Nanocrystalline TiAl-V Intermetallics Hot Pressed from Ball Milled Powders

Abstract

The TiAl-V alloys compositions were chosen from the γ-phase stability region up to maximum 20 at. % vanadium. The elemental powders were initially blended to desired composition of Ti-52.7Al-5V, Ti-55.4Al-10V, Ti-57.7Al-15V and Ti-60.4Al-20V (numbers indicate at. %). The milling was performed in a high energy planetary mill Fritsch Pulverisette P/4 resulted in formation of the almost fully amorphous structure after 40 hours. The formation of Al(Ti, V) solid solution was observed after 20 hours of milling. Longer milling caused amorphization of most of the material and formation of metastable phase in the alloy with the highest content of vanadium. TEM observation of powders after 40 hours of milling allowed to identify 50 nm wide nano-layers within individual particles. The origin of layered structure was explained as resulting from the mechanical alloying process, not from the thin foil preparation. The electron diffraction and bright field techniques revealed the amorphous and partially nanocrystalline structure within milled powders. Uni-axial hot pressing at 700°C under 200 MPa of milled powders allowed to obtain a bulk samples with density of about of 94 % of theoretical one. The X-ray diffraction revealed the two phases γ+α 2 structure. TEM studies have shown the nanometer range grain size structure with a mean size of about 20-30 nm. The diffraction pattern appeared in a ring like pattern without preferred pattern orientation. Some larger grains has shown a parallel aligned defects causing streaks in the diffraction indicating basal Ti 3 Al plane defects. Their existence indicate disordering within intermetallic grains in spite of 700°C annealing.