稀有金属材料与工程最新文献

Under different stress concentration factors K_t and stress ratios R, the high cycle fatigue (HCF) properties and fracture morphologies of Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) forgings and bars in longitudinal orientation were studied at room temperature. The results show that both the fatigue strength of forgings and bars increases with the increasing stress ratio and decreases with the increasing stress concentration factor. The fatigue strength of forgings is 1.08∼1.57 times greater than that of bars. There are primary α phase particles in the forgings, with more uniform distribution and smaller size. However, there exist residual casting primary α phase particles with the shape of bulk or strip in bars.

The La_xHo_2-xFe₁₇ (x=0.0, 0.2, 0.4, 0.6, 0.8) powders were prepared by arc melting and high energy ball milling. The influence of the La substitution on phase structure, morphology, magnetic properties and electromagnetic parameters were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating-sample magnetometry (VSM) and vector network analyzer (VNA), respectively. The results show that the saturation magnetization increases and the average particle size increases with the increase of La content. The minimum absorption peak frequency shifts towards a lower frequency region with the increase of La content. The minimum reflection loss (RL) of La_0.2Ho_1.8Fe₁₇ reaches –28.72 dB at 8.72 GHz, and the frequency bandwidth of RL<–10 dB reaches about 2.32 GHz with the best matching condition d=1.8 mm. The reflection loss with the thickness ranging of 1.2∼2.4 mm could reach –10 dB, which indicates that the particles are considered as the promising microwave absorbing materials with better absorption properties.

Al₂O₃-Pt/YSZ-Pt double layer composite coatings were prepared by cathode plasma electrolytic deposition (CPED) on the superalloy with a NiCoCrAlY bond-coat. The composite coatings consist of an Al₂O₃-Pt film and a YSZ-Pt top-coat and show a good adhesion to the metallic bond-coat. It is demonstrated, from the cyclic oxidation test and mechanical properties test, that such coatings possess good oxidation and spallation resistance. These beneficial results can be attributed to these effects: owing to the extremely low oxygen diffusion rate of Al₂O₃-Pt film, such composite coatings can inhibit further oxidation of the bond-coat; the mechanical properties of Al₂O₃-Pt/YSZ-Pt composite coating are improved by the toughening effect of Pt particles.

The Pd_71.5Cu₁₂Si_16.5 metallic glass was synthesized as wide ribbons by arc melting and copper roller spinning. The structures were determined by X-ray diffraction patterns using the conventional X-ray diffractometer and short wavelength X-ray stress analyzer, and the fully glassy state of the ribbons is confirmed. Multiple hydrogen absorption and desorption cycles at room temperature under 100 kPa were carried out on the samples. No destruction after more than 10 cycles is observed, which demonstrates good hydrogen embrittlement resistance. The hydrogen permeation properties of the Pd_71.5Cu₁₂Si_16.5 metallic glass and its crystallized counterpart were further tested using a direct permeation method. In the supercooled liquid region, the hydrogen permeation rate is obviously higher for the metallic glass form, which could be explained by the increasing free volumes introduced during isothermal stage in this range.

Al₂O₃/cerium oxide composite films were prepared on the surface of aluminum by an anodic oxidation and chemical conversion method. The aluminum was firstly anodized in sulfuric acid, and then the anodized aluminum was treated with the Ce(NO₃)₃ solution. The effect of different concentrations of 1H-Benzotriazole (BTA) on the corrosion resistance of Al₂O₃/cerium oxide composite films was investigated. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) with energy dispersive spectrometer (EDS), and electrochemical workstation were used to characterize the properties of the composite films. The results show that the deposition effect of cerium ions is optimal in 0.5 g/L BTA. At the same time, the surface uniformity and smoothness of the composite films are both enhanced. And the corrosion resistance of the composite film is strengthened.

A comprehensive performance of formed parts by AA7075-O sheets was studied in friction stir incremental forming (FSIF). Various tool rotation speeds were set to form two typical parts (truncated funnel and pyramid frustum). The formability, surface quality, tensile strength, micro-hardness and thickness distribution of the formed parts were investigated. Results show that the formability generally increases as tool rotation speeds increase. On the contact surface, variation trends of the surface roughness are different in horizontal direction (parallel to tool path) and vertical direction (perpendicular to tool path). However, the surface roughness on the non-contact surface is almost not affected by tool rotation speed. Furthermore, mechanical properties including tensile strength and surface micro-hardness in formed parts are obviously enhanced compared to those of the initial sheet, of which the hardness values begin to fall down gradually after rotation speed reaches 3000 r/min. As for thickness measurement, more uniform thickness distribution of formed parts can be obtained at relative high tool rotation speeds. In general, parts formed at high rotation speed have a better comprehensive performance except the mechanical property.

