The production of Al-Mg alloy/SiC metal matrix composites by pressureless infiltration

Metal matrix composites have been produced by pressureless infiltration of Al-Mg alloys into SiC preforms at 900°C under N₂ for different infiltration times. The wettability of the ceramic reinforcement by the Al-Mg alloy is crucial in determining whether an MMC can be produced by pressureless infiltration. Sessile drop results show that Al alloys with Mg contents greater than 8 wt% had a contact angle lower than 90°C after 5 minutes contact time. This was in agreement with the pressureless infiltration results as MMCs have been produced after 30 minutes with these alloys. Sessile drop experiments also show that SiC is similarly wetted by Al-Mg alloys under both N₂ and Ar. It is concluded that the infiltration process does not involve the intermediate nitride phase suggested by other authors.     

不同Mo含量对Al-Mg合金轧板微观组织和短时高温力学性能的影响

基于高强度耐火Al-Mg合金开发需求,设计并制备了6种Mo含量(质量分数)的Al-Mg合金,经变形热处理获得H3xx态轧板,结合光学显微镜、X射线衍射仪、拉伸试验机、带有能谱仪(EDS)的蔡司扫描电镜(SEM)等表征设备对各合金轧板微观组织和短时高温力学性能进行检测分析,揭示了微量Mo对Al-Mg合金的强韧化机理。结果表明:Mo合金化H3xx态Al-Mg合金具有较高的力学性能,这主要归功于与铝基体呈半共格关系的Al₁₂Mo弥散相起到的弥散强化效果和抑制再结晶作用,但过量的Mo易使Al-Mg合金形成较高Mg固溶度的难熔Al₁₂Mo结晶相,不利于合金性能提升。Al₁₂Mo弥散相具有一定的高温稳定性,高温状态下显著阻碍再结晶晶粒长大,进而提高Al-Mg合金高温性能。Mo含量为0.08%时的高温性能最佳,高温强度最大提升22.5%。随着Mo含量的增加,Al-Mg合金常温力学性能和短时高温力学性能都有所提高。     

Dynamic and metadynamic recrystallization in Al-Mg alloys

The investigation has been made of the microstructural changes which occur during the hot extrusion of single phase Al-Mg alloys containing 1.95, 3.76, 5.92 and 9.00% Mg. The microstructural changes during subsequent air cooling after the deformation have been observed. The billets were extruded at temperature of 415°C with a ram speed of 8.4 mm.s⁻¹ at an extrusion ratio of 4:1 followed by immediate water cooling and air cooling. Light microscopy has shown that the dynamic recrystallization is accelarated with the increase of Mg content. The significant metadynamic recrystallization takes place in atr cooling. Magnesium impedes dislocation mobility. The directional and straight dislocations appear in Al-5.92% Mg and Al-9.00% Mg alloys.     

Effect of magnesium on sliding wear performance of cast Al-8.3Fe-0.8V-0.9Si alloys

Abstract

The paper deals with the sliding wear behaviour of cast Al-Fe-V-Si alloys evaluated by 'pin on disc' tribometry. The alloys were castin cylindrical15 mm diameter metallic moulds from which 8 mmdiameter pins were machined. Volumetric wear loss and coefficient of friction were measured. Worn surfaces of the pins were examined by scanning electron microscopy. It was observed that by modifying the Al-Fe-V-Si alloys with Mg or Al-Mg master alloy the structure of Al₁₃Fe₄ precipitate changed from ten-armed star-like to hexagonal, rectangular, cuboidal, and other compact forms. The wear rate of the cast Al-Fe-V-Si alloys was much lower than that of eutectic Al-Si alloy. Modified Al-Fe-V-Si alloys exhibited lower wear rate and coefficient of friction than the unmodified Al-Fe-V-Si alloy. The wear rate decreased with increasing load for the modified Al-Fe-V-Si alloys. Both the wear rate and coefficient offriction were found to be load dependent. During wear extensive plastic deformation and work hardening occurred. The wear was shown to take place by delamination.     

