Infiltration-processed, functionally graded aluminium titanate/zirconia–alumina compositePart II Mechanical properties

The nature and degree of deformation-microfracture damage around Vickers indentations in a layered and graded aluminium titanate (AT)/(alumina–zirconia (AZ)) composite is studied. Samples with a homogeneous layer of AZ and a graded layer of heterogeneous AT/AZ are fabricated by an infiltration route. Depth profiling of the Vickers hardness shows that the hardness of the material is depth dependent with a relatively soft graded layer but a hard homogeneous layer. The microhardness of the graded layer is load dependent with 5.6 GPa as the asymptotic value at high loads. The evolution and accumulation of damage modes beneath Hertzian contacts are examined using "bonded-interface" sections. The stress–strain response of the material is monitored by Hertzian tests. The graded layer exhibits a distinctive "softening" in the stress–strain curve, indicating a microscale quasiplasticity which can be associated with grain debonding, grain sliding, diffuse microcracking, grain push-out and grain bridging. No contact-induced cracks are observed in the graded layer and the micro damage is widely distributed within the shear compression zone around and below the contacts. The capability of the graded material to absorb energy from the loading system and to distribute damage is somewhat akin to that of ceramics with heterogeneous microstructures. © 1998 Kluwer Academic Publishers     

Effects of cooling rate on latent heat released mode of near pure aluminium and aluminium–silicon alloys

Abstract

The information concerning solid fraction with respect to temperature in the mushy range is very important to solidification models which employ the effective specific heat method. The computer aided cooling curve analysis (CA–CCA) method is used in this study to measure the relationships between solid fraction (f_s) and temperature/time for aluminium alloys of different composition for various cooling rates. The present study examined near pure aluminium, A356.2, A390, and A413 (near eutectic) alloys. The results of the measurements and analyses show that a rather large temperature range is observed near the end of solidification for both near pure aluminium and A413 alloys. This mushy range becomes longer as cooling rate increases. For the A356.2 and A390 alloys, the solidus temperature, liquidus temperature, eutectic temperature, and maximum solid fraction at the eutectic temperature decrease as cooling rate increases. This is not true, however, for cooling rates higher than 9·5 K S^?1. It is also known that afunctional relationship of f_s with temperature is very convenient when it is applied to a solidification model. Two non-linearity factors n_e and n_p are required to define the function; n_e and n_p are found to increase as cooling rate increases. The relationship between n_e and n_p and cooling rate can also be obtained. A reasonable estimation of the solid fraction datafor the cooling rate not measured can then be made.

MST/3262     

Nextel™ 610 alumina fibre reinforced aluminium: influence of matrix and process on flow stress

Continuous alumina fibre reinforced aluminium matrix composites are produced using two different liquid metal infiltration methods, namely direct squeeze casting and gas pressure infiltration. Net-shape fibre performs for longitudinal parallel tensile bars are prepared by winding the Nextel™ 610 alumina fibre (3M, St Paul, MN) into graphite moulds. High purity aluminium, two binary (Al–6% Zn and Al–1% Mg) and one ternary (Al–6% Zn–0.5% Mg) aluminium alloys are used as matrix materials. The composite is tested in uniaxial tension–compression, using unload–reload loops to monitor the evolution of Young's modulus. A linear dependence between Young's modulus and strain is observed; this is attributed, by deduction, to intrinsic elastic non-linearity of the alumina fibre. This conclusion is then used to compare on the basis of the in situ matrix flow curve the influence of matrix composition and infiltration process on the composite stress–strain behaviour.     

