Materials Science Forum Vols. 467-470

Abstract: Compression tests were used to simulate simple deformation paths within the upper a-range of Zircaloy-4 (i.e. 500°C-750°C). The mechanical behaviour reveals two different domains : at low temperatures and large strain rates, strain hardening takes place before flow softening, whereas this first stage disappears at lower flow stress levels. Strain rate sensitivity and activation energy were determined for both domains. Dynamic recrystallization was investigated using the Electron BackScattering Diffraction (EBSD) technique. It appears that the mechanism involved here is continuous dynamic recrystallization (CDRX), based on the increasing misorientation of subgrain boundaries and their progressive transformation into large angle boundaries. At low strains (e £ 0.3), CDRX kinetics are similar whatever the deformation conditions, while higher temperatures and lower strain rates promote recrystallization at large strains.

Abstract: The aim of the present work was to undertake a detailed investigation of the softening mechanisms during hot deformation of a 21Cr-10Ni-3Mo (steel A) and a 21Cr-8Ni-3Mo (steel B) austenite/ferrite duplex stainless steels containing about 60% and 30% of austenite, respectively. The steels were subjected to hot deformation in torsion performed at 900 °C and 1200 °C using a strain rate of 0.7 s-1 to several strain levels. Quantitative optical and transmission electron microscopy were used in the investigation. Austenite was observed to soften via dynamic recovery (DRV) and dynamic recrystallisation (DRX) accompanied by DRV for the deformation temperatures of 900 °C and 1200 °C, respectively, for the both steels studied. DRX of austenite largely occurred through strain-induced grain boundary migration, complemented by (multiple) twinning, and developed significantly faster in steel A than in steel B, indicating that considerably larger strains partitioned into austenite in the former steel during deformation at 1200 °C. The above softening mechanism was accompanied by the formation of DRX grains from subgrains along the austenite/ferrite interface and by large-scale subgrain coalescence. At 900°C, stressassisted phase transitions between austenite and ferrite were observed, characterised by dissolution of the primary austenite, formation of Widmanstätten secondary austenite and gradual globularisation of the microstructure with increasing strain. These processes appeared to be significantly more widespread in steel B. The softening mechanism within ferrite for the both steels studied was classified as “continuous DRX”, characterised by a gradual increase in misorientations between neighbouring subgrains with strain, for the both deformation temperatures.

Abstract: A 304 austenitic stainless steel was deformed using hot torsion to study the evolution of dynamic recrystallization (DRX). The initial nucleation of dynamically recrystallization occurred by the bulging of pre-existing high angle grain boundaries at a strain much lower than the peak strain. At the peak stress, only a low fraction of the prior grain boundaries were covered with new DRX grains. Beyond the peak stress, new DRX grains formed layers near the initial DRX and a necklace structure was developed. Several different mechanisms appeared to be operative in the formation of new high angle boundaries and grains. The recrystallization behaviour after deformation showed a classic transition from strain dependent to strain independent softening. This occurred at a strain beyond the peak, where the fraction of dynamic recrystallization was only 50%.

Abstract: The dynamic recrystallization (DRX) behavior of a coarse grain sized Nb microalloyed austenite (∼ 800 µm), typical of thin slab casting processes, has been studied. Continuous torsion tests were carried out at different, Z, Zener-Hollomon parameter values. It has been observed that as Z increases the curves move to higher values of stress and both the peak, εp, and steady state, εss, occur at larger strains, with an increase in the εss - εp strain difference. Consequently, an increment in Z produces a delay in the beginning and the progression of the DRX process. In the present work, it has been found that the pre-existing grain boundaries are the most favorable nucleation sites for DRX for all Z values, the nucleation mechanisms being related to strain induced migration of high angle grain boundaries. However, in the case of high Z values, intragranular nucleation on defects generated during deformation is also observed. The microstructure analysis denotes also that dynamic recrystallization is a process dominated by repeated nucleation with limited growth.

