Polymorphism 1394。C 912。C δ-Fe Y-Fe a Fe bcc fec bcc nucleation,grow in size change in volume (thermal stress →fracture) ·change structure by heat treatment-→ property1394 °C 912 °C bcc fcc bcc δ - Fe γ - Fe α - Fe Polymorphism • nucleation, grow in size • change in volume (thermal stress →fracture ) • change structure by heat treatment→ property