Mesoscopic Design for Barrier Free Processing toward Up-Grade Recycling

摘要

The recyclable materials lower their grade inevitably after being reused many times. Various impurities and contaminants are housed into materials. Then, the whole recyclable materials, the grade of which becomes lower than the engineering tolerance, must be wasted and ejected from the recycling material loop. The point to be discussed here is how many times the same materials can be reused in this recycling loop or how to make up-grading of recyclable materials. In the barrier-free processing, mesoscopic material design is reconsidered for this up-grading without excessive use of energy. At the first stage, the light-weight metallic materials are employed as a target to discuss the suitable mesoscopic material design for many-time reuse of recyclable materials. The present mesoscopic design is based on the grain refinement as a subsidiary condition. Since the impurities or the additive recyclable materials are processed into fine dispersions in the refined material matrix, low strength as well as low ductility can be improved to meet the required grade level by mechanical engineering for reuse. In the present paper, the mesoscopic design principles for magnesium alloys are first discussed on the basis of crystallographic plasticity theory. The whole principles are expected to work for the microstructure-controlled or the grain-refined light-weight materials. The strengthening principle by fine, homogeneous dispersion is employed for experimental demonstration of the mesoscopic design for recyclable materials. AZ31 magnesium alloy chips as well as fragmented silicon chips from wasted Si-wafer are employed as a starting recycled material. The blended mixture is homogeneously mixed and refined for subsequent hot-extrusion process. Owing to homogeneous refining, the in-situ, solid-state reaction takes place at the relatively low temperature, so that the synthesized Mg_2Si finely disperses in the original magnesium alloy matrix without the coarsening of dispersions and their segregation along the grain boundaries. These microstructure control leads to high qualification of mechanical strength without loss of ductility. In the case of solid-state recycling for magnesium alloys, this type mesoscopic material design can be further extended to a new type of forming or redox-forming for various recyclable input materials.