The stability and reliability of braking system are essential factors for the safe operation of high-speed trains. In the proposed work, tribological properties of a newly developed brake disc material namely BD-1 were studied considering the thermal-mechanical effects, as well as the evolutions of wear debris, were particularly examined. The tribological properties were also compared with an existing commercial brake disc material namely BD-2 in text. Friction and wear tests were carried out on BD-1 and BD-2 against a commercial brake pad material (BP) to simulate the real emergence braking conditions of a 350 km/h high-speed railway. The thermal-mechanical coupling effects of the friction velocity, wear mass, temperatures and the friction coefficient were investigated. Local wear track and wear debris were analyzed by using SEM and EDS. Results show that the shape and size of wear debris evolve as the dominant wear mechanism varies during braking tests. As the sliding speed increases from 250 to 1250 rpm, the debris may become fine particles, then into a mixture of lamellar shape and flake shape, and finally becomes fine particles again at high speed. The maximum size of wear debris is first from 20 μm to 65 μm, and then down to 10 μm. As the local area temperature increased by more than 400 °C, debris adhere to the surface forming an adhesive layer that may act as a lubricant. Debris may help to form an adhesive lubrication layer and undertake plastics defor-mation at the speed range of 500–1000 rpm. The local area temperatures prompted the wear debris adhesion and oxidation. After reaching a certain speed limit, a uniform third body appears to protect the material surface from high speed and high temperature. Results suggested that the BD-1 could be a good candidate braking material for high-speed railway applications.

Effective braking in high-speed trains is one of the major bottlenecks in expediting the technology and possibilities to improve speed. Although substantial progress has been made to increase operating speed, perhaps, thermal fatigue cracking in brake discs is a primary constraint so far. Thermal fatigue cracking is the major cause of brake disc failure in high-speed trains, especially trains with a speed of 350 km/h or above. In this study, new material composition is proposed for brake discs of high-speed trains. A comprehensive investigation is presented based on fatigue crack initiation and propagation, along with wear and micro-hardness characterization. Thermal fatigue tests at various thermal cycles between 20 ℃ and 700 ℃ were performed and the experimental results are compared with fatigue properties of a commercial brake disc material. An experimental trial revealed that thermal cracks normally initiate and propagate along the oxidized grain boundaries; nevertheless, crack propagation is restricted by the fine precipitates and lath structure of martensitic. Moreover, crack length at the initiation and propagation stage is predicted through crack growth rate and favorable grain size in the crack vicinity. Thermal fatigue life can be improved by dictating the microstructure and precipitate morphology of cast steel by tailoring the alloying composition.

To investigate the microstructural evolution of wrought-nickel-based superalloy GH4169 from the original ingot to the finished product of manufacturing processes, different kinds of etchants and etching methods were used to show the fine precipitates and their morphologies. The obtained microstructures can vary in size, type, distribution, location, formation, and interactions of multiple phases, which were observed and analyzed by optical microscopy (OM), scanning electron microscopy (SEM), and an energy dispersive spectrometer (EDS). The dendrite segregation behavior of as-cast superalloy GH4169 was investigated. In addition, the microstructural evolution mechanism of second-phase particles during dynamic recrystallization was analyzed. This work sheds light on the evolution of the second-phase structure of nickel-based superalloys during the preparation process, providing guidance for process development and visual interpretation of the relationships between microstructure and properties.

This study systematically investigates the surface nanocrystallization of 35CrMo steel induced by Ultrasonic Surface Rolling Processing (USRP). It reveals the formation of a gradient nanostructure, where martensite lath fragmentation under high-frequency impacts leads to a surface layer of equiaxed nanocrystals and high-density dislocations. This novel microstructure yields exceptional surface integrity: roughness is minimized to 0.029 μm due to plastic flow, residual stress is transformed into high compressive stress, and surface microhardness is significantly enhanced by 32.3%, primarily governed by grain refinement and dislocation strengthening. Consequently, the treated material exhibits a 28.9% reduction in wear mass loss, which is directly attributed to the combined effects of the strengthened gradient layer’s improved load-bearing capacity and the effective suppression of crack initiation by compressive residual stresses. Our findings not only provide direct microstructural evidence for classic strengthening theories but also offer a practical guide for optimizing the surface performance of high-strength alloy components.

This study systematically investigates the surface nanocrystallization mechanism of 35CrMo high-strength low-alloy (HSLA) steel induced by ultrasonic surface rolling processing (USRP), with particular emphasis on elucidating its optimization effects on surface integrity, mechanical properties, and wear resistance. Through USRP treatment with varying static pressure parameters combined with multi-scale characterization techniques, we demonstrate that high-frequency impact and rolling effects promote martensite lath fragmentation and dislocation multiplication, thereby forming a gradient nanostructured layer composed of equiaxed nanocrystals and high-density dislocations in the surface region. After USRP treatment, the surface roughness was minimized to 0.029 μm, attributed to the synergistic "peak-cutting and valley-filling" effect and plastic flow-induced surface smoothening. Concurrently, compressive deformation during rolling induces lattice distortion effects, successfully transforming the residual stress state from tensile to high compressive stress. A remarkable 32.3% enhancement in surface microhardness was observed, primarily originating from multiple strengthening mechanisms including grain refinement, dislocation strengthening, and carbide dispersion strengthening, with an effective hardened layer depth reaching 300 μm. In terms of wear performance, the USRP-0.35 MPa sample exhibited optimal wear resistance with a 28.9% reduction in mass loss, owing to the significantly improved load-bearing capacity of the gradient nanostructured layer and effective suppression of crack initiation and propagation by compressive residual stresses. High-resolution electron microscopy and diffraction analyses captured the dynamic evolution process of dislocations, from their initiation at martensite boundaries to subsequent propagation and eventual formation of dislocation pile-ups and subgrain boundaries, providing direct experimental evidence for the Hall-Petch and Taylor strengthening theories. This work not only clarifies the microscopic mechanisms of USRP-induced surface strengthening in 35CrMo steel but also offers important theoretical guidance and practical references for optimizing surface treatment processes of high-strength alloy components.

Squeeze casting is an advanced technology to produce aluminum alloy components with excellent mechanical property. Due to good fluidity and castability, the Al-Si alloy is the most common aluminum material in squeeze casting. However, the segregation in solid α-Al phase and liquid eutectic Si phase happens during solidification, which leads to microstructural inhomogeneous in the final microstructure. In this study, the eutectic Si segregation of Al-12Si-1Cu alloy under different intensification pressure in squeeze casting was investigated by Optical Microscopy (OM). The results show that increasing intensification pressure promotes the eutectic Si segregation at center zone, while the eutectic Si phase at skin zone is not much influenced. Increasing the intensification pressure from 65 MPa to 140 MPa, the segregation degree of eutectic Si at center zone increases from 0.05 to 0.29, while the segregation degree at skin zone levels at 0.2. Under high intensification pressure, the feeding effect of liquid phase at center zone is prominent, which results to high tendency of eutectic Si segregation in the final microstructure.