Identification of Mechanisms Responsible for Degredation in Thin-Wall Stress-Rupture Properties

Abstract

A series of critical experiments was performed on single crystal CMSX-3 and equiaxed grain Mar-M246 specimens to identify the mechanisms responsible for the degradation in stress-rupture properties of 0.020 in. thick mini-flats machined from airfoils compared with 0.250 in. diameter standard test bars.Both materials exhibited approximately a factor of 3X life degradation at a stress level of 20,000 lb/in.2, when uncoated airfoil mini-flats were tested in air.

Creep Behavior of Thick and Thin Walled Structures of a Single-Crystal Nickel-Base Superalloy at High Temperatures: Experimental Method and Results

“…The time to reach 1% creep strain is more reliable to reveal tendencies than creep rupture lives. It is mentioned in [3,4,6] that the time to reach 1% strain is independent of wall-thickness. This is the case in our tests only for the higher stress level of 300 MPa.…”

“…For the polycrystalline nickel-base superalloy PWA 1484 Duhl [1] found a five fold reduction of creep rupture life if the specimen thickness is diminishing from 4 to 0.5 mm which mainly depends on grain size and micro structural defects. Doner and Heckler [3,4] observed in uncoated single crystal CMSX-3 a 30% loss in creep rupture life if the wall thickness was reduced from 3.18 to 0.76 mm at 982°C and if the stress level was below 275 MPa. They also found that the time to reach 1% strain was unaffected by wall thickness at a constant stress level.…”

“…They concluded that oxidation and the more constrained plastic deformation are the major contributions for early failure of thin specimens. Doner and Heckler [3,4] found that creep rupture properties of aluminized nickel-base superalloys are less influenced by the specimen thickness. By contrast Seetharaman and Cetel [6] mentioned a loss of creep rupture lifetime of 30-40% with a thickness reduction from 1.52 to 0.25 mm for coated Knowledge and understanding of the creep behavior is a fundamental prerequisite for component life-time predictions.…”

“…In order to optimize both the cooling efficiency and the weight of fast rotating turbine blades a general trend is to reduce the wall thickness of the hollow investment casting parts. The relation between creep properties and section thickness was rarely investigated [1][2][3][4][5][6][7][8][9][10]. For the polycrystalline nickel-base superalloy PWA 1484 Duhl [1] found a five fold reduction of creep rupture life if the specimen thickness is diminishing from 4 to 0.5 mm which mainly depends on grain size and micro structural defects.…”

Effect of Thermal Cycling on High Temperature Creep of Coated CMSX-4

“…In order to optimize both the cooling efficiency and the weight of fast rotating turbine blades a general trend is to reduce the wall thickness of the hollow investment casting parts. It has been shown that reducing the wall thickness leaded to lower lifetimes and higher minimum creep rates [16][17][18]. Besides in the case of thin-coated specimens it could be explained at least partly by the reduction of the load bearing because of the interdiffusion between coating and superalloy [18].…”