High Mechanical Strength of Rapidly Solidified Al<SUB>92</SUB>Mn<SUB>6</SUB>Ln<SUB>2</SUB> (Ln=Lanthanide Metal) Alloys with Finely Mixed Icosahedral and Al Phases

A finely mixed structure consisting of nanoscale icosahedral (I) particles embedded in an fcc-Al phase was formed in rapidly solidified Al92Mn6Ln2 (Ln=Y, La, Ce, Nd or Gd) alloys. The particle size and interparticle spacing of the I phase are about 50 to 100 and 5 to 25 nm, respectively, for the Al92Mn6La2 and Al92Mn6Ce2 alloys. Furthermore, the individual I-particle contains a high density of phason defects and approximant phases. These mixed phase alloys have good bending ductility and the Al–Mn–Ln alloys except Ln=Nd exhibit high tensile fracture strengths (σf) exceeding 700 MPa. The highest σf value reaches as high as 1320 MPa for the Al92Mn6Ce2 alloy. The simultaneous achievement of high σf and good ductility is obtained for the alloys containing the I-phase as a main phase. The good mechanical properties are presumed to result from the simultaneous achievement of the following structural effects: (1) the homogeneous distribution of nanoscale I-particles in the Al matrix, (2) the lower solute content in the I-particles as compared with the stoichiometric composition, and (3) further refinement of the nanoscale I-particles into the coexistent icosahedral and periodic approximant regions resulting from the generation of a high density of phasons. It is thus concluded that the unique structural modification of the I-phase is effective for the achievement of high σf and good ductility, though the stoichiometric I-phase itself has an extremely brittle nature.