Influence of Cr alloying on the microstructure of thermally sprayed quasicrystalline Al-Cu-Fe coatings

The present work reports the structural development of Al–Cu–Fe and Al–Cu–Fe–Cr coatings deposited by the high velocity oxy-fuel thermal spraying process and the influence of Cr alloying on the phase selection of Al–Cu–Fe coatings at various deposition temperatures. The porosity levels of the Al–Cu–Fe and Al–Cu–Fe–Cr coatings of the study are demonstrated to be lower than those reported for corresponding plasma-sprayed coatings. The results show that high velocity oxy-fuel spraying technique produces Al–Cu–Fe coatings that are phase structurally similar to plasma-sprayed Al–Cu–Fe coatings reported in literature. Al–Cu–Fe coatings are composed of a crystalline β-AlFe phase and a quasicrystalline i-Al65Cu20Fe15 phase as well as an oxidised form of either or both of these phases. Addition of Cr to Al–Cu–Fe alloys introduces coatings that are made up of the crystalline θ-Al2Cu phase and two quasicrystalline phases, the i1-Al80Cr13.5Fe6.5 and i2-Al13Cr3Cu4 phases. The formation of these icosahedral phases in Al–Cu–Fe–Cr alloys has not been reported before, although the occurrence of quasicrystal approximants with compositions close to those of the i1-Al80Cr13.5Fe6.5 and i2-Al13Cr3Cu4 phases has been demonstrated. On the basis of our results we propose that the icosahedral phase structure is greatly stabilised by the Cr addition to Al–Cu–Fe alloys.