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• The NS surface layer supported by CG substrate displays significant strain delocalization and achieves improved uniform elongation. • The strain delocalization is realized by forming dispersed stable strain bands (SBs) and promoting the deformation of non-strain banding zone. • The intra-layer microstructure inhomogeneity and the limited strain hardening capability of intra-layer domains lead to SBs nucleation. • The inter-layer microstructure inhomogeneity causes extra inter-layer shear constraints, which stabilize SBs and promotes the deformation of non-strain banding zone. • The NS indeed becomes ductile, due to the extensive nanotwinning and dislocation activity enabled by strain delocalization. How to suppress strain localization in tensioned nanograined layer and make it ductile? This remains a great challenge. Here we explore the effects of microstructure inhomogeneity on the plastic behavior of nanostructured layer. A nanostructured CrMnFeCoNi high entropy alloy layer supported by coarse-grained substrate (in gradient architecture) is taken as an example. In the tensile deformation, strain examinations find that the nanostructure layer experiences significant strain delocalization by activating dispersed stable strain bands (SBs) and promoting the deformation of non-strain banding zone. Finite element analysis reveals that it is the intra-layer microstructure inhomogeneity which enables dispersed SBs nucleation from the mechanical weak sites. The inter-layer microstructure inhomogeneity introduces extra shear constraints from the coarse-grained substrate, which stabilizes SBs and improves the stress of non-strain banding zone, thereby suppressing the early catastrophic strain localization. Importantly, the strong nanostructure layer indeed becomes ductile (enables further work hardening) due to the activation of hierarchical nanotwinning and dislocation activity. [Display omitted] [ABSTRACT FROM AUTHOR]

Here the combination of synergistic strengthening and strain delocalization effects is realized in high-manganese steel by gradient microstructure designing, which results in an excellent strength-ductility synergy. Specifically, the gradient structure consists of two gradient surface layers sandwiching a coarse-grained center. Hardness and microstructure examinations reveal that the synergistic strengthening is originated from the enhanced twinning behavior and extra work hardening in gradient layers. The formation of dispersed strain concentration bands is found in the nanostructured surface layer, which evolves steadily and undertakes large applied strain, thereby leading to delocalized straining response. The formation and stabilization mechanisms of strain concentration bands are analyzed in depth with respect to the microstructure gradient. These findings shed light on the deformation fundamentals of gradient structure, especially on the hetero-deformation induced effects. [ABSTRACT FROM AUTHOR]

Abstract 3D needle-punched C/C-SiC composites were fabricated from carbon fiber reinforced carbon (C/C) preforms, with densities of 1.05 g/cm3 and 1.28 g/cm3, by the gaseous silicon infiltration (GSI) method at fabrication temperatures from 1500 °C to 1800 °C. The compressive strengths and elastic moduli in transverse direction are larger than those measured under longitudinal compression except that samples fabricated from 1.28 g/cm3 density exhibit lower elastic moduli in transverse direction than in longitudinal direction. The compressive strength and modulus increase with fabrication temperature at 1500 °C and 1600 °C, and then decrease with higher fabrication temperature. Samples fabricated from the lower density C/C preforms have greater compressive strength and modulus. X-ray tomography was applied before and after the mechanical tests to characterize the microstructure and damage patterns, and the results indicated that for C/C-SiC composites fabricated at 1700 °C from 1.28 g/cm3 density C/C preform the matrix has a volume fraction (vol%) of 36.9%, and the initial intra-bundle cracks (0.6 vol%) display a space crossing structure while the inter-bundle pores (6.0 vol%) are special irregularly distributed. [ABSTRACT FROM AUTHOR]

Abstract Damage development has been observed in situ within a 3-dimensional needle-punched carbon fiber reinforced carbon and silicon carbide (C/C-SiC) ceramic composite, as compressive loads were applied longitudinally, transversely and obliquely to its anisotropic structure. Laboratory X-ray computed tomographs were analyzed using digital volume correlation to measure the local strains. Cracks exist within the microstructure before the application of load, due to thermal strains, and their propagation is affected by the heterogeneous microstructure and the direction of loading. The punched carbon fiber needles are observed to restrict the propagation of damage. [ABSTRACT FROM AUTHOR]

SiC ceramic materials were prepared using a combination of direct ink writing (DIW) and liquid silicon infiltration (LSI). In this study, we investigated the effects of the content of a copolymer of isobutene and maleic anhydride (ISOBAM) on the rheological properties of the slurry, microstructure of green bodies and SiC ceramics, and bending properties of SiC ceramics. ISOBAM significantly improved the rheological properties of the slurry and formability of the SiC green body. The yield stress and equilibrium modulus of the slurry without ISOBAM were 160.3 Pa and 1.12 × 105 Pa, respectively, and increased to 607.7 Pa and 2.37 × 106 Pa, respectively, after the addition of 0.46 wt% ISOBAM. LSI was effective in improving the density of the green body, and the densities of the sintered SiC ceramic materials ranged from 2.71 to 2.86 g/cm3. The bending strength of SiC ceramic materials initially increased and then decreased when the ISOBAM addition increased from 0.15, to 0.30, and 0.46 wt%, and the slurry with 0.30 wt% ISOBAM exhibited the highest bending strength of 287.7 ± 9.1 MPa. [ABSTRACT FROM AUTHOR]

