Severe plastic deformation (SPD) of biodegradable magnesium alloys and composites: A review of developments and prospects

M.Kasaeian-Naeini, M.Sedighi, R.Hashemi

Abstract The use of magnesium in orthopedic and cardiovascular applications has been widely attracted by diminishing the risk of abnormal interaction of the implant with the body tissue and eliminating the second surgery to remove it from the body.Nevertheless, the fast degradation rate and generally inhomogeneous corrosion subsequently caused a decline in the mechanical strength of Mg during the healing period.Numerous researches have been conducted on the influence of various severe plastic deformation (SPD) processes on magnesium bioalloys and biocomposites.This paper strives to summarize the various SPD techniques used to achieve magnesium with an ultrafine-graine(UFG) structure.Moreover, the effects of various severe plastic deformation methods on magnesium microstructure, mechanical properties,and corrosion behavior have been discussed.Overall, this review intends to clarify the different potentials of applying SPD processes to the magnesium alloys and composites to augment their usage in biomedical applications.

? 2021 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Peer review under responsibility of Chongqing University

Keywords: Magnesium; Severe plastic deformation; Biodegradable implant; Mechanical properties; Corrosion.

1.Introduction

Conventional metallic biomaterials such as Co-Cr alloys,stainless steel, and titanium alloys are commonly utilized as medical implants in fracture surgeries owing to their superior physical and mechanical properties and functional adaptations to biological environments [1,2].Nonetheless, the main disadvantages of these materials are: (I) the high elastic modulus differences between the implant and natural bone, which causes to stress shielding effect, (II) non-biodegradability in biological environments, which demands another surgery to remove the implant after the damaged bone tissue has adequately healed [3-5].Hence, these challenges have started a new era in the fiel of biomedical materials, and biodegradable materials have attracted much attention from researchers.Biodegradable materials - like biodegradable metals - dissolve entirely until the tissue is completely healed;afterwards,no implant residues could be detected due to the progressively corrode in vivo.Therefore, metallic ingredients are the significant components of biodegradable materials, and biological environments like the human body can metabolize them.Furthermore, the metallic elements are capable of controlling the degradation modes and rate in the human body [3,6].

1.1.Magnesium, an emerging biomaterial

Nowadays, owing to particular properties like high specifi strength, low density, proper casting properties, and high vibration absorption of magnesium and its alloys, they are extensively used in the aerospace, automobile, and electronics industries.Moreover, they have been presented as a substitute for traditional orthopedic and cardiovascular biomaterial implants thanks to their appropriate biocompatibility, biodegradation, and mechanical properties [7,8].One of the four most abundant elements in the human body is magnesium and it is indispensable for body metabolism.Besides, related studies have shown that approximately 50% of all magnesium storedin the adult human body is in bone tissue [9].As shown in Table 1, compared to other commercialized biomaterials,the density and Young's modulus of magnesium is closest to those of the human femoral cortical bone.Simultaneously,the fracture toughness of Mg is more significan than that of bioceramics like hydroxyapatite (HA) [5].

Table 1Mechanical properties of different biomaterials in comparison with femoral cortical bone [5].

Fig.1.Desirable degradation behavior of Mg implants in the healing period of bone tissues healing [11].

The most important limitation to employing Mg and Mg alloys as a biomedical material is their enormously high degradation rate in aqueous and physiological environments, even for a biodegradable implant.Thus, some problems such as reducing the mechanical integrity of the implant before the end of bone healing, the balloon effect that postpones the healing process of bone tissue, and osteolysis have occurred[10].The schematic diagram of optimal Mg implants degradation according to the mechanical strength versus the healing period of bone tissues is presented in Fig.1.Therefore, the researchers have been suggested many methods like designing new alloys, developing magnesium-based composites, surface modification and new processing technologies to overcome these problems.

