Joining ZTA ceramic by using Dy2O3-Al2O3-SiO2 glass ceramic filler

In this paper, a novel Dy₂O₃-Al₂O₃-SiO₂ (DAS) glass ceramic was designed and prepared for joining zirconia toughened alumina (ZTA) ceramic. The crystallization, thermal expansion behavior and wetting behavior of the DAS glass filler were studied. The effect of cooling rate and joining temperature on the microstructure and flexural strength of joints was investigated. The results show that slow cooling rate (15 °C/min) leads to crystallization of brazing seam, which causes the formation of pores in the joints due to the large density difference between the glass and the crystalline phases. The dissolution of ZrO₂ from ZTA substrate into the filler during joining process improves the mismatch of the coefficient of thermal expansion (CTE) between the brazing seam and substrate. The maximum flexural strength of 535 MPa is obtained when the joining temperature and cooling rate are 1475 °C and 50 °C/min, respectively.     

Influence of impurities on hot-rolled molybdenum for high temperature applications

Refractory metals are crucial for high-temperature applications such as fusion reactors. However, no systematic research has been reported for the influence of the impurities in refectory metals, especially at high temperatures. Our goal is an investigation of the influence of dissolved impurities on the recrystallization behavior of hot-rolled molybdenum (Mo). Two sets of 80%-rolled Mo plates with different purity levels pure Mo and non-pure Mo with the content of impurities, O, C, N, etc. are analyzed. Both isothermal and isochronous annealing experiments demonstrated that the impurities resulted in a reduction of recrystallization temperature and acceleration of texture transformation in a Mo plate. The recrystallization temperature of as-rolled non-pure Mo is about 100 °C lower than that of pure Mo. EBSD results show deformed γ-fiber texture gets weakened while {001}〈110〉 component gets strengthened at 1250 °C for pure Mo and 1150 °C for non-pure Mo, leading to the destabilization of fibrous grains. The XRD and TEM results reveal that the dislocation density of as-rolled non-pure Mo is ~2.4 times that of pure Mo. This discovery deviates from the conventional understanding of impurities or second phase particles in metals, providing better practical knowledge and opportunities to establish the technical parameters of refractory metals for high temperatures applications.     

Microstructure and mechanical properties of squeeze-cast Al−5.0Mg−3.0Zn−1.0Cu alloys in solution-treated and aged conditions

The microstructure and mechanical properties at different depths of squeeze-cast, solution-treated and aged Al−5.0Mg−3.0Zn−1.0Cu alloy were investigated. For squeeze-cast alloy, from casting surface to interior, the grain size of α(Al) matrix and width of T-Mg₃₂(AlZnCu)₄₉ phase increase significantly, while the volume fraction of T phase decreases. The related mechanical properties including ultimate tensile strength (UTS) and elongation decrease from 243.7 MPa and 2.3% to 217.9 MPa and 1.4%, respectively. After solution treatment at 470 °C for 36 h, T phase is dissolved into matrix, and the grain size increases so that the UTS and elongation from surface to interior are respectively reduced from 387.8 MPa and 18.6% to 348.9 MPa and 13.9%. After further peak-aging at 120 °C for 24 h, numerous G.P. II zone and η′ phase precipitate in matrix. Consequently, UTS values of the surface and interior increase to 449.5 and 421.4 MPa, while elongation values decrease to 12.5% and 8.1%, respectively.     

Properties of super heat-resistant silicon carbide fibres with in situ BN coating

An in situ BN coating was prepared on the surface of a nearly stoichiometric continuous SiC fibre with trademark Cansas-3301 (C3). The coated fibre was then subjected to continuous pyrolysis at 1800 °C, obtaining a fibre named Cansas-BN-1800 (C18). After annealing in Ar at 1500 °C for 1 h, the strength retention ratio of C3 was 49%, and that of C18 was almost unchanged. The strength decrease of the C3 fibre was mainly caused by the formation of surface defects resulting from fibre decomposition and active oxidation. However, the in situ BN coating on C18 protected the fibre from forming surface defects, resulting in high strength. Due to slight growth of the grain and purification of the grain boundary during fast heating at 1800 °C, C18 showed excellent creep resistance in the range of 1200–1500 °C.     

