Size and microstructural effects in the mechanical response of α-Fe and low alloyed steel

Compression experiments on pillars have been performed to study the mechanical response of α-Fe and DC04 steel with a focus on the effects of sample size and microstructure. DC04 steel is a low alloyed deep drawing steel that has the same crystal structure as α-Fe. Both materials exhibit a strengthening dependent on the pillar size similar to what is known for many bcc and fcc metals. The comparison of single crystalline with polycrystalline pillars shows that grain boundaries have only a weak effect on the strength of these metals. The detailed size scaling depends on sample orientation, the effect of alloying elements or the existence of particles in the material. Investigations on pillars made from cold rolled steel revealed a strong effect of plastic pre-strain on the strength and its size scaling. In pillars with sizes below ∼1 μm stored dislocations lead to a decrease in strength compared to heat treated steel whereas in larger pillars a strengthening due to the stored dislocations was found. Overall, only a weak dependence of the flow stress on sample size was measured for predeformed DC04 samples. This study shows that many factors influence the size dependent behavior of metals. The results indicate that the exponential exponents which are commonly used for describing the size scaling of the strengthening should only be used with care.

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