Mechanical milling and field assisted sintering consolidation of nanocrystalline Al-Si-Fe-X alloy powder

Abstract

Rapidly solidified pre-alloyed Al-17Si-5Fe-3.5Cu-1.1Mg-0.6Zr (wt.%) powder was mechanically milled using stearic acid and ethyl acetate as process control agents. The resulting nanocrystalline powder was degassed in a furnace at 350 degreesC for 5 hrs in vacuum. Both as-milled and degassed powders were consolidated using field-assisted sintering (FAST) in steel dies in vacuum at 375 to 500 degreesC. Some experiments were also carried out with an additional in-situ degassing step at 470-500 degreesC for 1 to 5 minutes. Shrinkage along the pressure axis, temperature and applied pressure were automatically stored through a data acquisition system. As-milled powder and sintered compacts were characterized using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and microhardness measurements. The crystal size, calculated from the line broadening of the X-ray diffraction peaks using Williamsonhall plots, of the mechanically alloyed powders and FAST sintered mechanically alloyed materials was in the range of 23-70 nm, depending on the type and amount of process control agent used during milling and the FAST parameters. As-milled powders with furnace degassing (350 C/5h) could not be fully densified due to remaining entrapped gasses. An in-situ degassing (500 degreesC/5min) step included during FAST sintering however resulted in the consolidation of fully densified bulk nanostructured compacts. Microstructure and hardness showed strong dependency on the type of process control agent used during milling.status: publishe