Fabrication of Al/Al_3Ni Matrix Composites with a Low Coefficient of Thermal Expansion

摘要

The present work describes the fabrication of PM aluminium based 20 and 56v percent SiC composites with a 25wt percent Ni powder addition to the Al matrix to promote in situ formation of the intermetallic Al_3Ni. For the higher loaded composite, the Ni addition was also alternatively obtained using Ni-coated SiC particles. The expected benefits from the formed Al_3Ni were to obtain a low Coefficient of Thermal Expansion (CTE) with a SiC content low enough for adequate powder metallurgy processing and to have the densification facilitated by the transient liquid phase formed due to the heat released by the exothermal reaction between Al and Ni. Cold isostatic pressed preforms of the composite mixtures and unreinforced matrix were canned, degassed at 450 deg C for 45 min and the exothermal reaction promoted during consolidation by Reactive Hot Isostatic Pressing (RHIP) at either 620 deg C-100 MPa-30 min or 620 deg C-150 MPa-60 min. Full density was obtained for the 20 percent SiC composite which showed the best balance between CTE and Thermal Conductivity (TC); the 56 percent SiC composites showed porosities higher than 10 percent that led to low TC but on the other hand reached the desired CTE < 10~(-5) K~(-1)