<Emphasis Type="Italic">In-situ</Emphasis> formation of SiC-reinforced Al-Si alloy composites using methane gas mixtures

Theoretical and experimental studies on the in-situ formation of an Al-Si alloy composite using a methane gas mixture were carried out. An Al-Si alloy composite with in-situ formed SiC as a reinforced phase was produced by bubbling methane gas at temperatures from 1223 to 1423 K. An optical microscope, scanning electron microscope (SEM), and electron microprobe were used for the product characterization. Primary and eutectic silicon were observed in the samples taken from the top part of the crucible, and only eutectic silicon was observed in the samples taken from the bottom of crucible. The SiC formation rate increased with the decrease in the bubble size. A silicon concentration gradient existed at different vertical positions of the liquid alloy. The silicon concentration close to the top of the liquid alloy was higher than that at the bottom. The SiC concentration was very low in the bulk alloy. The bubbling of the gas mixture in the melt resulted in the formation of a layer of foam on top of the crucible. Formed SiC particles were enriched in the foam and carried out of the crucible by the overflow foam to a composite collector located under the crucible. The foam in the composite collector was broken, and composites in the foam contained up to 30 wt pct SiC. The particle size of the SiC is in the range of 1 to 10 μm. The bubbling process resulted in the unevenness of the silicon concentration and the different crystallizing processes. The SiC formation rate was found to be about 12.5 mg/(L·s). A kinetic model was developed. The model-predicted results are in very good agreement with the experimental results.     

Processing,microstructure, and mechanical properties of cast <Emphasis Type="Italic">in-Situ</Emphasis> Al(Mg,Mn)-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>(MnO<Subscript>2</Subscript>) composite

Cast particulate composites, containing in-situ generated reinforcing particles of alumina, have been developed by solidification of slurry obtained by dispersion of externally added manganese dioxide particles (MnO₂) in molten aluminum, and alumina is formed by reaction of manganese dioxide with molten aluminum. The chemical reaction also releases manganese into molten aluminum. Magnesium is added to the melt in order to help wetting of alumina particles by molten aluminum and to retain the particles inside the melt. The present work aims to understand the influence of key parameters such as processing temperature, time, and the amount of MnO₂ particles added on the microstructure and mechanical properties of the resulting cast in-situ composites. The sequence of addition of MnO₂ particles and magnesium has significant influence on the microstructure and mechanical properties. Increasing processing temperature and time increases the extent of reduction of MnO₂ particles, generating more alumina particles as well as releasing more manganese to the matrix alloy. Alumina helps to nucleate finer and sometimes blocky MnAl₆ in the matrix of the composite and thereby results in relatively higher ductility and increased strength in the composite as compared to the base alloy of similar composition. Even in the presence of relatively higher porosity of 8 to 9 vol pct, one observes a percent elongation not below 7 to 8 pct, which is considerably higher than those observed in cast Al(Mg)-Al₂O₃ composite synthesized by externally added alumina particles.     

Study on electromagnetic force for preparation of in-situ Al/Mg2Si functionally graded materials by electromagnetic separation method

The aim of this article is to investigate the effects of electromagnetic force on primary particle distribution of in-situ Al/Mg₂Si functionally graded materials (FGMs) by electromagnetic separation method. Experimental results show that there is a critical value of electromagnetic force. The FGMs can be produced only when the electromagnetic force is beyond the critical value. With increasing the electromagnetic force, the particle volume fraction of the particle-packed regions increases, the length of the particle-packed regions decreases, the average gradient of particle volume fraction increases, and the primary particle size become smaller.     

