Enhanced mechanical properties and degradation rate of Mg–Ni–Y alloy by introducing LPSO phase for degradable fracturing ball applications

Jingfeng Wng, Shiqing Go, Xiuying Liu, Xing Peng, Kui Wng, Shijie Liu,Weiyn Jing, Shengfeng Guo, Fusheng Pn

aNational Engineering Research Center for Magnesium Alloys, College of Materials Science and Engineering, Chongqing University, Chongqing 400044,China

b Faculty of Materials and Energy, Southwest University, Chongqing 400715, China

Abstract In this work, as-cast Mg–Ni–Y alloys were proposed to develop a feasible material for fracturing balls, and their mechanical performance and corrosion behavior were systematically investigated. Long period stacking order (LPSO) phase was firstly introduced to improve both the mechanical properties and degradation rate of magnesium alloys. With the increase of LPSO phase, the compressive strength was improved significantly, while the elongation of the alloys decreased owing to the relatively brittle nature of LPSO phase. Due to the higher corrosion potential of LPSO phase, the LPSO phase can accelerate the corrosion process by providing more micro-couples. However, the LPSO phase would serve as the corrosion barrier between the corrosion medium and the matrix when the contents of LPSO phase are too high in Mg92.5Ni3Y4.5 and Mg87.5Ni5Y7.5 alloys. As-cast Mg97.5Ni1Y1.5 alloy with satisfactory mechanical properties and rapid degradation rate was successfully developed, exhibiting a high degradation rate of 6675mm/a (93 °C) in 3wt.% KCl solution and a favorable ultimate compressive strength of 410MPa. The degradation rate of Mg97.5Ni1Y1.5 alloy is 2–5 times of the current commercial magnesium alloy fracturing materials.

Keywords: Mg–Ni–Y magnesium alloys; Fracturing ball; LPSO phase; Degradation rate; Mechanical properties.?Corresponding author.

1. Introduction

Fracturing technology is the core technology to develop oil and gas resources, and fracturing ball is the key factor to determine the success of staged fracturing [1]. There are some problems with traditional fracturing materials,such as increasing the process due to the non-degradability [2] or blockage of the oil outlet channel caused by failure to degrade completely. The development of a lightweight fracturing ball that can withstand the high pressure and high temperature during fracturing process and can be controlled to conduct rapid corrosion in the oil well fluid environment, can effectively reduce the construction cost and risk, shorten the construction period, and improve the construction efficiency.

More recently, magnesium alloys are characterized by low density,high specific strength and stiffness,good damping and electromagnetic shielding properties, making them promising candidates for many practical applications [4–8]. Unfortunately, most researches focus on the mechanical properties and corrosion resistance of the resultant materials [9–13], and take the corrosion behavior of the magnesium alloys as its disadvantage restricted their further use in many applications [14–16]. In fact, the poor corrosion resistance of magnesium alloys can make them favorable for other functional applications, such as bio-medical application and petroleum fracturing application [17]. Therefore, the development of high strength and rapid degradation magnesium alloysprovides a new insight into the selection of fracturing materials. Recently, some magnesium alloys that can be used for degradable fracturing ball have been studied, such as Mg–17Al–3Zn–5Cu alloy [29], Mg–3Zn–1Y–4Cu alloy [30], Mg–20Al–5Zn–1.5Cu alloy [31] and Mg–17Al–7Cu–3Zn–1Gd alloy [32]. All of them speed up the degradation rate of alloys by adding copper to the high strength magnesium alloys,which may deteriorate the mechanical properties. Therefore,another new approach is needed to be develop to enhance both the mechanical performance and the degradation rate.

Table 1 Chemical composition of the as-cast Mg–Ni–Y alloys.

Among various magnesium alloys, the alloys with long period stacking ordered (LPSO) phases exhibit relatively high strength and good ductility [18–20]. Especially, the Mg–Ni–Y alloys with LPSO phase have drawn many attentions due to their excellent performance [21,22]. For example, Takaomi Itoi et al. [23] reported that the Mg90.5Ni3.25Y6.25alloy sheet exhibited outstanding mechanical property with an ultimate tensile strength of 526MPa and elongation of 8%. Considering the degradation behavior, the addition of Ni or Cu can effectively promote the corrosion process of magnesium alloys, due to the large potential difference between Ni/Cu and magnesium matrix. Besides, our previous work shows that magnesium alloys with only α-Mg and LPSO phase can be prepared by controlling the atomic ratio of Ni and Y in the Mg–Ni–Y alloy [24]. In the present study, we firstly proposed improving the mechanical properties and the degradation rate of Magnesium alloy by introducing particular LPSO phase. A series of Mg–Ni–Y alloys with favorable mechanical properties and slapping degradation rate were developed by appropriate addition of Ni and Y into magnesium alloys.The effects of LPSO phase on the microstructure, mechanical properties, and corrosion resistance of Mg100-2.5xNixY1.5xalloys were investigated.

