Gui-Rong Li

A novel coupled processing method (PDCT) that associated deep cryogenic treatment (DCT) with a high pulsed magnetic field (PMT) was investigated to improve the performance of an as-cast TC4 aero-titanium alloy. Through XRD, SEM, TEM, EBSD, and a properties test, its microstructure and tensile properties and their relationship were investigated. The results show that in comparison with the untreated samples, in the PDCT alloys, the amount of nano-sized precipitates and dislocation density are increased, and this phenomenon is characterized by their combed dislocation morphology. The grain sizes are refined and rounded, and the deformed grains are enhanced, together with the enhancement of low-angle grain boundaries in grains and the transformation from the β phase to the α phase. The (112) crystal orientation is apparently strengthened. The tensile strength, elongation, and fracture energy of the optimized PDCT sample are 921.4 MPa, 7.6% and 5.47 × 10⁷ J/m³, which increased by 4.9%, 28.8% and 80.5% compared with the untreated sample, respectively. The tensile fracture exhibits rheological deformation along the phase boundaries. The strength–toughness mechanisms are mainly attributed to the texture, precipitation, dislocation and fine grain strengthening, which stem from the cold contraction and lattice distortion of DCT and the main magneto-plasticity effect of PMT, together with their coupling effects.

This study investigates the effects of annealing and subsequent deep cryogenic treatment (DCT) on the microstructure and mechanical properties of TC27 titanium alloy. The results show that in the annealed alloy, the α(100) crystal plane grows preferentially along with an increased dislocation density. The size of α-phase significantly increases, forming a basketweave structure between the coarse colony laths. For the ADCT (annealed then DCT) sample, the strongest diffraction peak changes from β(110) after 24 hours of low temperature treatment (ADCT24) to α(101) after 36 hours of low temperature treatment (ADCT36). In the optimized ADCT36 sample, there are more needle-like precipitated α' phases, which result from the β phase supersaturated non-equilibrium densely packed hexagonal lattice formed by non-diffusion transformation of α. The tensile strength and elongation of ADCT36 are 1096.7MPa and 5.6%, respectively, and these improvements are attributed to the homogeneous structure, more isometric and finer secondary α phase, grain rotation, and high-density dislocations.

Model test is a common method to study the bearing peculiarity of pile foundation. The influence of overlying soil thickness and overburden pressure on the bearing capacity of soft rock-socketed pile should be considered in the physical model test of mini piles in soft rock. In this paper, the influence of coverage on the bearing characteristics of rock-socketed sections is studied by finite element analysis, and the modelling method of equivalent overburden pressure is proposed. This method can be used to study the carrying peculiarity of soft rock-socketed pile and reveal the failure mechanism of pile tip. The development of pile model test technology considering overburden pressure promotes more scientific design methods for pile foundation.

This study aimed at the preparation of a 2024 aluminum alloy (2024Al) matrix composite with high strength, high toughness and high magnetic properties that can be used in practical applications. Therefore, Nd₂Fe₁₄B_p/2024Al–Co composites with different Co contents (wt.%) were prepared by ball milling, cold isostatic pressing and microwave sintering. The effects of the Co content on the microstructure, mechanical properties and magnetic properties of the prepared composites were studied. Under the conditions of the sintering temperature of 490 °C, heating rate of 20 min/°C and soaking time of 30 min, it was found that with the increase in Co content (0→2.5%→5%→7.5%→10%), the grain size first decreased and then increased, and reached the optimal value of about 3–5 μm when the Co content was 7.5%, with the microstructure being relatively uniform. At the same time, the compactness of the composite arrived at a maximum of 95.4%. The main particle phases in the composite were Nd₂Fe₁₄B, Nd₂ (Fe, Co)₁₄B and Co particles. In the nanoindentation test, the interface strength of the 7.5% Co sample was significantly higher than that of the Co-free sample. In addition, the microhardness, yield strength and compressive strength of the 7.5% Co sample were 152 HV, 210 MPa and 269 MPa, respectively, which increased by 67%, 78% and 75%, respectively, compared with the Co-free sample. With the increase in Co content, the remanence (Br), coercivity (H_cj) and maximum magnetic energy product ((BH) max) of the composites first increased and then decreased. When the Co content was 7.5%, the three performance indicators reached their optimum values, which were Br: 0.20 (T), H_cj: 4.6 (kOe) and (BH)_max: 28.36 (kJ/m³). The expected goal of the lightweight magnetic materials was achieved, and the action mechanism of Co addition in the composites was also analyzed in detail.