The Influence of Mo Content and Annealing on the Oxidation Behavior of Arc-Melted Cr–xMo–8Si Alloys

Abstract

The influence of Mo (10-40at.%) additions to Cr-8at.%Si was systematically studied by producing arc melted alloys and analyzing the as-cast and annealed microstructures as well as the oxidation resistances. The majority of the alloys contained a solid solution A2 matrix with the intermetallic A15 = (Cr,Mo)3Si phase. The oxidation resistance was tested by thermogravimetric analysis (TGA) at 1200 °C in air for 50 and 100 h. Samples were characterized by XRD, SEM and EPMA (WDS). The studied Cr-xMo-8Si alloy series showed a beneficial effect of Mo on the corrosion behavior of the annealed arc melted alloys exposed at 1200 °C in air. An increased amount of A15 led to the formation of a continuous SiO2 scale beneath the Cr2O3 layer during high temperature exposure. The internal SiO2 layer inhibited further internal oxidation of the A15 phase. Prior annealing also influenced the oxidation behavior of the alloys. Higher annealing temperatures, resulted in an overall reduced fraction of the A15 phase, as well as coarsening, but also showed a more uniform size and distribution of the precipitates, which seems to benefit the formation of a protective duplex oxide scale. Cr-25Mo-8at.%Si annealed at 1350 °C for 100 h, showed the most promising oxidation behavior after exposure in air at 1200 °C for 100 h. For all of the investigated Mo contents, no nitridation was observed when compared to the Cr-8Si reference composition. The oxide scale adhesion also was improved. For Mo >25 at.% no spallation was observed. Evaluation of the TGA data showed it follows the paralinear law, with both the growth rate, kp, and volatilization rate, kv, of Cr2O3 being reduced by adding 10-25 at.% Mo. A binary Cr/Mo-Si phase diagram was generated for the studied alloy series in order to determine accurate annealing conditions and estimate the stability range of the metastable σ phase.