In-situ observation of nucleation, growth and interaction of deformation-induced α^″ martensite in metastable Ti–10V–2Fe–3Al

Microstructure evolution, dominated by the formation of deformation-induced α^″ martensite during the 3-point bending of a metastable β Ti–10V–2Fe–3Al alloy containing 5% α, was observed in-situ using forward-scatter electron imaging and electron backscattering diffraction. α^″ plates nucleated heterogeneously from {580}_α^″ habit planes at α-β interfaces and triggered the nucleation of additional α^″ plates at the advancing interface during growth and upon impingement with high-angle boundaries. Further thickening of the impinged α^″ plates led to a build-up of strain energy which induced 13031‾0_α^'' twinning within obstacle-forming α^″ plates. Until now, this twinning system was observed in Nb and Ta alloyed Ti shape memory alloys only. This in-situ study is also the first to document the activation of 13031‾0_α^'' twinning by α^″ impingement in a metastable β matrix. The first forming variants were {011}_α^″ compound twin-related variant pairs, each forming from up to two individual habit planes. These variant pairs were consistently predicted by determining the grain-average stress with finite element analysis and computing the available work from the transformation strain of the α^″ variants. At higher indenter loads, grains that were unfavourably oriented to martensite formation accommodated the local stress by forming aggregates of different α^″ variants, obeying either {111}_α^″ type I or 211_α^'' type II twin relationships. Scanning transmission electron microscopy revealed α^″ substructures comprising columnar domains oriented perpendicular to the habit plane along with the absence of lattice invariant deformation. The growth of α^″ plates involved ledge formation at α^″-β interfaces.