Crack growth behavior and failure micromechanisms in three heat-resistant materials at elevated temperature

Crack growth behavior for three heat resistant materials, namely, Alloy 800H, type 310 Stainless Steel, and Hastelloy X has been investigated under creep, fatigue, and fatigue with hold test conditions at the temperature range from 650 to 900$\sp\circ$C. The loading-unloading (fatigue) and hold (creep) in a hold time test represent a possible interaction between creep and fatigue. Frequency, R-ratio, environment, and hold-period and their synergistic effects on crack growth at elevated temperature were studied. All alloys show a predominantly transgranular crack advance mode when cycled at frequencies above 0.5 Hz. At lower frequencies, the failure in 310 SS transitions to a predominantly intergranular mode while Hastelloy X remains transgranular. Alloy 800H shows a mixed failure mode at intermediate and low frequencies. High R-ratio, temperature, and low frequency can combine to enhance the time-dependent crack growth at elevated temperature and creep damage are predominant in these cases.Creep crack growth rate correlates well with C* for all alloys, and can be extended to correlate fatigue with hold time loading conditions for Alloy 800H and 310 SS, but not for Hastelloy X under the conditions studied here. It was found that hold time crack growth were creep-controlled in Alloy 800H and 310 SS while fatigue-controlled in Hastelloy X. These crack growth observations were supplemented by microhardness, microstructural, and load response measurements. Creep crack growth rate predictions based on a local critical strain criterion were also examined and found to agree with experimental results in this study.U of I OnlyETDs are only available to UIUC Users without author permissio