Interdiffusion in binary and multicomponent silicides and aluminides of molybdenum at 1000 degrees C

Edward J Ciecko, Purdue University

Abstract

Interdiffusion, phase formation, and the development of diffusion structures were examined in the Al-Mo, Al-Mo-Si, and Al-B-Mo-Si systems with binary, ternary, and quaternary vapor-solid and solid-solid diffusion couples at 1000°C. Diffusion disks of Mo, MoSi2, Mo5Si3, Mo 5Si3Bx, and Mo3Al8 were employed in the assembly of diffusion couples. Powders of Mo3Al 8 and Mo3Al served as Al vapor sources. For all diffusion couples, concentration profiles were determined by electron microprobe analysis. Interdiffusion flux profiles calculated directly from the concentration profiles were integrated over selected concentration ranges in order to evaluate integrated, [special characters omitted], and average effective, [special characters omitted], diffusion coefficients for the individual components in various silicides and aluminides of Mo. An Al-Mo-Si isotherm at 1000°C was partially constructed on the basis of the experimental diffusion paths of selected diffusion couples. A simple model was developed to analyze two-phase diffusion layers of the monoclinic Mo3(Al,Si)8 and hexagonal (C40) h-Mo(Si,Al)2 phases developed in selected couples for the determination of [special characters omitted] for the diffusing species in the individual phases. While interdiffusion of Al and Mo was readily supported in both phases, [special characters omitted] for Si in the (C40) h-Mo(Si,Al)2 phase was more than an order of magnitude greater than that in the Mo3(Al,Si) 8 phase. B additions reduced [special characters omitted] of Al, Mo, and Si in the two-phase layers by about 70%. Interdiffusion of Al in columnar-grained MoSi2 was assessed in terms of a simple partition model for individual flux contributions from bulk and grain boundary diffusion. [special characters omitted] for grain boundary interdiffusion of Al was about 5 orders of magnitude greater than [special characters omitted] for volume interdiffusion of Al. An important observation of this study is the build-up of Al in the tetragonal (C11b) t -Mo(Si,Al)2 diffusion layer identified by relative maxima in the Al concentration profiles within the layer. Such Al build-up reflected diffusional contributions along grain boundaries and triple junctions, which may operate as “fast pipes”, enhancing diffusion if aligned parallel to the predominant diffusive flux, or conceivably as “traps”, if inclined at an angle to the predominant diffusive flux, resulting in segregation of Al. Another interesting observation was the formation of a porous Mo 5Si3 layer with concurrent loss of Si in MOSi 2 disks in contact with Mo3Al powder. The Kirkendall shifts arising from the large intrinsic flux of Si helped consolidate the outer regions of the Mo5Si3 layer.

Degree

Ph.D.

Advisors

Dayananda, Purdue University.

Subject Area

Materials science

COinS