稀有金属材料与工程最新文献

This paper aims to investigate the influential mechanism of grinding and sand blasting pretreatment of CoCr alloy manufactured by SLM (selective laser melting) process on the metal-porcelain bonding of the restorations, and then provides references for SLM manufacturing CoCr alloy dental crown and bridge restorations with good mechanical properties. Three groups of CoCr alloy substrates were manufactured by SLM process with optimal parameters. The samples were subjected to different grinding and sand blasting pretreatment, and the porcelain sintering was carried out with the same parameters. The samples of different groups were analyzed by three-point bending test, roughness test, SEM and EDS. Results show that the metal-porcelain bonding strength of samples in group a with grinding and sand blasting is 36.79±0.49 MPa, which is obviously higher than those of group b with no grinding but sand blasting and group c with only grinding. The average metal-porcelain bonding strength of the three group samples is higher than the ISO9693: 1999 standard minimum 25 MPa. The metal-porcelain bonding effect of group a and c is better than that of group b, and the element diffusion on metal-porcelain interface is observed in all three groups. The results also show that there are significant differences among the three groups on residues and element distribution of metal-porcelain peeling surface. The final conclusion is that the surface roughness obtained by the pretreatment of sand blasting and grinding can ensure the strong chemical combination of metal and porcelain and considerable mechanical properties combination, which can improve the performance of metal-porcelain restorations manufactured by SLM.

The mechanical state at the crack tip is one of the major factors affecting the stress corrosion cracking (SCC) growth rate in structural materials of the nuclear power plant. To understand the crack growth driving force and its effect on SCC growth rate in the whole process of SCC, a finite element model of the SCC growth process was built using a commercial software ABAQUS. Sequentially the working load, residual stress, and the film induced stress (FIS) produced by the oxide film formation in front of the crack tip during SCC initial stage were discussed in this paper. The results indicate that the FIS produced by the formation of oxide film is the main crack growth driving force during the initial stage of SCC. While the working load and residual stress gradually become the main crack growth driving force as SCC crack advances.

The evolution of metal flow, microstructure and corrosion properties of 5A06 alloy after friction stir welding were studied. Results show that the metal on the advancing side is curled directly into the threads and then is extruded down into the weld nugget zone by complex motions of torsion and swirl. However, the material on the retreating side encounters chaotic flow patterns. The electro backscattering diffraction results indicate that the grains in stir zone are pronouncedly refined by dynamic recrystallization, and the average grain size is about 6 μm. The scanning electron microscope analyses display that β-phase (Al₃Mg₂) and Al₆(Mn, Fe) are dispersedly distributed in the stir zone and the Al₆(Mn, Fe) is smaller than that in base material. The hardness profile is characterized by material softening along the cross-section and the minimum hardness (HV) value of 720 MPa is located in the interface between stirring zone and thermo-mechanically affected zone. It is evident that the stir zone exhibits better corrosion resistance than the base material. The corrosion potential of stir zone is 35 mV larger than that in base material (−0.725 V).

Cu/Ti₃SiC₂/C/MWCNTs/Graphene/La nanocomposites were fabricated by vacuum hot-pressing sintering and hot isostatic pressing. Effect of lanthanum (La) content on tribological behavior of Cu/Ti₃SiC₂/C/MWCNTs/Graphene/La nanocomposites against GCr15 was evaluated using a wear and abrasion tester. In addition, influence of La content, normal force and rotational speed on tribological behavior of the nanocomposite was investigated to analyze their interaction mechanism. The mutual influence of La content, normal force and rotational speed was analyzed by orthogonality analysis, variance analysis and range analysis. The morphologies of worn surfaces have been observed and analyzed by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). Results demonstrate that La content plays an exclusively significant role in the friction and wear performance of the Cu/Ti₃SiC₂/C/MWCNTs/Graphene/La nanocomposites. The wear mechanism of the nanocomposites with 0.05 wt% La is abrasive wear, delamination wear and oxidation wear. And wear mechanism of the nanocomposites with 0.1 wt% and 0.3 wt% La is adhesive wear and oxidation wear.

This paper presented a fundamental investigation on the formation mechanism and compatibility of microstructure/mechanical property of Fe-Cr-Ni-B steel samples, which were built by the divided-area forming and integral connection methods. Results indicate that the stress at the edge of the specimen produced in additive manufacturing is reduced by the divided-area forming and integral connection method. According to the microstructure analysis using stereology microscopy/optical microscopy/scanning electron microscope/X-ray diffractometer/Schaeffler diagram, the macrostructure is distributed in strip band geometry and the microstructures consist of dendrites with the intermetallic phases containing austenite phase, boride/matrix eutectic phase. Additionally, the macrostructure strips near the bonding line bend to the building direction and are discontinuous because of the unique forming method. However, the microstructures and composition of the samples are homogeneous. Due to the existence of boride and the finer microstructures, mechanical properties analysis shows that the alloy has high hardness, high ultimate strength and bad deformability. The hardness distribution is homogeneous apart from some positions of the re-melting zone and the heat-affected zone near the bonding line, which have a relatively lower hardness because of differences in microstructure.

