Recrystallization and Grain Growth

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Authors: S.Yu. Mironov, Gennady A. Salishchev, Sergey V. Zherebtsov
Abstract: The microstructure and texture evolutions in pure titanium during severe plastic deformation at T=400°C were investigated. Compressive deformation of prismatic samples was sequentially applied in three orthogonal directions up to 12 steps and a strain at each step of 40%. A radical microstructure refinement (from 20 to 0.2 µm) during strain has been found. The features of the deformation structure are a high level of internal stresses, high density of dislocations, a large number of deformation induced boundaries and the presence of twins. It is shown that during strain there is a significant change in disorientation angles and axes of individual high angle grain boundaries. At the same time the total set of high angle boundaries - Misorientation Distribution Function (MDF) and texture - does not change significantly with strain. The reasons for the change in disorientation angles and axes at new deformation-induced boundaries during plastic flow are discussed.
1211
Authors: Hiromi Miura, Tetsuo Sakai, R. Mogawa, Günter Gottstein
1217
Authors: Maria A. Murzinova, Gennady A. Salishchev, D.D. Afonichev
Abstract: The influence of hydrogen content on the mechanical properties and size of dynamically recrystallized grains in commercially pure (CP) titanium and Ti-5Al-2.5Sn alloy was investigated. The alloys with hydrogen contents from 0.1 to 5.2 at.% were deformed in the a-field at temperatures of 650°, 750°С with initial strain rates of 5×10-4 s-1. A decrease of the deformation temperature leads to a reduction in grain size and to a stress increase for all compositions. This is in good agreement with the well known relation between the recrystallized grain size (d) and the steady flow stress ss=kd-n. At a given test temperature the steady state flow stress is four times lower and the grain size is about ten times greater in CP titanium in comparison with the Ti-5Al- 2.5Sn alloy. Hydrogen alloying of the Ti-5Al-2.5Sn alloy does not lead to a noticeable change in ss and d. However, an increase in hydrogen content from 0.1 to 5.2 at.% in CP titanium leads not only to a decrease in grain size by a factor of 2 but also to a decrease in flow stress (about 28%). This result is not in agreement with the above relation. This unusual behaviour may be due to two reasons: the influence of hydrogen on grain growth and the hydrogen effect on dynamic strain ageing. Both these effects are stronger in CP titanium.
1223
Authors: Tarcisio R. Oliveira, Frank Montheillet
Abstract: The study was carried out to understand the mechanisms occurring during dynamic recrystallization of hot deformed 11% chromium stabilized ferritic stainless steels and to compare the behaviour induced by various types of stabilization. It was observed that continuous dynamic recrystallization (CDRX) operates in all materials starting at the onset of straining. Niobium has a more pronounced influence on hardening than titanium during hot deformation, which is due to solid solution strengthening and also to the reduction or stopping of grain boundary migration by solute drag effect. The D2 component, { 2 1 1 }<111>, was found as the major texture component at the steady state for the torsion tests carried along the negative shear direction. It was likely to be formed by the combination of straining and growth of the grains exhibiting both low stored energy and low rotation rate of the crystallographic axes.
1229
Authors: Siegfried Kleber, Christof Sommitsch
Abstract: The stress relaxation method has been applied to the nickel-based alloy 80A to predict meta-dynamic (MDRX) and static recrystallization (SRX) kinetics. Compression tests were performed on a Gleeble 3800 system atr different temperatures (950-1200°C). The strain rate was varied in the case of MDRX and the pre-strain in the case of SRX. To investigate MDRX, the prestrain was set to twice the peak strain in order to reach steady state before holding. To focus on the interaction of MDRX and SRX, the pre-strain was set to the peak strain, where dynamic recrystallization (DRX) starts but does not yet reach steady state. Avrami type equations for the prediction of both the MDRX and SRX were adapted to feed a semi-empirical grain structure model.
1237
Authors: John Wheeler, Zhenting Jiang, David J. Prior, Jan Tullis
Abstract: It is generally agreed that the driving force (plastic strain energy) is much too small to allow "classical" nucleation during static and dynamic recrystallisation, and that rotation/growth of subgrains is an alternative. The latter explanation predicts that new grains should begin at low angles to old grains. We have used electron backscatter diffraction on an experimentally deformed quartz polycrystal that has deformed by dislocation creep and partially recrystallised. In a region shortened by about 30% new grains are at high angles (much greater than 15º) to adjacent parent grains. A histogram of misorientation versus number of boundaries shows a gap at 15-20º. In its simple form we expect the subgrain rotation model to predict a spectrum of misorientations but with most of them being low angle. Instead, the histogram suggests that new boundaries began life as high-angle structures, so current models for deformation-induced nucleation require refinement.
