Mechanism of cube grain nucleation during recrystallization of deformed commercial purity aluminium

Cube texture is a sharp recrystallization texture component infcc metals like aluminium, copper, etc. It is described by an ideal orientation i.e. (100) (100). The subject of cube texture nucleation i.e. cube grain nucleation, from the deformed state of aluminium and copper is of scientific curiosity with concurrent technological implications. There are essentially two models currently in dispute over the mechanism of cube grain nucleation i.e. the differential stored energy model founded on the hypothesis proposed by Ridha and Hutchinson and the micro-growth selection model of Dugganet al. In this paper, calculations are made on the proposal of Ridha and Hutchinson model and the results are obtained in favour of the differential stored energy model. It is also shown that there is no need for the micro-growth model.     

Effects of initial grain size on hot deformation behavior of commercial pure aluminum

The effects of initial grain size of commercial pure aluminum on hot deformation behavior were investigated using hot compression tests. The hot compression tests were carried out on the pure aluminum samples with the initial grain sizes of 50, 150 and 450 μm using various strains, strain rates and different deformation temperatures. It was found that the hot deformation behavior of used material was sensitive to deformation conditions and initial microstructure. Results indicate that the initial grain size has significant effect on the flow stress. Flow stress decreases when the grain size decreases from 450 to 50 μm and when strain rate is lower than 0.05 s⁻¹. This procedure is reversed at strain rate of 0.5 s⁻¹. Furthermore, effects of other parameters like the strain rates and deformation temperatures on the flow stresses and hardening rates were investigated. It was also found that the inhomogeneity of microstructure distribution at different positions of the deformed specimens depended on the amount of deformation concentration at particular points and other processing parameters such as initial grain sizes, strain rates and deformation temperatures. In addition the geometric dynamic recrystallization (GDRX) was observed in the specimens highly strained (0.7) at elevated temperature (500 °C) using polarized light microscope and sensitive tint (PLM + ST).     

Dynamic recrystallisation phenomena of commercial purity aluminium during friction welding

Abstract

The microstructures of commercial purity aluminium near the friction weld interface were observed by transmission electron microscopy. Large plastic deformation of aluminium occurred near the weld interface and the microstructure of the aluminium was oriented from the centre to the periphery of the weld nugget. The aluminium grains were refined, and there were many recrystallised grains which were almost dislocation free. The refined grains, which were of size ~ 1 νm, formed very near the weld interface. The grain boundary was estimated to be a large angle tilt boundary. The refined grains were mainly formed by dynamic recrystallisation during the upset stage of the welding cycle. A large amount of shear strain and heat were introduced during the friction stage, and dynamic recrystallisation started during the upset stage. Grain growth occurred during air cooling after the upset stage. The grain size was larger in the central region than in the periphery owing to the variation in temperature. The Vickers microhardness of aluminium near the weld interface increased owing to the microstructural refinement.     

The corrosion behaviour of commercial purity titanium processed by high-pressure torsion

Commercial purity (CP) titanium was processed by high-pressure torsion (HPT) under an applied pressure of 6.0 GPa for different numbers of torsional revolutions and then exposed to a 3.5 % NaCl solution for open-circuit potential measurements followed by electrochemical impedance spectroscopy and potentiodynamic polarization tests. The electrochemical results exhibit a complicated relationship between the corrosion resistance and grain refinement. Thus, microhardness measurements reveal an improvement in hardness for CP titanium after processing by HPT but the corrosion resistance is lower in the NaCl solution than for the annealed coarse-grained Ti. It is shown that the corrosion susceptibility of the HPT-processed samples decreases with increasing torsional strain. The effect of grain size and microstructure on the corrosion properties of ultrafine-grained CP Ti is also examined.     

Evolution of ultrafine microstructures in commercial purity aluminum heavily deformed by torsion

Commercial purity aluminum (1100Al) bars were severely plastic deformed by torsion deformation at room temperature. The specimens were deformed to ultrahigh equivalent strain of 5.85 in maximum. Microstructure evolution during the torsion deformation was characterized using electron back scatter diffraction analysis on two different sections: the longitudinal section parallel to the torsion axis and transverse section perpendicular to the torsion axis. The grain size decreased and the fraction of high angle grain boundary increased with increasing equivalent strain. Elongated ultrafine grained structure was obtained after an equivalent strain of 3.27. We have found that the microstructure evolution in the specimen deformed by torsion exhibited similar behavior to those in the same material heavily deformed by accumulative roll bonding. The average grain size of 0.32 μm with the high angle boundary fraction of 0.76 was achieved in the specimen deformed to an equivalent strain of 5.27. Though the microstructure and hardness on the transverse section varied depending on the radial positions, they could be arranged as a simple function of equivalent strain. The present work confirmed that the torsion deformation worked as a kind of severe plastic deformation.     

