TiAl-B合金中初生TiB_2晶体的表面形貌和形成机制

Surface Morphology and Growth Mechanism of Primary TiB_2 in TiAl-B Alloys

用原位自生法制备了Ti-40Al-xB(r(B)t%)合金并利用XRD、SEM对合金的相组成和微观组织进行了研究.结果表明:初生TiB2颗粒呈六面棱柱状,在其(0001)面存在凸台或棒状分枝,而在其{1010}面存在分布较均匀的薄片状凸耳;有的初生TiB2的凸耳比较发达,凸耳呈一层层环状围绕初生TiB2晶体.这些凸台或棒状分枝及薄片状凸耳的晶面取向与母体的取向一致.分析表明,在不平衡凝固条件下,初生TiB2固-液界面前沿产生的富Al边界层和随着初生TiB2晶体的长大导致固-液界面上B原子过饱和度不均匀性增加的共同作用使固-液界面失稳,从而导致初生TiB2表面结构发生了变化,使初生TiB2(0001)面上的分枝具有较快的[0001]方向生长速度,呈棒状;而{1010}面上的凸耳具有较快的〈1010〉方向生长速度,使其呈片状.

Ti-54Al-xB alloys have been produced by an in-situ method. The microstructure and the phase institute of these alloys were investigated by XRD and SEM. The results show that there are pyramidal protrusion or rod shape dendrites on (0001) facet and some very thin convex flakes on {1010} facets of primary TiB2. The facets of these protrusions or rods and thin convex flakes are parallel to the facets of primary TiB2. The thickness and the height of these flakes are smaller than 02μm and 15μm, respectively. Some of these flakes are so developed that they exhibit ring shape layer upon layer surrounding primary TiB2. The Al rich layer and inhomogeneity in supersaturation at solid-liquid interface resulted from non-equilibrium solidification may make solid-liquid interface instable, and lead the primary TiB2 surface to engender rod shape dendrites on (0001) or very thin convex flakes on {1010} planes.