Towards extinction-free metallurgical powders for x-ray diffraction. An application to γTiAl

In the present work a new method will be presented for the routine generation of very fine ( < 5 micrometers) powders of metallic materials. This involves the use of mechanical methods to powder the material and the use of an acoustic sieve to produce powder with an average particle size that is sub 5 micrometers. The x-ray intensities generated from a Ti-51 at % Al alloy treated in this way will be analysed as an illustration of the effectiveness of the method. In the present work, the TiAl alloy studied contained about 700 ppm oxygen, and it is well known that high oxygen contents in this alloy can shift the liquid to liquid + alpha to alpha + gamma reaction to higher aluminum concentrations so that alpha can be present in an alloy of the composition studied here. In the coarse powder, where the surface area to volume value is relatively small no alpha was found to be present after annealing for one hour at 900 degree C. However, after annealing the sub five micrometer powder for the same amount of time a small amount of alpha was found to be present. Fortunately this did not interfere with the x-ray analysis. These comments also suggest that it is extremely important to have a very high quality vacuum (around 10 super(-4) Pa or better) in the powder annealing furnace.