Microstructural evaluation of dross formation on Mg- and non-Mg-containing Al alloys from industrial furnaces

Abstract

The microstructural aspects of dross formation in industrial Al re-melt furnaces for several different alloy compositions have been evaluated. The Al alloys used in this study included 1350 (no Mg), 3004 (low Mg), and 5182 (high Mg). Dross specimens were collected directly from the different Al alloy melts in industrial furnaces using a consistent sampling protocol in order to compare the microstructure and phase development of the dross as a function of melt temperature and composition. The results showed that the sequence of phases formed during the re-melt process was the same for all the alloys examined; amorphous-Al₂O₃ forms first followed by either α-Al₂O₃+AlN (for non-Mg-containing alloys) or cubic MgO, then MgAl₂O₄, and lastly α-Al₂O₃ (for low- or high-Mg content). The formation of MgAl₂O₄ is associated with accelerated oxidation rates (known as breakaway oxidation) and this reaction proceeds until the Mg is depleted at the molten surface. At this point, aluminum oxidation is predominant and occurs at a significantly lower oxidation rate. The results obtained in this study are consistent with models developed for dross grown on similar Al alloys in laboratory environments and show that Mg oxidation (and the accelerated formation of MgAl₂O₄) dominates the oxidation process during Al melting, whether the Al contains low or high Mg contents. The oxide morphology within the dross layer differed according to the particular alloy being melted and thus the amount of Al recovery from dross can vary with composition.     

Al-Mg合金的晶界内耗

研究了退火、淬火状态下 Al-Mg 合金的晶界内耗。实验表明,随 Mg 含量的增加,晶界内耗峰的峰宽和弛豫强度单调下降,峰位开始移向高温,后又移向低温;高温淬火处理能压低 Al-2.4wt-%Mg的晶界峰,但对其它低 Mg 含量试样不敏感,一定温度的保温能使压低的晶界峰回升。     

Ca-modified Al–Mg–Sc alloy with high strength at elevated temperatures due to a hierarchical microstructure

Al-Mg alloys are normally prone to lose part of their yield and tensile strength at high temperatures due to insufficient thermal stability of the microstructure. Here, we present a Ca-modified Al–Mg–Sc alloy demonstrating high strength at elevated temperatures. The microstructure contains Al₄Ca phases distributed as a network along the grain boundary and Al₃(Sc,Zr) nano-particles dispersed within the grains. The microstructure evolution and age-hardening analysis indicate that the combination of an Al₄Ca network and Sc-rich nano-particles leads to excellent thermal stability even upon aging at 300 °C. The tensile strength of the alloy for temperatures up to 250 °C is significantly improved by an aging treatment and is comparable with the commercial heat-resistant aluminum alloys, i.e., A356 and A319. At a high temperature of 300 °C, the tensile strength is superior to the above-mentioned commercial alloys, even more so when expressed as the specific strength due to the low density of Ca-modified Al–Mg–Sc alloy. The excellent high-temperature strength results from a synergistic effect of solid solution strengthening, grain boundary strengthening and nanoparticle order strengthening.

     

High-temperature oxidation of Fe-Cr alloys in air

The scales formed on the Fe-Cr (19, 29.5, and 46.8 at % Cr) and Fe₇₀Cr₁₅B₁₅ alloys in the initial stages of oxidation in air at 1470 K have been analyzed using depth profiling. The results demonstrate that the Cr/(Fe + Cr) ratio in the scales on the Fe-Cr alloys is close to 30 at %, independent of the bulk chromium concentration. The top scale layers on the Fe-Cr alloys consist predominantly of M₂O₃ (M = Cr, Fe) solid solutions and CrO₂, the intermediate layers are composed of (Fe, Cr)₃O₄ and FeO, and the internal oxidation and nitridation layers consist of chromium oxides and nitrides. The top scale layers on the oxidized Fe₇₀Cr₁₅B₁₅ alloy contain, in addition, boron oxides, and the internal nitridation product is BN instead of chromium nitrides.     

