Effect of Ti Addition and Microstructural Evolution on Toughening and Strengthening Behavior of as Cast or Annealed Nb–Si–Mo Based Hypoeutectic and Hypereutectic Alloys

Room temperature fracture toughness along with compressive deformation behavior at both room and high temperatures (900 °C, 1000 °C and 1100 °C) has been evaluated for ternary or quaternary hypoeutectic (Nb–12Si–5Mo and Nb–12Si–5Mo–20Ti) and hypereutectic (Nb–19Si–5Mo and Nb–19Si–5Mo–20Ti) Nb-silicide based intermetallic alloys to examine the effects of composition, microstructure, and annealing (100 hours at 1500 °C). On Ti-addition and annealing, the fracture toughness has increased by up to ~ 75 and ~ 63 pct, respectively with ~ 14 MPa√m being recorded for the annealed Nb–12Si–5Mo–20Ti alloy. Toughening is ascribed to formation of non-lamellar eutectic with coarse Nb_ss, which contributes to crack path tortuosity by bridging, arrest, branching and deflection of cracks. The room temperature compressive strengths are found as ~ 2200 to 2400 MPa for as-cast alloys, and ~ 1700 to 2000 MPa after annealing with the strength reduction being higher for the hypoeutectic compositions due to larger Nb_ss content. Further, the compressive ductility has varied from 5.7 to 6.5 pct. The fracture surfaces obtained from room temperature compression tests have revealed evidence of brittle failure with cleavage facets and river patterns in Nb_ss along with its decohesion at non-lamellar eutectic. The compressive yield stress decreases with increase in test temperature, with the hypoeutectic alloys exhibiting higher strength retention indicating the predominant role of solid solution strengthening of Nb_ss. The flow curves obtained from high temperature compression tests show initial work hardening, followed by a steady state regime indicating dynamic recovery involving the formation of low angle grain boundaries in the Nb_ss, as confirmed by electron backscattered diffraction of the annealed Nb–12Si–5Mo alloy compression tested at 1100 °C.

     

Development of a low-melting-point filler metal for brazing aluminum alloys

The study is concerned with developing low-melting-point filler metals for brazing aluminum alloys. For this purpose, thermal analyses of a series of Al-Si-Cu-Sn filler metals have been conducted and corresponding microstructures observed. The results showed that the liquidus temperature of Al-Si-Cu filler metals dropped from 593 °C to 534 °C, when the amount of copper was increased from 0 to 30 pct. As the copper content reached further to 40 pct, the liquidus temperature would rise to 572 °C. By adding 2 pct tin into the Al-Si-20Cu alloys, the liquidus and solidus temperature would fall from 543 °C to 526 °C and from 524 °C to 504 °C, respectively. The main microstructures of Al-Si-Cu alloys consist of the α-Al solid solution, silicon particles, the CuAl₂ (ϑ) intermetallic, and the eutectic structures of Al-Si, Al-Cu, and Al-Si-Cu. For further improvement of the brazability of this filler metal, magnesium was added as a wetting agent, which would remove the residual oxygen and moisture from the brazed aluminum surface and reduce the oxide film. Based on results gleaned from the thermal analyses, a new filler metal with the composition Al-7Si-20Cu-2Sn-1Mg is proposed, which possesses a melting temperature range of 501 °C to 522 °C and a microstructure that includes an Al-Si solid solution, silicon particles, a tin-rich phase, and CuAl₂, CuMgAl₂, and Mg₂Si intermetallic compounds. When this filler metal was used to braze the 6061-T6 aluminum alloy, an optimized bonding strength of 196 ± 19 MPa was achieved.     

Interaction of silicon-containing melts with single-crystal aluminum oxide

The sessile droplet method is used to study the wettability of aluminum oxide (single-crystal Al₂O₃, i.e. sapphire) by melts of Au-Si, Cu-Si, Ni-Si, Pd-Si, and Ge-Si in relation to silicon concentration, temperature and exposure time. Addition of silicon to melt leads to a fall in the wetting angle from 120–140° for Au, Cu, Ni, Pd and Ge to 70–90° for an alloy with 30–70 at.% Si. The adhesion activity of silicon (a nontransition element) is lower than for transition metals (for example, Ti, Zr). In the systems Au-Si and Pd-Si a phenomenon of dewetting (the wetting angle increases after the initial spreading) is observed. Interphase chemical processes responsible for the wetting angle are studies by mass spectrometry, microscope and profilographic analyses for metal alloys and the surface of solid sapphire.     

