Abstract
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 α phase coarse grain was changed to a bimodal α grain structure, formed nanocrystalline subsurface region and un-formed coarse grain inner region. SPS transformed from the powder with the bimodal α 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 (α+γ) two phase stainless steel exhibited a complex shell structure consisting of mid-shell and outer-shell regions. The α 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 α nano grains, followed by γ precipitation. On the other hand, the outer-shell is formed by the recovery of deformed α phase, and γ phase precipitates at the α subgrain boundaries. Consequently, the (α+γ) two phase stainless steel shows regularly aligned nano-duplex, micro-duplex and duplex structures, because of the competing procedure of recovery and recrystallization of α phase and γ precipitation.
