Xue Xiangyi

Study on Microstructure Characterization of YBCO Bulk Prepared by Directional Seeded Infiltration-Growth

Abstract The seeded infiltration-growth process is an alternative to replace conventional melt processing techniques for the preparation of bulk YBa2Cu3O7-s (Y-123) with finely dispersed small size of Y2BaCuO5 (Y-211) particles. The directional solidification is also a quite effective technique to produce well-oriented cylinder grains. Well-oriented single domain Y-123 bulks can be fabricated by combining seeded infiltration-growth and directional solidification technique (DSIG). The infiltration-growth process is able to control the size and distribution of Y-211 particles in the final Y-123 superconductor and can fabricate net-shape bulk. Y-211 particles and a liquid phase precursor were used as starting materials in the present investigation. The main parameters (temperature gradient, configuration and maximum processing temperature) of the directional seeded infiltration-growth process were discussed in the present paper. The result shows that Y-211 particle sizes upon using 035 liquid phase composition are smaller than those upon using other two liquid phase compositions.

Isothermal Oxidation Behavior of TiAl-Nb-W-B-Y Alloys with Different Lamellar Colony Sizes

Abstract The oxidation behavior of full-lamellar Ti-42.5Al-8Nb-0.2W-0.2B-0.1Y alloys with different colony sizes was investigated by a thermogravimetric method at 900 °C for 100 h in air. The microstructure and composition of the alloy as well as the oxide scales were analyzed by OM, SEM, EDS and XRD. It is found that the oxidation kinetic of the present TiAl alloys accords with the parabolic law, and the alloy with larger lamellar colony has a better oxidation resistance than the one with smaller lamellar colony. The mass gain of the two samples with large and small lamellar colony after 100 h oxidation is 6.45 and 7.62 g/cm 2 respectively. And correspondingly the thickness of oxide scale is about 5 and 7 μm, respectively. Based on the analysis of oxidation kinetic and microstructure characterization, we conclude that the structural differences between the two samples which affect the diffusion rate of O element into the matrix result in different oxidation behavior.

Phase Evolution of Diffusion Bonding Interface between High Nb Containing TiAl Alloy and Ni-Cr-W Superalloy

Abstract The diffusion bonding between high Nb containing TiAl alloy and Ni-Cr-W superalloy was carried out under the condition of 1000 °C-50 MPa-60 min. The microstructure and the composition of diffusion bonding interface were characterized by optical microscope (OM), scanning electron microscope (SEM), energy disperse spectroscopy (EDS) and X-Ray diffraction (XRD). The phase evolutions of bonds with and without Ti/Cu interlayers were analyzed. The results indicate that the interfacial phases of high Nb containing TiAl alloy/Ni-Cr-W superalloy joint consist of γ -TiAl, Ni 2 AlTi, Ni 3 Ti, (NiCr)ss (ss represents solid solution), γ phase, while the interfacial phases of joint with Ti/Cu interlayers consist of Ti 3 Al, Tiss, Ti 2 Ni, rich Cu-NiAlTi, α 2 -Wss, Ni 2 AlTi, (NiCr)ss, γ phase. Besides, Cr and W atoms are concentrated in the zone near Ni-Cr-W superalloy because their diffusion is difficult. Ti/Cu foils can promote diffusion and reaction; therefore the micro-cracks in the bonded joint will be avoided efficiently.

Interface Reaction between Ceramic Moulds and High Nb-TiAl Alloys

Abstract The interface reaction between Ti-44Al-8Nb-0.2W-0.1B-0.1Y (at%) alloy and Al 2 O 3 /ZrO 2 /Y 2 O 3 crucibles was investigated. The thicknesses of the interface reaction layers are about 40, 170 and 20 μm. The most interesting is the microstructure transformation of Ti-44Al-8Nb-0.2W-0.1B-0.1Y solidifying in different crucibles. After solidifying in Al 2 O 3 /ZrO 2 /Y 2 O 3 crucibles, the oxygen contents in Ti-44Al-8Nb-0.2W-0.1B-0.1Y alloy are measured to be 0.35, 0.41 and 0.11 (at%). During melting and solidification, oxygen in crucibles will diffuse into the alloy matrix. High oxygen content causes microstructure transformation. Peritectic reaction occurs in the alloys solidifying in Al 2 O 3 and ZrO 2 crucibles. For contrast, Ti-49.5Al-2Cr-2Nb alloy was chosen to make investigation. No visible microstructure difference is found in alloys solidified in various crucibles though the oxygen contents are determined as 0.40, 0.63 and 0.25 (at%).

