Microstructure and mechanical property of nano-duplex materials produced by HRS process

An SUS316L and a Ti-6Al-4V alloy powders are treated by Mechanical Milling (MM) process, which is one of SPD processes, and then sintered by Hot Roll Sintering (HRS) process. The FIRS process consolidates powder by hot rolling of an evacuated metal pipe filled with the powder at elevated temperatures. Those MM powders have a heavy deformed microstructure at the surface region and have a work hardened microstructure in the core region of the powder. In the surface region, a nano grain structure forms after the MM treatment in both materials. In case of the SUS316L powder, such a nano grain structure consists of an equiaxed nano ferrite (a) grains which has transformed from nano austenite (gamma) grains. Volume fraction of the (x phase decreases with distance from the surface of powder. During HRS the (alpha + gamma) nano-duplex structure changes to (sigma (sigma) + gamma) nano-duplex structure by transformation of the a to the a phase. Thus, the SUS316L HRS material consists of a hybrid structure. That is, a (sigma + gamma) nano-duplex structure at the powder shell region, and a work hardened gamma structure in the powder core region. In case of the Ti-6Al-4V MM powder, though no remarkable transformation occurs, a heavy deformed shell and work hardened core hybrid structure is also produced in the powder. By HRS the Ti-6Al-4V MM powder demonstrates a hybrid structure consists of an equiaxed nano grain structure and a coarse martensite structure. These two HRS materials indicate superior mechanical properties. Mechanical properties are improved by the HRS process. The proof stress and tensile strength in the SUS316L HRS material are x3.8 and x2.1 of the SUS316L conventional material, respectively. In the Ti-6Al-4V HRS material, they are x1.7 and x1.5 compared to the Ti-6Al-4V conventional material.