Hierarchical criteria to promote fast and selective αGB precipitation at β grain boundaries in β-metastable Ti-alloys

Abstract

Early grain boundary (GB) decoration was analyzed in a β-metastable titanium alloy by using electron backscatter imaging and diffraction. More than 1000 GBs were analyzed and classified as 'special' or 'general' GBs: A boundary is called 'special' if it is able to form an αGB layer satisfying the Burgers Orientation Relationship (BOR) with both grains within an angular deviation θ2-BOR; the others are called 'general' GBs. First, the result gives the probability of a GB to transform early according to a disorientation criterion. 49.5°, 60°/<110> special HAGBs are clearly identified as preferential GBs for early precipitations whereas 60°/<111> special HAGBs behave as general GBs. Second, the probability for transformed GBs to respect the double-BOR variant selection (VS) criterion according to θ2-BOR deviation is evaluated. The probability is close to 100% for GBs with θ2-BOR up to 10°, random for GBs with θ2-BOR over 16°, and decreases progressively but remains higher than random with θ2-BOR in between, i.e. an intermediate domain. In addition the habit plane (HP) VS criterion was considered based only on single trace analysis between HP and GB plane (GBP). In most cases, the HP closeness to the GBP was not a relevant VS criterion.