Improvement of mechanical properties in SUS304L steel through the control of bimodal microstructure characteristics

Abstract

The present work deals with achieving improvement in the mechanical properties of SUS304L stainless steel through the application of a unique microstructure design termed as 'Harmonic structure', and establishing a co-relationship between various microstructural characteristics and mechanical properties. Harmonic structure essentially means a bimodal grain size distribution with a specific periodic arrangement of coarse-and ultrafine-grain fractions. SUS304L stainless steel samples having such microstructure were fabricated by a powder metallurgy route involving the mechanical milling of pre-alloyed steel powder followed by spark plasma sintering. Due to these peculiar microstructural characteristics, the harmonic-structured SUS304L stainless steels demonstrated a winning combination of high strength, large uniform elongation, and large total elongation to failure, simultaneously. It was also found that the fraction of a shell area (a three-dimensional continuously connected network of ultrafine-grained structure) is an important parameter controlling the balance of the mechanical properties of the harmonic-structured SUS304L steel compacts.

Application of Al-Si Semi-Solid Reaction for Fabricating Harmonic Structured Al Based Alloy

“…It appears that the plastic deformation is only limited to the surface of the powder particles. Similar morphological observations have been reported also in literature 9,12,13) . Moreover, it is also noticed that the Si particles have fragmented to signi cantly smaller particle size after mechanical milling.…”

“…Recently, Ameyama and co-workers have proposed a microstructural design to improve mechanical properties by obtaining both high strength and high ductility in metallic materials, through the formation of so-called harmonic structured material [1][2][3][4][5][6][7][8][9][10][11][12][13] . Essentially, it is an exquisite heterogeneous microstructural design, consisting of bimodal grain size distribution, in which ductile coarse grained regions (termed as core ) are enclosed in a continuously connected three-dimensional networks regions with high strength ultra-ne grained structure, known as shell .…”

“…Considering the improved ductility in harmonic structured material, in earlier works, 9,14) it has been experimentally demonstrated that an improvement in the ductility is attributed to the large uniform deformation under tensile loading. In other words, strain hardening is kept to larger strain region, and thus resulting in the improvement of ductility.…”

Three-dimensionally gradient harmonic structure design: an integrated approach for high performance structural materials

“…To deal with the above-mentioned issues of controlled heterogeneous microstructure, Ameyama et al proposed an integrated approach to prepare bulk materials with a controlled and unique heterogeneous microstructure, called 'harmonic structure' (HS), with bimodal grain size distribution. [15][16][17][18][19][20][21][22][23][24][25] A schematic diagram illustrating the HS design is shown in Figure 1. The HS is a heterogeneous microstructure with a specific spatial distribution of fine and coarse grains, that is, the CG areas ('core') embedded in the matrix of three-dimensional continuously connected network ('shell') of fine-grained (FG) areas.…”

“…It has also been demonstrated that the shell/core fraction can be successfully controlled by controlling milling time and number of milling passes in BM process and JM process, respectively. [19][20][21][22] The milled bimodal powders are consolidated at elevated temperatures to prepare bulk materials. Recently, spark plasma sintering (SPS) has been used extensively for the consolidation due to rapid sintering together with avoidance of deformation induced texture during consolidation process.…”