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

An ultra-high strain rate (10⁴ s⁻¹) dynamic plastic deformation treatment at liquid nitrogen temperature (LNT-DPD) followed by annealing is carried out to obtain dual heterogeneity of grain size and dislocation density in an equiatomic CrCoNi medium entropy alloy (MEA). Such extreme loading conditions resulted in extensive phase transformation in this MEA. Subsequent annealing at 650 °C for 1 h further induced reverse phase transformation and partial recrystallization, forming a complex heterogeneous microstructure characterized by nested trimodal grain sizes and partitioned dislocation density. A superior yield strength of ∼800 MPa and a good ductility of ∼40% were simultaneously achieved in this heterogeneous alloy. In order to reveal the effects of grain size and dislocation density distributions on the mechanical property improvements, the underlying deformation mechanisms were systematically discussed. High density of geometrically necessary dislocations (GNDs) would be induced in complex heterogeneous structures during tensile deformation due to strain gradients or partitioning between different regions, which would lead to additional strengthening and work hardening. These results provide a novel approach to overcome the strength-ductility trade-off dilemma for FCC-structured MEAs.

To meet the service conditions and strength requirements of turbine stator blades and the inner and outer rings of aero engine casings, CoFeNiCrMn high-entropy alloy filler was used to braze SiC_f/SiC/GH536 joints. This study investigated the effects of holding time on the joints' microstructure and mechanical properties. Key phases identified in the welded joints include MoNiSi, FCC, and Cr₂₃C₆ near the composites, with brittle Ni₃Si and Fe₂Si intermetallic compounds forming due to filler diffusion. Optimal brazing parameters were found to be 1220 °C for 30 min with a shear strength of 64.28 MPa. The study also highlighted that increased holding time at the same temperature enhances diffusion at the joint, increasing brittle intermetallic compounds, initially improving shear strength, which then declines. Microstructural and fracture morphology analyses revealed that insufficient insulation time leads to poor welding and stress concentration at pores, causing cracks. Excessive insulation time results in joint fractures due to the brittleness of Ni₃Si and Fe₂Si intermetallic. Thus, joint shear strength correlates with welding quality and intermetallic compound distribution.

The semi-solid Al₈₀Mg₅Li₅Zn₅Cu₅ light weight high entropy alloys were prepared by strain induced melting activation method (SIMA), and the microstructure evolution, mechanical properties and corrosion resistance of semi-solid Al₈₀Mg₅Li₅Zn₅Cu₅ light weight high entropy alloys were investigated. The results indicate that the ideal globular or near-globular microstructures with an average grain size of 29.1 μm and a shape factor of 0.86 can be obtained when the Al₈₀Mg₅Li₅Zn₅Cu₅ light weight high entropy alloys with 20% deformation held at 500 °C for 15 min. The coarsening coefficient is 35.8 μm³/s when the temperature is 500 °C, which is lower than the traditional single major element alloys due to the sluggish diffusion effect of high entropy alloys. The compression strength of semi-solid Al₈₀Mg₅Li₅Zn₅Cu₅ light weight high entropy alloys is 558.4 MPa, which is 11% higher than that of as-cast state. The corrosion resistance of semi-solid Al₈₀Mg₅Li₅Zn₅Cu₅ light weight high entropy alloys is also significantly improved.

The accurate description of anisotropic plastic deformation is key to accurately predicting the stamping forming of metal sheets. The anisotropic yield criterion, when based on the assumption of isotropic hardening, often leads to significant inaccuracies. To address this issue, this paper observes the anisotropic hardening phenomenon through non-associated flow rules and evaluates the anisotropy of 7075-O aluminum alloy. Through a series of tensile tests, we determined the mechanical properties of 7075-O aluminum alloy in three distinct orientations. To describe the metal hardening behavior, we employed the Swift-Voce hardening criterion. From the hardening curves in three different directions, it was found that AA7075-O exhibits plastic anisotropy. Based on the VUMAT subroutine, finite element simulation of AA7075-O tensile tests was conducted through Abaqus. Compared with the Hill48 model, it was found that the simulated values of the S–Y2009 anisotropic hardening model have a higher degree of agreement with the experimental curves. The S–Y2009 anisotropic hardening model was adopted to predict the earing behavior of AA7075-O during circular cup deep drawing. The root mean square error between the predicted values of the S–Y2009 model and the experimental values was only 0.1795, which is far smaller than that of the Hill48 yield model. Therefore, the SY2009 model has important guiding significance for the stamping forming of metal sheets.

