Abstract
First principles calculations are performed on Zr2AlC and Cr2AlC MAX phases to compare their ability to accommodate point defects under irradiation. Interatomic bonding is stronger in Cr2AlC than Zr2AlC but contrary to expectation Zr2AlC exhibits higher vacancy and antisite pair formation energies. However, interstitials and Frenkel defects are generally more difficult to form in Cr2AlC. The results are attributed to the mixed covalent/ionic/metallic nature of the bonding. Detailed comparison of all the energies suggests that the preferred defects in Zr2AlC and Cr2AlC are the VAl+Ali Frenkel and CrAl+AlCr antisite respectively. Thus the potential response of the two phases to irradiation is different and taking account of other competing defects it is suggested that Zr2AlC is less susceptible to amorphization.
Highlights
A group of nanolaminated hexagonal materials called MAX phases have come under intense scrutiny in recent years due to their unusual physical properties that result from a combination of metallic and ceramic bonding characteristics[1]
The structural stability and irradiation tolerance of Zrn+1AlCn MAX phases can be assessed using density functional theory (DFT) calculations that focus on their formation enthalpy, bond strengths and propensity to form point defects[13]
That DFT calculations of defect energetics can only provide an indicator rather than a complete predictor of the susceptibility of a material to amorphize under irradiation since other process features such as kinetics are not taken into account
Summary
A group of nanolaminated hexagonal materials called MAX phases have come under intense scrutiny in recent years due to their unusual physical properties that result from a combination of metallic and ceramic bonding characteristics[1]. The mixed covalent/ionic/metallic bonding present in MAX phases results in another possible application as an in-core structural material or coating within the hostile environment of a nuclear reactor[3, 4] Under irradiation these phases have shown a remarkable ability to accommodate point defects and remain crystalline rather than becoming amorphous even when subjected to high levels of radiation or ion bombardment. Ti and Cr have relatively high neutron cross-sections (6.1 and 3.1 barn respectively) while Zr and Al are relatively low (0.184 and 0.233 barn) This suggests that Zrn+1AlCn MAX phases might be candidates for nuclear applications. For the nuclear applications of interest here, the materials are synthesized and used at temperatures significantly above the observed Curie temperature (~73 K)[20]
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