Analysis of heat-affected zone phase transformations usingin situ spatially resolved x-ray diffraction with synchrotron radiation

Abstract

Spatially resolved X-ray diffraction (SRXRD) consists of producing a submillimeter size X-ray beam from an intense synchrotron radiation source to perform real-time diffraction measurements on solid materials. This technique was used in this study to investigate the crystal phases surrounding a liquid weld pool in commercial purity titanium and to determine the location of the phase boundary separating the high-temperature body-centered-cubic (bcc) β phase from the low-temperature hexagonalclose-packed (hcp) α phase. The experiments were carried out at the Stanford Synchrotron Radiation Laboratory (SSRL) using a 0.25 × 0.50 mm X-ray probe that could be positioned with 10-µm precision on the surface of a quasistationary gas tungsten arc weld (GTAW). The SRXRD patterns were collected using a position-sensitive photodiode array in a φ-2φ geometry. For this probe size, integration times of 10 s/scan at each location on the specimen were found adequate to produce high signal-to-noise (S/N) ratios and quality diffraction patterns for phase identification, thus allowing real-time diffraction measurements to be made during welding. The SRXRD results showed characteristic hcp, bcc, and liquid diffraction patterns at various points along the sample, starting from the base metal through the heat-affected zone (HAZ) and into the weld pool, respectively. Analyses of the SRXRD data show the coexistence of bcc and hcp phases in the partially transformed (outer) region of the HAZ and single-phase bcc in the fully transformed (inner) region of the HAZ. Postweld metallographic examinations of the HAZ, combined with a conduction-based thermal model of the 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 microstructural conditions that may exist in the HAZ at temperature during welding. This work represents the first directin situ mapping of phase boundaries in fusion welds.