High temperature corrosion studies and interdiffusion modelling in TiAl based alloys coated with high performance surface layers

This work forms a part of a large EU project (InnovaTiAl) designed to increase the service temperature (up to 1273 K) of TiAl based intermetallics (Ti45Al8Nb) coated with high performance surface layers for applications in power generation and also in aerospace components. In this thesis the high temperature corrosion behaviour of some of these newly developed high performance coatings deposited on Ti45Al8Nb alloys using UBM and HIPIMS/UBM has been studied. The coatings studied included intermetallic coatings: TiAlCr, TiAlCrY, Al2Au, and ceramic coatings: CrAl2%YN, TiAlYN/CrN+Al2O3, TiAlN+Al2O3, CrAlYN/CrN+CrAlYON coated Ti45Al8Nb etched by Cr, CrAlYN/CrN+CrAlYON coated Ti45Al8Nb etched by CrAl, CrAlYN/CrN+CrAlYON coated Ti45Al8Nb etched by Y, CrAlYN/CrN coating Y etch, CrAl (thin), CrAlYN/CrN coating Cr etch, CrAl (thin), CrAlYN/CrN coating, Y etch, CrAlY (thick) coated Ti45Al8Nb. High temperature corrosion investigations were carried out by the exposing the coated materials in oxidising (static air, pO2 = 21278.25 Pa), sulphidising (pS2 = 10-1 Pa, and pO2 = 10-18 Pa) and hot corrosion (20%NaCl/80%Na2SO4) environments at temperatures range 1023 – 1223 K. The long term oxidation studies have been performed up to 5000 hours at 1023 K. Weight change data have been used to determine the corrosion kinetics. SEM studies have provided information on morphology; EDS analyses have given the information on concentration profiles and phase contents have been obtained by XRD analyses. The excellent high temperature oxidation resistance has been achieved for the CrAl2%YN coating at temperature rage 1023 – 1123 K for 500 hours. Also TiAlCr and TiAlCrY have shown moderate degree of corrosion resistance in oxidising environment after 500 hours of exposure at temperature range 1023 – 1223 K. The sulphidation experiments for 1000 and 675 hours at 1023 K and 1123 K respectively showed that the uncoated material developed a multilayered thick scale with poor resistance to sulphidation. The ceramic coatings developed a good protective Al2O3/Cr2O3 on places where the coatings remained intact and did not undergo cracking. However due to the cracks in the top coat, the development of non protective TiO2 occurred at 1023 and 1123 K. Hot corrosion studies performed at 1023 K for 150 hours under this thesis showed that all exposed materials (CrAl2%YN, TiAlN+Al2O3, and TiAlYN/CrN+Al2O3) suffered uneven attacks (crack formation, spallation, and the lack of Al2O3 formation). Interdiffusion studies of mass transport have been performed by GDM on TiAlCrY coated alloy after 500 hours oxidation at 1023 and 1123 K. A reasonable degree of agreement between the experimental and the simulated profiles has been achieved