S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Materials Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
Tailor-made design of complex concentrated alloys
컴플렉스 고용 합금의 특성 맞춤형 합금 설계법 개발
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- Complex concentrated alloy; Solid-solution strengthening; Atomic-level pressure; Chemical complexity; Electronegativity; Stacking fault energy
- 학위논문 (박사)-- 서울대학교 대학원 : 공과대학 재료공학부, 2018. 2. 박은수.
- Complex concentrated alloys (CCAs
CCAs that have more than four elements are also referred to as high entropy alloys) are a new alloy development philosophy in which the base alloy has a significant fraction of multiple principal elements. CCAs have attracted worldwide attention as strong candidates to solve problems owing to their useful performances, such as superior mechanical properties at all temperature ranges and good irradiation resistance.
Much of the interest in CCAs stems from the belief that the atomic-level complexity, which originates from the large number of principal elements would provide profound effects, such as the lattice distortion effect, the sluggish diffusion effect, the irradiation resistance, and the solid-solution strengthening. However, the correlation between the complexity and the resultant properties has not yet been thoroughly understood. As a result, the advantage of so many degrees of freedom for alloy design of CCAs is diminished by a lack of mixing rules, rendering alloy design an empirical try-and-error undertaking. Therefore, to make a useful guide for the new CCA designs, a simple parameter is required to reflect the atomic environments of CCAs in a physically meaningful way so that they can be directly related to properties.
In CCAs, all constituent elements are solute and solvent, and every element interacts with the stress field of dislocations, thereby increasing or decreasing the elastic strain energy of the system and resulting in solid-solution strengthening. Thus, the solid-solution strengthening effect is one of the representative phenomena that reflect atomic-level complexity in CCAs, which can also be related to macroscopic mechanical and functional properties influenced by the complexity. In this thesis, the relationship between atomic-level complexity and its influence on properties, in particular solid-solution strengthening, is investigated. CCAs with face-centered cubic (FCC) phase consisting of late 3d transition metal elements (i.e., V, Cr, Mn, Fe, Co, and Ni) are mainly discussed (Here we call them 3d CCAs). These alloys have been considered to have outstanding mechanical properties, and some commercial alloys, such as austenitic steels and γ matrix of a superalloy, belong to this group, which implies a high possibility of new commercial alloys.
First, we analyzed the local atomic structure of 3d CCAs by X-ray Diffraction (XRD) and Extended X-ray Absorption Fine Structure (EXAFS) to measure the elemental average atomic sizes of consisting elements. From the obtained structural information, we predicted the solid-solution strengths by applying the atomic size difference among the constituting elements to the existing model on the basis of elasticity theory. However, the predicted solid-solution strengths do not match well with the experimentally measured values. In order to interpret this mismatch, we calculated the atomic structure using density function theory (DFT) and found that the fluctuation of bond lengths due to the dissimilar local atomic configurations, which is usually ignored for dilute alloys, has a significant impact on the solid-solution strengthening of CCAs, which introduces higher degree of complexity problem in CCAs.
As a second approach, we calculated the atomic-level pressure of each element in 3d CCAs, which is the cause of solid-solution strengthening, using DFT calculation. We found that the atomic-level pressure of individual atoms originates from the charge transfer between a center atom and its surrounding atoms. This was also confirmed with an experimental study by measuring volume strain of each element in 3d CCAs using EXAFS and plotting with charge transfer. This means that the atomic-level pressure in 3d CCAs is attributed to the electronic effect rather than the elastic interaction of constituent elements.
In order to utilize this concept for an alloy design strategy, we tried descending the degree of complexities in 3d CCAs. Through a statistical approach, we found that both values of deviation of average elemental atomic-level pressure and deviation of atomic-level pressure due to the variance in local atomic configurations are linearly proportional to each other. This makes it possible to estimate a higher degree complexity (configurational deviations) using a lower degree complexity (elemental deviations), which can be identified experimentally. As a result, we were able to theoretically explain the proportional relationship between electronegativity difference and solid-solution strengths in the 3d CCAs.
Based on the aforementioned discussion, we constructed an electronegativity-mixing entropy diagram that shows the relationship between chemical complexity and complexity induced by deviation of atomic-level pressure, i.e., solid-solution strengthening. All possible combinations of 3d transition metal elements (V, Cr, Mn, Fe, Co, Ni) are included in the diagram. The area of the 3d CCAs has inverse C-shape boundary, which means that (1) the mixing entropy does not have a strong correlation with solid-solution strengthening, and (2) there is a region where the mixing entropy should be decreased to obtain greater solid-solution strengthening effects. Thus, we concluded that there is no strong correlation between the chemical complexity and the deviation (i.e. complexity) of atomic-level pressure in 3d CCAs.
One may argue that the chemical complexity is no longer important for CCAs as the complexity of atomic-level pressure are closely related to the lattice distortion effect and the sluggish diffusion effect, which are CCAs two core effects among the four. Changing the composition from the Cantor alloy, we developed twin-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) CCAs by decreasing stacking fault energies while maintaining the deviation of atomic-level pressure, i.e., solid-solution strength, in order to show that there are many factors that we can manipulate besides complexity of atomic-level pressure. The change of deformation mechanisms from dislocation gliding to TWIP and TRIP increases the strain hardening rate of the CCAs, enhancing both ultimate tensile strength and the percentage of uniform elongation without loss of yield strengths. The development of these new CCAs was possible due to the freedom in manipulating composition, which implies that chemical complexity is also important for the design of new CCAs for the vastness of composition space.
Additionally, we discussed asymmetry of atomic-level pressure-induced element-specific properties in CCAs. Atomic-level pressure of an element includes the information of anharmonicity of lattice potential and represents the resistance of it against displacement. As a result, element-specific properties, such as atomic displacements, diffusivity, and preferential site of interstitial elements show asymmetric behavior upon atomic-level pressure. Consequently, the deviation of atomic-level pressure dominantly affects the degree of lattice distortion, the diffusivity of substitutional elements, and the solubility of interstitial elements, which are crucial for engineering applications.
Through this research, we distinguished the previous concept of complexity in CCAs into two categories: chemical complexity for the vastness of composition space and complexity of atomic-level pressure reflecting fluctuation of lattice potential energies. We believe that the tailor-made design of CCAs is possible when both complexities are investigated well for the desired elemental combinations.