Scripta Materialia最新文献

Dynamic behavior of α-iron in the presence of hydrogen (H) under triaxial tension is investigated by molecular dynamic simulations. The process of hydrogen enhanced strain induced vacancy is directly captured at the atomic scale in a wide range of strain rates. Significant strain rate effects and H concentration dependence are observed in the hydrogen embrittlement of α-iron. When the concentration of H ($C_H$) is substantially high, the maximum tensile stress of iron falls. More local phase transitions are observed at the aggregation sites of H atoms, which plays a crucial role in ensuring that the material does not lose its load bearing capacity immediately after reaching the maximal stress. Our study sheds light on the intricate interplay of H, strain rate, phase transition (PT) in α-iron and provides valuable insights into the complex dynamics of H embrittlement.

Decoupling between calorimetric and dynamical glass transitions has been observed in high-entropy metallic glasses (HEMGs) owing to their complex entropy effects. Pressure is another versatile thermodynamic variable that can be used to control the properties of metallic glasses (MGs) owing to the reduced interatomic distances. However, how can it affect such anomalous decoupling behavior in HEMGs remains unclear. Here, molecular dynamics simulations are performed to investigate the influence of pressure on the decoupling behavior in several prototypical MGs and HEMGs. We find that pressure can enhance the sluggish atomic diffusion and depress structural α-relaxation, thus promoting the decoupling in HEMGs, even with less atomic size mismatch. Finally, incorporating pressure and configurational and mismatch entropies, we propose a general criterion based on an explainable machine-learning method to quantitatively classify MG's decoupling or not. These findings refine our understanding of glass transitions and suggest an avenue for manipulating decoupling behaviors in HEMGs.

Micro-level mechanical tests are of extreme sensitivity to the initial internal state and early response of materials. The stability of elastic property and yield behavior of macro-level polycrystal specimens fade significantly under micro-level uniaxial loading state. In this study, the incipient plasticity and underlying mechanisms of medium-entropy alloy CoCrFeNi were investigated by in-situ micropillar compression. Stress fluctuations at elastic stage of 〈001〉-oriented pillar were observed in the stress-strain curves and then evidenced by TEM characterization as the activation of leading partial dislocations. The discreteness of yield strength was thus attributed to the impediment of pre-formed stacking faults. Furthermore, a model illustrating the incipient plasticity and its consequences on micropillar compression tests was proposed after an evaluation on twinning stress of alloys with low stacking fault energy.

A novel protocol utilizing a phase-field model was used to process the reconstruction of a polycrystalline microstructure from synchrotron-based high-energy X-ray diffraction microscopy. This approach is an intuitive and standardized alternative to typical image processing routines. It preserves high-confidence regions by deploying a completeness-based mobility parameter in the phase-field model. Phase-field governing equations result in a space-filling grain map that adheres to the physics of the microstructure, i.e., it penalizes high-energy grain shapes and configurations and promotes grain boundary (GB) smoothing. We quantify GB smoothing by measuring, in 2D, the circularity of interior grains and the tortuosity of individual GBs. Results are also presented in 3D. This post-processing protocol can be applied to any X-ray diffraction microscopy reconstruction that consists of a spatial map of grains and corresponding confidence values. Furthermore, it can be adapted to accommodate other types of microstructures, including those that are polyphase.

We perform neutron diffraction on a bulk Ni_50.0Mn_27.5Ga_22.5 single crystal to investigate the evolution of its five-layered modulated (10M) martensite structure, from 300 K down to 10 K. Close to martensite transformation, the modulation is nearly commensurate, with q = 0.402 in a modulation vector (q, q, 0). Upon cooling, the shift in diffraction satellites indicates a transition to an incommensurate modulation with increasing q. However, the observed fifth-order diffraction satellites below 260 K and even more complex satellite landscape at 10 K cannot be explained by incommensurability alone. Our analysis reveals that the modulation function is highly anharmonic, encompassing Fourier components up to the eighth order. We have developed a single-parameter description of the evolution of modulation across the entire temperature interval of the 10M phase existence. Surprisingly, the structure evolution from commensurate to incommensurate modulation has no significant effect on twin boundary mobility.

