金属熔体中气泡形核的理论分析

Study on bubble nucleation in liquid metal

引入界面接触角,考虑表面张力对气泡形貌的影响,以熔体中均质形核、夹杂物的平表面上异质形核和圆锥形凹坑内异质形核三种典型模型对气泡形核机理进行理论研究.研究发现,三种形核模型下具有相等的微米量级的气泡临界形核半径,并随气压的增大而减小.结果表明,用以制备藕状规则多孔金属的Gasar工艺中能够形成的最小气孔的直径为微米量级(0.1—1.0MPa气压).在圆锥形凹坑内异质形核时存在最佳圆锥顶角(对应最小气泡体积),其值与气压无关,只随接触角的增大而增大.在接触角处于90°—180°范围内,最佳圆锥顶角下圆锥形凹坑内气泡异质形核最容易.

The nucleation mechanism of gas bubbles in melts was studied theoretically by three representative models, uamely the homogeneous nucleation in liquid metal, heterogeneous nucleation on the flat surface or in conic pits of refractory inclusions, under the assumption that the state of the system＇ s Gibbs free energy change being maximal is the critical state of bubble nucleation. Introducing the contact angle and considering the effect of interfacial tension on bubble morphology, the following conclusions can be drawn： the critical nucleation radius of bubbles in the three models is the same, which will be reduced with increasing gas pressure above the liquid metal. The diameter of embryo bubbles, which is also the minimum pore size in lotus- type porous metal ingots, has a magnitude of micrometers under the common pressure of 0.1-1.0 MPa for the Gasar process. For heterogeneous bubble nucleation in conic pits of inclusions, there exists an optimal cone apex angle corresponding to the smallest volume of the embryo bubble, which is independent of gas pressure above liquid metals and increases with the increase of the contact angle. For alumina inclusion in Mg, A1, Fe, Ni and Cu melts, the optimal apex angles are 8.0°, 19.5°, 20.6°, 42.7° and 51.5°, respectively. Among these three bubble nucleation models, homogeneous nucleation is the most difficult, heterogeneous nucleation in conic pits of inclusions is the easiest, and heterogeneous nucleation on the flat surface of inclusions is intermediate.