Effects of Microstructure on The Room-Temperature Strength and Ductility of the Alloy Ti-25Al-10Nb-3V-1Mo
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EFFECTS OF MICROSTRUCTURE ON THE ROOM-TEMPERATURE STRENGTH AND DUCTILITY OF THE ALLOY Ti-25Al-10Nb-3V-1Mo FRANCOIS-CHARLES DARY AND ANTHONY W. THOMPSON Carnegie Mellon University, Materials Science and Engineering Dept., Pittsburgh, PA 15213 ABSTRACT The role of the microstructure on the room-temperature tensile properties of the alloy Ti-25AllONb-3V-lMo (at.%) was investigated. A wide spectrum of microstructures was obtained by varying the cooling rate after a solutionizing treatment in the 13phase field. The strength was found to increase monotonically with increasing cooling rate. It is proposed that strength is controlled by (cc2+13R) boundary strengthening in 13-solutionized microstructures. The ductility was observed to go through a maximum at a cooling rate of 1.5°C/s. Different deformation and failure mechanisms were identified, depending on the cooling rate regime, and correlated to the ductility trends. The P3 Rnature, a 2 lath size, czf 2 lath arrangement and prior-P3 grain boundaries appeared to be the principal features governing the deformation and failure mechanisms. INTRODUCTION For many years, attention has been focused on developing Ti 3AI-based titanium aluminides for aerospace applications. Indeed, the low density of binary Ti 3AI alloys and their ability to retain excellent strength and creep properties at high temperature made them very attractive. However, the necessity to improve the poor ductility and toughness of these alloys called for additions of 13-stabilizing elements, such as Nb, V or Mo, which in turn increased significantly the complexity of the microstructure. To date, although mechanical properties have been found to be very sensitive to microstructural changes, only limited research has been aimed at trying to understand the microstructure/mechanical property relationships for given alloy compositions. The disparity of microstructures used in most studies has precluded thorough understanding of the role of microstructure upon mechanical properties. The focus of this work was to examine the dependence of room-temperature tensile properties upon microstructure in the titanium aluminide alloy, Ti-25Al-l0Nb-3V-lMo (at.%). Microstructural features were varied by means of heat treatments, and their effect on room temperature strength and ductility were investigated. EXPERIMENTAL PROCEDURE Details concerning the as-received material, heat treatments and method of tensile specimen preparation have been described in an earlier paper [1]. Specimens were solutionized in air in the 13-phase field, at 1150'C, for one hour and cooled at different rates, in order to vary the microstructure. Seven cooling rates were chosen, i.e., 25, 7.5, 4, 1.5, 0.5, 0.1 and 0.010 C/s. Round tensile specimens, with a 2.5 mm gage diameter and 10 mm gage length, and compression specimens, with a 6x6 mm square cross area and 9 mm length, were used. Prior to testing, tensile specimens were electropolished and compression specimens were mechanically polished to 0.5 ý.tm alumina. Room temperature tests were conducte
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