J.Wng,T.Li,H.X.Li,*,Y.Z.M,K.N.Zho,C.L.Yng,J.S.Zhng,*

aState Key Laboratory for Advanced Metals and Materials,University of Science and Technology Beijing,Beijing 100083,China

b Department of Petroleum Equipment of RIPED,PetroChina,Beijing 100083,China

c State Key Laboratory of Solidificatio Processing,Northwestern Polytechnical University,Xi’an 710072,China

Abstract Magnesium alloys,a novel functional material for the fabrication of fracturing tools,are being paid more and more attentions recently due to their relatively high mechanical properties and fast dissolubility ability after fracturing.In this study,the novel extruded Mg-10Gd-3Y-0.3Zr-xNi alloys will be reported and their microstructure,mechanical and corrosion behaviors will be also studied.The results show that Ni contents influenc phase precipitation behaviors.With adding 0.2wt% Ni,a large amount of Zr7Ni10 phases will be precipitated insides α-Mg matrix,directly leading to degradation of strength and large corrosion rate.With further increasing Ni contents,the precipitation phases can be changed from Mg5RE to 18R-LPSO structure,resulting in higher mechanical properties and faster corrosion rate.Moreover,adding Ni element also change the texture orientation by influencin the precipitation behavior of the alloys.The alloys invented in this paper have attained the highest compressive and tensile properties among all the reported dissoluble magnesium alloys.This work is beneficia in understanding the role of Ni in the magnesium alloys and provides more materials alternatives for the fabrication of dissoluble fracturing tools.? 2020 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:Dissoluble magnesium alloys;Microstructure;Mechanical properties;Corrosion behaviors;Fracturing tools.

1.Introduction

In recent years,magnesium alloys have become attractive candidates for automotive,aerospace and power transmission industry due to their lightweight,high specifi strength and other attractive properties[1-4].However,weak corrosion resistance of magnesium alloys restricts their application in these industrial field[5].On the other hand,magnesium alloys could be an ideal material as fracturing tools due to its large corrosion rate and relatively favorable mechanical properties.There have been some researches on the application of magnesium alloys as fracturing balls[6-16].

Admittedly,the corrosion properties of magnesium alloys are very sensitive to some impurity elements,e.g.Fe,Cu,Ni and Si,which exhibit significan effects in accelerating the corrosion rates of magnesium alloys,even at contents below 0.2wt% for some certain impurities[17].For example,Xiao et al.investigated the effects of Cu and Si on the mechanical and corrosion behaviors of as-cast Mg-Al-Zn alloys[7-11].They found that adding Cu in the Mg-17Al-3Zn alloys influence the precipitation behavior of T-phases and further affects the properties of the alloys.The highest compression strength(438MPa)and the largest corrosion rate(82.2 mg?cm?2?h?1in 3wt% KCl at 93°C)can be obtained for the Mg-17Al-3Zn-5Cu alloy[8].For the Mg-17Al-5Zn-xSi alloys,yield strength could reach a maximum value of 309MPa in the Mg-17Al-5Zn-0.5Si alloy while the dissoluble rate could attain a highest value in the Mg-17Al-5Zn-3Si alloy[10].

Compared with Fe,Cu or Si elements,Ni element exhibits the most obvious influenc on the corrosion behaviors of magnesium alloys[18].Niu et al.[12,13]prepared novel Mg-4Zn-xNi alloys.It is shown that Mg2Ni phase can be precipitated after Ni addition,which can exacerbate the corrosion rates of alloys due to the promotion of galvanic reaction.Han et al.[14]investigated the influenc of long-period stacking ordered(LPSO)phases on the mechanical and corrosion properties in the Mg-Gd-Ni alloys.The results show that as the volume fraction of the Ni-containing LPSO phase increased,the strength increased remarkably,but the plasticity decreased.Moreover,the LPSO phase containing Ni was preferentially eroded than the Mg matrix.Wang et al.[19]prepared as-cast Mg-Ni-Y alloys as a potential material for fracturing balls by introducing the Ni-containing LPSO phase to improve the mechanical properties and corrosion rate of the alloys.The LPSO phase with higher corrosion potential can form more micro-couples with the matrix and then accelerates the corrosion process.But when the contents of LPSO phase is too high,it will become a corrosion barrier between the corrosion medium and the substrate.Summarizing the current progress on Ni alloying in the dissoluble magnesium alloys,it is noted that the existing investigations are mainly being concentrated on the ternary alloy systems[6-16,19].In the multi-component magnesium alloys,i.e.more than quaternary alloy systems,the effect of Ni on the microstructure and properties,especially the mutual interaction between Ni and other elements,is still unclear.Meanwhile,the current study on Ni alloying is basically restricted to be as-cast alloys and up to date the role of Ni in the wrought magnesium alloys has rarely reported.On the other hand,since the dissoluble magnesium alloys are mostly as-cast alloys,the direct problem is that their mechanical properties are not high due to the coarseness of the microstructure and the presence of casting defects.Currently the development of dissoluble magnesium alloys with the better performance has become a necessity for ensuring the service of fracturing tools under all kinds of mining conditions.Therefore,it is essential to fabricate the wrought dissoluble magnesium alloys,especially aiming at the role of Ni in the extruded magnesium alloys.

