Achieving high-strain-rate and low-temperature superplasticity in an ECAP-processed Mg-Y-Er-Zn alloy via Ag addition

Horn Wu ,Jinghu Jing,b,? ,Zhnqun Yng ,Mngji Li ,H Hung ,Ningfi G ,Aibin M,Hun Liu,?

a College of Mechanicals and Materials, Hohai University, Nanjing 210000, China

b Suqian Institute, Hohai University, Suqian 223800, China

c Center of Advanced Analysis and Gene Sequencing, Zhengzhou University, Zhengzhou 450003, China

d School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450003, China

e Faculty of Georesources and Materials Engineering, RWTH Aachen University, 52062, Germany

Abstract The effect of adding a small amount of Ag on the microstructure evolution and superplastic properties of Mg-Y-Er-Zn (WEZ612) alloys was systematically studied.The basal texture of the refined WEZ612 alloy produced by equal channel angular pressing was altered to a non-basal structure upon the addition of Ag.Ag addition also refined the grain size and promoted the formation of a large number of nano-14H-long period stacking ordered phases.Using high-resolution transmission electron microscopy,many nano-precipitated phases were detected on the basal plane of the Mg-Y-Er-Zn-1Ag (WEZ612–1Ag) alloy,The nano-precipitated phases on the basal plane improved the thermal stability of the alloy,lowered the deformation activation energy (Q),and improved the stress sensitivity index (m).At 523 K with a strain rate of 10-2 s-1,the Q value of WEZ612 was higher than that of WEZ612–1Ag (299.14 and 128.5 kJ mol-1,respectively).In contrast,the m value of the WEZ612 alloy (0.16) was lower than that of the WEZ612–1Ag alloy (0.46).At 623 K with a tensile rate of 10-2 s-1,the WEZ612 and WEZ612–1Ag alloys were elongated by 182% and 495%,respectively,with the latter exhibiting high-strain-rate and low-temperature superplasticity.The improved superplasticity of the WEZ612-1Ag alloy is attributed to the nano-precipitated phases,which effectively limit the cavity extension during superplastic deformation.

Keywords: Magnesium alloys;Long period stacking ordered (LPSO);Ag addition;Nano-precipitates;Superplastic behavior.

1.Introduction

Lightweight metals and their alloys have emerged as important materials in automobile and aerospace engineering for improving fuel efficiency and reducing CO2emissions[1–4].Lightweight metallic materials are also needed for consumer electronics,communication devices,and computers [5–8].Magnesium alloys are some of the lightest structural metallic materials,however,their practical applications are limited owing to their low ductility and poor formability.At room temperature (298 K),slip systems activated in Mg alloys with hexagonal close-packed(hcp)lattice structures are rare [9,10].The poor room-temperature formability of Mg alloys significantly limits their practical applications,such as in cellphone and computer engineering [6,11].It is noteworthy that Mg alloys exhibit superplastic properties at elevated temperatures.Superplastic Mg alloys also exhibit uniform and high elongation without necking or pre-mature failure.Superplastic forming (SPF) has attracted significant attention in both academia and industry as a means of overcoming these drawbacks owing to its ability to fabricate large and complex products [3].

