モリブデンの加工硬化

抄録

Stress-strain curves of sintered pure molybdenum were analyzed by the Crussard and Jaoul method that assumes the Ludwik equation, σ=σ₀+hεⁿ. Also, the effects of temperature (292∼550°K) and stain rate (10⁻⁶∼10⁻² sec⁻¹) on the constants h and n were studied. Obviously, the Crussard and Jaoul plot exhibited a “double-n behavior”. Both temperature and strain rate had no influence upon the parameters n₁, n₂ and h₁, while an increase in temperature resulted in the decrease in h₂ (h at ε>ε₁) and ε₁, the latter being independent of strain rate. The double-n behavior was discussed with the aid of a dislocation model reported by Bergström, and it was concluded that ε₁ did not correspond necessarily to the plastic strain for the formation of cell structures.
σ₀ (=σ−hεⁿ) was independent of the plastic strain, but varied strongly with temperature and strain rate. Again, σ₀ could be divided into two components, i.e. σ₀=σ^*(T, \dotε)+σ_μ1, where σ_μ1 was estimated to be 5.0 kg/mm². Subsequently, σ^* was calculated to be 23.3, 10.0 and 3.5 kg/mm² at 292, 350 and 420°K, respectively for \dotε=3.3×10⁻⁴ sec⁻¹, and they are in a good agreement with the previous results from stress relaxation tests. Finally, logσ^* versus log\dotε plots showed a good linear relation, and the dislocation velocity-stress exponent, m^* (=dln\dotε⁄dlnσ^*) was deduced to be 6.5 independent of test temperatures above 292°K.