Papers by Keyword: Pesting

Abstract: The oxidation behavior of Mo, Nb, and Ti-silicides has received significant attention in past few decades for their potential to be used as high temperature structural materials. These Si-bearing intermetallic alloys have the ability to form an oxide scale containing SiO₂, which is protective if formed as a continuous and impervious layer, so that the ingress of oxygen from atmosphere to the underneath alloy is restricted. To form a continuous and stable SiO₂ scale, it is important to have sufficient activity of Si along with thermodynamic and kinetic conditions favoring its growth in comparison to that of oxides of other alloying elements. MoSi₂ has superior oxidation resistance compared to that of Mo₃Si or Mo₅Si₃, because of its higher Si content. Furthermore, a continuous film of SiO₂ is able to form at temperatures in the range of 800-1700 ^oC on MoSi₂ due to vaporization of MoO₃, but not on NbSi₂ or TiSi₂ due to competitive growth of Nb₂O₅ or TiO₂, respectively. During past two decades, a significant effort has been devoted to development of Mo-Si-B alloys containing Mo-rich solid solution, Mo₃Si and Mo₅SiB₂ as constituent phases, due to their ability to form a protective borosilicate scale. The presence of B₂O₃ contributes to fluidity of borosilicate scale, thereby contributing to closure of porosities. Efforts have been also made to develop multicomponent Nb-silicide based alloys with optimum combination of mechanical properties and high temperature oxidation resistance with limited success. There have been efforts to develop silicide based coatings for protection oxidation for Mo-rich Mo-Si-B alloys and Nb-Si based ternary or multicomponent alloys with inadequate oxidation resistance. Oxidation behavior of selected silicides with potential for structural application, along with mechanisms for protection against oxidation has been reviewed and discussed.

Abstract: The single and synergism effect of SiC and ZrO₂ nanoparticles on low temperature oxidation resistance of MoSi₂ were investigated by thermo-gravimetric analysis (TGA), phase and micrograph analysis of surface film. The results show that at the oxidation temperature of 600°C, the addition of 10%ZrO₂ results in obvious oxidation mass gain of MoSi_2, and discontinuous protective film forming on the surfaces, which makes “pesting” phenomenon still occur. The addition of 10%SiC accelerates the oxidation of MoSi₂, but compact SiO₂ protective film formed on the surface after some time of oxidation, avoiding the occurrence of large scope of “pesting” phenomenon. The synergism of 10%ZrO₂ and 10%SiC promotes the formation of compact, even silicate glass film on the surface of MoSi₂, as a result, significantly improves the low temperature oxidation resistance of MoSi₂.

Abstract: Several Nb based alloys (Nb-20Mo-15Si-25Cr, Nb-20Mo-15Si-25Cr-5B, Nb-20Si-20Cr-5Al, and Nb-20Cr-20Si-5Hf) have been prepared to evaluate the oxidation resistance from 700 to 1400°C in air. The phase identification was determined by calculating the isothermal sections in this temperature range using PANDATTM software. The experiments involve static heating for 24 hours (short term oxidation, STO) or 7 cycles of 24 hour heating (long term oxidation, LTO). Weight gain per unit area as a function of either temperature (STO) or time (LTO) has been used to determine their oxidation resistance. However, SEM, EDS on SEM, x-ray mapping, and XRD have been used to evaluate the oxide scale characterization and the influence of various microconstituent effects have been determined. It appears that B addition may be beneficial while Al is advantageous in comparison to Hf addition. The problem of pesting, typically, in a range of temperature from 900 to 1100°C needs to be controlled through minor additions since the alloys exhibit fairly good resistance at lower and higher temperatures up to 1400°C.