Dissociation Processes in the Orthorhombic O Phase
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DISSOCIATION PROCESSES IN THE ORTHORHOMBIC 0 PHASE JOEL DOUIN*, SHIGEHISA NAKA** AND MARC THOMAS** * LEM, CNRS/ONERA, 29, avenue de la division Leclerc, BP 72, 92332 ChAtillon Cedex,
France ** OM, ONERA, 29, avenue de la division Leclerc, BP 72, 92332 Chitillon Cedex, France.
ABSTRACT In the orthorhombic phase of Ti 2AlNb-type, the occurrence of different dissociation modes of dislocations, glissile in the (001) basal plane is reported. [100] and 1/2[1 10] dislocations usually dissociate in order to form antiphase boundaries. Detailed weak-beam observations show that [100] dislocations, as well as [010] dislocations formed by interaction of 1/2 dislocations, can dissociate into three-fold configurations containing a pair of stacking faults. INTRODUCTION Recently, Nb-rich titanium aluminides near the
0
Ti2AINb composition have received an increasing attention since they were found to exhibit an extremely Al high temperature strength associated with a good 0 0 room temperature ductility [1]. It is presumed that these promising mechanical properties are related to the intermetallic ordered orthorhombic phase desi. 0 gnated as 0 [2], since a high amount of this phase is present in Nb-rich alloys. Figure 1 : The unit cell of the 0 phase. The shortest perfect translations have Since then, a number of people have identified been outlined (cell parameters : the 0 phase in a large range of compositions [3-91. a = 5.9 A,b = 9.6 A,c = 4.5 A [21). One of the best known of these alloys is the so-called Super ot2 alloy which, depending upon thermomechanical and thermal treatments, may exhibit a microstructure dominated by the 0 phase. For instance, Super ca2 alloy is transformed into the 0 phase almost completely after extrusion [10] and the 0 matrix has the same composition as the bulk material, here Ti-25AI-0ONb-3V-1Mo at.%. As a preliminary investigation aimed at elucidating the deformation mechanisms of this phase, our observations were focussed on the dissociation mode of dislocations gliding in the basal plane during high temperature extrusion. Since the 0 phase has an ordered unit cell with large parameters (Fig.1), superdislocations are expected to dissociate in order to reduce their energy.
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MATERIAL AND METHODS Dislocations formed in the 0 phase during extrusion at 1050°C of a Super oa2 alloy have been studied using weak-beam transmission electron microscopy. After extrusion at a strain rate of 0.37m/s (ratio 16:1), the samples were aged 1 h at 800'C. This treatment promotes large grain formation which helps to identify the dislocation networks which originate from the deformation. The foils have been electropolished using a solution of 5% HCIO 4 , 35% 2butoxyethanol and 60% methanol. RESULTS General deformation microstructure According to previous investigations [7, 11], the deformation microstructure of the 0 phase is essentially characterized by the interaction of screw superdislocations with [100] and 1/2[110] (and equivalent 1/2[1 10]) Burgers vectors in the (001) basal plane (Figs. 2, 4-5). In spite of a m
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