Nucleation in Mg alloys

Burke and Turnbull [94] used to explain the nucleation of recrystallization with the phase transformations nucleation theory that nucleation is the result of random atomic fluctuations. However, this theory is not so convincing because the required energy to form the LAGBs is very high [95] while the driving force is low.

It is generally accepted that the nuclei come from the small volumes in the deformed condition. Some nucleation mechanisms of recrystallization are termed as strain-induced grain boundaries migration (SIBM) [96, 97], LAGBs migration [98, 99] and subgrains coalescence [100, 101]. In SIBM mode, the different dislocation densities between neighboring grains are the driving force of grain boundaries motion. As is shown in Figure. 1.3.2 (a), the thick black line separates the less deformed grain, grain A, and more deformed grains, grain B. During annealing, the grain boundaries can bow towards the interior of grains B due to the tendency of reducing the energy [102]. LAGBs migration mode describes the growth of subgrain by absorbing the neighboring LAGBs in Figure. 1.3.2 (b). Subgrain coalescence mechanism explains the formation of a large subgrain from small subgrain pair. As is shown in Figure. 1.3.2 (c), the rotation of small subgrains builds the coincide lattices, leading to the coalescence of subgrains [101].

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Figure. 1.3.2 Three different nucleation mechanisms: (a) SIBM, (b) LAGBs migration, (c) subgrains coalescence [103].

Effects of particles on the recrystallization process have been extensively investigated in recent years. Some theories are well established in Al alloys [104]. First, particles whether can be sheared or not will introduce the stress and strain field and deformation energy in the neighboring regions, which means the driving force for recrystallization is enhanced. Another advantage of the particles is that they provide extra surface for nucleation, decreasing the energy requirement of nucleation. However, the uniformly distributed particles with small spacing can pin the grain boundaries, which retard the grain growth process. There are some arguments when we correlate the weakened texture in Mg-RE alloys to PSN mechanisms. Robson [105]

reported a higher misorientation gradient in the vicinity of particles than the matrix away from particles, resulting in randomly oriented recrystallization nuclei, as is shown in Figure. 1.3.3. Talal [106] proposed the growth advantage of these PSN grains over other grains is responsible for the weakened texture.

However, Stanford [107] reported the texture was weakened in ME10 rather than AZ31 despite the fact that both alloys have comparable size and distribution of the particles. Besides, the dilute Mg-Zn-RE alloys with few precipitates exhibited a weak texture [16]. Thus, PSN may not be the major mechanisms for texture weakening.

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Figure. 1.3.3 EBSD maps of: (a) IPF map of M03 alloy in the region of particles, (b) corresponding line misorientation plot from (a), (c) recrystallized grains around particles in M16 alloy, (d) scatter orientation

of recrystallized grains in (0001) pole figure [105].

Shear bands are the severely deformed band structure with large internal misorientation. The origin of shear bands is not clear. Couling [108] suggested that the shear bands in Mg-RE alloys came from double twinning.

Sandlöbes [70] pointed that shear bands consist of narrow secondary twins and lamellar matrix. The lattice continuously rotated around <112�0> axis in the shear bands and the misorientation of shear bands to the surrounding matrix can reach 15°. The addition of Y modified the distribution of shear bands and decreased the potential of failure within shear bands. Stanford [92] found the recrystallized grains showed a random texture within shear bands while those formed at grain boundaries maintained the deformed texture. Basu [109] reported the grain growth advantage of non-basal grains within shear bands over basal grains. Guan [110] studied the shear bands related recrystallization during quasi in-situ annealing of WE43 alloy. He proposed that the basal texture was weakened rather than replaced by the RE texture. The scattered weak texture of recrystallization nuclei was preserved during grain growth. But there are some doubts regarding the explanation of texture weakening mechanisms of RE addition by SBIN. For example, the shear bands were observed in many commercial Mg alloys like AZ31 [73] and Mg-1Zn [93], which exhibited strong basal textures after rolling. What’s more, the texture weakening effect of RE addition was also reported even without the formation of shear band by many researchers [17, 111].

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Figure. 1.3.4 EBSD maps of Mg-Gd extruded at 415 °C: (a) full ebsd map, (b) recrystallized grains at grain boundaries and corresponding inverse pole figure, (c) recrystallized grains within shear bands and

corresponding inverse pole figure [92].

Apart from the significant impacts on the mechanical properties, deformation twins are also the favored sites for recrystallization. It should be noted that recrystallization in Mg alloys normally triggers within compression [112] and secondary twins [113] rather than tension twins. Unlike the stable compression or secondary twin boundaries, tension twin boundaries with a high mobility can release the strain accumulation during twin propagation [66, 114]. As a result, the driving force for recrystallization is much higher in compression and secondary twins than in tension twins. Li [112] and Martin [115] suggested that even though the orientations of nuclei within twins deviated from that of the matrix, their contribution to the recrystallization texture is minor due to the low volume fraction of the twins. Besides, the twin boundaries restricted the growth of nuclei into the matrix. Guan [113, 116] traced the whole recrystallization nucleation and grain growth of WE43 alloys during quasi in-situ annealing. He reported DTIN was the dominant recrystallization mechanism as the recrystallized grains originating from secondary twins account for a volume fraction of 69.9% in the fully recrystallized condition. The RE texture formed during nucleation was maintained afterwards while dynamic precipitation at twin and grain boundaries restricted the formation of basal oriented grains by SIBM.

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Figure. 1.3.5 EBSD maps of WE43 alloy during quasi in-situ annealing: (a-e) tension twins (f-k) primary tension twins and compression-tension twins, (l-q) compression-tension twins [116].