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.     

63Cu NMR analysis of microstructure evolution in Al–Cu–Mg alloys

⁶³Cu NMR spectroscopy has been used to detect metastable Guinier–Preston–Bagaryatsky (GPB) zones and nanoscale precipitates of equilibrium S-phase (Al₂CuMg) in dilute alloys of aluminium containing copper and magnesium with compositions which lie in the α?+?S phase field. The GPB zones are observed to form rapidly at room temperature with a time development closely related to the Vickers hardness. The final development of S-phase in the alloy has been confirmed by the observation of a line shape in the alloy identical to that observed in a specimen prepared from stoichiometric Al₂CuMg. Analysis of the hyperfine structure of the ⁶³Cu line shape observed for S-phase shows clearly that two Cu sites are present with approximately equal population. This result suggests that possibly two crystallographically distinct Al₂CuMg phases are present. The addition of small amounts of silver to Al–Cu–Mg alloys in the α?+?θ phase field is known to induce the formation of Ω-phase: a slight distortion of tetragonal θ-phase Al₂Cu. A hyperfine-structured ⁶³Cu line shape assigned to Ω-phase, indicating one distinct Cu site, has been observed in two separate Al–1.7?at.%?Cu–0.33?at.%?Mg alloys containing 0.1 and 0.18?at.%?Ag, but not in the same Al–Cu–Mg alloy without Ag.     

Control Al/Mg intermetallic compound formation during ultrasonic-assisted soldering Mg to Al

To prevent the formation of Al/Mg intermetallic compounds (IMCs) of Al₃Mg₂ and Al₁₂Mg₁₇, dissimilar Al/Mg were ultrasonic-assisted soldered using Sn-based filler metals. A new IMC of Mg₂Sn formed in the soldered joints during this process and it was prone to crack at large thickness. The thickness of Mg₂Sn was reduced to 22 μm at 285 °C when using Sn-3Cu as the filler metal. Cracks were still observed inside the blocky Mg₂Sn. The thickness of Mg₂Sn was significantly reduced when using Sn-9Zn as the filler metal. A 17 μm Mg₂Sn layer without crack was obtained at a temperature of 200 °C, ultrasonic power of Mode I, and ultrasonic time of 2 s. The shear strengths of the joints using Sn-9Zn was much higher than those using Sn-3Cu because of the thinner Mg₂Sn layer in the former joints. Sn whiskers were prevented by using Sn-9Zn. A cavitation model during ultrasonic assisted soldering was proposed.     

Rapid solidification of Al-Cu-Ag ternary alloy under the free fall condition

The rapid solidification of Al-30%Cu-18%Ag ternary alloy is investigated by using the free fall method. Its solidified microstructure is composed of θ(Al₂Cu), α(Al) and ξ(Ag₂Al) phases. The liquidus temperature and solidus temperature are determined as 778 and 827 K, respectively. The alloy melt undercooled amounts up to ΔT _Max=171 K (0.20T _L). Its microstructural evolution is investigated based on the theoretical analysis of undercooling behavior and nucleation mechanics. It is found that the undercooling increases with the decrease of the diameter of the alloy droplet. When ΔT<78 K, the primary θ(Al₂Cu) phase of the alloy grows into coarse dendrite. When 78 K⩽ΔT⩽171 K, its refined θ(Al₂Cu) phase grows alternatively with α(Al) phase. Once ΔT⩾171 K, its microstructure is characterized by the anomalous (θ+α+ξ) ternary eutectic. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101 and 50395105)     

Scanning transmission electron microscopy investigation of an Al–Mg–Si–Ge–Cu alloy

Precipitation in a Mg-rich Al–Mg–Si–Ge–Cu alloy was investigated using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. The precipitates were needle or lath shaped with the longest dimension parallel to ?001?_Al. The precipitates had no repeating unit cell when viewed along this direction. However, the precipitate structure in projection consisted of a hexagonal network of mixed Si and Ge columns, with Mg, Al, and Cu columns occupying specific sites in between the network columns. The Cu columns appeared with the same local arrangement of atomic columns as in Al–Mg–Si–Cu precipitates, and the Cu-free regions consisted of structural units with Mg and Al at specific sites. These structural units were often arranged in a locally ordered fashion, and in some cases the projected structure possessed and overall point symmetry. The amount of strain on the surrounding matrix was found to vary depending on the width of the precipitate cross section.     

