GRAIN BOUNDARIES IN HIGH TEMPERATURE DEFORMATION OF YTTRIA AND MAGNESIA CO-DOPED ALUMINA
Le Journal de Physique Colloques
Resume-La presence d'yttriurn dans des alurnines B grains fins dopees B la rnagnesie entraine des modifications importantes des lois de comportement et de la microstructure de ces materiaux lors d'une deformation en compression B haute tempkrature. Les premiers rksultats de I'etude en rnicroscopie Blectronique revdent une ammentation de la densit6 des dislocations a la fois dans les grains et dans les joints de grains. Ils mettent en evidence le r61e limitant de I'yttrium, fortement segreg6 aux
... rtement segreg6 aux joints, vis-a-vis des processus d'accommodation des dislocations intergranulaires; ces observations sont en bon accord avec fa diminution de la vitesse de d&formation observbe en presence d'yttrium. Abstract -This is a first investigation on the role of yttrium in the ductility under high temperature deformation of fine-grained Mg-doped aluminas. The changes in the constitutive laws of com~ressive tests associated to the modifications of the microstructure are supported by lhe electron microscopy observations. The most important effect of yttria CO-doping is an increase in the dislocation density both in the grains and in the grain boundaries; these observations reveal the hardening role of this element in the accommodation processes of grain boundary dislocations. This role of yttrium in grain boundary dislocation creep is in good agreement with the observed decrease in the deformation rate. I -INTRODUCTION Fine grained Mg-doped aluminas display a high ductility behavior under high temperature deformation. The constitutive laws and the mechanisms are characteristic of a structural superplasticity in which grain boundary (GB) diffusion limited GB sliding is the predominant process 11-41. A significant cavitation occurs as a non accommodation of GB sliding and grain growth takes place accounting for the observed decrease in the deformation rate. These processes involve different interface properties, such as their role as sources and sinks for vacancies and dislocations, that depend on GB crystallographic parameters and on the presence of segregated species. Apart from a purely diffusional process, the superplasticity mechanisms can be understood by means of GB dislocation creep model, that can be summarized as follows 15,61. The onset of a "pure" superplastic flow occurs with the creation and the motion of dislocations in the GB plane as a result of the shear stress. Different obstacles, triple points, ledges, subboundary junctions, induce a non uniformity of GB sliding and stress relaxations occur by emission of lattice dislocations. The absorption of these defects by other GB gives rise to a stimulation of GB sliding that is not only a deformation process but also a micrornechanisrn of GB recovery U/. Finally the displacement of steps associated with dislocations can be involved in GB migration 151. Although GB dislocation creep is frequently evoked, there are little transmission electron microscopy (TEM) observations of linear defects in superplastically deformed materials. An important result of our previous work on Mg-doped aluminas is the occurrence of non periodic dislocation networks in special near coincidence boundaries of the deformed samples that confirms the activity of GB dislocations during creep 181. Furthermore, it is shown that special GB play a limiting role in the transmission of sliding and do not behave as good sources and sinks for lattice dislocations. Defects are rarely observed in general GB where arcnrnrnnrlafinn nrnroccoc taka nlaro oacilv Article published online by EDP Sciences and available at http://dx.