Directionally freeze-cast titanium foam with aligned, elongated pores

Directional freeze-casting - a process used to create foams with elongated, aligned pores applied so far exclusively for ceramics - is demonstrated for titanium foams. An aqueous slurry of <45 pm titanium powders was directionally solidified, resulting in a powder preform consisting of elongated, aligned dendrites of pure ice separated by interdendritic regions with high powder content. After freeze-drying to remove the ice dendrites and sintering to density the powders, the resulting titanium foams exhibited 57-67% aligned pores (similar to 0.1 mm wide and several millimeters long) replicating the ice dendrites. Because of the high powder oxygen content, the foams display high compressive strength and signs of embrittlement. Lower contamination was achieved by using purer <125 mu m powders, but their larger size prevents the formation of pure ice dendrites (and thus elongated pores in the foam), in agreement with a model considering particle pushing and engulfment by a moving ice front. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.