Significant strengthening induced by heterogeneously generated widmanstätten structure in directional grains: Deformation behavior and strengthening mechanism

TiAl alloys with a fully lamellar (FL) microstructure exhibit promising high-temperature mechanical properties at 800 °C due to their closely packed lamellar architecture. However, equiaxed FL structures obtained by conventional casting and heat treatment are insufficient for prolonged service above 700 °C, making directional solidification (DS) an effective approach to improve performance at higher temperatures. The formation of metastable Widmanstätten structures during DS, caused by local perturbations, may disrupt the morphology of directional grains, warranting investigation into their influence on deformation behavior. In this work, the tensile performance of directional grains with and without embedded Widmanstätten laths (Wid-laths) was evaluated at 800 °C. High-performance specimens exhibited a clear advantage over some reported high-Nb-containing DS TiAl alloys. Lamellar misorientation relative to the loading axis was characterized using electron backscatter diffraction (EBSD), and a pseudo-in-situ EBSD experiment was designed to analyze deformation coordination between Widmanstätten structures and the lamellar matrix. Combined EBSD-FIB preparation and TEM observations were employed to investigate interfacial strengthening mechanisms and plasticity behaviors. Results reveal that the Widmanstätten structure primarily contributes to interfacial strengthening and multiple slip activation. Benefiting from refined lamellar spacing and favorable orientations, Wid-laths deform more readily than the matrix via dislocation slip, whereas the matrix deforms predominantly by twinning. Strengthening mechanisms involving twin-dislocation interactions, ∑11 interfacial strengthening, and stacking faults intersections are discussed. This study provides insights into the deformation coordination of Widmanstätten structures, facilitating the strengthening of directional grains through controlled application of this metastable microstructure.