Prismatic slip in beryllium

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Abstract
In situ experiments have been performed on beryllium between 80 and 423 K, in order to find the exact origin of the anomalous increase in the critical resolved shear stress (CRSS) of prismatic glide, as the temperature is increased between 160 and 330 K. In the whole temperature range, including the domain of the stress anomaly, the results clearly show that the CRSS is controlled by the locking-unlocking mechanism as described in Part I of this study. As in part I, quantitative measurements are also performed at different temperatures. In particular, local stress measurements reproduce very well the shape of the macroscopic CRSS as a function of temperature. All these results show that the origin of the stress anomaly is at variance with all proposed models. It is an individual dislocation mechanism, corresponding to an increase in the difficulty of cross-slipping from basal to prismatic planes at increasing temperatures (entropy effect). This effect may be due to an intrinsic decrease in stacking-fault energy. The latter hypothesis is supported by observations and theoretical considerations made by several researchers. The possibility of applying these results to different materials and superalloys is discussed.