Nucleation and Microalloying for Control of Nano-Structure Refinement

Abstract

Abstract : Kinetic transformation studies in rapidly-quenched Al88Y7Fe5 alloys, under previous AFOSR support raised concern that they were not amorphous. The substitution of 0.5-2.0 at.% of the Al by Ti, Zr, and V produced glasses with improved stability against crystallization. Under funding from this grant we demonstrated that the Al88Y7Fe5 alloys are amorphous, but this is masked in devitrification studies because of the extremely high nucleation rates and low growth rates of alpha-Al. We have explored further the influence of microalloying by systematically adding small additions of each element in the 3d transition metal series. Primary crystallization is to alpha-Al for the early (Ti, V, Cr) and late (Co, Ni, Cu) 3d transition metals. For Mn and Fe, an intermetallic phase forms first, which is closer in composition to the original glass. Kinetic calorimetric and microstructural studies of devitrification show that the microadditions raise the nucleation barrier of the crystal phase and decrease the rate of diffusion-limited growth. Scanning probe fluctuation microscopy studies show that the microadditions change the medium-range order. Complementary atom probe tomography studies, supported by our NSF grant (DMR-0606065), show that the Ti microaddition suppresses a nanoscale phase separation in the Al88Y7Fe5 glass.