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Abstract A gradient-structured Cu sample composed of grain-size gradient surface layer (GSL) and homogeneous coarse-grained (CG) core was fabricated by surface severe plastic deformation. Microhardness measurements revealed that both tension-induced softening in the top nanograined surface layer and hardening in the subsurface layer of integrated gradient sample were far more pronounced than that of a freestanding GSL, i.e. extraordinary softening and hardening occurred in gradient sample. The synergetic deformation between GSL and CG core produced accumulation of geometrically necessary dislocations and thereby back stress strengthening, resulting in extra flow stress and hardening. The extraordinary softening was attributed to the extra flow stress-assisted grain coarsening in nanograined layer. Furthermore, strain measurements proved that the CG matrix plays critical roles in stabilizing the grain coarsening and homogenizing the strain distribution in nanograined layer, which were favorable for the uniform ductility. [ABSTRACT FROM AUTHOR]

The effect of back stress on the mechanical behaviors of heterogeneous material is studied in two modeled heterogeneous laminates, i.e. laminated structure with a nanostructured (NS) Cu-Zn alloy layer sandwiched between two coarse-grained (CG) pure Cu layers. The improved tensile ductility of NS layer is revealed and attributed to the constraint from the stable CG layers. It is found that the elastic/plastic interaction between NS and CG layers is capable of significantly improving the back stress, which makes a significant contribution to the synergetic strain hardening in low strain stage. Furthermore, a higher mechanical incompatibility permits stronger and longer mutual interaction between layers, i.e. coupling effect, which contributes to a higher back stress. These results improve our understanding about the role of back stress on mechanical behaviors of heterogeneous laminate materials. [ABSTRACT FROM AUTHOR]

The effect of the pre-strain on hydrogen embrittlement (HE) of high strength steels was investigated by slow strain-rate tensile (SSRT). The specimens were electrochemically hydrogen-charged in 0.5 mol/L H 2 SO 4 with 1 g/L CH 4 N 2 S at room temperature for 24 h. It is found that the amount of hydrogen increases linearly with the exponent of pre-strain, i.e., C H = 11.963 + 0.00385 exp ( 1.144 ε P ) . The HE susceptibility of the alloy initially decreases and then increases with increasing pre-strain. The ultimate tensile strength for the hydrogen-charged specimen is highest in the 3% pre-strain, which is interpreted by competition between strain hardening and Hydrogen Enhanced Decohesion (HEDE) mechanism. The fracture mode shows a mixed mode fracture of quasi-cleavage and intergranular fracture for hydrogen-charged specimens while the fracture mode exhibits dimples for hydrogen-free specimens. [ABSTRACT FROM AUTHOR]

Abstract: The low fracture toughness of lanthanum zirconate (La2Zr2O7, LZ) greatly impedes its wide application as thermal barrier coatings (TBC). The 3mol% Y2O3-stabilized tetragonal zirconia polycrystals (3Y-TZP) have been introduced to toughen the brittle LZ ceramics. The dispersive 3Y-TZP undergoes a simultaneous t–m transformation upon cooling below a critical volume fraction x of 3Y-TZP, above which its tetragonal phases can however be preserved. The different stabilities of 3Y-TZP second phases arise from a variation of residual tensile stress within them. The fracture toughness has been greatly improved by dispersing the tetragonal particulates (t-3YSZ) in the LZ matrix and the primary toughening mechanisms are phase transformations of the dispersive second phases and the residual compressive stress within the matrix. An anticipated increase of fracture toughness from the ferroelastic toughening and the residual compressive stress toughening highlights the great potentials to improve coating durability by depositing t′-3YSZ/LZ composite TBCs by the industrial non-equilibrium route. [Copyright &y& Elsevier]

The low strength of pure Ti significantly limits its biomedical applications, although its biocompatibility is the best. Here, by controlled annealing and cryogenic drawing after rolling, we synthesize a set of pure Ti combining the strength of ultrafine-grained Ti and the ductility of coarse-grained Ti. The microstructure is characterized with fine grains (a few microns), abundant twins, and high dislocation density, which enable both short dislocation-slip path for strengthening and enough defects accumulation room for work hardening. Twins provide strengthening due to the subdivision effect of twin lamella on grains and dislocations accumulation near the twin boundaries. High flow stress helps to activate FCC phase transformation at large-strain stage. The excellent ductility is originated from the sustained work hardening enabled by the initial texture, fine grains, the interaction among dislocations, FCC lamellae, and twins. [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]

Heterostructured laminates have attracted significant attention in the materials community owing to their exceptional combination of strength and ductility. However, long-standing questions remain regarding the impact of the strength difference between the soft and hard layers on the synergistic strengthening in hetero-laminates with weak mechanical heterogeneity; the role played by soft/hard layers with various dislocation storage capacities on the strength–ductility synergy is also poorly understood. In this study, a series of typical heterostructured copper–brass laminates were fabricated using the diffusion welding + forging + rolling + annealing technique. The results demonstrated that larger microstructural and mechanical heterogeneities improved the strength–ductility combination of the hetero-laminates. The relatively weak mechanical heterogeneity and superior dislocation storage capacity of the soft copper and hard brass layers facilitated inter-layer synergistic deformation. This induced a gradual distribution of local strains and geometrically necessary dislocations (GNDs) near the hetero-interface. Thus, when evaluating the role of soft layers in synergistic strengthening, it is crucial to consider their generation and storage capabilities for dislocations. [ABSTRACT FROM AUTHOR]

The present work investigated the effect of cathodic hydrogen-charging current density on mechanical properties of prestrained high strength steels. This was done by tensile tests on both hydrogen-charged and -uncharged prestrained specimens at a cross-head displacement speed of 0.03 mm/min. The influence of prestrain on hydrogen behavior was also studied using an electrochemical permeation technique. The results show that the relationship between ultimate tensile strength of the hydrogen-charged specimens (UTS-H) and prestrain depends on current density. The UTS-H decreases with increasing current density independent on prestrain. With an increase in prestrain the diffusion coefficient of hydrogen gradually decreases, which is attributed to increasing dislocations density acted as hydrogen traps. SEM fractograph reveals that hydrogen charging causes a change from ductile to brittle failure. [ABSTRACT FROM AUTHOR]

Abstact The tensile properties and fracture behaviors of the pre-charged hydrogen low alloy steel subjected to various tempering temperatures were investigated by slow strain rate tensile tests. Hydrogen was introduced into the samples by electrochemical method in 0.5 mol L −1 NaOH solution with 1 g L −1 CH 4 N 2 S for 24 h at room temperature (298 K). The results show that with an increase in tempering temperature, the resistance to hydrogen embrittlement increases. For the sample tempered at 200 °C, hydrogen-assisted microcracks initiate at the lath boundaries and the interfaces between inclusions and matrix. Moreover, the void around the inclusion initiates at the matrix-inclusion interface and grows in the form of the interface decohesion for hydrogen free sample, while the void also starts at the interfaces and propagates into the steel matrix along vertical tensile stress direction in the presence of hydrogen. Fractography reveals that for hydrogen-charged samples, the crack originates from mixed O-Si-Al-Ca inclusions in the samples tempered at 200 °C and 400 °C, while it is not located at inclusions in the sample tempered at 650 °C. [ABSTRACT FROM AUTHOR]