The effect of strain rate on the mechanical properties of AA5xxx series aluminum alloys containing solute Mg atoms (AA5005, AA5021, AA5082 and AA5182) and pure aluminum (A1070) was investigated within a wide strain rate range of 1.0 × 10−4 to 1.0 × 103 s−1 at room temperature. The A1070 exhibited a positive strain rate dependence of material strength at the investigated strain rates. However, the AA5xxx series aluminum alloys primarily exhibited the negative strain rate dependence of material strength and serration caused by the Portevin-Le Chatelier effect on the Mg content and strain rate. As a result of using the material constitutive equation for the negative strain rate dependence, it was found that the flow stress may change in the dynamic strain rate range. However, it was found that the strain rate dependence of material strength differed in the AA5082 and the AA5182 alloys. It would be caused by less solute Mg of the Al phase in the AA5182 alloy than in the AA5082 alloy, because more Mg2Si compounds precipitated on Mn bearing particles as precipitation sites in the AA5182 alloy.

Indentation tests are used to determine the local mechanical properties of materials. Previously, the indentation strain rate was correlated with the strain rate in uniaxial tests based on the hardness, which was the obtained load divided by the cross-sectional area. However, the hardness can be influenced by pile-up of material after indentation. The purpose of this study was to relate the indentation strain rate with the uniaxial strain rate through serration behavior. The material used in this study was 5082 aluminum alloy, whose main alloying elements are aluminum and magnesium, and which is known to exhibit serration at certain temperatures and strain rates. Quasi-static uniaxial tensile tests were performed at strain rates from 10 -4 to 10 -1 s -1 at room temperature. Micro-indentation using a Berkovich indenter was performed at constant loading rates from 0.7 to 350 mN/s. The loading curvature, which was defined as the load divided by the square of the displacement, was used instead of the hardness to avoid the pile-up effect. As a result, the serrated loading curvature in the indentation tests was obtained as the decreasing loading rate. The effective strain rate, which was defined as the derivative of the load with respect to time divided by two times the applied load, decreased with increasing displacement. The serrated loading curvature changed its behavior as the effective strain rate decreased. It behaved similarly to the serration observed in uniaxial tensile tests. It was found that the indentation strain rate is correlated with the strain rate in uniaxial tensile tests through the serration behavior.

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The effect of strain rate on mechanical properties of Al-2.3wt.%Mg alloy (AA5021) and commercial pure aluminum (purity 99.7wt.%: A1070) was investigated at room temperature. The tensile tests were conducted at strain rates from 1.0×10−4 to 1.0×103 s−1. The universal testing machine was used for strain rate 1.0×10-4 to 1.0×10−1 s−1. For the strain rate 1.0×100 s-1, the servohydraulic testing machine, which was developed by our laboratory, was used. The impact strain rate 1.0×103 s−1 was obtained using the split Hopkinson pressure bar method. The pure aluminum showed positive strain rate dependence of material strength at the investigated strain rates. In contrast, the Al-2.3wt.%Mg alloy showed the negative strain rate dependence at strain rates from 1.0×10−4 to 1.0×100 s−1. However, Al-2.3wt.%Mg alloy showed the positive strain rate dependence at strain rates from 1.0×100 to 1.0×103 s−1. It was surmised that the effect of dislocation locking by the solute Mg atoms became negligible at strain rate of approximately 1.0×100 s−1. It was confirmed that material properties for the Al-Mg alloy at the strain rate of 1.0×100 s−1 were important, since the strain rate dependence changed negative to positive around this strain rate.

Abstract. Indentation is widely used to investigate the elastic and plastic properties of mechanical materials, which includes the strain rate sensitivity. The indentation exhibits an inhomogeneous strain distribution in contrast to compression and tensile tests with homogeneous deformation. Thus, the strain rate of the indentation may form the inhomogeneous distribution. Therefore, the effect of strain rate distribution of the indentation on pure aluminum with respect to the strain rate dependence of strength in order to clarify the effect of the strain rate on the indentation technique. First, the numerical simulation was established using the Cowper-Symonds equation as the dynamic constitutive equation. Secondary, the strain rate distribution was calculated from the equivalent plastic strain distribution. The strain rate distribution was quite different from the strain distribution, which showed that the strain rate at the crater rim was higher than that beneath the indenter. Finally, we try to perform the averaging of strain rate distribution in order to make an index of strain rate in the indentation. The average of strain rate distribution was calculated using the equivalent plastic strain above a boundary value that is the critical strain and the representative strain. There is correlation between the average strain rate and the loading curvature, which shows that the average strain rate can express as the representative of strain rate for the indentation technique.