Metallurgical and Materials Transactions A最新文献

The development of the microstructure during severe plastic deformation of an aluminum single crystal by complex shearing of the extruded tube (CSET) was studied in this paper. The research has demonstrated that even in a single crystal, an ultrafine-grained microstructure can be obtained during this one-step process. The size of the grains gradually changes and reaches the minimum size on the level of 1 μm at the inner surface of the resulting tube. Simultaneously, preferential orientations in individual parts of the deformed sample change in a complex way. The main mechanism affecting the final microstructure is continuous dynamic recrystallization. The microhardness also exhibits a gradient character with higher values at the inner surface of the tube compared to its center.

Graphical Abstract

The medium-entropy alloy (NbTa)(_{0.67})(HfZr)(_{0.33}) and the high-entropy alloy (NbTa)(_{0.67})(HfZrTi)(_{0.33}) were prepared by mechanical alloying using high-energy planetary ball mill. The results of X-ray diffraction, scanning electron microscopy, and positron annihilation lifetime spectroscopy measurements suggest that both as-prepared powders are multicomponent alloys in amorphous (or highly disordered) state. The magnetic and thermodynamic results obtained for these powders undoubtedly prove that bulk superconductivity is not observed at temperatures exceeding 2 K. Thermal treatment of both studied materials leads to decomposition of the amorphous phase and precipitation of several crystalline phases. In both annealed samples, the structure of the main crystalline phase was identified as body-centered cubic (bcc), and in this phase, bulk superconductivity was observed below 6.5 K.

Al–5Ti–1B master alloy has been used to refine the microstructure of a commercial high-strength aluminum alloy (B206). The effects of grain refinement and mold preheat on the semi-solid contraction behavior were investigated. A coupled thermal-stress model combined with displacements measured during solidification was utilized to determine the semi-solid contraction coefficients (SSCC). The simulation results show that Al5TiB additions and preheating the mold significantly reduced SSCC values and limited the semi-solid contraction of B206 alloy.

This study presents a comprehensive and innovative approach to tailoring the microstructure of AISI 9254 steel using a quenching and partitioning (Q&P) process, demonstrating significant improvements in mechanical and wear properties. The initial pearlitic microstructure was first heated to the fully austenitizing region before undergoing phase transformation into a multi-phase matrix using the novel Q&P process, resulting in the formation of ferrite, bainite, martensite, and retained austenite. Investigation revealed the central role of retained austenite in enhancing the mechanical properties through the Transformation Induced Plasticity (TRIP) effect. In particular, heat-treated specimens exhibited a mechanical resistance of up to 1.9 GPa and an elongation of approximately 12 pct. Furthermore, this study highlights remarkable enhancements in wear resistance of the treated AISI 9254 steel. A decreased wear rate, reduced volume loss, and improved contact area stability were achieved, attributed to debris aggregation, contact area changes, and the work hardening of the wear track. Consequently, the Q&P process can considerably enhance the in-service performance and life span of AISI 9254 steel components. The insights provided by this work into the potential benefits of a tailored Q&P process in AISI 9254 steel set forth a promising pathway towards reduced maintenance costs and heightened reliability across various applications. Exploring this process showcases the transformative potential of materials engineering for industrial applications.

A novel galvanic-polishing-assisted near net shape forming was proposed to improve the heat dissipation capacity of the liquid-cooled plates. The coarsest upper surface roughness of channels was reduced from 74 to 37 μm. The coolant flow rate and heat dissipation capacity of the liquid-cooled plates with the polished channels were increased by 35.2 and 51.0 pct compared with those of untreated liquid-cooled plates, respectively.