Comparison of the Microstructure, Tensile, and Creep Behavior for Ti-22Al-26Nb (At. Pct) and Ti-22Al-26Nb-5B (At. Pct)

The effect of the addition of 5 at. pct boron on the microstructure and creep behavior of a nominally Ti-22Al-26Nb (at. pct) alloy was investigated. The boron-modified alloy contained boride needles enriched in titanium and niobium, and because to these borides, this material was considered to be a discontinuously reinforced metal matrix composite. These needle-shaped borides made up to 2 pct of the volume and were up to 158-μm long and 22-μm wide. The effect of boron on the mechanical properties was evaluated through in-situ creep testing and tensile testing at room temperature (RT) and 650 °C. Overall, the addition of 5 at. pct boron proved to be detrimental to the tensile and creep behavior. The composite exhibited a brittle failure and lower elongations-to-failure than the monolithic material. The in-situ tensile and creep experiments revealed that the deformation process initiated in the boride needles, which cracked extensively, and significantly greater primary creep strains and creep rates were exhibited by the composite.

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Intermetallic-reinforced light-metal matrix in-situ composites

Transition-metal trialuminide intermetallics such as Al₃Zr and Al₃Ti, having low densities and high elastic moduli, are good candidates for the in-situ reinforcement of light-metal matrices based on Al and Mg alloys. In this work, in-situ composites based on Al and Al-Mg matrices reinforced with an Al₃Zr intermetallic were successfully processed by conventional ingot metallurgy. The microstructural studies showed that “needle” or “feathery”-like particles of Al₃Zr phase, whose volume fraction increased with increasing concentration of Zr, were formed in the Al matrix in the investigated range of Zr contents from 0.9 to 11.6 at. pct. Properties of Al-Zr alloys were investigated as a function of volume fraction of Al₃Zr. It is shown that the density, hardness, and yield strength of the in-situ Al/Al₃Zr composites can be quite adequately described by the composite rule-of-mixtures (ROM) behavior. Alloying of a binary Al-2.4 at. pct Zr alloy with Mg up to ∼25 at. pct reduces profoundly its density and, additionally, strengthens the matrix by a Mg solid-solution strengthening mechanism.     

Improving the Mechanical Properties of Fe-Nb-(Ni-Mn) Dendrite-Ultrafine Eutectic Composites via Controlling the Primary Phase Features

Tuning of microstructure by addition of austenite stabilizers effectively enhances the mechanical properties in Fe-Nb-(Ni-Mn) dendrite-ultrafine eutectic composites. The Fe₉₃Nb₇ alloy displays the improved plasticity up to 10?pct due to the introduction of a ductile ??-Fe dendrite into the ultrafine eutectic matrix. Meanwhile, the Fe₇₈Nb₇Ni₁₀Mn₅ alloy, which forms an in-situ martensitic ????-Fe dendritic phase reinforced ultrafine eutectic composite exhibits excellent combination of a high fracture strength of 1.6?GPa and a large plastic strain of 11?pct. The investigations reveal that the characteristics of the modulated primary dendrites in the dendrite-ultrafine eutectic composites play an important role in manipulating the generation and propagation of shear bands, thus resulting in the improved mechanical properties and plastic deformation behavior.     

The atomic-structure changes in Al-16 pct Si alloy above the liquidus

The atomic-structure changes in an Al-16 pct Si alloy above the liquidus have been studied by a θ-θ high-temperature X-ray diffractometer, rapid solidification, a vertical centrifugal casting apparatus, and differential scanning calorimetry (DSC) measurements. It was found that the diffraction intensity and structure-factor (S(Q)) curves for an Al-16 pct Si alloy have small prepeaks at small Q values when the temperature is high enough. Rapid-solidification and centrifugal-casting experimental results show that the primary silicon phase can easily coarsen and segregate under additive force after an overheat at high temperatures. The DSC measurements show that the temperature and latent heat of primary solidification rise with the temperature of overheating. These experimental results suggest that a strong interaction occurs between Si-Si atoms in a liquid Al-16 pct Si alloy at high temperatures, resulting in the microsegregation of Si atoms in the melt.     