2. Experimental procedures

2.1. Samples preparation

The nominal compositions of as-cast Mg–Ni–Y alloys with varying Ni and Y contents are listed in Table 1. To obtain the series alloys, commercial pure Mg (99.95wt.%), Mg-20.91 Y(wt.%) master alloy and Mg-61.02 Ni (wt.%) master alloys were mixed and then melted in a stainless steel crucible inside an electromagnetic induction furnace with the smelting power of 15kW under the protection of Argon. After the raw materials were completely melted, the smelting power is reduced to 10kW for 5 min. At last, the molten alloys were rapidly cooled in brine with the mild steel crucible.

2.2. Microstructural analysis

The actual chemical compositions of the as-cast alloys were carried out based on X-Ray Fluorescence (XRF-1800 CCDE). The microstructural observation of specimens were carried out in the Olympus optical microscope (OM, OLYMPUS OLS4000). The requirement of the specimens for OM observation is that specimens should be polished in waterproof abrasive paper and corroded with a solution of acetic acid and ethanol (1 mL: 7mL), and saturated picric acid. The morphologies and phase composition were observed by using scanning electron microscope (SEM, JEOL JSM-7800F)coupled with an energy dispersive X-ray spectrometer (EDS).To obtain reliable data of phase distribution and better image quality, specimens for SEM measures were mechanical grinded and electrochemical polished in an AC2 electrolyte undering 20 Volts of direct current for 90s. The phase compositions were also measured through X-ray diffraction(XRD,Rigaku D/MAX-2500PC) with Cu Kα radiation at a 10–90? scanning angle and 4?/min scanning speed.

2.3. Mechanics performance test

The compressive properties of the alloys were measured at room temperature on a universal testing machine (Shimadzu CMT-5105) at an engineering strain of 1mm/min. The specimens for compression test were machined to the particular size of5×10mm and all faces of the specimens were ground through 1000-grit SiC paper.

2.4. Immersion test

Samples prepared for immersion test were cut into 10×10×10mm3, and polished and cleaned ultrasonically in anhydrous ethanol (95wt.%), then immersed in 3wt.% KCl solution at 93 °C [25]. The ratio of initial surface area to 3wt.% KCl solution volume was 1 cm2: 10mL.

After 2h immersion, the samples were taken out of the solution, gently rinsed successively with purified water and anhydrous ethanol, and dried to constant weight in cold air.Short-term immersions for 30s were conducted to observe the corrosion characteristics of the samples. Corrosion products were removed by using 10g/L of AgNO3and 200g/L of chromic acid for 10min in ultrasonic cleaning apparatus.According to ASTM-G31-72 [26], the average corrosion rate(mm/a) can be calculated according to the equation below[26]:

where K is a constant 8.76×104, W is the weight loss (g),A is the initial surface area (cm2), T is the immersion time(h), and D is the density of the tested material (g/cm3).

2.5. Electrochemical measurements

Fig. 1. Microstructures of the as-cast Mg–Ni–Y alloys: OM image of Alloy 1 (a1), Alloy 2 (b1) and Alloy 3 (c1), respectively; SEM images of Alloy 1 (a2),Alloy 2 (b2) and Alloy 3 (c2), respectively.

To ensure the similarity with the actual application environment, the electrochemical measurements were performed in 3.0wt.%KCl solution at 93°C.Specimens used as working electrodes were mounted in the mixed self-solidifying denture acrylic and self-solidifying methyl methacrylate to ensure the exposed area of 1 cm2(10mm×10mm). A saturated calomel electrode (SCE) was used as the reference electrode and the platinum plate as the counter electrode. Before testing, all the specimens were polished by using SiC sandpaper to 2000#and then lightly rinsed the surfaces with anhydrous ethanol(95wt.%).Before the dynamical potential polarization testing,the open circuit potential (OPC) of the alloys was measured for a certain period of time to ensure that an approximately steady state was established, followed by polarization curve measurement at a constant scanning rate of 10mV/s.

