Diffusion Parameters in the Nb-Mo System: Revisited

Abstract

The variation of the interdiffusion coefficient with the change in composition in the Nb-Mo system is determined in the temperature range of 1800°C to 1900°C. It was found that the activation energy has a minimum at around 45 at. pct Nb. The values of the pre-exponential factor and the activation energy for diffusion are compared with the data available in the literature. Further, the impurity diffusion coefficients of Nb in Mo and Mo in Nb are calculated.Refractory 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][4][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.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 Aesa...

Interdiffusion in Nb-Mo, Nb-Ti and Nb-Zr Systems

“…Variation of interdiffusion coefficient with composition in terms of Nb at pct as calculated in our previous work [15] is shown in Fig. 3b.…”

“…In the present work, we have revisited the binary Nb-Ti and Nb-Zr systems to determine the interdiffusion coefficients as a function of composition at various temperatures in these systems. Further we have considered the results of Nb-Mo system for comparison [15]. First, the interdiffusion data calculated in different systems are presented separately and then the impurity diffusion coefficients and the activation energies from different systems are compared.…”