Study on the microstructure and mechanical properties of VW73B alloy at high extrusion ratio

The microstructure and mechanical characteristics of VW73B prepared by a high extrusion ratio (70:1) were studied. High-strength plastic VW73B alloy containing long-period structural phase (LPSO) was obtained by extrusion process. OM, SEM, TEM, and EBSD were used to investigate the relationship between microstructure and mechanical characteristics of VW73B. The findings indicate that VW73B presents complete dynamic recrystallization after deformation, and there are 14H-LPSO phases, SFs, and a large amount of β´ phase, which are precipitated after aging treatment. The UTS, YS, and EL of VW73B in peak aging state are 406 MPa, 339 MPa, and 9%, respectively. The β´ phase, LPSO phase, poor texture, and fine grain strengthening of VW73B contribute to its exceptional characteristics.


Introduction
Owing to its low density, high specific strength, and green environmental protection, magnesium alloy, the lightest metal structural-functional material, is utilized extensively in the aerospace, transportation, and 3C product industries, and other fields [1] .More exacting specifications have been made for the mechanics, particularly the mechanical characteristics of magnesium alloys due to the rapid advancement of the military manufacturing field.Strength and plasticity can be increased by adjusting the alloy composition and optimizing the deformation process [2] .
A beneficial method for enhancing the magnesium alloy's qualities is alloying.The most efficient Rare Earth (RE) elements to enhance the alloy's overall mechanical qualities are Gd and Y. Zn elements added to Mg-RE, which can produce an LPSO structural phase, which is a strengthening phase.The LPSO structural phase's bending and deformation mechanism can help to coordinate the magnesium alloy's plasticity and enhance its toughness [3] .Mg-RE-Zn alloy has great potential in achieving reasonable matching of alloy strength and plasticity.
The microstructure and characteristics of magnesium alloy are significantly impacted by the extrusion ratio.Guan et al. [4] investigated the characteristics of Mg-Sm-Zn-Zr at three different extrusion ratios of 6.9, 10.4, and 17.6.The results demonstrate that as the extrusion ratio increases, the alloy's DRX fraction rises along with the size of the grains, and basal texture decreases.Wang et al. [5] explored the impact of extrusion ratios of 7.6, 12.5, and 26.1 on Mg-2.5Nd-0.5Zn-0.5Zr.The DRX grain size initially reduces and subsequently increases with a rise in extrusion ratio, while the texture strength rises from 6.7 to 19.4.A higher extrusion ratio can provide a larger strain, which is conducive to obtaining a higher percentage of dynamically recrystallized grains and enhancing the alloy's general qualities.
At present, rare studies have been done on the ultra-high extrusion ratio method of creating Mg-RE-Zn-Zr alloys.We prepared Mg-7.0Gd-3.0Y-1.0Zn-0.5Zr(VW73B) by an extrusion ratio of 70:1, and the alloy was aged.The connection among the structure and characteristics was established by analyzing the alloy's microstructure, texture, degree of recrystallization, and precipitation equality.Additionally, the mechanical properties of VW73B were examined.

Experiment
VW73B alloy ingot with a large size of Φ400×1, 000 mm was prepared by die casting.The true composition of VW73B was determined by ICP as Mg-6.79Gd-3.36Y-1.12Zn-0.57Zr(wt.%).The homogenization of the as-cast alloy was carried out at 480℃×12 h+520℃×40 h.The small ingot with Φ120×150 mm was taken along the edge of the large ingot for hot extrusion deformation.Before extrusion, the ingot is preheated at 460℃ for 4 h, the extrusion temperature is selected at 460℃, the rate of extrusion is 0.4 mm/s, and the extrusion cylinder's diameter is 130 mm.Finally, the extrusion bar with Φ15.5 mm was obtained.The extruded alloy was aged at 200℃.The microstructure, precipitated phase, dynamic recrystallization degree, and texture of the alloy were examined using OM, SEM, EBSD, and TEM, and the mechanical characteristics of the VW73B were tested.

