Work-Hardening Model for the Effect of Grain Size on the Flow Stress of Niobium Strained by Rolling

摘要

Effect of grain size in the range of 32 to 435 microns on the dislocation density and structure and the flow stress of 125-microns niobium foil rolled various amounts at room temperature was investigated. For a given grain size d the dislocation density increased in an approximately linear manner with the rolling reduction, the rate of increase being proportional to 1/d. The shear flow stress tau increased with the square root of the dislocation density independent of grain size in accord with the relation tau - tau sub f + 0.60 G b rho to the 1/2 power. Effect of grain size on flow stress is thus explained in terms of a work-hardening model based on long-range stress fields of dislocations, rather than a Hall-Petch pile-up model. The strain at which a well-defined cellular network of dislocations first developed increased with an increase in grain size. Once the cell structure formed, the cell size tended to decrease with further straining. The cell size at a rolling reduction of 10% varied approximately as d to the 1/3 power. The results on the rolled niobium foil are compared with those obtained on identical material strained in tension. Although there are some differences between the two methods of straining, the general behavior is similar. (Author)