Texture transition in Al–Mg alloys: effect of magnesium

ABSTRACT

The evolution of deformation texture and microstructure in commercially pure Al (cp-Al) and two Al–Mg alloys (Al–4Mg and Al–6Mg) during cold rolling to a very large strain (true strain ε_t? ≈?3.9) was investigated. The development of deformation texture in cp-Al, after rolling, can be considered as pure metal or Copper-type, which is characterised mainly by the presence of Cu {112}<111>, Bs {110}<112> and S {123}<634> components. The deformation microstructure clearly indicates that deformation mechanism in this case remains slip dominated throughout the deformation range. In the Al–4Mg alloy, the initial slip mode of deformation is finally taken over by mechanism involving both slip and Copper-type shear bands, at higher deformation levels. In contrast, in the Al–6Mg alloy, the slip and twin mode of deformation in the initial stage is replaced by slip and Brass-type shear bands at higher deformation levels. Although a Copper-type deformation texture forms in the two Al–Mg alloys at the initial stage of deformation, there is a significant increase in the intensity of the Bs component and a noticeable decrease in the intensity of the Cu component at higher levels of deformation, particularly in the Al–6Mg alloy. This phenomenon indicates the possibility of transition of the deformation texture from Cu-type to Bs-type, which is concurrent with the addition of Mg. Using visco-plastic self-consistent modelling, the evolution of deformation texture could be simulated for all three materials.     

Multiscale characterization of dislocation processes in Al 5754

Multiscale characterization was performed on an Al–Mg alloy, Al 5754 O-temper, including in situ mechanical deformation in both the scanning electron microscope and the transmission electron microscope. Scanning electron microscopy characterization showed corresponding inhomogeneity in the dislocation and Mg distribution, with higher levels of Mg correlating with elevated levels of dislocation density. At the nanoscale, in situ transmission electron microscopy straining experiments showed that dislocation propagation through the Al matrix is characterized by frequent interactions with obstacles smaller than the imaging resolution that resulted in the formation of dislocation debris in the form of dislocation loops. Post-mortem chemical characterization and comparison to dislocation loop behaviour in an Al–Cr alloy suggests that these obstacles are small Mg clusters. Previous theoretical work and indirect experimental evidence have suggested that these Mg nanoclusters are important factors contributing to strain instabilities in Al–Mg alloys. This study provides direct experimental characterization of the interaction of glissile dislocations with these nanoclusters and the stress needed for dislocations to overcome them.     

Influence of ageing on the low cycle fatigue behaviour of an Al–Mg–Si alloy

Abstract

The low cycle fatigue (LCF) performance of AA6063 Al–Mg–Si alloy at under-aged (UA), peak-aged (PA) and over-aged (OA) conditions has been examined to understand the micromechanism of fatigue and the associated dynamic structural changes in this alloy. The LCF behaviour of the differently aged AA6063 alloys has been studied at strain amplitudes ranging between 0.2 and 1.0% under strain control mode. The UA state exhibits pronounced cyclic hardening unlike the PA and the OA states at strain amplitudes greater than 0.4%. The PA and the OA states show hardening only for a few cycles followed by prolonged softening. Characterisations of the micro- and the sub-structural alterations due to LCF establish that the phenomenon of dynamic precipitation results in cyclic hardening the UA alloy. The softening of PA alloy occurs due to shearing of precipitates and that in the OA alloy takes place owing to reversibility of slip by the formation and annihilation of the Orowan loops around the β (Mg₂Si) precipitates. Analyses of the hysteresis loops reveal Masing, nearly-Masing and non-Masing behaviour in the UA, OA and PA states, respectively. Analyses of the asymmetry factor of the hysteresis loops assist to infer that the Masing behaviour in the UA alloy is due to dislocation–dislocation interactions, whereas the nearly-Masing behaviour in the OA alloy and the non-Masing behaviour in the PA alloy are the consequence of varying degrees of dislocation–precipitate interactions associated with inhomogeneous deformation.     