The adiabatic shear band is a common dynamic failure mechanism in metal materials at high strain rates. In this work, a new approach was proposed to improve mechanical properties and inhibit the formation and propagation of the adiabatic shear band in the pure Mo single crystal. The microstructure evolution of the pure Mo and Mo-Nb single crystals subjected to the dynamic load was investigated by EBSD and TEM. At a strain rate of 2500 s⁻¹, the developed adiabatic shear band was totally distributed in the pure Mo single crystal, and the slip is the main deformation mechanism. A thinner adiabatic shear band was distributed in the Mo-3Nb single crystal accompanied by a few bands of deformation twins and then disappeared in the Mo-6Nb single crystal, replaced by the {112}<111> deformation twinning. At high strain rates, the dynamic deformation mechanism of the Mo single crystal is sensitive to the Nb element, which attributes to the reduction of the generalized stacking fault energy.

In this study, the effect of post heat treatment on microstructural evolution and tensile properties of 0.75% of carbon doped NiCoCr MEA alloy manufactured by laser powder bed fusion (LPBF) was investigated. The post heat treatment of LPBF-built 0.75C MEA alloy was selected and carried out at 700 °C for 1hr, respectively. The microstructural observation in heat-treated 0.75C MEA HT sample shows a lower stacking fault energy (SFE) with wider stacking fault width (SFW) compared to as-built condition. In-situ nano-sized precipitates are formed along the sub-boundaries of the as₋built 0.75C MEA and 0.75C MEA HT samples which are estimated as 1.25 % and 2.45 %, respectively. After post heat treatment process, the size and ratio of nano-sized precipitates enhanced and identified as Cr-rich M₂₃C₆ carbide. Subsequently, the yield and tensile strengths increase from 823.2 MPa to 872.7 MPa and 1.05 GPa–1.15 GPa for 0.75C MEA and 0.75C MEA HT conditions, respectively which is attributed to the prevailing solid solution strengthening and precipitation strengthening. The deformation twins in as−built specimen are transformed to twin bundle once the local strain increases from 5% to 20%. The 0.75C MEA HT deformed specimen exhibits high dislocation accumulation with retained stable substructure in local strain 20% area due to the pinning effect of nano-sized precipitates which stabilizes and strengthens the substructure boundary i.e., Dynamic Hall-Petch mechanism. These results provide a new perception for achieving better strength performance by post heat treatment process for NiCoCr-based medium entropy alloys (MEA) alloy.

Creep, as a long-term deformation characteristic of shotcrete, significantly influences the load-bearing performance and durability of tunnel support structures. This study conducted creep tests on shotcrete under varying stress levels. The results reveal that shotcrete exhibits linear creep response within the stress range of 30%–40%, while the critical stress range for nonlinear creep response occurs between 40% and 50%. Employing SEM-EDS and XRD analysis methods, the impact of accelerator on the microstructure evolution and creep mechanism of shotcrete is explored. The accelerator accelerates the hydration of C₃S, promoting the generation of AFt and C–S–H gel, leading to rapid solidification of the matrix and enhanced early strength. With the consumption of SO₄²⁻ ions, AFt gradually transforms into AFm, resulting in increased matrix porosity. This phenomenon contributes to the reduced later-stage strength and substantial creep deformation of shotcrete. Based on experimental outcomes, this study evaluates the applicability of existing creep calculation models to shotcrete.

In order to solve the embrittlement problem of welded joint of pipeline steel, this paper proposes to study the influence of Mo on M-A component of the subcritically reheated coarse-grained HAZ (SCGHAZ), and Mo on bainite transformation. Through the welding thermal simulation experiment, two kinds of X80 pipeline steel with different Mo content were set different secondary thermal cycle peak temperatures to simulate the heat affected zone (HAZ). Samples in the SCGHAZ were selected to observe the microstructure, M-A component, microalloy-element distribution and fracture crack trend, and comprehensively analyze the reasons for the deterioration of impact toughness caused by the changes of Mo content. The results show that the increase of Mo content leads to the formation of twinning martensite, which is one of the reasons for the deterioration of toughness. Mo is enriched at the interface of ferrite and M-A component, reducing the binding energy at the interface, resulting in the desorption of M-A component from the matrix, the formation of pores and becoming the nucleation site of cracks, resulting in the generation of micro-cracks and the decrease of toughness.