A STUDY OF THE STRUCTURE AND PHASE-COMPOSITION OF SURFACE-LAYERS FORMED ON OXIDATION OF NB-TI-AL-CR ALLOYS

The structural and phase changes occurring on oxidation in the surface layers of niobium alloys with -30% Ti, -5% Al. -5% V, -1% Zr and 0 to 15% Cr were studied using optical microscopy, X-ray diffraction and e lectron probe analyses and microhardness measurements. Chromium was fo und to reduce the rate of oxidation at 800 and 900 degrees C. Two surf ace layers are formed essentially during oxidation of the alloys: scal e and subscale layer with high microhardness (1700-1900 MPa). The scal e represents a mixture of two phases: alpha-TiO2 type with a rutile st ructure and mNb(2)O(5)-nTiO(2) type K-phase with a complex lattice, it s composition including all the metal elements of the alloy. Chromium promotes the increase of the a-phase in the scale, reduces the niobium content and increases the titanium content of the scale. At 900 degre es C chromium promotes intense enrichment of the scale in vanadium.