Development of a Nanoscale Precipitation-Strengthened Creep-Resistant Aluminum Alloy Containing Trialuminide Precipitates

Development of a Nanoscale Precipitation-Strengthened Creep-Resistant Aluminum Alloy Containing Trialuminide Precipitates Keith Edward Knipling This research is toward developing a castable andheat-treatable precipitation-strengthened aluminum alloy exhibiting coarseningand creep resistance at temperatures exceeding 400°C. Criteria for selecting alloying elements capable of producing such an alloy are established. Those systems forming Al3M trialuminide compounds with a cubic L12 crystal structure are favored, and based on a review of the existing literature, these are assessed in terms of solid-solubility and diffusivity in α-Al (satisfying the need for slow coarsening kinetics), and castability (which is discussed based on the binary phase diagrams). The first Group 3 element, Sc, and the second Group 4 element, Zr, are shown to be most promising. These expectations are confirmed by an initial study on the Al-Ti system, which demonstrates that conventionally-solidified alloys are not capable of precipitation strengthening. The Al-Zr system, by contrast, exhibits precipitation of nanometer-scale Al3Zr (L12) producing pronounced precipitation hardening when aged at 375, 400, or 425°C. The Al3Zr precipitates are coarsening resistant and have the metastable L12 structure up to 500°C, a result of very sluggish diffusion of Zr in α-Al. Ternary additions of Ti are also investigated, formingAl3(Zr1−xTix) (L12) precipitates with a reduced lattice parameter mismatch with α-Al, potentially improving the coarsening resistance. The composition ofAl3(Zr1−xTix) precipitates formed at 375 or 425°C aremeasured directly using 3-D atom-probe tomography. At these temperatures, the Zr:Ti atomic ratio in the precipitates is about 10 and 5, respectively, indicating that most of the available Ti fails to partition to the Al3(Zr1−xTix) phase. This is consistent with prior studies on Al-Sc alloys, where the slower-diffusing ternary solute species make up a small fraction of the Al3Sc-based precipitates. Despite the confirmed presence of Ti, Al3(Zr1−xTix) precipitates exhibit no improvement in terms of coarsening resistance compared to binaryAl3Zr. Mechanical properties of the Al-Zr andAl-Zr-Ti alloys are investigated utilizing Vickersmicrohardness and creep. The alloys deformed by creep at 300–400°C exhibit a dislocation climb-controlled threshold stress, ca. 6–12 MPa. The binary Al-Zr and ternary Al-Zr-Ti alloys behave similarly under ambientand high temperature loading, consistent with the similar microstructures of the two alloys.