Importance of Bimodal Structure Topology in the Control of Mechanical Properties of a Stainless Steel

Abstract

Refinement of microstructure from ordinary coarsegrained (CG; grain size d ! 10 mm) down to ultrafine-grained (UFG; grain size d 1 mm), and even further to nano-scale, is a well-proven approach for the strengthening of metallic materials. [1][2][3] However, the downside of bulk UFG metals is their rather poor ductility resulting from very limited capacity for dislocation storage and therefore early strain localization during mechanical loading. [2][3][4][5] Since both strength and ductility are important characteristics for the majority of structural applications, various strategies have been suggested to improve the ductility of bulk UFG and nano-structured metals and alloys. [5,6] Among these strategies, fabricating structures with rather wide, gradient or, better, bimodal grain size distributions appear to be very efficient for the enhancement of ductility at a little compromise in strength. [7,8] In such structures, the ultrafine-and nano-scale grains provide the high strength, while the coarser grains store dislocations, therefore, enhancing the ductility characteristics. [3][4][5] Recently, a substantial number of processing routes allowing the fabrication of metallic materials with bimodal grain size distributions have been developed. [8][9][10][11] These routes are based on severe plastic deformation (SPD) [10] and other thermo-mechanical processing techniques [8,9] as well as on powder metallurgy approach. The latter involves mechanical milling (MM) of metal powders, mixing of the MMed and the initial powders (IP), and finally consolidating the mixture. [11] Such processing routes, although effective in the control of UFG/CG fraction ratios, provide very limited control over the spatial topological distribution of the coarse and the fine grains. Therefore, these limited-control microstructures deliver only a limited control over properties.In order to improve the control over the microstructural characteristics, and thus properties, Ameyama and co-workers [12][13][14] recently proposed a novel approach based on the powder metallurgy route. Its peculiar features are: (i) the MM processing is optimized to induce heterogeneous structure in powder particles having CG and nano/UFG structures in their core and periphery, respectively; and (ii) the consolidation is optimized to preserve the heterogeneity. Bulk materials fabricated in this way have bimodal structure with a specific regular spatial distribution of nano/UFG and CG regions. Namely, CG structure is enclosed in a three-dimensional continuously connected network of nano/UFG structure. Therefore, we call them "harmonic structured" materials. Further details about the fabrication route, the "harmonic structure" (HS) design concept, and the benefits of them can be found elsewhere. [12][13][14] Although both the ordinary limited-control bimodal structures and the HSs exhibit a very appealing combination of strength and ductility, the variation of the mechanical properties can be quite large. [6] It is envisaged that the large variation of the mechanical ...

Application of High Pressure Gas Jet Mill Process to Fabricate High Performance Harmonic Structure Designed Pure Titanium

“…7,11,15) Hence, the controlled mechanical milling via the jet mill proved extremely effective in achieving controlled severe plastic deformation, limited to the sub-surface regions of the fine size powder particles, in the form of a featureless "shell" region. As a result, jet milling successfully created a bimodal microstructure in the milled powder particles, i.e.…”

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

“…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 .…”

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

“…Moreover, other important observations were also made from these studies: (a) Both random and HS bimodal exhibit same strength but HS exhibit higher ductility when compared to random bimodal; (b) the HS specimens show extremely small variation of properties when compared to random bimodal specimens. [19,24,25] Moreover, in general, it was also observed that the harmonic structure design promotes uniformity of deformation when compared to random bimodal structure, leading to suppressed strain localization and relatively higher tensile ductility.…”

“…The effect of topological distribution of fine-and coarse areas on the properties was also evaluated wherein random bimodal was created by mixing severely milled powders with un-milled initial powders. [19,24,25] It was observed that the random bimodal structured specimens demonstrated higher strengths when compared to bulk CG specimens. Moreover, other important observations were also made from these studies: (a) Both random and HS bimodal exhibit same strength but HS exhibit higher ductility when compared to random bimodal; (b) the HS specimens show extremely small variation of properties when compared to random bimodal specimens.…”

“…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.…”