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Oxidation of binary Hf-Nb alloys with 10-50wt%Nb and a few corresponding alloys with ternary additions of Cr, Ti, V, and W has been studied in the temperature range 800-1300 C. Most of the studies were carried out at 10 torr O2, but the oxidation of one binary alloy (Hf-34wt%Nb) was also studied as a function of oxygen pressure. The major reaction products are HfO2, Nb2O5, and Nb2O5 6HfO2. At and above 10 torr O2 the oxidation of the binary alloys increase with temperature up to 900-1000 C, where the oxidation goes through a maximum and then decreases up to about 1100 C. Above 1100 C the oxidation again increases with temperature. This maximum is interpreted as a result of an interplay between relative rates of formation of HfO2 and Nb2O5 and their subsequent reaction to form Nb2O5 6HfO2. The ternary alloy additions of Cr, Ti, and W did not significantly change the oxidation rates. Catastrophic oxidation was observed for Hf-25w/oNb-52/oV and Hf-20w/oNb-10w/oV alloys above 1000 and 900 C, respectively. The binary Hf-Nb alloys showed poor oxidation resistance, and at 900-1000 C the oxidation is as fast as for unalloyed niobium. (Author).
Experimental programs concerned with the oxidation of titanium and its alloys are reviewed and results compared with those predicted by theory. Wagner-Hauffe theory is used as the primary basis for comparison, and its inconsistencies are pointed out. Fifteen binary alloy systems involving titanium are covered, as well as a few ternary and commercial alloys. A short section discusses the effects of oxygen or nitrogen contamination on the mechanical properties of titanium and its alloys. (Author).
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.