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

In order to clarify the effect of microstructural change during ageing and creep on elastic modulus, the elastic moduli of aged, crept and serviced (service-exposed) materials were investigated. In ferritic steels, the elastic modulus obtained by ultrasonic pulse does not change after long-term ageing and creep although the formation of creep voids slightly decreases the elastic modulus. In SUS304 and SUS316, the elastic modulus evaluated by ultrasonic pulse increases during ageing due to precipitation of M23C6 carbides, Laves phase and σ phase though the elastic modulus decreases during creep because of the formation of creep voids on grain boundaries. The increased elastic modulus of aged material with a large amount of σ phase was regarded as that of composite consists of matrix and σ phase. For STBA24, the elastic modulus obtained by a high temperature tensile test decreases after long-term service. The tensile test at a higher strain rate was proposed to avoid underestimation of elastic modul...

Elastic properties of heat resistant steels after long-term creep exposure

An Fe-3%Si bicrystal was deformed in compression to a strain of 0.9 at ambient temperature. In the interior of deformation bands, characteristic band structures with high orientation gradients and low and high angle boundaries were formed during straining. Furthermore, isolated ultrafine grains were found in the matrix of the deformation bands. The morphology and crystallography of the ultrafine grains indicate that they are caused by slip system partitioning, i.e. local activity of a highly stressed slip system that is different from the active slip system in the surrounding crystal volume.

Slip System Partitioning as a Possible Mechanism for Ultrafine Grain Formation in Fe–3%Si Bicrystals

The phase relationships between the ferrite phase (α) and the austenite phase (γ) were investigated in the iron-rich corner of the Fe-Cr-Mn-Ni quaternary system at 1000°C. The phase boundary between the α and γ phases was determined using a diffusion-multiple technique. In this technique, three end members, FeCr, FeCrMn and FeCrNi alloys, were coupled inside a metal tube, and then heat treated at 1300°C for 24h to create a microstructural state with a composition gradient of Cr, Ni and Mn within the composition range of the end members. This was followed by long time heat treatment at 1000 °C for a period of 240 h to attain equilibrium states between the γ and α phase in the composition range. The phase boundary between the γ single phase field and the γ+α two-phase field was found to slightly shift in a way to expand the γ single phase field when the manganese content was increased from 0 to 5 wt%. The shift of the phase boundary was not obvious at chromium contents above 20 wt% and nickel contents above 8 wt%. These results indicate that manganese is a weak austenitizer at low chromium and nickel contents.

Determination of α / γ phase boundaries in the Fe-Cr-Ni-Mn quaternary system with a diffusion-multiple method

PROBLEM TO BE SOLVED: To suppress intergranular fracture caused by old austenite boundaries in a steel and to obtain excellent toughness by subjecting a steel to working specified in reduction of area in a temp. region lower than the recrystallization temp. of austenite and executing cooling without causing recrystallization and diffusion type phase transformation. SOLUTION: A steel is perfectly austenitized and is subjected to working of >=30% reduction of area in an austenite unrecrystallized region. By this working, into the austenitic grains, shearing deformation macroscopically uniform and microscopically ununiform is generated, and wavy ruggedness is introduced into the part of >=70% of the austenite boundaries. After the working, it is cooled about to a room temp. under the condition in which recrystallization and diffusion type phase transformation do not occur. A fine structure in which as for the boundaries corresponding to the old austenite boundaries before the working, the part of >=70% per boundary unit length on a view from the surface vertical to the boundary face has ruggedness of =200 nm amplitude can be obtd.

High toughness and high strength steel and its production

Here, we present a review of the hydrogen embrittlement behavior of face-centered cubic (FCC) alloys with short-range order (SRO) of solute atoms. In this paper, three types of FCC alloys are introduced: Fe–Mn–C austenitic steels, high-nitrogen steels, and CoCrFeMnNi high-entropy alloys. The Fe–Mn–C austenitic steels show dynamic strain aging associated with Mn–C SRO, which causes deformation localization and acceleration of premature fracture even without hydrogen effects. The disadvantageous effect of dynamic strain aging on ductility, which is associated with the deformation localization, amplify plasticity-assisted hydrogen embrittlement. Cr–N and Co–Cr–Ni SRO effects in high-nitrogen austenitic steels and high-entropy alloys enhance the dislocation planarity, which causes stress concentration in the grain interior and near the grain boundaries. The stress concentration coupled with hydrogen effects causes quasi-cleavage and intergranular fractures.

Kaneaki Tsuzaki

Papers

Elastic properties of heat resistant steels after long-term creep exposure

Slip System Partitioning as a Possible Mechanism for Ultrafine Grain Formation in Fe–3%Si Bicrystals

Determination of α / γ phase boundaries in the Fe-Cr-Ni-Mn quaternary system with a diffusion-multiple method

High toughness and high strength steel and its production

Potential Effects of Short-Range Order on Hydrogen Embrittlement of Stable Austenitic Steels—A Review