Mg-Gd-Y-Zr alloys has received extensive attention in recent years due to their excellent mechanical properties[20-22].Especially,Mg-10Gd-3Y-0.5Zr alloy(GW103K)exhibits the higher mechanical properties compared with other Mg-Gd-Y-Zr alloys[21].In addition,Zn element is often added into the Mg-Gd-Y-Zr alloys to form a LPSO phase,thereby further improving the performance of the alloys[23-25].As another important transition group element to form the LPSO structure,Ni addition into the Mg-Gd-Y-Zr alloys has not been investigated currently.Therefore,in this study different amounts of Ni were introduced into the commercial GW103K alloy to prepare novel extruded Mg-Gd-Y-Zr-Ni alloys,aiming at preparing a magnesium alloy with good mechanical properties and high corrosion rate for dissoluble fracturing tools application.The effect of Ni on the microstructure,mechanical and corrosion properties,especially its influenc on the texture of the alloy and the mutual interaction between Ni element and other elements,will be concentrated specially.

2.Materials and experiments

2.1.Specimen preparation

The nominal alloy compositions used in this study were Mg-10Gd-3Y-0.3Zr-xNi(x=0,0.2,0.6,0.8wt%,hereafter dominated as Ni0,Ni0.2,Ni0.6,Ni0.8 alloys).Commercial GW103K alloys and Mg-25Ni(wt%)master alloys were melted in an electrical furnace under a SF6/CO2(1:99)atmosphere.At 710°C,the melt was cast into a water-cooled steel mold with a diameter of 95mm.The analyzed compositions of the alloys are exhibited in Table 1.The alloy ingots were homogenized at 520±5°C for 10h,and then quenched in cold water.The homogenized ingots were preheated at 420±5°C for 20min,and then extruded at 425±5°C with an extrusion ratio of 22:1 at a rate of 0.4 mm?s?1.

Table 1Analyzed composition(wt%)of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys.

2.2.Microstructure characterization

The specimens for microstructure analysis were cut from the extrusion rods perpendicular to the extrusion direction.X-ray diffraction(XRD,Rigaku D/max 2550V diffractometer)was used for investigating the phase constitutions.The samples were ground through SiC paper and polished with diamond grinding paste for microstructure observation using scanning electron microscope(SEM,Zeiss Spura 55)equipped with an energy-dispersive X-ray spectrometer(EDS).The transmission electron microscope(TEM,Tecnai G2 F20)were used to give a detailed visual description about the types,morphologies and distributions of the precipitated phases.The samples for TEM test were prepared via ion thinning technology.For electron backscatter diffraction(EBSD)tests,the specimens were ground and finall electro-polished in an ACII solution at?20 °C cooled by liquid nitrogen with a 20V applied potential and a 0.2 A current.

2.3.Mechanical and corrosion properties

Tensile tests were performed along the extrusion direction(ED)using the dog-bone samples by an uniaxial tensile test at a strain rate of 1×10?3s?1.The compression test was performed on a CMT 4305 testing machine using the samples with a dimension ofΦ3mm×6mm at a strain rate of 1×10?3s?1.All the measurements were conducted at ambient temperature.The mechanical test of each composition was repeated at least three times and the morphologies of the fracture surface were observed by SEM.

Fig.1.XRD patterns of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys.

The corrosion behaviors of the extruded alloys were investigated by immersion tests and electrochemical tools in KCl solution.Immersion tests were performed in 1wt% and 3wt% KCl at 25°C and 93°C,respectively.The ratio of the surface area to solution volume was 1 cm2:20ml according to ASTM G31-72 and the corrosion morphologies were analysed by SEM.Open circuit potentials(OCP,V vs.SCE),polarization curves and electrochemical impedance spectroscopy(EIS)tests were performed in 1wt% KCl solution at 25°C with a PS-168A electrochemical testing system.Open circuit potentials measurements began immediately after the samples were immersed into the solution.The EIS and polarization experiments were also carried out after the OCP tests.EIS tests were done over a frequency range from 100 KHz to 100 mHz with an amplitude of 5mV.The polarization curves were conducted at a rate of 0.5mV/s.To measure the Volta potential of the precipitated phases andα-Mg matrix,a scanning kelvin probe force microscopy(SKPFM)were applied.The experiments were conducted at ambient temperature under a relative humidity of 40±5%.

3.Results

3.1.Microstructure characterization

Fig.1 exhibits the XRD pictures of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys.It reveals that the precipitated phases in the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys change significantl with the increase of Ni contents.The Ni0 and Ni0.2 alloys consist mainly ofα-Mg and Mg5RE phases and in addition,a spot of Zr7Ni10phase appears in the Ni0.2 alloy due to the Ni addition.As the Ni content further increases,the precipitation amount of Mg5RE phases and Zr7Ni10phase in the alloy decrease or even disappear,and a new phase with a LPSO structure was formed.

