Journal of Materials Research and Technology-Jmr&t最新文献

In order to satisfy the application requirements for strength and toughness of hydropower steels for large pumped storage power plants. In this study, based on the synergy of deformation and phase transition, the trade-off between strength and toughness was broken, and the GPa-grade nano-precipitated steel was prepared by direct quenching and tempering. The effects of Nb–V–Ti composite nano-precipitation behavior and martensite variant selectivity on the strengthening and toughening mechanisms were highlighted. The results show that directly quenched steel has the optimal comprehensive mechanical properties after tempering at 550 °C. Combining the contributions of various strengthening mechanisms, the increase in the yield strength relative to off-line quenching and tempering is mainly attributed to additional dislocation strengthening (∼33 MPa) and precipitation strengthening (∼174 MPa). Direct quenching enhances the homogeneous nucleation of finer mono- and binary precipitates and their stable growth during tempering, while the ternary precipitates do not suffer from elemental inhomogeneities due to lack of re-dissolution. Furthermore, the high-frequency variants of the dominant close-packed plane group tend to form high-angle grain boundaries with different orientations and planar alignments, derived from the refinement of Bain groups induced by the self-plasticity modulation of martensite transition due to the accumulation of deformed geometrically necessary dislocations and finite dislocation slip.

Rare earth (RE) elements not only modify inclusions but also refine the steel's microstructure. This study investigates the evolution mechanism of cerium on inclusions and microstructure in low-alloy cast steel through experimental and thermodynamic analyses. After cerium is added to the steel, strip-like inclusions transform into spherical ones, accompanied by a reduction in size. The evolution path of inclusions follows MnS + MnS–Al₂O₃→Ce₂O₂S–Ce₂S₃–MnS + CeAlO₃–Ce₂S₃–MnS with cerium addition. Notably, Ce₂S₃ and Ce₂O₂S exhibit an orientation relationship, with [$\bar{1}$ 22]_Ce2S3//[01 $\bar{1}$ 1]_Ce2O2S and (02 $\bar{2}$)_Ce2S3//($\bar{1}$ 011)_Ce2O2S. Compared to Ce-free steel, the Ce-0.06 steel shows a reduced grain size and the absence of needle-shaped Widmanstätten structures, which may be mainly attributed to rare earth inclusions as heterogeneous nucleation of δ-Fe. This work can provide a theoretical basis for improving the overall mechanical properties of low-alloy cast steel by adding cerium.

A comprehensive understanding of the diffusion coefficients and atomic mobilities in the Ti–Cr–Mo system is crucial for establishing an atomic mobility database for titanium-based high-strength titanium alloys. In the present work, nine sets of solid/solid diffusion couples were prepared and measured in bcc Ti–Cr–Mo alloys, with the temperatures ranging from 1373 to 1473 K. Scanning electron microscopy (SEM), and Electro Probe Micro-Analysis (EPMA) techniques were used to respectively test its microstructure and composition distance. The diffusion coefficients in bcc Ti–Cr–Mo alloys were determined using the Matano-Kirkaldy method and numerical inverse method respectively. The diffusion coefficients obtained from the above two methods exhibit good consistency. The atomic mobility parameters for the Ti–Cr–Mo systems were provided with the assistance of HitDIC software. A three-dimensional plot of interdiffusivities was generated for chromium and molybdenum contents ranging from 0 to 20 mol % and 0–14 mol %. Moreover, the application of the Arrhenius equation enabled the determination of changes in frequency factors and activation energies concerning temperature and composition.

In the fields of wind power, and other industries, the Al₂O₃/TiO₂ coating is prepared and ground on the bearing ring, in order to obtain the insulation resisting the electrical erosion for the motors at high voltage. A novel wet chemical mechanical grinding (WCMG) method is proposed to finish this hard-brittle composite material, utilizing the structured diamond abrasives pad and the NaOH solution. The softened workpiece's surface under the chemical reaction was demonstrated by means of Raman spectrum, indentation and scratching tests. The analytical model of surface roughness and fracture cracks is established based on the synthesis of geometry and kinematics, contact mechanics, material removal and indentation fracture mechanics. The surface roughness of coating was reduced from initial S_a 3.293 μm down to final S_a 0.049 μm in the WCMG process with grain size 3 μm and pH 12.01, mostly attributed to the ductile removal of chemically softened coating surface.

