Effect of Local Cellular Transformation on Fatigue Small Crack Growth in CMSX-4 and CMSX-2 at High Temperature

Abstract

In order to get fundamental understanding to establish a refurbishment technology for advanced gas turbine components, the cellular formation associated with a y/y microstructure coarsened in lamellar or equiaxed arrays in single crystal Ni-base superalloys, CMSX-2 and CMSX-4, have been studied, supposing the case in which they were previously subjected to a damage associated with local plastic deformation, followed by a re-heat treatment. During this study special attention was paid to understand the nucleation and the growth of the transformation from viewpoints of crystallo-plasticity and therm0 dynamics. The experimental evidence indicated that the transformation originated and developed with high anisotropy, being influenced by the following factors: the strain field produced by local plastic deformation, the crystallographic orientation, the re-heat treatment temperature and time, and the microsegregation in the material. It was shown that the transformation was reproduced in material previously subjected to fatigue and thermo-mechanical fatigue damage, in which the shearing of y precipitates resulting from the activation of { 11 l}4fO> slip systems was significant. It was shown that diffusion was controlling the growth rate of the transformation, accompanied with an activation energy of 36 kcal/mol. Furthermore, the effect of the local cellular transformation on the high temperature small fatigue crack propagation was quantified experimentally. supeml10ys 2oc0 Edited byT

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

“…Thus, repair, recoating, and refurbishment technologies, especially of the hot section components that are subjected to extremely severe conditions, have been inevitable for this decade. [1][2][3][4][5][6][7][8][9][10] The techniques of recoating, welding, and bonding, as well as new material design, are very important, although all of them are on the way of development. Nowadays many techniques have enabled us to repair polycrystalline superalloys and relatively lower temperature section components.…”

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) .…”

The Potential for the Improvement of High Performance Thermal Barrier Coatings

“…This kind of microstructure has been confirmed to be formed when the substrate alloys were exposed to actual service conditions of gas turbines. This microstructural change is also promoted by the machining which is aimed the refurbishment of the components [40]. Once the cellular microstructure is formed, the high temperature strength is remarkably reduced [40].…”

“…This microstructural change is also promoted by the machining which is aimed the refurbishment of the components [40]. Once the cellular microstructure is formed, the high temperature strength is remarkably reduced [40]. The composition of single crystal superalloys in which grain boundary strengthening elements are generally free is believed to be closely related to the undesirable effect [40].…”

“…Once the cellular microstructure is formed, the high temperature strength is remarkably reduced [40]. The composition of single crystal superalloys in which grain boundary strengthening elements are generally free is believed to be closely related to the undesirable effect [40]. More recently an interesting method has been developed to prevent the above problem [41].…”