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

Crack growth behavior for three high temperature materials, namely, Alloy 800H, type 310 Stainless Steel, and Hastelloy X, have been investigated under creep, fatigue, and hold time test conditions at 650°C. 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. 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. These crack growth observations were supplemented by microhardness, microstructural, and load response measurements. Crack growth rate predictions based on a local critical strain criterion were also examined and found to agree with experimental results.