Question

State and explain the different stages involved in refining the grain structure of metallic powders using high-energy ball milling.

Answer 1

In high energy ball milling the powder partcicles are subjected to high energy impact .The mechanical allyoing process mainly divided in to four stages( microsrurally)

1)intial stage

2)Intermediate stage

3)final stage

4)Completion stage

1)intial stage

In the intial stage the powder particles are flattenned by compressive forces due to collosion of the balls Micro forging leads to change in shape of the individual particles or cluster of these particles are impacted repeatedly by milling partcles with high kinetic energy.Here there is no net change in mass.

2) intermediare stage

At the intermediate stage significant changes occur.In mechanical alloying process intimate mixture of powder constituents decreases distance in diffusion distance to micrometer range.Fracturing and cold welding are the main milling processes at this stage.In this stage the chemical composition of the alloy powder still not homogeneous

3)Final stage

At final stage of the mechanical alloying process, there is coniderble refinement and reduction in particle size .The micro strcture of particle be more homogeneous.True alloys may have already been formed.

4) complition stage

In complition stage the powder particles possses an extremely deformed meta stable structure.At this stage the lamelllae are no longer resolvable by optical microscopy.Real alloy with composition similar to the starting is formed