Magnesium can be alloyed with several elements which classifie into three categories [12,13]: (I) toxic elements: Ba,Be, Cd, Pb, Th; (II) elements that cause allergic problems:Al, Cu, V, Ni, Ce, Cr, Pr, La, Co; (III) nutrient elements:Ca, Zn, Si, Mn, Sn, Sr.Moreover, there is a growing focus on Mg-based composites thanks to their valuable mechanical and corrosion properties obtained by selecting of the proper reinforcement material [14-16].Table 2 shows different reinforcement materials properties and applications.Besides,surface modificatio methods, including surface coating and other surface treatments (i.e., hydrothermal and alkaline heat treatments, chemical and mechanical surface modifications)have been employed to decrease the degradation rate of Mg and its alloys.On the other hand, many of the mentioned methods can bring other problems such as the decline of biocompatibility and mechanical properties and weak bonding between the coating layer and Mg surface, which its performance can be devastated [17,18].

Table 2The properties and applications of reinforcement materials.

1.2.SPD techniques, as supplementary process

In addition to all the mentioned procedures in the previous section, mechanical processing technologies have drawn great research attention on account of their simultaneous effects on improving mechanical properties,modifying textures,and refinin grains of Mg and its alloys that have various effects on corrosion resistance.Several techniques for mechanical processing containing rolling [32,33], forging [34],extrusion [35,36], drawing [37,38], and severe plastic deformation (SPD) methods have been studied on magnesium and its biodegradable alloys.

Until now, several number of severe plastic deformation (SPD) processes such as equal channel angular pressing (ECAP) [39-42], high pressure torsion (HPT) [43-45],multidirectional forging (MDF) [46,47], cyclic extrusion and compression(CEC)[48,49],accumulative roll bonding(ARB)[50,51], severe shot peening (SSP) [52], rotary swaging(RSW) [53], and parallel tubular channel angular pressing(PTCAP) [54]have been introduced by researchers to generate ultrafine-graine (UFG) or nanostructured materials.As illustrated by the known Hall-Petch relation, the strength of a material is reversely proportional to grain size.An essential benefi of SPD processes is producing UFG materials on various scales without the need for alloying elements and employing a variety of materials from single to multi-phase materials.Hence, this is particularly significan for biomaterials due to the biocompatibility limits the usage of diverse chemical compositions [55].Along with the desirable strengthening effects, the enhancement in tensile ductility has been gained by SPD processing of magnesium and its alloy.Other mechanical properties like fatigue strength, creep, superplastic-ity, formability can be influence by UFG structure [56- 58].The corrosion rate is another property of magnesium that is affected by grain size and microstructural characteristics.Although many papers concentrate on the type of corrosion and how SPD processes can alter the corrosion properties of magnesium and its alloys, its impact still remains uncertain.

1.3.Aims of the review

In the case of biodegradable magnesium alloys and composites, the ability of large-strain deformation processes to enhance the mechanical properties, control the corrosion rate,and modify the microstructure is a crucial point to access the operational magnesium-based implants.Therefore, this review focuses on the influenc of the SPD methods on the microstructure, corrosion resistance, and mechanical properties of biomedical magnesium alloys and composites.This review also presents a comparative study between different SPD parameters and methods applied to biodegradable magnesium.Overall, this paper attempts to peruse the various properties of SPD processed Mg-based alloys and composites to help enhance their use in medical applications.

2.The characterization of biodegradable magnesium after the SPD techniques

This section is dedicated to interpreting the effect of SPD processes on the properties of magnesium alloys and composites with the biomedical application.Firstly, microstructural studies such as grain shape and size, secondary phase distribution, dynamic recrystallization (DRX), and so on are scrutinized.Next, the ultimate and yield tensile and compression strength, elongation, and hardness of SPD-processed Mg alloys and composites are examined.Finally, the variations induced by SPD methods in magnesium's corrosion rate are analyzed.More than 100 articles have been reviewed in this section, and Fig.2 shows the number of investigated articles per year since 2005.As demonstrated, there has been more attention to using SPD techniques to improve the Mg properties in recent years.

2.1.Microstructure of Mg after SPD processing

Before SPD processing, the primary shape and size of asreceived Mg grains are often reported coarser and larger, but on the other hand, by using the SPD methods, the ultra-fin grains are produced owing to the strain-induced effect and dynamic recrystallization (DRX) [59-61].Besides, with increasing the mechanical deformation of Mg, the fraction of low angle grain boundaries(LAGBs)and misorientation angle have changed [62].Birbilis et al.[63]reported that the ECAP process of pure Mg caused the appearance of low angle grain boundaries, and their population decreased with subsequent passes.Additionally, they achieved a remarkable decline in the size of grains after ECAP down to 2.6 μm, as shown in Fig.3.

