Effect of extrusion temperature on the microstructure and mechanical properties of hot-extruded ZK60 magnesium alloy

ZK60 magnesium alloy was prepared by the hot-pressed sintering combined with the hot-extruded technique. The microstructure and mechanical properties evolution of ZK60 alloy with a reduced extrusion temperature from 350 °C to 250 °C were investigated in detailed. The extruded alloy was mainly composed of fine α-Mg grains and MgZn2 phase. The grains size gradually decreased with a reduction in extrusion temperature, and the yield strength obviously increased accordingly. The alloy extruded at 250 °C exhibited the smallest average grain size of 2.5 μm and the best combination of strength and plasticity, with a yield strength of 279 MPa and fracture strain of 18.1% in compressive condition, and 263 MPa and 13.7% in tensile condition, as well as a tension-compression yield asymmetry of 0.94. Both the fine grains formed by the continuous and twin induced dynamic crystallization mechanism and the precipitation strengthening contributed to the strength improvement.

Among various thermal deformation processes, hot extrusion has been widely applied in Mg alloys to efficiently produce solid or hollow structural components.Numerous studies focused on the optimization of extrusion process of Mg alloys to achieve desirable microstructure and mechanical properties, including extrusion temperature, extrusion speed and extrusion ratio [6-9, 16, 19-21].It was verified that the appropriate extrusion temperature contribute greatly to obtain the ultrafine-grained microstructure, which not only helped the increase of strength, but also offered the possibility for the plasticity improvement [8,[19][20][21].As for ZK60 alloy, when the extrusion temperature was higher than 350 °C, it was found that the grain size of the extruded alloy was not fine enough (generally > 5 μm), which caused an unsatisfactory yield strength (generally less than 150 MPa) [8,9,[19][20][21].Therefore, when the ZK60 alloy was prepared by the hot extrusion process, the extrusion temperature was generally considered to be lower than 350 °C.
In addition, in many Mg alloys, the tensile strength was found to be much higher than the compressive yield strength, which was described as the tension-compression yield asymmetry [22,23].Generally, the yield asymmetry of Mg alloys was mainly due to the occurrence of {10 12} twinning, which obviously reduced the compressive yield strength [22].Therefore, how to suppress the {10 12} twinning is the key point to the problem is crucial to improve the yield asymmetry.Luo et al [23] found that ultrafine-grained microstructure has contributed to suppressing the {10 12} twinning due to the fact that {10 12} twins was difficult to form in the fine grains compared with those in the coarse grains in compression process.Therefore, obtaining the fine grains was also crucial to achieve the improvement of strength and yield asymmetry.
In this work, the sintered ZK60 Mg alloy was hot-extruded in the temperature range of 250 °C-350 °C with an interval of 50 °C to obtain the fine grains.The effect of extrusion temperature on the microstructures and mechanical properties of the ZK60 Mg alloy were investigated systematically.The dynamic recrystallization mechanism, strengthening mechanisms and tension-compression yield asymmetry were studied in detail.This paper provided a comprehensive discussion on the association of process, property, and mechanism of ZK60 Mg alloy.