In this work, the deformation behavior of Al-8.1Zn-2.0Mg-1.0Cu-0.2Ag-0.15Zr alloy was studied under isothermal compression, with strain rate and temperature in the range of 0.001 s⁻¹−10 s⁻¹ and 330 °C−450 °C, respectively. First, it was found that the deformation temperature significantly affected the dynamic recrystallization (DRX) and phase transformation behaviors of this alloy. At 330 °C, discontinuous dynamic recrystallization (DDRX) and arched grain boundaries were observed, whereas dislocation entanglements dominated continuous dynamic recrystallization (CDRX) was notably detected at 370 °C. Temperatures exceeding 410 °C facilitated the CDRX behavior, as polygonal chain-like subgrains and block-like dislocation loops were found within the new grains. High deformation temperatures also resulted in the amalgamation and growth of adjacent subgrains. Moreover, we found the Ag-containing phase spheroidized and grew at high temperatures, while the deformation had little effect on the Al₃Zr. Strain rate also significantly affected the deformation behavior of this alloy. Samples deformed at 0.001 s⁻¹ exhibited prolonged deformation time with clear dynamic recovery (DRV) and DRX. At 1 s⁻¹, irregular dislocation distribution was observed, while at 10 s⁻¹, shear band formation induced by deformation instability was evident. Furthermore, the deformation induced η′/matrix interfacial transformation from Zn clusters to a Zn–Mg common interface. Particularly, the precipitation order at grain boundary was proved to be Zn atomic clusters → Zn, Mg atomic clusters (GP zone) → η′ (MgZn₂).

As a typical age-strengthened alloy, Cu–Cr–Zr alloys have high strength, good electrical conductivity, and excellent wear resistance. In this work, the Cu–Cr–Zr–Nb alloys were in different aging states after the aging process. After aging at 460 °C for 4 h, the alloy was peak-aging. The common effect of the existence of the FCC Cr particle, which was coherent with the Cu matrix, and the increase in hardness improved the wear property of the alloy. The results of the wear test showed that the optimal wear parameter was a speed of 300 rpm and a load of 5 N. Under these wear parameters, the alloy had the lowest mean coefficient of friction (COF) value and the least mass loss, having the best wear resistance. Furthermore, the wear mechanism of the alloy was mainly adhesive wear and abrasive wear.

The additive manufacturing of directionally solidified Ni-based superalloys remains a significant challenge due to the formation of stray equiaxed grains and their high susceptibility to cracking, particularly for high-volume γ´-type Ni-based superalloys. In this study, a novel strategy based on temperature field management was proposed to prevent hot cracking and control crystallographic texture during laser powder bed fusion (LPBF) of IN738LC superalloy. The influence of laser parameters and substrate preheating on crack density, melt pool morphology, and texture of IN738LC superalloy was investigated. The fluid dynamics and thermal behavior of melt pool was simulated using the discrete element model (DEM) and volume of fluid (VOF) method. The processing window for achieving defect-free IN738LC samples was established and was found to be highly affected by substrate preheating. Substrate preheating at 350 °C resulted in expanded processing window, with the volume energy density ranging from 42.9 to 62.5 J/mm³ without substrate preheating to 40–75 J/mm³. The enlarged processing window was achieved by reducing cracking susceptibility due to the reduced temperature gradient and cooling rate. A unique crystallographic lamellar microstructure (CLM), comprising a <110>-oriented major layer and <100>-oriented sub-layer along the building direction, was successfully achieved in the LPBF-processed IN738LC superalloy. The solidification conditions for obtaining such a CLM were discussed on the aspects of temperature gradient and solidification rate within the melt pool. This work provides new insights and methods for preparing crack-free γ´-Ni-based superalloys with specific textures, which is favorable for improving the high-temperature properties.

