A review of thermo-mechanical fatigue behaviour in polycrystalline nickel superalloys for turbine disc applications

Abstract

Within the gas turbine engine, the high transient thermal stresses resulting from throttle movement from idle to high settings give rise to the phenomenon of thermo-mechanical fatigue (TMF). These effects have been widely explored for turbine blade materials, typically single crystal nickel alloys. More recently however, a combination of thinner disc rims and further increases in turbine entry temperature has led to a situation where TMF in disc materials cannot be ignored. Turbine discs will usually be manufactured from polycrystalline nickel alloys, and as such it is now considered critical that TMF effects in this system of alloys is fully characterised. The current paper seeks to summarise the published work on TMF in polycrystalline nickel alloys for turbine disc applications, whilst introducing recent work at Swansea University. Previous research has concentrated particularly on the differences between in-phase (IP) and out-of-phase (OP) loading. The current work from Swansea indicates that this approach is overly simplistic and that for a complete evaluation of the alloy, intermediate phase angles should also be considered.

Enhancing the Accuracy of Advanced High Temperature Mechanical Testing through Thermography

“…A solution to this issue is to pre-oxidize the specimen to produce a stable oxide layer and hence stable surface emissivity [1]. However, pre-heat treatments and thus consistent oxide formation has previously been found to lack repeatability where previous work has found that the surface emissivity evolution with temperature is dependent on the material, surface preparation, thermal treatment, and the chemical reactions on the surface [25].…”

“…Depending on the specific temperature and wavelength, the energy radiated by a metallic surface is highly sensitive to any change in the surface condition and therefore is directly proportional to the spectral emissivity of the object [5]. Without a stable emissivity value, it is extremely difficult to achieve accurate temperature measurement with a non-invasive technique such as a pyrometer [1,5,[17][18][19][20][21][22]. Pyrometry does offer non-invasive temperature monitoring and control [23], however the measurement reading is limited to a particular location on the specimen, a limitation shared with TCs [24].…”

“…In order to try and maintain the rapid development of gas turbine performance and efficiency existing in-service materials continue to be used and subjected to more extreme thermal environments. As a consequence, aggressive high temperature damage mechanisms such as thermo-mechanical fatigue (TMF), become more prevalent and therefore require further consideration in advanced component lifing strategies [1].…”

Non-invasive temperature measurement and control techniques under thermomechanical fatigue loading

“…4,[5][6][7][8] A solution to this is to pre-oxidise the specimen in order to produce a stable oxide layer. 9 Preexposure of this kind requires careful consideration as this additional heat treatment may be detrimental to fatigue life in some alloys. A method of emissivity correction based on the direct processing of the IR camera output signal has been developed by Vellvehi et al 10 Pyrometry offers the possibility of non-invasive temperature monitoring and control; 11 however, the measurement reading is limited to a particular location on the specimen, a limitation shared with TCs.…”

Lifing the Effects of Crystallographic Orientation on the Thermo-Mechanical Fatigue Behaviour of a Single-Crystal Superalloy

“…One of the more traditional procedures for the estimation of TMF life is empirical correlation with isothermal low cycle fatigue (LCF) data [7,8]. For polycrystalline alloys, this method has been reasonably successful [4,9,10] but in the case of single crystal superalloys, there are many types of intricate TMF failure mechanisms that can contribute to material damage and make direct correlations with isothermal data difficult. In addition, given the strong anisotropic nature of directionally solidified materials, and the preferred <001> growth orientation of many Ni-based superalloys, an added complexity is present when attempting to derive a model for data that is generated from multiple crystallographic orientations.…”