HETEROGENEITIES IN COLD CRUCIBLE MELTED AND GAS-ATOMIZED TIAL BASED ALLOYS

Samples of atomised powder of Ti-48Al-2Mn-2Nb, produced using a bottom pouring cold wall induction furnace, have been examined using optical , scanning electron, and analytical transmission electron microscopy b efore and after hot isostatic pressing (hipping). It has been found th at individual atomised powders sometimes contain Ti rich dendritic fea tures which are on too coarse a scale to have formed during atomisatio n. Hipped samples are also found to contain regions up to 100 mu m in size which can have Ti contents as low as 42 at.%. The number density of these heterogeneities is very low, with typically one Ti rich regio n found in a 1 cm diameter sample. These Ti rich regions act as failur e sites when hipped samples are tested in tension at room temperature. Tt is argued that these regions are formed because some of the mushy zone, which must be present in a cold wall furnace, passes through the nozzle during atomisation. Hipped atomised powder from other sources, also manufactured using a cold wall process, has been examined and fo und to show similar features. (C) 1997 The Institute of Materials.