The influence of nano/micro sample size on the strain-rate sensitivity of plastic flow in tungsten

The plastic flow characteristics of body centered cubic metals, which is dominated by the motion of screw dislocations, generally exhibits strong strain rate dependence. In-situ scanning electron microscopy compression tests are performed on tungsten (W) single crystal nano and micro pillars, with diameters in the range 100-2000 nm. We demonstrate here that by reducing sample size, the strain rate sensitivity of plastic flow, including not only the yield strength but also the strain burst statistics, is reduced. It is found that the strain rate sensitivity of the flow stress scales with the sample size as 0.67. Statistical analysis of the strain burst size displays truncated power law scaling with a lower exponent at higher strain rate for micron size pillars. Nano size pillars (<500 nm) show near universal scaling exponent of 1.50 +/- 0.07 at all applied strain rates. The strain hardening rate and deformation morphology are found not to be very sensitive to the strain rate. This external pillar sample size effect is further compared with the internal grain size in nanocrystalline W in order to guide the design of new materials by tuning the characteristic length scale (external or grain size)