Papers by Author: R.A. Hobbs

Abstract: Fourth generation superalloys are characterised by the addition of Ru which contributes to improved creep resistance whilst improving the microstructural stability. However, Ru additions have a negative effect on coated Ni-base superalloys, promoting Secondary Reaction Zone (SRZ) formation. Formation of a layer of SRZ beneath an aluminised or Pt-aluminised coating has the potential to reduce the effective cross section of a blade by in excess of 100 μm or 10% of the wall thickness. In this paper the effects of alloy composition on the formation of the SRZ in Pt-Aluminised fourth generation alloys were investigated systematically. A series of experimental fourth generation alloys was used having two distinct compositions of Co, Mo, W and Ru and conforming to a four factorial 'Design of Experiments' model. These alloys showed significant and consistent changes in the SRZ depending on alloy composition. These were in distinct contrast to the effects of these elements on stability in the bulk. Mo was demonstrated to be by far the most effective element suppressing SRZ formation, followed by Co. In contrast, both W and Ru enhance SRZ formation.

Abstract: The influence of Ru, Co, Mo and W on the lattice misfit of eight highly alloyed Re containing single crystal nickel-base superalloys was investigated. High resolution X-ray diffraction (XRD) was used to relate the elemental partitioning behavior and the Vegard coefficients of the elements under investigation to the measured lattice parameter of the  and  phase. The residual chemical segregation and especially the coherency stress-induced tetragonal distortion of the  matrix lattice in the high Mo containing alloys results in the observation of two different lattice parameters for the  matrix phase. This leads to three overlapping, but clearly distinguishable {002} X-ray reflections.

Abstract: The demand for higher engine operating temperatures to improve aeroengine efficiency has meant that increasing levels of alloying additions are being added to single-crystal nickel-base superalloys for turbine blades applications. Whilst better mechanical and environmental performance may be obtained with these alloying additions, they also destabilise the alloys forming topologically closed-packed (TCP) phases. In this study, the formation of TCP phases has been studied in a series of four alloys designated LDSX1-4 which have a systematic variation in the levels of Co, Mo and W. The alloys were exposed to elevated temperatures between 900-1100°C for up to 1000 hours. This was followed by detailed analysis of the microstructures in the SEM. Identification of the TCP phases in selected alloys was also carried out. The effects of each alloying addition on TCP phase formation is discussed in light of these results.