The effect of strain-path reversal on cavitation during hot torsion of Ti-6Al-4V
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lk hot working of engineering alpha/beta titanium alloys such as Ti-6Al-4V is commonly used to convert coarse grain, colony-alpha ingot microstructures into fine, equiaxed-alpha billet structures. Despite the apparent simplicity of the processes employed (e.g., open-die forging, cogging, and upsetting), relatively complex states of stress and strain are typically generated within the workpiece; the complexity of industrial operations is also increased due to changes in strain path. Furthermore, processing may give rise to undesirable defects in finished wrought products. These defects include gross fracture, shear bands, and internal cavities. The initiation and growth of internal cavities is particularly important because such deleterious defects may eventually lead to gross fracture during subsequent manufacturing steps or in service.[1,2,3] A considerable amount of research has been devoted to developing an understanding of cavitation behavior for a wide range of metals and alloys.[4,5,6] Most of these efforts have focused on the determination of the conditions under which cavitation can be fully suppressed (or at least minimized) or quantifying cavity-growth kinetics as a function of a stress state.[7,8,9] In the majority of early research on cavitation kinetics, the stress ratio (i.e., the ratio of hydrostatic-to-effective stress) was usually equal to or greater than that corresponding to uniaxial tension, viz. 1/3. By contrast, Bae et al.[10] and Nicolaou et al.[11] have recently investigated cavitation under a macroscopic state of stress consisting of simple shear. For example, Nicolaou et al. used hot torsion testing to establish the effect of shear deformation on the cavitation behavior of the alpha/beta titanium alloy Ti-6Al-4V with a colony-alpha microstructure over a wide range of strains. Local texture and thus the P.D. NICOLAOU, Consultant, is with the Materials and Processes Division, UES, Inc., Dayton, OH 45432-1894. S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/ MLLM, Wright-Patterson Air Force Base, OH 45433-7817. Manuscript submitted March 20, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
nonuniform deformation associated with adjacent hard and soft colony orientations were taken into account in the analysis. The results were compared to other findings for the uniaxial- and notched-tension testing of Ti-6Al-4V with an identical (colony-alpha) microstructure.[7,12] The present work is a continuation of earlier efforts on cavitation during monotonic torsion testing. The specific objective of this research was to establish and model the effect of strain-path reversal on the cavitation behavior of the Ti-6Al-4V alloy. To this end, hot torsion tests comprising forward and reversed straining were conducted. The cavitation in deformed specimens was characterized using optical and scanning electron microscopy. The experimental measurements were interpreted in the context of several different
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