Fatigue property is an important index for novel high-entropy alloys (HEAs) before their engineering applications. Here we engineer a Al_0·3CoCrFeNi HEA with hierarchically heterogeneous microstructure by cold rolling and annealing treatment, which includes heterogeneous grains, annealing and deformation twins, residual dislocations and B2 precipitates with different morphologies, sizes and distributions. Stress-life (S–N) tests and characterization techniques including scanning electron microscope (SEM) and transmission electron microscope (TEM) were carried out to investigate fatigue properties as well as corresponding mechanisms. It is found that this HEA possesses good strength-ductility combination (i.e., yield strength of ∼870 MPa, ultimate tensile strength of ∼1060 MPa and ductility of ∼26 %) and fatigue resistance with fatigue ratio of ∼0.46 under stress ratio of −1. This fatigue ratio exceeds those of most reported HEAs. High strength renders the fatigue deformation mainly occurs in deformation twin regions decorated with B2 precipitates. Surface damage morphologies indicate that fatigue cracks initiate from persistent slip band-like shear bands. In addition, microstructural hierarchy results in the deflected fatigue crack propagation path, which is beneficial for the enhancement of fatigue resistance. Present results offer the guidance on future design for high fatigue-resistant HEAs by manipulating heterogeneous microstructure.

The introduction of twin lamellae and dislocation interaction through Multi-directional forging (MDF) at room temperature (RT) has been reported to significantly improve the mechanical properties of Mg alloys. However, the Mg alloys typically exhibited poor plasticity at RT, making it highly susceptible to cracking. This study further enhanced the deformation capacity of AZ80 Mg alloy by adding a pre-cryogenic step before MDF. Compared to direct MDF at RT, MDF at cryogenic temperature (CT, −196 °C) further increased the alloy's deformation capacity from 0.72 (4P) to 1.08 (6P), with the tensile yield strength (TYS) and ultimate tensile strength (UTS) increasing to ∼420 MPa and ∼512 MPa. The alloy subjected to cryogenic MDF exhibited denser twin lamellae and a higher proportion of {10–12}-{10–12} twin interactions. The more frequent twin interactions under CT inhibited excessive twin growth and further increased the dislocation density of the matrix, facilitating the nucleation of new twins. Additionally, with increasing deformation passes, the texture of the RT specimens became more single, while the CT specimen showed a trend toward texture dispersion. The study elucidated that poor forgeability of the alloy under MDF at RT was hindered by unfavorable texture and limited twin activation. However, this issue was mitigated under CT, where the activation of more atypical twins with low Schmid factors (SFs) significantly enhanced the deformation coordination ability. Furthermore, the activation of atypical twins and extensive twin interactions promoted texture dispersion in CT specimens, which facilitated the activation of various slips/twins and suppressed excessive twinning-induced hardening, thereby improving forgeability.

A low-alloyed Mg-1.2Zn-0.6Al-0.1Ca (wt.%) alloy was extruded at 200 °C with different ram speeds (0.5–4.0 mm/s), and the microstructure and mechanical properties were studied systematically. Heterostructures with fine dynamic recrystallized (DRXed) grains and coarse unDRXed grains were achieved at lower ram speeds of 0.5 mm/s and 1.0 mm/s, and fully-DRXed microstructure was attained at 4.0 mm/s. Increasing the extrusion speed resulted in an increase in DRXed grain size from 0.9 μm to 3.8 μm, and a transformation of the DRXed texture component from <10 to 10>−<11–20> to a new orientation that deviated by approximately 14°. The sample extruded at 0.5 mm/s presented an excellent tensile yield strength (TYS) of 369 MPa along with a 7.8% elongation, which was mainly due to the high hetero-deformation induced (HDI) strengthening provided by its heterostructures. Increasing ram speed resulted in an improved elongation despite a decreased TYS. The reasons for the decreased strength with increasing extrusion speed were mainly associated with grain growth, reduced dislocation density and weakened HDI strengthening. The reasons for the improved ductility with increasing extrusion speed were largely due to the increased DRXed grains fraction with soft orientations.