Papers by Keyword: Dynamic Recrystallization (DRX)

Abstract: Changes in strain path represent one of the most important processing parameters that characterise hot metal forming processes. In the present study, the effect of strain path change on dynamic recrystallisation, strain-induced precipitation processes and phase transformation behaviour in plain carbon and Nb-microalloyed steels was investigated. To assess the effect of strain-path change, forward/forward and forward/reverse torsion tests were conducted. It has been shown that the strain reversal delays the dynamic recrystallisation kinetics whereas its effect on strain-induced precipitation process of Nb(C,N) is rather negligible. Also the onset of austenite-ferrite transformation is delayed; its products however doesn’t change significantly. This can be due to the fact that ferrite nucleation density plays the second order role compared to the geometry of deformation.

Abstract: The aim of the paper is to determine the influence of hot deformation conditions on σ-ε curves and microstructure evolution of new-developed high-manganese C-Mn-Si-Al-Nb austenitic steel. The force-energetic parameters of hot-working were determined in continuous and multi-stage compression tests performed in a temperature range of 850 to 1100°C by the use of the Gleeble 3800 thermomechanical simulator. Evaluation of processes controlling work-hardening were identified by microstructure observations of the specimens water-quenched after various conditions of plastic deformation. Multi-stage compression tests with true strain of 0.29 permit to use the dynamic and metadynamic recrystallization for forming the fine-grained, austenite microstructure of steel in the whole range of deformation temperature.

Abstract: An austenitic Ni-30%Fe model alloy was employed to investigate the texture and substructure development within the deformed matrix and dynamically recrystallized (DRX) grains during hot torsion deformation. Both the deformed matrix and DRX grains predominantly displayed the crystallographic texture components expected for simple shear deformation. The characteristics of the deformed matrix texture evolution during deformation largely resulted from the preferred consumption of high Taylor factor components by new recrystallized grains. Likewise, the comparatively weaker crystallographic texture of DRX grains became increasingly dominated by low Taylor factor components as a result of their easier nucleation and lower consumption rate during DRX. There was a significant difference in the substructure formation mechanism between the deformed matrix and DRX grains for a given texture component. The deformed matrix substructure was largely characterized by “organized”, banded subgrain arrangements with alternating misorientations, while the substructure of DRX grains was more “random” in character and displayed complex, more equiaxed subgrain/cell arrangements characterized by a local accumulation of misorientations. Substructure characteristics of individual orientation components were principally consistent with the corresponding Taylor factor values.

Abstract: This experimental work deals with the influence of niobium additions to high purity nickel on dynamic recrystallization behavior during hot working. Various high-purity alloys were prepared (unalloyed Ni and Ni–0.01, 0.1, 1 and 10 wt % Nb) and deformed to high strains by hot torsion tests to characterize the rheological behavior within the range 800 – 1000°C at strain rates of 0.03, 0.1 and 0.3 s–1. Niobium additions strongly increased the flow stress. To quantify such behavior, the strain-hardening parameter h and dynamic-recovery parameter r in the Yoshie-Laasraoui-Jonas constitutive equation were determined from the initial part of the experimental stress-strain curves (i.e., at strains before the stress peak) in which dynamic recrystallization does not alter the mechanical behavior. A table showing the variation of h and r as a function of strain rate, temperature, and niobium content was compiled and used to fit a simple empirical model for predicting h and r from the deformation conditions and alloy composition. In addition, microstructures were determined by optical metallography and SEM/EBSD. Based on this work, it appears that niobium additions noticeably refine the steady-state grain size by considerably decreasing the kinetics of dynamic recrystallization in nickel.

Abstract: The paper is focused on application of multi-scale 2D CAFE method. CAFE approach consists of Cellular Automata (CA) model of microstructure development and the thermal-mechanical finite element (FE) code. Dynamic recrystallization phenomenon is taken into account in 2D CA model which takes advantage of explicit representation of microstructure, including individual grains and grain boundaries. Flow stress is the main material parameter in mechanical part of FE and is calculated on the basis of average dislocation density obtained from CA model. The results attained from the CAFE model were validated with the experimental data for austenitic steel X3CrNi18-9. The samples were subjected to axisymmetrical hot compression test. Compression forces were recorded during the tests and flow stresses were determined using inverse method. Light microscopy and EBSD analyses were performed for the initial and final microstructures of the samples.