Han and Langdon [64]observed through an optical microscope (OM) for the Mg-Zr alloy processed by ECAP, the average grain size of as-received specimens was 75 μm and 55 μm in longitudinal and transverse directions, respectively.After the 6-ECAP passes,the average grain size is diminished to 8.6 μm, and some dark discontinuous Zr-rich stringers are perceived.The optical micrographs of before and after multipass ECAP of the as-cast ZE41A alloy are investigated by Jiang et al.[65].For the as-cast sample, theα-Mg grains and a few second phases are distributed in the grain boundaries.With increasing ECAP passes to 60 passes, the grain size is reduced to 2.5 μm, and fin particles are dispersed much more homogeneously [65].Figure 4 presents the microstructure and reinforcement distribution of Mg-based biocomposite(Mg/HA) processed by 4-pass ECAP with different contents of Hydroxyapatite (HA).As reported, by applying the ECAP process to these composites,the decrement of grain length and efficien destruction of the biocomposite initial texture are ap-parent [66].Gholami et al.[67]reported the existence of the new fin grains which surrounded the coarse grains due to the dynamic recrystallization (DRX) after the 4-pass ECAP process of ZFW MP Magnesium alloy (Fig.5).Besides, Ly et al.[68]observed that DRX and severe shear deformation have significantl decreased the average grain size from 54.5 to 1.6 after four passes of ECAP.They concluded that fine-graine microstructure could also help to form a denser coating of Mg-Zn-Ca [68].

Fig.2.Distribution of total number of articles on the effects of SPD processes on the properties of biodegradable Mg alloys and composites in the past 15 years.

Fig.3.The distribution of a) grain size and b) grain boundary misorientation angle for pure Mg before and after multi-pass ECAP [63].

Torbati-Sarraf et al.[69]investigated the texture of HPT specimens obtained through the EBSD image, and it is shown that the prismatic {10ˉ10} plans of as-received samples changed to the basal {0001} fibe texture and illustrious dispersion of the grain boundary misorientations ～30° after fi e turns of HPT.The inverse pole figur maps of commercial purity (CP) polycrystalline magnesium processed by 1/8,1/2, 10 turns of HPT are shown in Fig.6.It is observed that pure Mg after 1/8 rotation at ambient temperature has a multimodal microstructure consisting of fin and coarse grains with twinnings.However,with increasing the number of HPT turns(N), the area fraction of coarser grains reduces, and refine grains are predominate in the microstructure.Therefore, it is difficul to fin grains with a size of 1μm after ten turns of HPT [70,71].Besides, it is reported for WE43 and ZX20 biodegradable Mg alloys processed by HPT that the grains are refine to submicron, and the microstructural homogeneity and suitable distribution of second phases are achieved[72-74].

Considerable grain refinemen of biodegradable Mg-Zn-YNd alloy is observed during the CEC process by Wu et al.[75].It has been reported that owing to the DRX occurring during the process, the grain size is decreased to 1 μm, and a large number of the second phase particles are dispersed at grain boundaries with grid shape [75].Besides, according to Lin et al.[76], the fraction of high angle grain boundaries (HAGBs) in as-received ZK60 alloy is comparatively low, while the fraction of HAGBs severely grew with the rise of the CEC strain.Equivalent results have been observed in AZ31 during the ECAP and CEC [77,78].The effects of the MDF process temperature for ZAXM4211 Mg alloy on texture and microstructure evolution are investigated by Bahmani et al.[79].It is remarked that the fraction of the sec-ond phase and the concentration of grain boundaries decrease with the rising temperature in the MDF process because of the solubility of the added element at different temperatures.Furthermore, performing the MDF process at temperatures below 200 °C is not able to refin the extruded grains and create a significan change in extrusion texture [79,80].The effect of SPD process parameters on the microstructure of biodegradable magnesium is summarized in Table 3.

Table 3Summary of the effect of some important parameters affiliate with SPD processes on the microstructure of the magnesium.

Fig.4.Microstructure of Mg/xHA composite ECAP processed a) x = 5 b)x = 10 c) x = 15 [66].

