Lu Wang

The grain boundary phase (GBP) has a significant influence on the magnetization behavior in nanocrystalline PrNd–Fe–B magnets. The current study demonstrates that reversible/irreversible magnetization behavior and the phenomenon of open recoil loops are related to both the nature of GBPs and the magnetization state by in situ observation. The optimization of GBPs nature (increase the volume fraction and improve the composition of GBPs) leads to the suppression of reversible magnetization behavior and the phenomenon of open recoil loops at low fields. Since the asymmetric magnetic domain structure appears only at low cycle fields, the openness phenomenon originates from the weak pinning grain boundaries (GBs). In addition, optimization of the GBPs also enhances pinning strength and uniformity, which contributes to the domain walls being pinned in the GBs at higher external fields. At this moment, the domain wall is dominated by irreversible magnetization behavior, and the openness phenomenon disappears. This proves that the coercivity mechanism is transformed from inhomogeneously weak pinning to homogeneously strong pinning with the optimization of GBPs. Consequently, the coercivity and squareness factor are significantly enhanced. This study sheds light on the understanding of the effects of GBP's nature on recoil loops and coercivity mechanism, and it also provides significant guidance for the development of advanced permanent magnets.

Complex concentrated alloys, including high-entropy alloys (HEAs) and medium-entropy alloys (MEAs), offer another pathway for developing metals with excellent mechanical properties. However, HEAs/MEAs of different structures often suffer from various drawbacks. So, investigations on the effect of phase and microstructure on their properties become necessary. In the present work, we adjust the phase constitution and microstructure by Al addition in a series of (Ti₂ZrHf)_100−xAl_x (x = 12, 14, 16, 18, 20, at.%, named Al_x) MEAs. Different from traditional titanium, Al shows a β-stabilizing effect, and the phase follows the evolution of α′(α)→α″→β + ω + B2 with Al increasing from 12 to 20 at.%, which could not be predicted by the CALPHAD (Calculate Phase Diagrams) method or the Bo-Md diagram because of the complex interactions among composition elements. At a low Al content, the solid solution strengthening of the HCP phase contributes to the extremely high strength with a σ_0.2 of 1528 MPa and σ_b of 1937 MPa for Al14. The appearance of α″ deteriorates the deformation capability with increasing Al content in the Al16 and Al18 MEAs. In the Al20 MEA, Al improves the formations of ordered B2 and metastable β. The phase transformation strengthening, including B2 to BCC and BCC to α″, together with the precipitation strengthening of ω, brings about a high work-hardening ratio (above 5 GPa) and improvements in ductility (6.8% elongation). This work provides guidelines for optimizing the properties of MEAs.

Developing advanced underwater welding technology for titanium, which is the key structural material for underwater applications, is of great significance for the design, fabrication, and maintenance of submarine equipment. In this study, in order to investigate the underwater welding microstructure and mechanical properties of Ti-4Al-2V alloy, underwater wet laser welding was conducted on Ti-4Al-2V alloy using varying laser power. The microstructure and properties of the welding joints were characterized and analyzed. The microstructure of the heat-affected zone and fusion zone in the welding joints are not significantly different from those of welding in air, but a mixed oxide layer composed of Al₂O₃ and TiO₂ is formed on the surface of the fusion zone. Due to internal stress, a large number of cracks initiate on the oxide layer and propagate to the joints. In the 4 kW and 5 kW joints, a penetrating crack formed due to the excessive accumulation of internal stress breaking up the α phase. The mechanical properties of the joints are significantly affected by the laser power. The tensile strength of the 3 kW and 4 kW joints is comparable to that of the base metal, which is about 600 MPa, while the 5 kW joint shows brittle fracture with no plastic deformation and 228 MPa strength. This research lays a solid foundation for understanding the underwater wet laser welding behavior of titanium alloys.

Laser welding of the near α-phase titanium alloy Ti-4Al-2V, used for complex components in the nuclear industry, has been rarely reported. In this study, butt weld joints made of Ti-4Al-2V alloy plates under different parameters, including the laser power, the welding speed, and the defocus distance, were manufactured and analyzed. The results showed that adjusting the combination of 4.2 kW of laser power, a 20 mm/s welding speed, and a −2 mm defocus distance could achieve a penetration depth exceeding 6 mm. Porosity defects were prone to forming in the middle and bottom parts of the fusion zone, due to rapid cooling. The microstructure of the fusion zone was mainly needle-like α martensite, which precipitated in the form of specific clusters. The interior of a cluster was composed of three types of variants with /60° phase interfaces to achieve the lower boundary’s energy. Affected by the microstructure and welding defects, the strength of the weld joint was basically similar under different welding conditions, namely about 720 MPa, slightly higher than that of the base metal, while the rupture elongation at breaking decreased by more than 50%. The micro-Vickers hardness of the weld joints was about 50–60 HV higher than that of the base metal, while the impact toughness was about 40 KJ, almost half that of the base metal. This research lays a solid foundation for the engineering application of laser welding of Ti-4Al-2V alloys.