Formation Mechanism of Non-Equilibrium Phases during Bulk Mechanical Alloying by Accumulative Roll Bonding Process

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Mechanical alloying (MA) of bulky sheet materials was tried by means of accumulative roll bonding (ARB). The MA by ARB made it possible to fabricate the bulky samples processed to various strain levels under well-controlled process conditions, such as temperature and strain rate, which enabled us to observe the microstructure and phase transition as a function of strain. The structure and composition of the obtained specimens were analyzed in nano-meter scale.The multilayered stacks of Cu+Zr and Fe+Al were deformed by the ARB process. Cu-Zr is a well-known amorphous formation system, while it has been thought that no amorphous phase can be obtained in Fe-Al system. In case of Zr+Cu multilayer, not only microstructural mixture but also atomistic level of micture were confirmed above 6 cycles ARB, so that supersaturated solid solution was obtained. It was also found that amorphous phase formed at interface regions between Cu and Zr. It was a surprising result since the severe plastic deformation (ARB) was conducted at ambient temperature in the present study. Nano-beam EDS analysis for the nano-lamellar structures clarified that a certain level of atomistic mixture is necessary for amorphous formation. During a heat treatment of the ARB sample, solid state amorphization proceeded to form the bulky composites of amorphous and nanocrystals. Atomistic mixture by ARB also proceeded in Fe-Al system, and supersaturated solid solution was obtained as well. However, amorphous phase was hardely observed in Fe-Al system. Only in the limited interface regiuons, small amount of amorphous was found. It was also clarified that the amorphous phase was formed via intermetallic compound.The present study indicated the potential of bulk mechanical alloying by the use of the ARB process and pointed out the importance of severe plastic deformation as well as the importance of interfaces for mechanical alloying reactions.