Effect of the primary phase on grain coarsening in undercooled Fe-Co alloys

Fe-Co alloy melts with Co contents of 10, 30, and 60 at. pct were undercooled to investigate the dependence of the primary phase on grain coarsening. A pronounced characteristic is that the metastable fcc phase in the Fe-10 at. pct Co alloy and the metastable bcc phase in the Fe-30 at. pct Co alloy will primarily nucleate when undercoolings of the melts are larger than the critical undercoolings for the formation of metastable phases in both alloys. No metastable bcc phase can be observed in the Fe-60 at. pct Co alloy, even when solidified at the maximum undercooling of ΔT = 312 K. Microstructural investigation shows that the grain size in Fe-10 and Fe-30 at. pct Co alloys increases with undercoolings when the undercoolings of the melts exceed the critical undercoolings. The grain size of the Fe-60 at. pct Co alloy solidified in the undercooling range of 30 to 312 K, in which no metastable phase can be produced, is much finer than those of the Fe-10 and Fe-30 at. pct Co alloys after the formation of metastable phases. The model for breakage of the primary metastable dendrite at the solid-liquid interface during recalescence and remelting of dendrite cores is suggested on the basis of microstructures observed in the Fe-10 and Fe-30 at. pct Co alloys. The grain coarsening after the formation of metastable phases is analyzed, indicating that the different crystal structures present after the crystallization of the primary phase may play a significant role in determining the final grain size in the undercooled Fe-Co melts.     

Microstructure/processing relationships in reaction-synthesized titanium aluminide intermetallic matrix composites

Near-γ TiAl- and Al₃Ti-based intermetallic matrix composites have been produced using in-situ reaction-synthesis techniques. The intermetallic matrices have been reinforced with relatively high loadings (e.g., 20 to 50 vol pct) of dispersed TiB₂ particulates. It is shown that the as-synthesized TiB₂ size is strongly dependent on the specific alloy formulation; specifically, the TiB₂ size tends to increase as the nominal volume percent of TiB₂ in the composite increases. The observed size effect is determined to be associated with the temperature that is attained during the synthesis event, which is established primarily by the net exothermicity of the participating synthesis reaction(s). The exothermicity of the reactions can be assessed through the calculation of a formulations’s adiabatic temperature, which is found to increase with the percentage of TiB₂ over the range of approximately 10 to 60 vol pct. The coupling of a composite’s characteristic adiabatic temperature with the resulting reinforcement size provides direct links among composition, processing, and mechanical performance, since the size of a reinforcing particle is influential in establishing the interparticle spacing, which, in turn, establishes the strengthening potency of the dispersed phase within the composite.     

Studies on Synthesis and Characterization of Mo Based <Emphasis Type="Italic">In Situ</Emphasis> Composite by Silicothermy Co-reduction Process

The results of an in situ synthesis of refractory metal–intermetallic composite (RMIC), Mo-16Cr-4Si (wt pct) multiphase alloy and its characterization, are presented in this study. The alloy was prepared from the oxides of molybdenum and chromium by their co-reduction with Si metal powder as a reductant. The exothermic nature of these reactions resulted in the formation of consolidated composite as a product in a single step. The thermodynamic aspects of exothermic reactions were studied by thermogravimetry/differential thermal analyzer. As-reduced alloys were remelted by arc melting and heat treated to obtain a homogenous microstructure. The evolution of phases and microstructures qA studied by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectrum analysis. The multiphase alloy consisted of Mo₃Si and discontinuous (Mo, Cr) (ss) phase with a volume percentage of 28 pct. The synthesized alloys were characterized with respect to composition, phases, microstructure, hardness, and oxidation behavior.     

Phase relationships in the Fe-Cr-Ni system at solidification temperatures

The phase relationships between the liquid phase and the primary solid phases were investigated in the iron-rich corner of the Fe-Cr-Ni system as part of a larger study of the Fe-Cr-Ni-C system. The investigation consisted of measurements and modeling of tie-lines and the liquidus surfaces of the liquid-delta (bcc) and liquid-gamma (fcc) equilibria and the peritectic surface involving all three phases in the iron-rich corner of the Gibbs triangle bounded by 0 to 25 wt pct Cr and 0 to 25 wt pct Ni (bal Fe). The temperature ranged from the melting point of iron (1811 K) to about 1750 K. Compositions for tie-lines were obtained from liquid-solid equilibrium couples and temperatures for the surfaces were obtained by differential thermal analysis. Parameters for modeling the system were then selected in the subregular solution model to minimize the square of the difference between experimental and calculated tie-lines. With one ternary parameter employed for each phase, calculations by the model are in excellent agreement with the tie-line and liquidus measurements and in fair agreement with the temperatures for the peritectic surfaceL + δ/L + δ + γ. The usefulness of the model is demonstrated by calculation of the solidification paths of selected alloys in the composition field investigated for the limiting cases of (a) complete equilibrium followed by the alloy system, and (b) no solid diffusion (i.e., segregation) with equilibrium maintained at the solidifying front and complete mixing in the liquid phase.     