Experimental procedure
ZK60 Mg alloy employed in this research was prepared by the hot-pressed sintering combined with the hotextruded technique.A gas-atomized ZK60 alloy powder (Tangshan Weihao Magnesium Powder Co. Ltd) was used as the raw material in the sintering process.The alloy powder, composed of varying particle sizes (thereinto, 25-45 μm for 25%, 45-75 μm for 50%, and 75-150 μm for 25%), was mixed by the way of ball milling under a high-purity Ar gas atmosphere.The speed of ball milling was 200 rpm, and the holding time was 2 h.Then the mixture was sintered using a ZT(Y) series vacuum hot pressing furnace under a constant pressure of 45 MPa.The sintering temperature was determined to be 450 °C with a heating rate of 10 °C•min −1 [5].The diameter and height of the sintered alloy were 58 mm and 46 mm, respectively.Then the sintered alloy was machined to the size of 52 mm in diameter and 32 mm in height for the subsequent extrusion process.After preheating, the sintered alloy was hot-extruded using a four-column hydraulic press machine (HP32-315, Jiangsu Tiankai Technology Co., Ltd) to a bar of 10 mm in diameter, with a ram speed of 20 mm•s −1 and an extrusion ratio of 27:1.The extrusion temperature was determined to be in the range of 250 °C-350 °C with an interval of 50 °C.
The phase structure of the extruded alloy was examined using x-ray diffractometer (XRD, D/Max 2500PC).The microstructure was observed using optical microscope (OM, DMI3000M) and scanning electron microscope (SEM, JSM-7900F).The regional morphology and structure of constituent phase were investigated using transmission electron microscope (TEM, JEOL JEM-2100) with a selected area electron diffraction (SAED) technique.The grain morphology and orientation were observed using electron backscatter diffraction (EBSD, TESCAN MAIA3) technique combined a Channel 5 software with a step size of 0.4 μm.Before observation, the samples were grinded and electrolytic polished successively.The direction of the samples in EBSD examination was aligned to the extrusion direction (ED), and the measured area was located in the center of samples.The quasi-static tensile and compressive tests were conducted on a CMT 4305 microcomputer electronic universal testing machine under a strain rate of 10 −3 s −1 at room temperature.The samples for tension test were flake with a size of 10 mm in gauge length, 3.2 mm in gauge width, and 1.2 mm in gauge thickness.The samples for compression test were cylindrical with a size of 4 mm in diameter and 6 mm in length.Both the tension and compression test samples were cut from the alloy along the axial direction.Three tests were performed on each set of samples to ensure the accuracy and reproducibility of data.

