1 Introduction
T
he processing and properties of ultrafine-grained (UFG) materials have generated considerable interest recently from both an industrial and a scientific standpoint. The impact of structure refinement to a submicron level on the substantial improvement of mechanical and performance properties of structural materials has been generally established.[
1] UFG metals and alloys exhibit extremely good mechanical properties characterized by high mechanical strength and good ductility,[
2]] as well as a possibility of superplastic forming at low temperature.[
3] Furthermore, a large fraction of high-angle grain boundaries (HAGBs) in a volume of bulk material is a key factor in the faster kinetics of diffusion-based phenomena leading to,
e.g., better corrosion resistance of UFG materials, as compared to their coarse-grained counterparts.[
4]
Severe plastic deformation (SPD) methods are one of the most efficient ways to produce bulk UFG metals. In these techniques, an extremely high strain is imposed on a material in order to obtain the structure refinement effect through formation of intersected shear bands[
5] or a dynamically activated recovery or recrystallization process.[
6]
So far, there are a few hydrostatic SPD methods that have been successfully used to fabricate UFG metals and alloys (
e.g., equal channel angular pressing, cyclic extrusion compression, or high pressure torsion). These and other methods were described recently in a comprehensive review article by Estrin and Vinogradov.[
7] However, despite positive results in terms of structure and properties control, hydrostatic SPD techniques have rather poor industrial potential due to the small size of processed samples and low manufacturing efficiency. Thus, development of SPD methods that are based on highly efficient, continuous plastic deformation processes has generated considerable research interest.
In recent years, some new SPD methods based on modified conventional manufacturing processes have been proposed. Differential speed rolling (DSR) is a modification of the rolling process that includes an application of different values of rotational speed for upper and lower rolls. An implementation of asymmetry of the rolling gap leads to an imposition of a high through thickness shear strain to the material. As a consequence of the change of the deformation gradient, the effects of both higher accumulated strain and quantitative/qualitative alteration of crystallographic texture, compared to a normal rolling process, are observed. Furthermore, due to the continuous character of the rolling process, the DSR method may be easily adopted to large scale industrial processing of materials. DSR has been widely analyzed as a tool of strain-induced microstructure refinement and as a method of crystallographic texture modification (and, thus, an improvement of anisotropy in the mechanical properties of rolled products).
The grain refinement effect by the DSR method was previously observed in numerous pure metals and alloys. Results reported by Zuo
et al.[
8] show that very fine grains, 50 nm in size, are obtained in pure aluminum through cold rolling with differentiation of roll speed. Furthermore, experimental results described by Kim
et al.[
9] revealed that commercially pure titanium subjected to a combination of warm and cold DSR processes is characterized not only by an UFG structure and high strength but also by superior corrosion resistance, with the corrosion rate in acid solutions almost two times lower than that of coarse-grained material. Additionally, in the case of precipitation-strengthened AZ91Mg-based alloy, an application of DSR results in both an ultrafine grain size (~300 nm) and more uniform distribution of strengthening particles upon postdeformation low-temperature aging treatment.[
10] The results of our previous study on commercially pure copper[
11] also point out that a shear strain imposed during rolling with different values of roll velocity allows obtainment of material with grain size in the submicron range and a high fraction of HAGBs, indicating dynamic transformation of the microstructure. Moreover, we recently showed that the DSR method also may be successfully applied to improvement of mechanical properties of hardly deformable materials such as intermetallic-based alloys.[
12]
Despite the limited reported data, similar results were also presented in a few articles on the DSR-processed low carbon steel. Suharto and Ko[
13] reported that the initial ferrite grain size of 35
μm was significantly refined to 700 nm after DSR processing under the roll speed ratio of 1:4 for lower and upper rolls. An even greater effect of grain refinement (to 500 nm) was presented in the recent work of Hamad
et al. on interstitial-free (IF) steel subjected to DSR operations.[
14] Additionally, these authors concluded that the finest and most uniform grain structure was obtained when a high value of reduction per pass was applied in the DSR process.
As previously mentioned, the DSR process was also successfully applied in the field of controlling the crystallographic texture of engineering materials. This issue is especially important for aluminum and magnesium alloys, which, due to formation of unfavorable rolling and recrystallization textures, possess a worse susceptibility to deep drawing than widely applied low carbon steels. However, it has been shown that a proper selection of DSR parameters allows improvement of the mechanical property anisotropy of these materials. Jin and Lloyd[
15] showed that an introduction of additional shear components (a so-called
γ-fiber {1 1 1}//ND in Euler angles space) to the deformation texture of AA5754 aluminum alloy also prominently affects the recrystallization texture formed during subsequent heat treatment. The presence of
γ-fiber inhibits the growth of the undesired {0 0 1}〈100〉 cubic component, which is recognized as the main reason for low susceptibility to deep drawing of aluminum alloys designed for car body applications. On the other hand, Lee
et al.[
16] reported that the DSR process not only enhances the plasticity of AZ31 magnesium alloy as compared to conventional rolling but also leads to a simultaneous improvement of in-plane isotropy through facilitating the activation of prismatic slip during deformation.
In the present article, existing experimental data are supplemented with the results of our electron backscatter diffraction (EBSD) examination of the microstructure, the texture, and the internal lattice strain evolution of Armco iron during DSR processing.
4 Conclusions
Generally, application of the DSR process leads to higher strengthening of the investigated Fe Armco than does application of the conventional rolling process, as confirmed by the results of microhardness measurements. Additionally, lower standard deviation values point to a more homogenous deformation course upon use of the DSR method. This strengthening effect results from structure refinement, which is a consequence of the DSR process; samples of iron processed with a high roll speed mismatch were characterized by a high fraction of grains with submicron size ~70 pct for the highest considered value of the R parameter. Based on the obtained experimental results and supported by appropriate data from the literature, it may be concluded that the structure refinement is an effect of both the shear strain-induced grain subdivision and the temperature rise, which supports activation of a subgrain evolution via continuous dynamic recovery. Results of the texture examination show that DSR processing of iron leads to an overall texture weakening and a displacement of bcc normal rolling texture components, rather than to a formation of completely new ones. Due to a change in the deformation gradient (an additional presence of a shear strain), main texture fibers undergo a shift to more stable positions. The results of EBSD lattice strain analysis show that, despite the higher hardness of the DSR samples, the estimated lattice strain (expressed as the minimum deformed fraction) decreases with an increase of the roll speed mismatch. This finding indicates that, upon the DSR process, some part of the storage energy of deformation is released due to a dynamic transformation of the material structure into a more stable state. However, since the observed structural changes take place inside deformation bands and do not lead directly to the formation of a fully equiaxed grain structure, it seems to be more reasonable to call the observed structure transformation a subgrain structure evolution through accumulated shear deformation, which may be related to the dynamic recovery process.