Investigation Of Solidification Behavior And Mechanical Properties Of K416B Alloy

K416B alloy is a typical low Cr high W superalloy,which is widely applied to the production of high-pressure turbine blades due to their excellent mechanical strength,low cost,good oxidation,and corrosion resistance.Previous research shows that although this alloy has outstanding properties,abnormal primary phases are always observed in the as-cast alloy.And this alloy exists the problem of un-stable properties and low qualification rate on key performance indicator.Therefore,it is necessary to study the microstructure evolution and composition optimization for this alloy.To understand the solidification behavior and improve the key performance stabilization,this study investigates the solidification behavior and how solidification structure influence on mechanical properties of K416B.Furthermore,the role of boron on the microstructure and mechanical properties of K416B is also investigated in this study.Related results provide a reference for optimizing the structure and composition of the alloy.The phase transformation and elemental segregation characteristic of K416B during solidification has been investigated by the DTA measurement,Thermo-calc simulation under Equilibrium and Scheil modes,and isothermal solidification experiment.The solidification path of K416B can be summarized as follows:L→L+γ(1371℃);L→L+M6C(1370℃);L→L+y+MC(1332 ℃);L→L+eutectic(γ+γ’)(1286℃);L→MC(high Hf)(1147℃).EPMA results imply that W and Co were enriched in the solid phase,while Cr,Al,Ti,Nb,and Hf were enriched in the residual liquid during the crystallization of primary γ dendrite.The positive segregation elements Nb and Hf piled up at the remaining liquid promoted the formation of MC carbides.MC carbides precipitated at the interdendritic region and broke the residual liquid network.The formation of eutectic(γ+γ’)is due to the intense redistribution of Al element in residual liquid.While eutectic(γ+γ’)seemed instability and easily translated into eutectic γ’ by the remelting of γ phase and the coarsening of γ’ phase during subsequent cooling.The effect of casting thickness on microstructure and mechanical properties of K416B has been investigated in this study.Five castings of different thicknesses were produced by a specimen module that shared the same "cast-mold-environment" to simulate the varied thicknesses across a turbine blade.The microstructure and mechanical properties difference of the specimen with different cast thicknesses have been investigated.The results indicated that a larger cast thickness caused a lower solidification rate and increased element microsegregation.The cast thickness H has exponent relation to secondary dendrite arm spacing λ2.The average area of eutectic γ’and MC carbide phases is linear with cast thickness.The grain size increased with the casting thickness,but abnormal fine grains were observed in the specimens with 20 mm and 40 mm cast thicknesses.Large size α-W phases could be observed in these regions and are suggested to be the cause of the grain refinement.The room temperature tensile result implies that γ’ size in the dendritic region has an obvious influence on room temperature tensile performance.With the increase of γ’ size,the deformation mechanism changes from dislocation pairs cutting γ’ phase to bypassing of γ’ phase.The 975℃/235 MPa rupture test results showed that the rupture property decreased in the 40 mm specimen due to abnormal hard phase,but rupture life was no significant alteration in the 3-20 mm specimens.The effect of trace element boron on the microstructure and properties of K416B alloy have been studied in this study.The alloys with 0 wt.%,0.01 wt.%,0.02 wt.%and 0.04 wt.%boron addition were prepared using a vacuum induction furnace.The effects of boron content on the solidification behavior of this alloy were studied.It is found that boron addition significantly reduces the eutectic γ’ formation temperature,but has little effect on the solidification temperature range.Boron addition promotes the formation of eutectic γ’ and M3B2 at the last stage of solidification.The precipitation behavior of grain boundary precipitates of different boron content samples during shortterm aging at 975℃ was studied.Boron element can inhibit the early nucleation of grain boundary M6C carbide and reduce their size.Boron addition slightly improves the yield strength and tensile strength of this alloy at room temperature tensile and significantly improves the tensile plasticity at 1000℃ tensile.The rupture life increases with increasing B content first to 0.02 wt%,then decreases,and the optimal amount of B addition is 0.02%.The grain boundary strength improves with the increase of B content,the proportion of dendritic morphology on fracture surface decreases gradually and transgranular fracture morphology becomes obvious.However,excessive boron addition will promote the formation of eutectic γ’ and M3B2 phase,which are harmful to rupture property.The local strain characteristics of the dendritic,interdendritic region,and grain boundary during rupture deformation have been investigated by preparing bicrystal samples with fixed grain orientation.The interdendritic regions firstly deform by formation of shear band during rupture deformation,the position of the shear band coincides with {111} plane slip trace,the location strain in the shear band is larger than matrix,and high dislocation accumulation can be observed at matrix/shear band interface.Comparing the deformation behavior of boron-containing and boron-free bicrystal samples,it is found that boron addition increases the steady-state creep rate,but restrains the formation of grain boundary creep damages and extends the steadystate stage.The rupture interrupt experiment shows that the grain boundary strain in boron-containing sample is lower than in boron-free sample under the same rupture time,which implies that B addition can improve the grain boundary dislocation motion.