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 electron probe analyses and microhardness measurements. Chromium was found to reduce the rate of oxidation at 800 and 900 degrees C. Two surface layers are formed essentially during oxidation of the alloys: scale and subscale layer with high microhardness (1700-1900 MPa). The scale represents a mixture of two phases: alpha-TiO2 type with a rutile structure and mNb(2)O(5)-nTiO(2) type K-phase with a complex lattice, its 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 degrees C chromium promotes intense enrichment of the scale in vanadium.