Fundamental analysis of deformation behavior in core-shell heterostructured materials

Abstract

Many studies have extensively investigated various microstructural features to understand heterogeneous microstructure effects with mechanical responses. In this study, a representative volume element modeling approach was used to control specific microstructure features accurately beyond the experimental difficulties. In particular, the deformation behavior of the core–shell structure was clearly visualized using the finite element method with the proposed dislocation-based constitutive model considering the evolution of geometrically necessary dislocations. From these findings, we suggest that the optimal heterogeneous microstructure should be designed based on the deformation behavior with respect to the geometry, size, and shape of soft and hard domains.

Introduction

Heterostructures have been extensively researched to improve conventional design strategies and achieve more advanced mechanical properties by controlling the microstructures using appropriate processing techniques [1], [2], [3], [4], [5]. The design of microstructural heterogeneity should consider several microstructural features to enhance strength, ductility, and fracture toughness. Although deformation behavior is mainly associated with factors that influence the microstructure–mechanical property relationship of heterostructures, the fundamental deformation behavior of these materials has not been quantitatively elucidated thus far.

Microstructural heterogeneities, such as grain size, phase, and morphology, lead to the formation of a strain gradient during plastic deformation owing to interfacial strain incompatibility between the soft and hard domains. Consequently, hetero-deformation induces a strain partition that preferentially deforms the soft domain by forming geometrically necessary dislocations (GNDs). Thus, the plastic incompatibility between the soft and hard domains is observable as a gradient distribution of GNDs in the soft domain at the interface [6]. As deformation proceeds, the accumulated GNDs promote hetero-deformation induced (HDI) hardening. This indicates that a heterogeneous interface diretly results in strain incompatibility and GND accumulations, and promotes HDI hardening [7], [8]. In a recent study, the phase boundaries in the designed dual-phase alloys were investigated with respect to their influence on mechanical properties due to strain accommodation that is dependent on phase morphology and size as an expansion of the grain level [9]. The phase morphology rather than phase orientation induces different microscopic deformation behaviors with respect to the width of the phase boundary affected zone where GNDs are piled-up; a narrow width of the soft phase enclosed by the hard phase can have a higher density of phase boundary affected zone in the entire soft phase region.

Furthermore, the importance of the phase arrangement was studied from the core–shell structure designed assuming the same phase interfacial region in this study. A structure composed of a soft matrix and hard particle is defined as a hard-core (HC), soft-shell structure [10], [11], [12], [13]. Conversely, a structure composed of a hard matrix and soft particle is defined as a soft-core (SC), hard-shell structure, which was proposed by Ameyama et al. based on the characteristic morphology and mechanical properties of these materials [14], [15]. The two types of core–shell structures are evaluated by applying a dislocation-based constitutive model using the finite element method (FEM) to verify the importance of the relationship between microstructural engineering and mechanical properties in the design of optimal heterostructures. In particular, because this proposed dislocation-based constitutive model can consider the evolution of GNDs, unlike the simple constitutive model that does not consider the physical meaning and spatial distribution of GNDs, the effect of additional HDI hardening by strain incompatibility was successfully estimated at the domain boundary.