1. Conclusion

In summary, it can be concluded that under the conditions of creep tests conducted according to the I variant, a crucial factor determining the increase of stability of superalloys with a bigger macrograin (Fig.3, 4) was levelled through an increase in grain size debilitating the actions of grain boundaries. At high temperatures, the grain boundaries absorb dislocations moving as a result of slipping and climbing, which leads to the reduction of strain hardening and weakening of the material. Similarly, weakening effect of grain boundary is associated with the deformation of the material due to the so-called slip across the grain boundaries facilitated in high temperature. The effect is that the grains can slide relative to each other rigidly without appreciable distortion inside the grains. As a result of these processes under these test conditions, a reduced stability and greaterdeformation rate of the fine-grained material is observed (Fig.3, 4).

Whereas, under the conditions of variant II of the creep test as a result of the increase in value of the normalized stress τN=τ/G a factor fundamentally conditioning the deformation and stability of the superalloys (Fig. 5, 6) is no longer a grain size (as can be inferred from the analysis of the deformation map). Under such thermal and mechanical loads a material deforming process takes place in the entire volume of the material, mainly due to a dislocation mechanism (due to the climb and dislocation slip). The role of the grain boundaries in this case is secondary. This is confirmed by the results of creep tests, which showed comparable stability of the samples both with smaller and bigger macrograin (Fig. 5, 6).