Effect of Titanium Addition on Alumina Growth Mechanism on Yttria-Containing FeCrAl-Base Alloy

FeCrAl-base alloys are well known for their excellent oxidation resistance due to formation of a slowly growing alumina surface scale during high-temperature service. The actual scale growth mechanism and especially adherence are strongly affected by the presence of oxygen-active elements such as yttrium, titanium or hafnium. In the present study, the effect of titanium addition on the scale growth mechanisms of an yttrium oxide dispersion-strengthened FeCrAl base alloy was studied during oxidation at 1200 degrees C in Ar-O-2. For microstructural characterization results of scanning electron microscopy and electron backscatter diffraction were combined with X-ray diffraction data. Scale growth mechanisms were investigated by two-stage oxidation using O-18 tracer with subsequent scale analyses using secondary neutrals mass spectrometry. The scale on the alloy without intentionally added titanium grew virtually exclusively by oxygen diffusion along oxide grain boundaries and exhibited a columnar structure with the grain size increasing in growth direction. The addition of titanium resulted in formation of an outer oxide zone of equiaxed grains on top of the inner columnar part. The equiaxed grains increased in size with increasing exposure time. Comparison with the tracer studies revealed that the titanium-induced equiaxed zone was the result of outer scale growth. Mechanisms for the initiation of outward aluminium transport are discussed. Indications were found that the effect of titanium on the scale growth mechanisms already occurred for titanium additions as low as 0.02 wt%.