ANALYSIS OF HEAT-AFFECTED ZONE PHASE-TRANSFORMATIONS USING IN-SITU SPATIALLY-RESOLVED X-RAY-DIFFRACTION WITH SYNCHROTRON-RADIATION

Spatially resolved X-ray diffraction (SRXRD) consists of producing a s ubmillimeter size X-ray beam from an intense synchrotron radiation sou rce to perform real-time diffraction measurements on solid materials. This technique was used in this study to investigate the crystal phase s surrounding a liquid weld pool in commercial purity titanium and to determine the location of the phase boundary separating the high-tempe rature body-centered-cubic (bcc) beta phase from the low-temperature h exagonal-close-packed (hcp) alpha phase. The experiments were carried out at the Stanford Synchrotron Radiation Laboratory (SSRL) using a 0. 25 x 0.50 mm X-ray probe that could be positioned with 10-mu m precisi on on the surface of a quasistationary gas tungsten are weld (GTAW). T he SRXRD patterns were collected using a position-sensitive photodiode array in a theta-2 theta geometry. For this probe size, integration t imes of 10 s/scan at each location on the specimen were found adequate to produce high signal-to-noise (SM) ratios and quality diffraction p atterns for phase identification, thus allowing real-time diffraction measurements to be made during welding. The SRXRD results showed chara cteristic hcp, bcc, and liquid diffraction patterns at various points along the sample, starting from the base metal through the heat-affect ed zone (HAZ) and into the weld pool, respectively. Analyses of the SR XRD data show the coexistence of bcc and hcp phases in the partially t ransformed (outer) region of the HAZ and single-phase bcc in the fully transformed (inner) region of the HAZ. Postweld metallographic examin ations of the HAZ, combined with a conduction-based thermal model of t he weld, were correlated with the SRXRD results. Finally, analysis of the diffraction intensities of the hcp and bcc phases was performed on the SRXRD data to provide additional information about the microstruc tural conditions that may exist in the HAZ at temperature during weldi ng. This work represents the first direct in situ mapping of phase bou ndaries in fusion welds.