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

A review of dynamic recrystallization phenomena in metallic materials

The double-edge effect of second-phase particles on the recrystallization behaviour and associated mechanical properties of metallic materials

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 transition from discontinuous to continuous recrystallization in some aluminium alloys: II – annealing behaviour

The transition from discontinuous to continuous recrystallization in some aluminium alloys I – the deformed state

The use of high-resolution electron backscatter diffraction in the scanning electron microscope to quantify the volume fraction of recrystallization and the recrystallization kinetics is discussed. Monitoring the changes of high-angle grain boundary (HAGB) content during annealing is shown to be a reliable method of determining the volume fraction of recrystallization during discontinuous recrystallization, where a large increase in the percentage of high-angle boundaries occurs during annealing. The results are shown to be consistent with the standard methods of studying recrystallization, such as quantitative metallography and hardness testing. Application of the method to a highly deformed material has shown that it can be used to identify the transition from discontinuous to continuous recrystallization during which there is no significant change in the percentage of HAGB during annealing.

Quantifying recrystallization by electron backscatter diffraction.

A sample of AISI type 316 stainless steel from a power station steam header, showing reheat cracking, was removed from service and has been examined by a combination of microscale X-ray computed tomography (CT), nanoscale serial section focused ion beam–scanning electron microscopy (FIB-SEM), energy dispersive X-ray (EDX) spectrum imaging and transmission electron microscopy (TEM). Multiscale three-dimensional analysis using correlative tomography allowed key regions to be found and analysed with high resolution techniques. The grain boundary analysed was decorated with micrometre sized, facetted cavities, M23C6 carbides, ferrite and G phase but no σ phase. Smaller intragranular M23C6 particles were also observed, close to the grain boundaries. This intimate coexistence suggests that the secondary phases will control the nucleation and growth of the cavities. Current models of cavitation, based on isolated idealised grain boundary cavities, are oversimplified.

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Multiscale 3D analysis of creep cavities in AISI type 316 stainless steel

Creep degradation in austenitic stainless steels is associated with nucleation and growth of cavities that can link up to form micro- and macro- cracks, usually along grain boundaries. A reheat crack found near a header nozzle weld removed from a nuclear power station has been examined using both electron backscatter diffraction (EBSD) and hardness mapping. The EBSD studies revealed higher levels of lattice misorientation towards the weld region where the crack initiated with strain particularly concentrated at grain boundaries. The pattern of deformation shown by the EBSD measurements was confirmed by the hardness survey.

H. Jazaeri

Papers

The transition from discontinuous to continuous recrystallization in some aluminium alloys: II – annealing behaviour

The transition from discontinuous to continuous recrystallization in some aluminium alloys I – the deformed state

Quantifying recrystallization by electron backscatter diffraction.

Multiscale 3D analysis of creep cavities in AISI type 316 stainless steel

Investigating plastic deformation around a reheat-crack in a 316H austenitic stainless steel weldment by misorientation mapping