Static recrystallisation kinetics of commercial aluminium: influence of hot deformation mode

Abstract

The kinetics of static recrystallisation and the recrystallised grain size of a commercial aluminium alloy have been determined after both hot torsion and axisymmetric compression within the steady state regime. Annealing of the specimens, quenched after hot deformation in both modes at 325 and 360°C, was carried out at 410°C. The presence of dynamically formed grains developed during deformation, which can act as pre-existing nuclei, exerts an influence on the subsequent static recrystallisation behaviour of the alloy. The increase in number of these dynamically formed grains with strain explains the significant effect of strain in the steady state. However, some differences result from the different modes of deformation. Hot axisymmetric compression leads to an important decrease of both the time to reach a recrystallised fraction of 50% and the recrystallised grain size in comparison with hot torsion. The different deformation modes, through their effect on the spatial distribution of particles, seem to affect the proportion of dynamically formed grains which become effective nuclei and therefore the recrystallisation kinetics. In addition, particle drag effects can influence greatly the static recrystallisation kinetics.

MST/1810     

High temperature compression testing and determination of warm working temperature for commercial purity aluminium

CP aluminium was subjected to axysymmetric compression testing at 303, 373, 473, 573 and 673 K. Strength coefficient and strain hardening exponent were determined from log-log plot of true stress and true strain at different temperatures. True stress vs temperature at different strains indicated that dynamic strain ageing (DSA) occurred between 473 and 673 K. Warm working is to be done between 423 K and 473 K, so that optimum substructure hardening and strain hardening can be achieved during mechanical processing.     

Effect of grain size on fatigue-crack growth in 2524 aluminium alloy

In this study, industrial 2524 aluminium alloy plates with various grain sizes (0.8–298 μm) were prepared by cold rolling and heat treatment. The fatigue-crack-growth rate was studied as a function of grain size through fatigue tests and microstructural observations. The results showed that grain refinement led to a decrease in the resistance against fatigue-crack growth. Besides, the levels of crack closure in coarse-grained samples were higher than those in fine-grained ones at low values of the range of the stress intensity factor K, ΔK. This phenomenon was predicted and explained well by the crack-deflection model.     

Effects of rare earth metals on electrical conductivity and mechanical properties of commercial purity aluminium

Abstract

The effects of the addition of rare earth metals on the electrical conductivity and mechanical properties of commercial purity aluminium have been investigated both in the laboratory and in industrial trials. The electrical conductivity of commercial purity aluminium is increased by about 1% on the international annealed copper standard by the addition of lanthanum, and both its tensile strength and elongation are improved considerably by the addition of cerium. Investigation of the microstructure of the alloys containing rare earth metals shows that the apparent improvement in the electrical conductivity of commercial purity aluminium is caused by a decrease in the solid solubility of impurities in the aluminium.

MST/2032     

Work hardening behaviour and subgrain size distribution of hot worked aluminium

Abstract

The subgrain structure of hot worked aluminium was found to remain geometrically self similar for various deformation conditions, undergoing only alteration in size as the flow stress changes. The stress and temperature related alterations of the subgrain size distributions have been described mathematically by the two parameters of the lognormal probability function which fits the experimental data. It has been found that the rate of scaling of the structure changes in stage IV work hardening. Experimental observations and statistical considerations suggest that the lognormal character of the subgrain size distribution arises as a result of a multiplicative interaction of recovery processes.     

Microstructures and tensile properties of commercial purity aluminium alloy AA1235 under different annealing conditions

Microstructures, hardness, tensile properties and texture of cold-rolled AA1235 alloy are investigated under different annealing conditions. Precipitation of Al₃Fe particles occurs during annealing of the alloy. These precipitates largely affect the microstructural behaviour, tensile properties and texture of the alloy. After complete recrystallization no change in mechanical properties is observed upon further annealing.     

Dynamic materials modelling concepts applied to analysis of hot rolling of commercial aluminium alloys

Abstract

A typical industrial hot rolling operation applied to a commercial Al–1%Mg alloy has been analysed in terms of some concepts from dynamic materials modelling (DMM), particularly the so called dissipator co-content J and the efficiency of power dissipation through microstructural changes η. The calculation of the parameter η for every deformation condition of the hot rolling schedule has been conducted assuming that this variable depends not only on the mean deformation temperature and strain rate but also on the strain applied to the material. All the analysis has been conducted on the basis of a constitutive equation previously determined for this material on a rational basis, which is capable of describing the strain, strain rate, and temperature dependence of the flow stress. It has been determined that, for this material, η can be significantly dependent on strain under certain deformation conditions, particularly low deformation temperatures and relatively high strain rates. It has also been shown that for the materials analysed, η is much more dependent on deformation temperature than on strain rate. A comparative analysis carried out with aluminium of various purities indicates that η is much lower for the alloy when it is deformed under similar conditions. In this sense, it has been suggested that such results could be interpreted in terms of the impurity content of the material and the interaction of the alloying atoms with the dislocation structure developed during high temperature deformation. However, this is only possible if the hypothesis advanced by Prasad and co-workers, that the power dissipation efficiency is associated with the dynamic microstructural processes that occur during deformation, is considered valid. The present results indicate that, contrary to previous findings, in terms of DMM concepts there are no specific conditions of temperature and strain rate for the optimum processing of the materials investigated.     

