Etude du frottement interieur dû aux joints de grains dans le magnesium

Abstract

We have studied the variation of internal friction in magnesium as a function of temperature in the range 20–350 °C by means of a low-frequency pendulum as developed by Ké. We used doubly sublimed magnesium containing less than 500 ppm of impurities, and also performed some experiments with zone-refined magnesium and with commercial (99.8%) magnesium. With the annealed doubly sublimed magnesium we observed around 185 °C at one c/s an internal friction peak which presents all the characteristics of a relaxation peak, and which we interpret as a grain-boundary peak. The activation energy for this peak, measured by changing the frequency, is 31.7 kcal/mole, whereas the activation energy for volume self-diffusion is 32 kcal/mole. The width of the peak exceeds the theoretical width by a factor of 2. We have studied the influence of irradiation and of deformation on the grain-boundary peak. After neutron-irradiation at low temperature (9 × 10 18 fast neutrons/ cm 2) the peak is slightly displaced towards higher temperatures and its profile is appreciably altered. When a specimen was plastically deformed in compression at low temperature, a very large increase in the internal friction was observed at the normal peak temperature, and this completely masked the grain-boundary peak itself; the appearance of this appears to be linked to recrystallisation.