Influence Of Pt-Al Coating On High-temperature Performances Of DD413 Nickel-Based Single Crystal Alloy

DD413,a single crystal superalloy,has been used in high-temperature structural components such as aeroengine turbine blades.Aeroengines are vulnerable to high temperature oxidation and molten salt corrosion during service.Pt-Al coating on the blades and vanes can improve the hot corrosion resistance and then the service temperature.However,for the coating is polycrystalline and the substrate is single crystal,and also their chemical compositions are different,Pt-Al coating may effect the tensile properties and low-cycle fatigue behaviors.In addition,as the environment is an important factor for the mechanical properties,the oxidation and degradation of coating may play important role during crack initiation at elevated temperatures.In the present thesis,high temperature properties of DD413 alloy with Pt-Al coating were systematically studied.It will lay a theoretical foundation for the application of Pt-Al coating on DD413 alloy.In this thesis,the static oxidation and hot corrosion tests of uncoated and Pt-Al coated DD413 samples were carried out at different temperatures.Tensile tests of DD413 samples with and without Pt-Al coating were performed at 760 and 980 °C.The effect of Pt-Al coating on the oxidation,hot corrosion and tensile properties of DD413 alloy were discussed.The effects of Pt-Al coating on the low-cycle fatigue properties at different temperatures were compared and the mechanisms were analyzed.The main results are as follows.Pt-Al coating samples show good high-temperature oxidation resistance at 760 and 980 °C,the oxidation products is mainly Al₂O₃.However,topological close packed（TCP）phases could be detected in the Pt-Al coating samples after oxidized for 70 h at 760 °C or 5 h at 980 °C.Pt inhibits the diffusion of refractory elements,and the refractory elements are enriched in the substrate,which leads to the formation of TCP.Pt-Al coating samples exhibit good hot corrosion resistance at 900 °C.It may be result from the Pt prevents outward diffusion of Cr to form Cr S_x and prevents the further inward diffusion of S.Pt-Al coating can inhibit the formation of pores at the interface of corrosion products and metal.Additionally,Pt can block a large number of Ta atoms in the inter diffusion zone.The tensile properties of Pt-Al coating samples at 760 and 980 °C are lower than those of uncoated samples.Pt-Al coating fracture characteristics at 760 °C is brittle,while ductile at 980°C.The ductile-to-brittle transition temperature（DBTT）ofβ-（Ni,Pt）Al phase in Pt-Al coating were detected to be between 760 and 980 °C.The ductile-to-brittle transition of Pt-Al coating is caused by the transition fromβ-（Ni,Pt）Al toγγ-Ni₃Al.Because of the tensile strength of Pt-Al coating is lower than that of the substrate at 760 and 980 °C under tensile loading condition,the cracks prefer to initiate from Pt-Al coating and propagate to the substrate.The cyclic stress amplitude of Pt-Al coating sample is lower than uncoated sample during cyclic deformation under the same applied total strain amplitude in the stable stage at 760 °C.The brittle fracture of Pt-Al coating leads to the initiation of multiple cracks near the surface of the sample.The decrease of effective bearing area leads to the increased stress.In addition,cyclic number of Pt-Al coating samples was around 19.9%shorter than the uncoated samples.Multiple cracks initiate at the interface between the coating and the matrix and propagate into the matrix,which results in the decrease of fatigue life of Pt-Al coating samples.The cyclic stress amplitude of Pt-Al coating and uncoated samples are basically the same under the same applied total strain amplitude at 980 °C.Theβphase of Pt-Al coating degenerates continuously under cyclic loading and formsγγ’/γγcoherent strengthening effect.The deformation resistance of Pt-Al coating under cyclic strain loading is improved by the combined effects of the pinning of the dislocations by Pt inγ.The low-cycle cyclic number of Pt-Al coating sample is about 18%～44%higher than uncoated one under the same applied total strain amplitude at 980 °C.Under low-cycle fatigue loading condition,Pt-Al coating induces the multiple source initiation and propagation of fatigue cracks at 760 °C.In contrast,more inhibit effect of Pt-Al coating on crack propagation was observed under low strain amplitude at 980 °C.