Ni-Ag偏晶合金凝固过程研究

STUDY OF THE SOLIDIFICATION OF Ni-Ag MONOTECTIC ALLOY

对Ni-Ag偏晶合金开展了喷铸快速凝固实验,获得了富Ag相粒子弥散分布于Ni基体相的复合凝固组织.建立了Ni-Ag偏晶合金喷铸快速凝固过程中组织演变的动力学模型,模拟分析了喷铸Ni-Ag合金凝固组织形成过程.结果表明:在喷铸快速凝固条件下,Ni-Ag合金液-液相变过程中富Ag相液滴的Ostwald粗化作用很弱,初生富Ag相粒子的弥散度主要由液-液相变过程中富Ag相液滴形核率决定,初生富Ag相粒子的数量密度和平均半径与液-液相变过程中富Ag相液滴形核阶段熔体冷却速率间满足N∝(?)和<R>∝(?)的指数关系.

The Ni-Ag alloy has good mechanical properties, high corrosion resistance and electrical conductivity. It is an excellent candidate to be used in many high-tech fields of aerospace, energy resource and chemical engineering etc. This alloy, however, is a typical monotectic system. Generally, the liquid-liquid phase transformation leads to the formation of a solidification microstructure with serious phase segregation. The manufacturing of this alloy is thus extremely difficult. Injection casting has already been carried out with the Ni-Ag monotectic alloy. The sample with composite microstructure, in which Ag-rich particles dispersed homogeneously in Ni matrix has been obtained. A model describing the microstructure evolution during injection casting of the Ni-Ag monotectic alloy has been proposed. The process of microstructure formation has been simulated and discussed in details. The results indicate that the Ostwald coarsening of Ag-rich droplets is very weak during cooling in miscibility gap under injection casting cooling conditions. The dispersivity of the primary Ag-rich phase is controlled by the nucleation of Ag-rich droplets during the liquid-liquid transformation. The number density （N） and average radius （（R/） of primary Ag-rich particles depend exponentially on the cooling rate of the alloy during the nucleation of Ag-rich droplets （TNuc） according to N TNuc^1.8 and （R） TNuc^-0.6.