Formation of (α+γ) nano-duplex structure by severe plastic deformation in an SUS316L stainless steel

Mechanical milling (MM) process is applied to SUS316L austenitic stainless steel powder and a nano-duplex structure formation behavior is investigated. A nano grain structure with BCC phase forms in surface region of the powder milled for 720 ks. Those nano grains have the grain size of less than 20 nm, The (alpha + gamma) nano-duplex structure, whose grain size is approximately 100 nm, forms in the inner region of the milling powder. The alpha grain in the surface region is an aggregated structure and consists of equiaxed grains, while those a grains in the inner region disperse homogeneously and indicate more spherical and smooth shape compared with the gamma grains around them. TEM/EDS examination revealed that there occurs two kinds of BCC transformation. Those a nano grains in the surface region are considered to form by increase of grain boundary energy. On the other hand, the spherical a nano grains in the inner region form by increase of the grain boundary energy as well as the chemical free energy. Such an increase of chemical free energy is assumed to be caused by the existence of a huge number of vacancies which are introduced by the MM treatment. Although the austenite phase in the SUS316L steel is meta-stable at the room temperature, it hardly transforms to martensite phase even by a heavy cold rolling. Thus, these two kinds of a transformation are extraordinary phenomena and are considered quite different from these conventional strain induced martensitic transformation. A (gamma + alpha) nano-duplex structure material obtained by sintering (alpha + gamma) nano-duplex powder has a good mechanical property.