The microstructure of diffusion aluminide coatings on a Ni based alloy

• Main Author: Dent, A. H.
• Published: University of Cambridge 1996
• Subjects: 546.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598506

The microstructures of both aluminide and platinum alumidide diffusion coatings on a single crystal Ni-based superalloy have been characterised in the 'as received' as well as the degraded state. X-Ray diffraction and a wide range of transmission electron microscopy (TEM) techniques including quantitative thin window EDX analysis have been used to examine the different coatings. A martensitic transformation from the β-phase of the coating which occurs as a result of a loss of aluminium during its degradation was examined in depth. This transformation from the B2 structure to a twinned form of the L1<SUB>0</SUB> structure was compared to a similar and related transformation which is known to occur for the binary β-NiA1 phase on cooling. A number of interesting differences between the two transformations are presented. Model alloys with a range of compositions, determined from using quantitative TEM EDX analysis of the martensite phase of the degraded coatings, were mechanically deformed both at room temperature and at 600°C. The test data and resultant microstructures have been used to suggest a possible reason for the ability of the diffusion coatings to reduce spalling of the surface oxide in service by the relaxation of stresses formed between the substrate, coat and surface scale.