The effect of Ag additions on the microstructure of aluminium–lithium alloys

Minor quantities of Ag have been added to Al–Li–Cu–Mg–Zr alloys. Their microstructure has been studied by means of optical metallography, transmission electron microscopy and X-ray diffraction. In the high Li, low Cu:Mg ratio alloys the main phases found were , , S and T1, while fewer T2 and Al7Cu2Fe precipitates were also observed. The addition of up to 0.5 wt% Ag diminishes the and T1 precipitates size. This is attributed to a small increase of Li solubility in the matrix. In the low Li, high Cu:Mg ratio alloy the addition of 0.2 wt% Ag resulted in the precipitation of phase simultaneously with , , S and T1 phases. Due to the low Li concentration an unusual growth of the / precipitates at the expense of the precipitates was also observed. © 1998 Kluwer Academic Publishers     

The effect of Ag additions on the microstructure of aluminium–lithium alloys

Minor quantities of Ag have been added to Al–Li–Cu–Mg–Zr alloys. Their microstructure has been studied by means of optical metallography, transmission electron microscopy and X-ray diffraction. In the high Li, low Cu : Mg ratio alloys the main phases found were , , S and T₁, while fewer T₂ and Al₇Cu₂Fe precipitates were also observed. The addition of up to 0.5 wt% Ag diminishes the and T₁ precipitates size. This is attributed to a small increase of Li solubility in the matrix. In the low Li, high Cu : Mg ratio alloy the addition of 0.2 wt % Ag resulted in the precipitation of phase simultaneously with , , S and T₁ phases. Due to the low Li concentration an unusual growth of the / precipitates at the expense of the precipitates was also observed.     

Preparation of monolithic aluminium titanate with well-defined macropores via a sol–gel process accompanied by phase separation

Monolithic aluminium titanate with well-defined macropores has been prepared through a sol–gel process accompanied by phase separation, using poly(ethylene oxide) (PEO) to induce the phase separation and formamide (FA) to control the gelation of Al₂O₃–TiO₂ system. Appropriate amounts of PEO and formamide allow the formation of aluminium titanate xerogel with cocontinuous macroporous structure and a monolithic shape. The pore size of the resultant dried gels is in the range of 2–3 μm and the porosity is above 60%. The as-dried gel is amorphous and completely transforms into a single phase Al₂TiO₅ after heat-treated at 1300 °C. The macroporous structure is well maintained while the skeleton becomes smooth after heat-treatment.     

Effect of calcinations of starting powder on mechanical properties of hydroxyapatite–alumina bioceramic composite

The effect of calcinations of starting powder on the mechanical properties of hydroxyapatite (HA)-based bioceramic composite was investigated. The calcinations of HA powder in air at 900 °C increased the crystallinity as well as the size of the powder. Ball milling after the calcinations was effective in eliminating large agglomerates in the powder. When the powder was mixed with reinforcing Al₂O₃ powder, the mixture became fine and homogeneous. The flexural strength of HA–Al₂O₃ composite was increased by the calcinations processes at all the Al₂O₃ concentration. However, the fracture toughness was not much influenced by the calcinations. These results lead to the conclusion that the calcinations process effectively reduced the critical flaw size in the body by removing the agglomerates in the HA powder.     

The influence of hybrid phosphate–alumina pigments on properties of Ethylene-propylene-diene rubber composites

In this work Ethylene-propylene-diene (EPDM) rubber vulcanizates were pigmented with a new hybrid pigment containing nano-phosphate layer deposited on surface of micronized alumina. This new pigment contains both single and double-ion phosphates. Different rheological, chemical and physical properties of the nano-pigmented EPDM vulcanizates were studied and compared to the non-pigmented EPDM composites. These pigmented composite properties were studied in the presence and absence of maleic anhydride (MAH) which was employed as a compatibilizer. The bound rubber and cross-linking density were calculated. The results revealed that composites pigmented with 3Zn·1Ca phosphate/alumina/EPDM and 1Zn·3Ca phosphate/alumina/EPDM exhibited the best properties compared to other pigmented composites.     

