resistance, and good formability. Consequently, such alloys are extensively used for applications requiring light-weight, chemically-inert components. In the mid 1980s, a + bphase Ti-6Al-4V was commonly regarded as an ideal material for medical implants.[1,2] However, recent research has revealed that Ti-6Al-4V debris contains Group V chemical elements, which are harmful to human health.[3] Accordingly, researchers have shown increasing interest in the potential of b-phase Ti alloy for medical applications.[4,5] In general, this alloy is known to have good biocompatibility characteristics and favorable mechanical properties. However, its dynamic mechanical behavior under the high-strain-rate and temperature conditions experienced during typical manufacturing processes, i.e., forging and machining, are poorly understood. As a result, the precise shape and mechanical properties of manufactured Ti-alloy components are not easily predicted. In general, strain rate has a crucial effect on the mechanical behavior of deformed materials. Although WOEI-SHYAN LEE, Distinguished Professor, TAO-HSING CHEN and HSIN-HWA HWANG, Graduate Students, are with the Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan R.O.C. Contact e-mail: wslee@mail. ncku.edu.tw Manuscript submitted October 15, 2007. Article published online April 5, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
the flow stress increases with increasing strain rate in engineering materials, the actual effect of the strain rate is very much dependent on the nature of the tested material.[6–9] Besides the strain rate, the temperature is also known to have a significant effect on the mechanical properties and microstructure of deformed materials. In general, a higher temperature reduces the internal resistance to dislocation movements and results in plastic flow. However, due to the combined effects of work hardening and work softening caused by impact loading and elevated deformation temperatures, respectively, predicting the overall plastic-flow response is problematic. Intuitively, it seems reasonable to assume that the variations observed in the stress-strain curves of impacted specimens are related to the microstructural evolution of the deformed material.[10] Accordingly, observations of the deformed microstructures of impacted specimens can provide useful insights into the strengthening mechanisms and plastic deformation behavior of materials subjected to deformation under different strain rate and temperature conditions. Dislocation slip is known to have a significant effect on the flow- stress curves of plastically deformed materials. Under high-strain-rate loading conditions, the rapid multiplication of dislocations suppresses the dislocationslip phenomenon and induces a strengthening effect.[11,12] However, under high-temperature deformation conditions, the increased temperature not only increases the annihilation of dislocations but also
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