Metal Matrix Composites with Several Particle Sizes

[Enthalpy]

Hello nice people!

Metal Matrix Composites (MMC) mix ceramic powder, nanotubes, other small things with a matrix of metal like magnesium, aluminium, titanium, iron alloys. The goal is to increase the stiffness, the compression resistance, have a small density, and hopefully keep a decent tensile resistance and toughness.

I haven't searched for long, but apparently they use a single range of particle size, to about 40% volume filing factor. Though, a suspension becomes quickly a slurry, and with about 40% volume of solid particles, it doesn't flow any more. I understand that particles touch an other as their arrangement isn't optimum, so they immobilize the suspension, and more particles create cracks.

A solution is already know and may apply to MMC. Concrete uses three very different ranges of particle sizes: pebbles, then grit to fill the voids between the pebbles, then sand to fill the voids between the pieces of grit. In this way, the liquid fraction is much smaller when concrete is mixed, and it still flows.

It is my hope that, by using several very different particle size ranges in MMC, a higher volume fraction can be used, and as the metal is still contiguous, the MMC retains a decent tensile resistance and toughness.

Marc Schaefer, aka Enthalpy

[Enthalpy]

If someone makes an aluminium matrix composite with 12ppm/K instead of presently 15.5pm/K for 25vol% of SiC, that will make excellent frames for many-strings music instruments like cimbaloms
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E=115GPa for ρ≈2900kg/m³ would give instruments more stable tuning for less weight than cast iron does for the piano.

I imagine that more ceramic in the composite reduces its thermal expansion and further increases its stiffness. Mix different particle sizes as suggested previously.

Of course, the composite must be cheap to produce and process, supposedly by casting. Flexural strength is interesting, but the stress isn't huge since stiffness decides the sections, and already the use in pure compression would be interesting.