The compressive creep deforrn;~tiort microstructure of polycrystalline Ni3AI h:is been observed by transnlission electron microscopy (TEM). ( 1 1 I ) rind ( 110) slip systems have been identified in primary creep. Dissociatiort of edge superparti:~l pairs perpendicular to the slip plane (both { 11 1 ) and ( 110)) is common in crept specimens. I'he formation of this superdislocation configuration is thought to limit the extent of primary creep deforniation. lnt rotluction The creep behaviour of

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... i3AI at intermediate temperatures does not display the three typical stages characterised as primary, steady-state and terrir~ry. Instead, after primary creep, the rate continually increases with creep strain [11,12],1.31. A model of the processes controlling the intermediate temperature creep behaviour of Ni3Al has been suggested by Hemker et al 131. In primary creep, the octahedral glide of superpartial dislocation p:rirs, which have ;I pl;in;ir dissociation, results in a comparatively high initial creep deformation rate. Subsequently, thermally activated crohs-slip of the screw segnients onto a cube plane to form K-W locks immobilizes the supcrdislocations. This leads to exhaustion of octahedral glide and reduces the strain rate. After the screw segments are locked, the edge segments continue to expand on the octahedral plane. As first proposed by Mills et :11.141 [tie dislocation str-ucturc associated with an expanded loop is a series of K-W locks connected by edge segntents on the octahedral plane. Each K-W lock has the capacity to be a Frank-Read dislocation source or1 the cube plane. Given sufficient time and temperature, the cross-slipped screw segments are able to bow out and glide on the cube plane. The production and multiplication of dislocations on the cube plane lead to ;in increasing creep rate, i.e. inverse creep. In the present investigation, superdislocation dissociation, especially of the edge segments, is studied by means of transmission electron microscopy (7'EM) in polycrystalli~lc Ni3AI crept at an intemiediate temperature. An attempt is made to understand the superdisl~atiort dissociation and its influence on the creep behaviour. E:xperimental Procedure A polycrystalline Ni3AI ingot was prepared for the present investigation. The ingot was homogenized at 1050°C for three days in vacuum. Specinlens with dimensions 4.5mmx4.5n~mx9.0rt~m were cut by spark machine. Creep tesrs were carried out on an ESH 2OOkN material tesring machine at 580°C in air. After the creep tests, the specimens were cluenched into an ice bath. 1'EM specimens were made with electrochemical polishing using a twin jet polisher and a solutiort of 55% ethanol, 3 2 8 butoxyethanol, 8% perchloric acid and 5% glycerol at --10°C. TEM observ:itions were c;~r~ied out with a JEOL 4000-FX microscope.