Microstructures and mechanical properties of Fe–Al–Ta alloys with strengthening Laves phase

The addition of Ta to Fe–Al alloys results in the formation of a stable Ta(Fe,Al)2 Laves phase with hexagonal C14 structure in the Fe–Al phase at temperatures of 800, 1000 and 1150 °C. It was found that the solubility of Ta in Fe–Al is generally low and the solubility of Ta varies with Al content. Respective isothermal sections of the Fe–Al–Ta system have been established. Particular attention has been given to precipitation in the Fe₃Al phase with a small addition of Ta. At intermediate temperatures, 600–750 °C, an additional Heusler-type phase with L2₁-structure precipitates, which transforms at longer times and high temperatures to the stable C14 Laves phase. The yield stress in compression and the creep behaviour of the Fe–Al–Ta alloys with various microstructures were studied. Due to the presence of the L2₁-Heusler phase, the yield stress and the creep resistance at temperatures below 700 °C was increased considerably.     

Structure of strengthening particles of niobium carbide in Fe–Cr–Ni cast refractory alloys

Methods of optical and electron microscopies were used to study the structure of particles of niobium carbide in a cast refractory Fe–Cr–Ni–C alloy modified by Nb and Ti. Particles of niobium carbide in the structure of the cast alloy are predominantly multiphase polycrystalline clusters that are inhomogeneous in the chemical composition and crystal structure. The misorientation angle between individual crystals that compose the carbide particles is 30°–60°. The polycrystalline character of carbides is probably associated with significant thermal stresses that arise at the interphase boundaries in the structure of the alloy upon the primary cooling of the ingot. To explain the polymorphism of the cluster of niobium carbide, a further analysis of the structural and geometrical crystallography is required.     

Structure and properties of cast and splat-quenched high-entropy Al–Cu–Fe–Ni–Si alloys

The effect of the composition and cooling rate of the melt on the microhardness, phase composition, and fine-structure parameters of as-cast and splat-quenched (SQ) high-entropy (HE) Al–Cu–Fe–Ni–Si alloys was studied. The quenching was performed by conventional splat-cooling technique. The cooling rate was estimated to be ~10⁶ K/s. Components of the studied HE alloys were selected taking into account both criteria for designing and estimating their phase composition, which are available in the literature and based on the calculations of the entropy and enthalpy of mixing, and the difference between atomic radii of components as well. According to X-ray diffraction data, the majority of studied Al–Cu–Fe–Ni–Si compositions are two-phase HE alloys, the structure of which consists of disordered solid solutions with bcc and fcc structures. At the same time, the Al_0.5CuFeNi alloy is single-phase in terms of X-ray diffraction and has an fcc structure. The studied alloys in the as-cast state have a dendritic structure, whereas, after splat quenching, the uniform small-grained structure is formed. It was found that, as the volume fraction of bcc solid solution in the studied HE alloys increases, the microhardness increases; the as-cast HE Al–Cu–Fe–Ni–Si alloys are characterized by higher microhardness compared to that of splat-quenched alloys. This is likely due to the more equilibrium multiphase state of as-cast alloys.     

Precipitation strengthening of stress-aged Al–xCu alloys

Effects of stress aging on yield strength and yield anisotropy of single crystal and cube-textured polycrystalline Al–xCu alloys were investigated. The resulting microstructures were correlated with the yield stress and analyzed with respect to continuum mechanic models and computer simulations. The yield stresses of the stress-aged specimens, were found to be lower than those of the stress-free-aged specimens whether the test direction was along or perpendicular to the aging-stress direction. This was attributed to the effect of stress-induced preferential orientation of the θ′{100}-plates as well as to their “detrimentally” different volume fraction and/or morphology in the stress-aged specimen subjected to the same thermal treatment as the stress-free-aged specimen. When compared with the continuum models, both of which account for only effects of the orientation and volume fraction of the θ′{100}-plates, the computer simulation of interaction between slip dislocation and unshearable plates, which additionally incorporates the effect of plate morphology, yields the closest results to those obtained experimentally.     

