A method for the preparation of a water-collapsible Al composite material comprising the steps of adding, to Al or an Al alloy, a low melting point metal or alloy selected from the group consisting of In, Sn, combinations of In and Sn and alloys thereof in an amount ranging from 0.1 to 20% by mass to thus give a raw material;
- fusing or melting the raw material such that the raw material has a uniform composition;
- and then quenching the molten raw material in a non-oxidizing atmosphere to thus induce or cause the inverse segregation of the foregoing low melting point metal or alloy on the external surface of small pieces or powder constructed from a single or a plurality of crystalline grains of Al or an Al alloy to thus cover the external surface of the small pieces or powder with the low melting point metal or alloy selected from the group consisting of In, Sn, a combination of In and Sn and alloys thereof.