As in Communution? I think you are refering to the ball mills as used in the crushing and attrition of ore bearing rock.
I belive that the P/M industry usually used a more energetic type of ball mill called a SPEC-mill. (its been a long time since I took that class thou.) I know that you can reduce particle size in them but only to a certain extent, at some point your particles stop getting smaller and then only change structure. there also is some alloying btw. the balls you use and the powder, (also the container walls.)
I believe that SPEC-mills are commmonly run in inert atmospheres.
nick
(I have no details for you about Hafnium however)