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Two ternary phases in Nb-Si-B system, T2(Cr5B3-type tetragonal structure) and D8(Mn5Si3-type hexagonal structure), were prepared. The oxidation resistances of both T2 and D88 were found better than binary Nb5Si3 at 1000 0C, but still inadequate. The oxide scale is the mixture of Nb2O5 and borosilicate glass. The severe cracks were developed in the matrix and scale due to the large volume expansion associated with the formation of Nb2O5, SiO2 and B203. In Nb-Mo-Si-B system, all observed phases contain both Mo and Nb because Nb/Mo substitution occurs. The suitable Nb/Mo substitution can avoid the formation of Mo3Si and offer the possibility of ductile phase toughening. The lattice parameters for T1, T2 and D88 increase linearly with the increase of the Nb/Mo ratio. Oxidation tests indicated that the substitution of Nb for Mo decreased the oxidation resistance of boron doped Mo5Si3 dramatically. All specimen exhibited catastrophic oxidation behavior at 800 0C and converted to oxides completely after 20-50 hours exposure to flowing air. Although a parabolic behavior was achieved for most compositions at 1200°C, the high rate constants indicated that the scales formed are not protective. The existence of four layers in scale implies that the oxidation mechanism is complex and need further study in detail.
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Abstract: With current high temperature structural materials such as nickel based superalloys being pushed to the limits of suitable operating conditions, there comes a need for replacement materials with even higher temperature capabilities. Niobium silicon based systems have been shown to have superior density normalized strength at elevated temperatures when compared to currently used alloys. The drawbacks associated with the niobium silicon system are due to catastrophic oxidation behavior at elevated temperatures. Alloying addition have been shown to increase the oxidation resistance near suitable levels, but also decrease the high temperature strength and increases creep rates when compared to the binary alloy system. The microstructure of the material is similar to metal matrix composites in which high melting temperature silicides are dispersed in a niobium based matrix phase. The silicides produce high temperature strength while the niobium based matrix increases the room temperature properties such as fracture toughness. The bulk of the research has been conducted on directionally solidified material which has a coarse microstructure due to the slow cooling rates associated with the processing condition. The current research uses a powder metallurgy process termed Laser Engineered Net Shaping, or LENS, to produce material with a significantly refined microstructure due to fast cooling rates associated with the laser process. Several compositions of alloys were examined and the ideal processing parameters were determined for each alloy. The resulting microstructures show a refinement of the microstructure as expected with a fine scale distribution of Nb5Si3 and Nb3Si dispersed in a niobium based matrix phase. The high temperature oxidation behavior of the LENS deposited alloys was comparable to alloys produced using other techniques. A non protective oxide scale formed on samples exposed for only 0.5 hours but was not protective and showed large amounts of spallation at extended exposure times. The increase in grain boundaries and interfaces did not significantly increase the internal oxidation rate despite increased oxidation rates along these defects. The high temperature compression behavior was comparable to other alloys and processing techniques despite having a lower silicon content and therefore a smaller volume fraction of strengthening phase present. Dissolved oxygen levels in the LENS deposits appeared to be responsible for the increased strength at elevated temperatures. The oxygen levels in LENS processed alloys were higher than material produced by other processing techniques. The current work illustrates that the LENS processing techniques is a viable processing method for niobium silicide based materials and potentially increases the strength of the material.
Microstructure and oxidation of M-Si-B alloys, where M=Nb, Mo and (Nb, Mo) with phase assemblies T1(Mo5Si3B[subscript x])-MoSi2-MoB, T1T2(Mo5SiB2)- Mo3Si, Mo- T2- Mo3Si in Mo-Si-B system, T2 (Nb5(Si, B)3), D88(Nb5Si3B[subscript x]) in Nb-Si-B system, and T1-T2-D88 in (Nb, Mo)-Si-B system were investigated. In Mo-Si-B compositions, alloys showed excellent oxidation stability and initial mass loss of alloy varied according to Mo content. Nb-Si-B and Nb-Mo-Si-B alloys displayed large parabolic rate constants (in the range of 0.5-120 mgs2/cm4.hr) indicating that these systems are not as oxidatively stable as Mo-Si-B alloys. Oxidation kinetics was significantly dependent on initial heating atmosphere. In the Nb-Si-B system T2 and D88 alloys were more resistant to oxidation when heated to test temperature in pure argon. Quaternary Nb-Mo-Si-B alloy containing less D88 phase was more oxidation resistant than that containing more D88 phase. Scales on the order of 20-80 [mu]m were observed on Mo-Si-B alloys and relatively thicker scales (on the order of 200-600 [mu]m) were observed on Nb-Si-B and Nb-Mo-Si-B alloys. Initial heating in argon resulted in denser scale and reduced the parabolic rate constants of ternary alloys by [difference]17-22% and quaternary compositions by [difference]30-40%. The difference in oxidation resistance between Mo-Si-B and Nb-Mo-Si-B may be interpreted in terms of the volatility of the metal oxide that forms. MoO3 evaporates from the surface scale, leaving an oxidation resistant borosilicate glassy scale. Nb2O5 persists as a rapidly growing condensed phase that overwhelms the ability of the borosilicate glass to form a protective layer. A novel chlorination processing was employed to selectively remove Nb2O5 from the scale of the quaternary alloy as volatile NbCl5 from the scale of alloy comprised of a three phase microstructure of (Nb, Mo)5Si3B[subscript x](T1)-(Nb, Mo)5(Si, B)3(T2)-(Nb, Mo)5Si3B[subscript x](D88). Results show that Nb2O5 can be selectively removed from the scale to leave a borosilicate rich scale. The chlorinated scale forms a dense scale after heat treatment at 1000°C in argon. Oxidation rate of the chlorinated alloys was about one-third that of the unchlorinated alloy under identical conditions. Alloy oxidation during heating to test temperature has been studied and a plausible mechanism for formation of porosity in oxide scale has been offered.