Yield point phenomenon caused by the interaction between solute carbon atoms and dislocations and the associated inhomogeneous Lüders deformation, are well-known to occur in iron and low carbon ferritic steels. The study of Lüders deformation is of scientific significance since it is related to the fundamental aspects of plastic deformation in metallic materials. In recent years, digital image correlation (DIC) techniques have been widely used to characterize heterogeneous deformation such as Lüders deformation in metals. Although it has been well documented that Lüders deformation in ferritic steels becomes more significant with decreasing grain size, quantitative details of the grain size effect on Lüders deformation have rarely been studied. Hence, the aim of this study is to clarify the effect of grain size on the yielding behaviour and the formation and propagation of Lüders band in polycrystalline high purity iron by investigating the tensile deformation behaviour with DIC analysis.

Specimens with three different mean grain sizes, i.e. coarse-grain (CG: 23.9 μm), fine-grain (FG: 3.1 μm), and ultra-fine-grain (UFG: 0.56 μm), are selected to show the grain size dependence of yielding behaviour in the material. The CG specimen exhibited a typical yield drop and stress plateau followed by normal strain hardening behaviour. In the FG specimen, the flow stress interestingly kept decreasing after the yield drop until the nominal strain reached about 20%, and then macroscopic necking followed by tensile failure occurred. In the UFG specimen, macroscopic necking occurred immediately after the yield drop, leading to a limited tensile elongation. In the CG specimen, Lüders band front propagated through the whole gauge part and plastic strain of the specimen was introduced by passing of the band front. In the FG specimen, a huge Lüders strain of 15% was observed. A notable necking occurred behind the Lüders band front even though the band was still propagating towards the other side in the gauge part. Such a simultaneous occurrence of two types of plastic instability (Lüders deformation and macroscopic necking) interpreted the decreasing nominal stress from lower yield point of the specimen. In the UFG specimen, severe strain localization occurred with the formation of Lüders band, but the band could not propagate through the gauge part. The grain size dependence of Lüders deformation behaviour will be systematically discussed in the context of strain localization caused by the yield drop and following strain-hardening, and an explanation of the propagation mechanism of Lüders band will be suggested in the presentation.