Harmonic Structure Design: A Strategy for Outstanding Mechanical Properties in Structural Materials

Abstract

Structured heterogeneous materials are ubiquitous in a biological system and are now adopted in structural engineering to achieve tailor-made properties in metallic materials. The present paper is an overview of the unique network type heterogeneous structure called Harmonic Structure (HS) consisting of a continuous three-dimensional network of strong ultrafine-grained (shell) skeleton filled with islands of soft coarse-grained (core) zones. The HS microstructure is realized by the strategic processing method involving severe plastic deformation (SPD) of micron-sized metallic powder particles and their subsequent sintering. The microstructure and properties of HS-designed materials can be controlled by altering a fraction of core and shell zones by controlling mechanical milling and sintering conditions depending on the inherent characteristics of a material. The HS-designed metallic materials exhibit an exceptional combination of high strength and ductility, resulting from optimized hierarchical features in the microstructure matrix. The experimental and numerical results demonstrate that the continuous network of gradient structure in addition to the large degree of microstructural heterogeneity leads to obvious mechanical incompatibility and strain partitioning, during plastic deformation. Therefore, in contrast to the conventional homogeneous (homo) structured materials, synergy effects, such as synergy strengthening, can be obtained in HS-designed materials. This review highlights recent developments in HS-structured materials as well as identifies further challenges and opportunities.

Local Deformation Behavior of the Copper Harmonic Structure near Grain Boundaries Investigated through Nanoindentation

“…Figure 6 a shows a schematic of the true stress–true strain curve for homogenous materials [ 9 ]. Generally, metals undergo strain hardening during deformation, which may assist in bearing loads during deformation.…”

“…It is notable that necking or localized deformation, which is accompanied by a reduction in the cross-sectional area of the tensile specimen, occurs when the increase in stress is higher than the increase in the load-carrying ability of the metal due to strain hardening [ 8 ]. Therefore, as shown in Figure 6 a, the point of necking under tensile stress is obtained at the point where the strain rate hardening equals the stress (dσ/dε σ) in the true stress–true strain curve [ 9 ]. Typically, homogenous materials exhibit high strength but poor ductility owing to their poor strain-hardening capabilities, as indicated in Figure 6 a.…”

“…For example, pure copper shows a higher strain-hardening rate up to fracture compared with other materials [ 3 ]. It is reasonable to assume that a higher strain rate leads to higher elongation or ductility based on the plastic instability condition [ 8 , 9 ]. However, the exact reasons for the higher strain rate and higher elongation have yet to be reported.…”

Mechanical Properties of Spark Plasma Sintering-Processed Pure Ti and Ti-6Al-4V Alloys: A Comparative Study between Harmonic and Non-Harmonic Microstructures

“…Harmonic microstructures lead to a homogeneous distribution of strain, while plastic activity occurs and avert plastic deformation localization due to an accommodation of plastic deformation induced by the strain distribution in the coarse grains domains, which lead to an accommodation of work hardening to large strain values [20,21]. All the studies carried out to date have shown that harmonic structures improved mechanical properties compared to conventional fine-grained or ultra-fine microstructures [20][21][22][23][24][25]. In particular, a much better synergy between mechanical strength and ductility was observed [18,21].…”

“…All the studies carried out to date have shown that harmonic structures improved mechanical properties compared to conventional fine-grained or ultra-fine microstructures [20][21][22][23][24][25]. In particular, a much better synergy between mechanical strength and ductility was observed [18,21]. This results from the microstructure's heterogeneous character, which induces the delocalization of the deformation due to the grain size gradient [26], thus delaying the plastic instability during uniaxial tensile tests, for example [17].…”