TiAl intermetallics are typical metallic materials involving complex solid-state phase transformations, with crystal orientations that are difficult to control due to multi-transformation variants. However, lamellar orientation control is crucial to the development of a polysynthetic twinned single crystal structure in TiAl-based alloys for jet engines or other high-temperature systems. In this study, β-solidifying TiAl alloys were used to study the relationships between the lamellar structure and the phase transformation process under directional solidification (referred to as the directional phase transformation, DPT). It was found that the β → α phase transition affects the lamellar orientations and that the subsequent process of α → α₂ + γ leads to the final formation of the polysynthetic lamellar structure. Detailed analyses based on crystallography show that the β/α phase interface is responsible for the different oriented lamellar structures with the 0° or 45° orientation. With a lower interfacial energy, the 0° oriented α phase nucleates more easily but grows much more slowly than the 45° oriented α phases during DPT, which makes it feasible to control the lamellar orientations for TiAl-based alloys. The crystallographic origin for the control of lamellar orientations was then studied and confirmed by using EBSD in a β-solidifying Ti–Al–Nb alloy.

Additive manufacturing (AM) is redefining the limits of directional solidification (DS) and single-crystal (SX) fabrication for nickel-based superalloys. By reconciling classical Bridgman theory with the extreme thermal gradients (G) and solidification velocities (V) inherent to AM, this review establishes a unified framework for controlled epitaxy. It dissects the kinetics of grain competition and the columnar-to-equiaxed transition (CET), highlighting how scan strategies and thermal management dictate melt pool geometry and the G/V ratio. A comparative assessment of laser powder bed fusion (L-PBF), electron beam powder bed fusion (EB-PBF), and directed energy deposition (DED) delineates distinct process windows for direct fabrication versus epitaxial repair. Addressing the conflict between printability and performance in high-$\gamma'$ alloys, the text links defect mechanisms, specifically stray grains (SGs) driven by constitutional supercooling, complex cracking modes, and surface anomalies, to targeted mitigation strategies ranging from virtual grain selection to alloy tailoring and synergistic post-processing. Ultimately, a multi-process hybrid architecture is proposed that positions EB-PBF as the foundational platform for bulk SX formation, augmented by L-PBF or DED, to close the "manufacture–service–repair–remanufacture" engineering loop.

Intermetallic compounds including TiAl and NiAl suffer from an abrupt brittle-ductile transition at a critical temperature, limiting their higher temperature applications. Conventional viewpoints attribute this transition to thermally activated dislocation actions or dynamic recrystallization. Here, we discover a novel brittle-ductile transition mechanism in polysynthetic twinned TiAl, driven by lamellar degradation, which significantly deviates from the conventional mechanisms. The results demonstrate that the lamellar degradation is primarily caused by the \( \alpha_2 \rightarrow \gamma \) phase transformation at the brittle-ductile transition temperature, which stems from a non-monotonic increase in the stacking fault energy of \( \alpha_2 \) phase with temperature. This unconventional transition can be described efficiently by an energy density-based strength model, offering new insights into the brittle-ductile transition behavior of intermetallic compounds at elevated temperatures.

The ingot defects of P92 martensitic steel could induce crack initiation during the critical normalization-tempering treatment, which deteriorates the properties of P92 pipe used in ultra-supercritical (USC) power plants greatly. Thus, this study conducted numerical simulation based on a modified Johnson-Cook fracture model to thoroughly explore the effects of ingot defects on crack initiation in P92 pipes under common cooling approaches. Tensile tests of smooth and notched P92 steel specimens were executed at wide range of temperatures and strain rates to assist in calibrating the parameters for Johnson-Cook constitutive and fracture models. The fracture criterion for P92 steel as a function of temperatures, strain rates, and stress states simultaneously was thus put forward. Based on calibrated fracture model, validation simulation and experiment were conducted with small P92 CT specimens, showing good consistency between predictions and experiments. Subsequently, cooling simulations of a large P92 pipe with four types of defects placed at various positions of the pipe were performed. Largest strain concentrations were observed around the tips of radial pre-cracks followed by axial pre-crack, void, and inclusion at all positions and cooling rates. Only radial pre-cracks located at inside and outside walls were predicted to initiate under water-spray cooling. Other pre-defects under both air and water-spray cooling pose no risk of crack initiation. The direction of the pre-crack proves to be a crucial factor inducing crack initiation.