Evolution of Harmonic Structure in Two Phase Stainless Steel by Mechanical Milling Process

This investigation clarified the microstructural evolution in a two-phase stainless steel (329J1 grade), prepared by a mechanical milling (MM) and spark plasma sintering (SPS). The as-received stainless steel powder was produced by a plasma rotating electrode process and the microstructure of that was fully ferritic structure. After the MM, the single alpha phase coarse grain was changed to a bimodal alpha grain structure, formed nanocrystalline subsurface region and un-formed coarse grain inner region. SPS transformed from the powder with the bimodal a grain structure into harmonic structure'' of sintered compact. Namely, the fine-grain areas three dimensionally interconnect (Shell'' network) and the coarse. grain area periodically disperse (Core''). The sintered (alpha+gamma) two phase stainless steel exhibited a complex shell structure consisting of mid. shell and outer. shell regions. The alpha phase in the mid. shell is high angle boundaries and, that in the outer-shell is low angle boundaries. As the considered mechanism for such a complex microstructural evolution, the mid-shell is formed by grain growth of the alpha nano grains, followed by gamma precipitation. On the other hand, the outer-shell is formed by the recovery of deformed alpha phase, and gamma phase precipitates at the alpha subgrain boundaries. Consequently, the (alpha+gamma) two phase stainless steel shows regularly aligned nano. duplex, micro. duplex and duplex structures, because of the competing procedure of recovery and recrystallization of a phase and g precipitation.