2.2.Mechanical behavior of Mg after SPD processing

The mechanical properties, including the yield, ultimate tensile, and compressive strengths, Young's modulus, hardness, ductility, and fatigue endurance limit of Mg-based bioimplant, must be adequate during the service life.Additionally, the grain refinemen and improving the mechanical properties of the magnesium can be affected by processing conditions like SPD processes.Torabi et al.[115]introduced a combined SPD method including ECAP,CEC,and extrusion to produce Mg-Zn alloy and investigated the effect of different content of Zn on the hardness and mechanical properties.As shown in Fig.7, they concluded that by adding the Zn element more than 3wt%, the hardness, ductility, and ultimate tensile strength reduce.Wenting Li et al.[116]studied the mechanical behavior of the pure Mg, Mg-1Ca, and Mg-2Sr processed by 6-pass ECAP.The ECAP process was accomplished at 400 °C and 350 °C on a die set withΦ=120°intersecting angle.They observed that ECAP processing is an efficien method to improve the strength and ductility of alloys, especially for Mg-1Ca and Mg-2Sr.The engineering stress-strain curves of three biomedical magnesium alloys are depicted in Fig.8.

Although the coarser grain size of extruded AZ31 Mg alloy than that from the ECAPed condition, the extruded alloy showed a higher yield strength(YS)which was ascribed to the rotation of basal plans in the ECAP process [117-119].On the other hand, the heat generation in lattice during the SPD processes causes DRX and creating a fin uniform structure.So, the ductility is increased significantl after the SPD methods.The UFGed ZE41A magnesium alloy was produced by 32 passes ECAP technique at 330 °C, and the results showed a 65% enhancement in ductility than the value of the as-cast specimen owing to change prevailing deformation mechanism to grain boundary sliding (GBS) and presence of HAGBs in ECAPed samples [120].

Fig.5.OM images of ZFW MP alloy in various conditions a) as-extruded b) 1-pass ECAP c) 3-pass ECAP d) 4-pass ECAP [67].

Fig.6.Inverse pole figur maps of Mg during the HPT [71].

Fig.7.Stress-Strain curves of the Mg-xZn produced by combined SPD method [115].

Fig.8.a) The stress-strain curves of magnesium biomedical alloys.b) yield strength, ultimate strength, and elongation of Mg alloys [116].

Huang et al.[90]fabricated Mg/HA composite by combined high solid shear solidificatio and multi-pass ECAP method at 300 °C.They found that Mg/HA composite with 3wt% and 5wt% hydroxyapatite after 2-4 passes have the best combination of ductility and compressive strength with maximum compressive reduction and yield strength of 13%and 210 MPa, respectively [90].Also, the yield strength and hardness of the Mg-Zn-Ca-Mn alloy are reduced owing to the weakened basal texture after the firs pass of ECAP, but with growing the number of ECAP passes, the enhancement in hardness, yield strength, and elongation is quite noticeable compared to extruded alloy (Fig.9) [99].Besides, the torsional and tensile properties of ECAPed WE43 Mg alloy were considered by Zhang et al.[121].They reported that the ECAP process could have different effects on torsional and tensile properties due to the different basal plans behavior on various loading conditions [121,122].

Figure 10 shows the effect of T4-treatment, one pass ECAP, and two pass ECAP on compressive properties of Mg-4% Ag.The ultimate compressive strength (UCS), compres-sive yield strength (CYS), and hardness of Mg-4% Ag alloy tend to rise when the number of ECAP passes increases.The maximum hardness, UCS, and CYS obtained after the second pass of ECAP are about 54 Vickers, 325 MPa, and 62 MPa,higher than those of T4 treatment by 31.7%, 12%, and 100%,respectively [123].

