SEM‐TEM
study of low carbon steel subjected to conventional and severe plastic deformation
Authors: Vilotic Marko , Sidjanin Leposava , Rajnovic Dragan , Novovic Milorad 
Published: December 2016

It is well known that plastic deformation induced by different conventional metal forming methods can significantly increase the strength of a metal. This increase of strength, at the same time, is always accompanied by a loss of ductility. However, a better combination of high strength and ductility can be obtained by severe plastic deformation (SPD) [1].

In the present work, both methods of plastic deformation are concerned. One is the Rastegaev upsetting method (conventional method) where cylindrical samples of Ø20x20 mm were compressed between parallel plates with imposed plastic strain φ
R
=0.4; 0.8; 1.2; 1.6; 1.91 (~2.0). The other is the discontinuous SPD method, which our lab developed, for upsetting square shaped billet by V‐shape dies. In fact, the V‐shape dies compression is a multistage process in which the sample is removed from the die after the first compression turn, rotated for 90° in an anti‐clockwise direction, and then is returned into the dies [2]. The samples of 14x14x70 mm were compressed in eighteen turns with imposed strain from φ
SPD1
=0.39 up to φ
SPD18
=3.38. Maximum plastic strain of the conventional upsetting method (φ
R
=1.91) is achieved after 4 turns in case of a discontinuous SPD method (φ
SPD4
=2). The experimental compression tests for both of the methods were done on a hydraulic press using a normalized low carbon rod steel C15E with 0.14%C. The influence of the processing parameters on the microstructure was evaluated by SEM (JEOL JSM‐6460LV) and by TEM (FEI Tecnai F20). The TEM sample (cross‐sections) were prepared by FIB (Quanta 3D FEG) and in situ lift‐out technique.

Fig. 1 shows the starting SEM microstructure of the undeformed low carbon steel C15E in a longitudinal and transversal direction. The microstructure consists of 85% ferrite and 15% pearlite. The starting average grain size of the ferrite was 19.1 µm. After upsetting by the Rastegaev method the microstructure is highly deformed in the direction normal to the applied compression, Fig. 2a. The deformation is a homogeneous trough the sample. On the other hand, during SPD the microstructure is deformed only in the central part of the sample (Fig. 2b), while in the region further from the center (about 6 mm) the deformation appearance of the microstructure is hardly visible. Furthermore, while the effective plastic strain is similar for both upsetting methods (Rastegaev φ
R
=1.91 and SPD φ
SPD4
=2) the microstructure of the SPD samples appears less deformed when compared to the Rastegaev sample. However, the TEM observation of the SPD samples reveals that the microstructure consists of elongated ferrite sub‐grains having a width 0.2‐0.3 µm, Fig. 3. The band boundaries are mostly in the low‐angle misorientation and in the bands an interior dislocation cell boundaries are present. Additionally, beside a low‐angle boundaries, the formation of grains with the high‐angle boundaries could be noticed (indicated with arrows in Fig. 3). The presence of the newly formed equiaxed grains is confirmed by the diffraction ring pattern.