Trigger Stress for Stress-Induced Martensitic Transformation during Tensile Deformation in Ti-Al-Nb Alloys: Effect of Gr
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INTRODUCTION
TITANIUM alloys containing b stabilizing elements such as Nb show sluggish b to a phase transformation. This allows a number of metastable phase transformations such as b to martensite (a¢¢) or b to omega to occur during quenching.[1] The occurrence of these transformations can significantly affect the mechanical properties of these alloys. Therefore, a detailed understanding of the factors affecting the instability of b in Ti-Al-Nb system leading to b to martensite (a¢¢) transformation is of particular significance. The b to martensite transformation can be either thermally induced or stress induced depending upon the alloy composition and test temperature. The stressinduced martensitic transformation (SIMT) in such alloys is manifested in a tensile stress-strain plot. The alloy undergoing SIMT exhibits a typical stress plateau in the tensile stress strain curve (Figure 1), as reported by Duerig et al. in the Ti-10V-2Fe-3Al alloy,[2,3] Grosdidier et al. in Ti-4Mo-2Cr-1Fe-5Al-5Zr-2Sn (b-Cez),[4,5] Sasano, and Suzuki in Ti-Mo-Al alloys,[6] Lee Park et al. in Ti-15.4V4Al,[7] Lei et al. in Ti-11Mo-3Al-2V-4Nb,[8] Wu et al. in Ti-10Mo-3Al-2V-4Nb,[9] and Margevicius et al.[10] in Ti-Ta alloys. The trigger stress required for such a stressinduced transformation can be estimated by a simple tensile test and is defined in Figure 1 as an intersection of elastic modulus line with the extended plateau. The primary factors that control the stress required to trigger the transformation are test temperature[3] and Dr. ARCHANA PARADKAR and Dr. S.V. KAMAT, Scientists, are with the Defence Metallurgical Research Laboratory, Hyderabad 500058, India. Contact e-mail: agparadkar@rediffmail. com Dr. A.K. GOGIA, Director, is with the Project Office (Materials), Kaveri Engine Programme, Hyderabad 500058, India. Dr. B.P. KASHYAP, Professor, is with the Department of Metallurgical Engineering and Material Science, Indian Institute of Technology, Mumbai 400076, India. Manuscript submitted September 15, 2006. Article published online February 5, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
grain size.[4,5,11,12] The urge to undergo phase transformation under stress is also related to the metastability of b, which in turn is dictated by b composition. Thus, trigger stress should also depend upon the composition of the alloy under consideration. The effect of temperature on trigger stress has been reported in Ti alloys.[3] It has been observed that the trigger stress varies with the testing temperature and goes through a minimum at about room temperature.[3] Such a temperature dependence of trigger stress has been attributed to the interplay of variation of chemical driving force with temperature and the occurrence of competitive phase transformations such as b to omega formation.[3] In contrast, no systematic experimentation has been carried out to study the effect of b grain size (at a constant test temperature) in any alloy system. Earlier studies by Grosdidier et al.[4,5] have shown that an increase in b grain size decreases the tri
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