International Journal of Plasticity最新文献

Additively manufacturing alloys by a selective laser melting (SLM) usually generates large temperature gradients and rapidly cooling, which enables a refined microstructure, an elemental segregation and high-density dislocations network to achieve an excellent strength-ductility synergy. In this study, the SLM fabricates FeCoNiAlTi high-entropy alloys (HEAs) with a cellular structure composed of high-density dislocations network and elemental segregation, which results in a noteworthy combination of a yield strength and a significant uniform plastic elongation. Strengthening mechanism and deformation behavior of SLM-prepared FeCoNiAlTi HEAs are investigated by a transmission electron microscopy in combination with an in-situ neutron diffraction technique. The results demonstrate that the high strength is mainly derived from cellular structure strengthening, which accounted for over 64 % of the yield strength. The cellular structure's capability to alleviate severe stress concentrations can facilitate deformation homogenization, and break a strength-ductility trade-off. This study provides essential insights into the underlying mechanisms governing the strength and ductility of additively manufactured HEAs.

Body-centered-cubic (BCC) high-entropy alloys (HEAs) encounter significant challenges in obtaining a high uniform tensile ductility (UTD). A dense dislocation-cell (DC) structure is produced in a heterogeneously grained HEA under tensile deformation, resulting from the anchored dislocation motion by grain interior elemental segregation. This fluctuation in elemental concentration is facilitated by thermomechanical processing. The activation of multiple-slip mechanisms, prompted by strain incompatibility among grains of varying sizes, significantly propels this process forward. This novel DC structure simultaneously increased the UTD (by 349.1 %) and yield strength (${\sigma }_{0.2}$, by 29.0 %) for a stable BCC HEA. Specifically, the single-phase alloy achieved a record-high UTD of 7.5 % and an ${\sigma }_{0.2}$ of > 1,200 MPa, outperforming the counterparts of all the single-phase BCC HEAs. We employed a combination of transmission electron microscopy, in-situ scanning electron microscopy tensile testing coupled with an electron backscatter diffraction technology to investigate underlying strengthening mechanisms and identified that the serious stress concentration as a result of prevalent planar slip caused premature failure and localized strain of common BCC HEAs. At the initial stage of deformation, the DC structure promoted the activation of multiple slip systems and facilitated the extension of a plastic flow across the sample volume, effectively weakening stress concentration and premature failure. The extended plasticity zone and intensified dislocation interactions contributed to the increased UTD and ${\sigma }_{0.2}$. These findings offer valuable inspiration for tailoring alloy properties via microstructure strategies and promoting their adoption in advanced manufacturing.

Cr-rich stainless steel sheets exhibit superior corrosion resistance but low ductility, which presents a trade-off between fabrication complexity and performance of the materials in multiple industrial applications, such as marine equipment and microreactors. By transitioning the Cr-rich (30 wt.% Cr) stainless steel component to SS 316 L with a smooth composition gradient in the thickness direction, the intrinsic homogeneous elongation of the Cr-rich layer was increased by 260 % while maintaining the naturally high corrosion resistance (100 %) and retaining most of the strength (more than 80 %). By employing in-situ tensile testing and electron backscatter diffraction analysis, it was revealed that the Cr-rich layer in the gradient structure underwent a profound deformation mechanism, including significant heterogeneous deformation-induced hardening and grain reorientation induced by multiplication and accumulation of geometrically necessary dislocations, in such a way to enable a substantial plastic strain and thereby retarding the occurrence of fracture. The proportion of the Cr-rich layer makes a significant impact on the magnitude of the strain gradient in the gradient specimens, therefore affecting the increment of density of geometrically necessary dislocations. The critical proportion value of the Cr-rich layer is found to be around 22 %. Before and after the critical value the gradient specimens showed different sensitivities to the proportion. This discovery underlines the significance of intrinsic plasticity in low-ductility metals and the role of compositional gradient materials in enhancing strength and ductility.