Vacuum Carbothermic Production of Aluminum and Al-Si Alloys From Kaolin Clay: A Thermodynamic Study

Examination of the thermodynamic constraints for the carbothermic reduction of iron-free kaolinite, Al₂Si₂O₅(OH)₄, or of its calcination product mullite, Al₆Si₂O₁₃, either at atmospheric pressure or under vacuum of 10^?3 to 10^?5 bar, indicates the conditions required at equilibrium to produce either elementary Al or Al-Si alloys. At atmospheric pressure, a very high temperature of 3200 K would be required to obtain from Al₂Si₂O₅(OH)₄ + 9C an Al-Si alloy with 39 wt.% Si. At 10^?4 bar and 1800 K, the predicted Al-Si alloy would contain 2.4 wt.% Si. From mullite, the reaction of Al₆Si₂O₁₃ + 13C at 10^?4 bar and either 1800 K or 2200 K should produce an Al-Si alloy with 0.65 or 24 wt.% Si. The CO produced by the carbothermic reactions may be by water-gas shift converted to syngas, and further either to methanol or by a Fischer–Tropsch reaction to liquid fuels or chemical intermediates. Concentrated solar energy may be used to supply the required process heat of these high-temperature reactions.     

Diffusion of Zn in Nanostructured Aluminum Alloys Produced by Surface Mechanical Attrition Treatment

After surface nanocrystallization of pure Al and a cast Al-Si alloy through surface mechanical attrition treatment (SMAT), 200- to 300-??m-thick Zn coatings were deposited on the nanostructured surface using the clod spray technique. Subsequently, diffusion of Zn into the Al substrate was induced by postspray annealing treatment at various temperatures for different times. The diffusion kinetics of Zn in the nanostructured surface layers was studied in terms of the Zn concentration profile in the substrate by using scanning electron microscopy (SEM) and electron probe microscopy analysis (EPMA). Experimental results show that not only the diffusivity of Zn in the nanocrystalline grains is significantly increased compared with the diffusion in the coarse grained counterpart, but the temperature at which noticeable Zn diffusion in Al alloys occurs is also reduced from 573?K (300?°C) in coarse-grained Al alloys to 523?K (250?°C) in nanostructured alloys. In addition, because the nanocrystalline grains produced by SMAT in Al-Si alloys are much smaller than those in pure Al due to the effect of eutectic Si, the diffusion of Zn in the SMATed Al-Si alloy is much faster than that in the SMATed pure Al. It is believed that the high diffusivity of Zn in the nanocrystalline Al grains is attributed to the large fraction of grain boundaries that act as fast diffusion channel. The effect of thermal stability of the nanocrystalline grains on Zn diffusion in the SMATed Al alloys is also discussed.     

Solidification microstructure and temperature field during direct chill casting of Al-16Si alloy

By properly controlling casting parameters such as pouring temperature, casting velocity and water flux, direct chill (DC) casting can be employed to produce refined microstructure in the hypereutectic Al-Si alloys without chemical modification. This refined microstructure is characterized of fine primary Si particles, fully developed dendritic Al halos and fine coupled eutectics. In this work, in situ measurements of temperature field in the mould during DC casting of Al-16Si alloy at casting velocity of 2.17 mm/s, 3.5 mm/s and 4.34 mm/s at a pouring temperature of 800°C were performed. The results show that the primary Si phase nucleated at considerable undercooling (about 27°C to 38°C) and the growth temperature of dendritic Al halos was 7°C to 8°C below the equilibrium eutectic temperature. In the center regions of the DC cast billet, halos are fully developed because the G_l/R value is low.     

Precipitates in As-Cast and Heat-Treated ASTM F75 Co-Cr-Mo-C Alloys Containing Si and/or Mn

