The size dependent strength of Fe, Nb and V micropillars at room and low temperature

The strength of focussed ion beam (FIB) machined [001] orientation single crystal micropillars of body centred cubic structured (bcc) metals Fe, Nb and V, with diameters 200-5000 nm, have been measured at 296 K and 193 K using nanoindentation equipment. The metals have similar critical temperatures for screw dislocation mobility, T-c, close to 350 K. All three metals deform on the (110) plane at both test temperatures and show very similar behaviour of the plastic flow stress, sigma(p), normalised by the shear modulus resolved on the slip plane, mu, as a function of pillar diameter, d, normalised by the Burgers vector, b. At room temperature all metals show a size effect on strength described by power law relation sigma(p)/mu = A(d/b)(n) with n approximate to -0.6 and this is shown to mirror the behaviour of face centred cubic metals. At 193 K the three metals show a higher strength and larger size exponent n approximate to -0.3 and again their normalised flow data is very similar. The behaviour is consistent with the observation that the deformation size effect in bcc materials converges to that seen with face centred cubic (fcc) structure metals as testing temperature approaches T-c. The temperature and size dependence of the experimental data shows similarity with the prediction of strength models based on the geometric confinement of dislocation sources (the single dislocation arm model) and a temperature dependent friction stress. However the model cannot consistently capture the behaviour for the three metals studied.