The HPT process is another severe plastic deformation method to improve the strength and hardness of Mg alloys but usually leads to a drastic decrease in ductility.For example,a ZEK100 alloy exhibited an ultimate tensile strength (UTS)of 339.3 MPa with an elongation of 1.5% when processed at room temperature (RT) [100], and a WE43 alloy demonstrated a yield strength (YS) of more than 300 MPa with a strain to failure of 1% [124].On the other hand, the tension behavior of pure magnesium is different from Mg alloys due to the simultaneous increase in ductility and strength after the HPT process.As reported, the GBS is the main deformation mechanism in UFG pure magnesium at RT [125].Figure 11shows the hardness variations of ZK60A disks after extrusion,6 GPa compression, and various turns of HPT processing under a pressure 6 GPa at RT along the disk diameters.As can be seen, the maximum value of hardness and excellent hardness homogeneity was achieved after fi e revolutions of HPT.However, the inhomogeneous hardness distribution is observed in other HPT turns because of the larger strain at the edges of the samples [126].Besides, the microhardness of Mg-Y-Gd-Zr alloys after ten turns HPT processed at RT,200 °C, and 300 °C could be further increased by aging treatment (Fig.12) [127].Furthermore, Mg-Zn-Ca alloy exhibits proper UTS (270 MPa) and adequate elongation (8.5%) after HPT processing and subsequent annealing at 200 °C as compared to the initial state owing to the dispersion hardening and activation of dislocation slip in non-basal plans [128].

Fig.9.(a) hardness (b) stress vs.strain curves of Mg alloy after multi-pass ECAP processing [99].

Fig.10.Compressive stress-strain curves of Mg-4%Ag in various conditions[123].

Fig.11.The microhardness values of ZK60A processed by HPT under 6 GPa and 0, 1/4, 1/2, 1, 3, and 5 turns [126].

Fig.12.Microhardness of Mg-Y-Gd-Zr alloy after HPT and aging treatment[127].

Recently, the CEC process was efficientl utilized to produce Mg alloys with UFG structures.Tian et al.[111]prepared Mg-1.5Zn-0.25Gd (at.%) alloy with appropriate mechanical properties and fine microstructure by multi-pass CEC process.The elongation of samples processed by eight passes CEC was about 4.5 times higher than that of as-cast alloy.CEC processing was also performed to improve the mechanical behavior of ZK60 alloy at 230 °C; the YS, UTS, and elongation after one pass CEC increased slightly because of fine-grai strengthening.After four passes CEC, the elongation raised to 38%, which is three times higher than before CEC processing, and this could be attributed to grain boundary sliding and the smaller fraction of basal poles for CEC processed samples [110,129].Amani and Faraji [130]usedCyclic Expansion Extrusion (CEE) at 330 °C and 400 °C to refin the microstructure of WE43 Mg alloy.The yield strength, ultimate tensile strength, and elongation of the alloy after two passes CEE improved by 200%, 90%, and 250% at the processing temperature of 330 °C, respectively (Fig.13).Recently, Kavyani et al.[131]showed that the half equal channel angular pressing(HECAP)process could improve the mechanical behavior of Mg-Zn-Ca-Mn alloy due to the proper distribution of precipitations and fined-grai microstructure.As shown in Fig.14, in addition to significantl improving the ultimate tensile and compressive strength after two pass HECAP, the elongation and compressive failure strain have risen about 101.7 and 60.8%, respectively [131].The mechanical properties of some Mg alloys processed by SPD processes are summarized in Table 4.

Table 4Summary of mechanical behavior of ultrafin grained biomedical Mg alloys and composites.

Fig.13.The mechanical properties of unprocessed and CEC processed the Mg based WE43 alloy [130].

2.3.Effect of SPD methods on corrosion behavior of Mg

Although Mg shows good corrosion resistance in the atmosphere at room temperature, the degradation rate of Mgbased alloys in the physiological environment is dramatically increased owing to its lowest standard potential (-2.37 V)among all engineering metals.Thus, the magnesium alloys are used comprehensively for the cathodic protection of other metals [140,141].The thin MgO and Mg(OH)2surface film cannot protect the substrate against corrosion in the physiological environment,adequately[142].The magnesium degradation behavior under physiological conditions is depicted in Fig.15.

Fig.15.Mg degradation behavior in physiological environment and different corrosion products [160].

