Al_2O_3-ZrO_2系熔融材料非平衡态显微结构

NON-EQUILIBRIUM MICROSTRUCTURE OF FUSED Al_2O_3-ZrO_2 SYSTEM MATERIALS

含75%质量分数Al2O3的Al2O3–ZrO2系熔融材料在缓慢冷却时,正常的显微结构由粗粒状初晶刚玉和组成少许波动的Al2O3–ZrO2共晶体组成。依冷凝速率梯度不同,该组成样品可以获得显微结构渐变的熔融材料。较快冷凝的同组成样品则呈微细的、均匀的两相结构,间或有少量不定组成的偏共晶。同组成的熔体与水冷钢容器接触激冷时,相当于淬火,只析出尺寸为5～10μm的立方体和柱状刚玉以及小于2μm的球状氧化锆晶体,而不存在共晶。以上表明:熔体不沿相图所示析晶路线形成共晶,而是形成了非平衡态的相组合。在激冷的样品中,ZrO2以立方相和四方相为主。缓慢冷凝的自由表面,析出自形刚玉,晶体尺寸达30～40μm,而<2μmZrO2呈微晶粒填充于其间。

Under slow cooling condition, the melt of the Al2O3-ZrO2 system with 75% in mass Al2O3 forms normal microstrncture consisting of well developed granular primary corundum and Al2O3-ZrO2 eutectic with a slight fluctuation in the eutectic composition. The cooling speed gradient leads to a gradually changed microstructure of the fused materials. When the same melt was rapidly solidified, the microstructure of the sample consists of two fine and uniformly distributed phases of corundum and zirconia, or doped with a small amount of unconstant composition of off-eutectic. In the melt contacting the water-cooled steel container, as the melt was subjected to a near-quenching effect, no eutectic could be observed, but cubic and prism-like corundum with sizes of 5-10μm and spherical zirconia crystals with sizes under 2μm were found. Under such conditions, the crystallization of the melt was not in accordance with the phase diagram of the Al2O3-ZrO2 system to yield a typical eutectic structure; instead a non-equilibrium phase assembly was formed. The ZrO2 consisted mainly of cubic and some tetragonal phases in the rapidly cooled sample. Idiomorphic corundum with sizes of 30-40μm was formed from the slow-solidified surface and spherical zirconia with sizes 〈2μm filled in between the corundum grains.