The effect of the addition of Si or Mn to ASTM F75 Co-28Cr-6Mo-0.25C alloys on precipitate formation as well as dissolution during solution treatment was investigated. Three alloys—Co-28Cr-6Mo-0.25C-1Si (1Si), Co-28Cr-6Mo-0.25C-1Mn (1Mn), and Co-28Cr-6Mo-0.25C-1Si-1Mn (1Si1Mn)—were heat treated from 1448 K to 1548 K (1175 °C to 1275 °C) for a holding time of up to 43.2 ks. In the case of the as-cast 1Si and 1Si1Mn alloys, the precipitates were M₂₃C₆-type carbide, η phase (M₆C-M₁₂C–type carbide), and π phase (M₂T₃X-type carbide with a β-Mn structure), while in the case of the as-cast 1Mn alloy, M₂₃C₆-type carbide and η phase were detected. The 1Si and 1Si1Mn alloys required longer heat-treatment times for complete precipitate dissolution than did the 1Mn alloys. During the solution treatment, blocky dense M₂₃C₆-type carbide was observed in all the alloys over the temperature range of 1448 K to 1498 K (1175 °C to 1225 °C). At the heat-treatment temperature of 1523 K (1250 °C), starlike precipitates with stripe patterns—comprising M₂₃C₆-type carbide and metallic face-centered-cubic (fcc) γ phase—were detected in the 1Si and 1Si1Mn alloys. A π phase was observed in the 1Si and 1Si1Mn alloys heat treated at 1523 K and 1548 K (1250 °C and 1275 °C) and in the 1Mn alloy heat treated at 1548 K (1275 °C); its morphology was starlike-dense. The addition of Si appeared to promote the formation of the π phase in Co-28Cr-6Mo-0.25C alloys at 1523 K and 1548 K (1250 °C and 1275 °C). Thus, the addition of Si and Mn affects the phase and morphology of the carbide precipitates in biomedical Co-Cr-Mo alloys.     

HVEM study of recrystallisation in aluminium alloys containing second phase particles

An investigation of the recrystallisation behavior of two Al alloys containing fine (Al-Fe) and coarse (Al-Si) particles is presented. In situ anneals in a 1 MeV electron microscope revealed different recrystallisation mechanisms in each case and allowed direct measurements of grain boundary velocities and subgrain sizes. Multiple nucleation around coarse particles is observed at 250°C for the Al-Si alloys. Conversely, small precipitates strongly pin a subgrain structure up to 400°C for the Al-Fe alloy. Grain boundary migration rates are compared to values deduced from bulk-annealed specimens for the Al-Si alloy. The stability of the subgrain structure in the Al-Fe alloy is discussed with respect to recent models.     

Microstructure and high-temperature tensile deformation of tiai(si) alloys made from elemental powders

Two ternary TiAl-based alloys with chemical compositions of Ti-46.4 at. pct Al-1.4 at. pct Si (Si poor) and Ti-45 at. pct Al-2.7 at. pct Si (Si rich), which were prepared by reaction powder processing, have been investigated. Both alloys consist of the intermetallic compounds y-TiAl, α₂-Ti₃Al, and ξ-Ti₅(Si, Al)₃. The microstructure can be described as a duplex structure(i.e., lamellar γ/α₂ regions distributed in γ matrix) containing ξ precipitates. The higher Si content leads to a larger amount of ξ precipitates and a finer y grain size in the Si-rich alloy. The tensile properties of both alloys depend on test temperature. At room temperature and 700 °C, the tensile properties of the Si-poor alloy are better than those of the Si-rich alloy. At 900 °C, the opposite is true. Examinations of tensile deformed specimens reveal ξ-Ti₅(Si, Al)₃ particle debonding and particle cracking at lower test temperatures. At 900 °C, nucleation of voids and microcracks along lamellar grain boundaries and evidence for recovery and dynamic recrystallization were observed. Due to these processes, the alloys can tolerate ξ-Ti₅(Si, Al)₃ particles at high temperature, where the positive effect of grain refinement on both strength and ductility can be utilized.     

Influence of Phosphorus on the Nucleation of Eutectic Silicon in Al-Si Alloys

The role of phosphorus (P) in the heterogeneous nucleation of eutectic silicon (Si) and the evolution of eutectic grains in hypoeutectic aluminum-silicon alloys were investigated. Systematic additions of P in the range of 0.5 to 20 ppm to Al-7 wt pct Si alloys of different purities have shown that the morphology of the eutectic Si changes from a fine plate- to a coarse flake-like structure. The growth of eutectic grains was investigated by interrupting the eutectic reaction by quenching experiments. Moreover, the macroscopic growth mode of the eutectic grains was characterized by electron backscatter diffraction. An increase in P concentration from 2 to 3 ppm resulted in a transition of the macroscopic growth mode of the Al-Si eutectic in high purity alloys from growth with a planar front with a strong dependence of the thermal gradient, to nucleation in the vicinity of the primary Al dendrites and subsequent growth of distinct eutectic grains. It is suggested that AlP particles are the key impurities acting as potential nucleation sites for eutectic Si. This is further substantiated as with increasing P concentration nucleation and growth of the Al-Si occurred at higher temperatures close the equilibrium Al-Si eutectic solidification temperature at 850 K (577 °C). In addition, the recalescence undercooling ΔT _R,eu was reduced from 4.5 K (0.5 ppm P) to 1.5 K (20 ppm P) in high purity alloys. This was accompanied by a drastic increase of the nucleation rate of the eutectic grains.     

