The effect of microstructure on localized melting at separation in Ti-6Al-4V tensile samples
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I.
INTRODUCTION
As
described in previous publications, tl-5~ local surface melting during the separation process in Ti-8Mn, Ti 10V-2Fe-3A1, and Ti-6A1-4V (Ti-6-4) tensile samples has been observed and investigated. This phenomenon, which occurs at quasi-static as well as high applied strain rates, is closely associated with the formation of concentrated shear bands located directly below the unusual heat-affected surface regions. Spheroidized surface debris and dimple walls, indications of local melting, are found on areas of the fracture surfaces inclined to the tensile axis at angles of high shear, typically at the outer boundaries of the shear lip. Initial studies were conducted on Ti-8Mn samples heattreated to contain a lenticular and grain boundary a microstructure in a/3 matrix with local regions of colonies of parallel a platelets. This microstructure was chosen because it has moderate strength and ductility and the lenticular a platelets clearly show localized shear. Shear strain values below the fracture surfaces can be measured by sectioning the samples parallel to the tensile axis and measuring the deflection of the platelets in scanning electron microscope (SEM) micrographs. In an earlier study, t3j Ti-6AI-4V samples were tested in the millannealed condition to check if localized melting occurs also in this commonly used titanium alloy. These samples showed heat-affected surface features in the same general locations as found in the Ti-8Mn samples. The present set of experiments was designed to investigate the effect of Ti-6A1-4V microstructure on the formation of a number of features associated with localized melting. In particular, the possible role of com-
DAVID D. MAKEL, Senior Research Scientist, is with the Department of Materials Science, University of Virginia, Charlottesville, VA 22903-2442. DANIEL EYLON, Professor, is with the Department of Chemical and Materials Engineering, University of Dayton, Dayton, OH 45469-0001. Manuscript submitted January 2, 1990. METALLURGICAL TRANSACTIONS A
mon shear across a colony plates was considered in relation to the concentrated shear found below local heataffected zones on the shear lip surfaces. Prior investigations into the effect of microstructure on deformation, unstable shear, and fracture in/3 processed a-/3 titanium alloyst6-~~ clearly show that local shear banding and crack growth behavior are significantly affected by the a plate colony microstructure in fatigue, fracture toughness, and tensile samples, with concentrated shear being promoted across the colonies. The possible role of common dislocation glide paths across a-/3 interfaces within the a plate colonies on the initiation and development of larger scale localized and adiabatic shear in the shear lip region was of particular interest. These mechanisms are considered to play a critical role in the phenomenon of local surface melting at separation.
II.
EXPERIMENTAL PROCEDURE
A. Ti 6AI-4V Sample Preparation Tensile samples were cut from a 2.3-mm-thick millannealed sheet of commercial g
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