Many researchers have acknowledged that Mg has unusual corrosion behavior like negative difference effect (NDE) and chunk effect of magnesium [22,143].The increase of the hydrogen evolution rate (a cathodic reaction) during the anodic polarization of magnesium and its alloys in various electrolytes is related to the "NDE" phenomena [144,145].In 1866, it was firs announced by Beetz [146], and various models such as metal spalling, Mg univalent ion formation,impurity enrichment,and film-breakd wn were proposed to illustrate this phenomenon from 1940 to 2000 [147].However,in the last decade,with growing attention to magnesium in the medical and industrial applicants, various studies have been conducted on NDE phenomena.In potentiodynamic analyses,determining the current density is difficul due to the occurrence of NDE in the anodic curve.Thus, the extrapolating method of the cathodic Tafel zone is used to obtain the current density[148].In addition to conventional methods for assessing NDE, such as electrochemical procedures, combined mass loss, and hydrogen collection [144], the atomic emission spectroelectrochemistry (AESEC) [149]and inductivelycoupled plasma mass spectrometry (ICP-MS) [150]have recently been utilized.Besides, the ability to create an integrated and stable surface fil by adding different alloying elements to magnesium to suppress NDE has been examined by researchers in recent years [147,151-153].Furthermore,numerous studies have focused on the conception of Mg corrosion behavior in various solutions and proposed different methods to decrease magnesium's degradation rate.Nevertheless, SPD processing has been considered due to improving the mechanical properties and proper impact on corrosion resistance simultaneously.Despite producing the UFG magnesium, these processing routes can alter the texture, residual stresses, and second phase distribution, which each of them can also change the degradation rate.

Fig.14.(a) Compressive and (b) tensile stress- strain curve of Mg-Zn-Ca-Mn at different conditions [131].

Izumi et al.[154]investigated the corrosion behavior of Mg-Zn-Y with various grain sizes obtained by different cool-ing rates solidification They demonstrated that the corrosion rate depended on grain size,and the filifor corrosion of Mg-Zn-Y has retarded by increasing the cooling rate because of forming a supersaturated singleα-Mg phase and grain refine ment.Filiform and pitting corrosions are the most common forms of corrosion in magnesium and its alloys, especially in chloride solutions, which usually coincide.Although filifor corrosion takes place in steels under coatings, it occurs in bare magnesium alloys.After the onset of pitting corrosion,the filifor corrosion creates as superficia and thin filament protrude from the pits [155,156].Lunder et al.[157]declared that filament grow due to differences in gas concentration at the tip and tail, and the electrochemical transition of chloride ions to the tip of the filamen is one of the most crucial components of filifor corrosion growth.Another reason for decreasing the corrosion rate with finin the grains is stress-relieving on the surface because of the unconformity between substrate and MgO or Mg(OH)2fil by grain refinemen [141,158].

Fig.16.The surface corrosion morphology of as-received AZ31 and processed alloys (W1 and W2) after 2 h, 24 h, 48 h, and 144 h immersed in PBS with the mean grain size of 20 μm, 2-6 μm, and 0.72 μm, respectively[165].

Nonetheless, many studies also showed that the grain size reduction is caused to accelerate the corrosion rate of Mg alloys.Song et al.[92]found that pure Mg after ECAP treatment showed an increase in corrosion rate compared with the as-cast samples.The same results on the corrosion rate were observed in ZFW MP and AE21 after the SPD processes[67,159].Thus, these researches demonstrated that the SPD processes effects on corrosion behavior of magnesium alloys and composites are still under dispute.

The Investigation of the effects of SPD methods on magnesium corrosion behavior can be complicated for various reasons.Firstly, an unstable and time-dependent passive fil is formed on Mg in an aqueous solution, and its permeability changes abnormally with the time of immersion [55,161].Additionally, the dynamic recrystallization occurs due to the relatively higher SPD processing temperature,and it may provide inhomogeneous Mg microstructures.Therefore, the poor corrosion resistance of UFG magnesium can be related to its heterogeneous microstructure [162].Finally, the texture dependency of corrosion in Mg and alloys is high, and it depends on mechanical processing or the SPD method.Besides,the basal planes have the most significan corrosion resistance owing to the lower propensity for pits formation and lower dissolution rate, so it is difficul to reach a general conclusion on corrosion behavior [163,164].

In addition to alloy composition and type of corrosive solution, the surface corrosion morphology hinges on the grain size.As shown in Fig.16, the surface morphology of highratio differential speed rolling (HRDSR) processed AZ31 Mg alloy after immersed in phosphate-buffered saline (PBS) solution changed by grain size.The grain size of AZ31 adjusted the density and diameter of precipitates, and a more compact corrosion product layer is formed by grain refinemen[164,165].