Precipitation Behavior of Second Phases during Isothermal Oxidation of Hastelloy C-2000 Alloys

Abstract The precipitation behavior of the second phase in Hastelloy C-2000 alloy matrix was studied after isothermal oxidation at 800 °C for 100 h in air. Mo-rich phase was precipitated in the alloy matrix after oxidation. According to the determination Mo-rich phase with diamond cubic structure is Mo 3 Ni 3 C type carbide. Morphology characteristics of Mo-rich phase with both no-continuous and continuous irregular strip are presented along grain boundaries and in matrix, respectively. However, some variation of morphologies will take place at both grains and grain boundaries after deep etching, i.e. a large number of white flocculent structure are distributed along grain boundaries, and the corrosion pits with a diamond structure appear in intragranular. Ni-rich and Cr-rich areas are more easily corroded during deep etching due to a more negative electrode potential in the areas.

Effect of Processing Techniques on Precipitation Behavior of Secondary Phase Particles in Zr-1Nb-0.01Cu Alloy

Abstract The precipitation behavior of secondary phase particles (SPPs) was investigated in Zr-1Nb-0.01Cu alloy prepared by various processing techniques. The results show that with the decrease of cold rolling and annealing steps, intermediate annealing temperatures, final annealing temperatures and time, the average sizes of SPPs decrease accordingly. The β -Zr phase formed at the intermediate annealing temperature (higher than 640 °C) is hard to dissolve in the matrix after final annealing treatment. During the long-time final annealing process, the SPPs with smaller size coalesce into larger particles by diffusion through the matrix gradually because of Ostwald ripening mechanism. It shows that the decrease of intermediate annealing temperature is more effective than other processing techniques for minimizing the size of SPPs. The average particle size smaller than 50 nm can be achieved by low intermediate/final annealing temperature (≤520 °C) or short annealing time (≤2 h) treatment. The investigations in the present paper are very meaningful to control the SPPs in Zr-Nb based alloys.

Grain Boundary Segregation and Mechanical Properties of an Aged Ni-20Cr-18W-1Mo Superalloy at Different Temperatures

Abstract The influence of aging temperature (200 to 800 °C) on grain boundary segregations in a Ni-20Cr-18W-1Mo superalloy was investigated by scanning electronic microscope (SEM), transmission electron microscopy (TEM), electron microprobe analyzer (EMPA) and a mechanical testing machine. Results indicate that the grain boundary segregation critical time of sulfur and phosphorus decreases with increasing of the aging temperature. Increasing of the aging temperature has a conspicuous effect on the concentration distribution in the grain boundary and the grain core. Grain boundary concentrations of sulfur and phosphorus increase with raising the testing temperature until a peak value is obtained at 650 and 400 °C, respectively, which is the essential reason to the declined mechanical properties from 200 to 600 °C

Effect of Spraying Parameters on Microstructure and Thermal Stability of Fe-Based Metallic Glass Coatings

Fe-based metallic glass coating was prepared on Ti-3Al-5Mo-5V-4Cr-2Zr substrate by air plasma spraying (APS). The effect of spraying power (current) on amorphous phase content and properties of the coating were investigated. It was revealed that appropriate spraying current was necessary to produce fully amorphous coating, and the amorphous phase content in the coatings decreased with the increase in spraying current. The thermal stability and cross-section Vickers microhardness of the coating were also studied by DSC and microhardness tester, respectively. The highest cross-section microhardness of the coating was HV0,51260, roughly four times as high as that of the substrate.