Growth kinetics of interface intermetallic compounds in stainless steel fibre reinforced aluminium matrix composites

Growth kinetics of binary intermetallic compounds in the fibre/matrix interface has been studied in stainless steel fibre reinforced aluminium matrix composites fabricated by the P/M hot pressing, squeeze casting, and infiltration techniques. As expected in most binary diffusion couples, more than one intermetallic compound of the type Fe_xAl_y forms at the interface. However, not all the iron-aluminide intermetallic compounds possible as dictated by the binary phase diagram are present. This is primarily the result of the non-equilibrium conditions at the interphase boundaries as the activation-controlled and diffusion-controlled interfacial reactions progress between the fibre and the matrix. Two equations have been established for the growth kinetics of the interface; one relates to hot pressing, the other to squeeze casting and infiltration. Parabolic rate constants have been determined. A rate constant of about 0.7 × 10^–16 m² sec^–1 for hot-pressed composites produces an optimum thickness of the interface of about 3 m and results in the maximum strength of the composites. In addition to the FeAl and Fe₂Al₅ that form at the interface, the presence of NiAl₃ intermetallic compound is also predicted. Further investigation is suggested for the determination of the rate constants in squeeze-cast and infiltrated composites.     

Fabrication of fibre composites using an aluminium superplastic alloy as matrix

An aluminium superplastic alloy has been used as the matrix for a variety of fibre reinforcements. It is shown that, by hot pressing in the superplastic regime of the alloy, a number of different reinforcements can be incorporated into the matrix. Tensile tests on composites with up to 25 vol % of reinforcement showed good agreement with the rule of mixtures.     

Evaluation of continuous alumina fibre reinforced composites based upon pure aluminium

Abstract

Composites of super purity aluminium unidirectionally reinforced with Altex or Nextel 610 continuous alumina basedfibre have been made by liquid metal infiltration. The composites were well consolidated, with fibre volume fractions V_f of 0.4 and 0.6 for the Altex composites and 0.7 for the Nextel composite, the higher values being obtained where preforming involved the use of sized fibre tows. Matrix porosity was very low and there was no evidence of any deleterious reaction product having formed at the fibre/matrix interface. Monotonic longitudinal tensile tests of the composites gave Youngs modulus values between 125 and 250 GPa, in line with rule of mixtures (ROM) predictions and evidence of effective load transfer between fibres. The onset of yielding in longitudinal composites was commensurate with the yield stress of unreinforced super purity aluminium for V_f = 0.4 (～20 MPa), but increased to 225 MPafor V_f =0.7. The tensile strengths of the Altex composites were 760 and 930 MPa, values in accord with ROM predictions based upon equal load sharing of fibres up to the mean filament failure stress. Although the Nextel composite had a higher tensile strength of 1250 MPa, this was significantly lower than the ROM value of 1650 MPa and was better described by fibre ‘bundle’ theory. Predictions of the accumulation of fibre damage, by statistical analysis, indicated that filament breakage commenced at an applied stress of ～50 MPa for the Altex composite and ～ 500 MPa for the Nextel composite. Despite damage at the lower stress, however, the Altex composites were able to tolerate many more ‘double’ fibre breaks than the Nextel composite, the failure of which coincided with the onset of the first double break. Transverse tensile tests of the composites gave Young's modulus values between 80 and 170 GPa, in line with ROM predictions. The yield stress increased with increasing V_f, from 10 to 60 MPa, this behaviour being attributed to plane strain deformation caused by the virtually non-deformable fibres constraining matrix flow. The tensile strengths showed a similar trend, with 84 MPa for V_f =0.4 increasing to 168 MPa for V_f = 0.7.     

Creep of intermetallic composites

The creep behavior of nickel and titanium aluminides, molybdenum silicides, and their composites was evaluated as a function of stress and temperature to identify the effect of reinforcements on the creep resistance of these compounds. The deformation behavior was analyzed using a power-law creep equation. The experimentally determined activation energies and stress exponents were related to the rate-controlling mechanisms for each system. With reinforcements, there is no improvement in the creep strength in TiAl, some improvement in NiAl, particularly at low stresses, and notable improvement in MoSi and its alloys. Comparative analysis of the creep resistance of aluminides, silicides and the currently used superalloys was also provided. On the basis of the creep resistance, it was concluded that MoSi and its composites have high potential for application at temperatures greater than 1000 °C, and that they are potential competitors to somewhat more brittle ceramic-ceramic composites.     