Physical and thermal characteristics of aluminium titanate dispersed with β-spodumene and zirconia

The effects of -spodumene (Li₂O·Al₂O₃·4SiO₂) and zirconia additives on the physical and thermal characteristics aluminium titanate (AT) ceramics have been investigated. XRD, DTA, SEM, TEM and dilatometry have been used to characterise the influence of these additives on phase relations, densification, microstructure, thermal expansion, thermal decomposition and thermal shock behaviour of AT. The results show that the presence of both -spodumene and zirconia significantly reduces porosity, lowers thermal expansion, improves densification, hardness and thermal stability, without degradation of thermal shock resistance. Microstructural examinations have revealed the presence of glassy phase due to the phase separation of -spodumene and spontaneous microcracks due to thermal expansion anisotropy of AT.     

Influence of aluminium on solidification structure and initial deformability of continuously cast C–Mn–Si–N steel

Abstract

The solidification structure and the initial deformability of continuously cast steel were investigated by assessment of cracks on billets and on rolled product. Some billets were rolled directly off the casting machine and some cooled to ambient temperature, then reheated to rolling temperature. On direct rolled steels, the number of defects increases with increasing aluminium content, while virtually no defects are found on steel rolled after reheating. By increasing the aluminium content, the solidification structure of steel is highly modified and a columnar structure obtained over the entire section of the billet. It was shown by chemical analysis and fracture examination that the increased hot shortness is not related to the effect of AIN. It is concluded that the hot shortness is related to the effect of aluminium on the solidification structure.

MST/761     

Influence of atmosphere and carbon contamination on activated pressureless infiltration of alumina–steel composites

Al₂O₃/steel metal–matrix composites (MMCs) were fabricated by activated pressureless infiltration at atmospheric pressure in different gas atmospheres; N₂, Ar and Ar–5%H₂. The infiltration quality was evaluated with examination of the microstructure, infiltrated area and remaining porosity. The atmosphere with the best infiltration quality was chosen for improvement of infiltration by varying infiltration parameters such as temperature, holding time and heating and cooling rates. Further improvement was achieved by addition of Si or SiO₂ powder to the preform in order to reduce the effect of the residual carbon. The results show that the activated melt infiltration can be successfully done at atmospheric pressure in inert gas.     

Effect of addition of graphite particulates on the wear behaviour in aluminium–silicon carbide–graphite composites

Aluminium matrix composites with multiple reinforcements (hybrid AMCs) are finding increased applications because of improved mechanical and tribological properties and hence are better substitutes for single reinforced composites. Few investigations have been reported on the tribological behaviour of these composites with % reinforcement above 10%. The present study focuses on the influence of addition of graphite (Gr) particulates as a second reinforcement on the tribological behaviour of aluminium matrix composites reinforced with silicon carbide (SiC) particulates. Dry sliding wear tests have been performed to study the influence of Gr particulates, load, sliding speed and sliding distance on the wear of hybrid composite specimens with combined % reinforcement of 2.5%, 5%, 7.5% and 10% with equal weight % of SiC and Gr particulates. Experiments are also conducted on composites with % reinforcement of SiC similar to hybrid composites for the sake of comparison. Parametric studies based on design of experiments (DOE) techniques indicate that the wear of hybrid composites decreases from 0.0234 g to 0.0221 g as the % reinforcement increases from 3% to 7.5%. But the wear has a tendency to increase beyond % reinforcement of 7.5% as its value is 0.0225 g at.% reinforcement of 10%. This trend is absent in case of composites reinforced with SiC alone. The values of wear of these composites are 0.0323 g, 0.0252 g and 0.0223 g, respectively, at.% reinforcement of 3%, 7.5% and 10% clearly indicating that hybrid composites exhibit better wear characteristics compared to composites reinforced with SiC alone. Load and sliding distance show a positive influence on wear implying increase of wear with increase of either load or sliding distance or both. Whereas speed shows a negative influence on wear indicating decrease of wear with increase of speed. Interactions among load, sliding speed and sliding distance are noticed in hybrid composites and this may be attributed to the addition of Gr particulates. Such interactions are not present in composite reinforced with SiC alone. Mathematical models are formulated to predict the wear of the composites.     