Geometric modelling of viscosity of copper-containing liquid alloys

In this work, viscosities of ternary Au–Ag–Cu and Al–Cu–Si liquid alloys have been calculated as a function of gold, aluminium and copper compositions for the sections Au–Ag–Cu (x_Ag/x_Cu = 0.543 at 1373 K), Al_x(Cu₅₀–Si₅₀)_(1–x) and Cu_x(Al₅₀–Si₅₀)_(1–x) at 1375 K using Chou’s general solution model, Muggianu, Kohler, Toop, Hillert, Budai et al., Kozlov et al., Schick et al. and Kaptay et al. models. The present study finds that a comparison of the predicted values of viscosities associated with the geometric and physical models indicate good mutual agreement. The Muggianu model indicates the best agreement with the results obtained for Au–Ag–Cu and Al_x–Cu₅₀–Si₅₀ alloy systems and the Kaptay et al. model, which is a physical model, indicates the best agreement with the results obtained for Al₅₀–Cu_x–Si₅₀.     

Effect of variable particle size reinforcement on mechanical and wear properties of 6061Al–SiCp composite

Abstract

The influence of reinforcement particle size variation plays a major role on the properties of Al–SiC_p composite. Therefore, this study aim to investigate the mechanical and wear performance of single particle size (SPS) and multiple particle size (MPS) Al–SiC_p composite prepared by stir casting process. The SPS comprises three categories; fine (15 μm), intermediate (40 μm) and coarse (80 μm) particle sizes and combination of the three sizes accounts for the MPS in the ratio 1:1:2, respectively. Oxidation of the SiC_p and addition of 1 wt% Mg during composite processing resulted to interface reaction products such as MgO (magnesium oxide) and MgAl₂O₄ (magnesium aluminate) which suppresses the potential formation of undesired Al₃C₄ (aluminium carbide). The study reveals that MPS composite improved the hardness and impact properties with enhanced wear performance compared to SPS composite. Characterization of the composite morphology and phases was performed using scanning electron microscope and X-ray diffraction analysis. This study provides an effective method of optimizing the properties of Al–SiC_p composite by integrating MPS with low volume fraction of reinforcement phase.     

The structure of an Al–Rh–Cu decagonal quasicrystal studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy

The structure of an Al–Rh–Cu decagonal quasicrystal formed with two quasiperiodic planes along the periodic axis in an Al₆₃Rh_18.5Cu_18.5 alloy has been studied by spherical aberration (Cs)-corrected high-angle annular detector dark-field (HAADF)- and annular bright-field (ABF)-scanning transmission electron microscopy (STEM). Heavy atoms of Rh and mixed sites (MSs) of Al and Cu atoms projected along the periodic axis can be clearly represented as separate bright dots in observed HAADF-STEM images, and consequently arrangements of Rh atoms and MSs on the two quasiperiodic planes can be directly determined from those of bright dots in the observed HAADF-STEM image. The Rh atoms are arranged in pentagonal tiling formed with pentagonal and star-shaped pentagonal tiles with an edge-length of 0.76 nm, and also MSs with a pentagonal arrangement are located in the pentagonal tiles with definite orientations. The star-shaped pentagonal tiles in the pentagonal tiling are arranged in τ²(τ: golden ratio)-inflated pentagonal tiling with a bond-length of 2 nm. From arrangements of Rh atoms placed in pentagonal tilings with a bond-length of 2 nm, which are generated by the projection of a five-dimensional hyper-cubic lattice, occupation domains in the perpendicular space are derived. Al atoms as well as Rh atoms and MSs are represented as dark dots in an observed ABF-STEM image, and arrangements of Al atoms in well-symmetric regions are discussed.     

