Transition behavior from Mode I cracking to crystallographic cracking in a Ni-base single crystal superalloy

Nickel-base superalloys are heat resisting materials typically used at service temperatures above 500°C. Typically, Ni-base superalloys possess exceptional properties: i.e. high temperature strength, and resistance to corrosive and oxidizing degradation, hence their application in harsh and critical environments (land-base power plants, jet engines and chemical processing plants). Reliability and safety demands for such applications demand the Ni-base superalloy components to be constantly monitored and their remaining life continuously evaluated. This calls for a full understanding of the fatigue crack propagation (FCP) behavior. As a result, the FCP behavior of Ni-base single crystal superalloys has been widely studied under various temperature and frequency regimes. Such studies have shown that high temperature and low-frequency lead to Mode I cracking, while low temperature and high-frequency result in crystallographic cracking. In other words, it has been established that the fatigue crack propagation of Ni-base single crystal superalloy strongly depends on the crystallographic orientation and usually shows transition behavior from Mode I cracking to crystallographic cracking at low temperature. Unfortunately, the transition process is not well understood in details.

Generally, comprehending this problem is vital for the life assessment of superalloys components, in which crystallographic cracking often causes a catastrophic failure associated with a rapid crack propagation. In this view, researchers from the Department of Mechanical Engineering at Tokyo Institute of Technology in Japan: Xiaosheng Chen (PhD candidate) and Professor Motoki Sakaguchi conducted in depth experimental investigations and crystal plasticity analysis, with the main purpose being to clarify the transition process. Their work is currently published in International Journal of Fatigue.

In their approach, fatigue crack propagation test at 450 °C and crystal plasticity analysis were conducted for a Ni-base single crystal superalloy. In addition, crystal plasticity finite element analysis for Mode I cracking, transition, and the crystallographic cracking were all conducted to investigate the slip activity of an octahedral slip system in front of the crack tip by carefully considering the actual 3D geometry of the crack tip.

The authors reported that for the Mode I cracking, the transition, and the crystallographic cracking were dependent on the level of stress intensity factor range, and were only affected by the crystallographic orientations in a compact specimen. Further, by taking account of the fatigue damage on the individual slip plane, the team reported that a damage parameter could provide reasonable explanations for the fatigue crack propagation rates during Mode I cracking and crystallographic cracking regardless of crystallographic orientation.

In summary, the study reported on fatigue crack propagation tests at 450 °C and crystal plasticity analysis conducted for Ni-base single crystal superalloy at low temperature. The aim was to clarify fatigue crack propagation behavior, especially the transition from Mode I to crystallographic cracking. In a statement to Advances in Engineering, Professor Motoki Sakaguchi pointed out that their work rationalized crack propagation rates during mode I and crystallographic. Also, he mentioned that the critical condition for transition was quantified by a damage parameter.