Fig.2.SEM micrographs of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys(a)Ni0,(b)Ni0.2,(c)Ni0.6 and(d)Ni0.8.

The SEM micrographs of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys are manifested in Fig.2.The Ni0(Fig.2(a))and Ni0.2(Fig.2(b))alloys consist ofα-Mg matrix and white particle phases.EDS results show that the constituent of the white particle phases in the alloys is mainly Mg,Gd,Y,which can be further characterized by TEM.In the Ni0.6 alloy(Fig.2(c)),due to the increase of Ni,a large amount of gray phases composed of Mg,Gd,Y,Ni elements are formed in the alloy.In addition,since the gray phase consumes rare earth elements,the precipitation amount of white particle phase decreases.When the Ni content is increased to be 0.8wt%,the precipitated phases change almost from the white particle phases to the fla e gray phases,only leaving a very small amount of white particle phases distributed at the edge of the gray phase.Moreover,a larger precipitation free zone is formed for the Ni0.8 alloy compared with other alloys with low Ni contents.

To further characterize the precipitated phases,the samples are analyzed by TEM and selected area electron diffraction(SAED).From the results,the Ni0 alloy(Fig.3(a))consist ofα-Mg and particle phases,which is in accordance with the SEM results.And the composition of the particle phases is estimated to be 84.69 at% Mg-11.16 at% Gd-3.89 at% Y-0.26 at% Ni,which is known as Mg5RE phase in Mg-Gd-Y-Zr based alloys[26].The particle Mg5RE phase is further confirme from the SAED patterns shown in Fig.3(f)(cubic structure,a=b=c=0.9767nm).Unlike the Ni0 alloy,besides theα-Mg and Mg5RE phases,many fin needle-like phases are observed inside the grains for the Ni0.2 alloy,shown by the dashed circle in Fig.3(b).The composition of these phases is 70.21 at% Mg-12.68 at% Zr-15.15 at% Ni-0.57 at% Y-1.38 at% Gd.The reason for the higher Mg concentration is speculated to be from the small width of the needle-like phases,so the Mg signal derived fromα-Mg is found inevitably.The needlelike phases are confirme to be Zr7Ni10(orthorhombic structure,a=b=0.7712nm,c=0.9234nm)combined with energy spectrum analysis and high-resolution image analysis(Fig.3(g)).When the Ni content increases to be 0.6wt%,the morphology of the precipitated phase in the alloy change significantl described in Fig.3(c).The density of Mg5RE in the alloy decreases,and the Zr7Ni10phase is hardly observed,and an irregular plate-like phase is formed with the composition of 86.89 at% Mg-3.81 at% Gd-3.33 at% Y-5.97 at% Ni,which can be identifie to be Mg6(Gd,Y)Ni phase.By analyzing the SAED pattern(Fig.3(h)),the plate-like Mg6(Gd,Y)Ni phase has a 18R-LPSO structure which is common to Mg-RE-TM magnesium alloys[26].The 18R-LPSO structures consist of 18 stacking sequences of ABCBCBCBCABABABCBC as shown in the high magnificatio BF images(Fig.3(i)).As the Ni content further increases to be 0.8wt%(Fig.3(d)),the amount of 18R-LPSO structure in the alloy continuously increases while only few Mg5RE phases can be seen at the edge of the 18R-LPSO structure in the TEM pictures.

Fig.3.TEM micrographs of the alloys(a)Ni0,(b)Ni0.2,(c)Ni0.6,(d)Ni0.8;selected area electron diffraction of(e)α-Mg,(f)Mg5RE,(h)18RLPSO,and high magnificatio BF images of(g)Zr7Ni10 and(i)18R-LPSO.

Fig.4.(a)The average grain size of the extruded alloys;(b-e)EBSD orientation(IPF-X)maps with the extrusion direction and corresponding(0002)pole figure(PFs)and inverse pole figure(IPFs)of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys:(b)Ni0,(c)Ni0.2,(d)Ni0.6 and(e)Ni0.8.