The critical resolved shear stress (CRSS) of the non-basal slip system in the hcp lattice structure of Mg alloys decreases at high temperatures [12–14].Superplastic properties may be achieved in the Mg alloys with the following microstructure characteristics: (1) a stable fine-grained or ultrafine-grained(UFG) structure [15];(2) a high fraction of high-angle grain boundaries (HAGBs);(3) abundant fine precipitates;and (4) a biphasic or multiphase structure [16,17].Superplasticity typically occurs at temperatures above 0.5Tm(Tmis the absolute melting point of the alloy) and strain rates ranging from 10-4to 10-1s-1.Grain boundary sliding (GBS),which is the governing deformation mechanism for achieving superplasticity [18],can be activated using an appropriate combination of microstructure and deformation conditions.Similar elongations are observed at low temperatures (≤0.5Tm) with low strain rates or at high temperatures (～0.8Tm) with high strain rates (above 10-3s-1);however,increasing the temperature can cause undesirable grain growth.In this case,a stable UFG structure or a uniformly distributed second phase can inhibit grain growth via the Zener pinning effect at the thermodynamic environment [5,19,20].The current focus of the research on SPF is achieving superplasticity at a low temperature with a high strain rate.Severe plastic deformation(SPD) techniques,including equal channel angular pressing(ECAP) [5,21],friction stir processing (FSP) [22,23],and high-pressure torsion (HPT) [24,25],can be used to fabricate magnesium alloys with a high fraction of HAGBs.Concurrently,SPD techniques produce a weak texture that can facilitate the high elongation in Mg alloys.Kai et al.[26] observed an excellent elongation of～800% in the AZ90 Mg alloy processed using HPT at 423 K.Vávra et al.[19] fabricated an WE43 Mg alloy with an ultra-fine grain size of 340 nm using ECAP,which exhibited an exceptional elongation of～1230%at a strain rate of 10-2s-1and a temperature between 623 and 673 K.Some Mg-rare earth (RE) alloys with high-strainrate superplasticity have attracted significant attention in recent years.A large number of heat-stable RE-rich phases are formed upon the addition of RE elements,improving thermal stability and reducing the texture intensity of SPD-processed Mg alloys [6,27–29].

Remarkably,the addition of Ag can induce an extremely significant grain refinement in Mg -RE alloys [30].Ag facilitates the precipitation of the second phase and the lamellar long period stacking ordered (LPSO) phase in the grain.A large lamellar LPSO phase inhibits recrystallization during the high-temperature tensile test (HTTT),due to its high thermal stability.The magnesium matrix restricted by LPSO phase does not grow significantly during superplastic deformation,which is beneficial to superplasticity of the alloy [30].Notably,the unique precipitation of theβ′andγphases has been observed in Mg-RE(-Zn) alloys.Mg-Gd(Y) and WE43 alloys exhibit the precipitation of theβ′phase on the prismatic plane,while that of theγphase on the basal plane is observed in the Mg-Gd(Y)-Ag and Mg-Ca-Zn alloys[31].Theaddition of Ag into the Mg-RE or Mg-RE-Zn alloy promotes grain refinement and forms a densely distributedγphase,while this dense precipitation ofγphase is not observed in Ag-free Mg-RE(-Zn) alloys.A largeγphase precipitated on the basal plane increases the CRSS of basal slip,effectively inhibits twinning,and avoids early fracture [32,33].

Table 1 Chemical compositions of the alloys studied.

Our previous studies have shown that a Mg-6.5Y-1.2Er-1.6Zn (WEZ612) alloy can obtain excellent strength and ductility via multi-pass rotary-die ECAP (RD-ECAP) processing[5].The microstructure and superplasticity of the Mg-Y-Er-Zn alloy during the HTTT have not been systematically studied,and the influence of Ag on the microstructure evolution and superplasticity of the alloy has not been reported in detail.Herein,we characterized the microstructure evolution of the ECAP-processed Mg-6.5Y-1.2Er-1.6Zn alloy after Ag addition during the HTTT.The evolution of the strain rate sensitivity index (m value) and the deformation activation energy(Q value) were discussed in depth.The mechanisms that limit the cavity extension during superplastic deformation were also investigated.

2.Materials and methods

As-cast ingots of Mg-6.5Y-1.2Er-1.6Zn (WEZ612) and Mg-6.5Y-1.2Er-1.6Zn-1Ag (WEZ612–1Ag) alloys were fabricated in an electric resistant furnace using commercial pure Mg (99.95 wt.%),Zn (99.95 wt.%),and the master alloys of Mg-30Y (wt.%) and Mg-20Er (wt.%).The compositions(wt.%) of the studied alloys were determined using inductively coupled plasma mass spectrometry (ICP-MS;Table 1).Specimens measuring 19.5 mm × 19.5 mm × 50 mm were cut from the ingot for rotary-die ECAP [34,35].The as-cast alloy was first homogenized at 723 K for 10 h and immediately quenched with warm water at 323 K.Based on the results of previous studies [5],the ECAP pass number was set to 16.The specimens were rough-polished before ECAP at 693 K for 16 passes.The cooling rate was controlled using an elaborate cooling procedure [5].

