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Microyielding and flow in niobium alloy crystals Wilson, F. Graham

Abstract

Oriented single crystals of niobium and dilute alloys with molybdenum and tantalum were deformed in tension between 77°K and 500°K, and the macroflow and slip parameters established. At high temperatures the main effect of alloying was to increase the flow stress, considerably more with molybdenum than with tantalum. The observed yield drop and subsequent plastic flow were explained in terms of a stability theory relating changes in yield and work hardening parameters with temperature and addition of solute. A technique was developed for measuring small plastic strains in the microflow region, and for recording the dynamic transition to macroflow. From studies on pure niobium between 77°K and 295°K, the nature of dislocation motion at small strains was established; microflow was explained in terms of a transition from edge dislocation motion to screw dislocation motion at the macroflow stress. Interstitial effects were found to be particularly significant during microflow, and are probably important in determining the low temperature flow stress in: even the highest purity bcc metals. A further low temperature contribution comes from a directional component of the internal stress field which depends on the distribution of dislocations rather than on their density. Microflow curves were obtained for niobium alloy crystals, land the interaction of dislocations with substitutional solute atoms established. In contradiction to previous suggestions, substitutional solute was found to restrict the mobility rather than the multiplication of dislocations. The elastic contribution of solute atoms to the internal stress field was confirmed, although a quantitative theory for bcc alloys does not yet exist. Peierls stress considerations alone were found to be incapable of explaining either the temperature sensitivity of flow or the low temperature solution softening; the short range interaction of interstitials with the dislocation core was considered to be more significant.

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