Heterogeneities in Cold Crucible Melted and Gas Atomised TiAl Based Alloys

Abstract

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 before and after hot isostatic pressing (hipping). It has been found that individual atomised powders sometimes contain Ti rich dendritic features which are on too coarse a scale to have formed during atomisation. Hipped samples are also found to contain regions up to 100 μm 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 region found in a 1 cm diameter sample. These Ti rich regions act as failure sites when hipped samples are tested in tension at room temperature. It 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 found to show similar features. PM/0753