The superplastic behaviors of the WEZ612 and WEZ612–1Ag alloys were investigated by a testing machine (SUNS UTM4204X).ECAP-processed alloys were machined into a dog-bone-shaped stick with a gage length of 3 mm,width of 2 mm and thickness of 2 mm,respectively,along the extrusion direction [34].A series of HTTTs was conducted at 573,623,and 673 K and six different strain rates (5 × 10-4,1 × 10-3,5 × 10-3,1 × 10-2,5 × 10-2,1 × 10-1s-1) to determine the values of m and Q.The samples were held for 10 min to reach the designated temperature before deformation.The accuracy of the furnace temperature was controlled at ±1 K during testing.At least three specimens in each group were tested.

Fig.1.Microstructures of the (a) WEZ612 and (b) WEZ612–1Ag alloys after solid solution treatment.

Before and after the HTTT (1 × 10-2s-1,623 K),the microstructures of the alloys were observed using atomicresolution high-angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM,FEI Tecnai F20)at 300 kV.Furthermore,the electron backscattered diffraction(EBSD) analyses of the alloys before and after the HTTT(1 × 10-2s-1,623 K and 5 × 10-2s-1,623 K) were conducted using scanning electron microscopy (SEM;ZEISS Gemini 300) together with a Symmetry S2 EBSD detector(Oxford Instruments).Samples for EBSD test were electronpolished at 223 K and 12 V,using a solution of 10 mL perchloric acid and 90 mL ethyl alcohol.X-ray diffraction(XRD)analysis was conducted to complete the phase identification before deformation (analysis at 273 K) and after deformation(in-situ analysis at 623 K).Specimens used for the HTTT at different temperatures were all obtained from the direction parallel to the ECAP direction.

3.Results

3.1.Microstructure evolution of the studied alloys

The metallograph of the microstructures of the WEZ612 and WEZ612–1Ag alloys after solid solution treatment at 723 K for 10 h reveal coarse dendriticα-Mg and LPSO phases radially arranged along grain boundaries (Fig.1).Upon the addition of Ag,a mass with a refined phase appears in the Mg matrix.After ECAP (Fig.2),a part of the 18R-LPSO phase,which is difficult to break using most SPD processing methods,was twisted (red arrow),while the 14HLPSO phase was broken (yellow arrow).The LPSO phase is banded along the ECAP extrusion direction.The grain sizes of the WEZ612 and WEZ612–1Ag alloys after ECAP were acquired from the EBSD inverse pole figure(IPF)maps(Fig.3a and b),owing to the difficulty of determining the grain size from SEM images.The LPSO phase protruded on the surface and was difficult to identify using EBSD;therefore,the black unrecognized area in the IPF map was attributed to the LPSO phase.The average grain sizes of the WEZ612 and WEZ612–1Ag alloys after ECAP were 5.22±0.6 and 2.41±0.3 μm,respectively.

In contrast to the grains in the refined WEZ612–1Ag alloy produced by ECAP,those in the refined WEZ612 alloy have a predominantly basal orientation.The (0001),(110),and(100) pole figures of the ECAP-processed alloys are shown in Fig.4(a) and (b).The basal plane of WEZ612 is tilted toward the transverse direction (TD) and deflected by 30–45°,which differs from the texture characteristics of the ECAP-processed Mg alloy.The WEZ612–1Ag alloys show a completely different texture,whereinc-axes are perpendicular to the TD,and thus manifest a non-basal texture [36,37].The misorientation angle distributions of the alloys after ECAP are also illustrated in Fig.4.WEZ612 alloy possesses a high fraction of HAGBs (misorientation angle>15°,fHAGBs=0.901);however,thefHAGBsof the WEZ612–1Ag alloy is lower at 0.797.

