Importance of Non-uniform Boundary Migration for Recrystallization Kinetics
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RECRYSTALLIZATION is of key importance for thermomechanical processing of metals and alloys. When a deformed metal is heat-treated, new nearly defect-free grains, here termed recrystallized grains, emerge and grow to replace the deformed matrix. After recrystallization, the density of dislocations introduced during deformation is largely reduced. Recrystallization typically also leads to a significant change in grain size, shape, and texture compared to the deformed and the original material. It is thus one of the most effective approaches to alter the mechanical and physical properties of metals and alloys. To control the recrystallization process, the transformation kinetics during recrystallization needs to be quantified. The classical model describing transformation kinetics was proposed by Johnson and Mehl,[1] Avrami,[2] and Kolmogorov[3] for phase transformations,
FENGXIANG LIN is with the Institute of Mechanics, Materials and Civil Engineering (iMMC), Universite´ catholique de Louvain, 1348 Louvain la Neuve, Belgium. Contact e-mail: [email protected] YUBIN ZHANG, WOLFGANG PANTLEON, and DORTE JUUL JENSEN are with the Department of Mechanical Engineering, Technical University of Denmark, 2800 Kongens Lyngby, Denmark. Manuscript submitted March 13, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
but is generally used also for recrystallization. In this so-called JMAK model, the increase of the recrystallized volume fraction (VV ) during isothermal annealing is expressed as VV ¼ 1 expðktn Þ;
½1
where t is the annealing time, and k and n are two parameters. n is often termed the Avrami exponent. The theoretical derivation of the JMAK model assumes that the nucleation sites are randomly distributed in space. When all the recrystallized grains grow in 3D at a constant boundary migration velocity, the Avrami exponent n equals 3 for site-saturated nucleation (i.e., all the nuclei appear instantaneously at the beginning of recrystallization), and n equals 4 for the situation of a constant nucleation rate. This is in the following referred to as the idealized JMAK model. Unfortunately, the idealized JMAK model often fails in two aspects: the exponent n has frequently been observed to have values much lower than 3 for 3D growth (e.g., Reference 4), and is not always constant throughout the recrystallization process, but varies as a function of time (e.g., Reference 5). The shortcomings of the idealized JMAK model to describe typical recrystallization kinetics can be attributed to its assumptions. In reality, nucleation sites are often not randomly distributed. Local regions with high stored energy and large misorientations are preferred
nucleation sites, e.g., regions near triple junctions,[6,7] transition bands,[8] shear bands,[9] and large secondary particles.[10,11] Clustered nucleation, which in turn leads to early impingement of recrystallized grains, on average retards the recrystallization process, as compared to random nucleation. Moreover, migration velocities of the recrystallizing boundar
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