Low-Cycle Fatigue Behaviour of a Ni-Based Single-Crystal Superalloy

Abstract

Abstract. In this study, low-cycle fatigue (LCF) tests at 500• C in the ⟨001⟩, ⟨011⟩ and ⟨111⟩ directions have been performed for the Ni-based single-crystal superalloy MD2. All tests were carried out in strain control with R ϵ =-1. The ⟨001⟩ direction has the lowest stiffness of the three directions and also shows the best fatigue properties in this study followed by the ⟨011⟩ and ⟨111⟩ directions, respectively. It is well recognised that Ni-based single-crystal superalloys show a tension/compression asymmetry in yield strength and this study shows that a tension/compression asymmetry also is prevalent during LCF conditions. At mid-life, the ⟨001⟩ direction generally has a higher stress in tension than in compression, while the opposite is true for the ⟨011⟩ direction. For the ⟨111⟩ direction the asymmetry is found to be strain range dependent. The ⟨011⟩ and ⟨111⟩ directions show a cyclic hardening behaviour when comparing cyclic stress-strain curves with monotonic stress-strain curves. In addition, the ⟨011⟩ and ⟨111⟩ directions show a serrated yielding behaviour for a number of cycles while the yielding of the ⟨001⟩ direction is more stable.

Aspects of Crack Growth in Single-Crystal Nickel-Base Superalloys

“…These components are often cast from nickel-base superalloys in the form of singlecrystals. The material anisotropy of single-crystal materials adds an extra dimension of complexity to the modelling, testing and understanding of the fracture mechanisms [3][4][5][6][7][8][9][10][11]. The service life of many hot components is not fully restricted by the number of cycles to crack initiation, which means that a certain amount of crack propagation is allowed before the component has to be replaced in service.…”

Evaluation of the crystallographic fatigue crack growth rate in a single-crystal nickel-base superalloy

“…These blades are therefore often manufactured from nickel-base superalloys cast as single-crystals, due to their superior material properties under these working conditions. However, the complex nature of the anisotropy in these materials is often challenging in terms of testing, modelling and understanding the fracture behaviour [2][3][4][5][6][7][8]. The crystallographic orientation of single-crystal materials has been shown to influence the crack growth behaviour [9][10][11], and is thus an important factor in the modelling.…”

Modelling of Crack Growth in Single-Crystal Nickel-Base Superalloys

“…These components are often cast from nickel-base superalloys in the form of single-crystals. The material anisotropy coupled with the strong influence of the crystallographic orientation on single-crystal materials adds an extra dimension of complexity to the modelling, testing and understanding of the fracture mechanisms [5][6][7][8][9][10][11][12][13], compared to their polycrystalline counterparts.…”