The Plaston Concept

Advanced structural materials are required to show both high strength and large ductility/toughness, but we have not yet acquired the guiding principle for that. The bulk nanostructured metals are polycrystalline metallic materials having bulky dimensions and average grain sizes smaller than 1 μm. Bulk nanostructured metals show very high strength compared with that of the coarse-grained counterparts, but usually exhibit limited tensile ductility, especially small uniform elongation below a few %, due to the early plastic instability. On the other hand, we have recently found that particular bulk nanostructured metals can manage high strength and large tensile ductility. In such bulk nanostructured metals, unusual deformation modes different from normal dislocation slips were unexpectedly activated. Unusual <c + a> dislocations, deformation twins with nano-scale thickness, and deformation-induced martensite nucleated from grain boundaries in the bulk nanostructured Mg alloy, high-Mn austenitic steel, and Ni-C metastable austenitic steel, respectively. Those unexpected deformation modes enhanced strain hardening of the materials, leading to high strength and large tensile ductility. It was considered that the nucleation of such unusual deformation modes was attributed to the scarcity of dislocations and dislocation sources in each recrystallized ultrafine grain, which also induced discontinuous yielding with clear yield drop universally recognized in bulk nanostructured metals having recrystallized structures. For discussing the nucleation of different deformation modes in atomistic scales, the new concept of plaston which considered local excitation of atoms under singular dynamic fields was proposed. Based on the findings in bulk nanostructured metals and the concept of plaston, we proposed a strategy for overcoming the strength-ductility trade-off in structural metallic materials. Sequential nucleation of different deformation modes would regenerate the strain-hardening ability of the material, leading to high strength and large tensile ductility. The strategy could be a guiding principle for realizing advanced structural materials that manage both high strength and large tensile ductility.

Plastic deformation proceeds through the nucleation and migration of localized atomistic defects, i.e., plastons (e.g., dislocations, disconnections, disclinations, and shear transformation zones), as plastic strain carriers. Revealing the atomistic details, as well as the kinetics and thermodynamics, of nucleation and migration of localized atomistic defects is crucial for achieving a fundamental understanding and control of plasticity. Free-energy-based atomistic modeling is a promising approach for achieving this task. In this paper, recent free-energy-based atomistic studies on the nucleation kinetics of defects—particularly, (1) shuffling-dominant deformation twinning in magnesium, (2) dislocation nucleation from grain boundaries, and (3) homogeneous dislocation nucleation in nanoindentation—are introduced to demonstrate the advantages of free-energy-based atomistic modeling.

Disclination is a line defect in which rotational symmetry is broken. Recently, such defects have been observed in nanostructured metals. Hence, disclinations can bring out the unique properties of nanostructured metals. This chapter shows two examples of disclination-mediated plastic phenomena observed in atomic simulations. The first one is the grain subdivision mechanism, which is related to the mobility of partial disclination under severe plastic deformation processes. The second one is a mechanism that improves the fracture toughness using the disclination shielding effect, which appears at grain boundaries after dislocation emission. These atomic simulations with the geometrical restrictions of boundary conditions showed the possibility of selecting a plastic deformation mode by designing structures, elements, and environments to obtain materials with excellent mechanical properties.

Fundamental information on the collective motion of atoms can be learned by tracing imaginary phonon modes in deformed crystals. This should be useful to investigate the atomic process of plaston in a logical manner. Here, we summarize our works on the collective motion of atoms in unary metallic crystals examined by first principles phonon calculations. A simple algorithm for automated searching of the structural transition pathway following the imaginary phonon modes has been constructed. We first show the construction of structure evolution diagrams in Cu, Mg, Ti, and Hf by taking a simple cubic (SC) structure as a starting structure. The pathway corresponds to a route connecting initial and final structures without any energy barrier. Effects of the hydrostatic pressure on the diagram have been examined as well. In the second part, the collective motion of atoms in HCP-Ti under homogeneous shear deformation corresponding to the {\(10\overline{1} 2\)} twinning mode is shown. The structural transition pathway from the parent to twin, which is widely accepted as the “shuffling”, is shown in detail.

Dislocation is one of the most representative plastons to determine the mechanical properties of crystalline materials. In this section, several computational approaches for of dislocations and dislocation-related properties are introduced within the framework of first principles calculations. The staking fault energy corresponding to the local interfacial misfit energy is one of the most important characteristics to determine the shape and motion of dislocations. The first principles calculations of the staking fault energy on various slip planes in HCP metals are provided compared with several materials. Peierls–Nabarro model is then introduced to describe the shape and motion of the dislocation. Finally, atomic configuration of a dislocation dipole in a periodic cell can be modeled based on the linear elasticity theory. First principles calculations are thus directly applied to dislocation core structure which enable us to evaluate the effect of solute element on the dislocation core structure and motion.

The concept of ‘plaston’ that involves cooperative atom motion under shear stress is discussed as a deformation carrier that nucleates and moves in the deformation front under shear stress in many different materials in general. The selection of a plaston of a particular type among many different plastons depends on stress level/state, crystallographic orientation, specimen size (grain size) and so on. The importance of the understanding of the activation of various plastons is discussed for the improvement of mechanical properties of existing structural materials and the development of new structural materials with high strength and high ductility.