On the fibre/matrix interface in boron/aluminium metal matrix composites

The fibre/matrix interface of B; AI metal matrix composites (MMCs) has been examined by transmission and scanning electron microscopy (TEM and SEW. As-fabricated samples show no fibre/matrix reaction whereas isothermal exposure for increasing periods of time leads to the formation of at least four distinct borides. The extent and location of the fibre/matrix reaction is strongly influenced by the presence of an oxide layer which is present at all the interfaces. The effect of these reaction products upon mechanical properties is considered.     

Glass formation adjacent to the intermetallic compounds in Cu-Zr binary system

On the contrary to the common belief that glass formation is unfeasible near terminal intermetallic compound due to fast crystallization kinetics, here we present our findings that bulk metallic glasses are readily formed near intermetallic compounds, far away from the traditional region of glass forming near eutectics. While the intermetallic compounds themselves are not possible glass formers, bulk metallic glasses can be quenched compositionally neighboring the intermetallic compounds as close as 0.5 at.%. Taking binary Cu-Zr as a model system, the phenomenon of two optimum glass forming compositions sandwiching the corresponding intermetallic compounds (Cu₅₁Zr₁₄, Cu₁₀Zr₇, CuZr, CuZr₂ and Cu₈Zr₃) is observed consistently. This new scenario of “intermetallic glass” is verified by the thermodynamic principle that the alloy liquids neighboring the intermetallic compounds possess lower Gibbs free energy than that of the compounds themselves. Furthermore, the sluggish crystallization behavior of these liquids provides an additional kinetic explanation.     

Rate effects in the fracture of glass fibre chopped strand mat composites

The effect of varying loading rate on the fracture properties of composites based on glass fibre chopped strand mats and epoxy-based vinyl ester resins has been studied. Initially, the fracture behaviour of the pure resins was studied by using simple flexure and fracture mechanics test techniques. Following this, a series of perforation tests was undertaken on small plate-like samples in order to assess the rate sensitivity of the perforation threshold and to determine if this value could be related to the fracture properties of the composite.

It was found that the work of fracture, W_f, increased with increasing loading rate over the wide range of conditions examined. It is believed that this increase is related to the rate-dependent fracture properties of the glass fibres. Increasing the loading rate results in an increase in fibre failure stress and, as a result, the stored elastic energy. This in turn results in increased crack bifurcation and the formation of a larger damage zone during fracture. Similar trends were observed in the plate impact data, with the threshold increasing rapidly with loading rate. It was found that the work of fracture determined on single-edge-notch bend samples correlated well with the drop-weight impact perforation data.     

Influence of microstructure on axial strength of unidirectional continuous fibre reinforced aluminium matrix composites

Abstract

Systematic empirical investigations on the relationship between microstructural features and mechanical performance of unidirectionally reinforced continuous fibre Al matrix composites (CFAMCs) carried out by the present authors in recent years are summarised. The employment of a high strength matrix alloy and the development of a strong fibre/matrix interface are beneficial to maximise the strengthening effect of the fibre reinforcement. Processing defects, such as second brittle phases in the matrix, non-infiltration defects, matrix solidification shrinkage voids, excessive interfacial reactions, the presence of reaction products on the interface, weak interfacial binding, and excessively high fibre volume fraction reduce composite strength to different extents via a number of different mechanisms. Criteria for the microstructure design of CFAMCs for optimum fibre strengthening efficiency are proposed.     

The formation mechanism of intermetallic compounds in Al/Mg friction-stir weld joint

The mechanical properties of dissimilar metal joints are strongly related to the brittle intermetallic compounds (IMCs) that form at the weld interfaces. In this study, the effects of temperature, local composition and plastic strain on the formation mechanism of IMCs in a welded joint between 6061 Al and AZ31 Mg were investigated. The results demonstrate that Al₃Mg₂ is formed when the strain rate reaches 10?s^?1, even though the temperature is lower than the Al–Mg eutectic temperature, which suggests that plastic deformation may accelerate the formation of IMCs in the deformation zone. Meanwhile, the formation of Al–Mg IMCs can be suppressed by Mg–Zn and Al–Mg–Zn compounds when Zn filler is added at the Al/Mg interfaces.     

