Work hardening and flow stress of ultrapure molybdenum single crystals

The temperature dependence of the flow stress of molybdenum, deduced by means of two independent methods from tensile tests and stress-relaxation measurements on ultrapure single crystals (residual resistivity ratio 4×105, Schmid factor of the [111](1̄01) slip system μ=0.5) in the temperature range 4 K≤T≤460 K, is shown to agree with that obtained by the cyclic-deformation technique. The analysis in terms of the kink-pair theory of screw-dislocation mobility gives the following results. Below T̆=60 K the elementary slip steps take place on (1̄10) planes (Peierls stress of the a0〈111〉/2 screw dislocations σP{110}= (870±10) MPa, formation energy of a pair of isolated kinks 2Hk{110}=(0.63±0.03) eV). At T>T̆ slip proceeds by cross slip on (2̄11 and 1̄1̄2) planes (σp{112}=(690±20) MPa, 2Hk{112}=(1.27±0.03) eV).