Effect of NiO scales on the creep behavior of Ni single crystals at 550 °C

The creep properties of Ni(111) monocrystals are studied in oxygen at 550 degreesC: Ni samples were covered with a thermally grown NiO scale prior to the experiments. Three distinct mechanical studies were performed. global creeps local creep (periodic cracking technique) and stress relaxation. The global creep behavior of preoxidized nickel is divided into two stress domains associated with a diffusion creep mechanism (up to 35-40 MPa) and a power law creep (above 40 MPa). These results are confirmed by local creep analysis based on a periodic cracking process of the thin oxidized zone ('periodic film cracking' technique). Also modifications of deformation mechanism. between bare samples and samples covered with a NiO outer scale, are observed and associated with the properties of defects in the substrate near the oxide scale. Stress relaxation serves to identify the two effective stresses that must be taken into account: a stress which can be relaxed (Deltasigma) and a permanent internal stress (sigmai). Deltasigma is the effective stress for diffusion of mobile species in creep. Above mechanical loading of 40 MPa, stress relaxation suggests that, after complete oxide layer destruction, either a new continuous NiO scale develops, or the healing of cracks occurs. (C) 2002 Elsevier Science B.V. All rights reserved.