Diffusion Parameters in the Nb-Mo System: Revisited
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ry metals draw special attention in hightemperature applications. The elements Mo, W, Ta, Re, and Nb are used in Ni-based superalloys to improve creep resistance for the application in air and land-based gas turbine applications.[1] Recently, there has been enormous effort to find a material system that can replace Ni-based superalloys operating at its limit. A further increase in operating temperature is difficult, because it seems now that all of the possible manufacturing methods or alloying additions have been explored. So there is a search for a completely new material system, which can operate at much higher temperature to increase the efficiency as well as decrease the emission of unwanted gases. In this regard, Nbbased silicides are considered as one of the best potential systems; they have an even higher strength-to-density ratio[1] compared to Ni-based superalloys. Room-temperature fracture toughness has reached the minimum required level with the help of alloying additions.[1] Further research is being carried out to improve the oxidation resistance. It was recently found that the addition of Mo increases the oxidation resistance at high temperature.[2] So, it is important to study the interdiffusion in the Nb-Mo system, which will be helpful in designing the alloy for the application. There are some studies available[3–5] in the literature; however, significant differences were found in different studies. With the renewed interest in this system, it is necessary to revisit the system to re-examine the data available in the literature. S. PRASAD, Ph.D. Student, and A. PAUL, Assistant Professor, are with the Department of Materials Engineering, Indian Institute of Science, Bangalore-560012, India. Contact e-mail: [email protected]. ernet.in Manuscript submitted March 9, 2009. Article published online May 14, 2009 1512—VOLUME 40A, JULY 2009
The diffusion couple technique is used to determine the interdiffusion coefficient at three different temperatures of 1800 C, 1875 C, and 1900 C in a vacuum level of ~10–6 mbar. Interdiffusion studies were conducted at a lower temperature range also; however, the interaction zone was found to be very small and was excluded from the present report. A very thin interdiffusion zone introduces significant error in the calculation because of the introduction of the inaccuracy in determining the composition gradient. For example, Vergasova et al. reported the interdiffusion data at 1100 C,[6] where the interdiffusion zone is actually less than 1 lm. It is almost impossible to determine the variation of interdiffusion coefficients with composition following conventional techniques, when the interdiffusion zone is so small. In the temperature range of our interest, the interaction zone was found to vary from 65 to 138 lm. The Nb with purity of 99.8 wt pct and Mo with purity of 99.95 wt pct supplied by Alfa Aesar, in the form of foil of 1-mm thickness, were used in the present study. Pieces with dimensions of around 7 9 7 mm2 were cut, and standard metallographic preparation was carried out to
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