Mechanical properties in the semi-solid state and hot tearing of aluminium alloys

This review represents a comprehensive coverage of results reported in the literature over last 50 years on the methods of studying hot tearing and mechanical properties of semi-solid aluminium alloys; the mechanical properties of these alloys in the semi-solid state; and hot tearing criteria. While compiling this review, the authors attempted to include in it all available sources including quite a few works never published in English before. The review consists of three parts. The first part introduces the reader to the phenomenon of hot tearing. The second part describes different techniques for testing metallic alloys in the semi-solid state and summarizes reported results on strength and ductility of semi-solid model and commercial aluminium alloys. The third part describes the methods for assessing hot tearing susceptibility of aluminium alloys, gives the results on hot cracking of various aluminium alloys and discusses different hot tearing criteria.     

Influence of grain size and precipitation on hot ductility of microalloyed steels

Abstract

The influence of grain size on the hot ducility of microalloyed steels (C–Mn–Al, C–Mn–V–Al, and C–Mn–Nb–Al) has been determined by heating them above their solution temperatures and cooling to the test temperature of 850°C. The C–Mn–Al steel showed excellent hot ductility which was independent of grain size. Dynamic recrystallization readily occurred and there was no evidence for AlN precipitation. Marked dynamic precipitation occurred during the tensile test for vanadium- and niobium-containing steels but this did not vary significantly with reheating temperature, provided complete dissolution of the precipitates had occurred. Isolating the influence of grain size from that of precipitation in these steels showed that a change in grain size from 150 to 300 μm reduced the reduction of area values by 15–20%. Precipitate distribution was also varied by heating to temperatures in the range 850–1330°C and tensile testing at 850°C. When present before testing at the γ grain boundaries in the form of a fine grain-refining precipitate, AlN reduced the hot ductility in the C–Mn–Al steel and delayed the onset of dynamic recrystallization. Coarser precipitates produced by raising the reheating temperature allowing dynamic recrystallization to occur gave improved ductility. For the niobium- and vanadium-containing steels, precipitate distributions which were in a coarse randomly precipitated form gave the best hot ductility. These occurred with the niobium-containing steel when heated to 1100°C and more generally in the vanadium-containing steel throughout a wide temperature range. The worst precipitate distribution occurred in the niobium containing steel when the NbCN was taken into solution before testing and reprecipitated in a fine form at the γ grain boundaries and within the matrix during the test.

MST/490     

Effects of specimen geometry on temperature rise and flow behavior of aluminium alloys in hot torsion testing

Inappropriate design of the test specimen in hot torsion testing may lead to a high accumulation of heat in the central region of the specimen gauge length and as a result flow localization may occur. To avoid this, knowledge of the variation of temperature rise due to plastic deformation over the specimen gauge length for different specimen geometries is necessary. It is also necessary to estimate the distribution of temperature rise in the event that heat generation is inevitable so that the constitutive equations used are modified accordingly. This paper is devoted to these points.     

Effect of grain size on cyclic microplasticity of ECAP processed commercial pure magnesium

Equal channel angular pressing (ECAP) was performed on the extruded commercial pure magnesium at 250 °C for 4 passes. Heat treatments were carried out to modify the microstructures. The cyclic plastic deformation behavior of pure Mg with different grain sizes in microstrain region was studied by tensile loading and unloading experiments. The microplastic deformation process of pure Mg can be divided into two stages. In the first stage, pronounced plastic deformation associated with dislocation motion on basal plane is initiated at several MPa. The materials are softened and characterized by low friction stresses and hardening exponents. The microplastic deformation enters into region II above the strain of about 8 × 10^?4. Annihilation and tangle of dislocations lead to the increase of hardening exponents and friction stresses. Pure Mg shows a very pronounced anelastic behavior during cyclic microplastic deformation, which results in a rapid increase of modulus defect, effectively decreasing the elastic modulus by up to 60 %. Grain size has a marked effect on microplastic deformation behavior of pure Mg. With increasing the grain size, the specimen shows a more pronounced microstrain and anelastic behavior.     

Numerical simulation of bubble size distribution of aluminium foams in liquid state

Abstract

A mathematical model for predicting the evolution of bubble size distribution in Al foams is presented, which takes into account effects of both the coarsening due to gas diffusion between bubbles and the liquid drainage. A bubble size distribution equation and a one-dimensional drainage equation are solved coupledly by a finite difference approach. Comparison with experimental results from the literature shows a general agreement. The model predictions indicate that the bubble size increases exponentially with time that is in good agreement with MacPherson's theory. Furthermore, computational results reveal that bubble size distributions are dependent strongly on the drainage behaviour, the Henry constant, gas diffusivity and surface tension of the Al foam in liquid state.