Oxidation and crystallographic features of the new prototype structure Ti₄NiSi₄

The oxidation resistance and the crystal structure of the new Ti₄NiSi₄ 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 Ti₄NiSi₄ (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, 1/4, 1/2 and 3/4; it can therefore be considered as a pseudo lamellar structure. It has been suggested that this peculiar feature induces a low activation energy for the phase transformation of Ti₄NiSi₄ into Ti₄Ni₄Si₇, 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 TiO₂ and SiO₂ oxidation products, which are responsible for the high environmental resistance observed at 650 C in air. (authors)