Influence of annealing on the mechanical properties of compocast Al(Mg)-Al2O3 particulate composite

The annealing of cast Al(Mg)-Al₂O₃ particulate composite at temperatures of 673, 723 and 773 K for varying periods of time from 1–5 h resulted in reduction of strength and hardness similar to that occurring in cast Al-Mg alloy, due to homogenization. However, the ductility of the composite increased on annealing, contrary to that observed in cast Al-Mg alloy, presumably because the solution of the eutectic in the composite causes magnesium diffusion to the alumina particles resulting in further reaction to form magnesium aluminate and does not increase the magnesium level of its primary phase, as occurs in Al-Mg alloy.     

Kinetics of directed oxidation of Al-Mg alloys in the initial and final stages of synthesis of Al2O3/Al composites

In the directed oxidation of Al-Mg alloys, MgO forms in the initial stage. The mechanism of formation of MgO from the Al-Mg alloy in the initial stage of oxidation was studied. The variables studied were the total pressure in the reaction chamber and partial pressure of oxygen. The oxidation rate in the initial stage was proportional to both the oxygen partial pressure and oxygen diffusivity. These results suggest that MgO forms by reaction-enhanced vaporization of Mg from the alloy followed by oxidation of the Mg vapour in the gas phase. The end of the initial stage corresponds to the arrival of the oxygen front close to the melt surface, when spinel formation occurs.

The kinetics of formation of Al₂O₃ in the growth stage of directed oxidation of the Al-5wt.% Mg alloy was also investigated as a function of time, temperature and oxygen partial pressure. The growth rate decreased as a function of time, was practically independent of oxygen pressure and exhibited an activation energy of 361 kJ mol⁻¹. In the growth stage, the kinetics of oxidation is controlled by the rate of transport of oxygen through the alloy layer near the surface to the alumina-alloy interface.     

Corrosion resistance of in-situ growth of nano-sized Mg(OH)2 on micro-arc oxidized magnesium alloy AZ31—Influence of EDTA

One of the major obstacles for the clinical use of biodegradable magnesium (Mg)-based materials is their high corrosion rate. Micro-arc oxidation (MAO) coatings on Mg alloys provide mild corrosion protection owing to their porous structure. Hence, in this study a dense Mg(OH)₂ film was fabricated on MAO-coated Mg alloy AZ31 in an alkaline electrolyte containing ethylenediamine tetraacetic acid disodium (EDTA-2Na) to reinforce the protection. Surface morphology, chemical composition and growth process of the MAO/Mg(OH)₂ hybrid coating were examined using field-emission scanning electron microscopy, energy dispersive X-ray spectrometer, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectrophotometer. Corrosion resistance of the coatings was evaluated via potentiodynamic polarization curves and hydrogen evolution tests. Results manifested that the Mg(OH)₂ coating possesses a porous nano-sized structure and completely seals the micro-pores and micro-cracks of the MAO coating. The intermetallic compound of AlMn phase in the substrate plays a key role in the growth of Mg(OH)₂ film. The current density of Mg(OH)₂-MAO composite coating decreases three orders of magnitude in comparison with that of bare substrate, indicating excellent corrosion resistance. The Mg(OH)₂-MAO composite coating is beneficial to the formation of calcium phosphate corrosion products on the surface of Mg alloy AZ31, demonstrating a great promise for orthopaedic applications.     

Optimizing the tensile properties of Al–Si–Cu–Mg 319-type alloys: Role of solution heat treatment

The work presented in this study was carried out on Al–Si–Cu–Mg 319-type alloys to investigate the role of solution heat treatment on the dissolution of copper-containing phases (CuAl₂ and Al₅Mg₈Cu₂Si₆) in 319-type alloys containing different Mg levels, to determine the optimum solution heat treatment with respect to the occurrence of incipient melting, in relation to the alloy properties. Two series of alloys were investigated: a series of experimental Al–7 wt% Si–3.5 wt% Cu alloys containing 0, 0.3, and 0.6 wt% Mg levels. The second series was based on industrial B319 alloy. The present results show that optimum combination of Mg and Sr in this study is 0.3 wt% Mg with 150 ppm Sr, viz. for the Y4S alloy. The corresponding tensile properties in the as-cast condition are 260 MPa (YS), 326 MPa (UTS), and 1.50% (%El), compared to 145 MPa (YS), 232 MPa (UTS), and 2.4% (%El) for the base alloy with no Mg. At 520 °C solution temperature, incipient melting of Al₅Mg₈Cu₂Si₆ phase and undissolved block-like Al₂Cu takes place. At the same time, the Si particles become rounder. Therefore, the tensile properties of Mg-containing alloys are controlled by the combined effects of dissolution of Al₂Cu, incipient melting of Al₅Mg₈Cu₂Si₆ phase and Al₂Cu phase, as well as the Si particle characteristics.     

