Dislocation Source Sensitivity of Plasticity and Fracture in Tungsten

We have examined the effect of dislocation sources introduced on the surface of single-crystal and polycrystalline tungsten on the plastic flow and fracture behavior at temperature in the range 77 K to 590 K. The experiments utilize the phenomenon of surface film softening observed in many body-centered cubic metals, such as observed by Sethi and Gibala [

1

–

3

]. In investigations like these, it is observed that application of surface films of approximately 50–200 nm in thickness to bcc metal substrates can decrease the yield and flow stress, increase the ductility, and correspondingly reduce the large temperature dependence of the yield and flow stress at homologous temperatures T below approximately 0.15T

m

, where T

m

is the absolute melting temperature. The large temperature dependence of the yield and flow stress at T/T

m

< 0.15 is associated with the high Peierls-Nabarro stress of screw dislocations in the bcc structure. By contrast, edge dislocations in bcc metals have high mobility at low temperatures. Mechanistically, it has been shown that for the coated materials under applied stress, large densities of mobile edge dislocations can be generated in the substrate metal at the film-substrate interface. These edge dislocations can move into the substrate and effect plasticity at the reduced flow stresses observed. In this investigation, additional use is made of surface modification (roughening) of the substrate surface to afford control over the density of potential dislocation sources at the film-substrate interface in coated materials. The results are described below.