| As-cast K4169 alloy has the same composition with Inconel 718, which is the iron-nickel superalloy strengthened by precipitated phase B.C.T.γ" and F.C.C.γ'. K4169 alloy has good comprehensive performance between -253°C to 650°C. However, when the service temperature is above 650°C, theγ" phase becomes incoherent with matrix alloy, and then aggregates, grows up and transforms to stableδphase, so many properties such as strength and plasticity rapidly deteriorate. The properties of superalloys depend on the chemical composition and microstructure. To improve the properties at elevated temperature, alloying methods were applied to increase the structure stability of K4169 alloy by adjust the content of Al, Ti, Nb, Fe, Mo, Cr, Ta, W, et al. Except above elements, there are small amounts of Co and trace quantities of Zr and Cu in K4169 alloys to influence the microstructure and properties. However, the effect of these trace elements in K4169 alloys is not clearly understood and need further research and analyse. In this paper, 14 kinds of superalloys with different Al, Co, Zr and Cu addition were prepared by manual electric arc furnace. The effect of Al, Co, Zr and Cu on microstructure and properties of as-cast, standard heat treatment and long-time aging K4169 supperalloy, especially on the precipitates in grain and boundaries, were investigated by metallurgical microscopy (OM), scanning electronic microscope (SEM), transmission electron microscopy (TEM), electron microprobe analysis (EMPA), X-ray diffractometer (XRD) and mechanics performance testings. The research conclusions are as follows:(1) When the addition of Co element increases from 0.1wt% to 0.3wt%, the elements of Mo, Ti, Nb segregate to grain boundaries. So Co promotes the precipitation of Laves phase, reduces interdendritic microsegregation area, decreases the solubility of Nb, Mo, Ti in matrix and increases the solubility in Laves phase. The addition of Co increases the incipient melting temperature of Laves phase and slightly decreases the melting temperature of alloys.(2) When the additions of Al and Co vary simultaneously, the segregation of Mo, Ti, Nb are reduced. The interaction of Al and Co decreases the precipitation of Laves phase without changing the incipient melting temperature of Laves phase, but the melting temperature of alloys is reduced. After standard heat treatment, the interaction of Al and Co eliminates the unfavorable effects of Co on the precipitation phases at grain boundaries; the precipitation phases in grains are precipitated as nano polycrystalline; theγ″phases are precipitated as spherical shape but theγ′/γ″clad structure isn't gotten. In the long-time aging process, the interaction of Al and Co increases the stability ofγ″phases and decreases the coarsening rate of precipitation phases in grains.(3) The addition of Zr element promotes the segregation of C atoms to grain boundaries and precipitation of carbides at grain boundaries. In the meantime, the segregation of Nb, Mo in MC phases inhibits the precipitation of Laves phases at grain boundaries. The addition of Zr increases the incipient melting temperature of Laves phase and decreases the melting temperature of alloys. The Zr element strongly changes morphology and size ofγ′andγ″phases. In the process of long-time aging, the addition of Zr can increase the high temperature stability ofγ″phase; theγ″/γ′clad structure is formed and no flakyδphase is generated when the Zr addition is 0.03wt%.(4) The addition of Cu element promotes the segregation of Nb, Mo and precipitation of Laves phase. The addition of Cu decreases the incipient melting temperature of Laves phase and decreases the melting temperature of alloys. The addition of Cu changes the morphology ofγ″andγ′precipitation. With the increasing of Cu addition, the size ofγ″phase shows the trend of increasing firstly and then decreasing. In the process of long-time aging, Cu shows the effect of inhibiting the change ofγ″toδphase, so the Cu element increases the high temperature stability ofγ″phase.(5) The separately adjusting of Co addition has slight effect on the hardness at room temperature but decrease the yield strength of the test alloy. When the content of Al and Co is adjusted simultaneously, the hardness and yield strength of tested alloy increase with Al content. However, after long-time aging, the hardness is decreased due to the overaging. The Zr addition decreases the hardness and yield strength of the test alloy. The addition of Cu element decreases the hardness and yield strength too.In conclusion, in K4169 alloys, the Co content should be strictly limited; the addition of Al element can be properly increased, and the better content of Al is 1.1 wt% according to the experiments in this paper; the Zr element helps to improve the precipitated phases in grains, so small amounts of Zr can be added, but the content should be less than 0.03 wt% in consideration of precipitated phases at grain boundaries and property of alloy; because the Cu element can promote the precipitation of Laves phase and improve the stability ofγ″phase, the Cu elements can be properly added but the content need be strictly control less than 0.1 wt%.
|
PDF Full Text Request