Optimization Of Microstructures And Mechanical Properties Of Corrosion-resistant Directionally Solidified Ni-based DZ444 Superalloy

Directionally solidified Ni-based DZ444 superalloy is a kind of newly developed alloy,which is used in industrial gas turbine applications,since it has high strength and excellent hot corrosion resistance at high temperature.Its microstructures and properties were examined in detail under as-cast,heat treated,long-term thermal exposures conditions.In the present work,eleven alloying contents with different C,B,Cr,Al and Ti and the additions of Zr and Y/Ce,and different kinds of heat treatments are designed,in order to investigate(1)the effects of C,Zr and Y/Ce on the microstructures and stress rupture properties of DZ444 superalloys,(2)the behavior of incipient melting and its effect on mechanical properties in the DZ444 superalloy with a high boron content,and(3)the effects of Cr Al and Ti contents on the microstructure and stress rupture property during long-term thermal exposures.The obtained findings are expected to offer guiding significance to design and optimize the alloy compositions.The morphology of MC carbides does not change in as-cast alloys,as the C content is increased or the Zr is added.However,the Y/Ce additions promote the transformation of MC carbide morphology from blocky to script-like type.Compared with that in as-cast alloys,there is no obvious change in the morphologies of MC carbide or chemical compositions after heat treatment.The increasing of C content and the addition of Zr or Y/Ce significantly decreases and increases the amount of the ?/?' eutectic,respectively.After a solid solution treatment and subsequently a two-stage aging treatment,the y' phase is characteristic with two kinds of shapes and sizes,and the volume fraction of cuboidal ?' phase is much larger than that of spherical ?'phases.More MC carbide and residual eutectic result in the decreasing of cuboidal ?' phase volume fraction.The decrease in volume fraction of cuboidal ?' phase is principally responsible for the reduction of the stress rupture property.In addition,script-like carbide and residual eutectic remaining after heat treatment play the secondary role in the degradation of the stress rupture properties.The less regularly arranged cuboidal ?' phase can damage the stress rupture life to some extent.The spherical ?' phase re-dissolves into ?' matrix widening the spacing between large cuboidal ?' phase,and the addition of Zr inhibits grain boundary sliding and cracking;these two facts are beneficial to the rupture elongation.As-cast DZ444 alloy with high boron contents exhibits many y'/y eutectics,MC carbides,and multi-phase pileups comprising boride,NisHf and ? in the interdendritic regions.During solution treatments,incipient melting does not firstly occur in boride or NisHf phase with low melting point,but appears in the y matrix around multi-phase eutectic-like constituent that is affected significantly by borides.The incipient melting temperature of DZ444 alloy with high boron contents is between 1160?1170?,which is significantly less than that of DZ444 alloy with normal boron contents.The incipient melting occurs more seriously with the increase of the solid solution temperature or time.When water quenching can be utilized by microstructure observation,Incipiently melted region(IMR)is composed by the y dendrite and mass of tiny precipitation particles.However,when air cooling can be utilized,IMR is composed of multi-phase pileups,? matrix and ?/?' eutectic in order,and multi-phase pileups in IMR is the same as that in as-cast alloy.With the solid solution temperature ranging from 1210? to 1230?,the amount of IMR increases slightly,as the solid solution temperature proceeds to 1250?,the IMR fractions and sizes present a significant increase trend,which damages high temperature mechanical properties seriously.IMR is affected by the high or low aging treatment a little due to the relatively low temperature during the high or low aging treatment.With the increases of the IMR fractions and sizes,they consume more solid solution strengthening elements,and provide masses of crack sources,which decreases the high temperature tensile properties slightly but the creep-rupture properties remarkably.Increasing of Cr content promotes the enrichments of much more Ti,Ta and W in the interdendritic regions,and W elements with large radii in the melt make Ti and Ta more difficult for the diffusion,which results in pile-up of ?/?' eutectic elements(Ti,Ta),leading to the rise of ?/?' eutectics.At the same time,increasing of Cr content also enlarges the average electron-hole concentrations(Nv)and then brings about a increased tendency in the precipitation of ?phase.The fluctuation of tensile properties at room temperature(RT)is caused by the weak deformation coordination ability of residual ?/?' eutectic and y matrix.The brittlement of the residual ?/?' eutectic and the orientation of ?' rafts around the residual ?/?' eutectic become changed and the continuity of ?' rafts is also disturbed resulting from the effect of residual ?/?'eutectic,and a few of ? phases are precipitated,giving rise to the reduction of the stress rupture property as well as to the fluctuation of its value after heat treatment.The increment of Cr content resulting in the excessive precipitation of ? phase is the principal reason for the catastrophic reduction of the stress rupture properties after thermal exposure.The amounts of rounded cuboidal ?' and its coalescence increase,but the tendency of ?' alignment decreases with the increment of Cr content,which leads to the reduction of the stress rupture property to some degree.The change of lattice misfit shows a good agreement with the change of ?'morphology and the tendency of ?' alignment.The shape change of ?' is accompanied by the increase of the misfit dislocation density,which leads to the rise of ?' coalescence.The increase of Cr content can promote the formation of M23C6 or M6C in the vicinity of primary MC degeneration.Thus,the increased Cr content facilitates the decomposition of primary MC,which deteriorates the stress rupture properties.In addition,the increase of Cr content can promote the formation M23C6 and M6C along gain boundaries(GBs),facilitating the formation of a semicontinuous chain to some degree and broadens GBs,which plays a certain role in the degradation of the stress rupture properties.Increasing Al and Ti contents in the alloy promotes the precipitation of y' after heat treatment,thus improving the mechanical properties of the alloy.High Al and Ti contents result in the existence of many residual ?/?' eutectics,which leads to the fluctuations in tensile property at room temperature and stress rupture property.After the optimized heat treatment,the dissolution of ?/?' eutectic takes place at a higher level,and the amount of of ?' raises.The optimized heat treatment reduces the segregation of the refractory elements(W,Mo and Ta,etc)to produce a more homogeneous microstructure,giving rise to an optimum stress rupture property.The Al and Ti contents change the value of Nv,and the ? phase is precipitated more readily in the alloy containing a large value of Nv.