FORMATION AND HIGH-STRENGTH OF NANOAMORPHOUS STRUCTURE IN RAPIDLY SOLIDIFIED AL-CR-CO-CE AND AL-V-FE ALLOYS

The suppression of the transformation from supercooled liquid to icosa hedral phase by rapid solidification was found to result in nanoscale amorphous phase regions in coexistence with icosahedral and fee-Al pha ses for the Al94.5Cr3Co1.5Ce1 (at%) alloy and with an fee-Al phase for the Al94V4Fe2 (at%) alloy. The amorphous phase regions in the Al-Cr-C o-Ce alloy have a size of about 1 to 3 nm and are embedded in a nanosc ale icosahedral phase with a size of 30 to 50 nm surrounded by Al-phas e, while the amorphous regions in the Al-V-Fe alloy have a size of 10 nm and are coexistent with Al-phase with a size of about 7 nm. The nan oscale amorphous phase regions are presumed to be formed by the amorph ization mechanism in which a high density of boundaries between icosah edral and approximant crystalline phases on a subnanoscale scale are i ntroduced by the generation of subnanoscale approximant crystalline ph ase regions. The approximant crystalline phase can be spontaneously fo rmed by the generation of phason strains caused by the deviation of al loy composition from the stoichiometric composition and the quenching- induced strains. These nanoscale mixed phase alloys have good bend duc tility and exhibit high tensile strength reaching 1340 MPa for the Al- Cr-Co-Ce alloy and 1390 MPa for the Al-V-Fe alloy. The high tensile st rength is mainly attributable to the homogeneous dispersion of nanosca le amorphous and icosahedral particles without periodic atomic configu rations. The formation of the nanoamorphous phase regions by the struc tural modification of the icosahedral phase can be regarded as a new a morphization method and the development of such high-strength material s by utilizing the new mechanism is expected to attract increasing int erest in the near future.