E. Nes

On the origin of recrystallization textures in aluminium

Abstract The electron back scattering pattern technique (EBSP) in SEM has been applied to follow in detail the orientation aspects of the nucleation and growth of recrystallization in cold rolled aluminium. The investigation includes both high purity metal and a commercial grade. The cube- and Goss-oriented recrystallized grains nucleate from transition bands. In the cube case these bands are found in the Cu- and ND-rotated-Cu deformation texture components while the Goss bands have been identified in the brass texture component. Shear bands in the S-deformation component have been identified as nucleation sites for recrystallization. The orientation inside the bands is also S (complementary to the matrix), causing the S-orientation to reappear as a recrystallization texture component. These types of deformation heterogeneities and others are frequently associated with a 40°C 〈111〉 orientation relationship to the surrounding matrix, making this specific orientation relationship primarily a property to be associated with the oriented nucleation concept. Isolated examples of a true 40° 〈111〉 growth selection due to the rapid growth of 〈111〉-tilt-boundaries have been observed. A characteristic aspect in these cases is a growth selection from a planar transformation front. From a careful literature review as well as from experimental observations no direct evidence has been found in support of the hypothesis that 40° 〈111〉 grains, when uniformly distributed in space, have a transformation rate potential exceeding that of grains belonging to other texture components. No growth selection has been found in the special case of the growth of cube oriented grains.

The roles of oriented nucleation and oriented growth on recrystallization textures in commercial purity aluminium

The development of the recrystallization texture in commercial purity Al (AA1145) containing large Al3Fe precipitates was investigated to determine the dependence on the degree of deformation and recrystallization temperature by means of X-ray macrotexture analysis. The amount of Fe in solid solution was varied by applying different heat treatments. The recrystallization textures develop from the competition between the Cube-orientation, the R-orientation (similar to the rolling texture S-orientation) and randomly oriented grains due to particle stimulated nucleation (PSN). The formation of these orientations is discussed in terms of their nucleation and subsequent growth behaviour. Particular attention is focused on the influence of the process parameters, i.e. strain, temperature and heating rate, on the ratio between the individual orientations in the recrystallization textures.

Subgrain growth in heavily deformed aluminium—experimental investigation and modelling treatment

Abstract A comprehensive study of the annealing behaviour of two variants of commercial purity aluminium alloys has been carried out. The primary objectives of this work have been to improve the understanding of the recovery processes in itself as well as to study the effect of recovery on the annealing behaviour of heavily deformed aluminium. The experimental part has focused, firstly on describing the as deformed structure with emphasis on sub-boundary misorientations and long range lattice curvature, secondly on following subgrain growth and characterization of the softening reaction. The modelling part treats subgrain growth as a reaction controlled by sub-boundary migration. The significance of sub-boundary flexibility in this context is emphasized, an aspect which requires lateral drift of ledges as an important element in the boundary migration mechanism. A subgrain growth model based on thermally activated migration of ledges has been developed and applied to the experimental results.

Substructure evolution during different hot deformation processes of commercial non-heat treatable aluminium alloys

Abstract The objective of the present work was to examine to what extent different laboratory testing equipment, such as plane strain compression (PSC), torsion, rolling and extrusion reveal the same substructure evolution during hot deformation of aluminium. The investigation included three non-heat treatable aluminium alloys; commercial purity aluminium (cp Al), cp Al + 1wt.%Mn and cp Al + 1wt.%Mg. The substructure was characterized by the subgrain size and the misorientation between nearest neighbour subgrains using a scanning electron microscope equipped with the SINTEF EBSP-system. The results showed that the subgrain size of the PSC-materials was significantly smaller as compared with the other deformation modes. The larger subgrain sizes in the hot rolled and extruded specimens were attributed to recovery effects owing to a delay in quenching time. In the case of the hot torsion specimens, the larger subgrain size as compared with the PSC-results was ascribed to texture effects by means of differences in the Taylor factor when deforming in shear and plane strain deformation conditions. The misorientation results for the different deformation conditions were inside the same scatter band.

Constitutive laws for steady state deformation of metals, a microstructural model

Based on extensive investigations of the steady state conditions for creep and hot working, Sellars and McG. Tegart demonstrated that the flow stress was well represented by the following relation: {sigma}{sub s} = 1/{alpha}{prime} Arc sinh (Z/A{prime}){sup 1/n} where {alpha}{prime}, A{prime}, n are temperature independent constants and Z is the Zener-Hollomon parameter, Z = {dot {var_epsilon}} exp U/kT, where {dot {var_epsilon}} is the applied strain rate and U is an activation energy. This relationship is widely used in the modelling of metal forming processes such as hot rolling, forging and extrusion. The objective this work is to explore the possibilities for deriving an alternative, physically based, relationship which is capable of correlating the steady state flow stress both to the alloy condition and the microstructural characteristics of steady state deformation.

Application of microstructurally based constitutive laws to hot deformation of aluminium alloys

In the preceding paper new constitutive laws for steady state deformation of metals were developed using an approach based on the assumption that during steady state deformation of a pure metal, or stable solid solution the substructure can be adequately described by a few microstructural elements, the two most important ones being the cell/subgrain size, {delta}{sub s}, and the dislocation density in the cell interior, {rho}{sub i}. Based on this microstructural description and the assumption that during steady state the principle of similitude applies, in the sense that the separation of dislocations within the cells (1/{radical}{rho}{sub i}) scales with the cell size {delta}{sub s} (i.e. {radical}{rho}{sub i} = C{sub {delta}}/{Delta}{sub s}) then the following flow stress relationship was rationalized: {tau}{sub s} = {tau}{sub i} + {alpha}{sub 3}Gb 1/{delta}{sub s}.

A model for recrystallisation kinetics, texture and grain size applied to multipass hot rolling of an AlMgMn aluminium alloy

Abstract A physically based model for primary recrystallisation, including recrystallisation kinetics, grain size and texture evolution, during hot rolling of aluminium is presented. The model is based on extensive experimental investigations that have been directed towards identifying the nature of the nucleation sites for recrystallised grains of different crystallographic orientations. Particle stimulated nucleation and nucleation from cube bands and grain boundary regions have been incorporated in the model. The model has been applied to the hot rolling of an AlMgMn alloy processed in 13 passes. Reasonable model predictions have been obtained for cases of reversing rolling and a 3-stand tandem mill, including variations in the exit gauge temperature.