Reaction mechanisms between Al and Fe3O4 powders in the formation of an Al-based metal matrix composite

The reaction mechanisms between Al and Fe₃O₄ powders were investigated. Differential thermal analysis revealed that a two-step displacement reaction between Al and Fe₃O₄ took place during sintering. Initially, the Fe₃O₄ was converted to amorphous FeO at ～720°C and some of the Al was oxidized to amorphous Al₂O₃. In the final stage, when the temperature reached ～840°C, crystalline Al₂O₃ particles were produced in the molten Al–Fe liquid. The effects of cooling rate on the microstructures were studied. When the Al–Fe liquid was furnace-cooled to room temperature, proeutectic Al₃Fe plates, plate-like divorced eutectic Al₃Fe and Al₂O₃ particles were in situ formed in the Al(Fe) matrix. While quenching from 700°C, nanometer-sized Al dendrites and Al–Al₆Fe eutectic lamellae were produced in the Al matrix. However, when it was rapidly quenched from 900°C, the size of the proeutectic Al₃Fe phases was further reduced and Al₆Fe nanorods were found in the Al–Al₆Fe eutectics. A model was proposed to describe the transformation of the Al–Fe intermetallics during solidification.     

Observation of iron impurity diffusion in silicon under bending stress by Mössbauer spectroscopy

In the present work, the formation of the Al₇₀Cu₂₀Fe₁₀ icosahedral phase by mechanical alloying the elemental powders in a high-energy planetary mill was investigated by X-ray diffraction and Mössbauer spectroscopy. It was verified that the sample milled for 80 h produces an icosahedral phase besides Al(Cu, Fe) solid solution (β-phase) and Al₂Cu intermetallic phase. The Mössbauer spectrum for this sample was fitted with a distribution of quadrupole splitting, a doublet and a sextet, revealing the presence of the icosahedral phase, β-phase and α-Fe, respectively. This compound is not a good hydrogen storage. The results of the X-ray diffraction and Mössbauer spectroscopy of the sample milled for 40 h and annealed at 623°C for 16 h shows essentially single i-phase and tetragonal Al₇Cu₂ Fe phase.     

Micromechanical model for fracture toughness prediction in Al–Zn–Mg–Cu alloy forgings

An attempt has been made to model the plane-strain fracture toughness, K _Ic, in Al–Zn–Mg–Cu alloy forgings subjected to overageing. The proposed model, based on the multiple micromechanisms, reveals the quantitative relations between fracture toughness, fraction of all fracture modes and microstructural parameters associated with multiscale-sized second-phase particles and precipitate-free zones. The new model is validated by the present quantitative data of microstructural and fractographic analysis performed along with mechanical tests on hot-forged plates in T73 condition. The relevant parameters changed by the compositional variations were determined in two orientations. It was found that the predicted K _Ic values represent the tendency of fracture toughness change well. The new model provides better agreement for the case of dominant transgranular fracture mode.     

The study of Al3(Mn,Pd) crystal and Al–Mn–Pd decagonal quasicrystal by spherical aberration-corrected scanning transmission microscopy and atomic-resolution energy dispersive X-ray spectroscopy

An Al₃Mn-type Al₃(Mn, Pd) crystal and an Al–Mn–Pd decagonal quasicrystal (DQC) in an Al₇₀Mn₂₀Pd₁₀ alloy are studied using a spherical aberration (Cs)-corrected scanning transmission electron microscope (STEM) with high-angle annular dark-field (HAADF) and annular bright-field (ABF) techniques, together with atomic-resolution energy dispersive X-ray spectroscopy (EDS). Mn and Pd atomic positions in the Al₃(Mn, Pd) structure projected along the b-axis (pseudo-tenfold rotational axis) are represented by separate bright dots in observed HAADF-STEM images. Besides, Al as well as Mn and Pd atomic positions are represented as dark dots in ABF-STEM images. Most Mn and Pd atomic positions in the Al₃(Mn, Pd) structure can be observed on atomic-resolution EDS maps. On the basis of the good correlation between the STEM images and the EDS maps, and also considering the structure of the Al₃(Mn, Pd) crystal, which was determined by X-ray diffraction using a single crystal, observed HAADF and ABF-STEM images of the Al–Mn–Pd DQC have been interpreted. Pd and Mn atomic positions in the Al–Mn–Pd DQC can be detected on the observed EDS maps. It can be seen that Pd is enriched around the centre of the columnar clusters, having a decagonal section with 2 nm in diameter. It can therefore be concluded that Pd plays an important role in the stabilization of the decagonal clusters, which form the Al–Mn–Pd DQC structure.     

