In this article, microstructural evolution during the solidification of Ti-48Al-2Cr-2Nb with current density, as well as the formation mechanisms, are discussed, along with the impacts on microhardness and hot compression properties. The applied electric current promotes the solidification from the α primary phase to a largely β solidification in Ti-48Al-2Cr-2Nb. With an increase in supercooling, the solidification process have a tendency to change from an α-led primary phase to (α + β)-led primary phase. The primary dendrites, grain size, and lamellar spacing show a tendency to decrease first before increasing with increasing current density. Microhardness and high-temperature yield strength increase with a decrease in primary dendrite spacing, grain size, and lamellar spacing. Correlations between primary dendrite spacing, lamellar spacing, microhardness, yield strength, and current density are described by a fitting formula. An increase of α
2
phase, due to the application of electric current, results in improved microhardness. The yield strength of Ti-48Al-2Cr-2Nb alloy increases linearly with microhardness. Yield stress increases with a decrease in microstructure parameters, in accordance with the Hall–Petch equation. The predominant modification mechanism with electric current application for TiAl solidification is the variation of supercooling and temperature gradients ahead of the mush zone due to Joule heating.
The hot deformation behavior of a novel TNM-RE alloy (RE=Y,La,Ce) was studied using a hot simulation machine (Gleeble-3800), and microstructural evolution was also characterized. Finally, 3D forging was carried out on isothermal forging equipment. It is shown that the as-cast lamellar colony size is about 20~30 μm, which is refined by the formation of rare earth oxides and borides at grain boundaries inhibiting grain growth. The peak stress of the TNM-RE alloy deformed at 1200 ℃/0.01s-1 is about 97 MPa, which is governed by the lamellar colony size and the B2 phase. Based on microstructure observation, it is found that the lamellar is bent and elongated to coordinate plastic deformation, where dynamic recrystallization nucleates preferentially, and full dynamic recrystallization is obtained at 1220 ℃/0.01s-1. The TNM-RE alloy was forged by 3D isothermal forging method, and fine grains with a size of 10~20 μm were obtained by controlling the process parameters. The novel TNM-RE alloy shows an excellent hot workability.
The rapid development of fusion-reactor technology calls for excellent anti-irradiation materials. Complex concentrated alloy (CCA) is a newly proposed alloy concept which is a promising candidate of nuclear fusion materials by virtue of its great phase stability under irradiation. This article summarizes anti-radiation mechanism and the microstructure evolution in HEAs. The effective factors on irradiation behavior of HEAs, including entropy, sample size and temperature, are discussed. Finally, the article introduces the potential ways to solve the economic and environmental problems which the HEAs faced for their applications in the future. In summary, the HEAs usually show better irradiation resistance than traditional alloys, such as less swelling, smaller size of defects, and more stable mechanical properties. One possible reason for the irradiation resistance of HEA is the self-healing effect induced by the high-entropy and atomic-level stress among the metal atoms. The activation of the principal element should be considered when selecting components of HEA, and the high throughput technique is a potential way to reduce the design and fabrication cost of HEAs. It is reasonable to expect that coming years will see the application of novel HEAs in fusion reactors.
A rapid cellular microstructure of Ti-48Al-2Cr-2Nb (in atom %) intermetallic was grown without peritectic reaction by the method of melt-quenching in Ga−In liquid. After characterization of the microstructures and phase constituents, it is observed that the cellular crystals mainly consist of the α 2 phase and directionally grew in the outer layer of the melt droplet, with the growth length and cellular spacing about 358−460 μm and 0.68−3.6 μm, respectively. Upon detailed analysis of the cellular growth process, it is found that the formation of this characteristic microstructure is derived from the extremely rapid cooling effect of Ga−In liquid, mainly determined by the heat transfer process; the dominant heat transfer mechanism changes from heat convection to heat conduction at a growth distance of about 70 μm. By using the semireverse method, the dependence of the cooling rate on the cellular growth distance can be estimated accurately and conveniently, which ranges from 2.61 × 10 6 to 1.26 × 10 5 K/s. The nanoindentation examination proved that the rapid cellular microstructure possesses excellent micromechanical properties (8.457 ± 0.336 GPa in nanohardness), with a significant improvement of about 15−60% than the general microstructures.
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