In situ X-ray tomographic microscopy of Kirkendall pore formation and evolution during homogenization of pack-aluminized Ni–Cr wires

Abstract

The Kirkendall effect, usually associated with the formation of undesirable fields of micron-size pores, can be used to fabricate hollow nano- and microstructures, where all pores have coalesced into a single cavity. Here, this concept is used with the goal of converting 50 μm diameter Ni-20wt.%Cr wires into Ni–Cr–Al tubes by diffusion of a surface-deposited aluminide coating. The formation and evolution of Kirkendall pores, created by imbalanced diffusion fluxes of Al, Ni and/or Cr, are investigated using both in situ X-ray tomographic microscopy and ex situ annealing studies. The tomography results indicate that the pores nucleate near the interface between the Ni–Cr core and β-NiAl(Cr) reaction layer that develops upon homogenization. They then grow and coalesce near the center of the radial cross-section of the wire. This is followed by a stagnation period until the pores migrate longitudinally due to the influence of a temperature gradient imposed by the laser heating used for annealing. The ex situ experiments are in qualitative agreement with the tomography study with the exception of the secondary pore migration along the wire length, as expected because the ex situ study is conducted under isothermal conditions. Detailed features of the microstructure were discernable only upon metallographic preparation and include Cr-rich precipitates and an α-Cr(Al,Ni) rejection region that formed during coating and homogenization, respectively. In situ X-ray tomographic microscopy is thus demonstrated to be a viable technique to use in conjunction with more conventional ex situ metallographic techniques to conduct interdiffusion and Kirkendall pore studies.