Wettability and interfacial bonding in AuSi/SiC system

Using the sessile drop method under vacuum, the wettability of monocrystalline α-SiC by AuSi alloys is studied at 1373 K. Additions of Si to Au lead to a strong decrease of the contact angle from θ ⪢ 90° toθ ⪡ 90°. This effect is obtained without significant reactivity and is due to adsorption of Si at the Au/SiC interface, with the formation of a strong chemical bond localised at the interface. Experimental evidence is given showing that oxygen, present in the furnace as an impurity, reinforces the beneficial effect of Si on wetting and can lead to nearly perfect wetting.     

Microstructure and Wear Behavior of Solidification Sonoprocessed B390 Hypereutectic Al-Si Alloy

The hypereutectic Al-Si alloys constitute an important family of alloys because of their excellent wear resistance and low thermal expansion. However, the optimal microstructure and hence the optimal service performance of these alloys cannot be achieved by the conventional melt treatments used in industry today, because of the chemical incompatibility between the primary-Si refiners and the eutectic-Si modifiers used in microstructure control. The current study aimed at using ultrasonic vibrations to improve the microstructure and the properties of these alloys. The results of the current study showed that for the B390 Al-Si alloy (i) the ultrasonic treatment has potential refining effect on the primary Si and Fe intermetallic phases, (ii) the primary Si particles become finer as the pouring temperature decreases from 1033 K (760 °C) to 938 K (665 °C), (iii) pouring and ultrasonic treatment at temperatures below the start of primary Si precipitation result in the coexistence of large and fine Si particles in microstructure, (iv) phosphorous additions of 50 ppm did not show any substantial effect in the ultrasonically treated ingots, (v) ultrasonic-treated samples have uniform hardness over the surface while the untreated samples show large scattering (high standard deviation) in hardness levels and (vi) ultrasonic-treated samples showed better wear resistance in the absence of phosphorous.     

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.     

Enhanced carbon solubility in solvent for SiC rapid solution growth:Thermodynamic evaluation of Cr-Ce-Si-C system

The carbon dissolution in solvent plays a key role in the process of solution growth route for SiC single crystal,which could determine the growth rate and quality of the products.However,the carbon dissolving ability of binary alloy solvent still needs to be improved.Here,we demonstrate the improved carbon dissolution and enlarged carbon supersaturation in Cr-Ce-Si ternary solvent,showing great potential for SiC solution growth.The phase relations of Cr-Ce-Si-C system were determined by using C...     

Premelting transition on 38°〈100〉 tilt grain boundaries in (Fe-10 at.% Si)-Zn alloys

Zinc penetration along a 38°〈100〉 tilt grain boundary was studied in an Fe-10 at.% Si alloy. For the entire temperature range studied (700–840°C) the grain boundaries are wetted by the melt. A region of accelerated grain boundary diffusion is observed in the single-phase region of the (Fe-10 at.% Si)-Zn diagram below the critical temperature T_cr = 790 ± 5°C. At a concentration C_bt the product of the segregation factor, the thickness and the diffusion coefficient of the grain boundary, sδD_b, abruptly falls to a “normal” value. Above T_cr no discontinuities in sδD_b are observed. A singularity is observed in the C_bt(T) curve at the peritectic temperature T_per. The data obtained confirm the previously proposed model of a grain boundary premelting transition. This model explains in particular, why there are no singularities in the C_bt(T) curve of the alloy studied below T_per. This feature distinguishes the Fe-10 at.% Si alloy from the previously studied FeSi alloys containing 5, 6 and 12 at.% Si, which undergo a bulk magnetic or concentrational ordering transition.     

