| Refractory transition-metal disilicides such as MoSi2 (C11b, 2020oC) and NbSi2 (C40, 1960oC) have been studied for many years for high temperature structural applications because of their low density and excellent high temperature strength. However, there are still two main problems to overcome in practical applications. One is the structural disintegration during oxidation at low temperature, which is known as the pesting phenomenon. Although many studies have been made, controversy still exists concerning the pesting mechanism. Another problem is poor ambient temperature toughness and inadequate high-temperature creep resistance. Recent studies show that Nb addition can obviously improve mechanical properties of MoSi2, while the dual-phase regime of MoSi2/NbSi2 is considered to be an ideal prototype to develop balancing strength and toughness alloys. Except the benefits mentioned above, Nb addition may introduce new problems on oxidation behaviors, especially at elevated temperatures. Such studies are relatively scarce at the moment.To deal with these problems, MoSi2 and NbSi2 have been chosen to study in this work. Both poly- and single crystalline specimens with different microstructures were prepared by arc melting, spark plasma sintering (SPS) and optical heating floating zone (OFZ) methods. Oxidation behaviors of MoSi2 and NbSi2 at low temperature were investigated to identify the effects of cracks, pores and grain boundaries on the pesting phenomenon. In addition, the methods such as XRD, SEM and XPS, etc were used to analyze the composition of oxides. Oxidation processes of MoSi2 and NbSi2 were observed in situ by high temperature optical microscope and X-ray diffraction to clarify the pesting mechanism. The oxidation kinetics of single-crystalline MoSi2 and NbSi2 with different crystal surfaces was studied in order to identify the role of crystallographic orientations. The anisotropic oxidation behaviors had been explained by ab-initio calculation of the process of oxygen adsorption on crystal surfaces. Besides that, the oxidation behaviors of (Mo1- xNb x)Si2 alloys with different Nb contents were studied at 1473K~1773K. The results show that:Microstructural characteristics are thought to be the key factor for the different oxidation behavior of alloys. For arc-melted poly-crystalline specimens containing micro-cracks, MoSi2 fully turned into powders after 160h exposure at 773K. As a comparison, no pesting was found in the dense SPS poly- and single crystalline specimens after 300h. A linear oxidation law was found in the SPS specimens whose oxide scale is porous and full of lateral cracks. The oxidation rate of single crystal is considerable slower than that of polycrystal. A dense continuous oxide layer was formed on the surface of single crystalline specimen, which oxidation kinetics can be described by the parabolic law.Similar to MoSi2, arc-melted NbSi2 fully turned into powders after 3h exposure at 1023K, but no pesting was found in the dense specimens after 89h. The oxide scales of all specimens consist of a mixture of Nb2O5 and SiO2, which spall easily from the matrix during oxidation due to high internal stress. Therefore, the oxidation kinetics of poly- and single crystalline NbSi2 follows the same linear law.According to the above results, pesting disintegration is mainly attributed to pre-existing cracks in the microstructure. This is also proved directly by in-situ observations for MoSi2 and NbSi2. Initiation of new cracks from the pre-existing cracking sites were observed during oxidation, which caused by stress concentration at crack tips due to the growth of oxides. Newborn cracks would propagate and be oxidized, then turn into sources of cracks. Such a repeated progress named as crack propagation and reproduction led to the final failure. Pores and grain boundaries are the preferential oxidation sites but do not cause crack initiation and propagation.A pronounced anisotropy in oxidation behavior was observed in MoSi2 and NbSi2 single crystal during isothermal oxidation. For MoSi2, the orientation-dependent oxidation rates ordered from big to small were as follows: (101), (100), (001) and (110). The ratio of (101)/(110) is about 6. The oxide layer formed on (101) consists of MoO3 and SiO2, no preferred orientation was found. Instead, the oxide layer on (110) consists of Mo4O11 and SiO2 with a notable preferred orientation. For NbSi2, the oxidation rate of (112 0) is the highest, while (0001) and (1010) are almost the same. The oxides on all these surfaces consist of Nb2O5 and SiO2, which spall from the specimens during oxidation. A linear law of oxidation kinetics was found for all NbSi2 specimens when oxidized at 1023K.The adsorption of oxygen on MoSi2 and NbSi2 surfaces was analyzed using the first-principle density functional theoretical (DFT) calculation. For MoSi2, the calculated chemisorption energies are–0.715eV, -0.252eV, -0.241eV and 0.09eV for (101), (100), (001) and (110) surfaces, respectively. Lowest chemisorption energy indicates that MoSi2 (101) is most easily adsorbed by oxygen atom, which corresponds well with experimental result of oxidation kinetics. Analysis of the distribution profiles of densities of states (DOS) shows there are two strong peaks close to the Fermi level obviously for each surface of MoSi2, indicating that these surfaces can give or accept electrons easily, and then react with oxygen. The oxidation rate of single crystalline MoSi2 is correlated with chemisorption energy of oxygen, no the density of crystal surface. As a contrast, the density of crystal surface should have more effect in NbSi2 which structure of C40 could have high solubility for oxygen.Effect of Nb content on oxidation behavior of (Mo1- xNb x)Si2 has been investigated. It was found that specimens with x≤0.5 contents show weight gain increased with temperature, and conform to the parabolic kinetic law, whereas those with x>0.5 show weight loss and conform to the linear law. Furthermore, it was revealed that the effect of Nb addition in (Mo1- xNb x)Si2 alloys was closely related to phase composition and types of oxidation products. With the increase of Nb content, the alloys transform from single-phase of C11b to dual-phase of C11b + C40, and then, to single-phase of C40 finally. The oxidation products comprised of SiO2 were observed for specimens with x<0.4, while a mixture of SiO2 and Nb2O5 was observed for those with x≥0.5. At the same temperature, weight gains of specimens rolled in the two-phase region were higher than those of specimens rolled in the single-phase region. Comparison to single-phase region, besides chemical composition, the phase interface is an important factor for two-phase region. (Mo0.7Nb0.3)Si2 is proved to show more oxidation resistance than other alloys between 1473K and 1773K. Based on the above results, a content of Nb x close to 0.2 was suggested to be the optimum composition for achieving the balance between mechanical properties and oxidation resistance at high temperature.
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