To examine the texture,the orientation maps,pole figure(PFs)and inverse pole figure(IPFs)with the projection plane perpendicular to the extrusion direction is depicted in Fig.4.All the alloys present an utterly recrystallized microstructure with some differences in grain size and texture.As shown in Fig.4(a),the mean size of the Ni0 alloy is 4.9μm.When Ni elements was added,the mean size of grains initially decreases to 4.1μm for the Ni0.2 alloy,and then gradually increases to be 5.3μm for the Ni0.8 alloy.In addition to the difference in average grain size,Fig.4(b-e)also shows the effect of Ni on the texture of the extruded alloys.From the corresponding(0002)PFs and IPFs,it shows that Ni addition has a remarkable impact on the grain orientation.The EBSD orientation map of Ni0 alloy(Fig.4(b))presents colorful,indicating a relatively random texture.The basal planes have no significan difference in orientation,with a relatively concentrated distribution parallel to the ED and orientated at～45°from the ED.The IPFs confir that the structure may be a mixed morphology composed of four different components,including〈10-10〉,〈2-1-10〉parallel to ED,the direction between〈0001〉and〈2-1-10〉parallel to the extrusion axis and a weaker direction close to〈0001〉.When 0.2wt% Ni was added into the Mg-Gd-Y-Zr alloy,the orientation changed obviously.The basal planes are changed from orientation at～45° from the ED to be perpendicular to the ED.The〈10-10〉texture component become weaker and the components between〈0001〉and〈2-1-10〉deflec to the direction of〈2-1-10〉.From the Fig.4(d),with the Ni content increases to be 0.6wt.%,the basal planes are distributed on the circumference in the PFs,indicating that the basal planes are parallel to the ED.A strong〈10-10〉and a relatively weaker〈2-1-10〉texture component are noticed.For the Ni0.8 alloy(Fig.4(e)),the basal planes distribute more dispersed and a texture component between〈10-10〉and〈2-1-10〉parallel to the ED is observed in the IPFs.In addition to the difference in distribution,the peak intensity of the textures also changes with the addition of Ni.It can increase from 3.2(Ni0)to 4.76(Ni0.6)and then drop to 4.58(Ni0.8).

Fig.5.(a)Engineering tensile stress-strain curves of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys,(b)Tensile properties of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys and the morphologies of tensile fracture surface for the alloys(c)Ni0,(d)Ni0.2,(e)Ni0.6 and(f)Ni0.8.

3.2.Mechanical properties

Fig.5 exhibits the engineering tensile stress-strain curves of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys and the corresponding fracture morphology of the alloys.Compared with Ni0 alloy,the addition of 0.2wt% Ni makes the ultimate tensile strength(UTS)and tensile yield strength(TYS)decrease from 326.7 to 312.7MPa and 276.8 to 253.1MPa,respectively,whereas the elongation increases from 11.3%to 13.7%.As the Ni content further increases,the strength and the plasticity of the alloys are further enhanced.The UTS and TYS of the Ni0.8 alloy reach 342.9MPa and 257.8MPa,respectively,and the plasticity can be maintained to 15.2%.The fracture of Ni0 alloy consists of plenty of fin dimples and cleavage planes(Fig.5(c)).As presented in Fig.5(d),in addition to the dimples and cleavage planes,some sharp fracture edges are also observed for Ni0.2 alloys.With the Ni content continues to increase,the dimples become larger and deeper but their number decreased and replaced by more cleavage planes(Fig.5(e)).Especially,in the Ni0.8 alloy,a large number of dimples are replaced by cleavage planes(Fig.5(f)).

The compression properties of the alloys are summarized in Fig.6(a).Similar to the tensile test results,as the Ni content increases,the ultimate compression strength(UCS)and compression yield strength(CYS)of the alloys firstl fall and then climb.As the addition of Ni increase to 0.2wt%,the UCS of the alloy does not change much,but the CYS reduces by 30MPa.Afterwards,the strength increases with the further addition of Ni.Ni0.8 alloy exhibits the highest compression mechanical properties,i.e.the UCS and YS can reach 596.5MPa and 296.5MPa,respectively.Different from the strength,the elongation of the alloy increase with the addition of Ni,from 14.4% of the Ni0 alloy to 17.8% of the Ni0.8 alloy.Fig.6(b-e)shows the fracture morphology of the materials after compression test.All the alloys exhibit similar fracture morphology with plenty of typical cleavage facets and cleavage steps.However,it can be clearly seen that as the Ni element increases,the cleavage facets is obviously increased.In addition,it can be seen that the cleavage steps of Ni0 and Ni0.2 alloy is mainly at the edge of Mg5RE phase,while the cleavage steps of Ni0.6 and Ni0.8 alloy is mainly near the LPSO structure.

Fig.6.(a)Compression properties of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys and the morphologies of fracture surface for the alloys(b)Ni0,(c)Ni0.2,(d)Ni0.6 and(e)Ni0.8.

3.3.Corrosion properties

Fig.7 shows a comparison of the mass loss rate of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys after immersion for 12h at different conditions.From the results,the mass loss speed of the Ni-containing alloys is noteworthy faster than that of the Ni-free alloy,demonstrating that the Ni-containing alloys are more corrosive.The mass loss rate is ranked as Ni0.2>Ni0.8>Ni0.6>Ni0 under the same conditions.Moreover,the environmental conditions,i.e.solution concentration and temperature,have a great influenc on corrosion rate of the alloys.Once the Ni-containing alloys were immersed in the solution,plenty of bubbles were severely generated on the surface.As time increases,the bubble generation became more violent,and this phenomenon persisted during the test.However,it is worth mentioning that at the beginning of the experiment,the bubbles presented on the surface of the Ni0.2 alloy were less than that of the alloys with a higher Ni content.As the reaction proceeds,the rate of the bubbles formation on the surface of the Ni0.2 alloy increased obviously.

