Fluage haute temperature du sesquioxyde d'yttrium: Y2O3

Abstract

HIGH-TEXPERATURE CREEP OF YTTRIUM SESQUIOXIDE : Y₂O₃ High-temperature and low-stress creep behaviour of yttrium oxide, Y₂O₃ has been studied by means of compressive experiments, conducted in air, between 1550 and 1800°C in the stress range 20 to 140 MPa. The creep testing apparatus and its technical limits are described in detail. Experimental results are related to a phenomenological analysis in terms of a mechanical creep law and macroscopic and microscopic aspects of the dislocation substructure of the crept samples. As in most high-temperature creep investigations, a phenomenological analysis of the creep curves leads to failure in finding a well-defined creep model. However, detailed analysis of the aspect of the faces of the crept samples, together with electron microscopy investigations of the dislocation substructure, suggest the following high-temperature creep behaviour for Y₂O₃. (a) The first step of the creep, which exhibits a short transient, involved glide of dislocations whose Burgers vector is b=a/2. The dislocations density increases and induces dislocation junctions having a Burgers vector b = a100 >. Such a behaviour gives rise to a three-dimensional network of dislocations inside the crystal. (b) The dislocation junctions, b = a100 >, dissociates in a sessile manner by climb and pins down the three-dimensional network. Further glide development of this network is therefore difficult. The climb deformation process becomes quite competitive with respect to glide. Then, there begins a steady-state creep corresponding to the equilibrium between a strain hardening due to the multiplication of dislocations probably from Bardeen-Herring sources, and a recovery due to annihilation of dislocation by climb. In such a model, the dislocations of Burgers vector b = a100 >, which may largely dissociate by pure climb, plays the major role. It is suggested that such a pure-climb dissociation, not only inhibits the glide process, but is also an important ‘driving force’ in favour of a pure-climb deformation mechanism.