3.2.Superplastic deformation

The high-temperature tensile deformation behavior of the WEZ612 alloy is sensitive to the deformation temperature and the strain rate (Fig.5(a)–(c)).Two different patterns were observed.At a lower temperature of 573 K,a high flow stress was observed at a strain rate of 1 × 10-2s-1,which gradually decreased until failure;however,at an elevated tensile temperature of 673 K,the flow stress significantly reduced and the superplastic behavior was obvious.This failure phenomenon at 573 K and the high strain rate can be attributed to the accumulation of a large number of dislocations at the grain boundaries,preventing the initiation of GBS and leading to the necking phenomenon at an early stage [3].GBS can be fully activated at temperatures at 673 K,and the flow stress behavior of the Ag-free alloy also shows a steady state tendency over the entire gage length,even at high strain rates.

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Fig.2.SEM images of the microstructures of ECAP-processed alloys: (a) WEZ612 and (b) WEZ612–1Ag.

Fig.3.Inverse pole figure (IPF) maps of the (a) WEZ612 and (b) WEZ612–1Ag alloys after ECAP.

Fig.4.Grain boundary misorientation distributions and (0001), (110),and (100) pole figures of the (a) WEZ612 and (b) WEZ612–1Ag alloys.

The superplastic properties of the WEZ612 alloy were significantly improved by the addition of Ag.The engineering stress-strain curves of the WEZ612–1Ag alloy demonstrate that the elongations of the Ag-containing alloy were superior to those of the Ag-free alloy (Fig.5).Even at 573 K and 1×10-2s-1,WEZ612–1Ag still had an elongation of 319%,demonstrating excellent low-temperature,high-strain-rate superplasticity.The superplastic properties of the WEZ612–1Ag alloy are better than those of WEZ612 owing to its finer grain size and strong non-basal texture.During the initial stage of deformation,the low CRSS of basal slip activates the codeformation of twins,leading to pre-mature fracture [38,39].Promoting non-basal slip through an earlier reduction of the CRSS of non-basal slip can effectively increase the elongation of the alloy [10,15,40,41].

Fig.5.Tensile stress-strain curves and the photographs of samples after HTTT.(a-c) WEZ612 alloy and (d-f) WEZ612–1Ag alloys deformed at 573–673 K and 5 × 10-4–5 × 10-2 s-1.

From the general trend,the elongation of the WEZ612 alloy and WEZ612–1Ag alloy decreased with an increasing strain rate,regardless of the temperature (Fig.6a and c).In contrast,at a constant strain rate,the elongation of the WEZ612 alloy and WEZ612–1Ag alloy gradually increased with increasing temperature.These results indicate that the elongation of the WEZ612 alloy and WEZ612–1Ag alloy are influenced by both the strain rate and the temperature.The Ag-containing alloys exhibit excellent superplastic properties at the investigated strain rates and temperatures.The WEZ612–1Ag alloy exhibited both high-strain-rate superplasticity with an elongation of 306% at 623 K and a strain rate of 5 × 10-2s-1,and low-temperature superplasticity with an elongation of 324% at 573 K and a strain rate of 1 × 10-2s-1.These results demonstrate that the WEZ612–1Ag alloy in this study achieves better dual-mode superplasticity than WEZ612 alloy.

3.3.Strain rate sensitivity and activation energy

Strain rate sensitivity (m) is calculated from the curve slopes in the plots of the peak flow stress of the WEZ612 alloy as a function of the tensile strain rate at different temperatures (Fig.7) using the following equation [42]:

The strain rate sensitivity of the WEZ612 alloy is in the range of 0.16–0.35.Our results determined thatmis 0.16 at a low temperature of 623 K with high strain rates in the range of 1 × 10-2–5 × 10-2s-1,which is characteristic of dislocation creep.

Activation energy (Q) is helpful in understanding and interpreting the deformation mechanism during the superplastic process.Qis calculated under a constant strain rate using the following equation [43]:

whereσis the peak flow stress,nis the stress exponent(n=1/m),Ris the gas constant (R=8.314 J mol-1K-1),Tis the absolute temperature,andcan be estimated from the slope of the curve in Fig.7b.The value ofQat a strain rate of 1 × 10-2s-1was 299.14 kJ mol-1,significantly higher than the activation energies of lattice diffusion(Q=135 kJ mol-1) and grain boundary self-diffusion (Q=75 kJ mol-1) in pure Mg.Similar results were reported for some Mg alloys containing RE elements [44,45],wherein GBS was assumed to be assisted by lattice diffusion.As a special strengthening phase in the RE-Mg alloy,LPSO phase with high hardness can be considered as a kind of the reinforced phases in magnesium matrix composites,similar to SiC.The Young’s modulus of the LPSO phase is significantly higher than that of the Mg matrix [5].The Mg matrix can transfer a certain stress to the LPSO phase,giving the alloy a higher apparent activation energy.

