Modification of the interface in SiC/Al composites

Methodologies both to avoid the formation of Al₄C₃ and to tailor the interfacial structures in a SiC/2014 Al composite were demonstrated. Modification of the interfacial structures in the SiC/2014 Al composite was made by forming SiO₂ layers on the surfaces of SiC via passive oxidation at elevated temperatures. In the 2014 Al composite reinforced with the oxidized SiC, MgAl₂O₄ and Si crystals were observed to be present at the interfacial region as a result of the reaction between the SiO₂ layer and the matrix. On the other hand, in the case of the 2014 Al composite reinforced with unoxidized SiC, SiC was found to react with the Al matrix to form both Al₄C₃ and Si. Qualitative measurements of the interfacial bonding strength were carried out on composites having various types of interfaces and thicknesses. Detailed interfacial structures and phase identifications, which were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), were presented.     

Fiber and interface fracture in singlecrystal aluminum/SiC fiber composites

Model metal-matrix composite tensile specimens, each containing a single SiC fiber in a single crystal of pure Al, were grown using a modified Bridgman method at two growth rates and with various fiber surface treatments in order to study their effect on fiber and interface strength. Using the load drops in tensile tests, we measured both fiber and interface strengthin situ. Acoustic emission (AE) was monitored to assist in determining the failure mechanisms. Both the fiber surface treatment and growth rate were found to significantly affect the fiber and interface strength. Fibers with carbon-rich outer surfaces had higher fiber strengths but lower interfacial strengths than untreated fibers. These results are discussed in terms of failure mechanisms and interfacial reactions occurring during growth of the composites.     

Microscopic examination of the interface region in 6061-Al/SiC composites reinforced with as-received and oxidized SiC particles

Aluminum (6061) matrix composites reinforced with different SiC particles were processed. Black SiC particles were used in their as-received form or after artificial oxidation, leading to a 50-nm-thick SiO₂ layer surrounding the SiC particles. The manufacturing route used was the compocasting technique, which allows maintenance of the semisolid slurry at relatively low temperatures (<650 °C) during the incorporation of the reinforcement. Before squeeze casting, the liquid alloy is held at 700 °C for 5 to 10 minutes. The interface between the aluminum matrix and SiC was characterized by transmission electron microscopy (TEM). Results show that no reaction takes place during compocasting between the as-received SiC particles and the molten aluminum. This is a consequence of the low temperatures and short holding times in the liquid state of the 6061 alloy, possible with this process. Prolonged holding at 800 °C of this material leads to extensive formation of A1₄C₃. In the case of artificially oxidized SiC particles, the SiO₂ layer surrounding the SiC particles reacts with the molten Al-Mg-Si alloy to produce MgAl₂O₄. However, the amount of Mg from the base alloy lost to form this spinel phase is not sufficient to prevent age hardening of the material.     

Structure and mechanism of bonding at a diffusion-bonded Al/SiC interface

Al/SiC interfaces were fabricated by diffusion bonding a pure Al foil between two blocks of SiC for temperatures ranging from 500 to 600°C. For samples bonded below 586°C, the interfacial strength was low and TEM speciments could not be fabricated due to separation of the Al and SiC pieces dring thinning. For samples bonded at and above 586°C, a strong bond was formed and conventional and high-resolution transmission electron microscopy revealed the formation of a thin amorphous phase at the interface. Compositional analysis showed that the interfacial phase contained Al, Si, C and O. Formation of the amorphous phase was demonstrated to occur by a solid state reaction and is discussed on the basis of thermodynamic and kinetic considerations. Lastly, some of the advantages of having an amorphous phase at a metal/ceramic interface are discussed.     

Diffusion bonding reactions between a SiC/SiC composite and two superalloys during joining by hot isostatic pressing

Diffusion reactions during solid state joining of a ceramic SiC/SiC composite to two superalloys, Hastelloy X and Incology 909, by hot isostatic pressing (HIP) have been investigated. The HIP pressure was 200 MPa in all joining cycles, and the temperature/dwell time were either 800°C/15 min, 900°C/1 h or 1000°C/ 1 h. The reaction zones formed consisted of a thin layer of carbides surrounded by several layers containing silicides and free carbon. The thickness of the reaction layers increased with increasing temperature, but were more affected by the composition of the alloy. With more carbide formers in the alloy, the thickness of the reaction layer decreased. The SiC composite was found to be considerably more prone to reactions with these superallys during HIP as compared to Si₃N₄ under similar conditions.     

Palladium-zirconia diffusion bonds: Mechanical properties and interface reactions

High strength and toughness diffusion bonds have been fabricated using palladium foils between TZP zirconia blocks at temperatures above 1000°C in vacuum. Bonds fabricated below 1000°C in vacuum and under all conditions in air showed negligible strength. Strong, vacuum made bonds lost almost all their strength on annealing in air above 1000°C, anneals in vacuum also resulted in a decrease in bond strength but with a much less marked effect. PdZrO₂ interfaces have been characterised by cross-sectional TEM and a thin reaction zone identified. Microanalysis identified the presence of Pd, Zr and O in a ratio of approximately 30:52:18 in the reaction interlayer. This composition has a Pd:Zr ratio close to that of a steep eutectic in the PdZr binary system and evidence for the presence of a liquid phase at the PdZrO₂ interface during bonding is presented. The strength and toughness of the bonds are shown to be strongly dependent on the perfection of the bonded interface with the presence of a small fraction of voids causing a significant reduction in bond strength and toughness. Simple slip-line field methods are used to illustrate the influence of interface voids on the plastic constraint of bonded thin ductile layers.     

