Strengthening Mechanism In Non-heat-treat Strengthening Aluminium Alloy

Hall-Petch relationship can well describe boundary strengthening of coarse-grained materials, but is not suitable for nanostrutured metals. In the past few decades, most of strength models existed are described as the superposition of different strengthening mechanisms, which are too complex to predict the strength of metals. In this project, a “combined defect density” has been defined by relating dislocation density(ρ) to “boundary density(l)” which means total length of grain boundaties per unit area, and a modeling of which the strength of aluminum alloys can be quantitatively characterized by “combined defect density” has been systematically studied.Samples of 1060 pure Al and 5A06 alloy have been produced by cold rolling to different strain followed by a heat treatment, where different annealing processes have been used to produce samples with large variations in structural parameters such as boundary density and dislocation density. These parameters have been quantified by a structural analysis applying OM, TEM, EBSD and XRD, and the mechanical properties have been determined by tensile testing at room temperature. Strength-struture relationships have been analysed based on the operation of grain boundary strengthening and dislocation strengthening firstly, and then, the quantitative relationship between l and ρ has be analyzed. Finally, the strengthening mechanisms have been established by relating the yield stress to the “combined defect density”, according to the strength and structure data of aluminum alloy cold rolled to different strain.The results of OM implied that the heavier deformation combining with the lower temperature would lead to smaller equiaxed grains during annealing, and vice versa. Additionally, tensile tests show that the yield strength of aluminum alloy significantly raises with the increase of grain boundary density, however, the tensile strength only increases slightly. The yield strength of the pure aluminums follows a linear relationship with l1/2.Tensile tests of light deformation cold-rolled aluminum alloy followed with low temperature annealing indicates that, yield strength exhibited a rapid drop with the first 30 min of annealing and the elongation has a opposite trend. The OM, EBSD, and TEM analysis shows that parameters like grain size, misorientation angle and grain morphology have not changed during the annealing process, but the recovery of dislocations occurred significantly. The XRD data has been analysed by Williamson-Hall method, and the results indicate that the yield strength increases with dislocation densities following a linear relationship with ρ1/2.It has been proved that the high strength of deformed aluminum alloy is attributed to the combination of boundary strenthening and dislocation strengthenting, so that the strength modeling of deformed alloy can be described as 1/ 2 1/ 20s=s+K￠l +aGbr. Based on the assumption of the combined defect theory, the relationship between boundary density and dislocation density can be deduced as r =2ml/ p. According to the conversion of l1/2 and ρ1/2 the combined defects strength model of aluminum alloy can be established as ( )1/ 21/ 20s=s+ éK￠ p/ 2m +aGbùr? ?.