A direct study of the decrease of the content of magnesium and zinc alloying elements in aluminium alloys during heat treatment

Abstract

Changes in the content of magnesium and zinc alloying elements during heat treatment in vacuum (1.3 × 10 −3 Pa) and in air in the bulk and on the surface of specimens of AlMg, AlZn, AlMgSi and AlZnMg alloys have been investigated using an electron probe microanalyser. At the temperatures used (500 °C and above) in vacuum a zone depleted in magnesium and zinc is formed near the surface. After heat treatment in air in alloys containing magnesium (AlMg, AlMgSi and AlZnMg) a zone depleted in magnesium appears and the surface becomes enriched in magnesium. In alloys containing only zinc the concentration remains uniform after heat treatment in air; this indicates the formation of a layer which prevents oxidation. The results obtained with heat treatment in air emphasize the significance of the structure of the oxide layers for this process. The mechanism of depleted-zone formation is discussed.