Influence of oxidation protective SiAl coatings on the tensile and fatigue behavior of Ti–48Al–2Cr–2Nb at elevated temperatures

Abstract

This study considers the influence of novel oxidation protective SiAl-coatings on the tensile and fatigue behavior of the γ-Titanium Aluminide alloy Ti–48Al–2Nb–2Cr. The SiAl-coatings were applied by pack cementation (PC) and magnetron sputtering (MS) processes. After coating, the specimens were pre-oxidized for 300 h at 850 °C. As reference, bare untreated and bare pre-oxidized specimens were tested. In ambient temperature tensile tests, pre-oxidation reduced both ultimate tensile strength and elongation. In comparison, PC-coated specimens showed higher ultimate tensile strength and elongation indicating that the coatings can effectively prevent embrittlement due to surface oxidation. The fatigue lifetimes of PC-coated, MS-coated and pre-oxidized specimens reduced drastically at higher stress amplitudes. Damage observations indicate that pre-oxidizing facilitates early crack initiation in the embrittled surface layer above a certain transition stress amplitude. Both coating types can attenuate embrittlement of the surface layer. However, the coatings themselves tend to initiate early cracks above a transition stress amplitude leading to a lifetime behavior similar to pre-oxidized specimens. Below the respective transition stress amplitude, the lifetimes of both coated specimen types reach almost those of bare untreated specimens. The results suggest that the benefits of the coatings in the form of reduced oxidation and embrittlement outweigh the slight fatigue life penalty at service relevant low stress amplitudes.