Processing aspects of glass-nicalon fibre and interconnected porous aluminium nitride ceramic and glass composites

Glass matrix-fibre and glass infiltrated ceramic composites with interconnected phases have been shown to have the potential for displaying optimum thermal conductivity and dielectric constant at 1 MHz making them useful as substrates for electronic packaging. Ceramic (Nicalon and silicon carbide grade (SCS)) fibre-borosilicate glass composites were fabricated using tape casting processes combined with pressure and pressureless sintering techniques. Experiments were also conducted to process AIN ceramics with interconnected porous channels which were then hot infiltrated with borosilicate glass. Results of optical characterization of the composites indicate that infiltration of Nicalon cloth with glass is achieved by hot pressing, while the tape casting and lamination approach followed by sintering is useful for fabricating composites of glass and Nicalon tows. The sintered aluminium nitride ceramics are comprised of 28% (volume fraction) interconnected pores. Hot infiltration yielded 100 m penetration of borosilicate glass into the pores of the nitride ceramic. The paper discusses the various scientific aspects involved in processing the glass-fibre and porous AIN composites containing 3-d interconnected pores. Results of the microstructural characterization of these composites are discussed particularly in regards to the desired microstructure essential for these composites to be useful as substrates in electronic packaging.     

Micropyretic synthesis of Ni-Al intermetallic composites

Ni3Al and NiAl intermetallic compounds and their composites are potential structural materials for high-temperature applications. Among the composites with different types of reinforcements, particulate-reinforced composites possess several advantages, such as isotropic properties, lower costs of reinforcement and easy fabrication. Particulate-reinforced composites also allow for a wider range of component geometry. In this article, Ni-Al-Cu composites with CeO2 particulates were prepared using the micropyretic synthesis techniques. The effect of chemical composition on the processing response parameters, the phases of products, the microstructure and mechanical properties of the composites were studied. X-ray diffraction results indicated that the phases of the synthesized composites were critically dependent upon the aluminium content. The final porosity of the composites decreased with an increase in the aluminium content. The flexural bending test showed a variation in the flexural strength of the composites with changing microstructure. The flexural strength and the elastic modulus increased with the aluminium content and the final density. This revised version was published online in November 2006 with corrections to the Cover Date.     

The fabrication and properties of metal-matrix composites based on aluminium alloy infiltrated alumina fibre preforms

Composites have been produced by infiltrating a porous preform of semi-random short alumina fibres with liquid aluminium alloys, using squeeze-casting techniques. The fibres were wetted by pure aluminium and several different alloys, and sound castings produced. Full infiltration was observed to take place very rapidly when moderate pressures were applied.

The composites produced have been shown to possess enhanced stiffness which is maintained at useful levels at temperatures approaching 400°C; at ambient temperature they were brittle, but at elevated temperatures the strength was significantly enhanced.     

Superconductivity in ternary Heusler intermetallic compounds

We report the discovery of superconductivity in a number of stoichiometric A₂BC ordered ternary intermetallic alloys isostructural to the prototype ferromagnetic Heusler alloy, Cu₂MnAl. Ni in the superconducting Ni₂BC phases appears to be behaving as a full d-band constituent. The highest superconducting critical temperature T_c, for the Ni phases surveyed is exhibited by Ni₂NbSn (T_c = 3.4 K).     

Short aluminosilicate fibre reinforced aluminium

Characteristic of cheap ceramic alumina-silica (Sibral) fibres and their application in aluminium to produce composite material is given. Strengths and Young's modulus of short (average length of 148 m) fibres are 0.8 GPa and 70 GPa, respectively and its density is 2600 kg m^–3. Composite materials Al-Sibral fibres were prepared by pressure infiltration of aluminium into the preform made with SiO₂ as bonding phase (by sol-gel method). Sibral fibres increase tensile strength and wear resistance of the composite. These fibres significantly reduce that coefficient of thermal expansion. Results from the interaction between fibres and matrix as well as from the fracture of composite are presented and mechanism of the interaction zone (-Al₂O₃ in crystalline form) formation is discussed.     

High-strength,high-temperature intermetallic compounds

Materials that are solid at high temperatures are in demand for high-temperature structural applications, and materials that have high values of strength-to-weight and stiffness-to-weight are desired for aircraft and space applications. Basic properties that are insensitive to processing history can be used to provide a preliminary ranking of single-phase substances. A compilation is presented of 293 intermetallic compounds (or metal-metalloid compounds) that melt at T 1500° C. By displaying the data by crystal structure on plots ofT _m against the specific gravity candidates for optimum specific strength and specific stiffness can be recognized for materials that are likely to have similar plastic properties.