In a comprehensive study, Silva et al.[136]compared the corrosion properties of pure Mg with various mechanical processing procedures containing as-cast, rolling, rolling and annealing, rolling and ECAP, and HPT and observed that the HPT and rolling plus ECAP samples have larger arcs in Nyquist diagrams than others which indicate a reduced incorrosion rate.Besides, the potentiodynamic polarization test in 3.5 wt%.NaCl revealed that the corrosion current density of material processed by SPD is strikingly lower than other conditions due to the formation of a thicker protective layer[136].Besides, it is found that the protective layer is restored faster in UFG materials with a homogenous microstructure,which leads to enhance corrosion resistance [161].

Fig.17.The polarization curves of ECAPed ZE41A after a) 1 hour b) 4 days immersion in Hank's solution [166].

Fig.18.The corrosion rate of WE43 Mg alloy for different conditions [168].

The effect of the multi-pass ECAP process on the electrochemical behavior of ZE41A magnesium alloy in Hank's solution is studied by Zhang et al.[166].Fig.17 demonstrates the potentiodynamic polarization curves for various passes ECAP process after immersion for 1 hour and four days.It is readily apparent that the corrosion potential quantities shift positive, and the current density is diminished with increasing the number of ECAP passes, which may be attributed to enhanced resistance against corrosion [166].Furthermore,a recent report showed that processing by ECAP could decrease the corrosion rate of ZE41 Mg alloy, but a reduction in processing temperature and an increase in dislocation density can deteriorate the corrosion resistance [167].

Zhang et al.[168]checked the effect of ECAP and subsequent aging treatment on biodegradable properties of WE43 Mg alloy.The EIS and potentiodynamic tests showed a higher corrosion resistance after one pass ECAP process but reduced dramatically with more ECAP passes.Nevertheless, the subsequent aging at 200 °C for 24 h further improves the corrosion resistance of one pass ECAPed samples, as shown in Fig.18 [168].A similar decrease in corrosion rate was also observed for ZK60 and AZ31 Mg alloys after aging of ECAPed samples.It may be related to the internal stressrelieving, better fil protection, and redistribution of second phases [83,169].Besides, the effects of the ECAP process on corrosion rate and stress corrosion cracking (SCC) of AZ31 alloy have been scrutinized by Peron et al.[88].As displayed in Fig.19, they reported that the firs pass of ECAP could decrease theicorrand quantity of hydrogen evolved by 77%and 20%, respectively, by immersing the samples in SBF solution.However, by increasing the number of ECAP passes,corrosion rate and SCC susceptibility have also increased because of the arrangement of the basal planes in the shearing direction [88].

There are some studies about the influence of the HPT method on the corrosion properties of Mg alloys.Recently,the comparison of electrochemical behavior between pure magnesium, ZK60 Mg alloy, and AZ31 Mg alloy under various conditions in 3.5% NaCl media is fulfille by Silva et al.[170].They observed that the high pressure torsion process decreases the corrosion rate of pure magnesium,but its impact on corrosion resistance of extruded AZ31 and ZK60 in 3.5%NaCl solution is negligible.Finally, they concluded that the HPT could play a more conspicuous role in enhancing the corrosion resistance when testing in less invasive solutions[170].Torbati-Sarraf et al.[171]investigated the corrosion behavior of ZK60 magnesium alloy after different numbers of HPT turns.The hydrogen evolution test in 0.1 M NaCl solution revealed that with increasing the number of rotations,the volume of hydrogen gas released is reduced (Fig.20).This can be assigned to the formation of more uniform and fine grains compared with other samples, which cause to improve the stability of smooth oxide fil and diminish the micro galvanic cell [73,171].

Fig.19.(a) Potentiodynamic polarization curves and (b) hydrogen evolution quantity of as-received and ECAPed magnesium alloy [88].

Fig.20.The hydrogen evolution for ZK60 magnesium alloy after extrusion and different turns of HPT process [171].

Few reports have exhaustively studied the corrosion behavior of Mg alloys after the multidirectional forging process.The electrochemical properties of ZAXM4211 magnesium alloy after MDF at various temperature is investigated by hydrogen evolution, weight loss, and potentiodynamic polarization tests in 3.5 wt%.NaCl solution.Thus,it is observed that higher temperature processing can reduce the corrosion rate owing to the decrement in the fraction of second phases and subsequently decrease the galvanic corrosion [172].

