ACTA

向上滑动阅览英文摘要

Irradiation Assisted Stress Corrosion Cracking (IASCC) is a material degradation phenomenon affecting austenitic stainless steels used in nuclear Pressurized Water Reactors (PWR), leading to the initiation and propagation of intergranular cracks. Such phenomenon belongs to the broader class of InterGranular Stress Corrosion Cracking (IGSCC). A micromechanical analysis of IGSCC of an irradiated austenitic stainless steel is performed in this study to assess local cracking conditions. A 304L proton irradiated sample tested in PWR environment and showing intergranular cracking is investigated. Serial sectioning, Electron BackScatter Diffraction (EBSD) and a two-step misalignment procedure are performed to reconstruct the 3D microstructure over an extended volume, to assess statistically cracking criteria. A methodology is also developed to compute Grain Boundary (GB) normal orientations based on the EBSD measurements. The statistical analysis shows that cracking occurs preferentially for GB normals aligned with the mechanical loading axis, but also for low values of the Luster-Morris slip transmission parameter. Micromechanical simulations based on the reconstructed 3D microstructure, FFT-based solver and crystal plasticity constitutive equations modified to account for slip transmission at grain boundaries are finally performed. These simulations rationalize the correlation obtained experimentally into a single stress-based criterion. The actual strengths and weaknesses of such micromechanical approach are discussed.

ACTA

向上滑动阅览英文摘要

Due to native deficiency of the interfacial mismatch between Al₂O₃ and Al-melt, as well as micron-Al₂O₃ segregation at matrix grain boundaries (GBs), achieving both high strength and fracture toughness has been the long-lasting challenge for Al-CuO composite fabricated by conventional casting. Herein, we report a novel manufacturing of Al-CuO composite via shift-speed ball-milling (SSBM) of Al-5 wt.% CuO powders and afterwards subjected to hot-pressing (HP) as well as heat treatment. Comprehensive characterization shows that in-situ generation of the two types of Al₂O₃ with intragranular distribution, including δ*-Al₂O₃ particles (~200 nm) and γ-Al₂O₃ whiskers (length of ~150 nm, thickness of ~20 nm), was governed by a diffusion-assisted nucleation-regime ascribing to the intense thermal effect of Al-CuO reaction. The size and spatial distribution of Al₂O₃ were emphasized to address their contribution to the high mechanical performance of the composite, which exhibits a tensile strength of ~481 MPa and fracture elongation of ~16.8 %. Meanwhile, the toughening mechanism of present Al-CuO composite was rationalized on basis of the "dislocation punched zone" and "plastic zone" affected by Al₂O₃. Both the theoretical analysis and fracture morphology support a reinforcements-matrix interface failure mechanism and the non-uniform distribution of Al₂O₃ consisting alternant "rich/poor zones" can markedly contribute to the high toughness. The present findings may provide a promising strategy to achieve an intragranular distribution of nano-sized reinforcements in Al-metal oxides composites, which enables increasing strength and ductility of the metal matrix.

ACTA

向上滑动阅览英文摘要

A parametric experimental study on the role played by the power (P), velocity (VL) and profile (top-hat or Gaussian) of the laser on the porosity, inclusion content and microstructural evolution of in-situ alloyed laser powder bed fusion (LPBF) manufactured Ti34Nb was conducted. For this, alloys were printed with three sets of processing parameters, in which the above-mentioned parameters were varied but the energy density was held constant. A detailed tomographic and microstructural investigation of these alloys was followed by tensile tests. Observations of melt pools in the single tracks and single stripes were complemented by thermal finite element method (FEM) simulations that studied their evolution. Results show that a top-hat laser profile combined with high P (> 650 W), high VL (> 650 mm/s) and short stripe width scanning strategy forms a large single stripe melt pool that moves slowly and has a low aspect ratio. The large and slow melt pool allows Nb to melt efficiently and minimizes its unmelted content while the low melt pool aspect ratio prevents keyholing that can otherwise result in porosity. These melt pool attributes also favor the formation of a columnar β-Ti microstructure with a strong {100} texture, which has the ideal combination of strength and elastic modulus. The mechanisms of microstructural evolution were explained and the scope of this parameter optimization principle was discussed in the context of further improving the properties of this alloy.