Nucleation of solid aluminum on inclusion particles injected into Al-Si-Fe alloys

Systematic inoculation experiments were carried out to study the influence of various inclusions on the nucleation of the α-Al phase in Al-Si-Fe alloys at different cooling rates. The results showed that in dilute alloys, containing less than 1.5 pct Si+Fe, almost all the inclusion types have high percentages of occurrence within the α-Al phase, indicating that nucleation can be promoted on the surface of such inclusions. In a hypoeutectic Al-Si alloy containing 6.3 pct Si, the inclusion particles of MgO, TiB₂, TiC, α-Al₂O₃, and SiC become mostly inactive nucleants and are pushed to the interdendritic regions because of the dominating poisoning effect of Si. The current results were used successfully to explain the efficiency differences between the commercial grain refiners in the hypoeutectic Al-Si alloys. Silicon is observed to preferentially segregate to the liquid-Al/inclusion interfaces so as to lower the free energy of such interfaces. A theoretical analysis of the poisoning effect of Si showed that Si segregation to the liquid/nucleant interface alters the interfacial energy balance so that the catalytic efficiency of the nucleant particles is dramatically reduced. Careful analysis showed that the poisoning effect of Si in the hypoeutectic alloy is overcome when the nucleant particles have active surface characteristics, as represented by the high catalytic potencies of γ-Al₂O₃, CaO, and Al₄C₃ particles in nucleating the α-Al phase of the hypoeutectic Al-Si alloy. Although some inclusions have comparable or higher occurrence levels than TiB₂ in the α-Al phase, they cannot be used as efficient nucleants because of either their poor wettability with liquid aluminum or their chemical reactivity, which can change the alloy chemistry.     

Spheroidization of Eutectic Silicon in Direct-Electrolytic Al-Si Alloy

The spheroidization process of direct-electrolytic Al-Si alloy (DEASA) containing Si content in the range of 7 to 12 pct heated at temperatures of 778 K to 803 K (505 °C to 530 °C) was studied. The width, length, and aspect ratio of Si particles were measured to quantitatively analyze the microstructural variety of Si phase during the heating process in terms of chemical composition and remelting. Compared to existing Al-Si alloy, the lower soaking temperature of 778 K to 783 K (505 °C to 510 °C) is required to obtain the full spheroidization of the Si phase of DEASA. When remelting DEASA, a satisfactory granulation rate can be achieved at a higher soak temperature of 788 K to 803 K (515 °C to 530 °C). The origin of the high spheroidizaton rate is attributed to the microstructural characteristic relative to the electrolysis process. It would be expected that high crystallographic defects of Si grain result in the complete spheroidization of Si phase at lower temperatures for a short period.     

Strength of Al-Zn-Mg-Cu matrix composite reinforced with SiC particles

The AA7075 alloys reinforced with SiC and without SiC particles were fabricated by a pressureless infiltration method, and then, their tensile properties and microstructures were analyzed. The spontaneous infiltration of molten metal at 800 °C for 1 hour under a nitrogen atmosphere made it possible to fabricate 7075 Al matrix composite reinforced with SiC, as well as a control 7075 Al without SiC. A significant strengthening even in the control alloy occurred due to the formation of in-situ AlN particle even without an addition of SiC particles. Composite reinforced with SiC particles exhibited higher strength values than the control alloy in all aging conditions (underaged (UA), peak-aged (PA), and overaged (OA)), as well as a solution treated condition. Spontaneous infiltration was further prompted owing to the combined effect of both Mg and Zn. This may lead to an enhancement of wetting between the molten alloy and the reinforcement. Consequently, strength improvement in a composite may be attributed to good bond strength via enhancement of wetting. The grain size of the control alloy is greatly decreased to about 2.5 μm compared to 10 μm for the commercial alloy. In addition, the grain size in the composite is further decreased to about 2 μm. These grain refinements contributed to strengthening of the control alloy and the composite.     

Wetting of grain boundaries in hard-magnetic Nd-Fe-B alloys

Samples of Nd-Fe-B alloys, which have been the main hard-magnetic materials with the highest magnetic energy since the end of the 1980s, are investigated. Magnets based on them are obtained by liquid-phase sintering or spin coating. In this article, wetting of the Nd₂Fe₁₄B grains by the neodymium-enriched liquid phase is investigated. The microstructure of the Fe-12.3 at %Nd-7.6 at %B alloy quenched after annealing at T = 700?1100°C in the presence of a neodymium-enriched melt is studied. The acquired data indicate that the transition from incomplete to complete wetting of grain boundaries occurs as the temperature increases. The results are compared with the reference data for alloys of the Nd-Fe-B system obtained by liquid-phase sintering. The relation between the wetting phase transition of grain boundaries and magnetic properties is also discussed.