Chromium Depletion in a Ni-30Cr Alloy During High-Temperature Oxidation

Abstract

Ni-30Cr alloy samples were oxidized at temperatures between 500 and 900 °C to investigate the link between the evolution of the microstructure and the chemical composition in the alloy substrate beneath a growing chromia layer. Before oxidation, a layer of ultrafine grains was observed between the surface and a thick lamellar layer. This structure was replaced by some larger recrystallized grains after oxidation. The growth kinetics of the recrystallized grains was described by a parabolic law with a kinetic constant following an Arrhenius law from 500 to 700 °C. For samples oxidized at 800 and 900 °C, all Cr profiles showed a gradient close to the shape predicted by Wagner models. In the samples oxidized between 500 and 700 °C, many Cr profiles showed a two-step shape, with the smaller Cr fraction in the step closer to the alloy/oxide interface. By considering a fast diffusion accelerated by grain boundaries in the zone of recrystallized grains, the two-step shape can be simulated by numerical resolution of diffusion problem.

Appendix: Table of variables

Variable	Unit	Description
${d}_{\mathrm{g}}$ and ${d}_{\mathrm{g},0}$	cm	Grain diameters at time $t$ and at the initial time
$D$ and ${D}_{0}$	cm−2.s−1	Diffusion coefficient of Ni or Cr in volume, in dislocations or in grain boundaries and its pre-exponential factor
$\widetilde{D}$	cm−2.s−1	Interdiffusion coefficient in volume or in grain boundaries or apparent interdiffusion coefficient
$e$	cm	Thickness of the oxide layer
${E}_{\mathrm{a}}$	J.mol−1	Activation energy
$f$	dimensionless	Volume fraction of dislocations or grain boundaries
${J}_{\mathrm{Cr},\mathrm{i}}$	mol.cm−2.s−1	Flux of Cr at the alloy/oxide interface
${k}_{\mathrm{d}}$	cm3.s−1	Kinetic constant for recovery
$k$, ${k}_{\mathrm{p}}$ and ${k}_{\mathrm{c}}$	cm−2.s−1	Parabolic kinetic constants for grain growth, oxidation and recession processes, respectively
$n$	dimensionless	Exponent of the kinetic law
${N}_{\mathrm{Cr}}$	at%	Molar fraction of Cr at the depth $x$, at the interface or in the bulk
$s$	dimensionless	Segregation factor
$t$	s	Time
$T$	K	Temperature
${V}_{\mathrm{m}}$	cm3.mol−1	Molar volumes of the Ni-30Cr alloy or the Cr2O3 oxide
$x$ and ${\Delta x}_{\mathrm{m}}$	cm	Depth in the alloy from the alloy/oxide interface and recession depth, respectively
$\delta$	cm	Grain boundary width
${\lambda }_{\mathrm{vol}}$	cm	Characteristic length of volume diffusion
${\rho }_{\mathrm{d}}$ and ${\rho }_{\mathrm{d},0}$	cm−2	Density of dislocations at time $t$ and at the initial time