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The influences of alloyed Lu, Hf and Cr on the oxidation and spallation behavior of NiAl were investigated. Although Lu has a "block effect" of Al as Hf does, the oxidation rate of Lu-doped NiAl is significantly higher, due to: (1) inhibitation of θ-to α-Al 2 O 3 transformation in early oxidation stage; (2) stronger scale grain refinement effect and enhanced oxygen transport rate. Segregation of S and C occurs not only at the alloy/scale interface, but also within the scale. Both Lu and Hf can significantly improve the scale adherence. However, This beneficial effect can be easily overcompensated by Cr.

Reviewer #1: Review of Acta Met A-18-1525 Effect of microstructure on early oxidation of MCrAlY coatings by Ying Chen et al. This paper describes the early oxidation of both an LPPS and HVOF bond coat. The materials is excellently presented in nearly flawless English. In the end the rapid transport of Aluminum by grain boundaries explains nearly all the is observed. Much background is already known and well review in the paper. The paper does an excellent job of putting all the pieces together and extensive experimental results are shown including PLPS (oxide too thin for XRD) SEM and TEM using multiple techniques in the TEM. They then model the results to explain in a semi quantitative way why the results are the way they are. Generally a very nice paper. This reviewer enjoyed reading this well written paper. One caveat is that this reviewer although very familiar with the results in this filed is not an expert on which exact references are most appropriate. The following things can be improved. 1. For less experienced readers they should give the composition and crystal structure of gamma and beta phases. Reply: We have added a table (Table 2) listing the compositions of the β and γ-phase determined by energy dispersive X-ray spectroscopy (see Table 2 and corresponding text in Line 11-12, Page 5) The crystal structures of the γ and β-phases are given in Introduction (Line 8-9, Page 2). 2. On page 12 they state that equilibrium imposes strict restrictions on the multi-phase alloy. True but how does this play into the discussed behavior. Reply: This statement is to stress that the thermodynamic equilibrium in the two-phase NiCoCrAlY coating prevents the interaction of , and in the alloy. We have added this point into the text (Line 7-8, Page 13) 3.p. 3 the bond coats were over 200 microns thick. Probably not important given the short heating but typical bond coats are much thinner. If there is a reason for this choice it is worth stating. Reply: The reason for using a relatively thick coating is to eliminate the effect of substrate/coating interfusion on the early oxidation of the coatings. This statement has been added into the text (Line 41-43, Page 3) 4.P.4 It says the samples were heated to 1150C at 200 C/minute and air cooling. At this rate could be a question of will the samples heat fast enough to follow the furnace temperature setting and the question of how fast the *Response to Reviewer Reply: "MCrAlY" has been replaced by "NiCoCrAlY" throughout the text after "2. Materials and Methods".

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A high performance NiCoCrAlY bond coat with dense dendritic microstructure was fabricated using laser powder deposition (LPD) technique. The thermally grown oxides (TGO) formed on the coating deposited by laser powder deposition is predominantly alumina instead of the mix oxides usually formed on the coatings prepared by air plasma spray and high velocity air fuel. Isothermal oxidation tests performed at 1150 °C reveal that the LPD bond coat shows significantly better spallation resistance and lower TGO growth rate. The superior spallation resistance of the TGO is further discussed in relation to the unique microstructure of the LPD bond coat.