Results and discussion
The OM of extruded VW73B alloy is shown in Figure 1.It is discovered that the extrusion process breaks both the lamellar and block-shaped LPSO structural phases.The larger extrusion ratio provides higher strain energy for the occurrence of recrystallization, and the microstructure distribution is relatively uniform, which can be found from the longitudinal cross-section microstructure in Figure 1 (b).The LPSO phase is distributed along the extrusion direction, and the alloy is almost completely dynamic recrystallization.Figure 2 shows the SEM of the extruded VW73B alloy.Following extrusion, the LPSO is twisted, fractured, and dispersed along the grain boundary, which encourages the occurrence of dynamic recrystallization.Lamellar LPSO is a dynamic precipitate in the extrusion process, distributed in the grain.The block-shaped and lamellar LPSO is fragmented and scattered throughout the matrix, as seen in the cross-section of Figure 2 (a).In Figure 2 (b), the block-shaped LPSO is elongated in the direction of extrusion, and the fractured LPSO is often twisted to coordinate deformation under the action of extrusion stress.The aging hardness curves of the VW73B alloy at 180℃, 200℃, and 220℃ were studied, and the results are shown in Figure 6.With the aging time, the hardness value showed a rising trend, and after reaching the peak value, the hardness value was maintained.Within 0-16 h, the changes in hardness values at the three temperatures were similar, mainly because the precipitated phase was uniformly ICAMIM-2023 Journal of Physics: Conference Series 2720 (2024) 012039 precipitated at the early stage of aging.After 16 h, the precipitation at a low temperature of 180℃ is less, while the aging precipitation at 220℃ coarsens with the aging time.At these two temperatures, the hardness value is lower.Therefore, the aging temperature of VW73B alloy at 200℃ was selected, and the hardness reached a peak of 114.4 HV after aging at 200℃ for 80 h.Therefore, the extruded VW73B alloy was aged at 200℃ for 80 h.After calibration, the phase is the β' phase precipitated by aging.β' phase presents a parallel fringe in the matrix, precipitates equivalently along the <112 0> α direction, and shows an orientation relationship of 120° angle between each other [6] .
. TEM analysis of peak-aging VW73B alloy.The mechanical characteristics of aged and extruded VW73B alloys were examined.VW73B alloy shows good precipitation strengthening response, with extrusion UTS of 316 MPa, YS of 243 MPa, and EL of 16%.After aging, the UTS and YS of VW73B alloy reached 406 MPa and 339 MPa, and the EL was 9.0%.After aging treatment, a good deal of uniformly dispersed β' phases were formed within the alloy matrix, which played a good strengthening role in the matrix.As a precipitation strengthening phase, β' phase can significantly strengthen VW73B alloy's toughness and can nail the grain boundary to inhibit the slip of the grain boundary, so that compared with the extruded alloy, the plasticity is reduced.

Conclusions
In this paper, large-scale VW73B is prepared, and the microstructure and mechanical characteristics of VW73B were studied after the deformation of the ultra-large extrusion ratio (70:1).
(1).After extrusion deformation, LPSO in VW73B alloy is dispersed alongside the extrusion direction and violently shattered.Lamellar LPSO is dispersed in the matrix.
(2).Complete dynamic recrystallization occurred approximately after the extrusion of VW73B alloy.Its texture strength was weak, orientation distribution was relatively random, and the grain size was 7.0 μm.
(3).VW73B alloy has a good aging strengthening effect, and the dispersing nanoscale β' phases can be precipitated in the peak aging stage.The mechanical characteristics of VW73B in an aging state are best, with UTS of 406 MPa, YS of 339 MPa, and EL of 9.0%.

Figure 1 .
Figure 1.OM structure of extruded VW73B alloy.Figure2shows the SEM of the extruded VW73B alloy.Following extrusion, the LPSO is twisted, fractured, and dispersed along the grain boundary, which encourages the occurrence of dynamic recrystallization.Lamellar LPSO is a dynamic precipitate in the extrusion process, distributed in the grain.The block-shaped and lamellar LPSO is fragmented and scattered throughout the matrix, as seen in the cross-section of Figure2(a).In Figure2(b), the block-shaped LPSO is elongated in the direction of extrusion, and the fractured LPSO is often twisted to coordinate deformation under the action of extrusion stress.

Figure 2 .
Figure 2. SEM photos of extruded VW73B alloy.The precipitated phase in extruded VW73B alloy was analyzed by TEM. Figure 3 (a) shows the TEM open-field phase.Lamellated LPSO is twisted, and plastic deformation can be coordinated during the deformation process, as shown by the red arrow.The short rod-like phase is shown by the yellow arrow, with a small size and a length of about 1 μm.The SAED result is shown in Figure 3 (c), which can be determined to be 14H-LPSO.The lamellar structure is shown in Figure 3 (a) in the red line area, the SAED map in the direction of [112 0] is shown in Figure 3 (d), and the results show that it is a stratified fault (SFs).From Figure 3 (b), the topography of the SF structure can be more clearly seen with varying lengths.

