Transitional Creep Mechanisms in Textured Low c/a-Ratio Hexagonal Close Packed Metals

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TECHNICAL SUMMARY: Ti and Zr alloys find extensive applications in transportation and energy technologies respectively in addition to other applications such as in biomedical and chemical industries. Particular needs arise in the basic understanding of the underlying deformation mechanisms in these textured alloys as lower stresses are encountered, comparable to those under service conditions. Recent investigations on fine grained Ti3Al2.5V alloy tubing revealed a new deformation mechanism at lower stresses, under which strain-rates were found to be orders of magnitude higher than those predicted by diffusional creep mechanisms such as Coble creep. In addition, under testing conditions where dislocation creep is predominant, it has been shown that climb of edge dislocations controls creep at lower stresses while jogged screw dislocations make significant contribution at higher stresses. Microstructures following creep are required to differentiate these mechanisms since the measurement of creep parameters such as the stress exponent and activation energy cannot distinguish between them. Further investigations using different loading conditions, on other Ti and Zr alloys, are imperative. These low c/a-ratio hexagonal metals exhibit crystallographic textures that lead to complex biaxial anisotropy that must be properly taken into account. The proposed research addresses these important aspects through studies involving not only standard uniaxial creep tests, but also the characterization of anisotropic biaxial creep using closed-end internally pressurized thin-walled tubing superimposed with axial loading.NON-TECHNICAL SUMMARY: Zr and Ti alloys are commonly used as structural materials in the nuclear and chemical industries as thin-walled tubing subjected to complex biaxial loadings. In order to predict the dimensional changes of the structures made from these materials, a thorough understanding of the deformation mechanisms is needed. This is especially true of the changes in the mechanism(s) as lower stresses, equivalent to those under typical operating conditions, are approached. These transitions in time-dependent deformation (i.e., creep) can lead to dangerous non-conservative estimates of the strain-rates and lifetimes by blind extrapolation of the short-term high-stress and high-temperature data to these low levels. The current proposed study emphasizes the transitional creep mechanisms in these important structural alloys. Students exposed to these experimental and modeling efforts will be able to appreciate the complex phenomena that need to be addressed in attacking real life problems. The project will actively participate in Young Investigator programs for high school students during the summers by involving these students in the research.