M.A. Pech-Canul

Microstructure and mechanical properties of Al/Sicp composites with multimodal size distribution of reinforcements

The effect of particle size distribution and particle size ratio of SiCp in SiCp/SiO2 preforms on the microstructure, microhardness of SiCp reinforcements, modulus of rupture, and superficial hardness of Al/SiCp composites produced by pressureless infiltration has been investigated. SiCp/SiO2 preforms in the form of plates (4cm x 3cm x 0.5cm) have been pressureless infiltrated by the alloy Al-15.52 Mg-13.62 Si (wt. %) at 1100 oC for 60 min under inert atmosphere. SiC powders with average particle size of 10, 68 and 140 μm are mixed with SiO2 powders and preforms of 40 % porosity with unimodal, bimodal and trimodal size distributions are prepared by uniaxial compaction. The bimodal (small: large) and trimodal (small: medium: large) preforms are prepared with different particle size ratios in the following levels: 1:1, 3:1, 1:3, 2:2:2, 3:2:1, 3:1:2. Results from characterization by XRD, SEM and energy dispersive X-ray spectrometry show that the typical microstructure of the composites contains the MgAl2O4 (spinel), AlN and MgO phases formed during processing as well as partially reacted silica, SiC, Si and Al. It is found that the density, reinforcement microhardness, modulus of rupture and superficial hardness of the composites increase all with wider particle size distribution. However, whilst the modulus of rupture is mainly affected on going from unimodal and bimodal to trimodal distribution, superficial hardness and microhardness are mostly influenced on going from unimodal to bimodal and trimodal distribution.

Young's modulus of Al/SiC P/MgAl2O4 composites with different particle size distribution of reinforcements

The effect of particle size distribution of SiC particulate reinforcements coated with colloidal SiO2 on Youngs modulus of Al/SiCp/MgAl2O4 composites fabricated by reactive infiltration was investigated. Composites were prepared from porous preforms of silica-coated α- SiC powders of 10, 54, 86, and 146 μm, 0.6 volume fraction of reinforcements and particle size distribution from monomodal to cuatrimodal. Infiltration tests with the alloy Al-13.3Mg-1.8Si (wt. %) were carried out in Ar→N2 atmosphere at 1100oC for 60 min. The composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). In addition to density and residual porosity measurements, Youngs modulus was evaluated by ultrasonic techniques. Results show that with increase in particles size distribution, residual porosity decreases and density and Youngs modulus of the composites are improved, the latter from 185.39 ±3.6 to 201.93 ±2.3 GPa. This is attributed to the increased metal-ceramic interfaces and to an enhanced matrix-reinforcement load transmission.

Degradation of Al/SiCp composites processed with fly ash via reactive infiltration

The degradation in ambient atmosphere of Al/SiCp composites prepared by the reactive infiltration of SiCp preforms containing fly ash has been investigated. SiCp/fly-ash preforms in the form of plates (3 cm x 4 cm x 0.5 cm) with 50 % porosity are infiltrated by an Al- 8 Si-15 Mg (wt. %) alloy under argon atmosphere at 1050, 1100 and 1150 °C, for 50, 60 and 70 min. Characterization by XRD, SEM and EDX of composite specimens shortly after processing do not reveal the presence of the unwanted Al4C3 phase. However, in addition to Al, Si and SiC, MgAl2O4 and Mg2Si phases are detected. One month after the infiltration trials, white and gray powders are present on the composite specimens, accompanied by pitting corrosion and cracks which propagate with time. Although analysis by XRD of the degradation products reveals only Al4C3 in addition to the above mentioned phases, results from SEM, IR absorption and ICP also suggest the presence of Al(OH)3 and Mg(OH)2, probably from the interaction of Al4C3 and Mg2Si with water. It is considered that Mg2Si in the powders acts as an anode in a galvanic couple with atmospheric moisture as the electrolyte. The crack pathway through SiC, intermetallic AlFeMnSi and Si rich zones implies that one or more of these phases worked as the cathode. In summary, degradation of the composites is explained by the combined effect of galvanic corrosion caused by second phases and the interaction of Al4C3 with atmospheric moisture.