Alumina (α-Al₂O₃) is one of the representative structural ceramics. To understand its mechanical responses, the lattice defect behavior of alumina has been investigated by transmission electron microscopy (TEM) for many years. In this report, we review our recent research progress on TEM structural analysis of lattice defects in alumina. In the first half, the core atomic structure and dissociation reaction of b = \(1/3<11\bar{2}0>\), \(<1\bar{1}00>\), and \(1/3<\bar{1}101>\) dislocations formed in low-angle grain boundaries are investigated by atomic-resolution TEM observations. Based on experimental results, the slip deformation behavior associated with those dislocations is discussed. In the second half, the formation of \(1/3<11\bar{2}0>\) dislocations and fracture of Zr-doped ∑13 grain boundary of alumina are observed by in situ TEM nanoindentation. Furthermore, these indented samples were observed by atomic-resolution scanning TEM. The mechanisms of the deformation and fracture phenomena are discussed in detail.

Mechanical behavior of metallic materials on nanoscale is characterized by using Nanoindentation and Transmission Electron Microscope (TEM) to understand the fundamental plasticity mechanisms associated with microstructural factors including dislocations. The advanced characterization techniques enable us to grasp the behavior on the nanoscale in detail. New knowledges are obtained for the plasticity initiation under the extremely high stress close to the theoretical strength in regions with defect-free matrix and pre-existing defects such as grain boundaries, in-solution elements, and dislocations. The grain boundaries act as an effective dislocation source, the in-solution elements retard a nucleation of dislocation, and the pre-existing dislocations assist a plasticity initiation. The deformation behavior associated with microstructures is also described. The dislocation structure with a certain density was observed right after indentation-induced strain burst, which is so-called “pop-in,” suggesting a dislocation avalanche upon the pop-in. It has been directly observed that the lower mobility screw dislocation causes the higher flow stress in a bcc metal. A remarkable strain softening can be understood by an increase in dislocation density based on conventional physical models. Phase stability for indentation-induced transformation depends on a constraint effect by inter-phase boundary and grain boundary.

Grain refinement is one of the methods applied to strengthen metallic materials, and various peculiar mechanical properties have been reported to be expressed when the grain size is reduced to less than submicron dimensions. This is considered to be due to a change in the behavior of dislocations that are associated with plastic deformation. In situ synchrotron radiation measurements of microstructural changes during deformation in face-centered cubic (fcc) metals with grain sizes of 20 μm to 5 nm were performed to systematically investigate the effects of grain size on dislocation behavior during plastic deformation. In pure aluminum with grain sizes of 20 to 3 μm, the dislocation density during plastic deformation was approximately 10¹⁴ m⁻², regardless of the grain size. However, when the grain size was less than 3 μm, the dislocation density increased monotonically in proportion to the grain size to the power of -1. Furthermore, in a nickel alloy with a grain size of less than 10 nm, this relationship was no longer satisfied, and the results suggested that deformation progresses due to partial dislocations. In materials with a grain size of less than 1 μm, the dislocation density after unloading became much smaller than that during loading.

In this chapter, we present a metallurgical–mechanical mechanism-based strategy for the design of fatigue-resistant metals. Specifically, we elucidate the importance of the metallurgical microstructure in a mechanical singular field (crack tip). The fatigue crack growth resistance is controlled through the crack tip “plasticity”, and the effect of the associated microstructure becomes significant when the crack is “small (or short)”. More importantly, the resistance to small crack growth determines a major portion of fatigue life and strength. Therefore, the microstructural crack tip plasticity is a key breakthrough to the development of fatigue-resistant metals. As successful examples of this concept, we introduce the effects of grain refinement, martensitic transformation, strain aging, dislocation planarity enhancement, and microstructure heterogeneity on small fatigue crack growths.

High-Mn austenitic steels undergo characteristic plasticity mechanisms of the γ-austenite with an FCC structure, such as extended dislocation glide, mechanical twinning, and mechanical martensitic transformation into ε-martensite with an HCP structure and/or α’-martensite with a BCC/BCT structure. Distortions of polyhedron models are used to describe these plasticity mechanisms. These are the smallest volumetric units occupying the lattices and reflect the crystallographic characteristics of the lattices. The complicated crossing shears are correlated to the fine crystal phases formed at the intersection of the ε-martensite variants. The unidirectionality of the {1 1 1} < 1 1 2 > _γ twinning shear provides reversibility to the dislocation motion under cyclic loading. Based on this knowledge, the design concept of high-Mn steels is described considering microstructural, thermodynamic, and crystallographic characteristics.

Magnesium alloys are one of the lightest commercial metals, and applications of wrought magnesium alloys may lead to a substantial weight reduction of transportation vehicles. However, applications of wrought magnesium alloys are limited due to processing costs. This chapter discusses the requirements to broaden their applications through a brief review of existing wrought alloys. Then, a heat treatable wrought magnesium alloy is proposed as a new design concept for the wrought magnesium alloy. As an industrially viable precipitation hardenable alloy, Mg–Ca–Al(–Zn) dilute alloy is developed. A high strength Mg–Al–Ca–Mn alloy extrudable at high speed and a bake-hardenable Mg–Al–Ca–Mn–Zn alloy sheet with excellent room temperature formability and satisfactory strength are demonstrated, indicating the promising potential to develop heat treatable wrought magnesium alloys.