Effects of fluidised bed quenching on heat treating characteristics of cast Al–Si–Mg and Al–Si–Mg–Cu alloys

Abstract

The quench sensitivity of Al–Si–Mg (D357 unmodified and Sr modified), and Al–Si–Mg–-Cu (354 and 319 Sr modified) cast alloys was investigated using a fluidised bed (FB). The average cooling rate of castings in the fluidised bed is lower than those quenched in water; the cooling rate first increases to a certain maximum and then decreases during quenching. The change in the cooling rate during quenching in water was more drastic, where the cooling rate varied from 0 to ?80 K s^?1 in less than 8 s, as compared with those quenched in FB, where the cooling rate varied from 0 to ?14 K s^?1 in 18 s. The FB quenching resulted in the formation of several metastable phases in Al–Si–Mg–Cu alloys; in contrast, no such transformation was observed during water quenching. The T4 yield strength of the FB quenched alloys was greater than water quenched alloys owing to the formation of a greater volume fraction of metastable phases in the FB quenched alloys. The tensile properties of T6 treated alloys show that Al–Si–Mg alloys (both unmodified and Sr modified) are more quench sensitive than Al–Si–Mg–Cu alloys. The high quench sensitivity of the Al–Si–Mg alloys is because GP zones are not formed, whereas GP zones are formed during quenching of the Al–Si–Mg–Cu alloys as predicted by time temperature transformation and continuous cooling transformation) diagrams.     

Influence of alloy composition and heat treatment on precipitate composition in Al–Zn–Mg–Cu alloys

The composition of precipitates in three alloys of the Al–Zn–Mg–Cu system has been investigated for different heat treatments, including peak-aged and over-aged states as well as near-equilibrium conditions, by combining atom probe tomography and systematic anomalous small-angle X-ray scattering experiments. We show that the concentration of Cu in the precipitates changes during heat treatments and is alloy dependent. At low ageing temperature (120 °C) the Cu content in the precipitates is close to the alloy content. The precipitate Cu content is shown to increase with increasing temperature and Cu alloy content. We show that in near-equilibrium conditions the precipitate compositions are 33 at.% in Mg, about 15 at.% in Al, about 13 at.% in Cu and balance Zn. Our results strongly suggest that the gradual incorporation of Cu in the precipitates during the heat treatment is essentially related to the slower diffusivity of this element in aluminium.     

Structure and phase composition of deposited tantalum–carbon films

Ion plasma sputtering and the subsequent codeposition of ultrafine tantalum and carbon particles were used to prepare coatings with 4.6–71.5 at % C. Structural studies of the coatings showed the existence of carbon solid solutions in β Ta at carbon contents to 4.6 at %, carbon solid solutions in α Ta at carbon contents of 4.6–10.3 at %, and direct synthesis of TaC at carbon contents of 44.7–71.5 at %. During heat treatments to 700°C, the substantial concentration widening of regions of the existence of Ta₂C and TaC was found. The lattice parameters of hexagonal Ta₂C and fcc TaC carbides were determined for composition ranges of the existence of phases during heating to 700°C. Upon heating above 600°C, the progressive transition of quasiamorphous Ta₂C carbide into the crystalline Ta₂C carbide was found to take place. The possibility of applying the direct synthesis of TaC carbide in engineering was noted.     

Microstrucure and strengthening of Al–Li–Cu–Mg alloys and MMCs: I. Analysis and Modelling of Microstructural changes

A complete and detailed analysis of the microstructural development during ageing in an 8090 (Al–2.3Li–1.2Cu–1Mg–0.1Zr) alloy, an 8090/20 wt% SiC_p MMC, an Al–1.5Li–Cu–Mg MMC and an Al–Cu–Mg MMC (all with similar Cu and Mg contents) has been performed. Volume fractions of all precipitates relevant for precipitation strengthening of the alloys (δ′ phase, S′ phase and GPB zones) have been determined using a recently derived method based on differential scanning calorimetry (DSC). The volume fractions have subsequently been successfully fitted using a novel model for transformation kinetics. The sizes of these precipitates have been analysed using newly derived expressions consistent with the latter model. As a result of dislocation generation around misfitting SiC particles the volume fractions of both GPB zones and S′ phase depend strongly on the presence of these particles. Also the amount of Li present in the alloys influences the volume fractions of the phases significantly. The sizes of S′ are similar for the four alloys.     

