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

Twenty-five years of ultrafine-grained materials: achieving exceptional properties through grain refinement

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

The innovation potential is high for bulk nanostructured materials (BNM) produced by methods of severe plastic deformation and accordingly this report focuses on very recent developments demonstrating the potential of using BNM for advanced and functional applications in engineering and medicine.

The innovation potential of bulk nanostructured materials

Deformation mechanics of differential-speed rolling with a high speed ratio between the upper and lower rolls, which was found to be effective in producing very fine-grained microstructure in Mg–3Al–1Zn sheets, has been investigated by means of the rigid-plastic finite element method. The shear strain (3.1) takes a great portion of the total effective strain (∼3.5) in average. The magnitude of total effective strain is equivalent to that obtained by equal channel angular pressing on rods after 3–4 passes.

Finite element analysis of severe deformation in Mg-3Al-1Zn sheets through differential-speed rolling with a high speed ratio

The superplastic properties and microstructural evolution of a 7055 aluminum alloy was examined in tension at temperatures ranging from 300 to 450 °C and strain rates ranging from 2.7 × 10 −5 to 5.6 × 10 −2  s −1 . A refined microstructure with an average grain size of ∼1.4 μm and a portion of high-angle grain boundaries (HAGBs) of ∼67% was produced in sheets by equal channel angular extrusion (ECAE) followed by isothermal rolling (IR) at 250 °C. The alloy exhibited a maximum elongation-to-failure of ∼820% at a temperature of 450 °C and an initial strain rate of 5.6 × 10 −3  s −1 . Superplastic elongation of ∼670% was recorded at strain rates higher than 10 −2  s −1 , where the strain rate sensitivity coefficient was around 0.44. The relationship between superplastic ductilities and microstructure stability was analyzed. It was shown that increasing the portion of HAGBs by IR results in enhancement of superplastic properties.

I. Nikulin

Papers

Superplasticity in a 7055 aluminum alloy processed by ECAE and subsequent isothermal rolling