Development of microstructure in cast Mg–AI–rare earth alloys

Abstract

The microstructures and age hardening behaviours of a series of Mg–Al–rare earth (RE) alloys that had been either pressure die cast or permanent mould cast were investigated by SEM and analytical TEM. Two types of phases, Al₄MM and Al₁₂Mg₁₇, were found in the as cast alloys and no pseudoternary Mg–Al–RE phases were present. The Al₄MM phase was thermally stable during solution treatment at temperatures as high as 500°C, whereas Al₁₂Mg₁₇ partially dissolved in the α-Mg matrix during solution treatment at 420°C. No rare earth containing precipitates formed during heat treatment of the investigated alloys but two types of Al₁₂Mg₁₇ precipitation took place. Colonies of discontinuous precipitation containing alternate lamellae of α-Mg and Al₁₂Mg₁₇ formed preferentially in regions α-Mg with high aluminium content. Spheroidisation and coarsening of the discontinuous precipitates occurred after aging at 200°C. Continuous precipitation of Al₁₂Mg₁₇ also occurred and these precipitates had a rodlike morphology and grew in preferred crystallographic directions.

MST/3382     

Internal oxidation of Fe-Si alloys in γ-phase region

Internal oxidation measurements of Fe-0.070, 0.219, 0.483, and 0.920 wt % Si alloys were made in the γ-phase region in order to discuss kinetics of internal oxidation, to evaluate the diffusion coefficient of oxygen in the internal oxidation layer, and to determine the diffusion coefficient of oxygen in γ-iron. Internal oxidation of these alloys was conducted at temperatures between 1223 and 1323 K using a powder mixture of iron and Fe₂O₃. The internal oxidation front in Fe-Si alloys with between 0.070 and 0.483 wt % Si advances in parallel to the specimen surface. The internal oxidation in these alloys obeys a parabolic rate law, which indicates that the internal oxidation is controlled by an oxygen diffusion process in the alloy. The diffusion coefficient of oxygen, D _O ^IO , in the internal oxidation layer where SiO₂ particles disperse was determined by using the thermodynamic data for the solution of oxygen in γ-iron. D _O ^IO increases with the increase of the volume fraction of the oxide, f ^IO, in the oxidation layer at a given temperature. The diffusion coefficient of oxygen, D _O, in γ-iron was evaluated by extrapolating D _O ^IO to f ^IO=0. D _O may be given by the following equation: $$D_O = \left( {6.42\begin{array}{*{20}c} { + 4.37} \\ { - 2.60} \\ \end{array} } \right) \times 10^{ - 5} exp \left[ { - \frac{{159 \pm 5(kj mol^{ - 1} )}}{{RT}}} \right]m^2 \sec ^{ - 1} $$ .     

Effect of microalloying with silicon on high temperature oxidation resistance of novel refractory high-entropy alloy Ta-Mo-Cr-Ti-Al

Abstract

The effect of 1 at.% Si addition to the refractory high-entropy alloy (HEA) Ta–Mo–Cr–Ti–Al on the high temperature oxidation resistance in air between 900 °C and 1100 °C was studied. Due to the formation of protective chromia-rich and alumina scales, the thermogravimetric curves for Ta–Mo–Cr–Ti–Al and Ta–Mo–Cr–Ti–Al–1Si showed small mass changes and low oxidation rates which are on the level of chromia-forming alloys. The oxide scales formed on both alloys at all temperatures are complex and consist of outermost TiO₂, intermediate Al₂O₃, and (Cr, Ta, Ti)-rich oxide at the interface oxide/substrate. The Si addition had a slightly detrimental effect on the oxidation resistance at all temperatures primarily as a result of increased internal corrosion attack observed in the Si-containing HEA. Large Laves phase particles distinctly found in the Si-containing alloy were identified to be responsible for the more rapid internal corrosion.     

