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[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Friction stir welding/processing of metals and alloys: A comprehensive review on microstructural evolution

Methods of deforming metals to large strains are reviewed and the process of equal channel angular extrusion is analysed in detail. The development of microstructure during large strain deformation is discussed, and it is concluded that the main criterion for the formation of a sub–micron grain structure is the generation of a sufficiently large fraction (> 0.7) of high–angle grain boundary during the deformation process. For aluminium alloys, it is found that a low–temperature anneal is required to convert the deformed microstructure into an equi–axed grain structure. The material, microstructural and processing factors that influence the formation of such fine–grain microstructures are discussed, and the stability of these microstructures at elevated temperatures is considered.

Developing stable fine-grain microstructures by large strain deformation - Discussion

Current developments in kinetic and thermodynamic stabilization of grains in nanostructured metals, alloys, and compounds are generalized and discussed in detail. As applied to the thermodynamic approach, attention has recently shifted from using the regular solution approximation to estimating thermodynamic properties of nanomaterials by considering both inner regions of nanograins and their grain boundaries. This situation is discussed and examples for alloys based on iron, copper, tungsten, and other elements are presented. Experimental information about behavior of nanomaterials subjected to radiation or oxidation is considered, along with recent experiments on abnormal grain growth.

Review of thermal stability of nanomaterials

Microstructural response of pure copper to cryogenic rolling

: A high-resolution electron-back-scatter-diffraction (EBSD) technique was applied to quantify grain-structure development and texture evolution during/after cryogenic rolling of pure copper. Microstructure evolution was found to be a complex process involving mainly geometrical effects associated with strain and discontinuous recrystallization but also including limited twinning and grain subdivision. Recrystallization was deduced to be static in nature and probably occurred during static storage of the material at room temperature after cryogenic rolling. The texture contained a pronounced {110} brass component; this observation was interpreted in terms of the suppression of cross slip at cryogenic temperatures. In general, cryogenic rolling was found to be ineffective for the formation of a nanocrystalline (NC) structure in pure copper.

Microstructural Response of Pure Copper to Cryogenic Rolling (PREPRINT)

The electron-backscatter-diffraction (EBSD) technique was applied to investigate room-temperature annealing processes in cryogenically rolled copper during long-term (∼1.5 years) storage at ambient temperature. Static recrystallization appeared to be nucleated as result of both grain-boundary bulging and recovery. A bimodal recrystallized grain size distribution appeared to be a result of these two competing mechanisms. The ultra-fine grain copper produced via cryogenic deformation was deduced to be prone to abnormal grain growth after long static storage at room-temperature.

On the room-temperature annealing of cryogenically rolled copper

Electron backscattered diffraction (EBSD) was used to analyze the evolution of microstructure and crystallographic texture during friction stir welding of dissimilar type 2205 duplex stainless steel (DSS) to type S275 low carbon-manganese structural steel. The results of microstructural analyses show that the temperature in the center of stirred zone reached temperatures between Ac1 and Ac3 during welding, resulting in a minor ferrite-to-austenite phase transformation in the S275 steel, and no changes in the fractions of ferrite and austenite in the DSS. Temperatures in the thermomechanically affected and shoulder-affected zones of both materials, in particular toward the root of the weld, did not exceed the Ac1 of S275 steel. The shear generated by the friction between the material and the rotating probe occurred in austenitic/ferritic phase field of the S275 and DSS. In the former, the transformed austenite regions of the microstructure were transformed to acicular ferrite, on cooling, while the dual-phase austenitic/ferritic structure of the latter was retained. Studying the development of crystallographic textures with regard to shear flow lines generated by the probe tool showed the dominance of simple shear components across the whole weld in both materials. The ferrite texture in S275 steel was dominated by D1, D2, E, $$ \bar{E} $$
 , and F, where the fraction of acicular ferrite formed on cooling showed a negligible deviation from the texture for the ideal shear texture components of bcc metals. The ferrite texture in DSS was dominated by D1, D2, I, $$ \bar{I} $$
 , and F, and that of austenite was dominated by the A, $$ \bar{A} $$
 , B, and $$ \bar{B} $$
 of the ideal shear texture components for bcc and fcc metals, respectively. While D1, D2, and F components of the ideal shear texture are common between the ferrite in S275 steel and that of dual-phase DSS, the preferential partitioning of strain into the ferrite phase of DSS led to the development of I and $$ \bar{I} $$
 components in DSS, as opposed to E and $$ \bar{E} $$
 in the S275 steel. The formations of fine and ultrafine equiaxed grains were observed in different regions of both materials that are believed to be due to strain-induced continuous dynamic recrystallization (CDRX) in ferrite of both DSS and S275 steel, and discontinuous dynamic recrystallization (DDRX) in austenite phase of DSS.

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Evolution of microstructure and crystallographic texture during dissimilar friction stir welding of duplex stainless steel to low carbon-manganese structural steel

Tatyana Konkova

Papers

Microstructural response of pure copper to cryogenic rolling

Microstructural Response of Pure Copper to Cryogenic Rolling (PREPRINT)

On the room-temperature annealing of cryogenically rolled copper

Evolution of microstructure and crystallographic texture during dissimilar friction stir welding of duplex stainless steel to low carbon-manganese structural steel

Microstructure instability in cryogenically deformed copper