Fig.7.Mass loss rate after immersion for 12h at different conditions of the alloys.

The open circuit potential curves and polarization curves of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys are given in Fig.8.The OCP value of the alloy increases in the initial stage of the reaction,and gradually becomes stable with increasing the immersion time.Moreover,the addition of Ni element significantl increases the OCP value of the alloys.The corrosion potential(Ecorr),corrosion current density(Icorr),the slope of the cathodic polarization curve(βc)and the slope of the anodic polarization curve(βa)are represented in Table 2.As demonstrated in Fig.8(b),with the addition of Ni,the alloys have shifted the corrosion potential(?1.48 VSCEfor Ni0)towards more positive direction with a value of?1.31 VSCEfor Ni0.2,?1.24 VSCEfor Ni0.6 and?1.27 VSCEfor Ni0.8.Ecorrmainly depicts the thermodynamic properties,which cannot evaluate the corrosion rate[12,27].In general,the anodic curves represent the dissolution of magnesium as reaction(1),and the cathodic polarization curves represent the cathodic hydrogen evolution through water reduction as reaction(2)[28]:

Fig.8.(a)Open circuit potential and(b)polarization curves of the alloys in 1wt% KCl at 25°C.

Table 2Results obtained from polarization curves of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys in 1wt% KCl solution.

Fig.9.EIS of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys after immersion in 1.0wt% KCl solution:(a)Nyquist plots curves,(b)Bode plots curves,(c)Equivalent circuits of the Ni0 and Ni0.2 alloys and(d)Equivalent circuits of the Ni0.6 and Ni0.8 alloys.

However,due to the negative different effects,as the polarization potential increases to the anode zone,the release of hydrogen does not stop[28].Conversely,when the polarization potential increases to a positive potential than theEcorr,the polarization potential becomes more intense[2,28].In addition,the Ni-containing alloys exhibit an evidently higher cathodic polarization current density compared with the Ni0 alloy,indicating a relatively higher cathodic hydrogen evolution.There is little difference between Ni0.6 and Ni0.8 alloys,but they are significantl higher than Ni0.2 alloy.Fig.8 also reveals that the difference between the anode curves of the Ni-containing alloy is not as obvious as the one in the cathode curve.It reveals that the effect of Ni on the dissolution reaction of magnesium is not as indispensable as its effect on the hydrogen evolution rate.

The EIS results of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys measured in 1wt% KCl are described in Fig.9.From Fig.9(a),the Nyquist plots curves of Ni0 and Ni0.2 alloys show the semblable characteristics:two capacitance loops in the high frequency and the intermediate frequency region.For the Ni0.6 and Ni0.8 alloys,besides the two capacitance loops,there is also an inductive loop in the low frequency region.Generally,the high frequency capacitance loop is attributed to the charge transfer resistance of the electric double layer at the interface of metal and solution[28].The intermediate frequency capacitance loop represents the dissolution of the matrix[29].The low frequency inductance loop might be resulted from the localized or pitting corrosion[30].Based on the Bode plots in Fig.9(b),the impedance of the alloys at low frequency scope drops with the increase of Ni content,implying that the Ni0 alloy shows the best corrosion resistance and Ni0.8 alloy demonstrates the highest corrosion rate.The Bode plots of frequency vs.phase angle(Fig.9(b))are introduced to estimate the difficult of charge transfer during corrosion[12].The phase angle variation at middle frequency capacity resistance reduces with the increase of Ni content,which suggests that the corrosion rate of Ni0 alloy is the slowest,and the corrosion rate of Ni0.8 alloy is the fastest,which is consistent with the Nyquist curve.To state the corrosion mechanism of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys,the equivalent circuits and the fittin data of the EIS spectra are set out in Fig.9(c,d)and Table 3.Rs,RctandRfrepresent solution resistance,charge transfer resistance and fil resistance,respectively[12].A capacitor to compensate for the inhomogeneity caused by the existence of second phases,scratches and oxide film on the surface is replaced by constant phase element(CPE)[31].CPE1represents the electric double layer capacitance at the interface of corrosive media and alloys,while CPE2represents the fil resistance and capacity[31].LandRLrepresent inductance and electric resistance,respectively,which means pitting corrosion in the alloy[32].With the content of Ni increased,the values ofRctandRfdecrease,and the values of CPE1and CPE2increase,which demonstrates that the corrosion rate of the alloy becomes large.

Table 3Fitting results of the EIS spectra of experimental alloys.

Table 4Comparison of mechanical properties and corrosion properties between some dissoluble magnesium and our newly developed alloy.

Fig.10.SEM micrographs on the surface of the alloys immersed in 1wt% KCl for 1min and 1h at 25°C,respectively:(a)(e)Ni0,(b)(f)Ni0.2,(c)(g)Ni0.6,(d)(h)Ni0.8.

