Research Outline

  • Research Project Number: 18H05475
  • Researcher Number: 70354222
  • Term of Project: FY2018-2022
  • Contact: Eiji Abe (The University of Tokyo, Graduate School of Engineering, Professor)

Purpose of the Research Project

In order to solve the energy problem and realize a sustainable society, one of the prominent issues in materials science is to develop high-strength, light-weight structural materials. In our research project, we establish the "Kink strengthening phenomenon" as a univerasal strengthen principle, which has been firstly discovered in the LPSO-structured Mg alloy that revealed unusual high-strength beyond theoretical predictions. The LPSO structure can be generally viewed as "Mille-feuille structure", in the sense that they are constructed by alternate stacking of microscopic hard- and soft-layer. Establishing a universal kink principle applicable to any mille-feuille structures will lead to a new academic, innovative area. Furthermore, based on the established "kink stengthening principle", we will be able to design new alloys including Ti and Al alloys and further new polymer materials, providing an exciting opportunity for the development of next generation structural materials.

Content of the Research Project

Since kink formation and stengthening are not fully understood yet along with the existing solid deformation theory, it is indispensable to provide cross-disciplinary opportunities beyond the conventional frameworks, in order to establsih a new academic field "Materials Science of a Mille-feuille structure". In our research project,　researchers participate across the wide reserach fields that are indispensable for the present tasks, the majour three of which are "Materials synthesis (monozukuri)" "Solving the kink mechanism (elucidation of fundamental properties)" "Theory construction (universal principle/concept)". We will be all together to form "Japan National Team" to tackle these challenging issues, creating a new universal academic field.

There are four research groups in our project. In A01 group, along the experiences with the LPSO-type Mg alloys, we will attempt to develope novel Mg alloys having various mill feuille structures. In A02 group, we will try to elucidate the kink mechanism by performing mechanical experiments, advances structural measurements and computation modeling. In A03 group, a kink strengthening theory will be constructed under the effective collaborations between multiple fields including materials science, mechanics, physics and mathematics. In A04 group, we will try to develop and synthesize novel metal- and polymer-base Mille-feuille materials according to a proposd kink strengthening theory.

Expected Research Achievements and Scientific Significance

  1. Establishing a novel strengthening principle of the Millefeil structure makes it possible to develop higher strength structural materials including new Mg, Ti and Al alloys, and further polymer based materials, contributing to an esablsihement of a ebergy-saving, sustanable society.
  2. Establishment of the systematic kink strengthening theory of the Millefeil structure is engraved in history as a new material strengthening method, and hence leads to worldwide reputation in a material science field.
  3. Elucidation of the kink strengthening mechanism, based on the hierarchical structure science from the atomic level to the mesoscopic structures, brings out a drastic extension into a new solid mechanics that includes novel geomotry and non-linear elastic theory.
  4. Establishment of a new academic field “Materials Science of a Mille-feuille Structure” brings a great influence on wide basic-reserach fields, as well as an effective growth of the engineering fields and the relevant industry.

Key Words

Mille-feuille Structure : A microscopic layered structure constructed by an alternate stack of hard-layer (strongly-bonded) and soft-layer (weakley-bonded). It is named after "Mille-feuille cake", which is composed of a pie layer (hard layer) and a cream layer (soft layer).

A01：Creation of a new Mg alloy with various millefeuille structures

By creating new Mg alloys that satisfy the mille-feuille conditions and developing effective kink introduction technology, we will establish development guidelines for new mille-feuille materials based on the new strengthening principle through control of the mille-feuille structure and kink formation.

A01-1 : Development of novel Mg alloys with millefeuille structure

We will create a new Mg-based millefeuille structural material that meets empirical millefeuille conditions and exhibits kink strengthening.

  1. Using the LPSO-type Mg alloy as a model alloy, the interlayer distance of the solute-enriched layer (hard layer) is controlled, and the characteristic length at which kink formation develops is clarified to refine the empirical millefeuille conditions and to refine the mille-feuille structure. By extracting the forming and deformation controlling factors, it contributes to the "deepening" of Millefeuille materials science.
  2. In the creation of new Mg alloys, after designing the optimum components by thermodynamics and phase diagram calculation, strain-induced precipitation by processing heat treatment control and new Milfille structural material by hierarchical higher-order structure control that combines phase transformation and precipitation Try to create. In addition, by making full use of new non-equilibrium processes such as rapid solidification and molten salt electrolysis, we will contribute to the "development" of this area by aiming to expand various Milfille material groups in Mg alloys.
  3. By conducting an experimental search for the selection principle of the optimum process path by large-scale quantum wire in-situ diffraction / scattering experiments, we will improve the efficiency of control technology and establish the guiding principle for the creation of Mg-based millefeuille materials.
A01-2：Kink control and material creation of various Mg-based millefeuille structures (fusion system)

