Creep crack growth behavior at 1033K of directionally solidified CM 247 LC — a cast nickel-base superalloy

Abstract: In-phase TMF crack growth testing with different lengths of the hold time at the maximum temperature of 550 °C has been conducted on Inconel 718 specimens Focus has been on establishing a method f

Abstract: The CM 247 LC superalloy was remelted and cast to obtain the desired polycrystalline test bars by controlling casting parameters, followed by the investigation of precipitation morphology and mechanical properties. The experimental results show that by well-controlled casting parameters the CM 247 LC owns excellent castability to form a superalloy with fine grain structure, high strength as well as superior creep resistance.

Abstract: In-phase (IP) and out-of-phase (OP) thermomechanical fatigue tests with T = 100–750 °C and optional dwells of 20 min at 750 °C were carried out on directionally solidified Ni-base Alloy 247 LC DS. Introducing dwells reduced the lifetime for both phase angles to about one sixth. Specific damage mechanisms were internal carbide and carbide–matrix interface cracks in IP tests and crack propagation along {1 1 1}-microtwin planes in OP tests. Introducing dwells intensified both effects, thus contributing to the lifetime reduction. During dwells, the gauge length may exhibit transversal creep because of extensometer forces distorting the strain measurement.

Abstract: In order to meet the 21st century challenges, the need to improve the high temperature load bearing capabilities or creep properties of nickel base superalloys is more important than ever. Thorough understandings of creep deformation micro-mechanisms are essential for improving high temperature performance of these alloys. Therefore, in the present study, creep deformation micro-mechanisms of CM 247 DS LC alloy used for fabricating turbine blades and vanes under relevant service conditions have been explored. The creep deformation behaviour at the investigated temperatures has been divided into three temperature regimes. In the lower temperature regime, a classical three stage creep curve is observed, i.e., primary creep followed by steady-state creep and then an accelerating creep stage leading to failure. While, in the intermediate temperature creep regime a relatively shorter secondary creep region is seen, in contrast, secondary creep stage is almost missing in the high temperature regime. Transmission electron microscopic (TEM) studies revealed that the deformation micro-mechanisms in the low temperature regime are dominated by γ′-precipitates shearing of anti phase boundary (APB) coupled dislocations pair. In the high temperature regime, the deformation is dominated by the extensive dislocation networks formation and severe γ′-rafting. In the intermediate temperature regime, the deformation micro-mechanisms are basically mixture of both low and high temperature deformation micro-mechanisms. The facts revealed by TEM observations were well supported by scanning electron based factographic and microstructural analysis.

Abstract: In order to clarify the crack propagation properties of an anisotropic material (Ni-based directionally solidified superalloy), longitudinally loaded specimens (L-specimens) and transversely loaded specimens (T-specimens) with a crack are subjected to high temperature fatigue. The crack propagation rate is reasonably well correlated with the effective stress intensity factor range regardless of the propagation direction (specimens L and T), the stress range and the stress ratio. However, the crack propagation rate shows a notable fluctuation particularly in the T-specimens. It is at most about five times faster than the average. The fracture surface features can be classified into four types with three trans-granular and one intergranular types. In the former, though the crack is along the {100} or {110} planes on a macroscopic scale, it threads through the {111} or {100} planes on a microscopic scale. Crack propagation is notably accelerated in the intergranular region, while deceleration is caused by crack branching.

Abstract: Introduction. Processes of deformation and fracture at high temperatures. Stress analysis of uncracked bodies. Stress analysis of cracked bodies. Models for creep crack initiation and growth. Creep-fatigue crack growth. Experimental determinations of high temperature crack growth. Practical applications. Index.