Influence of Al3Ni crystallisation origin particles on hot deformation behaviour of aluminium based alloys

Abstract

Binary Al–Ni, Al–Mg and ternary Al–Mg–Ni alloys containing various dispersions and volume fraction of second-phase particles of crystallisation origin were compressed in a temperature range of 200–500 °C and at strain rates of 0.1, 1, 10, 30 s^?1 using the Gleeble 3800 thermomechanical simulator. Verification of axisymmetric compression tests was made by finite-element modelling. Constitutive models of hot deformation were constructed and effective activation energy of hot deformation was determined. It was found that the flow stress is lowered by decreasing the Al₃Ni particle size in case of a low 0.03 volume fraction of particles in binary Al–Ni alloys. Intensive softening at large strains was achieved in the alloy with a 0.1 volume fraction of fine Al₃Ni particles. Microstructure investigations confirmed that softening is a result of the dynamic restoration processes which were accelerated by fine particles. In contrast, the size of the particles had no influence on the flow stress of ternary Al–Mg–Ni alloy due to significant work hardening of the aluminium solid solution. Atoms of Mg in the aluminium solid solution significantly affect the deformation process and lead to the growth of the effective activation energy from 130–150 kJ/mol in the binary Al–Ni alloys to 170–190 kJ/mol in the ternary Al–Mg–Ni alloy.     

Effect of alloying with boron on the phase composition and structure of Ni3Al

The structure of Ni₃Al alloy prepared by self-propagating high-temperature synthesis is studied by means of transmission electron diffraction, scanning microscopy, and X-ray structural analysis. The effect of alloying with boron on the morphology of the alloy structure is established. The grain and defect structures are investigated. The dislocation density and the internal stress are measured.     

High-temperature deformation and specific features of dislocation structure of Ti<Subscript>3</Subscript>Al

The transmission electron microscopy was used to examine the dislocation structure of intermetallic Ti₃Al after deformation at temperatures T = 1073–1273 K. It is established that its microstructure contains mobile 2c + a and superdislocations. Possible models describing the destruction of barriers associated with 2c + a superdislocations in pyramidal planes are discussed using the results of computer simulation of the superdislocation core structure in Ti₃Al.     

Temperature and velocity dependence of the mechanical properties of the intermetallic compound Ni3Al. I

The paper presents the results of theoretical and experimental studies of the deformation mechanism of alloy Ni₃Al, undertaken in order to elucidate the nature of the anomalous temperature dependence of the mechanical properties of that alloy. A study is made of the mechanisms of hardening which do not require diffusion migration of atoms for their occurrence; 1) indeterminacy of dislocation axis; 2) recombination and conservative sliding of jogs in superpartial dislocations (SPD); 3) cross-slip of SPD; 4) intersection of antiphased boundaries introduced by cubic slip; 5) work-hardening due to reactions between dislocations of cubic and octahedral planes. All these mechanisms are effective, even at room and lower temperatures, and could be responsible for the increase of the yield point with increase in the temperature of testing from –180 °C, as observed by Davies and Stoloff.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 10, pp. 39–45, October, 1971.     

Dislocation structure of intermetallic Ti3Al subjected to high-temperature deformation

Transmission electron microscopy was used to examine the dislocation structure of intermetallic Ti₃Al subjected to deformation at tempertures T = 1073–1273 K. The microstructure of samples subjected to high-temperature deformation is established to contain mobile superdislocations of a and 2c + a types, and single dislocations with Burgers vector [0001] are also observed on the prismatic planes. Possible models of destruction of barriers associated with 2c + a superdislocations on the pyramidal planes are discussed using the results of computer simulations of the structure of a superdislocation core in in Ti₃Al.     

Plastic deformation of a monoclinic Al13Fe4 at 300 ∼ 873 K

The plasticity of monoclinic Al₁₃Fe₄ particles grown in a spray-formed Al–Fe alloy is examined after being submitted to two deformation modes between 300 and 873 K by specially designed semi-in situ compression tests; one is uniform and the other is by indentation. In the uniform mode, cracks propagate through the Al₁₃Fe₄ particles along the pentagonal column planes, leaving extremely thin and heavily deformed plastic zones along the cracked faces at 300 and 473 K. In contrast, the generation of isolated dislocations and their motion govern the plastic deformation at 673 and 873 K. In the indentation mode, local deformation is achieved exclusively by individual dislocations over the whole range of temperature explored. A possible mechanism of dislocation motion in monoclinic Al₁₃Fe₄ is discussed based on Burgers vector analyses coupled with the three-dimensional observation of dislocation configurations.     

Microcrack formation upon interaction of superdislocations belonging to different slip planes in Ti<Subscript>3</Subscript>Al

The interaction reactions between the superdislocations accomplishing deformation in the basal, prismatic, and pyramidal (I and II types) planes in Ti₃Al single crystals have been investigated. The types of interactions leading to the formation of dislocation barriers (microcrack nuclei) are established. The microcrack types are classified according to the orientations of the deformation axes of single crystals. Stereographic triangle regions, characterized by a preferred type of crack opening, are found.     