Martynenko et al.[173]prepared ultrafine-graine Mg alloy containing 3.57%Y,2.2%Nd,and 0.47%Zr with a diameter of about 0.5- 0.8 nm through multiple steps rotary swaging(RSW)with growing stages of extrusion ratio from 2.56 to 2.78.The corrosion rate was appraised using the weight loss,emission of hydrogen, and potentiodynamic polarization tests,and the results indicated that the degradation rate of WE43 did not change after RSW significantl [173].However, it is reported that hybrid SPD processing involving ECAP and RSW can improve the corrosion resistance and fatigue corrosion of Mg-Zn-Ca alloy in 0.9% NaCl solution [174].Nevertheless, some inconsistent results on the effect of different SPD methods, including ECAP, HPT, MDF, CEC, and RSW,on Mg alloys and composites corrosion behavior are reviewed in Table 5.

Table 5 (continued)

Table 5Summary of the SPD processes effect on corrosion behavior of magnesium alloys and composites.

3.Conclusion and future prospects

According to the extensive developments and studies that have been done in recent years on the use of magnesium as biodegradable implants,various countries have begun the clinical use of Mg-based implants.The German company"Syntellix" is the firs manufacturer of the bioresorbable magnesium(Mg-Y-Re-Zr alloy) screws used in Hallux valgus surgeries,and in 2013, Communauté Européenne (CE) approval was issued for the use of these screws [183].Later, the Korea Food and Drug Administration (KFDA) approved the clinical usage of magnesium K-MET screws made by the U&I company in 2015 [184].Besides, the high-purity Mg screws have been fabricated by Eontec in china as internal fixatio implants.Due to the long-term (12 months) effectiveness, the multicenter clinical experiments of these screws were approved by the CHINA National Medical Products Administration (NMPA)in July 2019 [185,186].Despite the approval of magnesium's biomedical application by some health organizations globally,there are still challenges for their widespread use.The most important of these challenges are:

1 The high rate of degradation of magnesium screws and hole creation in the peri-implant space

2 Reduction of mechanical integrity after applying in the human body.

On the other hand, the research on biodegradable biomaterials revealed the potential of SPD methods as an appropriate procedure for improved the mechanical and corrosion properties of magnesium alloys and composites.In this study, the effects of severe plastic deformation processes have been reviewed concerning the microstructure, mechanical properties,and corrosion behavior of Mg-based alloys and composites.The most widely used SPD methods for biocompatible magnesium,viz.ECAP,HPT,CEC,MDF,and RSW were focused herein.

These processes are widely applied to decrease the grain size, which has generally demonstrated effectiveness in improving Mg-based alloys' mechanical strength and corrosion performance.Moreover, recrystallization during the process through the high process temperature and heat generated by plastic deformation has a significan role in ascertaining the size and morphology of the deformed grains.In addition to grain size, thermomechanical processes can drastically alter magnesium's internal stress state, twinnings, defects density,and crystallographic orientations.The changes made in the mentioned cases will have a significan impact on the corrosion rate and mechanical strength.According to the previous sections, the mechanical properties of magnesium are improved by increasing the dislocation accumulation, texture modification and obtaining a fine magnesium microstructure by increasing the plastic deformation degree.

Overall, it appears that using the appropriate SPD technique to produce a UFG microstructure magnesium with prominent basal texture and minimum residual stress has shown promising results in the fiel of biodegradable biomaterials.It can even make magnesium the superior metal used in biomedical applications.Hence, achieving the optimal values of SPD parameters for magnesium alloys and composites should be considered.Despite the ability to eliminate defects induced by the composite fabrication process,the employment of SPD techniques for magnesium matrix biocomposites has been received less attention than its alloys by researchers.Besides, although the results of in vivo and in vitro biocompatibility examinations show the favorable effect of SPD methods on magnesium, realizing its long-term performance requires precise and comprehensive tests.Therefore, future studies should concentrate on obtaining appropriate values of SPD parameters to achieve the best corrosion and mechanicalproperties,more comprehensive investigations on the effect of severe plastic deformation processes on magnesium biocomposites, and accurate and complete experiments to understand their long-term performance in biological conditions.

Declaration of Competing Interest

The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.