Developing stable fine–grain microstructures by large strain deformation

Abstract

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 (greater than 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 equiaxed 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.

Recrystallization mechanisms during friction stir welding/processing of aluminum alloys

“…The recent emphasis on production of ultrafine and nanocrystalline microstructures through severe plastic deformation (SPD) has accentuated the need for models of recrystallization occurring concomitantly with largestrain deformation [7]. However, most of the in-depth studies have involved, again, approximately uniform strains and isothermal conditions during SPD.…”

Grain Refinement through Shear Banding in Severely Plastic Deformed A206 Aluminum Alloy

“…The macro shear banding phenomena has been already reported for pure Al after different severe plastic deformation procedures (e.g., ECAP [43,44] and FSP [45,46] ). As is observed, the serrated boundaries may coincide with each other and give rise to newly recrystallized grains through geometric dynamic recrystallization (GDRX) mechanism.…”

Microstructure Evolution in Ferritic Stainless Steels during Large Strain Deformation

“…This could be simply explained by two reasons: reduction in the cross section of original grains that followed the change of sample geometry, and the development of new strain-induced (sub)boundaries, which aligned parallel to the rolling direction after sufficiently large strains. 3,17,25) It should be noted that the transverse size of structural elements could approach some constant value at large strains following a rapid reduction during preceding deformation. Such behaviour was observed in certain studies on plane rolling of two-phase alloys, when the shear banding frequently operated during plastic working.…”

“…This interest was mainly motivated by promising structural state with a grain size of submicron scale that could be developed in almost all metals and alloys under large strain plastic working. [1][2][3][4][5][6] Several techniques were proposed to provide severe plastic working, [7][8][9][10][11][12][13][14] including mechanical milling, torsion under high pressure, multiple forging, equal channel angular pressing, accumulative roll-bonding, etc. The structural changes leading to the submicrocrystalline states during severe deformation could be associated with the formation of strain-induced dislocation subboundaries, namely geometrically necessary boundaries; then a gradual increase in the misorientations between the strain-induced subgrains upon further straining resulted in the development of new ultra fine-grained microstructures.…”

“…The structural changes leading to the submicrocrystalline states during severe deformation could be associated with the formation of strain-induced dislocation subboundaries, namely geometrically necessary boundaries; then a gradual increase in the misorientations between the strain-induced subgrains upon further straining resulted in the development of new ultra fine-grained microstructures. 3,4,[15][16][17][18][19][20][21] The most of processing methods mentioned above are multiple techniques that based on changing (or reversing) the strain path for each cycle. Such multiple character of straining stimulates the variation of deformation mechanisms operating in each strain pass and seems to play an important role in the formation of rather equiaxed fine grains.…”