Small fatigue crack growth and failure mode transitions in a Ni-base superalloy at elevated temperature

A study of the long and small fatigue crack growth behavior in IN100 tested at 650 °C both with and without dwell periods is summarized. A significant small crack effect is evident in this alloy, and it is observed that the influence of loading variables on small crack behavior is profoundly different from that on long cracks. While a 6 s dwell has negligible effect on long crack growth rates, it results in more than an order of magnitude faster growth for small cracks (∼30 μm to 1 mm). Long crack growth is dominated by intergranular cracking both with and without 6 s dwell. Small crack growth mode depends on numerous factors including crack size, dwell time, exposure to environment, and character of initiation site. Transitions in small crack growth modes and the operative crack growth mechanisms are discussed.