Internal oxidation in gold alloys containing small amounts of Fe and Sn

Internal oxidation was observed in gold-rich alloys as substrates for porcelain veneers in dental restorations, which contain small amounts of Fe and Sn. The internal oxidation proceeded with oxygen ions diffusing to the inner part of the alloy through Fe₂O₃ formed at the grain boundaries of the alloy matrix. SnO₂ was formed internally together with the Fe₂O₃. The external oxidation zone was composed of only Fe₂O₃ in a wide range of Fe and Sn concentrations. Fe₃O₄ was formed with Fe₂O₃ in the Sn-rich composition range by reduction of Fe₂O₃ in the presence of Sn. A band mainly composed of SnO₂ was formed at the inside of the internal oxidation zone in the composition range where Fe₃O₄ formed. In the Sn-rich alloys this internal oxidation band of SnO₂ moved to the external oxidation zone.     

Understanding formation of Mg-depletion zones in Al-Mg alloys under high pressure torsion

Redistribution of elements may take place in alloys during severe plastic deformation, which significantly alters the mechanical properties of the alloys. Therefore, comprehensive knowledge about deformationinduced redistribution of elements has to be established. In the present paper, the distribution of Mg in an Al-Mg alloy processed by high pressure torsion was examined using atom probe tomography(APT).With crystallographic information extracted by APT data analysis, this research reveals that the movement of dislocations plays an important role in the formation of Mg-depletion zones in the deformed microstructure.     

Interfacial reactions in the liquid diffusion couples of Mg/Ni,Al/Ni and Al/(Ni)-Al2O3 systems

The interfacial reactions between various molten metals and solid plates were investigated in this diffusion couple study. The molten metals were pure magnesium, pure aluminium, aluminium-rich Al-Mg alloy, and aluminium-rich Al-Cu alloys, and the solid plates were pure nickel plate, alumina plate, and nickel-plated alumina plate. The interfacial reactions in the diffusion couples were determined by using optical microscopy, scanning electron microscopy and electron probe microanalysis in regard to the formation of intermetallic phases, the dissolution rates of the nickel plates, and the morphology of the interfaces. Mg₂Ni phase was found in the pure Mg/Ni plate diffusion couples, and the Al₃Ni and Al₃Ni₂ phases were observed in the pure Al/Ni plate and Al-alloys/Ni plate diffusion couples. In the Al-Cu alloy/Ni-plated alumina plate diffusion couple, Al₂O₃ formed at the interface, while spinel particles were found in the diffusion couples of Al-7.4wt% Mg alloy/Ni-plated alumina plate. Experimental difficulty was encountered in preparing the diffusion couples with alumina plate, and a gap existing at the interface prohibited reactions between the molten metal with alumina plate.     

Effect of additions of Mn,Ce, Nd,and Si on rate of dissolution of splat quenched Mg–Al and Mg–Zn alloys in 3%NaCl solution

Abstract

Weight loss resulting from immersion in 3%NaCl solution buffered with Mg(OH)₂ has been determined for Mg alloy splats containing different levels of alloying with Zn or Al with and without ternary additions of Mn, Ce, Nd, and Si. The results show that additions of Zn increase the rate of dissolution, whereas an addition of 15 wt-%Al significantly reduces the dissolution rate. Ternary additions of Mn significantly reduce the dissolution rate of both Mg–Zn and Mg–Al alloys, whereas Ce and Nd reduce the dissolution rate of Mg–Al alloys. Additions of Si to Mg–Zn slightly increase the dissolution rate. These results are discussed in the context of other reported observations on the corrosion behaviour of rapidly solidified Mg alloys.

MST/I098