Study On Microstructure And Properties Of A Novel Low-Cost Nickel-Based Single Crystal Superalloy

Nickel-based single crystal superalloys have been extensively utilized to manufacture the components of advanced aero-engines and gas turbines due to their excellent creep property,oxidation resistance as well as microstructural stability at elevated temperatures.Alloying has always been the most significant method to enhance the single crystal superalloys,Re is considered as representative element of the second and third generation single crystal alloys,and the creep performance of single crystal alloys can be remarkably enhanced by the addition of it.Nevertheless,Re is expensive rare element display in trace quantities in the earth's crust,thus the rising costs of the single crystal alloys by its addition have the necessity to be solved.In this work,therefore,the influence of Re on the creep behavior and microstructural stability of single crystal alloys has been firstly investigated to clarify the comprehensive effect of Re.Hence a novel single crystal alloy with high performance and lower Re content has been designed by composition optimization and microstructure modification.The tensile properties of the alloy were tested from room temperature to 1120?,and the creep tests were conducted under different conditions to mearsure the creep properties of the alloy.Additionally,the oxidation behavior of the alloy was tested at a elevated temperature of 1120?.The experimental results indicated that:with the increasing addition of Re,the dendritic spacing decreased and after standard heat treatments,the?'size declined but the volume fraction of ?' phase was raised.Larger number of TCP phases were precipitated with increasing addition of Re after long term aging at 1120?,additionally,the TCP phase evolved from rod-like shape to needle-like and blocky morphologies.All the TCP phases with different shapes were identified to be?phase.What was noteworthy was that the alloy with 1wt.%and 2wt.%addition of Re exhibited similar creep performance under high temperature condition,which could be rationalized by comprehensive effects of Re in the two alloys.The yield strength of the novel low-cost alloy increased firstly and then decreased with the increase of temperature,presenting remarkable strength peak and relatively poor ductility at760?.The temperature dependence on the deformation mechanism was analyzed by transmission electron microscopy.At room temperature,the deformation mechanism of was the interfacial dislocation shearing the?'phase with a stacking fault being generated.As the temperature increased to 980?,the deformation mechanism was changed to dislocation pairs coupled with APB shearing theg'precipitates,additionally,these two shearing modes were believed to coexist at 760?.At 1100?,the dislocation climbing and Orowan by-passing mechanisms began to prevail in the alloy,thus the plasticity of the alloy was improved.The creep life of the novel alloy under the condition of 1120?/137 MPa,1100?/137MPa,760?/800 MPa were 121.0 h,215.6 h and 200.5 h,respectively.The fracture surface displayed a ductile fracture at high temperature.At intermediate temperature,a mixed fracture characteristic was observed while the cleavage prevailed.In addition,a typical a<101>superdislocation was observed in the deformed structures after creep rupture at high temperatures,these dislocation pairs were short in length but they could shear into the?'phase during the third stage of creep.At the condition of 1120?/137 MPa,another a<010>superdislocation with a relative poor mobility was found,which was beneficial to lower the creep rate of the alloy.The oxidation kinetics curve of the novel alloy presented three distinctive stages at 1120?.The outer NiO grains,inner Al₂O₃ layer as well as several spinels took shape at first stage while the oxidation dynamics obeyed cubic law.However,the NiO particles fell off and the continuity of Al₂O₃ layer was partly destroyed quickly.The weight gaining rate was remarkblely raised at second stage which could be rationalized by the severe oxidation of W and Mo as well as the formation of non-protective NiWO₄ and NiMoO₄ spinels.With the exposure time prolonging,a dense Al₂O₃ layer had reformed,which was favorable to the hindrance of further oxidation process.