| Popis: |
An electron microscope study has been made of the dislocation arrays produced in the vicinity of second-phase particles in aluminium crystals, using an alloy of Al-0.5 wt.% Si-0.45 wt.% Cu aged to contain equiaxed silicon particles. At low temperatures the yield stress increment is consistent with the Orowan bypass mechanism, and the dislocation structures at small (~0.12 μm) particles are predominantly of primary Burgers vector, consisting of rows of prismatic loops and helices. There is a sharp fall in yield stress at temperatures between 0.45 and 0.5 T m , and the hardening rate in the parabolic region of the stress—strain curve is strongly temperature dependent, falling off at ~0.35 T m . The reduction in work-hardening rate is accompanied by a reduction in the number of primary prismatic loops observed at the particles. Dislocation arrays involving secondary Burgers vectors are produced at Si particles greater than ~0.12 μm in diameter, and as the particle size increases, the number of secondary Burgers vectors operating increases. The effect of increasing the temperature of deformation is to increase the critical size for secondary dislocation generation. The results at elevated temperature are consistent with the operation of a dislocation pipe-diffusion mechanism, as has previously been reported for copper-based alloys. |
