Journal of University of Science and Technology Beijing, Mineral, Metallurgy, Material最新文献

By optical inspection of macro-etched metallography and electron back-scattered diffraction (EBSD) mapping, this paper analyzed the microstructure of austenitic stainless steel strips produced with an equal-diameter twin-roll strip caster. The results indicate that the microstructure of the strips includes two columnar zones with highly compact dendrites and one equiaxed zone. The characteristics, such as grain size and growing direction of columnar grains and equiaxed grains, were investigated. An additional transitional area with many finer grains between the columnar zone and the equiaxed zone was found. As shown in EBSD analysis, small angle boundaries exist both in the columnar zone and the equiaxed zone, although they are especially more in the transitional area. Additionally, some 〈111〉 twin boundaries were found in the microstructure of the strips.

With the consideration of slip deformation mechanism and various slip systems of body centered cubic (BCC) metals, Taylor-type and finite element polycrystal models were embedded into the commercial finite element code ABAQUS to realize crystal plasticity finite element modeling, based on the rate dependent crystal constitutive equations. Initial orientations measured by electron backscatter diffraction (EBSD) were directly input into the crystal plasticity finite element model to simulate the development of rolling texture of interstitial-free steel (IF steel) at various reductions. The modeled results show a good agreement with the experimental results. With increasing reduction, the predicted and experimental rolling textures tend to sharper, and the results simulated by the Taylor-type model are stronger than those simulated by finite element model. Conclusions are obtained that rolling textures calculated with 48 {110}〈111〉+{112}〈111〉+{123}〈111〉 slip systems are more approximate to EBSD results.

In the stainless steel 304 (SUS 304) wire drawing process, optimizing the die life and wire tensile strength, which are the larger-the-better quality characteristics (QCH) types, is of main interest. Three control factors, involving reduction ratio, lubricant temperature, and drawing speed, were investigated utilizing L₉(3⁴) orthogonal array (OA). The grey relational analysis was conducted for the normalized signal-to-noise (S/N) ratios. The ordinal value of the grey grade was then used to decide optimal factor levels. The anticipated improvements in die life and wire tensile strength were estimated 25.31 h and 22.50 kg/mm², respectively. To decide the significant factor which had effect on each QCH and predict the average value of each QCH, analysis of variance (ANOVA) was performed for S/N ratio and QCH. Confirmation experiments were then conducted, where a good overlap was noticed between the predicted and confirmation intervals for each QCH. The Hotelling T² and the sample generalized variance control charts were finally utilized in controlling and monitoring future production. In conclusion, the grey relational analysis utilizing Taguchi method is an effective approach for optimizing the die life and wire tensile strength for SUS wire drawing process.

The cylindrical billets of a Mg-3Al-1Zn (AZ31) alloy were synthesized by spray deposition processing. The microstructure evolution and mechanical properties of the alloy were investigated. The results reveal that the microstructure of the AZ31 alloy is refined significantly by spray deposition processing. A homogeneous and equiaxial-grain structure with an average grain size of 17 μm is obtained. Further grain refinement with an average grain size of 5 μm is attributed to dynamic recrystallization during extrusion processing. The great increase in the density of grain boundary nucleation sites by the finer initial grain sizes makes the dislocation pile-ups near subgrain boundaries being absorbed easily by the boundaries, resulting in an accelerated recrystallization process. The average tensile ultimate and yield strengths of the extruded rods are 321 MPa and 237 MPa, respectively, with an elongation of 15.2% at room temperature, which are remarkably higher than those of the conventional as-cast AZ31 alloy.

It was reported that both dielectricity and magnetism at room temperature were appreciably improved in Bi₄Fe₂TiO₁₂ film compared with Bi₄Fe₂TiO₁₂ bulk. X-ray diffraction profiles reveal similar crystalline nature and random orientation of the two, but X-ray photoelectron spectroscopy (XPS) experiments indicate that it is 1.4 eV lower binding energy of core-state O1s in the film relative to that of the bulk, so the improvement of multiferroics in the film is attributed to oxygen vacancies and high fraction of interface. The results have promising applications in multifunctional integrated devices.

