Microstructure Evolution and Enhanced Hot Workability of TiC/Ti-6Al-4V Composites Fabricated by Melt Hydrogenation.

Improving the hot workability and reducing the processing cost are critical steps to expanding the application of TiC/Ti-6Al-4V composites. This study employed melt hydrogenation to fabricate TiC/Ti-6Al-4V composites under a mixed atmosphere of hydrogen and argon. Experimental results indicated that hydrogen had an obvious influence on the growth and morphology of eutectic TiC particles, and the size of eutectic TiC and primary ß grains was significantly increased. As a result, large-sized eutectic TiC was distributed along the grain boundaries of primary ß grains. Hot compression results showed that the peak flowing stress of composites was reduced by hydrogen, which resulted in an improvement of hot workability, especially in the (α + ß) phase region, and the best results were obtained at 900 °C/0.01 s-1, at which the peak stress decreased from 241 ± 9 to 190 ± 8 MPa (a decrease of 21.2%). Inspection of the microstructure after hot compression showed that hydrogen improved the proportion of DRX grains from ~62.7% to ~83.2%, and hydrogen also decreased the density of dislocations, which were attributed to hydrogen accelerating atomic diffusion. Enhanced hot workability resulted from hydrogen atoms decreasing the atomic bonding force of the titanium matrix, hydrogen reducing the ß/(α + ß) transition temperature, the higher proportion of DRX, and the higher mobility of dislocations. It is expected that the findings of this study may support the development of a simple and efficient method to reduce the processing cost of TiC/Ti-6Al-4V composites.

Microstructure Evolution and Toughening Mechanism of a Nb-18Si-5HfC Eutectic Alloy Created by Selective Laser Melting.

Because of their superior mechanical performance at ultra-high temperatures, refractory niobium-silicon-based alloys are attractive high-temperature structural alloys, particularly as structural components in gas turbine engines. However, the development of niobium-silicon-based alloys for applications is limited because of the trade-off between room temperature fracture toughness and high-temperature strength. Here, we report on the fabrication of a Nb-18Si alloy with dispersion of hafnium carbide (HfC) particles through selective laser melting (SLM). XRD and SEM-BSE were used to examine the effects of scanning speed on the microstructure and the phase structure of the deposited Nb-18Si-5HfC alloy. The results show that when the scanning speed rises, the solid solubility of the solid solution improves, the interlamellar spacing of eutectics slowly decrease into nano-scale magnitude, and the corresponding hafnium carbide distribution becomes more uniform. We also discover the hafnium carbide particles dispersion in the inter-lamella structure, which contributes to its high fracture toughness property of 20.7 MPa∙m1/2 at room temperature. Hardness and fracture toughness are simultaneously improved because of the control of microstructure morphology and carbide distribution.