ACTA

向上滑动阅览英文摘要

The {-1012} tensile twins terminating inside the grains of a deformed Mg-Y alloy were investigated by transmission electron microscopy. The crystallographic features of terminating twins and associated slip structures were quantified and correlated. The local stresses developed at a terminating {-1012} twin were computed using crystal plasticity simulations in order to interpret the observed slip patterns. Results indicate that both basal 〈a〉 and 〈c+a〉 matrix glide were involved in accommodating the plastic stresses developed in the vicinity of terminating twins. Along the twin boundary, the defect contrast consistent with that of lattice dislocations and twinning partials was observed. Based on these observations, a dislocation reaction is proposed that establishes an interrelationship between the observed matrix glide and {-1012} twinning in Mg-Y alloys.

ACTA

向上滑动阅览英文摘要

Ultra-fast 3D material microstructure reconstruction and quantitative structure-property mapping are crucial components of integrated computational material engineering (ICME). It is particularly challenging for modeling random heterogeneous materials such as alloys, composites, polymers, porous media, and granular matters, which exhibit strong randomness and variations of their material properties due to the hierarchical uncertainties associated with their complex microstructure at different length scales. An explicit mixture random field (MRF) model is proposed to characterize and reconstruct multi-phase stochastic material property and microstructure simultaneously. The proposed method is shown to have ultra-high computational efficiency and only requires minimal imaging and property input data. The material property field is modeled by this strongly non-Gaussian random field, which is generated by a nonlinear mapping from the underlying Gaussian random field explicitly. The corresponding microstructure is represented by discrete phase indicators obtained from the material property field. A decomposed Karhunen–Loève (K-L) expansion method is used to dramatically reduce the computational costs and memory requirement for high-dimensional and high-resolution generations. The feasibility and superior efficiency are demonstrated by reconstructing various materials, from 2D to 3D, bi-phase to multi-phase, isotropic to anisotropic materials. The results show that it only takes approximately seconds to reconstruct a 3D material with the resolution of 500×500×500 pixels. The elegant expression for explicit microstructure-property mapping with uncertainty quantification capability can be directly incorporated into the ICME framework for material design and optimization.

ACTA

向上滑动阅览英文摘要

Tungsten heavy alloys (WHAs) are a type of ductile phase toughened alloy that are becoming increasingly interesting as an alternative to polycrystalline tungsten for fusion reactor plasma facing material components due to their balanced strength and ductility. To justify their use in the extremely harsh environment of a fusion reactor, understanding detailed microstructural features of WHAs associated with their mechanical property changes is necessary. A 90W-7Ni-3Fe WHA alloy has been chosen to investigate the effect of thermomechanical treatment and microstructural manipulation on the overall effectiveness of deformation accommodation in these bi-phase metallic composites. Both in-situ tensile testing and 3D microstructural analysis of the samples reveal a predominance of microcracking at tungsten grain boundaries that are blunted and arrested by the ductile phase, while there remains little to no instances of interfacial debonding. Thermomechanical treatment of this alloy is found to alter the spherical brittle phase domains into elongated plates, drastically reducing the ductile phase connectivity, and changing the nature of material deformation. Characterization of the ductile phase toughening mechanisms in these materials has provided deeper insight into the underlying physics governing material behavior in these alloys; revealing a surprising interfacial strength between the different phases.

ACTA

向上滑动阅览英文摘要

Layer thickness dependent mechanical behaviors of metallic nanolaminates have been extensively investigated. In a recent study [1], we show that a particular defect network, consisting of layer interface, stacking faults and twin boundaries, plays an important role in achieving high strength in Cu/Co multilayers. Here, we report a follow-up study on the effect of layer thickness on this unique interplay of defect networks. To this end, we investigate the mechanical behavior of highly textured Cu (111)/Co (0002) multilayers with individual layer thickness of 5, 25 and 100 nm. In situ micropillar compression tests show that the Cu/Co 25 nm multilayers have a much higher strength than 100 nm and 5 nm multilayers. Post-deformation TEM analyses and MD simulations reveal the layer thickness dependent variations of defect density dominating the strengthening effect in multilayers. This study provides new perspectives on optimal defect networks for the design of high strength, deformable metallic materials.