Micrographs on the surface of the alloys immersed for 1min and 1h in 1wt% KCl at 25°C are present in Fig.10.The micrographs shown in Fig.10(a,b)indicate that no severe corrosion occurs around the Mg5RE particles of the Ni0 and Ni0.2 alloys after immersion for 1min.However,for the Ni0.6 and Ni0.8 alloys,severe corrosion occurs at the positions where 18R-LPSO structure exists(Fig.10(c,d)).Subsequently,the morphology of the alloys without removing corrosion product after immersion for 1h are also observed(Fig.10(e-h)).The surface morphology of Ni0 alloy(Fig.10(e))is relatively complete and almost no pores are formed,so its corrosion rate is extremely slow.For the Ni0.2 alloy(Fig.10(f)),as time increases,a lot of corrosion holes are presented in the grains,which is due to the presence of Zr7Ni10phase.With the increase of Ni contents,because of the formation of the 18R-LPSO structure,the pitting corrosion occurred in the alloys as shown in Fig.10(g,h).

To verify the influenc of precipitation on the corrosion of the alloy in the above results,Volta potential maps of the Ni0,Ni0.2 and Ni0.8 alloys are shown in Fig.11.The maps reveal that the precipitated phases formed in the alloys have different Volta potentials from the matrix.The Volta potential values of theα-Mg,Mg5RE,Zr7Ni10and 18R-LPSO structure are 10mV,68.57mV,?32.38mV and?157.26mV,respectively.This demonstrates that the Zr7Ni10or 18R-LPSO structure can be preferable sites for pitting corrosion in the corrosion process.The difference in the Volta potential for the18R-LPSO structure andα-Mg is 170mV,whereas the one between the Zr7Ni10andα-Mg in the alloy is 40mV,demonstrating that the presence of 18R-LPSO structure should have a more beneficia effect on accelerating corrosion rate than the presence of Zr7Ni10.

Fig.11.SKPFM image and Volta potential profile of the alloys(a)Ni0,(b)Ni0.2 and(c)Ni0.8.

4.Discussion

4.1.Microstructure and texture evolution

From the above results,it shows that the Ni addition could change the type of precipitated phases in the alloys.The Ni0 alloy consists ofα-Mg and Mg5RE phase.For the Zr element,the solubility in magnesium is 0.2wt% at room temperature,so the excess Zr is present in the alloy in the form ofα-Zr as the nucleation center during solidificatio[5].When 0.2wt%Ni is added,Ni andα-Zr can tend to form Zr7Ni10phase[33].With the Ni content increases to 0.6wt%,Mg,Gd,Y,Ni elements can form 18R-LPSO structure,leading to the decreased precipitation amount of the Mg5RE phase(Fig.2(c)).Especially,once the Ni content gets to 0.8wt%,the formation of a large number of 18R-LPSO structures can lead to a very low contents of Mg5RE(Fig.2(d)).To our knowledge,the 18R-LPSO structure is a metastable phase,and it may be converted to 14H-LPSO after heat treatment[34].However,the addition of Ni element can increase the stability of the 18R-LPSO structure[35].Thus,in present study the 18RLPSO structure cannot be transformed into 14H-LPSO structure after heat treatment.On the other hand,for the Ni0.6 and Ni0.8 alloys,the content of Zr element is less than 0.2wt%,i.e.below the solid solubility of Zr in Mg,and therefore the existence of the Zr7Ni10phases are not observed in these alloys.

Typically,the magnesium alloys will display a basal fibe texture with(0002)plane parallel to ED after hot extrusion[24].However,in this study it can be seen that the partial basal planes in the Ni0 alloy have a rotation angle of about 45°related to the basal planes in the RE-free alloys.In the Mg alloys containing RE elements such as Gd,Y,the presence of the RE-rich phases and the enrichment of RE elements at the grain boundaries may change the texture,by impeding recrystallization in solute drag effect on grain boundaries and pinning effects of the RE-rich phase[36].Therefore,in the dynamic recrystallization process after hot extrusion,a large number of grains in the alloy can only rotate at a certain angle,thereby forming the texture distribution.Unlike the Ni0 alloy,the basal planes in the Ni0.2 alloy deflecte by nearly 90°,which is substantially perpendicular to the extrusion direction.This may be due to the existence of Zr7Ni10phase in the alloy,which further inhibits the rotation of the grains during recrystallization.With the Ni content increases,the basal planes in the Ni0.6 and Ni0.8 alloys are as same as that in the non-RE magnesium alloys,which is parallel to the ED.On one hand,the formation of the LPSO structure consumes the RE element,thereby impairing the influenc of the RE element on the modifie texture during recrystallization.On the other hand,the LPSO structure contributes to dynamic recrystallization,making it easier to form the base texture((0002)//ED)[37].In addition,for all the extruded alloys,the texture intensity is relatively low and the orientation distribution is more diffuse because the high extrusion temperature and the large extrusion ratio create an entirely dynamic recrystallization microstructure with a comparatively random texture[13,24].Therefore,the difference of the composition changes the type of precipitates in the extruded Mg-Gd-YZr-Ni alloy,which is the main reason for the obvious change in the texture orientation of the alloys.

