Solidification of metal 金属的凝固,REVIEW,1 Basic Solidification Concepts,Glossary Homogeneous,hmdi:nis 自发的 Heterogeneous ,hetrudi:njs 非自发的 Equiaxed i:kwikst 等轴的 Columnar klmn 柱状的 Nucleus 晶核 Nuclei 晶核复数 Nucleation 形核 Nucleation agent 形核剂 Embryo embriu 晶芽 Insoluble 不溶解的 Planar 平面的 Cellular seljul 胞状的 Dentritic 树枝状的 Morphology m:fldi 形态 Radii 半径 Substrate 基材，母材 Temperature gradient 温度梯度 Free energy 自由能 Epitaxial ,epitksil 联生的 Epitaxial growth 联生结晶 Competitive kmpetitiv 竞争的 Competitive growth 竞争生长 Constitutional undercooling 成分过冷,The two main mechanisms by which the nucleation of soled particles in liquid metal occurs are homogeneous nucleation and heterogeneous nucleation. Homogeneous Nucleation Homogeneous nucleation is considered first since it is the simplest case of nucleation. Homogeneous nucleation in a liquid melt occurs when the metal itself provides the atoms needed to form a nuclei.,1.2 Nucleation - Homogeneous Nucleation,Homogeneous nucleation requires a considerable amount of undercooling, which may be as much as several hundred degrees Celsius for some metals (see Table 1.1).,Table 1.1 Values for the freezing temperature, heat of fusion, surface energy, and maximum undercooling for selected metal,1.2 Nucleation - Homogeneous Nucleation,Heterogeneous nucleation is nucleation that occurs in a liquid on the surfaces of its container, insoluble impurities, and other structural material that lower the critical free energy required to form a stable nucleus. Since large amounts of undercooling do not occur during industrial casting operations and usually range between 0.1 and 10,the nucleation must be heterogeneous and nothomogeneous.,1.2 Nucleation - Heterogeneous Nucleation,2.2 EPITAXIAL GROWTH AT FUSION BOUNDARY,2.2.1 Nucleation Theory,Figure 2.6 Epitaxial growth of weld metal near fusion line.,Such a growth initiation process, shown schematically in Figure 2.6, is called epitaxial growth or epitaxial nucleation. The arrow in each grain indicates its direction.,2.3 COMPETITIVE GROWTH IN BULK FUSION ZONE,Figure 2.8 Competitive growth in bulk fusion zone.,Therefore, during solidification grains with their easy-growth direction essentially perpendicular to the pool boundary will grow more easily and crowd out those less favorably oriented grains, as shown schematically in Figure 2.8. This mechanism of competitive growth dominates the grain structure of the bulk weld metal.,