Effect of Thermal-Mechanical Treatment on the Fatigue Crack Propagation Behavior of Newly Developed Allvac 718Plus Alloy

Abstract

The newly developed Allvac 718Plus alloy has shown mechanical properties superior to alloy 718 and comparable to Waspaloy at the temperature up to 704 C. Previous results showed that fatigue crack propagation (FCP) resistance without holding time has no significant difference between three alloys with 718Plus being the best and 718 the lowest. During the holdtime FCP tests, 718Plus shows comparable results to those of Waspaloy and better than Alloy 718. In this paper, the effect of various thermal-mechanical treatments, including direct aging, pre-treatment, and long-term exposure, on the hold-time fatigue crack propagation behavior of 718Plus alloy were investigated and the results are summarized as follows:(1) The hold-time fatigue crack propagation rates (FCPRs) of DA sample are almost the same as that of the alloy after conventional solution plus age heat treatment, while the FCP of DA sample is slower than conventional 718Plus alloy under the 650 C, 3S loading condition; (2) the longterm exposure tests show that the alloy's hold-time fatigue cracking resistance is improved after exposed at 760 C for 350 hours; (3) the fine-grain alloy shows better hold-time FCP resistance than that of coarse-grain alloy, which is attributed to the delta-phase effect; and (4) it is indicated that after pre-treatment at 857 C for up to 24 hours, the alloy's fatigue cracking resistance is improved because of delta-phase at grain boundaries.

Hold-Time Fatigue Crack Growth of Allvac 718Plus®

“…However, contrary to observations in [4,5], performed at 650°C with 100 s hold-time, an improvement in the hold-time FCG properties due to long-time exposure could not be observed in the present case. Here, the exposed material showed similar FCG rates under purely cyclic loading, but during the 90 s hold-time blocks at both 600°C and 700°C the FCG rate increased compared to the as-heat treated material.…”

“…Here, the exposed material showed similar FCG rates under purely cyclic loading, but during the 90 s hold-time blocks at both 600°C and 700°C the FCG rate increased compared to the as-heat treated material. Liu et al [4,5] reported a reduction in hold-time FCG of roughly one order of magnitude when Allvac 718Plus (in material which had not been subjected to pre-solution soaking) was exposed at 760°C for 350 h.…”

Environmental and Dwell Effects on the Damage Tolerance Properties of ATI 718Plus® Alloy

“…The observed differences between Waspaloy and 718Plus continue in the dwell-FCGR specimens with the W1 dwell fracture surface exhibiting transgranular fracture morphology, although to a lesser extent than in the non-dwell FCGR specimens. These results substantiate what was found in previous studies [5,11].…”

ALLVAC 718 Plus™ Superalloy for Aircraft Engine Applications

“…Alloys 718Plus, 718 and Waspaloy have similar fatigue crack growth rates under 3 seconds triangle loading at 650°C with 718Plus being slightly better. Waspalloy shows the best resistance to fatigue crack growth under hold time fatigue condition while the resistance of 718Plus is better than that of Alloy 718 (Liu et al, 2005).…”