Microstructure and sliding-wear behavior of tungsten-reinforced W-Ni-Si metal-silicide <Emphasis Type="Italic">in-situ</Emphasis> composites

W-Ni-Si metal-silicide-matrix in-situ composites reinforced by tungsten primary grains were fabricated using a water-cooled copper-mold laser-melting furnace by the LASMELT process. Main constitutional phases of the W/W-Ni-Si in-situ composites are the tungsten primary phase, peritectic W₂Ni₃Si, and the remaining W₂Ni₃Si/Ni₃₁Si₁₂ eutectics, depending on the alloy compositions. The sliding-wear resistance of the W/W-Ni-Si intermetallic composites was evaluated at room temperature and 600 °C. Wear mechanisms of the W/W₂Ni₃Si in-situ composites were discussed based on morphology observations of the worn surface and wear debris. Results show that the W/W-Ni-Si composites have excellent wear resistance under both room- and high-temperature sliding-wear-test conditions, because of the high yield strength and toughness of the tungsten-reinforcing phase and the high hardness and the covalent-dominated intermetallic atomic bonds of the W₂Ni₃Si and Ni₃₁Si₁₂ metal silicides. Tungsten-reinforcing grains played the dominant role in resisting abrasive-wear attacks of microcutting, plowing, and brittle spalling during the sliding-wear process, while the W₂Ni₃Si and Ni₃₁Si₁₂ metal silicides are responsible for the excellent adhesive wear resistance.     

Sliding wear behavior of some Al-Si alloys: Role of shape and size of Si particles and test conditions

In this investigation, effects of the shape and size of silicon particles have been studied on the sliding wear response of two Al-Si alloys, namely, LM13 and LM29. The LM13 alloy comprised 11.70 pct Si, 1.02 pct Cu, 1.50 pct Ni, 1.08 pct Mg, 0.70 pct Fe, 0.80 pct Mn, and remainder Al. The LM29 alloy contained 23.25 pct Si, 0.80 pct Cu, 1.10 pct Ni, 1.21 pct Mg, 0.71 pct Fe, 0.61 pct Mn, and remainder Al. Wear tests were conducted under the conditions of varying sliding speed and applied pressure. The alloys were also characterized for their microstructural features and mechanical properties. The presence of primary silicon particles in the alloy led to a higher hardness but lower tensile properties. Further, refinement in the size of the primary particles improved the mechanical properties of the alloy system. The wear behavior of the alloys was influenced by the presence of primary Si particles and was a function of their size. Samples with refined but identical microconstituents (e.g., pressure cast vs gravity cast LM29 in terms of the size of primary Si particles and dendritic arm spacing) exhibited better wear characteristics. Their overall effect was further controlled by the test conditions. It was observed that test conditions leading to the generation of an optimal degree of frictional heating offer the best wear resistance. This was attributed to the reduced microcracking tendency of the alloy system otherwise introduced by the Si particles. The reduced microcracking tendency in turn allows the Si phase to carry load more effectively and impart better thermal stability to the alloy system. This caused improved wear resistance under the circumstances. Further, the primary Si particles improved the wear resistance of the alloy system (e.g., gravity-cast LM29 vs gravity-cast LM13) under high operating temperature conditions. Additional thermal stability and protection offered to the matrix by the primary Si phase, under the conditions of reduced microcracking tendency, were the reasons for the improved wear characteristics of the alloy system. Conversely, a reverse effect was produced at low operating temperatures in view of the predominating microcracking tendency. The study suggests that shape, size, microcracking tendency, and thermal stability of different microconstituents greatly control the mechanical and tribological properties of these alloys. The extent of effective load transfer between the phases plays an important role in this regard. Further, the overall effect of these factors is significantly governed by the test conditions.     

