Blending irreconcilable properties of materials by macro directional design of microstructure

Recently, Prof. Yonghao Zhao's group from School of Materials Science and Engineering in Nanjing University of Schence and Technology (NJUST) have made great breakthroughs in breaking material property trade-offs via macro-design of microstructure, which were published in “Nano Letters” (Link to this paper:https://doi.org/10.1021/acs.nanolett.1c00451) and “Communication Materials” (Link to this paper:https://www.nature.com/articles/s43246-021-00150-1).


Many properties of a material are irreconcilable. They proposed a new concept of macro directional optimization/design of microstructure in view of material properties trade-off paradox. Their concept sets up a bridge from micro to macro, truly achieves the accurate use of materials performance according to their specific working conditions. By using this concept, they successfully solved the paradox of thermal stability, high strength and high conductivity of copper contact wire in high-speed train.


1. Presentation and analysis of scientific difficult problem

Many properties of a material in nature are inconsistent and even contradict each other, this is so-called material properties trade-off paradox. The most typical example is the relationship between strength and ductility of a material. Strength is not only contrary to ductility, but also ambivalent to formability, deformability, conductivity and thermal stability. The latter two cases will be discussed in details in our paper just published in Communications Materials (https://www.nature.com/articles/s43246-021-00150-1). In addition to structural materials, many important properties of functional materials also have similar trade-off rule, as listed in Table 1, such as energy and power densities of battery materials, electrical conductivity and Seebeck coefficient/thermal conductivity of thermoelectric materials, magnetization and coercivity of magnetic materials, polarization and breakdown strength  of dielectric materials, reactant mobility and catalytic active sites of catalytic materials, ferromagnetism and ferroelectricity of ferroelectric and ferromagnetic materials, transparency and conductivity of transparent conductors used in photovoltaics and optoelectronic materials, damping capacity and elastic modulus of damping materials, etc. It can be said that the trade-off paradox of material properties is a widespread and universal law of nature. It seems nature proposes a troublesome problem for human being which is impossible to be solved, and sets an insurmountable obstacle for human scientific and technological progress, as well as tests human wisdom all the time. In our daily life, we will also often encounter the dilemma of having to choose one from the two.

Table 1. Lists of typical trade-offs between conflicting properties I and II.

No.

Materials

Property I

Property II

Ref.

1

Battery materials


Energy density

Power density,

Cycle life,

Safety

2

2

Thermoelectric materials

Electrical conductivity

Seebeck coefficient,

Thermal conductivity

3

3

Dielectric materials

Polarization

Breakdown strength

4

4

Permanent

magnetic materials

Coercivity

Magnetization

5

5

Catalytic materials

Reactant mobility

Catalytic active sites

6

6

Ferroelectric /ferromagnetic materials

Ferromagnetism

Ferroelectricity

7

7

Optoelectronic materials

Transparency

Conductivity

8

8

Damping materials

Damping capability

Elastic modulus

9

9

Structural materials

Strength

Ductility,

Formability,

Deformability,

Conductivity,

Thermal stability

10-12

2. Solution to scientific problem

Everything is both opposite and unified, thus evolving into a harmonious nature. For the troublesome problem of material performance paradox left by nature to human being, let's see how the artist of nature himself solves it. In order to support the weight of the crown and transport nutrients longitudinally, trees have evolved a fiber structure along the longitudinal direction of trunk. In the same way, the shell has evolved a multilayer structure to resist the vertical fracture and the teeth have evolved nanostructures on their surfaces for wear resistance. We can see the microstructures of all biological materials have been macroscopically and directionally optimized according to their specific service requirements, thus solving the properties paradox. Compared with the biological materials, the current man-made materials are still simple. For example, from the performance point of view, most of them are isotropic materials, which is bound to cause the material property cannot be fully utilized, because the specific service parts are always macroscopic and directional. Therefore, the characteristics of directional design of biological materials in nature inspired us to solve the man-made material properties paradox via the concept of bionics. They thus propose a new concept that the microstructure of materials should be macro directionally optimized/designed according to their specific working conditions (Figure 1-3). By using this concept, the paradox of high strength and high conductivity of copper contact wire in high-speed train was successfully solved along axial direction. For details, please refer to their papers just published in “Nano Letters” (Link to this paper:https://doi.org/10.1021/acs.nanolett.1c00451) and “Communication Materials” (Link to this paper:https://www.nature.com/articles/s43246-021-00150-1).

