a b s t r a c tA crystal-plasticity finite-element analysis of the loading-unloading process under uniaxial tension of a rolled magnesium alloy sheet was carried out, and the mechanism of the inelastic response during unloading was examined, focusing on the effects of basal and nonbasal slip systems. The prismatic and basal slip systems were mainly activated during loading, but the activation of the prismatic slip systems was more dominant. Thus the overall stress level during loading was determined primarily by the prismatic slip systems. The prismatic slip systems were hardly activated during unloading because the stress level was of course lower than that during loading. On the other hand, because the strength of the basal slip systems was much lower than that of the prismatic slip systems, the basal slip systems would be easily activated under the stress level during unloading in the opposite direction when their Schmid's resolved shear stresses changed signs because of the inhomogeneity of the material. These results indicated that one explanation for the inelastic behavior during unloading was that the basal slip systems were primarily activated owing to their low strengths compared to that of the prismatic slip systems. Numerical tests using the sheets with random orientations and with the more pronounced texture were conducted to further examine the mechanism.

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a b s t r a c tThe collision of single water droplets with a hot Inconel 625 alloy surface was investigated by a two-directional flash photography technique using two digital still cameras and three flash units. The experiments were conducted under the following conditions: the pre-impact diameters of the droplets ranged from 0.53 to 0.60 mm, the impact velocities ranged from 1.7 m/s to 4.1 m/s, and the solid surface temperatures ranged from 170°C to 500°C. When a droplet impacted onto the solid at a temperature of 170°C, weak boiling was observed at the liquid/solid interface. At temperatures of 200 or 300°C, numerous vapor bubbles were formed. Numerous secondary droplets then jetted upward from the deforming droplet due to the blowout of the vapor bubbles into the atmosphere. No secondary droplets were observed for a surface temperature of 500°C at the low-impact Weber numbers ($30) associated with the impact inertia of the droplets. Experiments using 2.5-mm-diameter droplets were also conducted. The dimensionless collision behaviors of large and small droplets were compared under the same Weber number conditions. At temperatures of less than or equal to 300°C, the blowout of vapor bubbles occurred at early stages for a large droplet. At a surface temperature of 500°C, the two dimensionless deformation behaviors of the droplets were very similar to each other.

This paper presents the work-hardening behaviors of a rolled AZ31B magnesium alloy sheet during in-plane cyclic loading. The overall trend of the stress-strain curve was as follows. As established before, the yield stress under compression was considerably less than that under tension, and an inflected shape was observed in the stress-strain curve during the subsequent tension. Furthermore, an asymmetric evolution of work-hardening was observed as follows. The rate of work-hardening in the late stage of compression became gradually large with an increase in the number of cycles. Owing to this increase in the rate of work-hardening, the stress at the end of compression increased as the number of cycles increased. On the other hand, the rate of work-hardening in the late stage of tension became small as the number of cycles increased, yielding a decrease in the stress at the end of tension with the increase in the number of cycles. The results were almost the same when the cyclic loading test was carried out after tensile strain was applied to the sheet. On the other hand, when the cyclic loading test was carried out after compressive strain was applied, the increase in the rate of work-hardening in the late stage of compression was significantly more pronounced, whereas the inflected shape of the curve during tension was considerably less pronounced. The mechanisms of the above macroscopic behaviors were investigated in terms of twinning.

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