The role of microstructure of nickel–aluminium–bronze alloy on its cavitation corrosion behavior in natural seawater

An investigation was carried out in our laboratory to study the effect of the microstructure of nickel–aluminum–bronze (NAB) alloy on its cavitation corrosion behaviour in seawater using a 20-kHz ultrasonic induced cavitation facility. Cavitation tests were made under free corrosion conditions as a function of exposure time in natural seawater. Optical and scanning electron microscopy showed NAB immersed in stagnant seawater suffered from selective corrosion of the copper-rich α phase at boundaries with intermetallic κ precipitates. The κ precipitates and precipitate-free zones did not suffer corrosion. Cavitation made the surface of this alloy very rough, with large cavities or pits, ductile tearing and corrosion of the boundaries of the α columnar grains. In addition, the number of cavities and their size increased with exposure time. Microcracks 5–10 μm long were observed in the α phase adjacent to κ precipitates along the cross-section of the material. Selective phase corrosion and cavitation stresses were considered to be the cause of the cracks observed.     

Note on POD test parameters to study wear behaviour of alumina–titania coatings

Wear behaviour of atmospheric plasma sprayed (APS) alumina–titania coating is investigated using Pin-On-Disc (POD) test. Mean friction coefficient values are assessed using the cumulative probability plot. Results showed that the friction coefficient increased with both sliding velocity and applied load.     

Effect of rare earth–aluminium additions on the microstructure of a semisolid low carbon Fe–B cast alloy

Abstract

The present study investigates the effects of rare earth and aluminium on the microstructures of as cast and heat treated semisolid Fe–B cast alloys. The as cast microstructure of the semisolid modified Fe–B cast alloy consists of the eutectic boride, pearlite and ferrite. Moreover, compared to a net-like distribution of the coarse eutectic borides in the ordinary unmodified alloy, the eutectic boride structures in the semisolid modified alloy are greatly refined and less interconnected. After heat treatment, the phases in the semisolid modified Fe–B cast alloy consist of the boride and martensite. The additions of rare earth and aluminium help to promote the formation of the short rod shaped and round borides in the semisolid Fe–B cast alloy during heat treatment. Compared to the ordinary unmodified alloy, there is no significant change in the boride area fraction but an obvious decrease in average boride area in the semisolid modified alloy.     

Effect of La2O3 additive on the dielectric properties of barium strontium titanate glass–ceramics

The barium strontium titanate (Ba_xSr_1–xTiO₃) glass–ceramics doped with different content of La were prepared via controlled crystallization. Phase compositions, microstructure and dielectric behaviors were investigated systematically. The results revealed that La₂O₃ additives had little influence on the dielectric constant but significantly changed the microstructure of the glass–ceramics, which led to improved breakdown strength (BDS). The optimized energy-storage density of 3.18 J/cm³ was achieved in the glass–ceramics with 1.0 wt% La₂O₃ content which is 2.56 times higher than pure BST glass–ceramics, suggesting glass–ceramics of this composition could be an attractive candidate for energy-storage applications.     

Selective laser sintering of barium titanate–polymer composite films

A selective laser sintering process has been used to consolidate electro-ceramic thin films on silicon substrates. Methods of forming pre-positioned layers of barium titanate were investigated by spin-coating the feedstock powder mixed with a commercial polymer photo-resist. The ceramic–polymer composite was deposited directly onto a nickel film which was evaporated onto a silicon substrate, pre-oxidised to form an electrically insulating layer. A range of laser processing parameters was identified in which consolidated barium titanate layers could be formed. The laser power was found to be more influential in forming sintered microstructures than laser exposure time. The microstructure of barium titanate films is sensitive to the SLS laser-processing conditions, with the optimum laser powers for the processing of the BaTiO₃–polymer found to be in the range 17–20 W. This article highlights the possibility of using ‘direct write’ techniques to produce piezoelectric materials upon silicon substrates.