Short carbon fibers were treated at high temperatures around 1100°C through chemical vapor infiltration technology. A thinner layer of pyrocarbon was deposited on the fiber surface. The dispersion of carbon fibers in a cement matrix and the mechanical properties of carbon fiber/cement composites were investigated by scanning electron microscopy (SEM) and other tests. The reflectivity of electromagnetic waves by the composites was measured in the frequency range of 8.0–18 GHz for different carbon fiber contents of 0.2wt%, 0.4wt%, 0.6wt%, and 1.0wt%. The results show that the reflectivity tends to increase with the increase of fiber content above 0.4wt%. The minimum reflectivity is −19.3 dB and the composites exhibit wave-absorbing performances. After pyrocarbon is deposited on the fiber, all the reflectivity data are far greater. They are all above −10 dB and display mainly wave-reflecting performances.

The aspects of two pipeline steels with different technologies were investigated by using transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD). The microstructure presents a typical acicular ferrite characteristic with fine particles of martensite/austenite (M/A) constituent, which distributes in grains and at grain boundaries. The bulk textures of the pipeline steel plate are {112}〈110〉 and 〈111〉 fibers, respectively, and the {112}〈110〉 component is the favorable texture benefiting for drop weight tear test. Moreover, low angle boundaries and low coincidence site lattice boundaries are inactive and more resistant to fracture than high energy random boundaries.

Y₂O₃-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics with high-performance were prepared for dental application by use of the micro-emulsion and two-step sintering method. The crystal phase, morphology, and microstructure of the reaction products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). XRD results show that the ceramics mainly consist of tetragonal zirconia. Physical and mechanical properties test results show that the bending strength, fracture toughness, and the density of full sintered Y-TZP ceramics are 1150 MPa, 5.53 MPa·m^1/2, and 6.08 g/cm³, respectively, which suggest that the material is relatively suitable for dental restoration. The dental base crown machined with this material by CAD/CAM system exhibits a verisimilitude configuration and the material's expansion coefficient well matches that of the glaze. These results further indicate that the product can be used as a promising new ceramic material to fabricate dental base crowns and bridges.

The effect of cooling rate on the cooling γ′ precipitation behaviors was investigated in a Ni-base powder/metallurgy (P/M) superalloy (FGH4096). The empirical equations were established between the cooling rate and the average sizes of secondary and tertiary γ′ precipitates within grains and tertiary γ′ precipitates at grain boundaries, as well as the apparent width of grain boundaries. The results show that the average sizes of secondary or tertiary γ′ precipitates are inversely correlated with the cooling rate. The shape of secondary γ′ precipitates within grains changes from butterfly-like to spherical with the increase of cooling rate, but all the tertiary γ′ precipitates formed are spherical in shape. It is also found that tertiary γ′ may be precipitated in the latter part of the cooling cycle only if the cooling rate is not faster than 4.3°C/s, and the apparent width of grain boundaries decreases linearly with the increase of cooling rate.

The performances of a novel foaming agent used in the preparation process of aluminum foams were investigated, and the effects of some factors, such as addition of the foaming agent, foaming temperature on the porosity, and appearance of aluminum foams were also discussed. Experimental results show that the novel foaming agent has a wide decomposition temperature range and a mild decomposed rate; the foaming agent has the ability to enhance the viscosity of aluminum melt, as a result, an extra viscosifier such as Ca or SiC_p is unnecessary while using this foaming agent; the bubble-free zone in material decreases and the foaming efficiency increases with the increase of foaming agent; the bubble-free zone disappears and the foaming efficiency is near 100% when the addition of foaming agent is more than 1.4wt%; the porosity of the aluminum foam increases with the increase of foaming agent when the addition of foaming agent is less than 2.2wt%.