Characterization and tribological behavior of cast In-Situ Al (Mg,Mo)-Al2O3 (MoO3) composite

Aluminum alloy—based cast in-situ composite has been synthesized by dispersion of externally added molybdenum trioxide particles (MoO₃) in molten aluminum at the processing temperature of 850 °C. During processing, the displacement reaction between molten aluminum and MoO₃ particles results in formation of alumina particles in situ and also releases molybdenum into molten aluminum. A part of this molybdenum forms solid solution with aluminum and the remaining part reacts with aluminum to form intermetallic phase Mo(Al_1−x Fe _x )₁₂ of different morphologies. Magnesium (Mg) is added to the melt in order to help wetting of alumina particles generated in situ, by oxidation of molten aluminum by molybdenum trioxide, and helps to retain these particles inside the melt. The mechanical properties of the cast in-situ composite, as indicated by ultimate tensile stress, yield stress, percentage elongation, and hardness, are relatively higher than those observed either in cast commercial aluminum or in cast Al-Mo alloy. The wear and friction of the resulting cast in-situ Al(Mg,Mo)-Al₂O₃(MoO₃) composites have been investigated using a pin-on-disc wear testing machine under dry sliding conditions at different normal loads of 9.8N, 14.7N, 19.6N, 24.5N, 29.4N, 34.3N, and 39.2 N and a constant sliding speed of 1.05 m/s. The results of the current investigation indicate that the cumulative volume loss and wear rate of cast in-situ composites are significantly lower than those observed either in cast commercial aluminum or in cast Al-Mo alloy, under similar load and sliding conditions. Beyond about 30 to 35 N loads, there appears to be a higher rate of increase in the wear rate in the cast in-situ composite as well as in cast commercial aluminum and cast Al-Mo alloy. For a given normal load, the coefficient of friction of cast in-situ composite is significantly lower than those observed either in cast commercial aluminum or in cast Al-Mo alloy. The coefficient of friction of cast in-situ composite increases gradually with increasing normal load while those observed in cast commercial aluminum or in cast Al-Mo alloy remain more or less the same. Beyond a critical normal load of about 30 to 35 N, the coefficient of friction decreases with increasing normal load in all the three materials.     

Processing, microstructure, and mechanical properties of cast In-Situ Al(Mg, Ti)-Al2O3(TiO2) composite

In-situ particle-reinforced aluminum alloy-based cast composites have been synthesized by solidification of the slurry obtained by dispersion of externally added titanium dioxide (TiO₂) particles in molten aluminum at different processing temperatures. Alumina particles (Al₂O₃) form in situ through chemical reaction of TiO₂ particles with molten aluminum. Simultaneously, the chemical reaction also releases titanium, which dissolves into molten aluminum and results in the formation of intermetallic phase Ti(Al_1−x ,Fe _x )₃ during solidification. Increasing the processing temperature increases (1) the amount of elongated as well as blocky intermetallic phase Ti(Al_1−x ,Fe _x )₃, (2) the proportion of alumina particles in the reinforcing oxides, and (3) the porosity content in the resulting cast in-situ composite. The difference in particle content and porosity between the top and the bottom of the cast ingot increases with increasing processing temperature. The hardness of the cast in-situ composite is significantly more than that of the matrix alloy due to the presence of reinforcing particles, but the hardness is greatly impaired by the presence of porosity at the top of the cast ingot. The percent elongation of the cast in-situ composite decreases with increasing processing temperature possibly due to increasing porosity as well as an increasing amount of elongated intermetallic phase, which affects the percent elongation of the matrix alloy. The tensile and yield stresses of the cast in-situ composite decreases with increasing processing temperature again due to increasing porosity, which affects the ultimate tensile stress more than the yield stress. In the cast in-situ composite containing 3.31 ± 0.77 vol pct of porosity, the Brinell hardness is about 6 times its yield stress. The estimated yield stress of the cast in-situ composite at zero porosity as given by the linear least-squares fit appears to increase with particle content at a significantly higher rate than that predicted by the shear-lag model.     

