Materials Science Forum Vols. 747-748

Abstract: The Ni₃Al and Ni₃Al-B-Cr alloys were fabricated by the self-propagation high-temperature synthesis with hot extrusion method. Their microstructure and mechanical properties were studied by using combination of X-ray diffraction, optical microscopy, transmission electron microscopy and compression test. Analysis of X-ray spectra exhibited that the elemental powders had been transformed to the Ni₃Al phase after the self-propagation high-temperature synthesis processing. Microstructure examination showed that the alloy without extrusion consisted of coarse and fine grains, but the subsequent hot extrusion procedure homogenized the grain size and densified the alloy obviously. Transmission electron microscopy observations on the Ni₃Al alloy revealed that Ni₃Al, γ-Ni and Al₂O₃ particles were the main phases. When the boron and chromium were added, besides the β-NiAl phase, α-Cr phase and some Cr₇Ni₃ particles with stacking faults inside were observed. In addition, a lot of substructure and high-density dislocation arrays were observed in the extruded part, which indicated that the subsequent extrusion had led to great deformation and partly recrystallizing in the alloy. Moreover, the subsequent extrusion procedure redistributed the Al₂O₃ particles and eliminated the γ-Ni. These changes were helpful to refine the microstructure and weaken the misorientation. The mechanical test showed that the self-propagation high-temperature synthesis with hot extrusion improved the mechanical properties of the Ni₃Al alloy significantly. The addition of B and Cr in Ni₃Al alloy increased the mechanical properties further, but the compressive strength of the alloy was still lower than that synthesized by combustion. Finally, the self-propagation high-temperature synthesis with hot extrusion was a good method to prepare Ni₃Al alloy from powder.

Abstract: Compression and three-point bending tests were conducted at room temperature (RM) on the Mo₅SiB₂ (T2) alloy, which was prepared by sparking plasma sintering (SPS). It was found that almost no plastic deformation occured in the T2 alloy before failure but with a tremendous compressive strength of 2907 MPa. The fracture toughness determined from the single edge notch bend specimen is 3.34 MPa·m^1/2, similar to the value of 3.5 MPa·m^1/2 in ɑ-Al₂O₃. Once the cleavage crack initiated near the notch under continuous loads, it propagated on a certain plane (001) until the specimen completely fractured. In this work, the electronic structure was also calculated by the first-principles method, indicating that the contribution to RM brittleness is mainly caused by the covalent bonds which arrange alternately in the T2 phase.

Abstract: As a material intended for application in hot-dip galvanization, Ti28.15Al63.4Nb8.25Y (at %) coatings were deposited onto 316L stainless steel substrate using high velocity oxygen fuel (HVOF) and atmospheric plasma spray (APS), respectively. The influence of different thermal spraying techniques on the microstructure, phase transformation, porosity, bond strength and hardness values of Ti28.15Al63.4Nb8.25Y coatings was analyzed by scanning electron microscopy (SEM), X-ray diffraction, tensile test and other analysis methods. In addition, the thermal shock test of Ti28.15Al63.4Nb8.25Y coatings was carried out to evaluate the desquamation resistance and the model of invalidation. The results indicated that HVOF Ti28.15Al63.4Nb8.25Y coatings had more uniform and compact morphology than APS Ti28.15Al63.4Nb8.25Y coating and HVOF Ti28.15Al63.4Nb8.25Y coatings have lower porosity and oxide content. The coatings processed by HVOF had higher bond strength, microhardness and thermal shock resistance and displayed better mechanical properties than that prepared by APS.

Abstract: NiCr/Cr3C2-WS2-CaF2 mixed powders were prepared on 1Cr18Ni9Ti stainless steel substrate to fabricate high-temperature self-lubrication wear resistant composite coating by laser cladding. The microstructure of the coating was examined by means of XRD, SEM and EDS. The high-temperature dry sliding friction and wear behavior of the composite coating were evaluated by using a ball-on-block wear tester at room temperature, 300 and 600 , respectively. The results showed that the composite coating had a unique microstructure consisting of γ-(Ni,Fe) solid solution matrix, Cr7C3/WC reinforcement and CrS/WS2 lubricant particles. The hardness of NiCr/Cr3C2-WS2-CaF2 coating by laser cladding was lower than NiCr/Cr3C2 coating, but higher than stainless steel substrate significantly. It was because of the addition of solid lubricant. In the wear process, elevated temperature was helpful to form a lubricating transfer film. As a result, the NiCr/Cr₃C₂-WS₂-CaF₂ coating exhibited a notable anti-friction and wear resistance at 300 and 600 .