Results
The XRD patterns of the sintered and extruded ZK60 alloy are shown in figure 1.Only the diffraction peaks belong to α-Mg phase were detected in both the sintered and extruded alloy.However, the relative intensity of the diffraction peaks calibrated as (10 10) and (0002) crystal planes in the extruded alloy exhibited an apparent change by comparison with those in the sintered alloy, which illustrated that the extrusion process had an apparent influence on the grain orientation of alloy.
The OM and SEM images of the sintered and extruded ZK60 alloy were displayed in figure 2(1) and figure 2(2), respectively.As shown in figure 2, no obvious micropores were observed in both the sintered and extruded alloy, indicating a considerable density of the alloy.Figure 2(a1, a2) showed that the powder particle boundary (indicated by the blue arrows) in sintered alloy was continuous and thick, demonstrating an incomplete metallurgical bonding between the powder particle during sintering process [24].Compared with the powder particle boundary, the grain boundary (indicated by the yellow arrows) in sintered alloy was apparently thinner.Additionally, the size of powder particle mainly distributed within a range from 20-150 μm, which was totally consistent with that of the mixed alloy powder.By comparison, the size of grains was in a range of 5-30 μm, which was evidently smaller than that of the powder particle.Apparently, as shown in figure 2(b1-d1), the grain size of the extruded alloy was much smaller than that of the sintered alloy.In addition, the grain size of the extruded alloy exhibited a decreasing trend with a decrease in extrusion temperature.Most of grain size of the alloy extruded at 350 °C was ∼5 μm, and the grain size of the alloy extruded at 300 °C was slightly smaller than that of the alloy extruded at 350 °C.In comparison, the grain size of alloy extruded at 250 °C was generally less than 5 μm, which exhibited a smallest value in extruded alloy.These results demonstrated that the extrusion process also have an evident effect on the refining of grain size.Moreover, some small white particles (indicated by the red circles) were found distributing in the grain boundary of the extruded alloy, as shown in figure 2(b2-d2).These particles were inferred as the precipitated Mg-Zn phase on account of the components of alloy.
As shown in the SEM images of the extruded alloy, some tiny precipitated particles distributed in the grain boundary of alloy.To clarified the morphology and crystal structure of these tiny precipitated particles, TEM analysis was performed on the alloy extruded at 250 °C, and the results were shown in figure 3 e), the nano-sized precipitated particles were verified to be MgZn 2 phase, whose average size was calculated to be ∼23 nm.
To quantitatively investigate the influence of extrusion temperature on the microstructure of ZK60 alloy, the EBSD technique was used to further analyze the grain size and orientation evolution of the sintered and extruded alloy.The EBSD maps and the grain size distribution maps were shown in figure 4(1) and figure 4(2), respectively.Figure 4(a1, a2) show that the grain size of the sintered alloy mainly distributed in a range from 5 μm to 30 μm with an average grain size of 10.69 μm, which was consistent with the results shown in figure 2(a1, a2).As shown in figure 4(b1-b2)-(d1-d2), the average grain sizes of the extruded alloy were less than 5 μm, which exhibited an evident decrease compared with the sintered alloy.The average grain sizes of alloy extruded at 350 °C and 300 °C were 4.91 μm and 4.74 μm, respectively.The later exhibited a slightly decrease than the former.When the extrusion temperature further reduced to 250 °C, the number of fine grains evidently increased, resulted to a sharply reduction of the average grain size to 2.50 μm.The (0001) pole figure (PF) and IPF of the sintered and extruded alloy were shown in figures 4(3) and (4), respectively.Figure 4(a3) displayed that the grains of the sintered alloy exhibited a random orientation, indicated that no texture formed during the sintering process.After hot extrusion, all three extruded alloy exhibited a same fiber texture with an orientation of (0001) plane parallel to ED, which is common in various extruded Mg alloys [25][26][27][28].As seen in figure 4(b3-d3), the maximum texture intensity reduced from 4.97 to 4.33 with a decrease in extrusion temperature form 350 °C to 250 °C, which illustrated that the low extrusion temperature contributed to the weakening of texture.The intense extruded fiber texture also could be seen from the IPF, as shown in figure 4(b4-d4), the extruded alloy showed comparatively weak 〈11 20〉//ED texture and the strong 〈10 10〉//ED texture characteristics.
The distribution images including different types of grains and their frequency images of the extruded ZK60 alloy were shown in figure 5.The specific size and frequency of different types of grains were further quantified using the recrystallized fraction component in Channel 5.The results were shown in table 1. Figures 5(a)-(c) shows that both the proportion of deformed and sub-structured grains increased with a decrease in extrusion temperature, while that of the recrystallized one reduced remarkably.As summarized in figure 5(d), the recrystallized proportion of the alloy extruded at 350 °C reached up to 92%, while that of the alloy extruded at 250 °C exhibited a relatively low level, which was only 61%.Apparently, for all extruded alloy, the size of deformed and recrystallized grains was smaller than that of the sub-structured one, and was also smaller than the average grain size.Additionally, the size of all three types of grains exhibited a mildly decrease when the extrusion temperature fell from 350 °C to 300 °C, but an obvious reduction when the extrusion temperature further deduced to 250 °C.All of these results indicated that the decrease of extrusion temperature could not only inhibit the occurrence of dynamic recrystallization (DRX), but also hinder the growth of recrystallized grains.These effects were attributed to the difficulty of dislocation climb and cross slip, as well as the low migration rate of grain boundaries at relatively low temperature, which was also caused by the decrease of thermal oscillation and diffusion rates of atoms [19].
The grain boundary and Kernel average misorientation (KAM) maps of the extruded ZK60 alloy were shown in figures 6(1) and (2), respectively.The low-angle grain boundaries (LAGBs, 2°−15°) and the high-angle grain boundaries (HAGBs, 15°−180°) were marked by green and black lines accordingly.Figures 6(a Generally, the slip systems (including basal slip and non-basal slip) with a high average Schmid factor (SF) value, are readily activated to accommodate the plastic strain, and then the alloy exhibits good plasticity [29].The SF values of prismatic slip (0.435) and pyramidal slip (0.395) in alloy extruded at 350 °C were much higher than that of basal slip (0.187), which suggested that the prismatic and pyramidal slip systems were likely to be activated by the fine DXRed grains [30].In addition, the SF values of the extruded alloy did not obviously change with a decrease in extrusion temperature, which may be ascribed to the formation of fiber texture and will lead to an increasing elongation at break.
The quasi-static tensile and compressive engineering stress-strain curves of the extruded ZK60 alloy were shown in figures 8(a) and figure (b), respectively.The concrete mechanical property values were listed in table 2. As displayed in figure 8 and table 2, both the yield strength and ultimate strength in tensile and compressive conditions (denoted as TYS, CYS, UTS and UCS hereafter) of the extruded alloy increased with a decrease in extrusion temperature, especially evidently improved when the extrusion temperature decreased to 250 °C.In addition, the tensile fracture strain (δ T ) also improved with a decrease in extrusion temperature.The increase of strength and plasticity of alloy extruded at 250 °C could be attributed to the grain boundary strengthening and the improvement of coordination deformation ability caused by the refined grain size at the relatively low extrusion temperature.In comparison, the compressive fracture strain (δ C ) hardly changed with a decrease in extrusion temperature, which illustrated the difference in tensile and compressive deformation mechanism in Mg alloys.Overall, the alloy extruded at 250 °C exhibited a favorable combination between strength (279 MPa in TYS and 263 MPa in CYS) and plasticity (18.4% in δ T and 13.7% in δ C ), which were better than most of the reported extruded ZK60 alloy [7,[31][32][33].Moreover, the tension-compression yield asymmetry (CYS/TYS) ratio of the extruded ZK60 alloy also improved from 0.80 to 0.94 with a decrease in extrusion temperature, indicating that low extrusion temperature was contribute to improving the tension-compression yield asymmetry of the alloy.

