Microstructural study of fatigue and dwell fatigue crack growth behaviour of ATI 718Plus alloy

A new superalloy, ATI 718Plus alloy (718Plus), has been developed to have a 55 degrees C higher temperature capability over the traditionally used alloy 718, while maintaining favourable processing characteristics and intrinsic raw material costs. The fatigue and dwell fatigue behaviour of four microstructural variations of 718Plus are evaluated and compared with Waspaloy: standard heat treated condition (HT1), standard heat treated condition with thermal exposure at 732 degrees C (1350 degrees F) for 1000 h (HT2), fine grain condition with a modified delta phase (HT3) and fine grain overaged through modified heat treatment (HT4). Fatigue crack growth rate tests were performed at 649 degrees C (1200 degrees F) and 704 degrees C (1300 degrees F) under constant amplitude loading at 10 Hz or with a 100 s tension dwell. The results show that the steady state fatigue crack growth rates of each microstructural variation and both alloys are identical, with all of the 718Plus variations showing a clear advantage over Waspaloy in the fatigue crack threshold regime. The 718Plus variants HT1 and HT3 exhibited the highest fatigue crack growth threshold, followed closely by HT4 and finally HT2. At 649 degrees C (1200 degrees F), the 718Plus alloys had average threshold values ranging from 8.8 to 10.4 MPa m(1/2) whereas the Waspaloy material was 6.1-7.5 MPa m(1/2). At 704 degrees C (1300 degrees F), the advantage increased with the 718Plus alloys having average threshold values ranging from 10.1 to 11.6 MPa m(1/2) compared with Waspaloy, which exhibited average threshold values of 6.6-7.7 MPa m(1/2). Dwell fatigue results show that Waspaloy has better resistance to crack propagation under dwell conditions; however, optimisation of the precipitate phase (HT2) shows vast improvement in the dwell fatigue resistance of 718Plus.