There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

Friction stir welding (FSW) is a relatively new solid-state joining process. This joining technique is energy efficient, environment friendly, and versatile. In particular, it can be used to join high-strength aerospace aluminum alloys and other metallic alloys that are hard to weld by conventional fusion welding. FSW is considered to be the most significant development in metal joining in a decade. Recently, friction stir processing (FSP) was developed for microstructural modification of metallic materials. In this review article, the current state of understanding and development of the FSW and FSP are addressed. Particular emphasis has been given to: (a) mechanisms responsible for the formation of welds and microstructural refinement, and (b) effects of FSW/FSP parameters on resultant microstructure and final mechanical properties. While the bulk of the information is related to aluminum alloys, important results are now available for other metals and alloys. At this stage, the technology diffusion has significantly outpaced the fundamental understanding of microstructural evolution and microstructure–property relationships.

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Friction Stir Welding and Processing

Metals have been mankind's most essential materials for thousands of years; however, their use is affected by ecological and economical concerns Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency However, most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength-ductility trade-off Here we present a metastability-engineering strategy in which we design nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection, the concept is overturned We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase) This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels and massive solid-solution strengthening of high-entropy alloys In our transformation-induced plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength Moreover, the increased strain hardening capacity that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials This metastability-engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys

Metastable high-entropy dual-phase alloys overcome the strength–ductility trade-off

Dynamic and post-dynamic recrystallization under hot, cold and severe plastic deformation conditions

The present invention provides a method of designing a high fatigue strength material in high tensile strength steel, comprising the steps of: obtaining the values of tensile strength σ B (unit thereof is MPa) and Vickers hardness Hv of the steel; measuring the flaw area of inclusion, when the fracture origin is located only at the surface of the steel; and estimating, in designing the high fatigue strength material, that the fatigue limit σ w (unit thereof is MPa) of the steel satisfies either σ w ≧0.5σ B or σ w ≧1.6 Hv, when a square root of the flaw area, (area) ½ (unit thereof is m), contained in the steel is no larger than 45.8 /σ B 2 or 4.47 /Hv 2 . According to the present invention, a method of evaluating high fatigue strength material in high tensile strength steel, in which method the relationship between the flaw dimension (area) of ODA and the fatigue strength is considered, as well as a high fatigue strength material, can be Provided.

Method of evaluating high fatigue strength material in high tensile strength steel and creation of high fatigue strength material

We studied the statistical quantitative analysis of the hydrogen-assisted damage evolution behavior from nanoto micro-scale by combining positron annihilation spectroscopy (PAS) and scanning electron microscopy-based damage characterization in a dual-phase steel with a tensile strength of 960MPa. The total elongation was markedly decreased by hydrogen pre-charging (0.32mass ppm H) from 17% to 4%. We divided the damage evolution behavior into three stages: damage incubation; arrest; growth, and evaluated the effects of hydrogen pre-charging on each stage. The damage nucleation was caused by martensite fracture and enhanced by hydrogen pre-charging. However, PAS showed no enhancement of vacancy formation by hydrogen. The statistical damage quantitative analysis indicated in the damage arrest stage that the critical damage size corresponding to the blunt limit of the damage tip was decreased from 31μm2 in the uncharged specimen to 30.5 μm2 in the hydrogen pre-charged specimen. The damage growth in the third stage was accelerated by hydrogen pre-charging owing to quasi-brittle damage propagation through the ferrite cleavage plane or ferrite/martensite interface. Microstructure observation showed that the cleavage propagation in ferrite was accompanied by the local plastic deformation. To explain this fracture acceleration, we proposed cooperative contribution of the enhancement of the local plastic deformation through adsorption-induced dislocation emission mechanism and the cleavage fracture through hydrogen enhanced decohesion mechanism. [doi:10.2320/matertrans.MT-M2019196]

Evolution of Quasi-Brittle Hydrogen-Assisted Damages in a Dual-Phase Steel

Preface to the Special Issue on “Advanced Structural Steels”

Surface orientation dependence of hydrogen flux in lenticular martensite of an Fe-Ni-C alloy clarified through in situ silver decoration technique

Superior fatigue life of 8000 cycles at low-cycle fatigue with a total strain Δe=2% was found in the Fe–30Mn–4Si–2Al high-Mn alloy, as compared to Fe–30Mn–6Si–0Al and Fe–30Mn–3Si–3Al alloys with fatigue life of 2×103 cycles. Examination of microstructural evolution and cyclic hardening/softening behavior was shown that high fatigue resistance of Fe–30Mn–4Si–2Al alloy associated with delayed development of the deformation induced martensite and inhibited dislocation slip as compared to Fe–30Mn–6Si–0Al and Fe–30Mn–3Si–3Al alloys, respectively. Cyclic strain softening followed by secondary strain hardening was observed in the Fe–30Mn–4Si–2Al alloy after primary hardening. Primary hardening to about 40 cycles was associated with continuous increase in density of planar dislocations and the development of slip bands. The cyclic softening manifesting as the drop of the stress amplitude in the range of the cycles from 40 to 400 was accompanied by development of deformation induced e-martensite in place of the slip bands. At the N>400 cycles further increase in the volume fraction of deformation e-martensite leads to continuous hardening up to the failure. In the presentation we will discuss the details of microstructural evolution during LCF of the Fe–30Mn–4Si–2Al alloy.

Kaneaki Tsuzaki

Papers

Method of evaluating high fatigue strength material in high tensile strength steel and creation of high fatigue strength material

Evolution of Quasi-Brittle Hydrogen-Assisted Damages in a Dual-Phase Steel

Preface to the Special Issue on “Advanced Structural Steels”

Surface orientation dependence of hydrogen flux in lenticular martensite of an Fe-Ni-C alloy clarified through in situ silver decoration technique

Low-Cycle Fatigue Behavior and Microstructural Evolution of the Fe–30Mn–4Si–2Al Alloy