DRX mechanism of the extruded ZK60 alloy
Generally, the DRX mechanism of Mg alloys involves three types: continuous dynamic recrystallization (CDRX), discontinuous dynamic recrystallization (DDRX), and twin induced dynamic crystallization (TDRX) [2].The dominant DRX mechanisms of Mg alloys during the deformation process varied with the deformation temperature.DDRX is the main recrystallization mechanism of Mg alloys when deformed at a relatively high temperature of 350 °C, and CDRX and TDRX are dominant when deformed below this temperature [17].During the hot extrusion process, large amounts of dislocations generate first, and then accumulate at grain boundaries (GBs), which promote the formation of sub-grains and grains with low angle grain boundaries (LAGBs).With an increase in strain, the sub-GBs and LAGBs transform into high angle grain boundaries (HAGBs), causing the formation of new grains with distinct boundaries, which is defined as CDRX [34].In contrast, DDRX is a process with the serrated nucleation at HAGBs, which bulge and grow through the grain boundary migration [35].As for TDRX mechanism, the deformation twins provide additional grain boundaries for either DDRX or CDRX to occur [36].TDRX mechanism is also closely associated with the extrusion speed.Nie et al [19] declared that the Mg-2.9Zn-1.1Ca-0.5MnMg alloy extruded at 270 °C with increasing extrusion speed gave rise to an increase of deformation temperature, further resulting in a fewer twins in extruded alloy, namely CDRX mechanism was dominant in deformation process.
Figure 9 shows the BF-TEM images of the ZK60 alloy extruded at 250 °C.As can be seen in figure 9(a), twins were observed and marked by the white dotted frame, which confirmed that TDRX mechanism occurred in deformation process when the extrusion temperatures below 350 °C.Additionally, it was found that dislocations with a high density tangled near the twin boundary, which also contributed to the occurrence of CDRX in the subsequent thermal deformation process.Meanwhile, the consumption of twin boundaries was beneficial to the formation of DRX grains.Besides, as shown in figure 9(b), the dynamic precipitation also happened in the region  with the dislocation entanglement, which could reduce the nucleation energy barrier, and act as the heterogeneous nucleation sites for MgZn 2 precipitation [9].In summary, the low extrusion temperature caused the strong plastic deformation, which contributed to the formation of fine grain structure in the ZK60 alloy.