Formation of nanostructured states in ternary TiNiFe-based shape memory alloys during megaplastic deformation and subsequent heat treatment

The ternary metastable TiNiFe alloys that exhibit a low-temperature shape memory effect and are subjected to plastic deformation by rolling or high-pressure torsion followed by heat treatment are studied by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and electrical resistivity measurements. It is found that moderate plastic deformation of a Ti₅₀Ni₄₉Fe₁ alloy at room temperature initiates the thermoelastic B2 ? B19’ martensitic transformation and the formation of a developed banded dislocation and twin substructure in the B19’ martensite. This deformation of a Ti₅₀Ni₄₇Fe₃ alloy forms a similar dislocation substructure but in B2 austenite. Megaplastic deformation by high-pressure torsion causes amorphization in the Ti₅₀Ni₄₉Fe₁ alloy and nanofragmentation in the Ti₅₀Ni₄₇Fe₃ alloy. The evolution of the nanostructure and the martensitic transformations in TiNiFe-based ternary alloys is studied during plastic deformation and subsequent annealing at various temperatures.     

Effect of alloying and temperature of deformation on the grain structure and phase composition of Ni3Al intermetallic

The structure of Ni₃Al intermetallic alloyed with boron and hafnium is studied at various temperatures of deformation by means of diffraction and scan electron microscopy. The alloy was produced through self-propagating synthesis. The phase composition of the alloy is determined. The effect of doping with boron and hafnium on the alloy structure morphology is revealed.     

Experimental and theoretical investigations of the brittle fracture of Ti<Subscript>3</Subscript>Al

The formation of microcracks upon dislocation interactions was studied by means of TEM analysis. The types of microcracks were classified depending on the orientation of deformation axes in single crystals. The Griffith surface energy was simulated, and the energy of unstable stacking faults was calculated by the molecular dynamics method using the N-particle EAM potentials of the interatomic interaction for Ti₃Al. The Rice—Thompson model was used to study the relationship between the tendency toward cleavage and the plastic relaxation of stresses near a crack tip by the emission of dislocations in Ti₃Al single crystals.     

Nucleation of deformation twins in nanocrystalline fcc alloys

An earlier dislocation model for predicting the grain size effect on deformation twinning in nanocrystalline (nc) face-centred-cubic (fcc) metals has been found valid for pure metals but problematic for alloys. The problem arises from the assumption that the stacking-fault energy (γ_SF) is twice the coherent twin-boundary energy (γ_fcc), which is approximately correct for pure fcc metals, but not for alloys. Here we developed a modified dislocation model to explain the deformation twinning nucleation in fcc alloy systems, where γ_SF ≠ 2γ_twin. This model can explain the differences in the formations of deformation twins in pure metals and alloys, which is significant in low stacking-fault energy alloys. We also describe the procedure to calculate the optimum grain size for twinning in alloy systems and present a method to estimate γ_twin.     

Experimental and computational creep characterization of Al–Mg solid-solution alloy through instrumented indentation

Carefully designed indentation creep experiments and detailed finite-element computations were carried out in order to establish a robust and systematic method to extract creep properties accurately during indentation creep tests. Samples made from an Al–5.3?mol%?Mg solid-solution alloy were tested at temperatures ranging from 573 to 773?K. Finite-element simulations confirmed that, for a power-law creep material, the indentation creep strain field is indeed self-similar in a constant-load indentation creep test, except during short transient periods at the initial loading stage and when there is a deformation mechanism change. Self-similar indentation creep leads to a constitutive equation from which the power-law creep exponent n, the activation energy Q _c for creep, the back or internal stress and so on can be evaluated robustly. The creep stress exponent n was found to change distinctively from 4.8 to 3.2 below a critical stress level, while this critical stress decreases rapidly with increasing temperature. The activation energy for creep in the stress range of n = 3.2 was evaluated to be 123?kJ?mol^?1, close to the activation energy for mutual diffusion of this alloy, 130?kJ?mol^?1. Experimental results suggest that, within the n = 3.2 regime, the creep is rate controlled by viscous glide of dislocations which drag solute atmosphere and the back or internal stress is proportional to the average applied stress. These results are in good agreement with those obtained from conventional uniaxial creep tests in the dislocation creep regime. It is thus confirmed that indentation creep tests of Al–5.3?mol%?Mg solid-solution alloy at temperatures ranging from 573 to 773?K can be effectively used to extract material parameters equivalent to those obtained from conventional uniaxial creep tests in the dislocation creep regime.     

