Expansion evolution during foaming of amorphous metals

Amorphous Pd43Ni10Cu27P20 foam of 38, 49, and 70% porosity is produced by expanding a 25% porosity amorphous precursor in the supercooled liquid state in three isothermal stages of varying durations. Metallographic examination suggests that expansion evolves by bubble growth towards a limit at which bubbles become critically packed. This limit is found to be close to the limit of random close packing of spheres of 63.7%. Beyond this critical limit, bubbles tend to impinge and coalesce and expansion progresses by growth of bubble clusters via stretching of intracellular membranes. The expansion evolution is modeled by means of a dynamic treatment of over-damped growth of individual bubbles. The model captures the expansion evolution reasonably well up to porosities between 50–70%, hence verifying that a transition from a bubble growth mechanism to a cluster growth mechanism takes place at some intermediate porosity.