Abstract
There is a continuing demand to raise the operating temperature of jet engine turbine blades to meet the need for higher turbine entry temperatures (TET) in order to increase thermal efficiency and thrust. Modern, high-pressure turbine blades are made from Ni-based superalloys in single-crystal form via the investment casting process. One important post-cast surface defect, known as 'surface scale', has been investigated on the alloy CMSX-10N. This is an area of distinct discolouration of the aerofoil seen after casting. Auger electron and X-ray photoelectron spectroscopy analysis were carried out on both scaled and un-scaled areas. In the scaled region, a thin layer (∼800nm) of Ni oxide is evident. In the un-scaled regions there is a thicker Al2O3 layer. It is shown that, as the blade cools during casting, differential thermal contraction of mould and alloy causes the solid blade to 'detach' from the mould in these scaled areas. The formation of Ni Oxides is facilitated by this separation.
Highlights
Superalloys are a class of material that have been developed for high temperature applications for use in aero-engines and land-based power generation
Turbine components are cast in SX form via investment casting and following cast, a thin layer with “fish-scale” appearance and texture is always present across a portion of the turbine blade (TB) aerofoil
It is observed that there is a marked presence of particles on the un-scaled surface of the aerofoil, seen as white “patches”, whilst these particles are absent from the scaled surface
Summary
Superalloys are a class of material that have been developed for high temperature applications for use in aero-engines and land-based power generation. The main benefit of these refractory elements is to improve solid solution hardening and to enhance microstructural stability at elevated temperatures. Turbine components are cast in SX form via investment casting and following cast, a thin layer with “fish-scale” appearance and texture is always present across a portion of the turbine blade (TB) aerofoil. Owing to this texture, it is commonly referred to as “surface scale”. The remainder of the aerofoil is referred to as “un-scaled”
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