A review of dynamic recrystallization phenomena in metallic materials
Graphical abstract
Introduction
Most of the metallic parts have been, during their processing cycle, subjected to hot deformation, during which dynamic recrystallization (DRX) often takes place. The final microstructure and mechanical properties of the alloys are largely determined by the recrystallization and related annealing phenomena, and the research on recrystallization can date back to 150 year ago [1]. The fast development of the DRX theory from 1960s was summarized by McQueen in 2004 [2]. A lot of important factors can have a significant effect on DRX, these include the stacking fault energy (SFE), the thermo-mechanical processing (TMP) conditions, the initial grain size, chemistry and microchemistry of the material in terms of solute level and second phase particles etc., which is also the reason why a vast amount of related works can be found in the literature.
During hot deformation, discontinuous dynamic recrystallization (DDRX) is frequently observed for low SFE materials, where nucleation of new strain-free grains occurs and these grains grow at the expense of regions full of dislocations. Cell or subgrain structures with low angle grain boundaries (LAGBs) are formed during deformation for materials with high SFE due to the efficient dynamic recovery, they progressively evolve into high angle grain boundaries (HAGBs) at larger deformations, a process which is known as continuous dynamic recrystallization (CDRX) [1]. Besides DDRX and CDRX, another relatively new concept of geometric dynamic recrystallization (GDRX) was also observed on deforming aluminium to large strains at elevated temperatures. In this case, the deformed grains become elongated with local serrations but remain distinguishable during deformation to large strains unless their thickness is below 1–2 subgrain size, at which time the developed serrations become pinched off and equiaxed grains with HAGBs are formed. Substantial grain refinement is thus obtained through the grain elongation and thinning. Numerical models are developed for these three types of DRX, most of them are focused on DDRX [3], with sparse models on CDRX [4] and mostly unexploited GDRX models [5].
It should be noted that there is no strict dividing line between these three types of behaviors. For example, CDRX was observed during rolling of fine-grained 304 austenitic stainless steel which is of low SFE [6], DDRX was reported for high SFE high purity Al [7], CDRX and DDRX can even co-exist during hot working of Mg3Al1Zn [8] or duplex stainless steel [9]. When designing new alloys, the addition of alloying elements to the base material may modify its SFE [10] and thus change the recrystallization mode. Even for the same material, changing the TMP conditions [11] or initial grain size [12] can also lead to the transition from DDRX to CDRX. Meanwhile, CDRX and GDRX can also operate concurrently, e.g., in Zircaloy-4 [13]. Due to the increasing requirements on the formability of the metallic parts, products which were previously formed at room temperature, where DRX is generally irrelevant, are now frequently processed at warm or hot temperatures [14], [15]. It is not easy to simply identify which type of the three DRX processes is operating at certain TMP conditions since they share some similarities and can take place concurrently and/or transitionally. There is actually also a hot debate between researchers within the recrystallization field on whether CDRX or GDRX should be responsible for the grain refinement of aluminium [16], the core issue is whether the HAGBs observed after large deformation are transformed from LAGBs, microshear/deformation bands, or original HAGBs.
The recrystallization phenomenon in general was reviewed in 1997 by the top experts in this area [17], since then the EBSD technique, which can provide invaluable information on the evolution of the crystallographic orientations and facilitates the understanding of different DRX processes, has been widely spread. CDRX and DDRX were recently reviewed in an excellent and extensive review paper [18], however, GDRX was not covered and the related DRX numerical models were only briefly described. The comparison of the three types of DRX processes was only occasionally mentioned in a few articles [19], [20], [21]. This short review paper covers seminal basic works as well as very recent contributions to all the three DRX processes. It differs from the above mentioned review papers since it updates key aspects on DRX from affecting factors, characterization methods to mechanisms and numerical models.
The main objective of this paper is to provide a short review of the different types of DRX observed during hot deformation for different types of metallic alloys, i.e., DDRX, CDRX and GDRX. The review presented is intended to equip the beginners in metallurgy with a concise insight into the DRX phenomenon. For more details on this topic, the interested readers are referred to the classic textbook on recrystallization [1] and the two excellent but longer review papers [17], [18]. In Section 2, the terminologies used in this field are firstly summarized, together with the key factors influencing the DRX processes, as well as the corresponding characterization methods. The transitions between the various types of DRX processes are only briefly discussed due to the lack of literature data. From 3 Discontinuous dynamic recrystallization, 4 Continuous dynamic recrystallization, 5 Geometric dynamic recrystallization, more details on the three types of DRX are given, including their mechanisms and related numerical models. Finally, in Section 6, further studies within the DRX field are suggested.
Section snippets
Terminology
There are different phenomenological categories of recrystallization processes, many of them are interconnected and the borderlines between them are often unclear. It is beneficial to recall all these processes, even though some of them will not be covered in this review work. In addition, there are also some terminologies in recrystallization field which are worth mentioning before going into the details of DRX.
Defects like dislocations and interfaces increase during deformation which makes
Summary
The three DRX processes, i.e., DDRX, CDRX and GDRX, occurring in different TMP conditions at high temperatures for various metallic materials have been reviewed. The terminologies used in DRX field were summarized, together with the key factors influencing the DRX processes, and the experimental techniques to characterize them. An emphasis was given on the mechanisms and the existing numerical models.
DDRX, also known as conventional DRX, is obviously the most extensively studied DRX process.
Acknowledgements
The authors acknowledge the financial support from PX group in Switzerland. The constructive and detailed comments from the reviewers are also highly appreciated.
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