Amorphization process induced by mechanical alloying in the immiscible Cu-Ta system

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The mechanism for the amorphization induced by mechanical alloying (MA) has been studied in the immiscible Cu- Ta system. A mixture of copper and tantalum powders at a composition ratio of Ta: Cu = 7:3 was used. The first 30 h of milling essentially results in the reduction in Ta and Cu grain size down to ∼10 nm without measurable formation of an amorphous phase. The thermally assisted amorphization (TAA) becomes noticeable after 60 h of milling. The higher the ambient temperature, the faster the amorphization proceeds. The TAA effect is also observed by annealing a partially amorphous MA powder. The microstructure after 30 h of milling is such that fine Cu crystallites are embedded in a fine- grained Ta matrix. Here an interfacial energy contribution is large enough to raise the Gibbs energy to that of an amorphous phase. Now high temperature milling or annealing allows an energetically downhill process to occur. This is most likely responsible for the observed TAA effect in the Cu- Ta system characterized by a positive heat of mixing.