Abstract: A ZK60 magnesium alloy was subjected to isothermal rolling (IR) at 275 and 300°C. This processing resulted in grain refinement through dynamic recrystallization (DRX) at both temperatures. The recrystallized volume fractions of 82 and 95% and average sizes of fine grains of 2.5 and 3.7 µm were achieved after IR at 275 and 300°C, respectively. It was shown that the ultrafine-grained structure produced by DRX at 300°C exhibited higher stability under following static and dynamic annealing than that produced at 275°C. This fact was attributed with the formation of a less constrained DRX structure at higher temperature of IR. As a result, the sheet produced from the ZK60 alloy at 300°C showed superior superplastic properties. Conversely, it was not feasible to enhance the superplastic properties in the ultrafine-grained alloy produced at 275°C because significant grain growth occurred during further processing of the as-rolled alloy.

Abstract: Only few models explain the development of a steady state grain size during dynamic recrystallization, and their microphysical basis is poorly understood. In this study, we investigate mechanical and microstructural data on dry and wet NaCl, deformed at a range of stresses and temperatures at elevated pressure, with the aim to evaluate the different models. The results show that dry NaCl continuously work hardens and shows evidence for recrystallization dominated by progressive subgrain rotation, while the wet material shows, at similar conditions, oscillating stressstrain behaviour and recrystallization dominated by grain boundary migration. Taking into account the distribution of grain size, deformation of wet NaCl is best described by flow laws based on composite grain size sensitive (GSS) solution-precipitation creep and grain size insensitive (GSI) dislocation creep. The recrystallized grain size data in wet NaCl can be modeled with the hypothesis that recrystallized grain size organises itself in the boundary between the GSS and GSS creep domains.

Abstract: Dynamic recrystallization, DRX, has become an increasingly important softening mechanism both from fundamental and industrial points of view. During finishing rolling of strips or wire rods, strain is accumulated from pass to pass so that DRX can be triggered. The time need for 50% of material to recrystallize, t50DRX, is strongly dependent on temperature and to a lesser extent on strain rate at which deformation occurs. Few studies report results on the kinetics of DRX and how this softening mechanism can be predicted for a given set of hot deformation conditions, namely strain, strain rate and pass temperature. The purpose of this paper was to investigate how the chemical composition of IF austenite can affect the kinetics of DRX by measuring the apparent activation energy for DRX, QDRX, for alloys with additions of Ti and a combination of Ti-Nb contents. Predicted and measured values of t50DRX, were compared and an empirical expression was proposed to model measured values.

Abstract: The mechanism of new grain evolution during equal channel angular extrusion (ECAE) up to a total strain of ~12 in an Al-Cu-Mn-Zr alloy at a temperature of 475oC (0.75Tm) was examined. It was shown that the new grains with an average size of about 15 µm result from a specific process of geometric dynamic recrystallization (GRX) which can be considered as a type of continuous dynamic recrystallization (CDRX). This process involves three elementary mechanisms. At moderate strains, extensive elongation of initial grains takes place; old grain boundaries become progressively serrated. Upon further ECAE processing, transverse low-angle boundaries (LAB) with misorientation ranging from 5 to 15o are evolved between grain boundary irregularities subdividing the initial elongated grains on crystallites with essentially equiaxed shape. The misorientation of these transverse subboundaries rapidly increases with increasing strain, resulting in the formation of true recrystallized grains outlined by high-angle boundaries from all sides. In the same time, the average misorientation of deformation-induced boundaries remains essentially unchanged during ECAE. It is caused by the fact that the evolution of LABs with misorientation less than 4o occurs continuously during severe plastic deformation. The mechanism maintaining the stability of the transverse subboundaries that is a prerequisite condition for their further transformation into highangle boundaries (HABs) is discussed.

Abstract: This work has studied the influence of different Ti and N compositions on hot deformation strength by determining the peak stress of flow curves and the activation energy (dynamic recrystallisation). It has also assessed their influence on static recrystallisation by means of the statically recrystallised fraction versus time and the activation energy. A precipitate study performed by SEM and TEM has yielded a better understanding of the influence of the Ti/N ratio and precipitation state in hot deformation (dynamic and static recrystallisation). A correlation was found between for the finer distribution of precipitates, Ti/N ratio close to 1.5, smaller austenite grain, maximum activation energy for hot deformation (dynamic recrystallisation) and maximum activation energy for static recrystallisation.