Interdiffusion in the Nickel–Tungsten and Thoria-Dispersed Nickel–Tungsten Systems

Abstract

The interdiffusion coefficient in the α solid solution of tungsten in nickel has been determined for the nickel–tungsten and TD (thoria-dispersed) Nickel–tungsten systems over the range 1000–1316° C. The Boltzmann–Matano analysis was employed to compute diffusivities from concentration vs. penetration curves measured with the electron microanalyser. The interdiffusion co-efficient is concentration-dependent in the nickel–tungsten system, decreasing with increasing tungsten content and can be represented by the equation: <disp-formula> <mml:math> <mml:mrow> <mml:msup> <mml:mi>D</mml:mi> <mml:mi>~</mml:mi> </mml:msup> <mml:mo>=</mml:mo> <mml:mn>1·19</mml:mn> <mml:mtext> exp</mml:mtext> <mml:mo>(</mml:mo> <mml:mo>−</mml:mo> <mml:mn>0·0469</mml:mn> <mml:msub> <mml:mi>C</mml:mi> <mml:mtext>w</mml:mtext> </mml:msub> <mml:mo>)</mml:mo> <mml:mtext> exp</mml:mtext> <mml:mfrac> <mml:mrow> <mml:mo>(</mml:mo> <mml:mo>−</mml:mo> <mml:mn>3·69</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>4</mml:mn> </mml:msup> <mml:mo>)</mml:mo> </mml:mrow> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> </mml:mfrac> </mml:mrow> </mml:math> </disp-formula> The data at low solute concentrations agree well with literature values and are consistent with a vacancy mechanism for diffusion. Interdiffusion in the TD Nickel–tungsten system is concentration-dependent and shows a bimodal temperature-dependence. The high-temperature behaviour is explained on the basis of a vacancy mechanism, while the behaviour at lower temperatures suggests enhanced diffusion due to short-circuiting diffusion paths. A large Kirkendall effect in TD Nickel/tungsten diffusion couples indicates that nickel is the more rapidly moving species.