Themvalues of WEZ612–1Ag are in the range of 0.34–0.62 (Fig.7(c) and (d)).The addition of Ag increases the strain stress sensitivity of the alloy at low temperatures and high strain rates,and significantly reduces the activation energy.The WEZ612–1Ag alloy exhibitsmandQvalues of 0.46 and 128.50 kJ mol-1,respectively,at 623 K and a strain rate of 1 × 10-2s-1.The significant reduction in the activation energy may be related to the CRSS of non-basal slip [46–49].The factors affecting the superplasticity of the WEZ612–1Ag alloy will be discussed in Section 4.

Fig.6.Elongations of the WEZ612 alloy (a and b) and WEZ612–1Ag (c and d) alloys at various strain rates and temperatures.(a and c) elongation vs.strain rate at various temperatures;(b and d) elongation vs.temperature at various strain rates.

3.4.EBSD analyze of alloys after superplastic deformation

The grain growth of the WEZ612 alloy is insignificant at 623 K.Some researchers have reported that the grain size of the alloy should not significantly increase during the superplastic deformation at high temperatures,because this is not conducive to superplasticity [3,50].Our results indicate that at 623 K,the LPSO phase and fine precipitates can firmly pin the grain boundary,resulting in the observed insignificant increase in the grain size.At 623 K,the grain sizes of WEZ612–1Ag alloys significantly decreased after the HTTT,whereupon a larger average grain size was observed with a strain rate of 1 × 10-2s-1than that with a strain rate of 5 × 10-2s-1(1.83±0.4 and 1.31±0.2 μm,respectively),as shown in Fig.9.This may be due to the longer exposure time at 623 K for the samples at the strain rate of 1 × 10-2s-1.The internal strain level of the grains at a tensile rate of 5 × 10-2s-1is significantly higher than that observed at a tensile rate of 1 × 10-2s-1.

The addition of Ag improves the thermal stability of a given grain size at high temperatures.At the same temperature and tensile rate,the WEZ612–1Ag alloy exhibits superior thermal stability compared to that of the WEZ612 alloy.At 623 K and a strain rate of 5 × 10-2s-1,the WEZ612–1Ag alloy has a finer grain size of 1.31 μm and exhibits superior dual-mode superplasticity.The WEZ612–1Ag alloy has an activation energy of 128.5 kJ mol-1,which is lower than that of the lattice diffusion in pure Mg.Zhang et al.[44] reported a similar range of activation energies in an extruded Mg-Gd-YZr alloy.Such a reduction inQindicates that dislocation creep exerts a stronger influence on GBS than lattice diffusion.

Fig.7.(a,c) Peak flow stress of the WEZ612 and WEZ612–1Ag alloys as a function of the shear strain rate at different temperatures (573,623,and 673 K);(b,d) lnσ as a function of 1000/T at a strain rate of 1 × 10-2 s-1.

4.Discussion

4.1.Factors affecting high-strain-rate and low-temperature superplasticity

The effects of the presence of Ag in the ECAP-processed WEZ612–1Ag alloys can be summarized as follows: (i) The textures of ECAP-processed alloys significantly changed;(ii)the addition of Ag promoted the precipitation of theγphase;and (iii) a large number of nano-precipitates exist both at grain boundaries and in the grain interior.