A model for the thermodynamics of and strengthening due to co-clusters in Al–Mg–Si-based alloys

An expanded model for the thermodynamics of co-clusters and their strengthening is presented and applied to predict co-cluster formation and strengthening in Al–Mg–Si alloys. The models were tested against data on a wide range of Al–Mg–Si alloys aged at room temperature. The strengthening due to co-clusters is well predicted. The formation of co-clusters was studied in an Al–0.5 at.% Mg–1 at.% Si alloy using three-dimensional atom probe analysis. The results correspond well with the model. It is shown that in general (short-range) order strengthening due to co-clusters will be the main strengthening mechanism in these alloys. Apart from the co-clusters, Si clusters also form, but due to their low enthalpy of formation they contribute little to the strength.     

Microstructure and phase composition of as-cast Mg–9Er–6Y–xZn–0.6Zr alloys

The microstructure and phase composition of as-cast Mg–9Er–6Y–xZn–0.6Zr (x=1, 2, 3, 4; normal mass fraction in %) alloys were investigated. In low Zn content, aside from the major second phase of Mg₂₄(Er, Y, Zn)₅, there are a few lamellar phases that grow parallel with each other from the grain boundaries to the grain interior. With Zn content increasing, the Mg₂₄(Er,Y,Zn)₅ phase decreases, but the Mg₁₂Zn(Y, Er) phase and lamellar phases continuously increase. When Zn content reaches 4% (normal mass fraction), the Mg₁₂Zn(Y,Er) phase mainly exists as large bulks, and some α-Mg grains are thoroughly penetrated by the lamellar phases. Moreover, the crystallography structures of the Mg₁₂Zn(Y,Er) and Mg₂₄(Er,Y,Zn)₅ phases are confirmed as 18R-type long-period stacking ordered structure and body-centred cubic structure, respectively.     

On thermal analysis,macrostructure and microstructure of grain refined Al–Si–Mg cast alloys: role of Sr addition

The present study aimed at investigating the influence of grain refinement in combination with Sr modification on the solidification behaviour of A356·2 alloy, and the resulting micro- and macrostructures obtained. Grain refinement of A356·2 alloy using Ti and B additions in the ranges of 0·02–0·5% and 0·01–0·5% respectively was studied using five different grain refiners in the form of Al–10%Ti, Al–5%Ti–1%B, Al–2·5%Ti–2·5%B, Al–1·7%Ti–1·4%%B and Al–4%B aluminium master alloys. Sr modification of the alloys was carried out using Sr additions of 30 and 200 ppm in the form of Al–10%Sr master alloy. The probable interactions between Sr and Ti and Sr and B were investigated using different metallographic techniques. Thermal analysis was also used to evaluate these interactions. Electron microprobe analysis revealed that adding >0·1%B to the A356·2 alloy may lead to formation of particles predominantly containing B and Sr, with a composition approaching SrB₆.     

Effect of minor Zr and Sc on microstructures and mechanical properties of Al–Mg–Si–Cu–Cr–V alloys

The effects of minor contents of Zr and Sc on the microstructures and mechanical properties of Al–Mg–Si–Cu–Cr–V alloy were studied. The results show that the effects of minor Zr and Sc on the as-cast grain refinement in the ingots, the improvement in the strength of the as-extruded alloys and the restriction of high angle grain boundaries in the aged alloys can be sorted as Al₃Sc>Al₃(Zr,Sc)>Al₃Zr. None of them could stop the nucleation of recrystallization, but Al₃(Zr,Sc) phase is a more effective inhibitor of dislocation movement compared to Al₃Sc in the aged alloys. Compared with the mechanical properties of the aged alloy added only 0.15% Sc, the joint addition of Zr and Sc to the alloy leads to a very slight decrease in strength with even no cost of ductility. Taking both the production cost and the little bad influence on mechanical properties into consideration, an optimal content of Zr and Sc in the Al–Mg–Si–Cu–Cr–V alloy to substitute 0.15% Sc is 0.13% Zr+0.03% Sc.     

