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Fatigue crack growth (FCG) behaviors of a nickel-based single crystal superalloy were tested in 6 crystal orientations at 600 °C, and the effect of secondary orientations on FCG behaviors were investigated through theoretical analysis. FCG test results showed significant differences in FCG rates between different secondary orientations, albeit SEM observations on their fracture surfaces did not reveal evident distinctions. To evaluate the anisotropic FCG behaviors, a methodology was implemented based on the elastic energy release rate, which is successful in reducing the scatter caused by various orientations. In addition, the proposed energy release rate coefficient is proven to be effective in evaluating the resistance to long fatigue crack growth in different secondary crystal orientations.

Fatigue crack growth (FCG) behaviors of a widely used nickel-based directionally solidified (DS) superalloy have been investigated. Standard compact tension (CT) specimens in longitudinal, transverse and diagonal directions are cast and tested at 25 °C, 600 °C and 850 °C to reveal the orientation and temperature dependence. The post-test fractography is observed through scanning electron microscope (SEM) and optical microscope (OM) to understand the underlying mechanism responsible for the fracture modes. Results indicate that cracks in all three orientations exhibit a similar propagating behavior, while the temperature shows a significant effect on the crack propagation regardless of the influence of orientation. It has been found that a higher temperature leads to a faster propagation rate in the initial stage due to the cyclic softening response of materials. However, the FCG rates of specimens at lower temperature speed up more rapidly and exceed those at higher temperature in the following stage. This is attributed to the crack closure effect induced by the oxidation at a much higher temperature. Therefore, a new model based on thermal activation is proposed to get a better ability for the FCG rate prediction of the DS superalloy under different temperatures.

Small fatigue crack growth behaviors of a nickel-base single crystal (SX) are studied by in situ observations with scanning electron microscope (SEM). Two different crack propagation modes are identified, i.e., cracks at room temperature and 300 °C prefer non-crystallographic propagation mode, while those at a higher temperature of 600 °C propagate preferentially along crystallographic slip systems, besides, evident “small crack anomaly” has been partly found. Further computation of crack growth rate reveals that conventional Paris law is found no longer capable of characterizing the crystallographic crack propagation behaviors. Subsequently, with the help of finite element analysis (FEA), the crack tip stress fields are reconstructed and resolved shear stresses in each octahedral slip systems are calculated. It is found that the maximum resolved shear stress intensity parameter, first proposed by Chen and Liu, is able to characterize the growth behaviors of those small cracks.

In high temperature creep testing the conventionally cast alloy IN 738 LC exhibits a comparably large property scatter which requires high safety margins in design and dimensioning and subsequently causes an incomplete exploitation of the materials potential. The reasons for this property scatter were investigated and traced back to different influencing factors. Parallel to investigations on the microstructure of post-exposure material and conventional scatter band analysis, artificial neural networks were successfully applied to discover relations between the chemical composition of the individual melt and the position of the corresponding test results within the global scatter band.

Based on in situ observations with scanning electron microscopy (SEM), we investigated the low cycle fatigue (LCF) propagation behaviors of a Ta-rich single crystal nickel-based superalloy at 550oC in vacuum. Specimens in three different orientations, i.e. [010], [011] and [111], were tested in stress-controlled tension-tension LCF tests with R=0.1. Slip traces can be observed ahead of the crack and distinct crystallographic fracture was dominated. By analysis on the surface slip traces and the crack propagation planes, the operated slip systems were examined to be octahedral slip and the active octahedral plane was identified in this context. The whole cracking process of small fatigue cracks were meticulously tracked and recorded. The evaluation of short crack growth rate for specimens in three orientations was carried out by fracture mechanics approach. The influence of crystallographic orientations on fatigue crack growth are analyzed and discussed. Especially, the interaction between the short crack and local microstructure such as the carbides ahead of the crack were analyzed, altering the propagation path as well as influencing the crack growth rate. The parameter of crack opening displacement was examined to correlate well the crack growth data in different orientations.