Simulations of Grain Refinement in Various Steels Using the Three-Scale Crystal Plasticity Model
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NTRODUCTION
OBTAINING very small grains in polycrystalline metals and alloys has been attracting increasing attention of scientists and engineers mainly due to the involved improvement of the materials strength. Specifically, it was shown that the Hall–Petch relationship coupling the inverse of the square root of the grain size with the increasing yield strength is valid down to grain sizes as small as 10 to 25 nm.[1] Therefore, in the range of grain sizes attainable by conventional or severe plastic deformation processes, decreasing grain size should normally lead to considerable improvement in the strength. Many authors studied the grain-refinement (GR) phenomenon occurring in severely deformed metals and alloys. However, most of the researchers were concerned with experiments and modeling of singlephase face-centered cubic (FCC) materials, such as copper and aluminum.[2,3] In such materials, grain refinement at large plastic strains can occur by continuous dynamic recrystallization (CDRX), where the misorientations between dislocation cells continuously increase leading to the formation of low- and high-angle grain boundaries (LABs and HABs). One should be careful not to mistake the CDRX with the ordinary or discontinuous dynamic recrystallization (DDRX) which is a process of nucleation and growth of new grains. For an extensive comparison and overview of CDRX and DDRX, see References 4 through 6.
KAROL FRYDRYCH is with the Institute of Fundamental Technological Research (IPPT), Polish Academy of Sciences, Pawinskiego 5B, 02-106 Warsaw, Poland. Contact e-mail: [email protected]. Manuscript submitted February 21, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
On the other hand, considerably less attention was paid to grain refinement in steels, despite their wide range of industrial applications. It seems that there are two reasons for this situation. First, tracking the microstructural evolution in single-phase FCC metals is relatively easy due to small number of phenomena influencing the process. On the contrary, in steels, there can be single or multiple phases, with different crystallographic structures, e.g., FCC austenite, body-centered cubic (BCC) ferrite and highly strained body-centered tetragonal martensite. This is challenging mainly from the point of view of the modeling. While it is not very hard to deal with single-phase FCC austenitic steel, considering microstructural evolution of dual-phase (DP) steels, especially in the presence of phase transformations, demands more effort. Second, the term ‘‘steel’’ covers a wide range of materials. Thus, it is much easier to compare studies by different groups conducted, e.g., on copper (since it is a more or less similar material in every case) than to compare studies conducted on steel. One has to choose a particular steel, and there is little likelihood that a different group studied the same material. The microstructural evolution in various stainless steels subjected to cold and warm deformation was experimentally studied in the series of papers by Belyakov
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