Papers by Author: Yves Bréchet

Abstract: A model describing the evolution of the misorientation angle between dislocation cells with plastic strain is proposed. The model is applied to the case of equal channel angular pressing (ECAP) of copper. In a basic version of the model, the evolution of the average misorientation angle is traced. A way of handling the evolution of the misorientation angle distribution function using a probabilistic description is also outlined.

Abstract: A physically based model is used to analyze quantitatively, the relative contributions of solute Nb and strain-induced NbC precipitation to the retardation of static recrystallization during the interpass time. The model explicitly takes into account the time evolution of strain-induced precipitation and its interaction with recovery and recrystallization. It is thus possible to quantitatively model the recrystallization kinetics taking into account: i) the effect of solute drag on the boundary mobility, ii) the effect of particle pinning (Zener drag) on the driving force for boundary motion, and iii) the effect of dislocation pinning by strain-induced precipitates, on the recovery kinetics and the nucleation of recrystallization. The analysis shows that there is an optimum partitioning of Nb between matrix solute and strain induced precipitates. This optimum partitioning maximizes particle pinning while ensuring an adequate solute drag effect to prevent the boundary from breaking away from solute atmosphere. The optimum partitioning of Nb between the matrix and the precipitates is shown to depend upon the temperature window of rolling, pass reduction and interpass time. The effect of delaying the kinetics of strain-induced precipitation of NbC through large Mn addition is shown to be an advantage for ensuring adequate solute drag in the low temperature, large pass deformation schedule used in near-net shape processing of thin slab or thick strip castings.

Abstract: The conditions for nucleation of abnormal grain growth have been investigated using a 2D vertex simulation allowing for the presence of pinning centers. The topological characteristics of an “almost pinned” structure, compared to the classical grain growth with no pinning force, show a striking correlation between the grain size and the size of the neighbors of this potential abnormal grain. The main finding of this contribution is to stress the crucial importance of pinning particles, and to point to the stochastic nature of the nucleation event in abnormal grain growth.

Abstract: The observation of inhomogeneous, ‘sluggish’ recrystallization in ferritic steels has been extensively documented and discussed, particularly with reference to low carbon steels. Stabilized ferritic stainless steels are also prone to this phenomenon and, in many cases, exhibit the effect more strongly than their carbon counterparts. The situation for stainless steels is exacerbated in part by the topology of the cold rolled microstructure, which is composed of highly elongated and layered grains. In this work an attempt has been made to probe the key features of this process by means of a two-dimensional vertex simulation. Key microstructural characteristics such as subgrain and grain size, topology, misorientation and energy are varied in these simulations in an attempt to elucidate the mechanisms responsible for the final recrystallization. These simulations are compared and contrasted with experimental observations from the recrystallization of an AISI409 stainless steel.

Abstract: During transport of spent Zircaloy-4 fuel rods, cladding temperatures can be expected to rise well over 400°C for transportation periods longer than 10 days. The kinetics of creep under these conditions will be controlled by both strain hardening and the softening effect of static annealing of cold work and irradiation defects. This paper will focus on the development of a coupled recovery/recrystallisation model for Zircaloy-4 from 400 – 520°C.