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‾0α'' 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‾0α'' 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 211α'' 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.