The microstructure and high temperature oxidation behavior of Co-Cr-Y₂O₃ modified pack aluminide coatings on Ni-based superalloy GH586 were investigated. The results show that the specific Co-Cr-Y₂O₃ modified aluminide coating has a mass gain of only 0.36 mg/cm² after oxidation at 1000 °C for 100 h, which is much less than that of the substrate at 1000 °C. From X-ray diffraction, the phase of the coatings is mainly AlNi, and after oxidation at 1000 °C for 100 h the denser oxidation scale is composed of Al₂O₃, Cr₂O₃, and CoCr₂O₄. Surface and cross sectional morphologies were characterized by scanning electron microscope (SEM). The coating exhibits a better resistance to high temperature oxidation, compared to the oxidation film of GH586 without coating. Moreover, the growing Cr (W) rich phase is gradually gathered at the grain boundaries during the oxidation, and it is beneficial to providing more Cr element for the dense oxidation film, which is mainly attributed to the excellent high temperature oxidation resistance.

The space holder technique is widely used to fabricate metal foams, especially titanium foam. However, how to obtain the desired porosities is a big challenge for this technique, because they are not always equal to the expected ones. The results of the previous study (i.e., P = ax + b, where a = 1/(1 + δ), b = δ/(1 + δ)) give a very interesting conclusion that is the change rate of pore volume (δ) is an indefinite mathematical constant. Based on the research work, we obtains a new result by establishing a mathematical model, which can be expressed as equation δ = ϕ − 1. Here, ϕ is the length index product of the ratio between the actual length and the designed length of the sintered metal foam. It reveals that the length index product (ϕ) is also an indefinite mathematical constant and we can measure its value. Therefore, solving δ means both a and b are solved, so the porosity (P) of titanium foam can be predicted by the equation P = ax + b, depending on the spacer content (x). This indicates that in the absence of porosity measurements, the macroscopic dimensions of the sintered metal foam can be measured to obtain a controlling equation for porosity.

A mathematic model was built to describe the cladding casting process and the casting parametric influence on the cladding billet. The effect of casting speed, casting temperature and internal cooling water rate on the casting process performance was discussed. Based on the numerical simulation results, the cladding billet was prepared successfully. Moreover, the model was verified by the temperature measurements during the cladding casting process. The interfacial characteristics were investigated by metallographic examination, energy dispersive spectrometer and universal testing machine. The results show that there is a good agreement between the measured and calculated results. The comprehensively considered casting process is: casting speed 100 mm/min, internal cooling water rate 35 L/min, casting temperature 1020 K (AA4045) and 1000 K (AA3003). Based on the simulation results, the AA4045/AA3003 cladding billet with no defects and Φ140 mm/Φ110 mm in size is fabricated successfully. A diffusion layer with 15 μm in width is formed around the bonding interface. The interfacial bonding strength is 107.3 MPa, higher than that of AA3003, indicating that the two alloys have been bonded metallurgically.

The hot deformation behavior of a typical nickel-based superalloy was investigated by isothermal compression tests in the temperature range of 1010∼1160 °C and strain rate range of 0.001∼1 s⁻¹. The results indicate that the work hardening, dynamic recovery (DRV) and dynamic recrystallization (DRX) occurred in the alloy during hot deformation. Considering the coupled effects of deformation parameters on the flow behaviors of the alloy, the constitutive models were established to predict the flow stresses during the work hardening-DRV period and DRX periods. In the DRX period, the modified DRX kinetic equation was used to develop the constitutive models, and the strain for maximum softening rate was used in this equation. Additionally, the material constants in the constitutive models were expressed as the functions of Zener-Hollomon parameter by using a linear fitting method. Meanwhile, comparisons between the measured and the predicted flow stresses were carried out, while the correlation coefficient (R) and average absolute relative error (AARE) between the measured and predicted values were also calculated. The results confirm that the developed models could give an accurate estimation of the flow stresses.