Intrinsic point defects in ternary MgCaSi: Ab initio investigation
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Nine possible native point defects in MgCaSi have been studied by employing density functional theory based ab initio calculations. The complex chemical potential limits are first determined using a two-dimension (ΔlMg, ΔlCa) diagram, then the defect formation energies as a function of the atomic chemical potential are gained. The energetic results show that under Mg-rich conditions, the most favorable defect is MgCa rather than MgSi, while CaMg is predominant compared to CaSi under Ca-rich conditions. The bonding energy is first introduced to uncover the intrinsic feature of defect formation energy. The local geometric distortion around CaMg, MgSi, and CaSi antisite defects gradually increases due to the smaller atomic radii from Ca to Mg and Si, showing the important role of the geometrical mismatch. The density of states indicates that the higher stability of CaMg and MgCa originates from the smaller deviation of the Fermi level from the pseudo-gap.
I. INTRODUCTION
Mg-based lightweight alloy has been attracting large research interests for potential applications in modern automobiles and aerospace industries over the past decades.1–4 However, uses of Mg alloys are still limited because of the restrained properties such as inferior tensile strength, the limited creep, and corrosion resistance.5–7 To further improve mechanical properties of Mg alloys, addition of alloying elements is found to be an effective way. Calcium element improves creep resistance and strength of Mg alloys and reduces the cost,8 and silicon is also considered to improve mechanical properties,9 so binary Mg–Ca and Mg–Si alloy systems have been developed and studied extensively.10,11 Furthermore, several industrial Mg alloys with simultaneous addition of Ca and Si have been developed, and many ternary phases, such as MgCaSi, Ca7Mg7.56dSi14, Ca2Mg3Si, and Ca2MgSi39,12 have been attracting a significant research interesting due to the superior mechanical properties. Especially, MgCaSi is the crucial strengthening phase that could increase tensile strength of alloys and so is of great interest. Moreover, MgCaSi is also a promising thermoelectric (TE) material. However, the research on the ternary phase is very scarce. Various point defects in crystals, such as vacancies and antisite defects in intermetallic compounds have been a hot topic of theoretical and experimental research. Vacancy defects are crucial mediums in atomic diffusions in intermetallics, so the study of native point defects is Contributing Editor: Susan B. Sinnott a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.303
useful to understand the diffusion behavior.13,14 Furthermore, point defects affect the thermodynamic behavior and formation of materials and further have an important impact on the microstructure and properties of materials. Especially, point defects have a crucial significance on ductility, strength,15 and creep resistance16 of materials. It is shown that the defect concentration is related closely to ductility, strengt
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