Abstract: In this study, the dry sliding wear behavior of Ni-based self-lubricating wear-resistant coating was characterized under various loads at 300 . Morphologies and compositions of the worn surface were analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The experimental results showed that with the increase of load, both the frictional coefficient and wear rate decreased firstly, then slightly increased. A small amount of debris dispersed on the worn surface of the coating under low load (2 N). Since the effects of oxide layer and lubricant particles spreading onto the worn surface, the coating exhibited superior friction, and improved the wear properties under moderate load (5 N). As the load further increased, till up to 10N, the worn surface started to appear some shallow grooves and craters. This was contributed to the dispersion of carbides and lubricant particles.

Abstract: In this study, Mg-7Gd-5Y-1Nd-0.5Zr alloy (EW75) was produced by melting method and then press-forged into large size plate. The properties of the Mg-7Gd-5Y-1.2Nd-0.5Zr alloy were optimized through T6 heat treatment. The microstructures of alloy were observed by means of optical microscopy (OM), scanning electron microscopy (SEM). Its mechanical properties under different heat treatment conditions were determined by tensile tests. The results indicated that increasing the solid solution temperature and prolonging the solid solution time can both lead to the dissolution of second phase in the alloy back into the matrix. The solid solution temperature affects the dissolution process more than the solid solution time. Grain growth occurred during the solid solution process. The grain size of the matrix enlarges with the increase of solid solution temperature. The tensile test result showed that the tensile strength of the alloy was significantly improved after T6 heat treatment. Its tensile strength in the same direction was nearly 40% up after T6 heat treatment. The analysis shows that T6 heat treatment can effectively eliminate the larger deformed precipitates and beneficial to the formation of hard precipitates, which leads to an improvement in the alloys tensile strength.

Abstract: One kind of deformed magnesium alloy with high strength and high plasticity was made to be bicycle parts. Meantime, the microstructure and mechanical properties of the magnesium alloy were analyzed. In addition, the deformation and welding technology of taper pipe and shaped tube of bicycle were studied. The results showed that the bicycle parts manufactured through extruding and welding of the new type magnesium alloy have high strength, good plasticity. The tensile strength can reach 321.3 MPa and the elongation is more than 20%. By adopting the new welding process of internal mending and cuttage grafting, the after-welding-strength is 90.2%. of the base metal Meanwhile, the industrialization level can meet the requirements of European Union (EU) vehicle standards.

Abstract: Cooling curves and residual deflection during spray quench process are measured by using thermometric instrument and spiral micrometer, separately, in extruded EW75 magnesium alloy. By using Numerical analysis, temperature and stress with the change of time during quenching were also investigated. The results showed that: when the spray speed was 6L/min, the steam film stage was obvious, as the speed reached to 9L/min, steam stage disappeared. The residual deflection increased with the spray speed increased. When the spray speed was 6L/min, the residual deflection was 3.38mm and increased to 3.88mm as the spray speed increased to 9L/min. But with the increase of spray speed, the growth rate of deflection induced. The results of the numerical simulation indicated that during the quench process, the surface cools faster than body and shrink more severely at first, leading stress distribution is compress at surface while tensile in body. Plastic deformation happens at this time. And then along with the surface cooling speed reduced, the body cool speed is greater than surface; the shrink in body is also greater, thus the compress stress turn to be tensile and contrary change take place at surface. With the increasing distance from the cross section, the tensile stress is reduced gradually, in 5.3mm, there exists a zero stress layer, then turns to compressive stress.

Abstract: Micro arc oxidation coatings were prepared on AZ91D magnesium alloy in order to evaluate the role of electrical parameters under different work mode. Taking HV hardness and pit corrosion time as investigation index, the optimum process parameters were obtained through orthogonal experiment method. The morphology of coating was observed by scanning electron microscopy (SEM). The results showed that the coatings with dense and few micro pores can be obtained under constant current and power mode, and this kind coatings are of good corrosion resistance and have HV micro-hardness compared with those under constant voltage mode.

Abstract: Evolution of microstructure and mechanical properties of the Mg-9Gd-3Y-0.5Zr base alloys without 0.5% Zn addition (GWK930) and with Zn addition (GWZK9300) have been investigated during processing by extrusion, rolling and ageing. It was found that the yield/ultimate strength of the two as-rolled alloys increased with the rolling strains. Twinning was the main deformation mechanism in GWK930 alloy while few twinning could be detected in the GWZK9300 alloy due to the dense secondary phases such as LPSO structure. The (0002) texture intensity in two as-rolled alloys increased with the rolling reduction, however, a typical rolling basal plane texture with single peak was found in GWZK9300 while a bimodal texture was found in the GWK930 alloy. Compared with GWK930 alloy, GWZK9300 alloy exhibited higher strength and ductility after rolling, annealing and ageing treatments. The finer grain size was due to more recrystallization fraction, while more fine secondary phases dispersed in matrix. And the basal texture strengthening after rolling deformation and subsequent heat treatment are suggested to be responsible for the superior comprehensive mechanical properties of GWZK9300 alloy.