Phase Transformation in Nanometer‐Sized γ‐Alumina by Mechanical Milling

Nanocrystalline γ‐alumina powders of 50‐nm size were milled by high‐energy ball milling. It was found that the pure γ‐alumina phase showed great stability and did not transform to any other phase even after a long milling time. On the other hand, γ‐alumina, which contained a small amount of the α‐alumina phase, showed a gradual phase transformation from γ‐ to α‐alumina on milling. The phase transformation mechanism during milling appears to be nucleation and growth type and promoted by the α‐alumina seed.