An experimental and theoretical investigation of the effect of local colony orientations and misorientation on cavitatio
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I. INTRODUCTION
THE hot working of metals is often accompanied by the formation of internal cavities. For example, one of the principal objectives of the hot working of / titanium alloys is to convert the transformed, colony- microstructure developed during primary processing of ingots into a uniform, fine equiaxed- microstructure. During the open-die forging processes that are typically used for such operations, secondary tensile stresses are often generated and may lead to the nucleation and growth of internal cavities at the prior- grain boundaries. Similarly, cavitation is often observed during superplastic forming of a number of engineering alloys. Such cavities may lead to premature failure or result in finished parts with degraded mechanical properties, thus severely restricting service integrity.[1,2,3] In previous work, the kinetics and mechanisms of cavitation during hot working of Ti-6Al-4V with a colony- microstructure (i.e., lamellar in equiaxed colonies) were established.[4,5,6] In polished sections of deformed hot tension samples, for instance, cavities were found to nucleate preferentially on prior grain boundaries perpendicular to the tension axis. Colonies adjacent to such boundaries were oriented with their laths nearly parallel to the tension axis. The lamellar- phase tends to have an {hk.0} plane parallel to the - THOMAS R. BIELER, Associate Professor, is with the Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824-1226. Contact e-mail: [email protected] P.D. NICOLAOU, R&D Scientist, is with S&B S.A., 106 72 Athens, Greece. 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 October 21, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
interface;[7] these planes contain the c-axis of the phase. It was thus hypothesized that cavities were most likely to form at boundaries between colonies that have the c-axis nearly parallel to the tension axis. When the c-axis is aligned with the tension axis, the resolved shear stress for a slip on basal or prism planes vanishes. Therefore, only slip systems with much higher values of critical resolved shear stress (CRSS) such as c a slip (or twinning) can operate.[8] The preceding hypothesis could not be proven, however, because an optical micrograph is not sufficient to identify the orientation of the c-axis. Hence, the present work was undertaken to examine how local texture affects the initiation and growth of cavities during hot deformation. For this purpose, orientation-imaging microscopy (OIM) was used to unambiguously identify crystallographic orientations and to correlate them to cavitation observations. This information was used to formulate and validate a simple model for predicting the influence of local texture on cavity growth and thus the cavity sizes that may be developed during hot de
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