稀有金属材料与工程最新文献

Cu-Nb alloys with a Nb concentration range of 0 wt%∼30 wt% were prepared by mechanical alloying (MA) at room temperature. The effects of Nb content on the crystalline refinement process and mechanical properties of the immiscible Cu-Nb system were investigated by X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) with energy-dispersive X-ray detection (EDX), transmission electron microscopy (TEM) and microhardness measurement. Results show that the completed dissolution of Nb in Cu can be achieved in the samples with Nb content less than ∼11 wt% after 100 h milling, although the equilibrium solubility level is nearly zero. The grain size refinement capability of MA-ed Cu-Nb powder is enhanced with increasing Nb content up to 30 wt%. This is because the susceptibility to recovery process becomes reduced, when the amount of Nb solutes segregated into the dislocation of Cu phase is increased. Cu-30wt%Nb powder milled for 100 h, the average Cu grain size is only ∼6 nm. The microhardness of the samples shows an enhancement with increasing Nb concentration. The main strengthening mechanisms of the MA-ed Cu-Nb alloys are linked to the grain size reduction and the dissolution of Nb into Cu matrix.

The penetration process of Ti-Al₃Ti metal-intermetallic laminate composites impacted by a projectile was numerically investigated. The ballistic performance, stress distribution, failure and energy absorbing mechanisms of Ti-Al₃Ti metal-intermetallic laminate composites under high-speed impact were examined in detail. The results show that Ti-Al₃Ti metal-intermetallic laminate composites under high-speed impact is mostly under tensile stress, since the compressive wave is reflected back as a tensile wave. During projectile penetration, transverse, inclined, and vertical cracks are formed in the Al₃Ti phase, which can dramatically absorb the kinetic energy of projectile.

To investigate the compatibility and diffusion behavior between U-Zr alloys and Zr-4 alloys, solid-to-solid U-10wt% Zr/Zr-4 diffusion couples were assembled by vacuum hot pressing and then annealed under vacuum at temperatures ranging from 580 °C to 1100 °C for various time. Scanning and transmission electron microscopes were employed to analyze the microstructures and composition profiles at the interfaces of the couples. The compatibility between the two alloys was investigated. δ-UZr₂ and ∼20-nm-thin U-rich layers existed in the vacuum-hot-pressed samples. The interdiffusion coefficient constant and activation energy are found to be (4.23±0.63)×10⁻⁶ m²/s and (160.73±1.67) kJ/mol, respectively. The interdiffusion coefficients of the U-10wt% Zr/Zr-4 alloy couples are higher than those of U—Zr alloys, especially at low temperature.

The evolution of microstructure in Al-Mn alloy with a low ratio of Fe/Si produced by twin-roll casting (TRC) during various homogenization treatments was investigated. The grain structure near the surface transforms from fibrous to coarse elongated recrystallized grain structure during homogenization at 550 °C for 4 h. The grain in the interior tends to be equiaxed with the temperature increasing. The primary particles are broken up after homogenization at 450 and 500 °C for 4 h, and coarsened when the temperature increases to 550 °C. The evolution of dispersoids is controlled by nucleation and growth mechanisms at 450 and 500 °C for 4 h, whereas the coarsening and dissolution are predominant mechanisms during homogenization at 550 °C for 4 h. Many small Zr-bearing dispersoids are obtained during homogenization at 500 °C for 4 h.

For development of low-Ag lead free solder alloys for microelectronic packaging, the correlation of creep properties with microstructure of novel Sn-0.3Ag-0.7Cu-0.5Ga (SAC-Ga) solder alloys bearing Pr has been investigated using nanoindentation. The results show that the creep deformation of SAC-Ga, SAC-Ga-0.06Pr, SAC-Ga-0.5Pr is 1717, 1144, and 1472 nm, respectively, which indicates that Pr addition could significantly enhance the creep resistance of SAC-Ga solders due to the refinement and uniform distribution of Cu₆Sn₅ intermetallic compounds (IMCs). However, compared with the SAC-Ga-0.06Pr solder alloy, the SAC-Ga-0.5Pr alloy shows poorer creep resistance which is mainly attributed to the surface oxidation of excess rare earth Pr. In addition, Dorn model has been used to describe the creep behavior and to obtain stress exponents of the SAC-Ga solder alloys bearing Pr. The strengthening mechanism of creep resistance in SAC-Ga solder alloys bearing Pr is that when encountering refined and well-distributed Cu₆Sn₅ IMCs, a dislocation line cannot climb through the IMCs but bypass the IMCs, thus leading to a decrease in the creep deformation of the solder alloys bearing Pr.

The welding of 15 mm thick TC18 titanium alloy thick plate was realized by electron beam welding. The effect of different welding speeds (10, 20, 30 mm/s) on the fatigue properties of the electron beam welded joints for TC18 titanium alloy was investigated. The macroscopic morphology, microstructure and fracture characteristics of the joints were analyzed by optical microscope, scanning electron microscopy and transmission electron microscopy, and the fatigue properties of welded joints were studied and tested by an electronic universal testing machine. The results show that the weld fusion zone is mainly composed of columnar β phase and acicular α martensite phase. The upper melting width, the middle melting width and the lower melting width are obviously reduced, and the grain size gradually decreases with the increase of welding speed, which results in the increase of fatigue properties of welded joints. At N_f =10⁷ the fatigue limit of the weld increases by nearly 29% with the welding speed from 10 mm/s to 30 mm/s. The fatigue fracture of the joints can be divided into three typical regions of fatigue crack source zone, expansion zone and instantaneous zone, and the fatigue cracks all originate from the surface of the specimen. With the increase of welding speed, the proportion of instantaneous area decreases and the fatigue performance increases.

Section flattening is an inevitable physical phenomenon in the forming process of tube bending, and severe section flattening will affect the reasonable assembly of the tube fittings, and therefore restricts its wide application. Finite element (FE) model for numerical control (NC) bending of titanium tube considering the variation law of contractile strain ratio (CSR) and the Young's modulus (E) was established in the present paper. The section flattening behaviors of TA18 tube under different geometric conditions and different process conditions were investigated. The results show that considering the variation law of CSR-E can change the cross-section, which has no remarkable influence on the change law. The reasonable range of geometric and process parameters are obtained, which provides a basis for studying the forming prediction and controlling the final precision for the NC bending of TA18 tube.

Specimens of as-cast Mg-1Ag-Zn, Mg-3Ag-Zn and Mg-5Ag-Zn alloys were fabricated, and then the specimens' thermal conductivity was calculated. With the increase of Ag content, the thermal conductivity of as-cast alloys decreases significantly. After that, directionally solidified specimens of the above mentioned as-cast Mg-Ag-Zn alloys were made at three different pulling velocities: V=25 μm/s, V=50 μm/s, V=75 μm/s, and then their thermal conductivity was calculated again. The results show that with the increase of the pulling velocity, the thermal conductivity of alloys decreases significantly. Variations in the solute content and the pulling velocities are believed to play important roles in the thermal conductivity of Mg-Ag-Zn alloys. Since these two factors can strengthen the electron's scattering process, reduce their free path, and thereby reduce the alloys' thermal conductivity.