Enhanced microstructure homogeneity of extruded Mg-6Zn-0.5Zr alloy by Ca addition

Commercial Mg-6Zn-0.5Zr (ZK60) alloys with different contents of the alloy Ca were subjected to indirect extrusion processing at an initial temperature of 250 C, and subsequently the effect of Ca alloy on the microstructure of the extruded alloys was investigated with special emphasis on recrystallization and grain growth during the hot extrusion. It was found that alloying with Ca results in refinement of recrystallized α-Mg grains and it also contributes to increase the homogeneity of grain size in the extruded state. Such microstructural changes in grain size and its homogeneity achieved by Ca addition are mainly attributed to the presence of Ca2Mg6Zn3.

Microstructure developed in a partially extruded Mg-3Al-1Zn billet and correspondingly a finite element method simulation indicating the flow pattern and the stresses obtained. Optical micrographs of extruded (a) ZK60 and (b) ZKX600 alloys. a) TEM micrograph, (b,c) EDS spectra and (d,e) electron diffraction patterns from (b,d) A and (c,e) B precipitates in the ZK60 alloy. a) TEM micrograph, (b,c) EDS spectra and (d,e) electron diffraction patterns from (b,d) C and (c,e) D precipitates in the ZKX600 alloy. Macroscopic optical micrograph showing the cross-section of ZK60 alloy impact and (b–g) optical micrographs taken at the marked points 1–6 in (a).

List of table

Introduction

Binary Mg-Zn alloying can involve the formation of Guinier-Preston (GP) zones, metastable β'1 (Mg4Zn7), β'2 (MgZn2), and finally the equilibrium β (MgZn) phase. However, there is a large microstructural difference in the extruded Mg-Zn alloy with and without Zr. Currently, many studies have been carried out on extruded Mg-Zn-based alloys containing a small amount of Ca [10-17].

Also, there is a higher amount of precipitation in the Ca-containing Mg-Zn binary alloy after extrusion. Moreover, the ternary Ca2Mg6Zn3 phase is formed in the Mg-Zn-Ca based alloy [18, 19], which is mainly located at dynamically recrystallized (DRXed) grain boundaries due to the dynamic precipitation during extrusion.

Table 1 The precipitation process during isothermal aging of Mg-Zn alloy system [6].

Background

When the bill is placed in a container and a ram compresses the wrapper, it begins to flow through the cover to the opposite end of the container. As the ram reaches the mushroom, the end of the ticket that cannot be pushed through the pile remains. However, the main problem with direct extrusion is that there is considerable friction created between the metal surface and the walls of the container as the billet is pushed towards the cover.

Therefore, the indirect extrusion process facilitates faster extrusion and more refinement of the DRXed microstructure than the direct extrusion process. As the metals continue to deform, newly dynamically recrystallized (DRXed) grains are nucleated at the old grain boundaries, the dislocation density of the newly DRXed grains increases, reducing the driving force for further growth, and the DRXed grains eventually stop growing. 5(c)), where the stress is highest, the DRX fraction increases significantly, reducing the overall grain size of the alloy.

Increasing the stress reduces the time required for the precipitate to nucleate below the recrystallization temperature, and dislocation motion during deformation improves the diffusion rate of atoms. Moreover, in the case of the deformed region, the dynamic inclusions in the DRXed region have a spherical shape, while the rod-like inclusions in the aged Mg-Zn-based alloys are deformation-free [40, 41]. The reason for the formation of spherical precipitates is attributed to the relative soft Mg matrix, where fine spherical precipitates can reduce the interfacial energy and may not increase the strain energy because the interface characteristics between the precipitates and the Mg matrix are incoherent.

Therefore, the DRXed grain size distribution may be uneven, depending on the non-uniform distribution of dynamic precipitates. However, fine precipitates can act to prevent movement of grain boundaries by exerting a pinning pressure, called Zener pinning. 7 (a) shows the grain growth behavior when second-phase particles are present in the Mg matrix.

7 (a), the moving boundaries become trapped in the particles as the grain grows, so the particles exert a force to constrain the movement of the grain boundary. This pressure means the value multiplied by the maximum force per particle and the number of particles per unit area of ​​the boundary.

Fig. 2. Types of extrusion process: (a) direct extrusion and (b) indirect extrusion.