C f /Hf 0.5 Zr 0.5 C-SiC composites were prepared by introducing Hf 0.5 Zr 0.5 C matrix (11 cycles) and SiC matrix (9 cycles) into the carbon cloth preform through precursor impregnation and pyrolysis (PIP) process. The influence of the introduction time of SiC matrix on the microstructure and mechanical properties of C f /Hf 0.5 Zr 0.5 C-SiC composites was studied, and the results show that with the increase of the PIP cycles of the SiC matrix introduced before Hf 0.5 Zr 0.5 C matrix, the composite open porosity decreased, and the flexural strength and modulus presented an obvious upward trend. CS45 sample, which has 4 cycles of PIP SiC introduced in advance, has the highest flexural strength, flexural modulus and interfacial shear strength of 402.73 ± 35.73 MPa, 56.92 ± 3.97 GPa and 100.88 ± 7.79 MPa, respectively. Hf 0.5 Zr 0.5 C matrix has a loose and porous structure, so when more SiC matrix was introduced in advance, its covering effect on the surface of fibers led to less intra-bundle pores and thusly denser composite structure, and due to the compactness of SiC matrix, better overall bonding of fiber, interface and matrix was achieved, as well as better load transfer effect, which led to obvious interfacial debonding and cracking based on the in-situ SEM observation during flexural tests. While in the sample without pre-introduced SiC, the cracking occurred mainly between the interface and porous matrix and the overall performance of the material was poor. [ABSTRACT FROM AUTHOR]

Duplex stainless steels (DSS) are subjected to a deleterious effect known as hydrogen assisted cracking (HAC). Revealing the hydrogen diffusion behavior in DSS is the key to understanding the mechanism of HAC, since hydrogen diffusivity and solubility are different in ferrite and austenite. In this study, finite element (FE) analyses on hydrogen diffusion behavior in DSS were performed by considering the combined effect of heterogeneous microstructure and hydrostatic stress with the help of finite element program-ABAQUS. Two-dimensional dual-phase models with representative volume element were applied based on the obtained microstructure using optical microscopy. The constant loading catholic charging experiments were also carried out to provide comparative results for characterization on the susceptibility of DSS to HAC. Results indicate that hydrogen diffusion in DSS is strongly dependent on the morphological diversity of microstructure. Compared to the coupled analysis with stress effect, hydrogen diffusivity and concentration are observed with an increase, and experimental results show that most of the HAC cracks initiate in the ferrite phase and arrested by austenite. Image 1 • Two-dimensional dual-phase hydrogen diffusion models were applied based on the obtained microstructure. • A constant-load hydrogen cathodic charging experiment of 2205 U-notch sample was carried out by considering stress effect. [ABSTRACT FROM AUTHOR]

Abstract Polymer film blends of hydroxypropyl methylcellulose (HPMC) and hydroxypropyl starch (HPS) were produced with an incorporation of three different plasticizers, including polyethylene glycol (PEG), glycerol and 1, 2-propylene glycol (PG) respectively, and the influence of these plasticizers were compared and studied by X-ray Diffraction (XRD), small angel X-ray Scattering (SAXS), optical microscope and dynamic mechanical analysis (DMA). Results showed that multi-scale structure of the blends were greatly affected by the plasticizers and therefore caused significant changes in mechanical properties. In the course, blends with glycerol presented lamellar structure rather than self-similar structure. Moreover, crystalline degree was increased with an order of glycerol > PEG > PG, whereas the compactness of amorphous region of all samples was decreased by PEG and PG. [ABSTRACT FROM AUTHOR]

Since hydroxypropyl methylcellulose (HPMC) is a thermal gel while hydroxypropyl starch (HPS) is a cool gel, their blends show some unique and interesting behaviors of phase transitions and microstructures, which control their performances. This work focused on the effect of processing conditions on the microstructure and mechanical properties of these blends with various ratios of HPMC/HPS, in particular for incubation and drying conditions. To clearly identify the changes of starch phase in such blending system, the technique of dyeing HPS with iodine was developed and used. Rheometer, small angel X-ray scattering (SAXS), X-ray diffraction (XRD), dynamic mechanical analysis (DMA) and mechanical testing apparatus were used to study the changes of microstructure and the relative properties of the blending gels and dried films incubated at different temperatures. It was found that the higher incubation temperature resulted in higher storage modulus, more solid-like behavior and more compact self-similar structure of the blending gel. Moreover, the films prepared from the gels dried at high temperature showed higher crystalline degree and more compact amorphous structure, and therefore result in increasing tensile strength and modulus but decreasing elongation. [ABSTRACT FROM AUTHOR]