Size effects in small-scale structures of body-centered cubic metals

O. T. Abad

(2016) 152-152

Small-scale metal structures play a crucial role in a broad range of technological applications. However, knowledge of mechanical properties at this size scale is lacking. Size strengthening effects are generally experienced at the microscale. Compression of non-free defect body centered cubic (BCC) metal micropillars has revealed that the size effect of these metals scales with a temperature ratio that signifies how much the yield strength is governed by screw dislocation mobility. So far, no effort has been made to systematically study the effect of screw dislocation mobility and lattice resistance on the size effect in BCC-based metals. Thus, this work investigated this in BCC tungsten (W) and tantalum (Ta), as well as B2 beta-brass (β-CuZn) and nickel aluminide (NiAl). The influence of temperature on the size effect in W and Ta was studied up to 400 °C, whereas the room-temperature size effect in β-CuZn and NiAl was studied as a function crystal orientation and deformation rate. It was found that the size effect scaled with the magnitude of the lattice resistance, which is strongly related to the screw dislocation mobility. Direct evidence of the mobility of screw dislocations was observed for the first time. The results also showed that plastic anisotropy vanishes with decreasing sample size and that ductility is considerably improved, thus highlighting the importance of dislocation-nucleation controlled deformation and screw dislocation mobility at the sub-micron scale.