The effects of the Mg content and bonding temperature on the morphology and crystalline structure of interfacial oxides in the diffusion-bonded joint have been investigated mainly by TEM observations for Al-Mg binary alloys with Mg contents from 0.06 to 2.0 at percent. For all the alloys employed, the oxide at the joint interface altered gradually from amorphous films to crystalline particles 10-100 nm in size, as the bonding temperature was increased, and the amorphous oxide film was annihilated at lower bonding temperatures, as the Mg content was increased. The crystalline oxide particles at the joint interface were identified as Al_2MgO_4 at Mg contents from 0.06 to 0.6 at percent and as MgO at Mg contents from 1.3 to 2.0 at percent, independently of the bonding temperature. At Mg contents from 0.9 to 1.1 at percent, the crystalline oxide particles changed from MgO to Al_2MgO_4, passing through a range where MgO and Al_2MgO_4 coexisted, as the bonding temperature was increased. The ranges of Mg contents and bonding temperatures where the crystalline oxides of Al_2MgO_4 and MgO formed can be roughly explained thermodynamically by assuming that these crystalline oxides were formed through reductive reactions of the superficial oxide film of aluminum by Mg. It was suggested that the slight difference between the thermodynamically calculated ranges and the experimental could be attributed to the contribution of the interfacial energies between the crystalline oxides and the Al matrix. Auger electron spectroscopic analyses of a fractured surface of a joint bonded at a low bonding temperature revealed that Mg atoms highly concentrated in the amorphous oxide film prior to the formation of the crystalline oxide particles, suggesting that the concentration of Mg caused a driving force for the formation of the crystalline oxide particle from the amorphous fiilm.
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