Precipitation of grain boundary alpha in a laser deposited compositionally graded Ti-8Al-xV alloy - an orientation microscopy study

摘要

A graded ternary Ti-8Al-xV alloy (all compositions in wt percent) has been deposited using the laser engineered net-shaping (LENS~(TM)) process. A compositional gradient in the alloy, from binary Ti-8Al to Ti-8Al-20V, has been achieved within a length of approx 25 mm. The feedstock used for depositing the graded alloy consisted of elemental Ti, Al, and V powders. Due to the columnar growth morphology of the beta grains in these LENS~(TM) deposited Ti alloys, the same prior beta grain boundary often extends across lengths approx 10 mm. Using orientation microscopy techniques in a scanning electron microscope, the crystallography of precipitation of grain boundary alpha across the same boundary with changing composition has been investigated in detail. It was observed that while most grain boundary alpha precipitates maintain a Burgers or near-Burgers orientation relationship with only one of the beta grains, a few of these precipitates develop a Burgers orientation relationship with the other beta grain. In some rare instances, the grain boundary alpha did not develop a Burgers or near-Burgers orientation relationship with either beta grains. Interestingly, in many cases while the grain boundary alpha maintained Burgers relationship with one of the beta grains, precipitates of two different variants decorated the boundary, in a near-alternate fashion.