Microstructure And Mechanical Properties Of Cast Nickel-based Superalloy With High Tungsten Content Using For High-temperature Dies

Cast Ni-based superalloys with tungsten content higher than 10 wt.% has been recongized as excellent microstructure stability,high temperature strength and good high temperature stress rupture properties.Its initial melting temperature of ?? phase is above 1250? diriving from the excellent solid solution strengthening caused by W and dispersion strengthening with sufficient carbides.Although it is not suitable for producing aero engine components because of the high density of the alloy,it can be made for high temperature die due to its relatively low cost.With the development of isothermal forging technology,especially,this kind of alloy has become important isothermal forging die materials that was used in between 1000 ? and 1100?.However,such alloys are prone to be formation of abnormal phases(e.g.?,M6 C,etc.)in as-cast microstructure,which results in microstructural instability and insufficiency of high-temperature strength during long-term thermal exposure under the working conditions of the dies,corresponding to shortening the service lives.Therefore,the effects of W and Mo on the microstructure and properties,the microstructure stability of long-term thermal exposure,the tensile deformation behavior of the alloy at high temperatures and the thermal fatigue behavior of the alloy under different conditions were studied in this paper,which provided a foundation for the optimization of the chemical composition,microstructure and properties of high tungsten Ni-based cast superalloy.The purpose of the research is engineering application.The 1100?/70 MPa stress-rupture life of the 17W-0Mo(wt.%)alloy with the cast states was 48.9h due to the existence of ? phase.The life of the optimized 14W-1Mo(wt.%)alloy with eliminating ? phase reached 145.3h.The tensile strength at 1100? attained 460 MPa.The microstructure stability was also improved.No acicular M6 C carbide precipitates were observed after 1100?/1000 h thermal exposure.The precipitation of primary ?-(W,Mo)phase depends tremendously on the amount of W and Mo additives.With increasing of Mo/(Mo+W)ratio,the dendrite-like ?-(W,Mo)phases are apt to convert into small bars or blocky-like phases at the vicinities of ??/? eutectic.The morphological changes of ?-(W,Mo)phase can be interpreted as the non-equilibrium solidification of W and Mo in the alloy.Because large sized ?-(W,Mo)phase has detrimental effects on stress-rupture properties in as-cast conditions,secondary cracks may mainly initiate at and then propagate along the interfaces of brittle phases and soft matrix.During exposing at 1100? for 1000 h,the ?-(W,Mo)phases transformed gradually into bigger and harder M6 C carbide,which results in decreasing of stress-rupture properties of the alloy.Finally,the alloy with an additive of 14W-1Mo(wt.%)maintains the longest stress lives at high temperatures and therefore it revealed the best microstructure stability after 1100?/1000 h thermal exposure.Based on the microstructure evolutions,stress-rupture life of the optimized 14W-1Mo(wt.%)alloy were examined under 1100?/70 MPa,where the alloy was evaluated after thermally exposed at 1000,1050,and 1100 ? for 100–1000h,respectively.Results showed that increasing of exposure temperature can cause coarsening and coalescing of ?? precipitates along with a decreasing in their volume fraction,especially at 1100?.The coarsening rate of the ?? precipitates increased with the exposure temperature and followed by the modified Lifshitz-Slyozov-Wagner coarsening theory of Ostwald ripening.The coarsening rate coefficient(k)at 1000,1050,and 1100? was calculated to be 5.72,7.40,and 8.71 nm/s1/3,respectively.In addition,at those temperatures,it found the blocky M6 C carbides in the interdendritic area to be precipitated and coarsened.Furthermore,the M6 C carbides linked with each other like a chain wrapped around the grain boundary and along the eutectic ?? phase.Because of the degradation of the microstructure,the stress-rupture life decreased after thermal exposure,from 94.8h at 1000? to 49.3 h at 1100? for 1000 h.Finally,in view of the experimental results,a relationship between the microstructural evolution and the stress-rupture life of the alloy was discussed in details.Tensile tests for optimized 14W-1Mo(wt.%)alloy were carried out at temperatures ranging from 20? to 1100?.Temperature was varied in increments of 100? after reaching 500?.The tensile strength of the superalloy used in this study was 150–200 MPa higher than that of other low W-content cast Ni-based superalloys at 1100?.Microstructural observations revealed that the tensile behavior of the alloy was controlled by the cross-slip mechanism which optionally operated in most of the ?? precipitates and a large number of isolated stacking faults(with high densities)with two different orientations were formed as temperature reached 1000?.At temperatures higher than 1000?,the applied stress facilitated the strain-induced precipitation of tungsten-rich MC type carbide particles in the matrix and along the ?/?? interface.On the other hand,the low lattice misfit reduced the nucleation barrier,which resulted in the formation of nanosized ?? phases in the matrix.The strong solid solution effect along with the formation of the tungsten-rich MC precipitates and nanosized ?? phases improved the high-temperature(temperatures higher than 1000?)strength of the superalloy.The thermal fatigue properties of cycling from 20 to 1100? of the 14W-1Mo(wt.%)Ni-based cast superalloy were studied,and the results are compared with those of other alloys,namely,K4002 and K403,corresponding to simulation analysis.The results show that the cracking models of the three alloys are basically the same;that is,when the temperature is alternately changed,cracks are easily formed at the interface of the matrix and carbide due to differences in the thermal expansion coefficients of the matrix and carbide.The investigated alloy contains a block-shaped(?+??)eutectic with a volume fraction of 15.5%,which effectively hinders crack propagation,thereby resulting in the best thermal fatigue performance among the investigated alloys.The K4002 alloy also has a sunflower-like eutectic with a volume fraction of 12.5%,which also shows good structural stability and excellent thermal fatigue properties.However,the K403 alloy exhibits poor thermal fatigue performance for the transformation of the carbides during thermal fatigue;in this case,the carbides form chain-like M23C6 carbides at the grain boundaries or interdendritic area because of unstable intrinsic character of the alloy.The results also demonstrate that a low eutectic content with a small volume fraction(2.2%)of the K403 alloy does not hinder crack propagation.