Effect of thermomechanical treatment on the microstructure,phase composition,and mechanical properties of Al–Cu–Mn–Mg–Zr alloy

The effect of the thermomechanical treatment on the microstructure, phase composition, and mechanical properties of heat-treatable AA2519 aluminum alloy (according to the classification of the Aluminum Association) has been considered. After solid-solution treatment, quenching, and artificial aging (T6 treatment) at 180°C for the peak strength, the yield stress, ultimate tensile strength, and elongation to failure are ~300 MPa, 435 MPa, and 21.7%, respectively. It has been shown that treatments that include intermediate plastic deformations with degrees of 7 and 15% (T87 and T815 treatments, respectively) have a significant effect on the phase composition and morphology of strengthening particles precipitated during peak aging T8X type, where X is pre-strain percent, treatments initiate the precipitation of significant amounts of particles of the θ′- and Ω-phases. After T6 treatment, predominantly homogeneously distributed particles of θ″-phase have been observed. Changes in the microstructure and phase composition of the AA2519 alloy, which are caused by intermediate deformation, lead to a significant increase in the yield stress and ultimate tensile strength (by ~40 and ~8%, respectively), whereas the plasticity decreases by 40–50%.     

The thermodynamics of and strengthening due to co-clusters: General theory and application to the case of Al–Cu–Mg alloys

Co-clusters in ternary or higher order metallic alloys are metastable structures involving two or more distinct alloying atoms that retain the structure of the host lattice. A thermodynamic model based on a single interaction energy of dissimilar nearest neighbour interaction energy is presented, and a model for the strengthening due to these co-cluster dimers is derived. The model includes a new treatment of (short-range) order strengthening relevant to these co-clusters and further encompasses modulus hardening and chemical hardening. The models are tested against data on a wide range of Al–Cu–Mg alloys treated at temperatures between 20 and 220 °C. Both quantitative calorimetry data on the enthalpy change due to co-cluster formation and strengthening due to co-clusters is predicted well. It is shown that in general (short-range) order strengthening will be the main strengthening mechanism.     

Solidification structure and mechanical properties of Al–Li–Cu–Zr cast alloys

Abstract

The microstructure and properties of three Al–3Li–1Cu ternary alloys have been studied, in particular the effect of Zr additions on the microstructure, precipitation and mechanical properties. The results showed that, for these Al–Li casting alloys, Zr content up to 0.2 wt-% was acceptable, and the Zr additions appeared to refine the grain structure. During aging, the Zr rich phase provided nucleation sites for δ' phase and promoted δ' phase refinement and homogenisation. Under optimised conditions, the tensile strength and elongation to failure of the Al–Li–Cu–Zr casting alloys were 400 MPa and 2.5%, respectively.     

Effect of Mg composition on sintering behaviors and mechanical properties of Al–Cu–Mg alloy

Al–3Cu–Mg alloy was fabricated by the powder metallurgy (P/M) processes. Air-atomized powders of each alloying element were blended with various Mg contents (0.5%, 1.5%, and 2.5%, mass fraction). The compaction pressure was selected to achieve the elastic deformation, local plastic deformation, and plastic deformation of powders, respectively, and the sintering temperatures for each composition were determined, where the liquid phase sintering of Cu is dominant. The microstructural analysis of sintered materials was performed using optical microscope (OM) and scanning electron microscope (SEM) to investigate the sintering behaviors and fracture characteristics. The transverse rupture strength (TRS) of sintered materials decreased with greater Mg content (Al–3Cu–2.5Mg). However, Al–3Cu–0.5Mg alloy exhibited moderate TRS but higher specific strength than Al–3Cu without Mg addition.