Grain Size Dependence of the Yield and Flow Stresses of Molybdenum in the Range from 300 to 500 K

Activation analysis of deformation in evaporated molybdenum thin films

“…Other researchers have shown that in bulk molybdenum, the value of A * measured using the flow stress is independent of grain size, at least for grain sizes above 1 µm [18]. A * measured in terms of the hardness (figure 13) for our present data of the SA films show that it is about 20b 2 and is nearly independent of grain size extending down to 25 nm.…”

“…dH/d ln εeff for the SA films are approximately equal to those of bulk molybdenum in the literature obtained using uniaxial loading. The literature data explore the effects of grain size on activation area [18] and work hardening on the rate sensitivity of the flow stress [17]. The activation volume for molybdenum is multiplied by a factor of 3 to convert the activation volume measured from the flow stress to activation volume measured from hardness [17].…”

“…Molybdenum is chosen as a model material because the properties of the bulk, large-grained material are well known and reasonably well understood [15][16][17][18][19][20]. Near room temperature, the glide of screw dislocations rate-limits plastic deformation in bulk molybdenum, giving rise to a Peierls stress that is sensitive to both strain rate and temperature.…”

“…The Peierls stress adds toand is independent of-the other two flow stress contributions. Experimentally, the key evidence for this additive rule is the fact that the derivative of the flow stress (τ ) with respect to strain rate ( γ ) remains constant, independent of the level of work hardening [17] and grain size in large-grained materials [18]. Excluding grain size effects, it is often stated that the flow stress of molybdenum consists of a sum of two components: τ * , the Peierls or friction stress, and τ I , the internal stress.…”

“…Here k B is Boltzmann's constant, T is the absolute temperature, and b is the Burgers vector. For molybdenum at room temperature and at a strain rate of 10 −4 s −1 , A * is about 20b 2 [15,18]. A * is usually interpreted (perhaps rather crudely [23]) as the area swept out by dislocations during thermal activation.…”