Analysis of heat-affected zone phase transformations using in situ spatially resolved x-ray diffraction with synchrotron
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INTRODUCTION
HIGH-intensity heat sources used for fusion welding create steep thermal gradients in materials as they are rapidly heated and cooled to and from their melting points. The rapid thermal cycling that takes place induces melting and solidification in those parts of the weld where the liquidus temperature has been exceeded, as well as solid state phase transformations on both heating and cooling during welding. Microstructural discontinuities exist at (or near) the location of each phase transformation isotherm, and two distinct microstructural regions form during the welding process: the fusion zone (FZ), in which melting, solidification, and solid state phase transformations take place, and the heat-affected zone (HAZ), where only phase transformations in the solid state take place. In each zone, metastable microstructures may be created that can enhance or degrade the quality of the weld, depending on the material and the application. Currently, there are only a handful of in situ, real-time
J.W. ELMER, Metallurgist, Materials Science and Technology Division, JOE WONG, Physical Chemist, Materials Science and Technology Division, M. FROBA, Humboldt postdoctoral fellow, Materials Science and Technology Division, and P.A. WAIDE, Technical Associate, Mechanical Engineering Department, are with Lawrence Livermore National Laboratory, University of California, Livermore, CA 94551. E.M. LARSON, Assistant Professor, is with Grand Canyon University College of Science and Allied Health, Phoenix, AZ 85107. Manuscript submitted January 11, 1995. METALLURGICAL AND MATERIALSTRANSACTIONS A
studies of phase transformations and chemical dynamics in high-temperature reaction systems such as solid combustion. u~l In welding processes where steep thermal gradients exist in the material, no direct methods are available for investigating the solid state phase transformations that take place. For example, conventional methods for studying general phase transformation behavior include calorimetry,IS] dilatometry,[61 and resistivity, while other methods such as Jominy end-quench testingt71 and Gleeble testing[8.9] exist for special applications. All of these methods are indirect in that they measure the response of the sample (changes in enthalpy, length, resistivity, magnetization, or hardness) to the imposed thermal cycle and do not in any way determine the phases that are present during the test. Furthermore, these methods only provide phase transformation data for heating and cooling rates on the order of 1 ~ which is much less than those of arc welds (10 to 103 ~ and laser and electron beam welds (102 to 104 ~ In the absence of phase transformation information at welding conditions, the microstructure and integrity of the weld cannot be accurately predicted. For example, welding related problems such as subsolidus cracking occur when the inherent welding stresses exceed the hot ductility of the material in the HAZ of certain stainless steelsy ~ intermetallic alloys,u~ and titanium alloys,t~ Since the hot ductility of
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