Outstanding Mechanical Properties in the Materials with a Nano / Meso Hybrid Microstructure

Grain refinement is well known to influence the mechanical properties of materials, especially the strength characteristics. The promising method for grain refinement is a SPD process and it produces the homogenized nano grain material which exhibits very high strength and limited ductility. Recently the grain refinement technique by the SPD in powder metallurgy (PM) field has received much attention. The SPD-PM process is one of new processes combining mechanical milling (MM) or alloying (MA), heat treatment and sintering processes. Microstructure of the SPD-PM materials is easily controlled by the MM condition, and hence we can intentionally make a heterogeneous microstructure. In the present study, commercially pure titanium, Ti-6Al-4V alloy and SUS316L stainless steel powders are applied to the SPD-PM process. These MM powders are sintered by Hot Roll Sintering (HRS) process. These SPD-PM materials demonstrate a heterogeneous microstructure and high strength and advanced plastic strain. The microstructure of materials consists of a shell and core hybrid microstructure, that is, a shell structure with nano grains and a core structure with work-hardened coarse grains. All of the materials fabricated by these processes demonstrate not only superior strength but also enough elongation. The mechanical properties are strongly influenced by the shell / core microstructure. The nano / meso hybrid microstructure by these processes has been proved to be very effective to improve mechanical properties.