The Evolution of Prior-BCC Grain Boundary Cracking During In-situ Creep Deformation of a Ti-15Al-33Nb(at%) Alloy
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0980-II05-05
The Evolution of Prior-BCC Grain Boundary Cracking During In-situ Creep Deformation of a Ti-15Al-33Nb(at%) Alloy Christopher J. Cowen and Carl J. Boehlert Chemical Engineering and Materials Science, Michigan State University, 2527 Engineering Building, East Lansing, MI, 48824-1226 ABSTRACT In-situ tensile-creep experiments were performed on a Ti-15Al-33Nb(at%) alloy using a specialized tensile stage placed within the vacuum chamber of a Camscan 44 FE scanning electron microscope (SEM). The creep damage evolution on the sample surface was chronicled through backscattered electron (BSE) imaging as a function of stress, time, and creep displacement at 650°C. The experiments revealed that the prior-BCC grain boundaries were the locus of damage accumulation during creep and significant grain boundary cracking was observed. The grain boundary cracking was verified to occur within the bulk of the material through post-mortem analysis. INTRODUCTION The creep stress exponent (n) and apparent activation energy (Qapp) values for many orthorhombic-based titanium-aluminium-niobium (Ti-Al-Nb) intermetallic alloys have suggested grain boundary sliding to be the dominant secondary creep deformation mechanism for low-tointermediate applied stresses [1-5]. One study has shown that grain boundary sliding and grain boundary cracking contribute significantly to the overall creep strain of a Ti-23Al-27Nb(at%) alloy [1]. Such observations were made through interrupted experiments performed inside a creep testing apparatus containing a vacuum chamber. The sample was removed from the chamber after achieving 4% and 10% creep strain and imaged using an SEM. Thus, true in-situ imaging was not obtained. The main objective of this work was to observe and characterize the deformation events in-situ during creep deformation of a Ti-15Al-33Nb(at%) alloy. To accomplish this objective a testing technique was developed to perform the creep experiments inside a SEM chamber while simultaneously imaging the sample’s surface using BSE imaging. EXPERIMENTAL DETAILS The processing, constituent phases, microstructure, heat treatments, and mechanical properties of the Ti-15Al-33Nb(at%) alloy have been reported previously [5]. For the constantload in-situ tensile-creep experiments, flat dog-bone-shaped samples, with gage section dimensions of 3 mm wide by 2.5 mm thick by 10 mm long, were cut by electron discharge machining (EDM). The EDM recast layers were removed through grinding. Images of the sample geometry and tensile stage are available in reference [6]. The specimens were glued to a metallic platen and polished to a metallographic finish using an automatic polishing machine. Colloidal silica with a 60 nm particle size was used for the final polish.
The tests were performed using a screw-driven tensile stage built by Ernest F. Fullam, Inc. (Lantham, NY) placed inside the chamber of the CamScan 44FE SEM. The stage allows for tensile or tensile-creep testing at temperatures ranging from RT to 1000°C and loads up to 4.4 kN (1000 lbs). A 4.4
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