Microtexture Analysis and Modeling of Ambient Fatigue and Creep-Fatigue Damages in Ti-6Al-4V Alloy
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INTRODUCTION
TITANIUM alloys have been widely used in aerospace, biomedical, and high-performance sporting good applications. A wide range of physical and mechanical properties in these alloys can be achieved by alloying Ti. Depending on the predominant phase or phases in their microstructures, titanium alloys are categorized as a, a/b, or b alloys. Among these alloys, Ti-6Al-4V has the largest share of the present aerospace market, mainly because it exhibits reasonable strength and ductility due to modest quantities of a and b stabilizers (Al and V, respectively).[1] This alloy has been extensively used in the compressor modules of aero-engines. A large number of compressor components are life-critical. The aero-engine experiences a period of dwell at high speeds during the initial phases of operation, which leads to the ambient creep-fatigue conditions in the first stage of compressor modules. The fatigue and the creep-fatigue damages may initiate at geometrical constraints such as bolt–hole, blade roots, etc. The fatigue /creep-fatigue damages are quite restricted at these locations so that their effects on global parameters, e.g., vibration signatures, are detectably minimal. As the large part of operation life is spent in the incubation and crack-initiation phases, a comprehensive approach is required to JALAJ KUMAR, Scientist, is with the Defence Metallurgical Research Laboratory, Hyderabad, 500058, India, and also with the Metallurgical and Materials Engineering Department, IIT Madras, Chennai, 600036, India. Contact e-mails: [email protected], k_jalaj@ yahoo.com A.K. SINGH and VIKAS KUMAR, Scientists, are with the Defence Metallurgical Research Laboratory. S. GANESH SUNDARA RAMAN, Professor, is with the Metallurgical and Materials Engineering Department, IIT Madras. Manuscript submitted June 2, 2016. Article published online December 2, 2016 648—VOLUME 48A, FEBRUARY 2017
address these issues. This involves detailed understanding of deformation and damage on a microscale before the development of a detectable crack, especially so for titanium alloys with relatively large grain size, e.g., Ti-6Al-4V.[2] While state-of-the-art microstructure characterization facilities are available, analytical tool for widespread applications requires addressing the universal laws such as the first and the second law during loading/unloading operations of materials. The input mechanical energy during the fatigue and the creep-fatigue loadings mostly gets dissipated in the form of heat and the rest is stored in the material. This stored energy is accountable for all fatigue- and creep-fatigue-related micromechanisms involving dislocation and defects.[3–5] Fatigue phenomena have been previously studied using empirical correlation involving these energies.[6] This is based on the fact that an error in stress level and inelastic strain can be minimized with the application of energy formulations. However, most of these formulations consider damage as a mechanical variable with less emphasis on the details of nitty-gritty of its e
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