Materials Science Forum Vols. 467-470

Abstract: As cast and precipitation annealed variants of Al-Mn-Zr-alloys with and without Sc have been extruded in order to study the effect of Sc on the extrudability and the recrystallisation resistance after extrusion and subsequent annealing. Both Zr and Sc form dispersoids, which retard recrystallisation very effectively in many aluminium alloys. However, while Al3Zr often is heterogeneously distributed, a dense and homogeneous distribution of Al3(Sc,Zr)-dispersoids is obtained when Sc is added. This was also the case in these alloys, and the Sc-containing variants consequently displayed a far higher recrystallisation resistance than the Sc-free variants during extrusion and subsequent annealing. Another advantage by adding Sc is that precipitation annealing no longer seems to be necessary in order to obtain a high recrystallisation resistance, as the Sccontaining variants displayed an identical structural stability. The Sc-free alloy, on the other hand, had to be precipitation annealed in order to be able to resist recrystallisation during extrusion. However, an addition of Sc leads to a lower extrudability, as the Sc-containing variants displayed significantly higher extrusion pressures than the Sc-free alloys.

Abstract: The strain path undergone by a material can have a significant influence on the deformation behaviour, recrystallisation kinetics, grain size and crystallographic texture. To study the influence of strain path, samples of an aluminium-1%manganese alloy have been subjected to a number of strain path changes. These have been achieved using combinations of plane strain and free compression to give strain paths of 0, 90, and 180°. The development of the dislocation substructure resulting from each stage of deformation was studied using Electron Back Scatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). Following deformation annealing was carried out to determine the effects of the strain path changes on the recrystallisation kinetics and grain size. Results have shown that the strain path angles cannot be used to satisfactorily describe the material behaviour when mixtures of deformation modes are used.

Abstract: AA5754 sheet has been processed by asymmetric rolling and the development of grain structure and texture in subsequent annealing studied at 240-500°C. It has been found that asymmetric rolling facilitates the formation of ultra-fine grain structure (1-2µm grain size) by shear strain promoted continuous recrystallization, which is a process of extended recovery and subgrain/grain growth. The ultra-fine grain structure is not thermally stable, and when the annealing temperature or time increases, the grain size eventually grows to its Zener limit. The deformation texture is similar to the typical f.c.c. cold rolling texture but rotated about the transverse direction. Along with the formation of an ultra-fine grain structure and subsequent grain growth, the deformation texture is retained.

Abstract: The transition from discontinuous recrystallization to a continuous recovery process during annealing of aluminium deformed to high plastic strains has been investigated by following several different microstructural parameters. Samples were examined following equal channel angular extrusion (ECAE), accumulated roll bonding (ARB) and high-strain conventional rolling (HSCR). Local variations in the uniformity of coarsening have been characterized by an analysis of grain size distributions, obtained from electron backscatter pattern investigations. Except for at the lowest strains the distributions were not obviously bimodal. Methods of separating the distributions into two parts based on the mean value have therefore also been examined. The results suggest that the presence of a high fraction of high angle boundaries (>15o) in the microstructure may be a necessary but not sufficient condition for continuous recovery during annealing. The annealing behaviour can be related in part to the heterogeneity of the deformed microstructures, which can be pronounced even after strains of ≈ 10 (ECAE). The presence of oxide particles in the ARB sample has a beneficial effect in limiting the grain growth.

Abstract: The recrystallisation kinetics experimentally measured on a supersaturated AA3103 after hot rolling is analysed using a numerical model. It is shown that temperature and nucleation inhibition due to precipitation on the subgrain structure are the two most important parameters in controlling the recrystallisation kinetics. On the opposite, particle pinning of recrystallised grain boundaries is negligible while pinning of subgrain boundaries during recovery and solute drag are relevant but second order effects.

