The influence of sulphidizing attack on the mechanism of failure of coated superalloy under cyclic loading conditions.
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Date
1998
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Abstract
A systematic study of the effect of sulphidizing atmosphere on the High Temperature Low
Cycle Fatigue (HTLCF) properties of coated and uncoated unidirectionally solidified MARM002
nickel base superalloy was performed at 870°C. The coating systems investigated
were, aluminide coating, three types of platinum modified aluminide coatings, and platinum
coating.
The creep-plasticity mode of the strain range partitioning method was used for creep-fatigue
loading. A constant loading regime (Strain range 6.6 x 10-3
) was used to test the samples
in argon, air and Ar + 5%S02 and a lower strain range of3.8 x 10-3 was used to investigate
the creep-fatigue properties in Ar + 5%S02 only. The results were analysed using scanning
electron microscopy including spot analyses (SEM-EDS), Auger electron spectroscopy
(AES) and X-ray diffraction (XRD) techniques.
The synergistic effect of sulphidizing environment and the creep fatigue loading (Strain
range - 0.66%) resulted in accelerated failure in all the materials systems tested, except for
the TYPE I platinum aluminide coated sample. This coating displayed a "self-healing"
mechanism which enhanced its fatigue life under sulphidizing conditions.
In general, the coatings had an adverse effect on the fatigue properties of the material
systems. This was due to the poor mechanical properties of the coating. The mechanical
properties of the coating was influenced by the coating microstructure and the chemical
composition. The modification of the NiAI zone with platinum in the platinum aluminide
coatings improved the fatigue properties of the coating by altering the crack propagation
mechanism in the NiAl zone. The higher the platinum content in this region the more brittle
it became.
The platinum modified aluminide coating showed an improvement in the corrosion fatigue
properties in the S02 containing environment at the higher strain range when compared with
the uncoated, aluminide coated and platinum coated samples.
However, at the lower strain range all the coating systems performed worse than the
uncoated alloy. This was mainly due to the brittle failure of the coating. The platinum
modified aluminides performed the worst due to the presence of brittle platinum aluminide
phases.
The interdiffusion and interaction of platinum with the substrate alloying elements, resulted
in this coating being ineffective for corrosion protection. The resultant coating layer
produced poor corrosion-fatigue properties.
Although the coating systems did show evidence of resistance to sulphidation and oxidation
there were relatively ineffective under the combination of sulphidizing environment and
fatigue loading due to their poor mechanical properties.
The mechanism of sulphidation was consistent for all the material systems tested with
oxidation proceeding first and sulphidation proceeding at the corrosion scale/substrate
interface. The crack propagation in the coating and substrate was controlled by the
sulphidation attack at the crack tip and failure of the oxide scales formed in the cracks.
Description
Thesis(M.Sc.Eng.)- University of Natal, Durban, 1998.
Keywords
Nickel alloys., Heat resistant alloys--Fatigue., Heat resistant alloys--Creep., Theses--Mechanical engineering.