The Effect of Atomic Size and Chemical Randomness on Phase Stability of Chemically Complex Alloys


Student thesis: Doctoral Thesis

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Award date28 Aug 2019


Unlike the conventional alloys, multi principle chemically complex alloys or high entropy alloys (HEAs) usually comprise more than five constituent elements in an equal or near equal molar fractions. Being deemed as a major paradigmatic shift, the design of HEAs without base elements poses challenges to the existing thermodynamic models and theories that were long established for traditional alloys. As a new research field, there are many fundamental questions yet to be fully understood such as the entropic stabilization, chemical short range order and lattice distortion etc. In this thesis, we addressed several fundamental questions in the HEAs community to provide a comprehensive understanding of the phase stability of HEAs. Starting from the thermodynamic principles and statistical mechanics, we developed different thermodynamic models to account for the configurational entropy of mixing in non-ideal solid solution alloys, chemical short range order and lattice distortion in HEAs. Various molecular dynamics and density function theory simulations were also adopted to verify our theoretical models.

Neglecting the atomic size effect and the interactions among constituent elements, the simple ideal mixing rule has been commonly used to calculate the configurational entropy of mixing for HEAs. However, there has been increasing experimental evidence showing that the ideal mixing rule tends to overestimate the configurational entropy of mixing in HEAs, particularly at a low temperature. In contrast to the ideal mixing rule, here we develop a thermodynamic model to assess the configurational entropy of mixing in random solid-solution multicomponent alloys by considering the possible correlations among the constituent elements due to various factors, such as atomic size misfit and chemic bond misfit, which may disturb the potential energy of an alloy system and thus reduce the configurational entropy of mixing. With our new thermodynamic model, the correlation is explored between the configuration entropy of mixing of different alloys and the general character of the phases formed, such as single- or multiple-phased crystalline phase versus amorphous phase. This correlation has been verified by the simulation and experimental results, which can be further used to explore phase stability in complex multicomponent alloys.

In addition, recent atomistic simulations clearly indicated the presence of chemical short-range order (CSRO) in a number of solid solutions HEAs, however, it still remains elusive of how such CSROs could form and affect the thermodynamics of phase selection in these alloys. To address this issue, we develop a theoretical model by generalizing the CSRO model originally put forward for binary alloys to account for the formation of CSROs in HEAs. Our model provides a comprehensive assessment of local chemical ordering versus de-mixing between different atomic pairs in concentrated multicomponent alloys and enables one to compute the alloys’ configurational entropy of mixing as a function of temperature and chemical compositions. In turn, this sheds quantitative insights into the chemistry of incipient ordered phases or incipient phase separation in HEAs.

Aside from CSRO in HEAs, we also studied the issue of lattice distortion and phase transition in HEAs. To quantify the degree of lattice distortion, here we carried out extensive first-principles calculations on a series of equimolar complex alloys and characterized their atomic-scale lattice distortions in terms of the local residual strains. Albeit the confounding chemical/geometric complexities, we can show that the average attributes of such an atomic-scale distorted lattice can be predicted very well by a simple physical model considering the efficient packing of different sized atoms interacting in an effective elastic medium. This theoretical model unveils the details of locally distorted lattices and sheds quantitative insights into the unusual strengthening mechanism as recently discovered in HEAs.