Investigation of real-time microstructure evolution in steep thermal gradients using in-situ spatially resolved X-ray diffraction: A case study for Ti fusion welds

A recently developed spatially resolved X-ray diffraction (SRXRD) technique utilizing intense synchrotron radiation has been refined to yield phase an d microstructural information down to 200 mu m in spatial extent in materia ls subjected to steep thermal gradients during processing. This SRXRD techn ique has been applied to map completely the phases and their solid-state tr ansformation in the so-called heat-affected zone (HAZ) in titanium fusion w elds in situ during the welding process. Detailed profile analysis of the S RXRD patterns revealed four principal microstructural regions at temperatur e in the vicinity of the HAZ surrounding the liquid weld pool: (i) a comple tely transformed beta-Ti zone 2-3 mm adjacent to the liquid weld pool; (ii) a mixed alpha+beta-Ti region surrounding the pure beta-Ti zone, (iii) a ba ck-transformed alpha-Ti zone on the backside of the HAZ where pure beta-Ti once existed at temperature well above the alpha --> beta transformation is otherm, and (iv) a more diffused region outside the HAZ where annealing and recrystallization of the alpha-Ti base metal occur. The high-temperature m icrostructures so derived corroborate well the expected transformation kine tics in pure titanium, and the observed phase transformation boundaries are in good agreement with those predicted from the transformation isotherms c alculated from a simplified heat-flow model. Based on a detailed assessment of the SRXRD setup employed, improved experimentations such as a smaller b eam spot emitted from third generation synchrotron sources, better mechanic al stability (tighter scattering geometry), and use of an area detector wou ld enable more quantitative structural information for future phase dynamic s studies exemplified by this work.