Figure 3 .
Figure 3. TEM results of extruded VW73B alloy.The EBSD analysis of VW73B magnesium alloy was carried out.The IPF diagram can be seen in Figure 4 (a).Figure 4 (b) shows the grain size distribution diagram of VW73B.The orientation graphic presented it obviously under the extrusion ratio of 70:1, the alloy underwent complete dynamic recrystallization, the orientation distribution was relatively random, and the average grain size was 7.0 μm.The distribution of grain boundary orientation difference is shown in Figure 4 (c).The proportion of large angle grain boundaries LAGBs (>15°) is 93.8%, which is relatively high, so it can be concluded that a large extrusion ratio promotes the occurrence of dynamic recrystallization.Figure 4 (d) is the KAM diagram of the superposition size angle grain boundary.The LAGBs (15°~100°) are represented by the black line, while the small angle grain boundaries (2°~15°) are represented by a white line.The overall dislocation density in the DRX region is small.

Figure 4 (
Figure 3. TEM results of extruded VW73B alloy.The EBSD analysis of VW73B magnesium alloy was carried out.The IPF diagram can be seen in Figure 4 (a).Figure 4 (b) shows the grain size distribution diagram of VW73B.The orientation graphic presented it obviously under the extrusion ratio of 70:1, the alloy underwent complete dynamic recrystallization, the orientation distribution was relatively random, and the average grain size was 7.0 μm.The distribution of grain boundary orientation difference is shown in Figure 4 (c).The proportion of large angle grain boundaries LAGBs (>15°) is 93.8%, which is relatively high, so it can be concluded that a large extrusion ratio promotes the occurrence of dynamic recrystallization.Figure 4 (d) is the KAM diagram of the superposition size angle grain boundary.The LAGBs (15°~100°) are represented by the black line, while the small angle grain boundaries (2°~15°) are represented by a white line.The overall dislocation density in the DRX region is small.
Figure 3. TEM results of extruded VW73B alloy.The EBSD analysis of VW73B magnesium alloy was carried out.The IPF diagram can be seen in Figure 4 (a).Figure 4 (b) shows the grain size distribution diagram of VW73B.The orientation graphic presented it obviously under the extrusion ratio of 70:1, the alloy underwent complete dynamic recrystallization, the orientation distribution was relatively random, and the average grain size was 7.0 μm.The distribution of grain boundary orientation difference is shown in Figure 4 (c).The proportion of large angle grain boundaries LAGBs (>15°) is 93.8%, which is relatively high, so it can be concluded that a large extrusion ratio promotes the occurrence of dynamic recrystallization.Figure 4 (d) is the KAM diagram of the superposition size angle grain boundary.The LAGBs (15°~100°) are represented by the black line, while the small angle grain boundaries (2°~15°) are represented by a white line.The overall dislocation density in the DRX region is small.
(a) IPF diagram; (b) Grain size distribution map; (c) Distribution map of grain boundary orientation difference; (d) KAM diagram.

Figure 4 .Figure 5 .
Figure 4. EBSD analysis of extruded VW73B alloy.The texture of VW73B alloy has been analyzed.Figures 5 (a) and (b) show the polar and inverse polar diagrams of VW73B magnesium alloy, and the texture strength is 3.603.Combined with the OM results in Figure1and EBSD analysis in Figure4, the alloy is in a complete dynamic recrystallization state, consisting of fine equilateral crystals, whose grain orientation is more random, so the texture strength is weak.

Figure 6 .
Figure 6.Hardness curves of VW73B alloy at different aging temperatures.Figure 7 shows the TEM results of peak-aging VW73B alloy.TEM bright field images and HAADF-STEM images are displayed in Figures 7 (a) and (b), respectively.After aging treatment, a large number of uniformly distributed precipitated phases are deposited inside the matrix.The corresponding electron diffraction patterns in the [0001] direction and [112 0] direction are displayed in Figures 7 (c) and (d).After calibration, the phase is the β' phase precipitated by aging.β' phase presents a parallel fringe in the matrix, precipitates equivalently along the <112 0> α direction, and shows an orientation relationship of 120° angle between each other[6] .