4.2.Mechanical properties

It can be seen from the Fig.5 that the tensile properties of the alloys could change obviously with the Ni contents.It is noted that the main factors affecting the mechanical properties of the extruded Mg-Gd-Y-Zr-Ni alloy are grain refinement precipitation strengthening and texture strengthening.Firstly,the effect of grain size on YTS can be obtained from the Hall-Petch relationship[38]:σs=σ0+,whereσ0andkare material constants anddis the average grain size of the alloy.According to the results in Fig.4(a),the difference in grain size of the alloy is not very obvious(ranging from 4.1μm to 5.3μm).Thus for the extruded Mg-Gd-YZr-Ni alloy,the grain refinemen is not a main factor affecting the mechanical properties.Secondly,at ambient temperature,there are only two slip modes in the magnesium alloys,i.e.the(0002)basal plane slips along the two directions〈11-20〉and〈2-1-10〉[39].During the extrusion process,the grains in the magnesium alloy have different texture orientations.In the tensile test,the Schmid factor for the basal slip system of the grains with basal texture is 0,and the basal slip would be hardly started.In this study,the strength of the basal texture of the alloy is as follows:Ni0.8>Ni0.6>Ni0>Ni0.2,which is consistent with the strength of the alloy.It indicates that the texture strengthening plays an important role in the Mg-Gd-Y-Zr-Ni alloys.Thirdly,precipitation strengthening may be another main factor affecting the mechanical properties of the alloys.From the fracture surface morphology of the alloys,the Ni0 alloy exhibits a mixed fracture surface with both dimples and cleavage planes.Under the action of external force,the dislocation moves and piles up between the grain boundary and the Mg5RE phase.When the force exceeds a certain limit,the micro-cracks generated at the interface,and the crack propagation causes the alloy to fracture,resulting in dimples formed at the location where the Mg5RE phase is located.In addition,a small amount of transgranular cracks is present so that the cleavage plane can be seen in the fracture morphology.Compared with the Ni0 alloy,the dispersed Zr7Ni10phases inside the grains of the N0.2 alloy can serve as a potential crack source during the tensile test,leading to a decrease in strength.On the other hand,the plasticity of the Ni0.2 alloy increases because the dispersed Zr7Ni10phase causes more transgranular fractures during the fracture process[40].When the Ni content increases to 0.6wt%,the 18R-LPSO structure is formed.The LPSO structure and matrix is coherent at the interface,so they can withstand higher stresses.On the other hand,the coherent interface also avoids the plugging of dislocations.Therefore,the strength and plasticity of the alloy are apparently enhanced[41,42].With the higher Ni contents,the strength of the Ni0.8 alloy increases because of more precipitation of the 18R-LPSO structure.However,the Mg/LPSO interface increases significantl in the Ni0.8 alloy,which makes it easier to generate stress concentration and promotes the generation of cracks,which leads to the slight decline of the plasticity compared with Ni0.6 alloys.

During the compression process,the deformation of the magnesium alloys is mainly along the(0002)basal plane,which is similar to the tensile process.Since the slip systems is less,the alloys exhibit brittle behavior and the compression fracture exhibits typical cleavage characteristics.During the slipping process,the dislocation motion is blocked by Mg5RE phase,and dislocations are generated to initiate cracking,so the compressive plasticity of the Ni0 alloy is lower and a large number of cleavage planes are observed.Similar to the tensile results,due to the formation of the dispersed Zr7Ni10phase,the strength of the Ni0.2 alloy decrease.The further addition of Ni element reduces the Mg5RE phase in the alloy and forms the 18R-LPSO structure.Most of the block-like or fibe-like LPSO structures are uniform along the ED with their(0002)basal planes parallel to ED.The primary deformation mode of the LPSO structure is(0002)<11?20>slip.In addition,the Schmid factor for basal slip is extremely small during deformation.Therefore,the LPSO structure may be considered as an external reinforcing phase[39].Furthermore,due to the weakening role of the Mg5RE phase on the dislocation motion and the LPSO structure undergoing kink deformation during the compression process,the LPSO-containing alloys exhibit much higher strength and better plasticity[43].