Effects of Ca/Al ratio and extrusion process on Mg–Al–Ca alloys to produce a high toughness in-situ composite

The microstructures and tensile properties of Mg–Al₂Ca–Mg₂Ca in situ composites (Mg–17Al–8Ca, Mg–14Al–11Ca and Mg–12.5Al–12.5Ca) with different Ca/Al ratios have been studied in both as-cast and extruded conditions. The results indicated that by increasing Ca/Al ratio, new Mg₂Ca intermetallic introduces to the Al₂Ca phase in eutectic structure. Computer-aided cooling curve analysis confirmed the formation of these phases during solidification. Extrusion process not only altered the size of large bulk Al₂Ca intermetallic, but also changed the size and morphology of intermetallics in eutectic structure considerably. The results showed that with increasing Ca/Al ratio, tensile properties of cast composites changes slightly, but significant enhancement is observed after extrusion process. The strength and elongation values of Mg–12.5Al–12.5Ca (Ca/Al = 1) alloy improved from 166 MPa and 2% in as-cast condition to 465 MPa and 12% in hot-extruded condition. The reason for the improved toughness may be attributed to the formation of finer and well-dispersed distribution of hard (Al₂Ca) and ductile (Mg₂Ca) phases. It was found that hot extrusion easily deforms ductile Mg₂Ca phase in comparison with Al₂Ca phase. In as-extruded condition, there are more very fine dimples than as-casted condition because extrusion process leads to formation of fragmented tiny particles and more uniformity distribution of Al₂Ca particles.     

Nucleation of microcracks during dislocation interactions in a Ti<Subscript>3</Subscript>Al single crystal

Reactions between superdislocations involved in deformation in the basal, prismatic, and type-I and II pyramidal planes in single-crystal Ti₃Al are considered. The types of dislocation interactions are established that result in the formation dislocation barriers (microcrack nuclei). The force and energy conditions for microcracks to arise are found. The interaction between a and 2c + a superdislocations results in microcracks with the plane of opening lying in basal and pyramidal planes; the interaction of 2c + a superdislocations in different pyramidal planes results in the formation of microcracks in prismatic and pyramidal planes; and the interaction of a superdislocations in basal and/or pyramidal planes does not cause the formation of dislocation barriers. The types of microcracks are classified in terms of the orientation of deformation axes of single crystals, and the regions of the stereographic triangle are determined characterized by a preferential type of crack opening.     

Vibrational entropy of dislocations in Al

The region nearest to a lattice defect must be described by an atomistic model, while a continuum model suffices further away from the defect. We study such a separation into two regions for an edge dislocation. In particular we focus on the excess defect energy and vibrational entropy, when the dislocation core is described by a cluster of about 500–100?atoms, embedded in a large discrete and relaxed, but static, lattice. The interaction between the atoms is given by a potential of the embedded-atom model type referring to Al. The dynamic matrix of the vibrations in the cluster is fully diagonalized. The excess entropy ΔS near the core has positive and negative contributions, depending on the sign of the local strain. Typically, ΔS/k _B ≈ 2 per atomic repeat length along the dislocation core in fcc Al. In the elastic continuum region far from the dislocation core the excess entropy shows the same logarithmic divergence as the elastic energy. Although the work refers to a specific material and defect type, the results are of a generic nature.     

Study of creep and features of the dislocation structure in single crystals of Ni3Ge alloy

Data from investigating the dislocation structure of Ni₃Ge alloy single crystals formed during creep are presented. The creep of the Ni₃Ge single crystals with the [001] orientation of the deformation axis was studied. It was found that a fragmented substructure with varying degrees of disorientation occurs in the areas of macrolocalized deformation. A polycrystalline substructure consisting of fragments with a low dislocation density is formed in the local areas.     

Grain refinement in coarse-grained 7475 Al alloy during severe hot forging

Grain refinement taking place in a coarse-grained 7475 Al alloy was studied in multidirectional compression at 490°C and at a strain rate of 3?×?10^?4?s^?1. The integrated flow curve displays significant work softening just after yielding and an apparent steady-state plastic flow at high strains. The structural changes are characterized by the development of deformation or microshear bands in coarse-grain interiors, followed by homogeneous evolution of new grains at high strains. The new grains are considered to be developed by a kind of continuous reaction through grain fragmentation that is similar to continuous dynamic recrystallization (cDRX). The mechanism of fine grain production and the factors controlling grain refinement during hot multidirectional deformation are discussed in detail.