Phase reaction and diffusion path of the SiC/Ti system

Bonding of SiC to SiC was conducted using Ti foil at bonding temperatures from 1373 to 1773 K in vacuum. The total diffusion path between SiC and Ti was investigated in detail at 1673 K using Ti foil with a thickness of 50 μm. At a bonding time of 0.3 ks, TiC at the Ti side and a mixture of Ti₅Si₃C _x and TiC at the SiC side were formed, yielding the structure sequence of β-Ti/Ti+TiC/Ti₅Si₃C _x +TiC/SiC. Furthermore, at the bonding time of 0.9 ks, a Ti₅Si₃C _x layer phase appeared between SiC and the mixture of Ti₅Si₃C _x and TiC. Upon the formation of Ti₃SiC₂ (T phase) after the bonding time of 3.6 ks, the complete diffusion path was observed as follows: β-Ti/Ti+TiC/Ti₅Si₃C _x +TiC/Ti₅Si₃C _x /Ti₃SiC₂/SiC. The activation energies for growth of TiC, Ti₅Si₃C _x , and Ti₃SiC₂ were 194, 242, and 358 kJ/mol, respectively.     

The effects of temperature and pressure on the interface chemistry in the graphitealuminum composite system

Mechanical property measurements were made on the graphite-aluminum composite after thermal exposure, creep-testing, and consolidation into larger structures. Chemical analyses of the interfaces of the composites were conducted, and relationships between the chemical makeup of the interface and the mechanical properties of the composite were found.     

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...     

Characterization of a diffusion-bonded Al-Mg alloy/SiC interface by high resolution and analytical electron microscopy

The interfacial structure of a diffusion-bonded Al-4.55 at. pct Mg/SiC interface was examined by conventional and high-resolution transmission electron microscopy. Formation of Mg₂Si, MgO, and Al₂MgO₄ was observed. The monoclinic Mg₂Si phase formed at the Al/SiC interface, while the oxides MgO and Al₂MgO₄ formed at the monoclinic Mg₂Si/Al interface. It is shown that the formation of these phases can be predicted using simple thermodynamic criteria such as the relative bond strengths between Al, Si, C, O, and Mg. In addition, precipitation of some equilibrium Al₈Mg₅ precipitate was also observed at the interface. The interfacial structure observed in the Al-Mg/SiC system is contrasted with that observed in the pure Al/SiC system.     

反应烧结SiC/Co-Si体系的润湿性及界面反应

采用座滴法研究反应烧结(Reaction bonded)SiC/Co-Si体系在真空中的润湿性及界面反应,并研究Si含量和实验温度对润湿角的影响。结果表明,元素Si对反应烧结(RB)SiC/Co-Si体系的润湿性有显著影响,当Co-Si钎料粉体中Si含量(质量分数)为6.7%和60%时,体系的最终润湿角都低于SiC/纯Co体系。SiC/Co-Si体系的润湿过程属于反应性润湿,随着温度升高,润湿角明显减小。微观结构研究和XRD相分析表明,对于SiC/Co-3Si体系(Co-3Si钎料中Si的质量分数为3%),界面区域发生了化学反应,反应产物为CoSi和碳,同时发生元素的互扩散,形成反应中间层;对于SiC/Co-60Si体系,界面反应产物只有CoSi2,界面区域没有存留碳。界面反应改变体系的界面结构,从而改善体系的润湿性。     

Wetting and brazing zirconia ceramic with alloys of the CuGaTi system

Experiments on the wetting of zirconia ceramics by CuGaTi alloys have been carried out. A method was developed for brazing by capillary infiltration of a layer of titanium powder with CuGa alloy. Specimens of brazed joints were prepared and their shear strength was measured. That strength reached 620 MPa, the average hardness being about 380 MPa. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaye Metallurgiya, Nos. 7–8(408), pp. 52–54, July–August, 1999.     

Wetting and the contact reaction in the LaB6-(Cu-Ni) system

Conclusions The wetting of lanthanum hexaboride by copper is accompanied by the establishment of a bond between them which has a chemical character. Raising the temperature markedly improves wetting, full wetting being attained at a temperature of 1770°K. Nickel in the LaB₆-(Cu-Ni) system constitutes a weak interface-active addition. The chemical reaction of Cu+5% Ni, Cu+10% Ni, and Cu+15% Ni alloys with lanthanum hexaboride takes place with the formation of a new phase enriched in nickel.Translated from Poroshkovaya Metallurgiya, No. 7(223), pp. 68–72, July, 1981.