Investigation On The Effect Of Annealing Induced Hardening For Commercial Pure Aluminum Processed By Severely Plastic Deformation

The mechanical properties and micro-structural of commercial pure aluminum (99.7%), with grains size in range of sub-micrometer, processed by methods of severe plastic deformation (SPD) before and after the annealing are respectively studied by uniaxial tensile test and transmission electron microscopy. From the tensile testing experiments with the tensile axis along both direction of 0°, 90°from the rolling direction , the cold-rolled aluminum to large equivalent strain (from 40mm to 1mm with ?V M?4.3) exhibit obvious hardening phenomenon after furnace annealing treatment, namely the yield stress and ultimate tensile stress both increase while the ductility decreases, which is totally opposite to the traditional materials with coarse grains.From the aspects of different equivalent strains, annealing temperatures and times, , An arrange of furnace annealing experiments are conducted to find out the critical point from annealing hardening to annealing softening. It is found that along with the decline of equivalent strain, the degree of annealing induced hardening decreases along with the decline of equivalent strain, with the increase of annealing temperature or time, the cold rolling samples with large equivalent strain (ε_VM=4.3) exhibit the evident turning point from the annealing induce hardening to annealing induce softening which is in the range of 150℃to 200℃with annealing time in 30min or in the range of 30min to 2h with annealing temperature at 150℃, the optimal heat treatment for furnace annealing hardening is at 150℃+30min. Based on the microstructures observation of different samples, it can be calculated that the grain size and dislocation density are both critical role in appearance of annealing induce hardening for sub-mircon materials. .Tensile behavior of pure aluminum under electric pulse annealing treatment was also studied at room temperature. interestingly, The strength increase becomes more pronounced if electric pulse annealing (EPA) is applied on as-rolled aluminum for a short period of time, its optimal annealing time and temperature to get the best combination of strength and ductility are respectively reduced from 30min to 2min and from 150℃to 127℃compared to common annealing. Microstructure characterizations indicate that the origin of this annealing induced hardening could be attributed to a"dislocation source-limited hardening"mechanism, and that the more pronounced effect by EPA treatment is a result of the significant reduction of dislocation density without simultaneous grain growth. Though the stress relaxation tests, the activation volumes of samples under situation are calculated, it was found that the physical activation volume of the annealed samples are larger than those of as cold rolled, and that of EPA samples is larger than that of furnace annealed samples. It is postulated that"the limit-grain boundary dislocation source hardening"is the essential mechanism of annealing induced hardening.