A chromium carbide coating in-situ formed on C/C composites by the novel reactive wetting strategy to enhance oxidation resistance

A chromium carbide (Cr-C) coating in-situ formed on the C/C substrate is successfully prepared by a novel reactive wetting strategy. The interfacial microstructure and oxidation resistance of coated C/C composites are investigated in detail. The as-prepared coating mainly consists of Cr₂₃C₆ and Cr₇C₃, forming a tight joining with the C/C substrate. Compared to uncoated samples, the oxidation weight loss of coated C/C composites is substantially reduced at high temperatures. Furthermore, the hardness of coated C/C composites is significantly increased, enhancing their ability to resist external damage. This reactive wetting strategy can also be used to prepare uniform coatings on C/C composites with complex grooved structure or large size. Surprisingly, coated C/C composites possess a low weight gain of 3.7% due to thin coating (< 10 µm), which can maintain their advantage of low density.     

A 2D image-based multiphysics model for lifetime evaluation and failure scenario analysis of self-healing ceramic-matrix mini-composites under a tensile load

We propose a multi-physics numerical model for a self-healing ceramic matrix mini-composite under tensile load. Crack averaged PDEs are proposed for the transport of oxygen and of all the chemical species involved in the healing process and studied in the dimensionless form to perform the most appropriate discretization choices concerning time integration, and boundary conditions. Concerning the fibres’ degradation, a slow crack growth model explicitly dependent on the environmental parameters is calibrated using a particular exact solution and integrated numerically in the general case. The tow failure results from the statistical distribution of the fibres’ initial strength, the slow crack growth kinetics, and the load transfer following fibres breakage. The lifetime prediction capabilities of the model, as well as the effect of temperature, spatial variation of the statistical distribution of fibres strength, and applied load, are investigated highlighting the influence of the diffusion/reaction processes (healing) on the fibre breakage scenarios.     

Physicochemical and thermo physical characterization of CaO–CaF2–SiO2 and CaO–TiO2–SiO2 based electrode coating for offshore welds

This paper investigates the physicochemical and thermo-physical properties of CaO–CaF₂–SiO₂ and CaO–TiO₂–SiO₂ based electrode coating for welding offshore structures. Twenty-one electrode coating compositions have been formulated using extreme vertices design method. The coating was crushed to powder form. The powder was characterized for weight loss, density, specific heat, enthalpy, thermal conductivity, diffusivity, and specific heat. Coating's structural analysis was done using X-Ray Diffraction and Fourier transformation. X-Ray Fluorescence, Thermogravimetric Analyzer, and Hot disc have been used to characterize the coating mixture. The regression analysis has been used to study the effect of individual constituents and their binary, tertiary interactions on the properties. The obtained output of properties has been optimized using multi-response optimization.     

A physically-based model for global collision avoidance in 5-axis point milling

Although 5-axis free form surface machining is commonly proposed in CAD/CAM software, several issues still need to be addressed and especially collision avoidance between the tool and the part. Indeed, advanced user skills are often required to define smooth tool axis orientations along the tool path in high speed machining. In the literature, the problem of collision avoidance is mainly treated as an iterative process based on local and global collision tests with a geometrical method. In this paper, an innovative method based on physical modeling is used to generate 5-axis collision-free smooth tool paths. In the proposed approach, the ball-end tool is considered as a rigid body moving in the 3D space on which repulsive forces, deriving from a scalar potential field attached to the check surfaces, and attractive forces are acting. A study of the check surface tessellation is carried out to ensure smooth variations of the tool axis orientation. The proposed algorithm is applied to open pocket parts such as an impeller to emphasize the effectiveness of this method to avoid collision.