4.3.Corrosion behavior

Based on the above results,the corrosion rate of the alloys after Ni addition is greatly enhanced due to the presence of precipitated phases.The schematic diagram of corrosion mechanism of Ni0,Ni0.2 and Ni0.8 alloys is depicted in Fig.12.For the Ni0 alloy,the Mg5RE phase has a higher Volta potential than the matrix,which may act as galvanic cathodes to increase the corrosion rate of the alloy.But Chang et al.reported that it is difficul for the pits to occur on theα-Mg matrix because of the formation of protective oxide film which contains RE elements[28].Therefore,the corrosion of the Ni0 alloy is very weak after immersion in the KCl solution for 12h.The Ni addition can remarkably accelerate the corrosion of the alloys owing to the formation of Zr7Ni10phase and the 18R-LPSO structure.The Volta potential of these phases are lower than that of the matrix,thereby forming a galvanic reaction with the matrix to accelerate corrosion process of the alloys.For the Ni0.2 alloy(Fig.12(b)),owing to the small volume fraction of the Zr7Ni10phase in the alloy,no evident corrosion occurs at the beginning of the immersion.However,as time goes on,galvanic corrosion occurs betweenα-Mg and Zr7Ni10phase,destroying the complete grains and causing the alloy to form a large number of holes.Thus,the KCl solution can enter the inside of the grains,which greatly increases the contact region between the alloy and the corrosive solution,resulting in an accelerated corrosion of the Ni0.2 alloy.Unlike the Ni0.2 alloy,the precipitated phase in the Ni0.8 alloy is mainly the 18R-LPSO structure.It has a lower potential(?157.26mV),which is more susceptible to electrochemical reaction with the matrix.Therefore,in the early stage of corrosion as shown in Fig.12(c),more serious localized corrosion easily occurs near the 18R-LPSO structure.However,since the 18R-LPSO structure is formed only at the grain boundary and there is a large precipitation free zone,which is not easy to corrode,the corrosion rate of Ni0.8 alloy is gradually slower than that of Ni0.2 alloy with the time prolonged.The corrosion rate of Ni0.6 alloy is in the middle value of Ni0.2 and Ni0.8 because of less 18R-LPSO structure and almost without Zr7Ni10phase.On the other hand,for all the alloys containing Ni,the corrosion rate is higher at 93°C than that at 25°C since the anode and cathode reactions and ion diffusion are thermally activated at high temperature[44].It is suggested in Fig.7 that the 18R-LPSO structure is more sensitive to temperature during the corrosion process than the Zr7Ni10phase due to its lower corrosion potential.

Fig.12.Schematic diagram of corrosion mechanism of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys(a)Ni0,(b)Ni0.2 and(c)Ni0.8.

From the above results and discussions,it can be seen that due to the precipitation of a large amount of 18R-LPSO and its larger susceptibility to corrosion,Ni0.8 alloy attained the highest mechanical properties and moderate corrosion rate.Table 4 compares the mechanical and corrosion properties of our newly developed Mg-10Gd-3Y-0.2Zr-0.8Ni alloy with other reported typical dissoluble magnesium alloys[6-11,13-16].It can be seen that among all the invented dissoluble magnesium alloys so far,the Ni0.8 alloy obtains the highest ultimate compressive strength(596.5MPa)and the largest ultimate tensile strength(342.9MPa).Considering its better elongation and a relatively decent corrosion rate,the Ni0.8 alloy is promising to provide an excellent alternative for the fabrication of fracturing tools applied in the exploitation of shale oil and gas.

5.Conclusions

In this study,the influenc of Ni addition on the microstructure,mechanical and corrosion properties of the extruded Mg-10Gd-3Y-0.3Zr-xNi alloys is investigated.Based on the observation and analyses,the conclusions can be summarized as follows:

(1)The Ni addition can influenc the precipitation behaviors of the phases in Mg-Gd-Y-Zr alloy.With the addition of 0.2wt% Ni,a new Zr7Ni10phase,having a size of several hundred nanometers,is formed inside the grains.With the further increase of Ni contents,the 18R-LPSO structure is formed in the alloys and the precipitation amount of Mg5RE phase is decreased for the Ni0.6 alloy.For the Ni0.8 alloy,the Mg5RE phase is almost converted into the 18R-LPSO structure.Besides,the addition of Ni not only changes the precipitated phase of the alloys but also influence the texture.

(2)Compared with the Mg-Gd-Y-Zr alloy,the mechanical properties of the Ni0.2 alloy are degraded due to the presence of Zr7Ni10.As the Ni content increases,the strength of the alloy further increases due to the strengthening effect of the 18R-LPSO structure in the alloys.Ni0.8 alloy obtains the highest ultimate compressive strength(596.5MPa)and the largest ultimate tensile strength(342.9MPa)among all the reported dissoluble magnesium alloys so far.

(3)The corrosion rate of the alloys is clearly increased after the Ni addition due to the formation of Zr7Ni10and/or the 18R-LPSO structure.The dissoluble rate of Mg-10Gd-3Y-0.3Zr-0.8Ni alloy at 93°C in 3wt% KCl reaches an excellent value of 24.42 mg?cm?2?h?1.

Declaration of Competing Interest

Authors have no conflic of interest to declare.

Acknowledgments

The National Natural Science Foundation of China(Nos.51671017,51971020),the Major State Research and Development Program of China(No.2016YFB0300801),Beijing Municipal Natural Science Foundation(No.2202033),Beijing Laboratory of Metallic Materials and Processing for Modern Transportation,the fund of the State Key Laboratory of Solidificatio Processing in NWPU(No.SKLSP201835),the Fundamental Research Funds for the Central Universities(No.FRF-IC-19-010,FRF-IC-19-015),and the Opening Research Fund of State Key Laboratory for Advanced Metals and Materials(2018-Z04)are deeply appreciated for their financia support.