Strengthening mechanisms of the extruded ZK60 alloy
The microstructure and mechanical properties of ZK60 alloy extruded at different temperatures (figures 2-8) show that the alloy extruded at 250 °C exhibited a relatively favorable comprehensive mechanical properties, which was related to its fine grains and dispersed precipitated phase.Therefore, the grain boundary strengthening and the precipitation strengthening were considered to be the main strengthening mechanisms.The refinement of grains resulted in an increase of the total grain boundary area, which conduced to the improvement of strength and could be expressed by the Hall-Petch relationship as follows [37]: Where k is the material constant which equals 0.22 MPa•m −1/2 , and d is the grain size.Zhou et al [38] studied Mg-Li-Al-Sn alloy and found a typical bimodal structure composed by the fine DRX grains and the elongated non-DRX grains.They deemed that both two types of grains acted to the grain boundary strengthening.Therefore, the strengthening effect was calculated using the average size of all grains.Wu et al [39] studied the effect of Yb addition on the microstructure and mechanical properties of ZK60 alloy during extrusion process.They also found a typical bimodal structure forming after extrusion.However, they deemed that only the fine DRX grains contribute to the grain boundary strengthening, which could be calculated as the following formula [39]: Where d and f are the average size and volume fraction of DRX grains, respectively.Both these two formulas were used to calculate the contribution of grain boundary strengthening in this work.The values of d and f are shown in table 1 and figure 5, and the calculation results are shown in table 3.As shown in table 3, the obviously difference existed between the two calculated ways.Apparently, the difference between two calculated values in the extrusion temperature of 250 °C was more significant than those of 300 °C and 350 °C.This visible difference was deemed to be related to the size of unDRXed grains and the recrystallization degree considered in formula (2).Firstly, the unDRXed grain size of ZK60 alloy extruded at 250 °C was significantly smaller than those of 300 °C and 350 °C, as can be seen in figure 4(2) and figures 5(a)-(c).Secondly, the recrystallization degree of ZK60 alloy extruded at 250 °C was also significantly lower than those of 300 °C and 350 °C, as can be seen in figure 5(d).Since the unDRXed grain size of the ZK60 alloy extruded at 250 °C was also small, more grain boundary strengthening contributions could be generated.However, its low recrystallization degree will result in more grain boundary contributions being ignored.Thereby, the grain boundary strengthening contribution was calculated by formula (1) to be the most accurate As shown in figure 3, a certain amount of nano-sized MgZn 2 phase uniformly precipitated in the ZK60 alloy during the extrusion process, which could hinder the slip of dislocations and inhibit the growth of DRXed grains, thereby improve the yield strength of alloy.Therefore, the contribution of precipitation strengthening caused by the MgZn 2 phase in ZK60 alloy extruded at 250 °C was calculated through Orowan mechanism with the following formula [39]: Where Δτ ps is the increment of critical resolved shear stress (CRSS), M is the Taylor factor, G is the shear modulus of α-Mg (17.2 GPa), b is the Burgers vector (0.32 nm), ν is the Poisson ratio (0.35), λ p is the effective inter-particle spacing, d p is the average size of particles, and r 0 is the core radius of the dislocation (r 0 = b).
Generally, the value of M was associated with the texture intensity of Mg alloys.It was reported that when the alloy mainly exhibited a strong texture, the value of M ranged from 2.1 to 3.5.When the alloy exhibited a weak texture, the value of M varied from 4 to 5.5 [9,19].Combing with literature [39], the value of M in this paper was selected as 2.0, and the λ p was calculated by the following formula:   where f is the volume fraction of the precipitated phase.The f values in ZK60 alloy extruded at 250 °C was calculated as 4.3% using Image-Pro Plus 6.0 software.Therefore, the contribution of precipitation strengthening was calculated to be 112 MPa.Apparently, the calculated contribution value (252 MPa) of grain boundary strengthening and precipitation strengthening for the alloy extruded at 250 °C was smaller than their experimental value (279 MPa), which may be owing to the neglect of other strengthening mechanism.

Tension−compression yield asymmetry of the extruded ZK60 alloy
The CYS/TYS ratio was used to quantitatively evaluate the tension-compression yield asymmetry of Mg alloys [22].The closer to 1 that the CYS/TYS ratio was, the less the tension-compression yield asymmetry exhibited.It was reported that the CYS was usually lower than the TYS for majority wrought Mg alloys, indicating that the CYS/TYS ratio was less than 1 [40][41][42].This situation was generally caused by the difference in main deformation mechanism during the tension or compression process: namely that slip is the main deformation mechanism in the tension process and twinning is the main deformation mechanism in the compression process [22].Previous reports indicated the fine grains and weak fiber texture can inhibit {10 12} twinning in the compression process along ED, thereby increase the substantial increase in the compressive yield strength, and finally resulting in the improvement of yielding symmetry [41,43].Figure 10 shows the comparison of CYS/TYS ratio between the present work and other previous literatures [32,[44][45][46][47]. Apparently, the ZK60 alloy extruded at 250 °C presented the favorable yield symmetry due to the formation of fine grains and weak texture.Besides, the ZK60 alloy extruded at 250 °C exhibited a good combination between strength and ductility, with a UTS of 341 MPa and an elongation of 18.1% respectively, which was mainly caused by grain refinement and dispersion distribution of precipitated particles.

