G. Roudini (Sp), R. Tavangar, L. Weber, A. Mortensen, Swiss Federal Institute of Technology, Lausanne (Switzerland)
The influence of ceramic phase contiguity on the coefficient of thermal expansion of metal matrix composites is
investigated using composite samples made by gas pressure assisted infiltration with molten pure aluminium.
Prior to infiltration, alumina powder beds are sintered to produce various degrees of connectivity among the
particles. The degree of ceramic phase connectivity is characterized using quantitative metallography, and the
composite coefficient of thermal expansion (CTE) is measured in the range from 100 to 275°C. This range is
sufficient to cause the pure aluminum matrix to exceed its elastic limit during the imposed temperature cycle.
The results show that the CTE of the composites decreases noticeably with increasing contiguity of the particulate
phase at constant volume fraction of inclusion phase. The results are compared to existing models for the CTE of
isotropic composites, and it is shown that for moderate contiguity of the ceramic in the composites the CTE
increases from the Schapery lower bound to the Schapery upper bound; this transition is attributed to plastic
deformation in the matrix. The CTEs of composites with the highest values of contiguity are in agreement with
those predicted by the Schapery lower bound over the entire temperature range; showing the effect of the
additional constraint that is placed on the expansion of the ductile phase by the increasingly continuous rigid
phase. For the lowest contiguity values, including the limiting case of non-connected particles, the CTE of the
composite varies between the Schapery upper bound and rule of mixtures; this transition is also attributed to the
influence of matrix plasticity.