Firstly,the effect of texture change is analyzed.An evident texture change occurred in the WEZ612 alloy upon the addition of Ag.The Ag-free alloy exhibited a strong basal texture,while the alloy containing Ag exhibited a non-basal structure(Fig.4).The superplastic properties of the WEZ612 alloy are primarily affected by its basal texture.The XRD analysis of the texture evolution in the Ag-free and Ag-containing alloys before and after the deformation at 623 K (in-situ analysis)shows that the prismatic(100),basal (0002),and pyramidal(101) planes ofα-Mg were distributed in the angle range of 30–40° (Fig.10).The relative intensities of the peaks corresponding to the prismatic(100),basal(0002),and pyramidal(101) planes are perhaps a more intuitive way of displaying the texture changes of the three samples before and after superplastic deformation (Table 2).The (0002) peak of theWEZ612 alloy exhibits a relatively high intensity (56.07%)compared to those of the other two peaks,which is characteristic of its strong basal texture.After the superplastic deformation at 623 K,the relative intensity of the peak corresponding to the pyramidal(101) plane increases from 12.32% to 34.23%,suggesting that the proportion of the non-basal texture in the alloy increases.Nevertheless,the intensity of the peak corresponding to the (0002) basal plane is still rather high at 46.52%,and the tensile fracture elongation of the WEZ612 alloy at 623 K is only 192%,indicating that dislocation climbing is dominant [51].The peak representing the pyramidal(101) plane of WEZ612–1Ag alloys before deformation is rather intense at 50.59%.After the deformation at 623 K,the relative intensities of the peaks corresponding to the prismatic(100) and pyramidal(101) planes sharply increased,while the relative intensity of that corresponding to the basal (0002) plane significantly reduced.In other words,the non-basal texture of the WEZ612–1Ag alloy was significantly developed after deformation,leading to an essentially random texture (Fig.11).The superior superplasticity of the Ag-containing alloy can be explained by the fact that the GBS mechanism was often accompanied by the formation of a random texture during superplastic deformation [3,51,52].

Table 2 Relative intensity of the XRD results in the WEZ612 and WEZ612–1Ag alloys before and after deformation at 623 K.

Fig.8.EBSD orientation and KAM profiles of the WEZ612 alloy after the HTTT at the strain rates of (a) 1 × 10-2 and (b) 5 × 10-2 s-1 at 623 K.

The gage sections of the WEZ612 and WEZ612–1Ag alloys were analyzed using EBSD after the tensile deformation at 623 K.The gage section subjected to the HTTT had a similar grain size after processing with ECAP,showing a sluggish grain growth at 623 K (Figs.8 and 9).A longer deformation time was observed at a tensile rate of 10-2s-1;therefore,the exposure time to high temperature was longer,resulting in a slight increase in the grain size.The average grain size in the WEZ612–1Ag alloy was 1.83 μm.The gage sections of the WEZ612–1Ag alloy subjected to the HTTT at a strain rate of 5 × 10-2s-1showed a smaller grain size than those of the samples before deformation,with an average grain size of 1.31 μm.Dynamic recrystallization (DRX) was conducted to elucidate the grain refinement during deformation [53,54].The corresponding pole figures indicate that Ag-containing WEZ612 alloys exhibited a more random,weaker texture after superplastic deformation (Fig.11).The WEZ612–1Ag alloy,under the strain rates of 10-2and 5 × 10-2s-1,exhibited the maximum texture intensities of 2.99 and 3.62,respectively,indicating that the texture intensity is influenced by the strain rate.The weaker texture after deformation further reflects the occurrence of GBS in the gage section during the HTTT.The GBS-controlled deformation can significantly weaken the texture in the superplastic region [4,52].

Fig.9.EBSD orientation and KAM profiles of the WEZ612–1Ag alloy after the HTTT at the strain rates of (a) 1 × 10-2 and (b) 5 × 10-2 s-1 at 623 K.

Fig.10.X-ray diffraction patterns of the surface of the samples before superplastic deformation (room-temperature analysis) and after deformation at 623 K(high-temperature in-situ analysis).

Fig.11.Pole figures of the (0002) and (100) planes of the WEZ612 and WEZ612–1Ag alloys after deformation at 623 K.Tensile deformations of the(a and b) WEZ612 alloy at the strain rates of 10-2 and 5 × 10-2 s-1,respectively and the (c and d) WEZ612–1Ag alloy at the strain rates of 10-2 and 5 × 10-2 s-1,respectively.

Fig.12.TEM images of the WEZ612–1Ag and WEZ612 alloy.(a and b) γ phase of the WEZ612–1Ag alloy,(c and d) Low-magnification image of the nano-precipitate phase and the cubic RE-rich phase in WEZ612 alloy.The electron beam in (a and b) is parallel to[100]Mg.