Conclusion
In this work, the ZK60 alloy was prepared by the hot-pressed sintering combined with the hot-extruded within a temperature range of 250 °C-350 °C and an interval of 50 °C.The microstructure and mechanical properties variation of the extruded alloy were investigated in detailed.The main conclusions can be drawn as follows.
(1) The extruded ZK alloy mainly exhibited an α-Mg phase structure along with a small amount of tiny MgZn 2 phases, as well as an equiaxed grain microstructure.The grains size of alloy gradually decreased with a reduction in extrusion temperature.The average grain size of the alloy extruded at 250 °C was just 2.50 μm, which contributed to a favorable mechanical property, with a tensile and compressive yield strength of 279 MPa and 263 MPa, a tensile and compressive fracture strain of 18.1% and 13.8%, and a tension-compression yield asymmetry 0.94.
(2) The main dynamic recrystallization mechanism of ZK60 alloy during low temperature extrusion was continuous and twin induced dynamic recrystallization.Both the grain boundary strengthening and precipitation strengthening contributed to the strength improvement of the extruded alloy.In addition, the low extrusion temperature contributed to improving the tension-compression yield asymmetry of the alloy and exhibiting a good strength-ductility combination, which was mainly caused by grain refinement and dispersion distribution of precipitated particles.The calculated contribution value of grain boundary strengthening and precipitation strengthening for the alloy extruded at 250 °C was reached to 140 MPa and 112 MPa, respectively.
. As clearly displayed in the bright field (BF) image and the selected area electron diffraction (SAED) pattern shown in figure 3(a), a certain amount of the nano-sized precipitated particles randomly dispersed in both the grain boundary and the intragranular of the α-Mg matrix.In addition, a certain number of dislocations (pointed by the yellow arrows) distributed inside of grains, indicating an incomplete recrystallization.Combining the measuring result of lattice fringes in the HRTEM image shown in figure 3(b) and the EDS maps shown in figures 3(c)-(

Figure 1 .
Figure 1.XRD patterns of the sintered and extruded ZK60 alloy.
)-(b) show that the alloy extruded at 350 °C and 300 °C exhibited a low LAGBs density, as well as a low geometrically necessary dislocations (GNDs) density.By comparison, figure 6(c) shows that the alloy extruded at 250 °C exhibited a high LAGBs and GNDs density, illustrating a high deformation energy storage.This tendency verified well the results shown in figures 5(a)-(b), that is the alloy extruded at 350 °C and 300 °C displayed a relatively complete DRX while the alloy extruded at 250 °C displayed an incomplete DRX.

Figure 7
shows the 〈a〉 basal slip {0001}〈11 20〉, 〈a〉 prismatic slip {10 10}〈11 20〉, and 〈c+a〉 pyramidal slip {11 22}〈 ¯1 123〉 IPF maps and the corresponding SF values calculated by using Channel 5 software of the sintered and extruded ZK60 alloy.As shown in figure 7(a), three slip systems in the sintered ZK60 alloy presented the approximate SF values.Compared with the sintered alloy, the extruded alloy exhibited the evidently different SF values in three slip systems, as shown in figures 7(b)-(d).

Figure 3 .
Figure 3. (a) BF-TEM image with inset SAED pattern, (b) HRTEM image, and (c)-(e) EDS maps of Mg, Zn and Zr elements in the ZK60 alloy extruded at 250 °C.

Figure 5 .
Figure 5. Distribution images of different types of grains (deformed grains in yellow, sub-structured grains in red and recrystallized grains in blue) and their frequency of the ZK60 alloy extruded at (a) 350 °C, (b) 300 °C, and (c) 250 °C; (d) Grain type frequency of the extruded ZK60 alloy.

Figure 8 .
Figure 8. Quasi-static (a) tensile and (b) compressive engineering stress-strain curves of the ZK60 alloy extruded at different temperatures.

Table 1 .
The size and frequency of different types of grains in extruded ZK60 alloy.

Table 3 .
The contribution of grain boundary strengthening of the extruded ZK60 alloy.

Table 2 .
Mechanical properties of the extruded ZK60 alloy.