Effect of grain boundary misorientation angle on diffusion behavior in molybdenum-tungsten systems

Grain boundary (GB) diffusion plays a key role in the long-term service processes of engineering materials at elevated temperatures. GB diffusion processes are strongly relying on the misorientation angle of the two adjacent grains. However, the orientation dependence of GB diffusion is not well understood. In this work, two types of tungsten coating were deposited onto a substrate consisting of a single molybdenum crystal to forming diffusion couples. The diffusion profiles of isolated GBs have been measured by electron backscatter diffraction and energy dispersive X-ray spectroscopy. The effect of the tungsten GB misorientation angle (theta) on the diffusivity of molybdenum was investigated. GB structures with different misorientation angles were resolved using high-resolution transmission electron microscopy. As theta increased from 5 degrees to 18 degrees, the spacing between the dislocations composing the GBs decreased from greater than 10 to 3 atomic spaces. Furthermore, when theta was greater than 18 degrees a 0.53 nm wide ordered structure was formed, and this structural change increased the GB energy. Consequently, the GB diffusion coefficient increased by 3- and 7- orders of magnitude fold as compared to the volume diffusion coefficient for low- and high-angle GBs, respectively. The GB energies were approximately 1.1 and 2.1 J m-2 in low- and high-angle GBs, respectively. This is the first time that tungsten GB energies were determined through experimental method. (C) 2019 Published by Elsevier B.V.