1243
Authors: Rustam Kaibyshev, I. Mazurina
Abstract: The mechanisms of grain refinement during severe plastic deformation have been studied, by comparing the microstructure evolution in an AA2219 aluminium alloy, containing Al3Zr nanoscale particles, with that in a dilute Al-3%Cu alloy deformed identically by equalchannel angular extrusion (ECAE) at 250oC to a maximum strain of ~12. Transmission electron microscopy (TEM) was used on the AA2219 alloy to reveal the misorientations of deformationinduced boundaries. Microstructural evolution in the Al-3%Cu alloy was studied by electron-back scattering diffraction (EBSD) orientation mapping. It was shown that the mechanism of grain refinement in the AA2219 alloy is continuous dynamic recrystallization (CDRX) consisting of two main elementary processes. In the initial stages of plastic deformation, the formation of threedimensional arrays of low-angle boundaries (LABs) takes place. Further strain results in increasing misorientation of these boundaries providing their gradual transformation into high-angle boundaries (HABs). A fully recrystallized structure with an average grain size of ~0.9 µm is evolved after a total strain of ~12. In the dilute Al-Cu alloy the evolution of ultrafine grains with an average size of ~6 µm is attributed to the formation of deformation bands outlined by HABs and extended medium to high-angle boundaries at moderate strains. The subdivision of these deformation bands into fine grains rarely occurs through the mechanism of geometric recrystallization (GRX). In this alloy the main contribution in the grain refinement gives CDRX occurring within fibrous structural features. At e~12, a partially recrystallized structure is formed in the Al-3%Cu alloy.
1251
Authors: Philip B. Prangnell, Jacob R. Bowen, M. Berta, P.J. Apps, Pete S. Bate
Abstract: Severe deformation techniques allow metallic alloys to be deformed to ultra-high plastic strains, without any geometrical change to the work piece. They thus offer potential for the cheap production of submicron grained materials, in a bulk form. After processing severely deformed materials do not have conventional, idealized, grain structures, contain significant fractions of low angle boundaries, and are often heterogeneous. Due to their high stored energy, they are unstable on annealing and in most cases can be thought of as continuously recrystallizing. However, locally discontinuous behaviors are often observed, due to the retained less mobile low angle boundaries, as well as abnormal grain growth at elevated temperatures. Monte-Carlo-Potts models have been used to show the sensitivity of the annealing behavior to the initial starting structure present after deformation. The effect of coarse (~1µm) particles and fine dispersoid particles are also discussed.
1261
Authors: G. Azevedo, Ronaldo Barbosa, Dagoberto Brandão Santos
Abstract: In the last years, several studies concerning ultra refinement of ferrite grains have been conducted using different experimental techniques (ECAP, ARB, HPT). The aim of all investigations was to provide an optmized relationship between mechanical properties and microstructure of steels. The present work, likewise, deals with strain induced dynamic transformation of ferrite. Samples of low C-Mn steel were intensely deformed in hot torsion aiming at the production of ultrafine grains of ferrite thereby enhancing the mechanical properties when compared to hot rolled products. After soaking during 5min at 900°C, the samples were quenched and then reheated and submitted to hot torsion deformation at temperatures of 700 and 740°C. The torsion schedule consisted of 7 isothermal passes leading to a total strain of ≈1, generating an ultrafine microstructure with grain sizes of the order of 1µm. The shape of stress-strain curves so obtained suggested that ferrite refinement occurred by dynamic recrystallization. The various constituents present in the microstructure as well as ferrite grain size and morphology were examined by optical and scanning electron microscopy. Microhardness tests were performed to evaluate mechanical properties.
1271
Authors: Sergey V. Dobatkin, V.I. Kopylov, Reinhard Pippan, O.V. Vasil'eva
Abstract: At present, the possibility of the formation of high-angle grain boundaries upon severe cold deformation, in particular, equal-channel angular (ECA) pressing is reliably proved. The structure formation upon multi-cycle ECA pressing substantially depends on the route determining the shear plane in the sample upon repeated passes. The route is defined by the rotation of the sample around its axis upon the multi-cycle ECA pressing. There are four main routes: route A, in which the sample is deformed by many passes without any rotations; route Ba, in which the sample is rotated by ± 90°; route Bc, in which the sample is sequentially rotated in the same direction by 90°and route C, in which the sample is rotated by 180° about its axis before each subsequent pass. By the methods of SEM, TEM and EBSD analysis it was shown that the fraction of high-angle boundaries in a-Fe upon cold ECA pressing with an angle of 90° between the channels and N=4 depends on the deformation route and increases according to the route sequence: Ba-C-Bc.
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