アルミニウム多結晶の引張り変形能におよぼすひずみ速度の影響

Abstract

In a temperature range from −196 to 400°C, a commercially pure polycrystalline aluminium, 2S-Al, was tested in tension to study its ductility as a function of strain rate over a range from about 10−3 to 102/sec. Findings in the present study are given as follows:(1) The total elongation measured in tension at a high strain rate, i.e., an impact tension, became larger than that measured in a quasi-static tension over the temperature range from −78 to 300°C, especially at room temperature to 200°C. This remarkable increase in elongation at a high strain rate may be attributed not to variations in grain boundary behaviours but to changes in behaviours of the matrix itself. The larger total elongation in the impact tension was brought, at least at room temperature, by the increase in both uniform and local elongations.The reduction in area increased gradually with temperature and no dip was observed on the reduction-temperature curves in all cases, though a deep dip around about 100°C was found on the total elongation-temperature curves obtained in tension at lower strain rates.(2) At room temperature, when the specimen was stretched incrementally to fracture, the total elongation was larger than that measured in a continuous tension, i.e., in a tension without unloading. The increase in total elongation due to an incremental deformation became larger in the impact tension and in tensions with larger repeated numbers of incremental deformation and with larger duration of unloading.(3) The total elongation at −196°C decreased abruptly beyond the strain rate of 1/sec. This decrease was due mainly to the decrease of local elongation. Since at −196°C the aluminium specimen has a small value of specific heat and its strength decreases rapidly with increasing temperature, a necked portion of the specimen subjected to a rapid tension at −196°C becomes remakably weakened under the effect of an adiabatic temperature rise, as compared with other portions which are not necked. The necking generated in rapid tension at −196°C is, therefore, restricted within a shorter length of the specimen, showing a local elongation smaller than that observed in a quasistatic tension at the same temperature.