The evolution of γ-Mg17Al12 intermetallic compound during accumulative back extrusion and subsequent ageing treatment

Accumulative back extrusion (ABE) processing, as a novel severe plastic deformation (SPD) method, has been recently justified to be capable of modifying the microstructural characteristics of alloys. In line to its ongoing researches, the present work has been planned to study the evolution of γ-Mg₁₇Al₁₂ intermetallic phase during ABE and subsequent ageing treatment in a high Al-bearing Mg–Al–Zn alloy. The behaviour of γ intermetallic has been systematically examined as following points of view: (i) strain–temperature-dependent morphology changes, (ii) strain-induced dissolution, and (iii) re-ageing behaviour as a function of time and temperature. Aiming to analyse the morphology of eutectic γ compound with respect to the strain and temperature, 2D projections of effective diameter, shape factor and globularity have been made in strain/temperature graphs. The processing conditions (strain and temperature) corresponding to the desired and undesired morphologies are introduced and microstructurally explained through underlying plasticity mechanisms, i.e., ‘necking-thinning-particle separation’ and ‘brittle fragmentation.’ The former mechanism is suggested to be in relation with partial strain-induced dissolution of eutectic γ phase, leading to generation of a supersaturated solid solution. This has resulted to the observation of ‘off-stoichiometry’ phenomena in Mg₁₇Al₁₂ phase and has been justified through dislocation-assisted deformation mechanism at elevated temperature. Surprisingly, a unique re-ageing behaviour has been found for the obtained solid solutions, where a modified kinetics and morphology of γ phase precipitation were characterized. The altered precipitation behaviour is attributed to the specific defect structure achieved by SPD acting as fast diffusion channel for Al solutes.     

Mg NMR site analysis in metals and intermetallics

Nuclear magnetic resonance (NMR) of the low abundance and low gyromagnetic ratio isotope ²⁵Mg, I=5/2, 2.606 MHz/T, 10% abundant, is shown here to provide an informative probe for phase identification, site symmetry and site multiplicity of the intermetallic compounds which occur as strengthening precipitate phases in lightweight alloys. The intermetallics discussed here, Mg₁₇Al₁₂, MgZn₂, Mg₂Al₃ and Al₂CuMg, are the final equilibrium precipitate phases in a number of Mg- and Al-based heat-treatable alloys. The ²⁵Mg spectra of Mg in Al–10 at%Mg alloy show the progressive precipitation of Mg₂Al₃ from Mg in solid solution as a function of annealing time at 150 °C. Also reported are ²⁵Mg spectra for CuMg₂, Mg₄₄Al₁₅Zn₄₁ and Mg₂Sn, along with the counter atom ⁶⁷Zn and ⁶³Cu NMR spectra for MgZn₂ and CuMg₂. All spectra are simulated to determine nuclear interaction parameters and confirm site occupancy.     

Hume-Rothery electron concentration rule across a whole solid solution range in a series of gamma-brasses in Cu–Zn,Cu–Cd,Cu–Al,Cu–Ga,Ni–Zn and Co–Zn alloy systems

The aim of the present work is to examine if the Hume-Rothery stabilisation mechanism holds across whole solid solution ranges in a series of gamma-brasses with especial attention to the role of vacancies introduced into the large unit cell. The concentration dependence of the number of atoms in the unit cell, N, for gamma-brasses in the Cu–Zn, Cu–Cd, Cu–Al, Cu–Ga, Ni–Zn and Co–Zn alloy systems was determined by measuring the density and lattice constants at room temperature. The number of itinerant electrons in the unit cell, e/uc, is evaluated by taking a product of N and the number of itinerant electrons per atom, e/a, for the transition metal element deduced earlier from the full-potential linearised augmented plane wave (FLAPW)-Fourier analysis. The results are discussed within the rigid-band model using as a host the density of states (DOS) derived earlier from the FLAPW band calculations for the stoichiometric gamma-brasses Cu₅Zn₈, Cu₉Al₄ and TM₂Zn₁₁ (TM = Co and Ni). A solid solution range of gamma-brasses in Cu–Zn, Cu–Cd, Cu–Al, Cu–Ga and Ni–Zn alloy systems is found to fall inside the existing pseudogap at the Fermi level. This is taken as confirmation of the validity of the Hume-Rothery stability mechanism for a whole solute concentration range of these gamma-brasses. An exception to this behaviour was found in the Co–Zn gamma-brasses, where orbital hybridisation effects are claimed to play a crucial role in stabilisation.     

