Mechanical Behaviour of Single Crystal Superalloy SC16 under Hold Time LCF Loading

摘要

The behaviour of single crystal nickel base superalloy SC16 has been studied under hold time LCF loading with various hold periods from 60 to 1800 s in compression region at 1223 K. The strain rate was 10~(-3) s~(-1) and the strain amplitudes were 0.4 % and 1.0 %. The alloy showed asymmetrical cyclic stress response behaviour. In the first few load cycles the alloy showed a cyclic hardening in tensile region and a cyclic softening in compression region. Both the stress amplitudes stabilised, however, after several load cycles. Based on the analysis of relaxation behaviour of the alloy during hold time the stress response behaviour observed in this study could be understood qualitatively. The introduction of hold periods at maximum compressive strain leads to a morphological change of γ' precipitates (rafting) in the alloy from initially cuboidal to a rod-like geometry with its rod axis parallel to load direction. In contrast to polycrystalline superalloy IN738LC, which has a similar chemical composition and microstructure, the cyclic life of the alloy SC16 is less influenced by the introduction of compressive hold periods at lower strain amplitudes. At higher strain amplitudes a decrease in cyclic life due to the introduction of hold periods was observed.