The mechanism of the nanocrystalline structure evolution for AZ31 Mg base

In the past decade, severe plastic deformation (SPD) approaches, such as equal channel angular extrusion (ECAE) [51,52], accumulative roll bonding (ARB) [47] and high-pressure torsion (HPT), have been applied to the grain refinement of Mg alloys on bulk materials.

Twinning and dynamic recrystallization (DRX) were found to be responsible for grain refinement for most processes. But in most of these cases, the grain size of the final refinement structure is in the micrometer or submicro-meter range. It is not clear what dominates the grain refinement mechanism if grain size can be further divided into the nanometer scale by SPD. So it is of interest from both the scientific and the practical points of view to investigate the strain induced grain refinement mechanism in the Mg system by using FSP, which, in Al alloy systems, has been proven to be able to produce a nanocrystalline structure in the nugget regions [94].

Under the cooling system with liquid nitrogen, the generated heat from the input of works can be conducted rapidly away by effective heat sink, therefore the effect of heat on

the growth of the resulted microstructure can be decreased significantly as compared to air cooling. In this study, the newly designed cooling system has demonstrated efficient cooling and thus favors to the refinement of resulting microstructure during FSP. Furthermore, it is interested to observe that FSP pass is an important parameter to determine the scale of the resultant microstructure for the tested alloy. Figure 4-5 shows the TEM images of the microstructure for the one-pass FSPed AZ31 alloy, which shows that the microstructure can be only refined to submicron scale, with the mean grain size of 100~300 nm. Besides the submicro-scale fine grains, some clear subgrains and dislocation walls can also be observed in the resultant microstructure.

However, followed by subsequent second pass with a lower heat input, the two-pass FSPed AZ31 alloy can produce a nano-scale microstructure (fine grains with an average size of less than 100 nm). Figure 4-6 shows the typical microstructure observed in the two-pass FSPed AZ31 alloy in the as processed condition, together with a select area diffraction (SAD) pattern. The pattern exhibits rings, indicating that there were many small grains with random misorientations in the selected regions. Figure 3-61 shows the grain size distribution of the FSPed specimens, which is summarized from numerous SEM micrographs. The resulting microstructure exhibits equiaxed grains ranging from 40 nm to 200 nm with an average grain size of less than 100 nm, clearly illustrating that a nanocrystalline structure is produced in the two-pass FSPed AZ31 alloy.

The foregoing observations demonstrate that for AZ31 alloy, during friction stir processing, the severe plastic deformation in the processed region under the high strain rate results in a progressive refinement of coarse grains into a nanometer regime. First, we characterize the microstructure of the two-pass FSPed samples. Figs. 4-7(a), (b), and (c) show that clear nanometer grains have formed within a highly deformed subgrain along the

boundary or the triple-connected point of the recrystals. This clean nanometer grain implies the elimination of strain even through there is still high strain in the surrounding area. The only possible way to generate such clean nanometer grains within a highly deformed area is DRX. Fig. 4-7(d) is another TEM micrograph taken from this region. The “clean” and strain-free equiaxed grains can only be the result of recrystallization.

Dynamic recrystallization is generally acknowledged to happen within temperature ranges from 0.5–0.6 to 0.9–0.98 Tm [187,188]. For pure Mg, 0.5 Tm corresponds to 193^oC; the DRX temperature for AZ31D, therefore, could be even lower than this temperature.

Moreover, Kaibyshev et al. [189] even claimed that DRX occurred at room temperature, which corresponds to 0.3 Tm in pure Mg when the strain is quite high. During the FSP process, the rapid rotation of work pin generates severe plastic deformation and strain at the nugget region. The high strain in the sample will decrease the DRX temperature. As mentioned above, the Mg alloy has a low melting point and therefore a low recrystallization temperature.

The heavy plastic deformation at high strain rates during the FSP also leads to the generation of heat. The combination of heat and heavy plastic deformation leads to the occurrence of DRX for recrystallized grains.

