Oxidation and crystallographic features of the new prototype structure Ti4NiSi4

Abstract The oxidation resistance and the crystal structure of the new Ti 4 NiSi 4 compound were investigated. This compound was manufactured both as a single-phase and as the outer layer of a protective coating for vanadium alloys. The recorded oxidation rates were very low at 650 °C and 750 °C in air, which represents a more severe environment than those envisaged for the targeted application: fuel cladding for sodium-cooled fast reactors. The powder XRD measurements performed at the synchrotron SOLEIL allowed for the crystallographic structure determination of Ti 4 NiSi 4 (S.G. Pnma; a  = 15.63931; b  = 5.08321; c  = 12.75151) with high confidence factors ( R wp  = 0.07; R Bragg  = 0.067). Its structure consists of atomic planes stacking along the b-axis with planes at coordinates y  = 0, ¼, ½ and ¾; it can therefore be considered as a pseudolamellar structure. It has been suggested that this peculiar feature induces a low activation energy for the phase transformation of Ti 4 NiSi 4 into Ti 4 Ni 4 Si 7 , which thus renders the transformation possible at moderate temperature. This structural rearrangement is accompanied by the liberation of Ti and Si, which leads to the formation of both TiO 2 and SiO 2 oxidation products, which are responsible for the high environmental resistance observed at 650 °C in air.