Strain rate sensitivity and ductile-brittle behavior of polycrystalline Fe-Si alloys with 2.5, 3.5, and 4.5 wt pct Si

The strain rate sensitivity of the tensile properties of polycrystalline Fe-Si alloys with 2.5, 3.5, or 4.5 wt pct Si and a C+N content of 0.005 to 0.010 wt pct has been determined at room temperature. From these and previous results the influence of silicon on the athermal and thermal components of the yield (proof) stress of ferrite has been deduced. Both components increase with silicon content, but at the highest strain rates (and lowest temperatures) the thermal component varies little with silicon content and is smaller than that reported for pure iron. Transmission electron microscopy of ductile alloys has shown that the density and arrangements of dislocations are not greatly influenced by silicon content or strain-rate within the range investigated. At room temperature the dislocation velocity exponent,m*, increases with silicon content, being 4.5, 7.4, and 10.6 (±1.0) in the three alloys. For the grain size studied (5 to 7 ASTM) it appears that the stress for yielding by glide must not exceed about 700 MN/m² (100 ksi) if brittle behavior is to be avoided. The combinations of temperature and strain rate at which the ductile-brittle transition occurs are indicated for different silicon contents and different grain sizes. Formerly with the same Institute