Hydrogen is known to embrittle austenitic stainless steels, which are widely used in high-pressure hydrogen storage and delivery systems, but the mechanisms that lead to such material degradation are still being elucidated. The current work investigates the deformation behavior of single crystal austenitic stainless steel 316L through combined uniaxial tensile testing, characterization and atomistic simulations. Thermally precharged hydrogen is shown to increase the critical resolved shear stress (CRSS) without previously reported deviations from Schmid’s law. Molecular dynamics simulations further expose the statistical nature of the hydrogen and vacancy contributions to the CRSS in the presence of alloying. Slip distribution quantification over large in-plane distances ($>$1 $\mathrm{mm}$), achieved via atomic force microscopy (AFM), highlights the role of hydrogen increasing the degree of slip localization in both single and multiple slip configurations. The most active slip bands accumulate significantly more deformation in hydrogen precharged specimens, with potential implications for damage nucleation. For $〈110〉$ tensile loading, slip localization further enhances the activity of secondary slip, increases the density of geometrically necessary dislocations and leads to a distinct lattice rotation behavior compared to hydrogen-free specimens, as evidenced by electron backscatter diffraction (EBSD) maps. The results of this study provide a more comprehensive picture of the deformation aspect of hydrogen embrittlement in austenitic stainless steels.

Texture, microstructure, and local grain neighbourhood contribute to the development of localized stresses in polycrystals. For hexagonal close-packed materials, crystal's elastic and plastic anisotropy can also be a major contributing factor, yet there is a paucity of experimental studies focusing on the extent of contribution of such parameters on the magnitude of localized stresses at microscales. This study focuses on addressing this knowledge gap by deforming double-edge-notched soft-textured α-zirconium specimens in-situ, while measuring grain scale tensorial stresses using high energy synchrotron X-ray diffraction. The specimens were subjected to cyclic loads to study the evolution of stresses in the vicinity of both shallow and deep notches. The soft-texture of the specimens is such that there are no c-axes of grains aligned along the macroscopic loading direction thereby inhibiting deformation twinning. The “as-measured” microstructures and notch geometries were imported into a crystal plasticity finite element model for further analysis. Results show that despite the absence of c-axes of grains aligned along loading direction, the developed stresses were substantially influenced by crystallographic orientations. Stress drop was observed near the onset of plasticity with further loading and the orientation and position effects were highlighted. A plastic deformation mechanism was revealed where, upon specimen loading, the mechanical constraints enforced during grain-grain interactions led to hardening. Accordingly, a parameter was devised to quantify the grain level hardening arising from this mechanism. It was shown that grain-scale stress concentration factors vary significantly before the onset of plasticity, but they settle in the plastic zone and with the progression of cycles.

Elucidating the effect of local strain on the mechanical properties is of great significance for the design of high-performance layered metals. For this purpose, we conceived the present study, featured by tailoring the local strain by layer thickness design, and simultaneous monitoring of local strain and geometrically necessary dislocations (GNDs) via coupling in-situ electron backscatter diffraction (EBSD) and high-resolution digital image correlation (DIC). In addition, synchrotron X-ray micro-computed tomography (μCT) was employed to analyze the microcracks that serve as another form of strain localization. Such detailed experimental studies revealed that the interfacial strain gradient was rapidly elevated, and the strain localization band was effectively dispersed as the layer thickness decreased. This leads to two typical transitions, from grain-boundary-related to layer-interface-related plastic deformation mode, and from macroscopic shear to zig-zag fracture mode. Their influences on the mechanical properties, as well as underlying mechanisms, were discussed based on the relationship among the layer thickness, strain gradient, strain localization, GND density, and microcracks. Our work not only contributes to the fundamental understanding of the mechanical behavior of multilayered metals but also offers guidance for the structural design of high-performance metals aimed at achieving superior strength-ductility combinations.