Fig.13.SEM-SE images of the WEZ612–1Ag alloy.(a) Low-magnification image showing the distribution of the precipitated phase,(c) High-magnification image of the white dotted area shown in (b).

Next,the effect of precipitation phases is discussed.The addition of Ag promoted the precipitation of theγphase(Fig.12a and b).Concurrently,the ECAP process refined the 14H-LPSO phase and produced a large number of submicronsized 14H phases in the alloy (Fig.13a).The high thermal stability during the HTTT is assumed to be a result of the pinning effect of the particles at grain boundaries (Fig.13b and c)and the submicron-sized 14H-LPSO phase.Bright-field TEM images (Fig.12a and b) demonstrate that many parallel nanosizedγphases were distributed throughout the grain interiors [55,56].In contrast to the WEZ612 alloy,WEZ612–1Ag alloys exhibited the presence of numerous fine particles,as shown in HAADF-STEM images(Fig.14).Ag co-precipitates with Gd and Y elements via atomic clustering across the grain boundaries.Ag addition effectively suppresses the solute drag effect of the segregated Gd and Y elements at the grain boundaries via producing new second-phases or cluster of atoms at these potential sites [57].Given that,the DRX would proceed with an enhanced rate for the WEZ612–1Ag alloy after ECAP with respect to the Ag-free counterpart.The DRX rate of the Ag-containing alloy is higher than that of the Ag-free alloy,and the fine grain size and random texture are maintained by DRX coordinated deformation during superplastic deformation.As a RE-rich phase,LPSO phase has higher hardness than magnesium matrix.The 18RLPSO phase has a large volume and usually penetrates several grains,and the lamellar structure is conducive to coordinated sliding deformation.The grains surrounded by 18R-LPSO phase are difficult to recover and grow during superplastic deformation.The fractured 14H-LPSO phase has a finer lamellar structure and is uniformly distributed near grain boundaries,which can coordinate grain boundary slip and hinder grain growth during superplastic deformation.These particles contributed to thermal stability,while the submicron-sized 14HLPSO phase and precipitates helped reduce the connection of the cavities generated during the HTTT (Section 4.2),thus preventing fracture during superplastic deformation.Precipitation hardening further improved the superplastic properties of Ag-containing alloys.The high density of the nanoscale basalγphase precipitates in the grain interiors of WEZ612–1Ag alloys effectively increased the CRSS of basal slip [31,58] and hindered dislocation motion,promoting the GBS mechanism to coordinate superplastic deformation.

4.2.Cavitation behavior of the WEZ612–1Ag alloys during superplastic deformation

The evolution of cavitation during superplastic deformation involves three stages: (i) crack nucleation;(ii) propagation of cavitation;and (iii) coalescence of the cavitation.The second stage can help elucidate the evolution of the cavity during superplastic deformation and determine the mechanism of fracture failure.Related studies [50,58,59] demonstrated that the different types of cavitation have different formation mechanisms and unique morphologies.Once the cavity development enters the third stage,the material often rapidly fractures.The mechanisms of fracture development during superplastic deformation can be divided into three categories [50]:diffusion-controlled,plasticity-controlled,and superplasticity diffusion-controlled growths.Diffusion-controlled growth typically produces circular cavities and does not produce an extended orientation in the tensile direction.Plasticity-controlled growth tends to produce elongated cavities,which are often connected along the slip direction of superplastic deformation.Superplasticity diffusion-controlled growth only occurs if the grains are extremely small (<5 μm);cavities penetrate grain boundaries and the rate at which adjacent cavities are connected is accelerated.The growth rate can be calculated using the following equation [60]:

whereΩis the atomic volume,δis the thickness of the grain boundary,Dgbis the grain boundary diffusion coefficient,kis the Boltzmann constant,Tis absolute temperature,σis the peak flow stress,and ˙εis the strain rate.At grain sizes of<5 μm,if the cavity radius is greater than half the average grain size,this equation can successfully distinguish between superplastic diffusion and plasticity-controlled growths.