In the present study, our research target materials are several mille-feuilled Mg system materials, which is satisfied with experimental mille-feuille condition. In order to produce the bulked Mg alloys with controlling kink-morphologies and/or kink-structures, we apply “a multiple high-dimensional plastic processes”, i.e., not only conventional wrought process (extrusion, forging, rolling) but also unique process (caliber rolling, multiple forging) as well as severe plastic deformation (high pressure torsion etc), to these mille-feuilled Mg system materials. We, from the atomistic scales to the macroscopic scales, control kink-morphologies (density, size, angle, stress distribution in kink) and kink-structures (matrix/kink boundary structure and strain distribution at interface) via managing of many process parameters, such as temperature, forming speed, applied strain and so on. We also use numerical simulation methods to predict applied strains during this multiple high-dimensional plastic forming processes. In addition, we observe detailed microstructures of bulked Mg alloys using several methods, e.g., transmission electron microscopy and electron back-scatter diffraction pattern. We have two challenging goals via this study; at the first point, we try to develop a suitable process technique to induce kink-morphologies and its structures, through mutual understanding and complement the present results. At the other point, we try to propose suitable kink-morphologies and its structures, which are shown superior mechanical properties to those of conventional metallic materials, based on the results of mechanical testing in several types bulked Mg alloys.

A02：Deepening and development of areas by elucidating the mechanism of kink formation and kink strengthening

Multi-scale mechanical property evaluation, structural / microstructure analysis, and modeling are integrated to experimentally elucidate the kink formation mechanism and strengthening mechanism of the millefeuille structure, and the creation of various new Mg-based and new metal / polymer-based millefeuille materials (A01, A04) ・ Contribute to theory construction (A03).

A02-1 : Clarification of formation and strengthening mechanisms of deformation kink by the mechanical analyses

The formation mechanism (formation criteria) of deformation kink bands, which is drawing attention as the controlling factor of the unique mechanical properties exhibited in the Mille-feuille structure, and the resultant strengthening mechanism by the kink band formation are still unknown. In this research, attention has been paid to the "Mille-feuille criteria" for kink formation / strengthening proposed from the empirical viewpoint, and we will verify the validity of "Mille-feuille criteria" from the experimental point of view.
By focusing on the various materials with different microstructural factors in the Mille-feuille structure, such as size ratio of hard layer and soft layer, crystal structure, interface structure between the layers which governs the continuity of deformation through the interface, crystal orientation relationship between layers, etc., the controlling factors of the deformation behavior; active deformation mode, deformation mechanism etc. will clarified.
In addition, we will examine the temperature, strain rate and crystal orientation dependencies of the deformation behavior in the Mille-feuille materials to further clarify the kink formation / strengthening mechanism in them, by using various novel multi-scale mechanical analysis methods including micro-pillar test, in-situ observation with ultrahigh-speed video camera combined with acoustic emission (AE) measurement, and mechanical testing of the oriented single crystals.
Through these, we will provide guidelines for developing the high-performance Mille-feuille materials by elucidating the elementary processes of the kink band formation and controlling factors of the mechanical properties induced by the Mille-feuille structure.

A02-2 : Solving kink formation/strengthening mechanism through precise structure analyses

In various novel Mg-based and novel metal-based materials, in the layered structure satisfying the Millefeuille condition, a space-filled kink region is formed without peeling between the hard layer and the soft layer. From this multi-scale multi-scale precision analysis of the kink structure, we aim to elucidate the structural properties of these two points, how these kinks were formed, and how the kink contributes to material reinforcement.

Micro analysis by electron beam
n the kink region, in the vicinity of the interface formed by lattice rotation, the three-dimensional structure of the lattice continuity of hard and soft layers, distribution / bonding state of additive elements, lattice strain, and dislocations is spread from the atomic level to the grain scale (10-10～10-6 m). Systematically analyze.

Macro analysis by quantum wire
Neutron and synchrotron radiation Using the most advanced measurement methods for large-scale facilities, the interaction and stress distribution between hard and soft layers of the Milfille structure, kink strain analysis, in-situ observation of kink formation, and bulk kink three-dimensional morphology from the microscale (10-6～10-2 m). Systematically analyze up to the scale.

