Controlled synthesis of primary phase nanostructures in amorphous aluminum alloys

Thermally induced devitrification of amorphous aluminum alloys yields a microstructure composed of a high density of primary phase nanocrystals embedded in an amorphous matrix. These unique materials, with a primary precipitate density ranging from 1021--1023 m-3 (several orders of magnitude larger than that in conventional high strength aluminum alloys), possess tensile strengths at room temperature approaching 1500 MPa. Unusually rapid nanocrystal growth rates are measured at the onset of primary crystallization. However, since this effect is not consistent with commonly observed parabolic growth, nucleation and growth kinetics of the primary phase in aluminum-based glasses are studied to quantify this distinctive behavior.; Rapidly quenched Al92Sm8 samples are shown to be reproducibly amorphous and the growth behavior of aluminum nanocrystals at the onset of the primary phase transformation has been probed with several thermal and microstructural characterization techniques. Nuclear magnetic resonance studies of replicate Al92Sm8 samples reveal differences in intra-sample local atomic configuration in the amorphous starting material. Although the local structure of the amorphous solid is shown to be sensitive to differences in thermal history encountered during rapid solidification processing, further characterization of partially devitrified samples confirm that the microstructure approaches a common metastable equilibrium consisting of approximately thirty volume percent aluminum nanocrystals embedded in the residual aluminum-samarium amorphous matrix. With unique high heating rate differential scanning calorimetry measurements, the glass transition temperature in this alloy has been successfully measured at 115°C. Moreover, heat capacity measurements in the glassy, liquid and crystalline states serve to classify this material as kinetically fragile in comparison to other bulk metallic glasses. In a similar material system, select substitution of less than one atomic percent Cu for Ni in Al88Ni8Sm 4 has been shown to enhance the nucleation density and retard the growth rate of the evolving nanocrystalline microstructure.; This study has revealed that a heterogeneous nucleation reaction drives the unusually high primary phase nanocrystal density observed in partially devitrified aluminum alloys. Further, knowledge of the kinetic fragility of these materials as demonstrated by transient growth behavior and fluctuating local atomic configurations during devitrification, is necessary in designing nanocrystalline aluminum alloys from amorphous precursors.