采用电磁场近液相线熔体保温工艺对Ni-20Cr-18W(质量分数,％)高温合金进行处理,获得了平均晶粒尺寸d=127 μm,凝固组织为球状晶粒的60kg铸锭.借助于OM,SEM和EBSD等手段,针对所获得的电磁场对原子团簇、界面稳定化以及树枝晶向球状晶转变的研究结果,从形核热力学方面对近液相线处理后高温合金铸锭晶粒细化机制进行了研究.结果表明:该工艺能够细化凝固组织;其细化机制为原子团簇与球晶分别作为后续凝固过程中的形核核心,促进凝固过程中晶粒形核,细化组织.此外,在改善熔体宏观温度场均匀性的基础上,电磁场促进了球晶稳定化以及树枝晶向球状晶的转变,并提高其形核率.
The effects of electromagnetic field and melt treatment near liquidus on the grain refinement of the Ni-20Cr-18W (mass fraction,％) superalloy have been studied.The average grain size of 60 kg ingot can be refined to 127 μm and the grains are both fine and globular.Based on the effect of electromagnetic field on atomic cluster,interface stability and transformation from dendritic grain to globular grain,the mechanism of grain refinement has been studied from the aspect of nucleation thermodynamics by OM,SEM,EBSD.The results show that electromagnetic melt treatment near liquidus results in significant refinement.The grain refinement can be attributed to the mechanism that atomic clusters and globular structures can become the nucleus when the initial undercooling reaches certain level.Electromagnetic field not only improves homogeneity of the macroscopic temperature field,but also plays a positive role in interface stability and dendritic to globular transformation,which increases the nucleation rate.
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