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

Deformation microstructures were studied in ferritic stainless steels during cold bar rolling and swaging to total true strains about 7. Two steels, i.e. Fe-22Cr-3Ni and Fe-18Cr-7Ni with coarse-grained ferritic and fine-grained martensitic initial microstructures, respectively, were selected as starting materials. Microstructure evolution in the both steels was characterized by the development of highly elongated (sub)grains aligned along the rolling/swaging axis. The transverse size of these (sub)grains in the Fe-22Cr-3Ni steel gradually decreased to about 0.1 μm with increasing the strain. On the other hand, the transverse (sub)grain size in the Fe-18Cr-7Ni steel decreased to its minimal value of 0.07 μm with straining to about 3 followed by a little coarsening under further working. The strengthening of worked steels that revealed by hardness tests correlated with the microstructure evolution. The hardness of the Fe-22Cr-3Ni steel increased with cold working within the studied strain range, while that of the Fe-18Cr-7Ni approached a saturation after fast work hardening at strains below 3, leading to an apparent steady-state behaviour. Development of strain-induced (sub)grain boundaries and internal stresses in the steels with different initial microstructures during severe deformation is discussed in some detail.

Microstructure Evolution in Ferritic Stainless Steels during Large Strain Deformation

Effect of α-Al/Al3Ni microstructure on the corrosion behaviour of Al–5.4 wt% Ni alloy fabricated by equal-channel angular pressing

We investigated the cause for poor ductility in austenitic Fe-Mn-C steels under a specific condition. Tensile tests were performed on an Fe-17Mn-0.3C steel at 273 K, 294 K, 323 K, 373 K, 423 K, 473 K, and 523 K (0 °C, 21 °C, 50 °C, 100 °C, 150 °C, 200 °C, and 250 °C). Microstructural observations were conducted by optical microscopy, atomic force microscopy, scanning electron microscopy and the X-ray diffraction method. e-martensitic transformation was concluded to be the major cause for the poor ductility. The cracks were initiated from the annealing twin boundaries that interacted with the e-martensite.

Premature Fracture Mechanism in an Fe-Mn-C Austenitic Steel

Grain refinement effect on hydrogen embrittlement resistance of an equiatomic CoCrFeMnNi high-entropy alloy

Nanoindentation techniques were applied to evaluate the matrix strength of the as-quenched and the tempered martensite of an Fe-0.4 wt% C binary alloy. The matrix strength was compared with the micro Vickers hardness H V. The nanohardness Hn of the matrix decreases with increasing tempering temperature, which is similar to the temper softening observed in the micro Vickers hardness. The ratio H n/H V, which corresponds to the contribution of the matrix strength to the macroscopic strength, increases markedly when the specimen is tempered at 450°C. This is due to a reduction in the grain size effect caused by the grain growth of the blocks that are analogous to the effective grain. The pop-in behaviour that appears in the specimens tempered at or above 550°C is due to the reduction in the dislocation density caused by recrystallization.

Kaneaki Tsuzaki

Papers

Microstructure Evolution in Ferritic Stainless Steels during Large Strain Deformation

Effect of α-Al/Al3Ni microstructure on the corrosion behaviour of Al–5.4 wt% Ni alloy fabricated by equal-channel angular pressing

Premature Fracture Mechanism in an Fe-Mn-C Austenitic Steel

Grain refinement effect on hydrogen embrittlement resistance of an equiatomic CoCrFeMnNi high-entropy alloy

Evaluation of the matrix strength of Fe-0.4 wt% C tempered martensite using nanoindentation techniques