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.

To further determine the brittle-to-ductile transition temperature, the microstructures and mechanical properties of typical forged β-solidifying Ti-42Al-5Mn (TiAlMn) and Ti-43Al-4Nb-1Mo-0.5B (TNM) alloys were studied. The results show that the microstructures of both heat-treated alloys consist of γ/α₂ lamellar colony, equiaxed γ phase, and β₀ phase. In addition, the globular α₂ phase appears in the TNM alloy. The yield strength of TiAlMn alloy increases gradually with the testing temperature, whereas a significant drop from 605 MPa to 469 MPa occurs between 650 °C and 700 °C. In contrast, the TNM alloy exhibits a declining trend with the increasing testing temperature, and a remarkable reduction is observed in the temperature range of 700 °C–750 °C. Moreover, the fracture mode transition from transcrystalline cleavage to intercrystalline with increasing testing temperature was applied to the auxiliary judgment of brittle-to-ductile transition. As a result, the brittle-to-ductile transition temperatures of TiAlMn alloy and TNM alloy are about 650 °C–700 °C and 700 °C–750 °C, which may provide a reference for service temperature range of both alloys.

Directional heat treatment reduces the number of transverse grain boundaries in pure copper wires at suitable temperatures, thereby promoting the formation of columnar or even single-crystal structures. This process significantly enhances the electrical conductivity of the wires and has become a research focus. Fundamentally, directional heat treatment is a secondary recrystallization process, involving key microstructural evolutions such as grain growth and grain boundary migration. Investigating its mechanism in pure copper wires is essential for optimizing their conductive performance. In this study, pure copper wires were subjected to directional heat treatment and systematically characterized using electron backscatter diffraction (EBSD). The effects of treatment on grain growth and boundary migration were analyzed, clarifying the evolution of grain boundary structures and crystallographic textures during columnar grain development. It was revealed that grains with a orientation preferentially develop into columnar structures, with most inter-columnar grain boundaries being low-energy ∑ 3 and ∑ 9 types. The novelty of this work lies in revealing the mechanism of directional grain boundary migration in pure copper wires and elucidating the formation mechanism of island grains after directional heat treatment.

The Ohon Continuous Casting is the main method for preparing single crystal copper wire, and it is also the research hotspot at present, but it is difficult to directly cast ultrafine single crystal copper wire (diameter < 0.05 mm). The copper wire obtained by continuous casting must be drawn and deformed before it can be used in practice, but this will bring a series of problems such as single crystal structure destruction and conductivity deterioration. Directional heat treatment technology can control the direction of heat flow at a low temperature, realize the directional migration of grain boundaries in the recrystallization process, and form columnar crystals or single crystals, which is of great significance for improving electrical conductivity. In this paper, the directional heat treatment method was used to investigate the microstructure evolution and influencing factors of pure copper wire, the process parameters were optimized, and the conductivity of pure copper wire was measured. It was found that the conductivity of pure copper wire increased by 5% when the heating temperature was 750 °C and the withdrawing velocity was 15 μm/s, which laid a foundation for the improvement of conductivity of pure copper wire.

The transverse grain boundaries in pure copper wires increase resistivity, generating capacitance and inductance effects, leading to a decrease in the electrical conductivity of pure copper wires. Directional heat treatment technology can eliminate transverse grain boundaries in pure copper conductors, which is of great significance for improving electrical conductivity. Directional heat treatment is essentially a secondary recrystallization process, with influencing factors involving microstructure, texture, etc. This study systematically investigated the effects of cold-drawing deformation rate and heat treatment processes on secondary recrystallization microstructure, grain boundary structure, and crystallographic texture in pure copper wires. The results demonstrate that higher deformation levels (≥89%) facilitate secondary recrystallization and enhance texture development. Moreover, the heat treatment temperature exerts a more significant influence on secondary recrystallization than the heat treatment duration. The grain coarsening temperature for pure copper wires with a deformation degree of 89% was determined to be 400 °C. These findings provide a fundamental basis for formulating directed heat treatment processes for pure copper wires.