Superlattice intrinsic stacking faults in γ′ precipitates

After 2% tensile and creep and tensile deformation of MA6000 at 760 and 790 deg C, respectively, a/3 < 112 > partial dislocations have been identified to deform gamma ' precipitates. No evidence was found for a/2 < 112 > partial dislocations. However, fault configurations typical for a/2 < 112 > partial dislocations are observed after the first LCF experiments at 760 deg C. Upon further deformation, SISF's expand in the matrix which means that the energy of an extended fault is lower than the energy of faulted precipitates separated by unfaulted matrix. The extension of faults in the matrix might be stimulated by the annihilation of interfacial Shockley dislocations. From the results it might be concluded that upon deformation at 760 and 790 deg C, a/3 < 112 > partial dislocations are mobile in spite of a high Peierls stress. At room temp. deformation till fracture produces a rather low density of twins. As a matter of fact, this density is comparable with the density of twins in the as-received material. Calculations show a/3 < 112 > partial dislocations may have glissile and sessile core structures. Therefore, the explanation might be that at room temp. a/3 < 112 > partial dislocations are sessile. The microstructure shows that superlattice dislocations coupled together by APB only play a minor role in deformation of MA6000 at 760 and 790 deg C. Yet a few superdislocations inside gamma ' precipitates are found. Measuring the distance between the two dislocations is a suitable method to obtain an estimate of the APB energy. Unfortunately, the reliability of the estimate is influenced by residual stresses. Nevertheless, the antiphase boundary energy on {111} of gamma ' precipitates is calculated to be 144 ( plus/minus 10%) mJ/m exp 2 . 15 ref.--AA