Experimental

Result

In particular, there was a larger amount of precipitate in the ZKX600 alloy than in ZK60. To identify what kinds of precipitates are present in SEM micrographs (Fig. 11), TEM analysis of the extruded alloys was performed, as shown in Fig. In the bright field TEM image of the ZK60 alloy (Fig. 12), energy dispersive spectrometry (EDS) showed that the precipitates were present in the form of Mg-Zn with or without Zr.

The analysis result of electron diffraction patterns obtained from Mg-Zn phase shown in Fig. In the TEM bright-field image of ZKX600 alloy (Fig. 13), EDS analysis indicates that Zn-Mg-Zr contains 2.1 at.% Ca. and Mg-Zn-Ca precipitate containing 3.1 at.% Ca was observed at the DRX grain boundary. By the analysis of electron diffraction patterns, the Zn-Mg-Zr and Mg-Zn-Ca phases coincide well with MgZn2 and Ca2Mg6Zn3, respectively.

MgZn2 precipitates, known to be typically found in wrought ZK60 alloy [9, 40], are also present in ZKX600 alloy. It is known that the second phase particles in the Mg-Zn-Ca alloy exist in the form of Mg2Ca or Ca2Mg6Zn3, which depends on the atomic ratio of Zn/Ca. The Ca2Mg6Zn3 phase predominates when the Zn/Ca atomic ratio is greater than 1.23 [44]. However, the Ca2Mg6Zn3 phase, which is present in the ZKX600 alloy, does not have the Zn/Ca atomic ratio of 1.5.

Several studies suggest that the Zn/Ca atomic ratio does not have to be 1.5 depending on the situation [45, 46]. a) TEM micrograph, (b,c) EDS spectra and (d,e) electron diffraction patterns of the (b,d) C and (c,e) D precipitates in the ZK60 alloy. a) TEM image, (b,c) EDS spectra and (d,e) electron diffraction patterns of the (b,d) C and (c,e) D precipitates in the ZKX600 alloy. To investigate the microstructural evolution during extrusion, the microstructure of the residual sample after extrusion, known as butt, was observed. However, as the billet passed through the die, some of the fine DRX grains grew up coarsely during extrusion. a) Macroscopic optical micrograph showing the cross-section of the end of the ZK60 alloy and (b-g) optical micrographs taken at marked points 1-6 in (a).

Fig. 11 SEM micrographs in the DRXed regions of extruded ZK60 and ZKX600 alloys: (a-b) ZK60 and (c) ZKX600

Discussion

These phases are dynamically precipitated in the form of MgZn2 during extrusion, which can be precipitated even below the static precipitation temperature through deformation. Therefore, to determine the thermal stability of dynamic precipitates in the extruded alloys, artificial heat treatment was performed for 2 min within the increasing temperature range during extrusion. 16 shows SEM and optical micrographs of the extruded alloys with and without heat treatment at 330 °C for 2 min.

Fine precipitates were usually found to exist in the extruded state of the extruded alloys in Fig. However, when the temperature was increased to 330 °C in the ZK60 alloy, most of the fine precipitates were almost dissolved in the matrix. TEM analysis of the precipitates (see Figure 12) found them to be in the MgZn2 phase.

As a result, some of the DRXed grains appear to grow sharply in the ZK60 alloy because the grain-growth inhibiting MgZn2 precipitates were almost dissolved into the matrix by the frictional heat generated during extrusion. It can be concluded that the microstructure immediately after extrusion in the ZKX600 alloy would be almost identical to that of the ZK60 alloy, although the microstructural evolution was not observed because the butt sample of the extruded ZKX600 alloy did not exist. However, the amount of precipitates was slightly reduced in the heat-treated condition of ZKX600 alloy, compared to the extruded condition.

The TEM-EDS results for the ZKX600 alloy (Fig. 13) show that the fine precipitates contained a small amount of Ca, and most of the other precipitates also contained Ca. This indicates that there are smaller amounts of precipitates at 330 °C than those in the extruded state because Ca-free MgZn2 precipitates. In fact, the thermal stability of Ca2Mg6Zn3 is better than that of MgZn2 precipitates, suggesting that grain growth can be prevented more effectively when it occurs after recrystallization during extrusion in the ZKX600 alloy compared to the ZK60 alloy.

Fig. 15. (a) Non-equilibrium thermodynamic calculation showing the fraction of phases as a function of temperature for the ZKX600 alloy

Conclusion

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