TEM observations of dislocations in aluminium nitride after high temperature deformation

AlN ceramics were deformed up to 10% strain in compression at elevated temp eratures (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 disloca tions were identified by TEM in deformed crystals. Most of them have (a) ov er right arrow type Burgers vectors and were seen to glide in the basal pla ne or in prismatic planes. They looked usually undissociated but large diss ociations were observed after deformation tests of long durations. In such cases, (a) over right arrow dislocations could be dissociated either by pur e glide in two Shockley partials or by combined glide plus climb in two Fra nk-Shockley partials. In any case, the enclosed stacking fault was in the b asal plane. (c) over right arrow+(a) over right arrow dislocations were als o observed bur seldom. Different kinds of faulted loops are reported, delin eated by Shockley, Frank or Frank-Shockley partials. Indications are given that oxygen segregation should play a role in the dissociation of dislocati ons. (C) 2000 Elsevier Science S.A. All rights reserved.