The relevant calculations and observations of the cavity morphology reveal that a large number of cavities extend along the tensile direction,some of which intersect grain boundaries,and that the grain size near the cavities is<5 μm (Fig.15).The mechanism of cavity expansion in the WEZ612–1Ag alloy is considered to be controlled by two types of growth: plasticity-controlled growth and superplasticity diffusion-controlled growth.Most cavities were found to be blocked by the precipitated phase in the alloy along the expansion path,effectively delaying the cavity-to-cavity bonding effect,which effectively improves the superplasticity of the alloy.The 18R-LPSO phase prevents the cavity from extending further along the tensile direction (Fig.15a).In addition,the cavity is blocked by the spherical precipitated phase (Fig.15(b) and (c));therefore,it expands around the spherical precipitated phase,effectively delaying the cavity extension in the tensile direction.If the precipitate is sufficiently small,it can separate the cavities,thereby reducing their porosities.The cavity shown in Fig.15(d) is hindered by two different types of RE-rich precipitates [5],making it difficult for the cavity to expand along the tensile direction and form a triangular cavity in the cracks.The addition of Ag significantly promotes the precipitation of WEZ612,forming a large number of LPSO phases,along with circular and cubic precipitates.The formation of a large number of precipitates effectively hinders cavity expansion and further improves the elongation of the WEZ612–1Ag alloy.

5.Conclusions

The microstructure,texture evolution,and superplastic behavior of the WEZ612 and WEZ612–1Ag alloys processed by ECAP were investigated.The main conclusions are summarized as follows:

(1) Upon the addition of Ag,the texture of the WEZ612 alloy after 16 passes of ECAP significantly changed from a basal texture to a non-basal texture.This change in the texture type was responsible for the significantly improved superplasticity of the alloy.

Fig.15.SEM images of the WEZ612–1Ag alloy after the HTTT at a strain rate of 1 × 10-2 s-1 and 623 K.The white arrows indicate the LPSO phase and spherical/cubic precipitates.

(2) Upon the addition of Ag,the superplasticity of the WEZ612–1Ag alloy at a low temperature with a high strain rate significantly increased,and the strain rate sensitivity increased from 0.16 to 0.46 at 623 K and 1 × 10-2s-1.The activation energies of the WEZ612 and WEZ612–1Ag alloys were 299.14 and 147.89 kJ mol-1,respectively.The predominant superplastic deformation mechanism changed from a dislocation slip mechanism to a lattice diffusion-assisted grain boundary slip mechanism.

(3) The addition of Ag promoted grain refinement,precipitating a large number of nanoscale phases on the basal plane,which effectively increased the CRSS of basal slip,promoted grain boundary slip at high temperatures and improved elongation.Simultaneously,a large number of precipitates,including an LPSO phase along with spherical and cubic precipitates,effectively hindered the expansion of the cavities along the tensile direction,delayed the connection of cracks and improved elongation,significantly increasing the superplasticity of the magnesium alloy.

Declaration of competing interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work,there is no professional or other personal interest of any nature or kind in any product,service and/or company that could be construed as influencing the position presented in,or the review of,the manuscript entitled“Achieving high-strain-rate and low-temperature superplasticity in an ECAP-processed Mg-Y-Er-Zn alloy via Ag addition”by Haoran Wu,Jinghua Jiang,Zhenquan Yang,Mengjia Li,He Huang,Ningfei Ge,Aibin Ma,Huan Liu.

Acknowledgments

The study was supported by the Postgraduate Research and Practice Innovation Program of Jiangsu Province(SJKY19_0460),the National Natural Science Foundation of China (Grant No.51979099 &51774109),Natural Science Foundation of Jiangsu Province of China (Grant No.BK20191303),The Key Research and Development Project of Jiangsu Province of China (Grant No.BE2017148),and Postgraduate Education Reform Project of Jiangsu Province(JGLX19_027).Thanks to the Yangtze Delta Region Institute of Advanced Materials,the Center of Advanced Analysis &Gene Sequencing of Zhengzhou University and Nanjing Institute of Technology for their support in electron microscope characterization in this paper.We also thank Mr.Huang lu from Shiyanjia Lab (www.shiyanjia.com) for XRD analysis.