TEM observations of dislocations in aluminium nitride after high temperature deformation

AlN ceramics were deformed up to 10% strain in compression at elevated temperatures (1820-1920 K) under constant stress in the range 150-250 MPa or at a constant fixed strain rate of 5 x 10(-6) s(-1). Several kinds of dislocations were identified by TEM in deformed crystals. Most of them have (a) over right arrow type Burgers vectors and were seen to glide in the basal plane or in prismatic planes. They looked usually undissociated but large dissociations were observed after deformation tests of long durations. In such cases, (a) over right arrow dislocations could be dissociated either by pure glide in two Shockley partials or by combined glide plus climb in two Frank-Shockley partials. In any case, the enclosed stacking fault was in the basal plane. (c) over right arrow+(a) over right arrow dislocations were also observed bur seldom. Different kinds of faulted loops are reported, delineated by Shockley, Frank or Frank-Shockley partials. Indications are given that oxygen segregation should play a role in the dissociation of dislocations. (C) 2000 Elsevier Science S.A. All rights reserved.