The slip transmission across an interface is essential for the mechanical properties of dual-phase alloys like Ti-6Al-4 V. However, the correlation between the dislocation-interface interaction and the strength and strain hardening anisotropy remains unclear due to the lack of direct experimental evidence. Via in situ scanning electron microscopy micropillar compression, prismatic plane dislocations were preferentially activated and interacted with an individual α/β interface at different angles. Based on transmission electron microscopy characterization, this study suggests that α/β interface shows a more pronounced strengthening effect when the coordinated slip system is more difficult to be activated and the slip deflection angle is larger. Differently, its higher strain hardening rate is initially determined by the larger Burger vector magnitude of interfacial residual dislocation after slip transmission. These results provide a unique basis for understanding the contribution of the interface to the mechanical properties of dual-phase alloys.

In light of the steel development trend—plainification and high performance, we designed a gradient nanostructured plain steel with ultrahigh mechanical performance and exceptional cost performance. This multiscale design is achieved by using surface mechanical attrition treatment (SMAT) in a plain steel matrix subjected to quenching-partitioning-tempering (Q-P-T) process. The integration of Q-P-T and SMAT processes effectively achieves the gradient surface nanocrystallization on the high mechanical performance matrix. This gradient nanostructure exhibits an environment with gradient compressive stress accompanying with the refinement of grains, which prevent crack formation and propagation effectively. Consequently, an ultrahigh fatigue strength (up to 820 MPa) at high-cycle (10⁷) can be achieved with remarkable cost performance (1653.1 MPa·kg/USD) at the same time, surpassing maraging steel by 14 times. The multiscale design of gradient nanostructured plain steel not only breaks the endurance-cost trade-off but also paves the way to imparting significant endurance on high carbon plain steel.

Controlling the shape and orientation of metal nanoparticles constitutes fundamental strategies employed to tune their distinctive properties. However, singular high-energy surfaces are notably missing from nanoparticles fabricated using existing synthesis techniques. In this work, we fabricated supported (111)-oriented Pt nanoparticles that expose low-energy surfaces and subsequently reshaped them using a metal forming technique. We found that the orientations of deformed particles span a continuum encompassing (111), (110) and (112) and observed a linear dependence of this re-orientation on the plastic strain. We proposed a model describing particle rotation during deformation in terms of synergistic interplay between slip and diffusion.

In high-molybdenum (Mo) maraging steels, the formation of coarse Mo-enriched precipitates during conventional hot processing can significantly compromise both strength and ductility. In this contribution, we utilized the laser powder bed fusion (L-PBF) technique to produce a high-Mo maraging steel with a nominal chemical composition of Fe-13Ni-12Co-10Mo-1W-1Ti (wt.%). Notably, the ultrafast cooling rate inherent to L-PBF successfully suppresses the formation of coarse Mo-enriched precipitates. The co-precipitation of high-density Ni₃Ti and Mo-enriched nanoprecipitates within the direct-aged martensitic matrix was observed. As a result, despite containing ∼16% soft reverted austenite, the direct-aged samples exhibit an ultrahigh yield strength (YS) of ∼ 2.34 GPa and an ultrahigh ultimate tensile strength (UTS) of ∼2.57 GPa, with an acceptable uniform elongation (UE) of around 2.7%. This work may provide a new pathway for the development of ultrahigh-strength maraging steels.

High temperature (HT) strength and room temperature (RT) ductility trade-offs are always unavoidable in the design of refractory high entropy alloys (RHEAs). Exploring the vast chemistry space to find optimally strong and ductile RHEAs remains a highly challenging task. Herein, we formulate a machine learning-based design strategy fusing uncertainty estimation and clustering analysis to explore a special alloy system (NbTaZrHfMo) for collaborative optimization of HT strength and RT ductility. Four non-equimolar alloys with a superior combination of HT strength, RT ductility and HT specific yield strength were discovered and synthetized. The influence of elements on mechanical properties are analyzed and an optimal composition range is identified based on model prediction. This work provides a general design approach enabling the concurrent optimization of conflicting properties with a small data-trained machine learning model, thereby producing a recipe to accelerate the discovery of desired RHEAs and other materials within a vast search space.