Critical Assesment 37:  Harmonic-structure materials - idea, status and perspectives

Abstract

Recent efforts in engineering metals with high structural efficiency have resulted in developing a new category of artificial materials with heterogeneous microstructures architected across multiple scales. In this critical assessment, a relatively new concept of heterogeneous bimodal harmonic-structure (bHS) materials is introduced and analysed. It is shown that the bHS concept is applicable to a large variety of metallic materials and is most efficient in scenarios where changes in the chemical composition of materials are restricted for some reason. Basic principles, weaknesses and advantages along with present development status and perspectives are discussed. The overview of critical performance characteristics of various bHS materials is provided, and interesting directions for future research, development and applications are proposed.

Peridynamic Modelling of Harmonic Structured Materials Under High Strain Rate Deformation

“…A proposed solution is to modify the structure heterogeneously, by featuring both CG and UFG microstructures in a bimodal fashion [19]. The maintained ductility originate from the coarse grains storing the lattice defects, while the UFG provide the increased strength [5].…”

“…The fabrication process to achieve the harmonic-structured material in a reproducible fashion is as follows [5]:…”

“…The deformation process consists of an initial homogeneous elastic deformation (elastic properties of the UFG and CG materials in the HS material are the same), followed by a plasticity in the CG regions. This initial plasticity affects the stress strain-curve moderately, changing the slope inclination but maintaining a nearly linear behaviour during much of the stress increase, according to [5]. Knowledge of the behaviour following yielding in the CG region is as of yet not established fully, but is believed to Theodor S. de Sousa, Aylin Ahadi, Elis Sjögren, and Dmytro Orlov be attributed to dislocation movement [1].…”

“…One of the methods for increasing strength is to decrease the grain size in the material, from coarse-grained (CG, d CG ≥ 10 µm) [1] down to ultra-fine-grained (UFG, d UFG ≤ 1.0 µm) [1], but this method also decreases ductility [2]. A possible solution proposed is to create a heterogeneous grain size distribution in a certain topology, creating what is denoted as a Harmonicstructured Material (HS) [1,[3][4][5]; The harmonic structure consists of evenly distributed cores of CG material inside a network of UFG material, as seen in Figure 1 on the following page. The grain sizes should differ at least by one order of magnitude, and the elastic behaviour and chemical composition must be the same for both UFG and CG materials.…”

Ratcheting-Fatigue Behavior of Harmonic-Structure-Designed SUS316L Stainless Steel

“…In recent years, structural materials with heterogenous microstructures have been proposed to achieve excellent mechanical properties and good fatigue resistance [7][8][9][10][11][12][13][14][15]. Mechanical properties and deformation mechanisms of some typical heterogenous microstructures, such as gradient nanograined (GNG) structure [9][10][11], lamellar structure [12][13][14], hierarchical and laminated grains and twins structure [15], and harmonic structure [16,17], have been investigated. As indicated in Figure 1a, the conventional bimodal structure has an irregular coarse grain (CG) and ultrafine grain (UFG) distribution.…”