Modification of Cast Al-Mg<Subscript>2</Subscript>Si Metal Matrix Composite by Li

The effects of both Li modification and cooling rate on the microstructure and tensile properties of an in-situ prepared Al-15 pct Mg₂Si composite were investigated. Adding 0.3 pct Li reduced the average size of Mg₂Si primary particles from ~30 to ~6 μm. The effect of cooling rate was investigated by the use of a mold with different section thicknesses from 3 to 9 mm. The results show a refinement of primary particle size as a result of both Li additions and cooling rate increases, and their effects are additive. Similarly, both effects increased ultimate tensile stress (UTS) and elongation values. The thin sections show somewhat unexpectedly low and scattered tensile results attributed to the casting defects observed in fracture surfaces. The Li-modified alloy displays serrated yielding behavior that is not fully explained here. The refinement by Li and enhanced cooling rate is explained in terms of an analogy with the effect of Sr and cooling rate in Al-Si alloys, and is ultimately attributed to the effect of the alkali and alkaline earth metals deactivating oxide double films (bifilms) suspended in Al melts as favored substrates for intermetallics.     

Effects of Melt Thermal-Rate Treatment on Fe-Containing Phases in Hypereutectic Al-Si Alloy

In this paper, effects of melt thermal-rate treatment (MTRT) on Fe-containing phases in hypereutectic Al-Si alloy were investigated. Results show that MTRT can refine microstructures and improve castability, mechanical properties, wear characteristics, and corrosion resistance of Fe-containing Al-Si alloy. When Al-15Si-2.7Fe alloy is treated with MTRT by 1203 K (930 °C) melt: coarse primary Si and plate-like Fe-containing phase both can be refined to small blocky morphology, and the long needle-like Fe-containing phase disappears almost entirely; ultimate tensile strength and elongation are 195 MPa and 1.8 pct, and increase by 12.7 and 50 pct, respectively; and the wear loss and coefficient of friction decrease 7 to 17 and 24 to 30 pct, respectively, compared with that obtained with conventional casting technique. Corrosion resistance of the alloy treated with MTRT by 1203 K (930 °C) melt is the best, that is it has the lowest i _corr value and the highest E _corr value. Besides, effects of MTRT on Al-15Si-xFe (x = 0.2, 0.7, 1.7, 3.7, 4.7) alloys were also studied, MTRT can only refine microstructure and improve mechanical properties of Al-15Si alloy with 0.7 to 3.7 pct Fe content greatly in the present work.     

Fracture behavior of thixoformed 357-T5 Al alloys

The effects of microstructural features on the fracture behaviors, including impact, high-cycle fatigue, fatigue crack propagation, and stress corrosion cracking, of thixoformed 357-T5 (Al-7 pct Si-0.6 pct Mg) alloy were examined. The resistance to impact and high-cycle fatigue of thixoformed 357-T5 tended to improve greatly with increasing volume fraction of primary α. An almost threefold increase in impact energy value was, for example, observed with increasing volume fraction of primary α from 59 to 70 pct. The improvement in both impact and fatigue properties of thixoformed 357-T5 with increasing volume fraction of primary α in the present study appears to be related to the magnitude of stress concentration at the interface between primary α and eutectic phase, by which the fracture process is largely influenced. The higher volume fraction of primary α was also beneficial for improving the resistance to stress corrosion cracking (SCC) in 3.5 pct NaCl solution. The in-situ slow strain rate test results of thixoformed 357-T5 in air and 3.5 pct NaCl solution at various applied potential values demonstrated that the percent change in tensile elongation with exposure decreased linearly with increasing volume fraction of primary α within the range studied in the present study. Based on the fractographic and micrographic observations, the mechanism associated with the beneficial effect of high volume fraction of primary α in thixoformed 357-T5 alloy was discussed.