Blending irreconcilable properties of materials by macro directional design of microstructure

Many properties of a material are irreconcilable. We propose a new concept of macro directional optimization/design of microstructure in view of material properties trade-off paradox. Our concept sets up a bridge from micro to macro, truly achieves the accurate use of materials performance.

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  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. As summarized in Figure 1 for Ni and its alloys, the tensile ductility for coarse-grained Ni is about 60 % (about 40 % for cast Ni)1-4. Conventional four strengthening mechanisms including grain refinement, deformation, precipitate/particle and solid solution routinely enhance the yield strength with the expense of their ductility, following the strength-ductility trade-off paradox1,3-13. From the extensive research on nanostructured and ultrafine grained Ni in the past three decades, the grain refinement and deformation result in the worst loss in ductility. The nanostructured and ultrafine grained Ni prepared by electro-deposition (ED) and severe plastic deformation (SPD) as well as cryo-rolling could possess more than 10 times higher yield strength than the coarse-grained Ni, but disappointing low ductility (< 10%)3-10. The particle hardened Ni alloys (Nicorros Al k-500 and Nicrofer 4823 Ti) have better yield strength and ductility combinations (700-800 MPa and 20%) than the nanostructured and ultrafine grained Ni1. For solid solution strengthened Ni alloys as well as high-entropy alloys, all data still deviate to the left with the lowest ductility expense rate1,11-13. Strength and ductility are just like the two ends of a seesaw, as shown in Figure 2. Increasing the strength has to lose the ductility, and in turn, obtaining the ductility must sacrifice the strength, but it is difficult to achieve both. In our daily life, hard ceramics can't be deformed, while deformable metals and plastics are soft, which is the similar principle.

 

Figure 1. Comparison of yield strength and ductility of four strengthening mechanisms (grain refinement, deformation, particle and solid solution) in Ni1-13. CG - coarse-grained, ED - electro-deposition, SPD - severe plastic deformation, SS - solid solution.

Figure 2. Schematic representation of the trade-off relationship between strength and ductility. Strength and ductility consume each other. The increase of strength is accompanied by the decrease of ductility. On the contrary, only the decrease of strength can reward the increase of ductility.

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://doi.org/10.1038/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 materials17, electrical conductivity and Seebeck coefficient/thermal conductivity of thermoelectric materials18, magnetization and coercivity of magnetic materials19, polarization and breakdown strength  of dielectric materials20, reactant mobility and catalytic active sites of catalytic materials21, ferromagnetism and ferroelectricity of ferroelectric and ferromagnetic materials22, transparency and conductivity of transparent conductors used in photovoltaics and optoelectronic materials23, damping capacity and elastic modulus of damping materials24, 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 mutually contradictory properties I and II of structural and functional materials14-24.

No. Materials Property I Property II Refs.
1 Stuctural materials Strength

Ductility/Formability/Deformability/Conductivity/Thermal stability

14-16
2

Battery materials

Energy density Power density/Cycle life/Safety 17
3 Thermoelectric materials Electrical conductivity Seebeck coefficient/Thermal conductivity 18
4

Permanent magnetic materials

Coercivity Magnetization 19
5 Dielectric materials Polarization Breakdown strength 20
6 Catalytic materials Reactant mobility Catalytic active sites 21
7 Ferroelectric/Ferromagnetic materials Ferromagnetism Ferroelectricity 22
8 Optoelectronic materials Transparency Conductivity 23
9 Damping materials Damping capability Elastic modulus 24
  1. Solution to scientific problem

There is an old Chinese saying that “fish is what I want, bear's paw is also what I want, however, I will select the bear’s paw and give up the fish in the case that only one of them can be choosen”. The wisdom of the ancients told us to weigh the pros and cons at this time, to maximize the benefits. In the past half century, the emerging nanotechnology has enabled human beings to challenge the paradox of material properties tenaciously and unremittingly. The general guiding ideology is to combine the microstructures corresponding to the two mutually inconsequent properties, i.e. the concepts of composite materials14,25. In the past 20 years, in response to the challenge of low ductility of nanostructured materials for practical applications, we have proposed various schemes and strategies to improve the low ductility of nanostructured materials by tailoring their microstructures26. Detailed strategies include eliminating processing artifacts27, bi-modal and multi-modal grain size distributions3, nanoscale second-phase precipitates/particles dispersed in NS grains28, nanoscale twin boundary29, transformation-induced-plasticity (TRIP) and twinning-induced-plasticity (TWIP) effects30, etc. Moreover, in view of the paradox of strength and thermal stability, we propose a strategy to introduce low-energy grain boundary (i.e. low-angle grain boundary) and change the grain morphology to reduce the stored energy for grain growth31.

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.32 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.32 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 can not 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. We thus propose a new concept that the microstructure of materials should be macro directionally optimized/designed according to their specific working conditions. 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 our paper just published in Communications Materials (https://doi.org/10.1038/s43246-021-00150-1).

  1. Significance and prospect of our research

Different from the traditional concepts of composite in the literature, our 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, we 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 3, our 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, we 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, we do not need copper wire conducting along the radial direction. Therefore, the soul of our 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, we don't have to spend energy to make perfect materials in all directions, but use steel on the blade. 

Figure 3. Schematic representation of the MDDM infulence on the strength and conductivity trade-off relationship. Our 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.

Our concept provides an alternative design idea to combine the mutually exclusive properties. Usually, mechanical design engineers only design the macro-structure on the macro level (> 0.1 mm) without considering the microstructure of materials, such as drilling spare parts into holes; while material scientists only design the microstructure on the micro level (< 0.1 mm) without considering the macro-structure, such as improving the low ductility of nanostructured materials by tailoring their microstructures26-31. Our MDDM design concept sets up a bridge from micro to macro, as shown in Figure 4. We give an overall consideration and design from the composition and microstructure of upstream materials to the manufacture and use of downstream parts. Therefore, our design concept further optimizes the structure and performance of materials from micro to macro, and truly achieves the reasonable and accurate use of materials, and gives full play to the material performance.

Figure 4. Schematic representation of our macro directional design of microstructure (MDDM, or micro-macro design). Usually, mechanical design engineers only macro-design the macro-structure without considering the microstructure of materials; while material scientists only micro-design the micro-structure without considering the macro-structure. Our MDDM design concept sets up a bridge from micro to macro. The circular pattern in the center adopts the creation idea of Taiji diagram in the ancient Chinese Philosophy book of Changes. The original meaning is that all things have evolved from the intersection of Yin and Yang. There is Yang in Yin and Yin in Yang, and the Yin and Yang always transform each other. Here we use similar idea to express the relationship between macrodesign and microdesign.

Our 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. For battery, thermoelectric, catalytic etc. functional and structural materials, we can design their microstructures purposefully according to the macro working directions of charge and discharge, heat conduction and conduction, reaction and loading, respectively. Therefore, our research will promote material design into a new space. For the generalized material field, we can make a large class of bionic advanced materials with our design concept, and then achieve a breakthrough in performance. Finally, the macro directional design of microstructure (including grain size, morphology and orientation, etc.) can be further expanded into the macro-design of composition (different elements) and phase (different structures as face-centered cubic, body-center cubic and hexagonal-close packed), etc. In this respect, gradient materials33 and hierarchical microstructures34 can also be classified into this category.

To learn more about this work, please read our article "Enhanced electrical conductivity and mechanical properties in thermally stable fine-grained copper wire" in Communications Materials: https://doi.org/10.1038/s43246-021-00150-1

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

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Yonghao Zhao

Professor, Nanjing University of Science and Technology