Graphical Abstract

Prediction of texture changes during cold rolling is important because they affect the recrystallization and anisotropy of an aluminum sheet during successive forming steps. During cold rolling of aluminum alloys, the through-thickness textural change in the subsurface layer depends heavily on the shear stresses exerted on the material. The intensity of this shear stress is determined by the value of and change in the coefficient of friction as the contact length between the rolls and metallic sheet changes. The quality of the texture prediction under constant and variable coefficients of friction are assessed for three established texture models: the grain interaction (GIA) model, the viscoplastic self-consistent (VPSC) approach, and the full-field crystal plasticity Düsseldorf Advanced Material Simulation Kit (DAMASK) code. The simulation results are compared with subsurface layer textures obtained from conducting experimental cold-rolling trials on an aluminum alloy, which are designed to maximize shear in a single rolling pass. The formulation of a variable coefficient of friction is crucial for ensuring both the reasonable prediction of rolling forces and changes in texture. GIA and DAMASK yield the best texture prediction results for a variable coefficient of friction model.

The effect of peak temperature for post-weld heat treatment (PWHT) on the dissolution of Nb-rich carbonitrides, Nb(C,N), in the weldline of an electric resistance welded (ERW) X70 grade pipeline steel was investigated. Selected-area electron diffraction patterns and corresponding electron energy loss spectroscopy mappings revealed both Nb-rich carbonitrides and Ti-rich nitrides, (Ti,Nb)N, in the sample heat-treated to 1080 °C peak temperature. However, only Ti-rich nitrides were revealed in the sample heat-treated to 1220 °C peak temperature. During continuous heating to higher than the Ac3 temperature, i.e., near 1100 °C temperature, the dissolution of Nb(C,N) was observed in dilation and derivative of dilation. This experimental result agreed with the predictions by a DICTRA model. Furthermore, a phenomenological model was developed to explain the volume shrinkage during the dissolution of Nb(C,N) precipitates.

During the laser cladding process, complex composition distribution in the molten pool was caused from the Marangoni flow, which leading to one kind of defect termed as macrosegregation. Therefore, comprehensive understanding on the flow behaviors in the molten pool is particularly important for predicting and regulating macrosegregation. In this paper, a three-phase Eulerian model based on the volume averaging method is developed to investigate the effect of Marangoni coefficient variations on the flow and compositional distribution. The flow patterns were photographed and analyzed using a high-speed camera, and then the simulated and experimental results for chromium concentration distribution, size of the cladding layer, depth of the molten pool, and flow pattern were compared and found to be in agreement with each other; thus, the accuracy of the model was validated. Meanwhile, the effects of flow direction and intensity on the composition distribution are comprehensively studied. The results show that the flow direction extremely affects the flow path and evolution process of Cr-enriched melt in the molten pool. At the same time, the flow intensity will change the flow time of Cr-enriched melt in the molten pool and then affect the homogenization of composition distribution.

The effects of cryorolling (CR), room-temperature rolling (RTR), and subsequent aging on the mechanical, electrical, and wear characteristics of Cu-6Ni-6Sn alloys were examined. Compared to the RTR specimens, those subjected to CR displayed a higher dislocation density and a greater number of nanosized deformation twins. The CR facilitated the uniform and dense formation of precipitates, enabling the CR-treated alloys to achieve an optimal balance of strength (1004 MPa), electrical conductivity (11.8 pct IACS), and wear resistance.

Graphical Abstract

Gas nitriding is a thermochemical surface hardening process that is widely used in industry but suffers from a low decomposition rate of ammonia (NH₃) and extremely long processing cycles. To enhance the gas nitriding efficiency, laser vibrational excitation of NH₃ is introduced into the gas nitriding of 38CrMoAl steels. By matching the laser wavelength with the N–H wagging mode of NH₃, laser energy can be preferentially deposited into NH₃ molecules and facilitates their dissociation. The morphology, composition, and mechanical properties of nitrided 38CrMoAl plates strongly depend on the nitriding time length. Nitrided surfaces experience phase transition from α-Fe solid solution with N through γ′-Fe₄N to ε-Fe_2~3N phase as prolonging the nitriding time length. A 38CrMoAl plate nitrided with laser vibrational excitation of NH₃ for 6 hours possesses a higher nitrogen content (7.1 wt pct) and a harder surface (944 HV_0.1) than that nitrided without laser (6.3 wt pct, and 811 HV_0.1). The study concludes that laser vibrational excitation of NH₃ is an effective way to promote nitrogen diffusion kinetics and shorten the nitriding cycle of gas nitriding, which holds great promise in the development of gas nitriding.