Microstructure and mechanical properties of transient liquid phase (TLP) bonded joints and the post bonding heat treatment (PBHT) for CB2 heat resistant steel using BNi-2 insert alloy were investigated. The Cr-rich borides and Cr-Mo borides generated in the transition region (TZ) and diffusion-affected zone (DAZ) reach the peak values in size and quantity when the isothermal solidification is completed. Subsequent elevation in bonding temperature and increment in bonding time result in gradual disappearance of such Cr-rich and Cr-Mo borides, as well as increased percent of BN precipitates. After PBHT the Cr-rich borides almost disappear while the size and amount of BN precipitates rise. The maximum tensile strength reaches to 934 MPa for the joint bonded at 1150 °C for 1800 s with elongation of only 5.3%. PBHT result in significant improvement in ductility of the joint as elongation of 20% is achieved with a decreased strength of 720 MPa, while the fracture takes place at base materials.

Isothermal compression tests in the temperature range of 1123∼1273 K and the strain rate range of 0.001∼1 s⁻¹ were conducted to analyze the elevated temperature deformation behavior of TA7 titanium alloy for the purpose of acquiring optimum processing parameters. Besides, a modified parallel constitutive model was put forward to illustrate the high temperature flow stress as a function of the strain rate, deformation temperature and true strain. Thereafter, the processing map which was on the basis of dynamic materials model was established. The investigation was made from the microstructure of compressed TA7 titanium alloy specimens to validate processing map. According to the results, the deformation temperature of 1223 K and strain rate of 0.001 s⁻¹ are the best processing parameters for the TA7 alloy. In addition, the instability regions are in a relatively lower temperature-higher strain rate region.

Hastelloy C-276 was subjected to 10% deformation and annealed at 1100 °C for different time. The fractions of Σ3ⁿ (n=1, 2, 3) boundaries and the connectivity of random boundary networks were characterized by electron back-scatter diffraction (EBSD). The results show that the Σ3 boundary fraction is hardly increased during the annealing period. Instead, the fractions of Σ9 and Σ27 boundaries have a dramatic change with annealing time, whose peaks are arisen after annealing for 15 min. Meanwhile, the corresponding connectivity of random boundary network is effectively disrupted. This can be attributed to the incoherent Σ3 boundary producing more Σ9 and Σ27 boundaries and incorporating those boundaries into the random boundary networks. Actually, the connectivity of random boundary networks can be definitely identified by Σ(9+27)/Σ3 radio.

Ultra-low-density bulk nanoporous gold (NPG) is required in inertial confined fusion (ICF) experiments. Existing bulk NPG has some limitations in ICF application due to non-uniformity structure or residues of C originated from organic template. Here, combined with dealloying, NPG with submicron cavities and nanopores was prepared by nano-SiO₂ templates. However, the smaller templates bring greater challenges for the volume shrinkage control during drying as gold content on each shell is extremely low for ultra-low density. Therefore, the influence of drying methods on volume shrinkage was investigated. The results show that volume shrinkage is up to 86.41% during conventional drying, but it is controlled below 4% by supercritical CO₂ drying. Finally, NPG with ultra-low-density of 0.35 g/cm³ was fabricated. Compared to NPG prepared by micro templates, the NPG with 500 nm spherical cavities and 2∼70 nm nanopores reduces the size difference between large cavities and nanopores and significantly improves the uniformity of its structure.

During the machining of powder metallurgy (PM) superalloy parts, the machined surface bears severe plastic deformation which leads to the transformation of microstructure and mechanical properties. White layer which reflects the transformation of microstructure and mechanical properties, often appears on the top of machined surface during hard machining of PM superalloy. White layer has significant effect on the machined surface integrity. The effects of cutting speed on the white layer formation have been investigated in order to reveal the transformation of the microstructure and mechanical properties in the machined surface of PM superalloy FGH95. Results show that white layer thickness increases with the increasing of cutting speed. The machined surface exhibits densification with no obvious structural characteristics. FGH95 superalloy bulk material exists in the form of Ni-based solid solution, while the microstructure of white layer is significantly different from that of bulk materials. It's because of the microstructure of Ni-based solid solution which has transformed during the cutting of FGH95. The higher the cutting speed is, the more obvious the grain refinement is. A higher cutting speed could also lead to higher values of hardness in white layer. Residual stresses in the machined surface of FGH95 are tensile in all cutting conditions, which show an increasing trend with the increasing of cutting speed. This research can provide the theoretical basis for the investigation and controlling of machined surface quality.