Diffusion Studies On Systems Related to Nickel Based Superalloys

Abstract

Superalloys offer high temperature strength, excellent creep, corrosion and oxidation resistances, microstructural stability and good fatigue life at elevated temperatures. The composition of the superalloys has been modified continuously to improve the properties. The addition of Pt improves oxidation resistance without compromising the mechanical properties of the superalloys. To further enhance the performance of the superalloy components, various coatings are applied on them. The-(NiPt)Al intermetallic compound bond coats, which are presently utilized, have certain drawbacks. Diffusion of Al from the bond coat to superalloy during service leads to accumulation of stress near the bond coat. The refractory elements present in superalloy precipitate as topological close packed (TCP) phases in the interdiffusion zone. Consequently, a Pt enriched γ(Ni) + γ’(Ni3Al) phase mixture has been proposed as a possible alternative since TCP phases do not form in the interdiffusion zone. In this thesis, diffusion studies are performed on several binary and ternary systems with the primary purpose of understanding the effect of Pt in Ni based superalloys and also in γ + γ’ phase mixture bond coats. Further, a detailed interdiffusion study is conducted in Mo- and W- based binary and ternary systems to understand the growth of the TCP phases. By performing bulk and multifoil diffusion couple experiments, different diffusion parameters like, inter, intrinsic, tracer, impurity diffusion coefficients and activation energy that are necessary to understand the diffusion mechanism are determined. Additionally using the nanoindentation technique on diffusion couples, variation of mechanical properties such as, hardness and modulus with composition is studied. First, interdiffusion in Ni-Pt, Co-Pt, Co-Ni, Ni-Fe and Co-Fe binary systems is examined. In Ni-Pt and Co-Pt, experimental results show that Pt is the slower diffusing species at all compositions. In both the systems, driving force is found to be the reason for higher values of intrinsic diffusion coefficients observed in the range of 40-60 at. % Pt. Contribution of vacancy wind effect on diffusion parameters is found to be negligible. It is found from the multifoil diffusion couple experiments that Ni is the faster diffusing species in the Co-Ni system. Bulk diffusion couple experiments are conducted in the Co-Ni-Pt and Co-Ni-Fe systems, by coupling binary alloys with the third element. Uphill diffusion is observed for Co and Ni in Pt rich corner of the Co-Ni-Pt system. Main and cross interdiffusion coefficients are calculated at the compositions where two diffusion profiles intersect. In both the systems, the main interdiffusion coefficients are positive over the whole composition range and the cross diffusion coefficients show both positive and negative values at different regions. Hardness measured by performing the nanoindentations on diffusion couples of both the systems, shows the higher values at intermediate compositions. The effect of Pt in and’ phases of Ni-Al system are examined by conducting interdiffusion experiments between Ni(xPt) alloys and (NixPt)40Al alloy of β phase, so that both and’ phases grow in the interdiffusion zone. The interdiffusion coefficients in Ni-Al binary system increases with the Al content in the -phase, and they do not vary significantly with composition in the ’ phase. The average effective interdiffusion coefficients of Ni and Al in the and ’ phases increase with the addition of Pt. Nanoindentation studies on diffusion couples show that the hardness of both and ’ phase increases with the addition of Pt. In the +’ phase mixture bond coats, effect of Pt on interdiffusion of major alloying elements of CMSX4 superalloys are discussed. A phase mixture of and ’ with increasing Pt content is coupled with CMSX4 superalloy. The addition of Pt to the +’ phase mixture increases the diffusion rate of Ni, while the diffusion rate of Al, decreases with the addition of 5% Pt, and increases with further addition of Pt. No significant change in the diffusion rates of Co or Cr is observed. The growth kinetics and diffusion in systems (both binary and ternary) with TCP phases are examined. Interdiffusion studies performed in Co-Mo system show significant volume change because of the growth of the phase. Intrinsic diffusion coefficient of Mo is found to be higher than that of Co. Diffusion studies conducted in Ni-Mo system show reasonably low activation energy in the phase, indicating the grain boundary controlled diffusion process. The Co-Ni-Mo and Co-Ni-W ternary phase diagrams are revisited and the phase boundary composition of the TCP phases are found to be different from those reported earlier. Following, the average effective interdiffusion coefficients are calculated and compared with the data calculated in the binary systems to examine the role of the third element. It is noticed that the average effective interdiffusion coefficients in the Co(Ni,Mo) and Co(Ni,W) solid solution increases with the addition of Ni. On the other hand, these diffusion coefficients decrease with the addition of Ni in thephase in both the systems. The role of the driving force for diffusion and possible change in defect concentrations on different sublattices are discussed.