Microstructural and Environmental Effects On the Mechanical Behaviour of AllvacŪ 718PlusŪ

Four microstructural variants of 718Plus^® are produced via modified heat treatments to elucidate the effects of grain size, precipitate size, morphology, and phase fraction (δ and γ'), on the mechanical properties of low cycle fatigue (LCF) life, fatigue crack growth rate (FCGR) properties, and creep-fatigue crack growth rate (CFCGR) behaviour at both 650ºC and 704ºC under 100s dwell and no dwell conditions. Similar testing is also performed on Waspaloy in two comparative microstructural conditions. FCGR results show that at both test temperatures, all microstructural conditions of 718Plus^® and Waspaloy exhibit identical behavior in the steady state regime, except that 718Plus^® exhibits a much higher threshold stress intensity (ΔK_TH). However, the CFCGR results show that Waspaloy displays better steady state crack growth resistance under dwell conditions, with an optimized precipitate microstructure of 718Plus^® showing considerable improvement. Environmental effects are also investigated by performing FCGR and CFCGR tests in both air and high vacuum conditions. LCF test results, with and without 100s dwell, demonstrate that all four microstructural conditions of 718Plus^® have superior life compared to Waspaloy under all test temperatures and total strain ranges investigated.