Dynamic recrystallization

Geometric dynamic recrystallization occurs in grains with local serrations. Upon deformation, grains undergoing GDRX elongate until the thickness of the grain falls below a threshold (below which the serration boundaries intersect and small grains pinch off into equiaxed grains). Geometric Dynamic Recrystallization has 6 main characteristics: More sophisticated models consider complex initial grain geometries, local pressures along grain boundaries, and hot working temperature, Lastly, recrystallization can be accelerated as grains are shifted and stretched, causing subgrain boundaries to become grain boundaries (angle increases). The affected grains are thinner and longer, and thus more easily undergo deformation. == Discontinuous Dynamic Recrystallization ==

Discontinuous recrystallization is heterogeneous; there are distinct nucleation and growth stages. It is common in materials with low stacking-fault energy. Nucleation then occurs, generating new strain-free grains which absorb the pre-existing strained grains. It occurs more easily at grain boundaries, decreasing the grain size and thereby increasing the amount of nucleation sites. This further increases the rate of discontinuous dynamic recrystallization. == Continuous Dynamic Recrystallization ==

Continuous dynamic recrystallization is common in materials with high stacking-fault energies. It occurs when low angle grain boundaries form and evolve into high angle boundaries, forming new grains in the process. For continuous dynamic recrystallization there is no clear distinction between nucleation and growth phases of the new grains. Continuous Dynamic Recrystallization has 4 main characteristics: • As strain increases, stress increases • As strain increases, subgrain boundary misorientation increases • As low angle grain boundaries evolve into high angle grain boundaries, the misorientation increases homogeneously • As deformation increases, crystallite size decreases There are three main mechanisms of continuous dynamic recrystallization: First, continuous dynamic recrystallization can occur when low angle grain boundaries are assembled from dislocations formed within the grain. When the material is subjected to continued stress, the misorientation angle increases until the critical angle is achieved, creating a high angle grain boundary. This evolution can be promoted by the pinning of subgrain boundaries. Second, continuous dynamic recrystallization can occur through subgrain rotation recrystallization; subgrains rotate increasing the misorientation angle. Once the misorientation angle exceeds the critical angle, the former subgrains qualify as independent grains. Third, continuous dynamic recrystallization can occur due to deformation caused by microshear bands. Subgrains are assembled by dislocations within the grain formed during work hardening. If microshear bands are formed within the grain, the stress they introduce rapidly increases the misorientation of low angle grain boundaries, transforming them into high angle grain boundaries. However, the impact of microshear bands are localized, so this mechanism preferentially impacts regions which deform heterogeneously, such as microshear bands or areas near pre-existing grain boundaries. As recrystallization proceeds, it spreads out from these zones, generating a homogenous, equiaxed microstructure. ==Mathematical Formulas==

Based on the method developed by Poliak and Jonas, a few models are developed in order to describe the critical strain for the onset of DRX as a function of the peak strain of the stress–strain curve. The models are derived for the systems with single peak, i.e. for the materials with medium to low stacking fault energy values. The models can be found in the following papers: • Determination of flow stress and the critical strain for the onset of dynamic recrystallization using a sine function • Determination of flow stress and the critical strain for the onset of dynamic recrystallization using a hyperbolic tangent function • Determination of critical strain for initiation of dynamic recrystallization • Characteristic points of stress–strain curve at high temperature The DRX behavior for systems with multiple peaks (and single peak as well) can be modeled considering the interaction of multiple grains during deformation. I. e. the ensemble model describes the transition between single and multi peak behavior based on the initial grain size. It can also describe the effect of transient changes of the strain rate on the shape of the flow curve. The model can be found in the following paper: • A new unified approach for modeling recrystallization during hot rolling of steel ==Literature==

• A one-parmenter approach to determining the critical conditions for the initiation of dynamic recrystallization, onset of DRX • Flow Curve Analysis of 17–4 PH Stainless Steel under Hot Compression Test, comprehensive study of DRX • Constitutive relations to model the hot flow of commercial purity copper, chapter 6, doctoral thesis by V.G. García, UPC (2004) • A review of dynamic recrystallization phenomena in metallic materials, Latest review paper on DRX • A Cellular Automaton Model of Dynamic Recrystallization: Introduction & Source Code, Software simulating DRX by CA: Introduction, Video of software run ==References==