Xiangyi Xue

Microstructure and hydrogenation kinetics of Mg2Ni-based alloys with addition of Nd, Zn and Ti

Abstract In order to enhance the hydrogen absorption kinetics of the Mg 2 Ni-based alloys, metal elements (Nd, Zn, and Ti) were added during melting process, respectively. The Mg 2 Ni-based alloys were melted using an electric resistance furnace under the protection of the covering reagent to prevent the oxidation and the evaporation of magnesium. Phase compositions and microstructures of as-cast alloys were characterized by XRD and SEM equipped with EDS. Hydrogenation kinetics of experimental alloys were investigated by the constant volume method using a Sievert-type apparatus. The addition of Nd, Zn or Ti elements to Mg 2 Ni results in the formation of minor phases Mg 6 Ni and Ni 3 Ti. Nd and Zn are dissolved in α -Mg, Mg 2 Ni and MgNi 2 phases in Mg 2 Ni-based alloys. With the addition of Nd, the hydrogen content of the first absorption is 2.86% in mass fraction, which is higher than that of the Mg 2 Ni. Hydrogen absorption kinetics and activation properties of Mg 2 Ni-based alloys are improved evidently. During the initial three hydrogenation/dehydrogenation cycles, the hydrogen absorption capacity and kinetics properties have been improved for alloys with the addition of transition element Zn or Ti. The kinetics properties of the experimental alloys and absorbing reaction mechanism were also analyzed with the help of the Hirooka kinetics model.

Deposition of Fe-based metallic glass coatings by Air Plasma Spraying process

Fe-based metallic glass coatings were prepared on Ti?3Al?5Mo?5V?4Cr?2Zr substrate by Air Plasma Spraying (APS). The effect of spraying power (arc current) on amorphous phase content and properties of the coatings was investigated. It was revealed that the coating sprayed at arc current of 150 A was nearly fully amorphous although the spraying powders were not. The amorphous phase content in the coatings decreased with increasing arc current. Vickers microhardness and corrosion behaviour of the sprayed coatings were also studied. The highest cross-section microhardness of the coatings was Hv0.05908, roughly three times as high as that of the substrate. The coating sprayed at arc current of 300 A exhibited the best excellent corrosion resistance.

Effects of hot compression on carbide precipitation behavior of Ni—20Cr—18W—1Mo superalloy

Abstract The effect of hot compression on the grain boundary segregation and precipitation behavior of M6C carbide in the Ni–20Cr–18W–1Mo superalloy was investigated by thermomechanical simulator, scanning electronic microscope (SEM) and X-ray diffraction (XRD). Results indicate that the amount of M6C carbides obviously increases in the experimental alloy after hot compression. Composition analyses reveal that secondary M6C carbides at grain boundaries are highly enriched in tungsten. Meanwhile, the secondary carbide size of compressive samples is 3–5 μm in 10% deformation degree, while the carbide size of undeformed specimens is less than 1 μm under aging treatment at 900 and 1000 °C. According to the thermodynamic calculation results, the Gibbs free energy of γ-matrix and carbides decreases with increase of the compression temperature, and the W-rich M6C carbide is more stable than Cr-rich M23C6. Compared with the experimental results, it is found that compressive stress accelerates the W segregation rate in grain boundary region, and further rises the rapid growth of W-rich M6C as compared with the undeformed one.

Precipitation Behavior of σ-FeCr Phases in Hastelloy C-2000 Superalloy Under Plastic Deformation and Aging Treatment

Precipitation behavior and structural feature of the σ-FeCr phase in Hastelloy C-2000 alloy under plastic deformation and aging treatment are investigated using scanning electron microscopy, transmission electron microscope, and x-ray diffraction. The plate-shaped σ-FeCr phase can precipitate in the alloy by plastic deformation and aging treatment because of the increase of both defect density and heterogeneity of alloying elements. This phase is distributed along the interface between matrix and deformation twins. The appearance of unidirectional faults on surface of the σ-FeCr phase indicates that it may be a metastable-phase. Because of the precipitation of the brittle σ-FeCr phase and the increase of defects’ density such as dislocation and deformation twins in the matrix, mechanical properties of the alloy are improved remarkably.

Effect of hot-forging on beta phase transformation of a high niobium containing titanium aluminide alloy

In this paper, ingot breakdown process of a high Nb containing TiAl alloy with a chemical composition of Ti–42.63Al–8.11Nb–0.21W–0.09Y (at.%) has been investigated under conventional forging conditions. It was found that the present alloy possesses superior hot-workability that can be successfully forged by conventional upsetting route due to the appearance of large amount of β/B2 phase, though shear band was observed in the forged-pancake. Further studies revealed that hot-working performed in (α + β) phase region which can effectively impede the β → α transformation and thus significantly increase the volume fraction of β/B2 phase. In contrast, the amount of β/B2 phase was notably reduced by heat treatment at the same conditions. This stress-induced effect is considered to be responsible to the superior hot-workability of the present alloy and the mechanism has been discussed and reasonably clarified. It was also suggested that the stress-induced effect has practical significance that it allows the implementation of conventional multi-step forging process which can develop fine and uniform microstructures suitable for secondary processing.