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• Nanoscale digital image correlation (HR-DIC) strain mapping of HCF up to 670 °C. • Microplasticity mainly in softmode colonies: parallel to lamellae, near interfaces. • Poor strain transfer across colony boundaries at both temperatures. • At 25 °C, maximum stress influenced the slip band count more than did the cycle count. Microplasticity in a lamellar TiAl alloy upon high cycle tensile fatigue was measured by high resolution digital image correlation strain mapping at several stress and cycle increments, at 25 °C and 670 °C, for two lamellar thicknesses. Plastic deformation occurred primarily in soft-mode colonies at both temperatures, operating by slip parallel to the lamellae, and near lamellar interfaces. Plastic strains generally decreased to zero by the colony boundary; strain transfer across such boundaries was rare at both temperatures. At 25 °C, the maximum applied stress influenced the number of slip bands more so than did the number of loading cycles. [ABSTRACT FROM AUTHOR]

Detailed microstructure characterisation and in-situ micropillar compression were coupled with crystal plasticity-based finite element modelling (CP-FEM) to study the micro-mechanisms of plastic anisotropy in lamellar TiAl alloys. The consideration of microstructure in both simulation and in-situ experiments enables in-depth understanding of micro-mechanisms responsible for the highly anisotropic deformation response of TiAl on the intra-lamella and inter-lamella scales. This study focuses on two specific configurations of γ / α 2 lamellar microstructure with the γ / α 2 interfaces being aligned 25 o and 55 o to the loading direction. Microstructure-based CP-FEM shows that longituginal slip of super and ordinary dislocations are most responsible for the plastic anisotropy in the 25 o micropillar while the anisotropy of the 55 o micropillar is due to longitudinal superdislocations and longitudinal twins. In addition, transversal superdislocations were more active, making the deformation in the 25 o micropillar less localised than that in the 55 o micropillar. Moreover, the CP-FEM model successfully predicted substantial build-up of internal stresses at γ / α 2 interfaces, which is believed to be detrimental to the ductility in TiAl. However, as evidenced by the model, the detrimental internal stresses can be significantly relieved by the activation of transverse deformation twinning, suggesting that the ductility of TiAl can be improved by promoting transverse twins. Deformation behaviour of (ϕ = 25 o )- and (ϕ = 55 o )-pillars: in-situ observation versus CP-FEM results. Image 1 • Crystal plasticity of lamellar TiAl is studied by in-situ testing and modelling. • CP-FEM well predicts the constitutive response and localisation of lamellar TiAl. • This study reveals the micro-mechanisms of anisotropy of lamellar TiAl. • Deformation twinning significantly relieves internal stresses at γ / α 2 interfaces. [ABSTRACT FROM AUTHOR]