Structural Transitions Induced by Ion Irradiation in V₂AlC and Cr₂AlC

Abstract

Nanolayered structural metallic ceramics, MAX phases, possess unique and highly attractive properties, including excellent radiation tolerance for some of them, whereas little is known about the detailed process of irradiation-induced structural transitions. In this study, the microstructural transformations and the stabilities of V 2 AlC and Cr 2 AlC induced by 1 MeV Au + ions irradiation over a wide range of fluences were investigated by grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM). GIXRD analyses show different processes of phase transitions and amorphization tolerance under irradiation between these two MAX phases, which are consistent with the selected area electron diffraction (SAED) results and the high-resolution observations. TEM observations reveal that the nanolamellar structures are disturbed and respective phase transitions occur at relatively low fluences, with the formation of stacking faults. As the fluence increases, Cr 2 AlC becomes completely amorphous, while V 2 AlC are gradually transformed into facecentered cubic (fcc) structure from the original hexagonal close-packed (hcp) structure without amorphization, indicating that V 2 AlC is more tolerant of irradiation than Cr 2 AlC. Based on the phase contrast images and the electron-diffraction pattern (EDP) simulation of the microstructures, mechanisms of the phase transitions of V 2 AlC and Cr 2 AlC are proposed and the difference of the irradiation tolerance between them is discussed.

Point defect formation in M2AlC (M = Zr,Cr) MAX phases and their tendency to disorder and amorphize

“…Point defect calculations have only been performed on Cr 2 AlC where it is found that the Cr-Al antisite pair has the lowest formation energy of all possible point defect combinations thus providing a defect mechanism for recovery during irradiation 19 . This result, as well as the finding that the C Frenkel pair also has relatively low formation energy, is consistent with the recent observations on ion irradiated Cr 2 AlC 15, 16 . Although the calculations were based on summing the energies of the individual defects they showed that despite the low magnetic moment of Cr, a non-magnetic treatment of Cr 2 AlC is sufficient if only trends in defect formation behavior are required.…”

“…Chromium is chosen as an alternative M element since it should enhance the high temperature oxidation resistance of the material 14 and has a lower neutron cross-section (3.1barn) than Ti. In addition Cr 2 AlC has been successfully synthesized and the main radiation induced defects have been determined 15, 16 . Depending on the processing conditions, small amounts of Cr 3 C 2 , Cr 7 C 3 and Cr 5 Al 8 can be present in the material 17 although generally speaking Cr 2 AlC has been synthesized (at the moment) with higher purity than Zr 2 AlC.…”

Behaviors of helium in Cr2AlC from first principles

“…However, the structure of Cr–C–Cr still remains. Therefore, the decomposition products of Cr 2 AlC mainly consist of CrC x , and the results are supported by experiments …”

“…Most of the recent investigations about Cr 2 AlC primarily focus on its synthesis and intrinsic properties . Some of the studies about irradiation resistance of Cr 2 AlC show that structural transition occurred and saturation of ion irradiation effects existed in Cr 2 AlC induced by irradiation using Xe, Au, and He ions . Theoretical calculations show that the unique properties of Cr 2 AlC are contributed to nanolaminated structure and the chemical bonding characters consisting of covalent, ionic and metallic .…”

“…[12][13][14][15][16] Some of the studies about irradiation resistance of Cr 2 AlC show that structural transition occurred and saturation of ion irradiation effects existed in Cr 2 AlC induced by irradiation using Xe, Au, and He ions. 17,18 Theoretical calculations show that the unique properties of Cr 2 AlC are contributed to nanolaminated structure and the chemical bonding characters consisting of covalent, ionic and metallic. 19 However, little research of He existence and diffusion behaviors in Cr 2 AlC have been reported in relevant literatures.…”