Performance of Alloy 709 under creep-fatigue at various dwell times

A comprehensive experimental evaluation of the creep-fatigue behavior of Alloy 709 at 750 °C is reported in this study. Alloy 709 is a 20Cr–25Ni austenitic stainless steel, with high temperature creep strength and corrosion resistance, which can potentially be used in structural components of nuclear power plants. Creep-fatigue crack growth (CFCG) experiments were conducted using an in-situ heating-loading and Scanning Electron Microscope (SEM) equipped with Electron Backscatter Diffraction (EBSD) detector. To study the “real-time” CFCG behavior of Alloy 709 at 750 °C with varying dwell times in vacuum, flat dog bone samples were prepared. A starter notch was added, and a pre-crack was introduced by high frequency fatigue cycles at room temperature. Prior to loading and heating the entire area ahead of the crack tip was mapped using EBSD. These maps were utilized to generate a set of Coincident Site Lattice (CSL) boundary maps from the area ahead of the crack tip. Upon completion of the EBSD and CSL mapping, the heating and loading of samples took place in the SEM. During the experiment, crack growth was monitored on the surface of the sample using SEM imaging and data was transferred over to the CSL maps, to highlight the crack path with respect to the grain boundary and precipitations arrangement in the sample. Some samples went through EBSD-CSL mapping before heating and loading along with Transmission Electron Microscopy (TEM) imaging post heating and loading. Comparing the CSL maps before and after crack growth provided additional details about the dependence of crack path and crack growth mode on microstructure, primarily grain boundary character, and dwell time. TEM analysis of the microstructure after the crack growth was employed to validate the findings of the in-situ heating and loading SEM data. Real-time monitoring of microstructural phenomena, such as void nucleation, grain boundary cavitation, slip activation, and resistance of twin boundaries to cracking during CFCG tests, sheds a new light on the crack growth mechanism. The results indicated that at lower dwell times, the crack mainly propagates in a transgranular fashion, with the aid of slip lines. At higher dwell time, intergranular cavitation dominates the crack growth. However, as more than 50% of grain boundaries are coherent twin boundaries, which are low energy boundaries resistant to cavitation, crack growth is delayed when reaching such boundaries and hence twin boundaries impart