Magnesium alloys with low density are important light metals, widely used in the aerospace and automotive industries and in the manufacture of communication devices, consumer-electronics appliances and computers products in recent years. However magnesium and magnesium alloys are the most reactive metals so that oxide films easily form on them during the melting or pouring process. These films are difficult to observe by optical micrograph. In this study we propose a simple method to observe oxide films entrapped in cast magnesium alloys. The oxide films are fractured as a result of cavitation erosion on the sample surface that occurs during ultrasonic-vibration treatment. The eroded areas become visible as differently shaped foggy marks. This method of observing and identifying foggy marks is shown to be useful in the diagnosis of oxide films in cast magnesium alloys. In addition, the presented method in the diagnosis of oxide films that formed on magnesium and aluminum alloys are also compared.

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Pure aluminum (99.999%) cubes were polished by abrasive papers and then heated in a furnace at 873 K for 25 h in order to grow oxide on the polished surfaces, coded as Al/oxide. These Al/oxide samples were stacked with a pure Al cube and a Al-7 mass%Si cube, respectively, then heated in a furnace at 1023 K for 1200 s in an Ar+H 2 atmospheric gas. The sandwiched samples were sectioned and polished after the heated sample was cooled to room temperature. The morphologies of the interface (or junction of the sandwich samples) were recorded photographically. Based on the recorded cavities shown at the interface, we measured both the radii of curvatures and contact angles of the cavities. When the Al/oxide stacking with pure Al sandwich samples was heated in Ar plus H 2 gas, cavities were readily shown at the interface; very few cavities have been observed when samples were heated in Ar gas. The cavities were formed when an air-pocket was initiated at the microchannels by hydrogen diffusion, then grew and coalesced at the interface. The air-pockets remained at the interface of the heated Al/oxide stacking with pure Al sandwich sample and were entrapped as cavities after samples solidified. Microbubbles detached from the airpocket forming micropores trapped in a matrix of the Al/oxide stacking with Al-7 mass% Si cube sample.

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