Effects of Re microalloying on glass formation and mechanical properties of Zr–Cu–Al alloys

The effect of microalloying with rhenium on a metallic glass-forming alloy (Cu₄₆Zr₄₆Al₈)_{100? x} Re _x (x?=?1,?2) was investigated. Re possesses a positive enthalpy of mixing within the Cu–Re terminal system. Splat quenched foils of ≈40?µm in thickness display an amorphous structure. Their crystallisation temperature increases from T _x?=?504 to 513°C with addition of Re at nearly constant glass formation temperature T _g?=?445°C for the amorphous samples. In contrast, injection cast rods consist of B2-CuZr type phase dendrites, minor fractions of a cubic phase CuZrAl, and randomly distributed small particles of a Re-rich phase. This represents a novel concept in microalloying where Re-rich precipitates trigger the B2 phase formation. It leads to a unique combination of mechanical properties for as-cast rods, which display high strength at sizeable plastic deformation up to ε _p?≈?4% and an extended range of work-hardening prior to failure.     

Ab-initio study of long-period superstructures and anti-phase boundaries in Al-rich γ-TiAl (L10)-based alloys

In this work, we report first-principles investigation of structural stability of all experimentally observed ordered long-period superstructures (LPSs), viz., r-Al₂Ti, h-Al₂Ti, Al₅Ti₃ along with Al₅Ti₃′, Al₁₁Ti₇ and Al₃Ti₂ LPSs, which are observed only as short-range ordered clusters at nanoscale level in Al-rich TiAl-based alloys. We adopt a procedure based on space-filling tiling arrangement of ordered Ti₂Al, Ti₃Al, Ti₄Al motifs and their combination along with a symmetry analysis programme to determine the unit cell and the crystallographic information of Al₅Ti₃′, Al₁₁Ti₇ and Al₃Ti₂ LPSs in terms of L1₀ fcc unit cell. First-principles calculations are performed to further refine these crystallographic parameters (Wyckoff positions and lattice parameters) obtained from the above procedure. Moreover, it is found that the family of five LPSs have subgroup–supergroup relationships with γ-TiAl (Sp. gr. P4/mmm) and among themselves. Further, we find the inherent stability of r-Al₂Ti?+?γ-TiAl and 2Al₅Ti₃?+?γ-TiAl phase mixtures at 0?K compared to isomolecular Al₃Ti₂ and Al₁₁Ti₇ LPSs at their respective concentrations. The calculations of single-crystal elastic constants of Al₅Ti₃, Al₁₁Ti₇, Al₃Ti₂ and Al₅Ti₃′ LPSs show all these four structures are mechanically stable. We also calculate antiphase boundary (APB) formation energies for two types of APBs, viz., type-A and type-C in ordered Al₅Ti₃ LPS using the supercell approach. The relaxed APB energies for type-A and type-C APBs are 15.44 and 124.16?mJ/m², respectively.     

Structural and optical properties of MgxAl1-xHy gradient thin films: a combinatorial approach

The structural, optical and dc electrical properties of Mg_xAl_1-x (0.2≤x≤0.9) gradient thin films covered with Pd/Mg are investigated before and after exposure to hydrogen. We use hydrogenography, a novel high-throughput optical technique, to map simultaneously all the hydride forming compositions and the kinetics thereof in the gradient thin film. Metallic Mg in the Mg_xAl_1-x layer undergoes a metal-to-semiconductor transition and MgH₂ is formed for all Mg fractions x investigated. The presence of an amorphous Mg-Al phase in the thin film phase diagram enhances strongly the kinetics of hydrogenation. In the Al-rich part of the film, a complex H-induced segregation of MgH₂ and Al occurs. This uncommon large-scale segregation is evidenced by metal and hydrogen profiling using Rutherford backscattering spectrometry and resonant nuclear analysis based on the reaction ¹H(¹⁵N,αγ)¹²C. Besides MgH₂, an additional semiconducting phase is found by electrical conductivity measurements around an atomic [Al]/[Mg] ratio of 2 (x=0.33). This suggests that the film is partially transformed into Mg(AlH₄)₂ at around this composition. PACS 78.20.-e; 68.55.-a; 64.75.+g