Mechanical properties of ultra fine grained steels

Mechanical milling (MM) of iron powder is one of the useful techniques to achieve ultra grain refining to nanosize. For instance, MM treatment using high energy ball-mill makes the grain refining to 20-30nm possible and such an ultra grain refining results in marked hardening of iron powder to around Hv 9.5GPa. On the annealing of the powder with MM treatment (MM powder), very fine oxide particles (Fe₃O₄) play an important role to keep the grain size fine through the effect of grain boundary pinning. Thus, MM powder with sufficient initial hardness can be successfully consolidated at around 950K with keeping the grain size below 1μm. Tensile strength of consolidated bulk iron increases with grain refining and Hall-Petch relationship was realized to about 0.2μm as to 0.2% proof stress (σ_0.2(MPa) = 100+0.6d^-1/2; d (m)). In the test pieces with the grain size less than 1μm, uniform elongation completely disappears due to the marked strengthening. On the hardness, Hall-Petch relationship was also confirmed to around 0.1μm at least (Hv(MPa) = 330+2.0d^-1/2). In the grain size range below 0.1μm, however, experimental data tended to deviate toward lower side from the Hall-Petch line and hardening was seemed to level off at around Hv 12GPa. Ductile brittle transition temperature was lowered with grain refining (DBTT(K) = 330-0.33d^-1/2) but upper shelf energy becomes smaller when the grain size was refined below 5μm.