Generally, there are two types of dynamic recrystallization (DRX) discussed in the literature: (i) continuous DRX and (ii) discontinuous DRX. During continuous DRX, new grains develop via a gradual increase in misorientation between subgrains. In contrast, during discontinuous DRX, new grains exhibiting large-angle boundaries evolve; for example, dynamic nucleation followed by grain growth from migration of high-angle boundaries. It is worth mentioning that the development of nanostructures with high-angle grain boundaries via continuous DRX during severe plastic deformation (SPD) process, which result in qualitative changes in properties, is a rather difficult task. For example, equal channel angular

pressing (ECAP), which currently offers maximum potential for scaling, can reduce the grain size to 0.5–1.0 μm in aluminum alloys, but requires a strain of >4.0 [190,191].Generally it takes >8 ECAP passes to achieve very-fine grains exhibiting high grain-boundary misorientations [192]. In the past decade, severe plastic deformation (SPD) approaches have also been applied to the grain refinement of Mg alloys on bulk materials. But in most of these cases, the grain size of the final refinement structure is in the micrometer or submicro-meter range.

But, how the nanocrystalline structure is achieved for the two-pass FSPed AZ31 alloy?

Based on the published papers, the assumption of the occurrence of discontinuous DRX is reasonable for the current study. The difference between FSP and the previously reported severe plastic processes, may lie in the strain rate. Wang et al. [193] have reported that strain rate plays an important role in refining grains into the nanometer region during the SPD processes. The strain rates of ECAP and HPT are too low to induce enough twinning activity and to form nanometer grains. However, in FSP, the strain rate is estimated to be about 10⁰ - 10² s^-1. It is believed that a complex stress state and strain components with very-large strain gradients will be induced by the high strain rate. Furthermore, large amounts of dislocations are also introduced to accommodate the strain incompatibility. The complex stress state, complicated strain patterns and dislocation configurations, and high density of geometrically necessary dislocations provide the necessary conditions for the occurrence of discontinuous DRX process, which is beneficial in allowing copious nucleation.

However, a notable feature of this study is that dislocation walls and subgrains can be observed in the one-pass FSPed samples, as shown in Fig. 4-5. This suggests that the continuous DRX plays a dominant role in the evolution of microstructure. As what we have known, nuclei form preferentially in regions where the local degree of deformation is highest,

such as grain boundaries, deformation bands, inclusions, twin intersections and free surfaces.

Due to the paucity of the “site” for nucleation originating from the coarse microstructure of first-pass FSPed samples, more dislocation walls even subboundaries form to accommodate the high strain incompatibility. Therefore, continuous DRX may be dominant process for the first-pass FSPed samples. Due to the rapid cooling, these microstructual features can be remained in the first-pass FSPed samples. Then during second-pass FSP, the remained high-density dislocation walls and submicro-scale subgrains as well as recrystallzed grains all can become the “site” for the nucleation of recrystals. Then under the high strain rate of FSP, the discontinuous DRX may be dominant, which results in the formation of copious nuclei. It should be mentioned that discontinuous DRX process can significantly refine the grains, but the newly formed grains will be subject to growth process which is controlled by volume diffusion under the condition of severe plastic deformation. Then the working temperatures are crucial for the refinement of the resulted microstructure of the FSPed AZ31 alloy.

However, in this current study, under the cooling system with liquid nitrogen, the generated heat from the input of works can be conducted rapidly away by effective heat sink, therefore, the working temperature is decreased significantly. Thus, nanocrystalline microstructure can be remained.

On the basis of microstructure observation, the evolution of the nanostructure can be described as follows: (i) In the first-pass stage of FSP, submicro-sale grains are introduced in the processed sheet via continuous dynamic recrystallization (ii) In the second-pass stage of FSP, copious nuclei form via discontinuous dynamic recrystallization (DDRX) due to the existence of submicro-sale grains, subgrains and a high density of dislocation walls (iii) The growth of the recrystallized grians is limited due to the effective heat sink from the liquid nitrogen cooling system. This fundamental understanding of the nanocrystalline evolution makes it possible to control the final microstructure, including grain size, grain-boundary

structure, and dislocation density, by changing the processing parameters and cooling rate.

Based on the above discussions, the process and mechanism of nanocrystallization of AZ31 alloy during the FSP with subsequent second pass can be proposed. Fig. 4-8 is a schematic diagram showing the process. A two type recrystallization mechanism is proposed.

During the first pass, the continuous recrystallization is preferable, in which a lot of dislocation walls and subgrains as well as recrystals is remained. During the second-pass FSP, discontinuous DRX process is chosen due to the combining effects from low heat input, high total strain, high strain rate and copious sites for nucleation.