材料期刊网

Fe-35Ni-15Cr型铁基高温合金中的点状偏析,元素偏析是第一位的,碳硼化物及其它金属间化合物偏聚是第二位的,二者是因果关系.点状偏析是由若干枝晶组成,和枝晶间一样聚集了Ti,Al,Si,Mo,Ni和微量元素C,B,S等,这样就促使偏析区生成较多的γ′相、Ti(C,N)和M_3B_2相,此外还生成脆性相σ;点状偏析区硬度比基体高,熔化温度比基体区大约低40℃;高温长时间均匀化处理不能完全消除,因此,它给合金的工艺性能和力学性能带来了不良影响。点状偏析实际上属于重力偏析的一种,它是在合金液凝固结晶过程中,溶质元素偏析形成的上浮液流造成的,其形成倾向正比于凝固过程中金属的熔池深度.

Investigation of freckles observed in Fe-35Ni-15Cr-2. 4Al-2. 3Ti-2W-2Mowrought superalloy has been carried out. The freckles were actually dendrites whichmay appear as small spots or streaks depending on whether cross- or longitudinalsections of the bar are taken. Enrichment of Ti, Al, Si, Mo, Ni and minute C, B,S, etc. in interdendritic spaces in the freckles' region may result in the formationof γ', Ti(C,N) and M_3B_2 as well as in the growth of σ-phase which was observedonly in the freckle areas. In the case of the freckles' region, the temperature offusion would be low, being probably about 1190℃ which is nearly 40℃ belowthat of the matrix. If the hot working of the alloy is carried out at 1190℃ orthereabout, microcracks may be formed in such freckles' region. The freckles cannot be eliminated completebly even after soaking for a long time at elevated tem-peratures. The higher hardness associated with the freckles can be troublesome inmachining and in giving a rough machined surface. In vie of the fact that the', the brittle intermetallic σ-phase and the non-metallic inclusions are somewhatlocally concentrated, the mechanical properties of such an alloy might be expectedto be adversely affected. The freckle segregation is essentially associated with gravity segregation. It iscaused by upward movement of jets of liquid metals enriched with light elementsin the mushy zone during solidification. The tendency of forming freckles seemsto be related to the melting rate of the consumable electrode as well as to thedepth of the liquid metallic pool.