Damage repair in CMSX-4 alloy without fatigue life reduction penalty

Abstract

The microstructural changes in a single-crystal Ni-base superalloy, CMSX-4, that might occur during the processes of repair and recoating of hot section components for advanced gas turbines were studied. It is shown that the cellular ␥/␥Ј microstructure is formed when the material is subjected to local plastic straining, followed by the reheat treatments during the course of damage recovery. The formation of cellular microstructure in the material led to the remarkably reduced fatigue strength. In order to reduce or prevent the preceding undesirable effect resulting from cellular microstructure, a new method based on applying overlay coating technique was developed. The method is based on an idea that the alloying elements that are depleted in base alloys could be supplemented via the overlay coating. An X alloy, which contains grain boundary strengthening elements, was selected and coated on the CMSX-4 with the cellular microstructure by low-pressure plasma spraying. The fatigue tests on the coated CMSX-4 specimens demonstrated the effectiveness of the method. The observations of the crack initiation site, the fatigue fracture mode, the crack density in the cellular transformed area, and the crack propagation morphologies near the prior interface strongly supported the validity of this approach. The method is expected to build a road to a so-called damage cure (or recovery) coating.

Effect of Bond Coat Materials on Thermal Fatigue Failure of EB-PVD Thermal Barrier Coatings

“…The reason why the latter two alloys were selected is from the following reasons: whereas it is possible to get these MCrAlY alloys on commercial base, they are containing boron in significant amount in their chemical compositions (see Table 2). An addition of boron; one of the grain boundary strengthening elements, is expected to have a specific role to minimize the undesirable effects of the cellular microstructure formed in the superalloy substrate (18) (20) . The cellular microstructure might be originated in some cases in new generation Ni-base superalloys during a grit blasting process; an important process to fabricate the TBCs, but it may reduce the high temperature strength of superalloy substrates (18) .…”

“…Many types of precipitates are appearing, depending on the alloy systems. These precipitates were identified by CoAl in the CoNiCrAlY, CoAl and borides in the CoNiCrAlY+B, and Ni 3 Al and NiAl with a little borides in the HaynesC22 (20) . Induction heating system (2) Mechanical and Physical properties Mechanical properties of the virgin and aged bond coat alloys were measured by means of the self-standing MCrAlY alloy specimens, where the aging was conducted at 1000℃ for 1000 hours in air.…”

Effect of Surface Treatment and Crystal Orientation on Microstructural Changes in Aluminized Ni-Based Single-Crystal Superalloy

“…There is another important report which revealed that surface re-crystallization formed during service at elevated temperatures was promoted by the residual stress that had been applied to the components during machining. 9) It is thus easily anticipated that surface crystal orientation, and/or pre-surface treatments prior to coating can affect the SRZ formation kinetics. However, to the best of our knowledge, there are very few reports investigating such effects, except for the authors' group.…”

Creep-Fatigue and Thermo-Mechanical Fatigue of Friction-Welded IN 718/MarM247 Dissimilar Joint

“…The directions of gas turbine development are not only to achieve high thermal efficiency but also cost reduction for production and maintenance [13]. Thus, repair, recoating and refurbishment technologies for the hot section components are important.…”