Effect Of Grain Refinement On Microstructure And Properties Of Two Superalloys

Ploycrystalline superalloys have the advantages of low cost and excellent mechanical properties at medium and low temperatures,and are widely used in aero-engine components that serve at medium and low temperature conditions.With the rapid development of aviation and aerospace,the thrust-weight ratio of aero-engines has been gradually improved,and the performance requirements for superalloy have been improved.Since the 1980s,the development of fine grain casting technology is an effective way to improve the performance of superalloys.In this work,the effect of grain refinement on two common cast superalloys are studied,including the high Cr superalloy and nickel-based cast superalloy K417G;The grain size of the alloys was refined by different process,and the effect of grain refinement on the microstructure evolution of the alloy,the tensile properties and the stress rupture properties were investigated.The change of grain size and the mechanism of crack initiation were investigated by changing the casting temperature and adding cobalt aluminate refiner to the shell for high Cr superalloys,respectively,and the main factors affecting the tensile properties and he stress rupture properties were comprehensively analyzed.The results show that:lowering the casting temperature or adding cobalt aluminate refiner to the shell would reduce the grain size of the high-Cr superalloy;when the casting temperature is lowered,the secondary dendrite spacing of the alloy decreases;the secondary dendrite spacing changed little with the addition cobalt aluminate refiner;the yield strength and tensile strength of the alloy at room temperature increased with the grain refinement;and the stress rupture properties at 800℃/206 MPa were improved with the grain refinement This could be explained as the following:the M₂₃C₆phase changed from chain-like to grain-like with the grain refinement,which reduced the grain boundary brittleness and led to the improvement of the alloy endurance life.The method of adding cobalt aluminate refiner to the shell of K417G alloy is adopted,The effect of grain refinement on microstructures,tensile properties and endurance performance of K417G alloy were systematically investigated.The results showed that the grain size of K417G alloy decreased with the addition of cobalt aluminate refinement agent on the mold shells,leading to a gradual increase in the yield strength and tensile strength of K417G alloy at room temperature;the yield strength and tensile strength of the alloy did not change significantly at900℃.The stress rupture properties at 760℃/645 MPa,were also gradually increased with the reduction of grain size.