Effects of process control agents on the mechanical alloying behavior of Nb-Ti-Si based alloy

Powder mixtures with the composition of Nb-20Ti-15Si-5Cr-3Hf-3Al (at.%) were mechanically milled in a planetary ball mill. Normal hexane (5 mass%), methyl alcohol (5 mass%) and different amount of stearic acid (0.625, 1.25, 2.5 and 5 mass%) were used respectively as the process control agent (PCA). The effects of the type and amount of PCAs on the mechanical alloying (MA) behavior of the powder mixtures have been investigated systematically. It has been shown that, compared with using 5 mass% methyl alcohol or stearic acid, ball milling with the same amount of normal hexane shows the fastest MA progress. However, the final powder particles have a coarse size as a result of severe cold welding. Ball milling with 5 mass% stearic acid shows a hyperslow MA progress due to its excessive lubrication effect. Milling with 5 mass% methyl alcohol yields a moderate MA rate and much fine particle size. However, HfC formed during milling due to the high carbon contamination of methyl alcohol. Moreover, it is found that decreasing the amount of stearic acid could accelerate the MA progress significantly. A suitable amount of PCA is necessary to restrain the excessive cold welding and avoid the formation of HfC. Finally, 1.25 mass% stearic acid is adopted as the PCA to fabricate the powder mixtures used for hot pressed sintering.