A02-3 : Modeling-based investigation of mechanism of kink deformation and strengthening

In the present project, we will elucidate (i) the controlling factors for the kink formation at the micro- to meso-scale and (ii) the dynamic state of the kink formation at the meso- to macro-scale in various mille-feuille structures. These issues are explored by applying a new computational-mechanics method which bridges the spatial and/or temporal scales, such as large-scale electronic-state calculations, coarse-grained atomistic simulations, and higher-order strain gradient crystal plasticity models.
From the microscopic analysis, we obtain the energetics of nucleation and migration of dislocations that constitute the kink boundaries. In particular, we focus on the atomic shuffling with local lattice rotation, which corresponds to an elementary process of kink formation, to clarify the peculiar deformation mode under the limitation of active slip and twin systems. From the macroscopic analysis, we identify the size effect and dislocation patterning in the kink formation process and their influence on the mechanical properties to capture a physical scenario how the formation of kink bands leads to strengthening the system through possible cooperation or competition with other deformation mechanisms. Finally, we will establish guidelines for the design of hard and soft layers in mille-feuille structures and for the control of a kink-deformation mode in mille-feuille structured materials.

A03 : Formulation of the kink formation and strengthening theory for mille-feuille structured materials

We will investigate the kink formation during the deformation of mille-feuille structured materials through an interdisciplinary perspective involving mathematics, physics, mechanics, and materials science. We will further elucidate the mechanism of kink formation and establish a theory of the unique strengthening mechanism due to the kink formation.

A03-1 : Formulation of a comprehensive theory of kink formation and strengthening via an interdisciplinary approach (fusion of theory and experiment)

Kink formation during the deformation of mille-feuille structured materials is considered to be one of nonlinear phenomena in dislocation dynamics. Based on mathematics and materials science and from the multidisciplinary and multilateral perspectives, a kink formation theory is developed as follows. (1) A mathematical model for the kink formation mechanism is developed. (2) The geometric conditions of the kink interface compatibility are found on the basis of physical crystallography. (3) The kink formation is visualized according to the disclination theory, including dislocation theory and microstructural characterization of materials. After the evaluation of the mathematical model via numerical simulations, a kink formation theory is proposed. (4) Furthermore, the kink formation is modeled on the basis of the field theory in plasticity. (5) Next, nonlinear partial differential equations describing the elastic interaction between kinks and lattice defects are derived. (6) Subsequently, a model for kink strengthening in mille-feuille structural materials is developed from materials physics. Through a comparison with experimental results, these findings are refined and integrated to formulate a kink strengthening theory for mille-feuille structural materials. Finally, we will propose the design guidelines for mille-feuille structural materials using metals as well as organic and inorganic materials.

A04：構造制御&キンク制御による領域の展開

New metal- and polymer-based materials were developed to satisfy the requirements of mille-feuille which guidelines to create the new concept in strengthening mechanism through the control of kink formation and the mille-feuille structure.

A04-1 : Structural Control and Development of New Materials Based on Metals and Polymers

The purposes of this research (A04-1 group) are the control of the structural formation and development of new materials satisfying the desired properties of the mille-feuille products inspired by strengthening mechanisms acting in Mg-based long-period stacking ordered (LPSO)-phase alloys. In other words, we aim to explore, design and realize a group of materials to fabricate the laminated structures composed of alternative soft and hard layers with similar the Mg-based LPSO phases.
From the phase diagram investigation of binary or higher order system of the metals including Ti alloys, Al alloys, Co alloys, etc., in the novel metallic system, the close-packed planes having highest atomic density and interplanar distance is large hence it becomes the favorable plane for dislocation motion or slip occur between the hard layer/soft layer interface. For these reasons, it is necessary to control the morphological structure of metallic mille-feuille by the segregation of the heterostructures of the elements, process conditions, heat treatment methods, and so on.
In the polymer system, it has been synthesized a block copolymer having the layered microphase-separated structure composed of butadiene and styrene. Moreover, polyethylene crystals having a layered structure, polypropylene/polybutadiene blends, polypropylene/copper composites, are subjected to control the molecular orientation by the injection molding and melt drawing, for investigating the structural formation of the mille-feuille products. In this research, we will investigate the formation behaviors of mille-feuille structures aim for guiding the establishment of the principle to control these new mille-feuille structures.

A04-2 : Control of the kink formation and development of metal/polymer materials for the novel type of mille-feuille structure

New mille-feuille materials such as metals, polymers, and its composites will be fabricated using various processing techniques such as conventional rolling, hot rolling with precise temperature control, accumulative roll bonding (ARB), nano-imprinting, drawing process, stretching in the supercritical fluid and/or under ultraviolet (UV) conditions, and so on. Various mille-feuille materials were newly found and proposed by the A04-1 group to obtain the ultra-fine layer structures with high-density kink structures.
In this research, we will clarify the relation between the process conditions and the mille-feuille morphological structures (layer structure, kink shape and its structure) by using various evaluation methods of the morphological observation and structural analysis of the obtained new mille-feuille materials.
This research aims to propose the novel fabrication methods of the mille-feuille structure caused by the strengthening mechanism of kink formation. The relationship between process conditions, structural formation behaviors, and physical properties will clarify for understanding the fundamental of the novel strengthening mechanism. The process conditions will optimize for improving the properties of the novel mille-feuille material.