PRECIPITATE STRENGTHENING OF A GAMMA'-(NI,CO)(3)(SI,TI)-INTERMETALLICALLOY

Homogeneously distributed, coherent precipitation of the disordered fe e gamma-phase has been observed in an L1(2)long-range ordered gamma'-i ntermetallic (Ni,Co)(3)(Si,Ti)-alloy with the composition Ni58Co20Si10 Ti12. This leads to precipitate strengthening of the alloy after aging in the temperature range from 973 to 1073 K, as revealed by hardness measurements. In order to understand the strengthening mechanism, in-s itu tensile tests of thin foils of the aged alloys were carried out in a transmission electron microscope. The motion of the superdislocatio ns in the gamma'-matrix and their interaction with the disordered gamm a-precipitates were analyzed. It was found that most of the [110] supe rdislocations were of screw character. They were dissociated dominantl y into two unit 1/2[110] dislocations separated by an antiphase bounda ry (APE) on the {111} slip planes. Their movement was jerky. They were strongly pinned by the gamma-precipitates due to the absence of the A PE in the disordered gamma-phase. The pinned configurations of the sup erdislocations have been simulated in a computer. (C) 1997 Elsevier Sc ience S.A.