Figure 1.Microstructures of the swaged Cu with ε = 2.5. a, Picture of initial coarse-grained (CG) and swaged Cu rods. b, EBSD crystal orientation map from side view. c, EBSD crystal orientation maps (c-1, c-3) and GB maps (c-2) from top (c-1, c-3) and side (c-3) views. The insets are inverse pole figures and color code, respectively. Black and red lines in c-2 represent high-angle GBs (> 15°) and low angle GBs between 2°and 15°, respectively. d, TEM images from top (d-1) and side (d-2) views. The insets are selected area electron diffraction (SAED) patterns. e, High-resolution TEM image of low-angle GBs. Inverse Fourier transformation (e-4) revealed zigzag low-angle GBs formed by polygonized dislocation walls.

Figure 2. Mechanical, thermal and conductive properties of the swaged Cu with ε ;= 2.5. a, Quasi-static tensile curves of the swaged and CG Cu tested at room and liquid nitrogen (LN) temperatures. The necking onsets are marked by empty squares. b, Yield strength and ductility (elongation to fracture) versus swaging deformation stain. c, Micro-hardness evolution during isothermal annealing process at 473, 523 and 573 K. d, Conductivity of CG, swaged and annealed Cu. e, Microstructural evolutions during isothermal annealing.

Figure 3. Literature review for relations of yield strength and conductivity/thermal stability of pure Cu prepared by different severe plastic deformation and powder sintering methods.21-32 a, Yield strength versus conductivity. b, Grain size versus annealing temperature. ED – electrodeposition, SPS – spark plasma sintering, ECAP/R/E – equal-channel angular pressing/rolling/extrusion, DCT – deep cryogenic treatment, LP/D – liquid pressing/drawing, DPD – dynamic plastic deformation, CR – cold rolling, SMGT – surface mechanical grinding treatment, LSEM – large strain extrusion machining. c, Schematic representations of microstructural evolutions of Cu during swaging and annealing processes and their influences on mechanical and conductive properties. d, Schematic representations of traditional optimization of trade-off properties and our concept of macro directional design of microstructure according to the service direction.

3. Significance and prospect of their research

Different from the traditional concepts of composite in the literature, the concept of macro directional design of microstructure (MDDM, or micro-macro design) aims to make design according to the specific working direction so that the performance of the material can be fully used. The two sides of contradiction can transform each other. Through directional design, they introduce the electrical conductivity in radial direction, which is in contradiction with the strength and axial conductivity. The strength and conductivity are no longer contradictory in the direction of axis, but increase and decrease harmoniously. As schematically shown in Figure 4, the MDDM ingeniously places the strength and conductivity at the two high ends of the seesaws along the copper wire axis, achieving super compatible conductivity and mechanical properties, and even breaking the paradox of strength and conductivity. Therefore, the directional design skillfully realizes the mutual transformation of contradictions, and makes the opposition evolve into harmony. Nevertheless, they have not changed the essential law of strengthening mechanisms and electric conduction. The excellent axial conductivity of copper wire is at the expense of radial conductivity. Fortunately, they do not need copper wire conducting along the radial direction. Therefore, the soul of the design concept is to optimize the good performance of materials to the required place according to the actual working conditions and use environment. Of course, this is at the cost of sacrificing the performance of other places. In other words, they don't have to spend energy to make perfect materials in all directions, but use steel on the blade. The MDDM concept will give new inspiration to scientists in different fields and can be applied to solve other paradoxes of material properties from a new way of thinking.

Figure 4. Schematic representation of the MDDM influence on the strength and conductivity trade-off relationship. The macro directional design ingeniously places the strength and conductivity at the two high ends of the seesaws in the axial direction of the copper wire, while the conductivity in the radial direction is placed at the low ends of the seesaws.

ACKNOWLEDGMENT

This work was supported by the National Key R&D Program of China (Grant No. 2017YFA0204403), National Natural Science Foundation of China (Grant No. 51971112 and 51225102) and the Fundamental Research Funds for the Central Universities (Grant No. 30919011405). The Jiangsu Key Laboratory of Advanced Nanomaterials and Technologies (TEM, APT and SESI) is acknowledged.

More about Zhao's group and publication:https://publons.com/researcher/2872257/yonghao-zhao/publications/