With the increase in computational costs driven by the use of artificial intelligence, enhancing the performance of semiconductor systems while improving efficiency has become an inevitable challenge. Due to the fine pitch limits of micro bumps, bumpless Cu-Cu bonding is emerging as the next-generation core technology. This study aims to analyze the effects of individual temperature and pressure on both large- and local-scale behaviors of material in the Cu-Cu bonding process with experiments and numerical analysis. The motivation of this study is to compensate the deficiencies in reported studies on process optimization, particularly the lack of exploration of the separated effects of temperature and pressure on large- and local-scale Cu-Cu bonding. Furthermore, reports on the thermodynamic modeling of Cu-Cu bonding behavior are not sufficient, making it challenging to find suitable models. Bonding experiments were performed by independently controlling the temperature and pressure using blank Cu films treated by precise chemical mechanical polishing (CMP) processes. The large-scale bonded area under each condition was measured, and transmission electron microscope (TEM) images were captured to observe the patterns of local void formation under various temperature and pressure conditions. In the experiments, it was observed that the temperature increase had a greater impact on the bonded area at a larger scale than the increase in pressure. However, for nanoscale-local voids, an increase in pressure had a more dominant effect. To discuss the experimental results, a thermodynamic modeling framework that considers coupled heat-induced deformation, plastic deformation, and volumetric changes caused by material flux was proposed. The proposed model has been implemented in the user-defined material subroutine (UMAT) of the ABAQUS program for finite element (FE) analysis. Numerical analysis using the proposed model captures the experimental data well. In large-scale simulations, temperature conditions have a significant impact, with plastic deformation being the primary mode of deformation, while the pressure conditions dominate the material flux, making substantial contributions to reducing voids at local-scale. To achieve complete closure of the void, the simulation demonstrated that maintaining a sufficient pressure gradient until the complete closure is required. The study findings provide an explicit understanding of how the temperature and pressure conditions differently affect large-scale bonding and local voids for semiconductor package manufacturing.

Metal matrix composites (MMCs) are the materials-of-choice for a large range of important applications under harsh service conditions. However, owing to the high phase contrast between the matrix and the reinforcements, the strength-ductility conflict of MMCs is still outstanding. Here we fabricated a novel aluminum (Al) matrix composite reinforced by deformable, cobalt-zirconium-boron (CoZrB) metallic glass nanoparticles. The amorphous CoZrB/Al composite with only 2.0 vol.% particle reinforcements possessed a uniaxial tensile strength of 387.0 ± 1.2 MPa, showing over 80 % improvement over the unreinforced pure Al matrix at a similar uniform elongation. The strength-ductility synergy of the composite was also significantly superior to that of the composite reinforced by fully crystallized nanoparticles. These findings were rationalized by the unique multi-functionality of the amorphous particle/matrix interfaces, which effectively transferred the load from the matrix to the particles, coordinated the co-deformation of the nanoparticles and the matrix, and imparted a transgranular fracture mode in the composite with extensive matrix plastic deformation. The methodology developed in this study was shown to be generally effective for other matrix and metallic glass nanoparticle compositions, and our work may shed new light on the development of high-performance metal matrix composites for advanced structural applications.

This study develops a novel crystal plasticity (CP) model incorporating deformation-induced martensitic transformation (DIMT) and transformation-induced plasticity (TRIP) effect to predict the complex interplay between microstructural evolution and mechanical behavior in a third-generation advanced high-strength steel QP980. This model introduces phenomenological theory of martensite crystallography (PTMC) based TRIP theory and DIMT kinetics grounded on nucleation-controlled phenomenon. Notably, the DIMT model is improved by utilizing a geometric approach for calculating shear band intersections. A virtual multiphase representative volume element (RVE) based on the Voronoi tessellation is generated for the QP980 steel, which comprises ferrite, martensite, and retained austenite (RA). The study highlights how phase transformation affects mechanical properties, notably the strengthening from transformed martensite and the mechanical alterations in RA due to the TRIP effect. The DIMT kinetics dependent on stress states such as uniaxial tension (UT), uniaxial compression (UC), plane strain tension (PST), and equi-biaxial tension (EBT) are analyzed using the developed model. The role of microstructural surroundings on martensitic transformation is also examined. Furthermore, analysis under biaxial loading angles using the model reveals an asymmetric yield surface, with more pronounced changes in yield stress in the tensile region due to accelerated transformation behaviors, as opposed to the more gradual transformations in the compressive region. This study provides valuable insights into the deformation mechanisms of the third-generation advanced high-strength steels including relationship between plastic anisotropy, transformation kinetics, and microstructural evolution.