Plastic deformation of oxide scales at elevated temperatures
- PDF / 348,798 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 88 Downloads / 245 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Plastic deformation of oxide scales at elevated temperatures Yifan Zhang, William W. Gerberich, and David A. Shores Corrosion Research Center, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received 5 June 1995; accepted 12 August 1996)
The atomic force microscope (AFM) has been used to observe and characterize for the first time surface steps and grooves on the faces of Cr2 O3 grains formed as an oxide scale on Ni –30Cr and Ni –30Cr–0.5Y alloys during high temperature oxidation. The very high spatial resolution of the AFM is required to characterize these features. We propose that these surface features, whose dimensions are in the range of nanometers and tens of nanometers, may be interpreted as evidence of highly localized plastic deformation of the oxide scale. The size and spacing of the steps and grooves are consistent with models of plastic deformation based on slip bands derived from dislocation climb or dislocation glide. Mechanical twinning and the models for stress-driven surface instability are also possibly responsible for some surface features. The addition of yttrium to the alloy seemed to enable enhanced plastic deformation of the scale. The strain corresponding to the observed features, estimated by simple models, could relax a significant part of oxide growth and thermal stresses.
I. INTRODUCTION
The protection of alloys against high temperature oxidation is often provided by the selective oxidation of chromium or aluminum to form a compact, adherent, and slow-growing protective scale. Under practical conditions, the protective oxide scales may be subjected to stresses induced by the scale growth process during isothermal oxidation and by thermal expansion mismatch stresses during thermal cycling.1–5 These stresses will cause scale fracture and hence will lead to accelerated oxidation if they cannot be relaxed in a nondestructive way, such as by plastic deformation of the metallic substrate and/or of the oxide scale. The plastic deformation of the scale could contribute to the scale integrity by dissipating some of the elastic stress of the scale, as suggested by Schu¨ tze.5 Previously suggested forms of oxide scale plastic deformation include grain boundary sliding, mechanical twinning, and dislocation climb.1 Dislocation glide in polycrystalline oxides is generally not considered to be a favored form of plastic deformation due to the insufficiency of independent operating slip systems at normal temperatures1 ; however, some evidence for this mechanism has been found in NiO by TEM.6 Recently, it has been proposed that stresses in thin films can be relaxed by surface shape changes caused by stress-driven surface diffusion. The process has been called stress-driven surface instability.7–10 For example, Harvey et al.11,12 have applied this theory in the surface morphology study of In0.25 Ga0.75 As thin films grown on GaAs(100) surfaces. Due to the structural heterogeneity of the typical
Data Loading...