Abstract: A simple mesoscale model was developed for discontinuous dynamic recrystallization. The material is described on a grain scale as a set of (variable) spherical grains. Each grain is characterized by two internal variables: its diameter and dislocation density (assumed homogeneous within the grain). Each grain is then considered in turn as an inclusion, embedded in a homogeneous equivalent matrix, the properties of which are obtained by averaging over all the grains. The model includes: (i) a grain boundary migration equation driving the evolution of grain size via the mobility of grain boundaries, which is coupled with (ii) a dislocation-density evolution equation, such as the Yoshie–Laasraoui–Jonas or Kocks–Mecking relationship, involving strain hardening and dynamic recovery, and (iii) an equation governing the total number of grains in the system due to the nucleation of new grains. The model can be used to predict transient and steady-state flow stresses, recrystallized fractions, and grain-size distributions. A method to fit the model coefficients is also described. The application of the model to pure Ni is presented.

Abstract: Compression test for GH4586 superalloy was carried out at 1273 K and 1373 K with stain from 20% to 60%. The characteristics of dynamic recrystallization were investigated by confocal laser scanning microscope, TEM and EBSD. It was found that at 1273 K dynamic recrystallization mainly took place along preexisting grain boundaries creating a necklace structure. It was confirmed by TEM observations that the first layer of the necklace structure was formed by the mechanism of bulging of preexisting grain boundaries and the following layers were nucleated at the triple junctions of former dynamic recrystallization grains. When specimens were deformed with different strains at 1373 K, dynamic recrystallization was almost completed. Our observation showed that twinning played an important role during dynamic recrystallization by promoting dynamic recrystallization grains growing. Substructures of grains were constructed by the reconfiguration of dislocations hindered at large ' particles in interior of initial grains during dynamic recovery.

Abstract: A crystalline modeling of deformation implemented in the Finite Element code Abaqus® coupled to a recrystallization Cellular Automaton code is proposed and applied to the hot forging process. A sequential modeling is used in order to obtain a better understanding of the experimental observations and to improve our knowledge of the dynamic recrystallization process. Modeling is performed on aggregates built up from Electron Back Scattered Diffraction measurements. At the deformation temperature, the material presents two phases with a γ matrix of FCC structure and a γ’ hardening phase under a precipitate shape (Ni3(Ti,Al)) of SC structure. The crystalline approach can describe the interactions between the two phases and can compute the evolution of the local strain and stress fields as well as the dislocation density and the lattice rotation in the different grains. A Cellular Automaton algorithm is used for simulating the microstructure evolution during dynamic recrystallization. Nucleation and grain boundary mobility depend on the misorientation and on the local variation in stored energy. This presentation mainly details the different assumptions introduced in the recrystallization code and their influences on the microstructure evolution.

Abstract: A simple analytical model is proposed for estimating grain boundary mobility during dynamic recrystallization in metallic alloys. The combined effects of solutes (solute drag) and second phase particles (Zener pinning) on mobility are considered. The approach is based on (and is consistent with) a recently published mesoscale model of discontinuous dynamic recrystallization. The dependence of grain boundary mobility on solute concentration and particle size is summarized in the form of two-dimensional maps.

Abstract: Through the analysis of many creep rate-time or creep rate-strain curves of -single phase Ni-20mass% Cr alloy single crystals with various stress axes, it was clarified that the creep deformation manners at lower stresses are drastically different to those at higher stresses. These creep features at lower stresses are summarized into three ones as follows. (i) The fully extended transient stage occupies the considerable ratio of rupture life. (ii) The steady state stage disappears, because the transient stage directly connected with the accelerating stage. (iii) The origin of the onset of accelerating stage is regarded as the formation of the dynamic recrystallized grain. These difference in creep deformation manner were caused by the predominant operation of the primary slip system and then the homogeneous evolution of dislocation substructures.