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The post-dynamic recrystallization behavior of ultrafine-grained (UFG: 0.44 μm) cp-Ti under annealing, room temperature (RT) monotonic and cyclic loading was investigated across a range of temperatures and deformation rates wherever appropriate. By characterizing the grain and boundary structures, it was confirmed that recrystallization and grain growth occurred due to annealing (≥ 600 °C) and R = − 1 fatigue at RT. There was a noticeable 30 deg aggregation in misorientation distribution, along with the increased grain size. However, the hypothetical correlation between 30 deg aggregation and Σ13a or the other characteristic coincidence site lattice boundaries was found to be weak. The fatigue-induced grain growth is particularly intriguing for two reasons. First, the large monotonic deformation with low strain rate cannot trigger grain growth. Second, fatigue sharpened the basal intensity around the ND and caused a weaker texture component close to TD (load axis along the LD, perpendicular to the TD–ND plane). By contrast, high-temperature annealing only strengthened the UFG processing induced basal pole but without affecting its location. Novel insights into this fatigue-induced texture evolution in UFG cp-Ti has been provided. The lattice rotation during fatigue can be attributed to the combined effect of activation of prismatic $$ \langle a\rangle $$ ⟨ a ⟩ slip parallel to LD, and basal $$ \langle a\rangle $$ ⟨ a ⟩ slip perpendicular to it. The theoretically calculated stress to activate dislocation slip by assuming a non-equilibrium grain boundary state lent support to the above assertion. Moreover, the TEM observation evidently showed the characteristics of dislocation cross-slip and multiple slip in the grain interior. Graphical Abstract

Gamma-TiAl (> 99 pct) sample with composite structural design was fabricated by a combined powder metallurgical approach of the SEBM capsule and HIP. Selective electron beam melting (SEBM) was used to create the pre-sintered powder bed, composite structure and the capsule, followed by hot isostatic pressing (HIP) at 1250 °C. A mixture of plasma rotating electrode (PREP) and gas atomization (AA) processed powders, with the respective higher (49.66 at. pct) and lower Al (47.61 at. pct) concentrations, was employed successfully to track the transition from powder to bulk sample, providing enriched information to elucidate the microstructure formation mechanism. The selective melting created composite structure consisted of the fine equiaxed γ-grains, while the rest of the powder bed that had been subjected to preheat only and then HIP was characterized by the triple microstructure. Formation mechanisms of such unique microstructure, consisting of primarily the coarse γ-grains and fine γ-grains with the previous particle boundaries (PPBs), were correlated to the Al concentration. The Al-rich powder was transformed into the coarse γ-grains, while the Al-depleted powder was responsible for the fine γ-grains with the PPBs. This finding suggests that the detrimental PPBs can be eliminated by increasing the Al concentration in the nascent powder. In the post-heat treated condition (1350 °C), no PPBs was found, suggesting that heat treatment is beneficial by eliminating the PPBs. For the designed composite structures, there was evidence to suggest desired distribution of the hard and soft regions, with the fine γ-grains associated with the composite structure being responsible for the high hardness region.

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