Feng Huang

High Nb-TiAl alloys have attracted increasing attention in aerospace applications due to their superior high-temperature performance compared with conventional TiAl alloys. The combined addition of B and C can further enhance alloy properties; however, as-cast TiAl alloys typically contain numerous solidification-related defects, making heat treatment essential for microstructural optimization. In this study, two different heat-treatment schedules were applied to an as-cast Ti-45Al-8Nb-0.5B-0.6C alloy to investigate the effects of heat-treatment parameters on microstructure and mechanical properties. The results indicate that heat treatment significantly modifies both the microstructure and phase constitution. The as-cast alloy exhibits a coarse dendritic network structure consisting mainly of γ-TiAl, α2-Ti3Al, Ti2AlC, TiB2, and the B2 phase. After solution treatment in the same α single-phase region, low-temperature long-time aging produces a fine near-lamellar microstructure, whereas high-temperature short-time aging results in a coarse fully lamellar structure. Mechanical testing reveals that both room- and high-temperature properties are improved after heat treatment. The low-temperature long-time aged alloy exhibits the best room-temperature tensile properties (388.3 MPa, 1.41%), while the high-temperature short-time aged alloy shows the optimal high-temperature compressive performance (1342.8 MPa, 12.5%). These findings establish the relationships between microstructure evolution and mechanical properties, providing critical insights into the heat treatment regulation of the room-temperature and high-temperature mechanical properties of high Nb-TiAl alloys.

To solve the problem of inadequate plasticity of traditional processing routes in improving the plasticity of novel Co-saving 18Ni (300) maraging steel, a cold deformation-cycle solution treatment process was developed. Through systematic characterization and tensile property testing, the study focuses on elucidating the impact of the number of solution treatments on the microstructure and mechanical behavior. The results showed that with a 30% cold deformation, three times of solution treatment at 860 °C for 10 min refined the original austenite grains (equivalent circle radius: 3.3 μm) and martensite structure (length and width: 7 μm and 1.3 μm, respectively) to the utmost extent. The grains became uniformly equiaxed, and the texture was eliminated, and a moderate content (4.5%) of retained austenite was formed. At this time, the material achieves the best match between strength (tensile strength of 1240 MPa) and plasticity (elongation of 9.93%), which are increased by 15.3% and 94.3%, respectively, compared with the traditional process. Mechanistic analysis revealed that grain refinement and uniform equiaxialization were the primary drivers for enhancing strength and plasticity. This study has demonstrated that the cold deformation-cyclic solution treatment process is an effective methodology for tailoring the microstructure and mechanical properties of maraging steel.

In this paper, a bidirectional temperature gradient directional annealing process for growing TiAl columnar crystals was proposed, and the influences of structural parameters and process parameters on the temperature distributions of TiAl rods were discussed through numerical simulation. The results indicate that the α phase zone is expanded and its boundary becomes planar as the thickness of graphite ring (b) and gap width (d) decrease. Increasing the graphite rod length (l) and the height of the graphite ring from the Ga-In coolant surface (h) results in an expanded α phase zone with flattened boundaries, but the temperature gradient decreases. Taking all the α phase zone height, its boundary shape, and the temperature gradient into consideration, the optimal b, d, l, and h are 10 mm, 5 mm, 50 mm, and 50 mm, respectively. The higher heating temperature within the α phase temperature range, such as 1375 °C, is favorable for the establishment of the required temperature field during directional annealing. The effect of drawing speed is more complicated. Although its effect on the temperature field of the TiAl rod is almost negligible, it will seriously affect the microstructure of the annealed alloy, and it needs to be optimized by subsequent experiments.

Combining a comparative experimental study and theoretical analysis, the feasibility and corresponding action mechanisms of the pre-hardened hot forming (PHF) process applied to 6082-0.2Zr-0.3Er alloy were discussed. The results indicate that unlike a traditional process, after PHF treatment, both the grains and the precipitates at the grain boundaries show a certain directional arrangement, and the texture coefficients of (220) and (200) are enhanced. Although the average size of the second phase is almost the same, the proportion of the fine second phase in the PHF-treated sample is larger. In addition to shortening the processing time and improving the production efficiency, the PHF process was proven to be beneficial for the improvement in the comprehensive mechanical properties of this 6082-0.2Zr-0.3Er alloy, with the product of strength and elongation increased by about 12.1% under the given conditions. Furthermore, with the optimization of process parameters, the improvement effect is expected to be further enhanced.