Abstract: A study was carried out to determine the role of additives such as Mg and Cu on the microstructural characteristics of grain refined, Sr-modified eutectic A413.1 alloy (Al-11.7% Si) during solution heat treatment. For comparison purposes, some of the alloys were also studied in the non-modified condition. The alloys were cast in a steel permanent mold preheated at 425 °C that provided a microstructure with an average dendrite arm spacing (DAS) of ~ 22 µm. Castings were solution heat treated at 500 ± 2 °C for time up 24 h, followed by quenching in warm water (at 60 °C). Microstructural analysis of the as-cast and heat-treated castings was carried out using optical microscopy in conjunction with image analysis. Phase identifications were done using the electron probe microanalysis (EPMA) technique. In the as-cast condition, the addition of 0.42 wt% Mg to the unmodified alloy produced relatively large Si particles compared to the base A413.1 alloy. The Si particle size remained more or less the same with increase in solution treatment time and Mg level. Both Mg2Si and Al2Cu phases were observed to dissolve almost completely after 8 h solution time, while the Al5Cu2Mg8Si6 phase was found to persist even after 24 h.

Abstract: Extrusions experience large deformations at discontinuities when they traverse the die land, leading to considerable modifications to the average deformation parameters when compared to the remainder of the extrusion. The distribution of structure is therefore greatly inhomogeneous. Reference to both empirical and physical models of the recrystallisation process indicate that nucleation and growth will differ at these locations in those alloys that are usually solution treated and aged subsequent to the deformation process. Since static recrystallisation has a significant influence on many of the properties of the extrudate, it is therefore essential to provide the methodology to predict these variations. In the work presented, a physical model based on dislocation density, subgrain size and misorientation is integrated into the commercial FEM codes, FORGE2® and FORGE3® to study the microstructure changes. Axi-symmetrical and shape extrusion are presented as examples. The evolution of the substructure influencing static recrystallisation is studied. The metallurgical behaviour of axi-symmetric extrusion and that of shape extrusion are compared. The predicted results show good agreement with experimental measurement. The distribution of equivalent strain, temperature compensated strain rate and temperatures are also presented to aid in interpretation.

Abstract: Strain-induced grain refinement in a coarse-grained 7475Al alloy was studied by means of multidirectional forging (MDF) carried out at T = 490oC under a strain rate of 3 x 10-4 s-1. Integrated flow curves exhibit significant work softening just after yielding, followed by steady-state-like behavior at high strains. The evolution of new fine grain structure during deformation can be assisted by grain-boundary sliding, resulting in frequent formation of high strain gradients and subsequently microshear bands in grain interiors. Microshear bands developed in various directions are intersected with each other, subdividing original grains into misoriented small domains. The number and the misorientation angle of microshear bands progressively increase during deformation, finally followed by their transformation into high-angle boundaries. It is concluded that grain refinement under hot MDF conditions occurs by a series of deformation-induced continuous reactions; that is essentially similar to continuous dynamic recrystallization.

Abstract: The mechanical anisotropy of wrought Mg alloys is very high. For example the yield stress of extruded Mg-3Al-1Zn tested in tension can be as high as twice that obtained in compression [1]. To solve the problems this creates for product design it is necessary to understand the sensitivity of texture to processing parameters. Uniaxial compression tests at different temperatures were performed on cylindrical samples of an extruded Mg-3Al-1Zn bar. The texture during this deformation changes from a situation where all crystal c-axes are nearly perpendicular to the sample axis to one where the c-axes are all nearly parallel to this axis. Compression was stopped at different strains to examine the rate of this texture change. Textures were examined using EBSD measurements. It was found that different mechanisms operate depending on the temperature of deformation and that a variety of textures can be created. Also it was seen that an annealing treatment performed after compression has an influence on the texture. Afterwards the samples were subjected to another uniaxial compression test to examine the influence of texture on room temperature properties.

Abstract: The influence of grain size on the deformation of extruded Mg-3Al-1Zn tested in tension at temperatures between room temperature and 300°C is investigated. The results enable estimation of the deformation conditions for the transition from slip to twinning dominated flow and for the initiation and completion of dynamic recrystallization. A map illustrating these critical parameters is constructed and it is shown that the operating conditions of the common wrought processes straddle key transitions in microstructure behaviour.