3. Results and discussion

3.1. Microstructures of as-cast Mg–Ni–Y alloys

Fig. 1 shows the microstructures of as-cast Mg100-2.5xNixY1.5x(x=1,3,5) alloys named Alloy 1, Alloy 2 and Alloy 3, respectively. Seen from the SEM images, the alloys are all mainly made up of the matrix phases (the dark areas) and white secondary phases. However, the volume fraction anddistribution patterns of each phase were changed with the alloying elements content increases. For Alloy 1, where the Ni and Y content is the lowest, the white secondary phases were distributed along the grain boundary. Increasing Ni and Y content in Alloy 2,the volume fraction of the white secondary phase increases. At the same time, the secondary phase distributes staggered as a strip form.For Alloy 3,whose alloying elements content is the highest, the secondary phase is almost full of the entire alloy, which is in agreement with the previous studies [24,34]. EDS results (see Table 2) revealed that the atom ratios of Ni to Y in the white areas of alloy 1, alloy 2 and alloy 3 are 1.4, 0.66 and 0.76 respectively. According to the previous study [24,34], these results can demonstrate that all of the white phases in the three alloys are mainly composed of Mg12NiY (LPSO) phase with minor Mg2Ni or Mg24Y5phases, which is in agreement with the XRD patterns showed in Fig. 2. For alloy 1, there were almost no yttrium element in the α-Mg matrix. The XRD results also demonstrated that the diffraction peak corresponding to the LPSO phase increases with the increase of Ni and Y while the alloy phase types does not change. The volume fraction and distribution of LPSO phase or second phases would have a great influence on the mechanical and corrosion resistance of magnesium alloys [27,28].

Table 2 Energy dispersive X-ray spectrometer (EDS) results of the points selected from Fig. 1.

Fig. 2. XRD patterns of as-cast Mg–Ni–Y alloys.

Fig. 3. Compressive stress-strain curves of the as-cast alloy.

3.2. Mechanical properties of the as-cast Mg–Ni–Y alloys

The compressive stress-strain curves and the corresponding data for the as-cast Mg–Ni–Y alloys are shown in Fig. 3 and Table 3. With the increase of the content of the LPSO phase,the compressive mechanical properties of the alloys show an increasing trend, while the plasticity of the Mg–Ni–Y alloys deteriorate obviously. Alloy 1 presents the highest plasticity with the elongation of 34%. However, the Compressive strain(?) of Alloy 3 is 17% that is almost the half of that in Alloy 1. All of the three alloys present excellent compression mechanical properties.The ultimate compressive strengths(UCS)of three alloys are 410MPa, 536MPa and 575MPa, respectively. For Alloy 1, the good compressive performance can be explained by the existence of dispersed LPSO phase in the grain boundaries, which can refine the grain and impede the movement of the dislocation within the grain. Owing to the coherent interface between LPSO phase and matrix, the compressive strength has been improved with the increase of LPSO phase [3,35]. And the reduction in plasticity can be attributed to the brittle nature of the LPSO phase.

Table 3 Compressive performance of the as-cast alloy.

Fig. 4. Summary of the mechanical properties.

The microhardness test results shown in Fig. 4 presents the same trend compared to the compressive tests. Chino et al. previously reported that the Vickers hardness of LPSO phase in Mg-Zn-Y alloy is 137±34 HV, while the Vickers hardness of α-Mg matrix is 31±2 HV [33]. Therefore,with the introduce of LPSO phase, the hardness of the alloys increase. The Vickers hardness of Mg97.5Ni1Y1.5and Mg92.5Ni3Y4.5alloys were 76±3 HV and 88±3 HV, respectively. For the Mg87.5Ni5Y7.5alloy with plentiful LPSO phase covered in the matrix, it shows an obviously high Vickers hardness of 110±3 HV, which is nearly close to that of single LPSO phase. And all the three alloys can meet the sealing and surface stability requirements of fracturing ball materials.

3.3. Corrosion behavior of the as-cast Mg–Ni–Y alloys

Considering that the working environment of fracturing ball is operating in deep strata where the temperature is well above the room temperature and the main corrosive ion is chloride, the corrosion medium was 3wt.% KCl solution at 366K (93 °C) used to simulate the corrosion environment.

Fig. 5. Corrosion rates of the samples from immersion test.

Fig. 6. The comparison of the corrosion rate and UCS between the Mg97.5Ni1Y1.5 alloy and the SoluMag magnesium, Mg–17Al–3Zn–1Gd–7Cu alloy [32], Mg–3Zn–1Y–4Cu alloy [30], Mg–17Al–3Zn–5Cu alloy [29].

The corrosion performance of the Mg–Ni–Y alloys are mainly studied by means of immersion tests and electrochemical polarization tests. Fig. 5 shows the results of immersion tests of the Mg–Ni–Y alloys at 366K (93 °C). Unlike the previous study,the Alloy 1 with the less LPSO phase shows an extraordinary high degradation rate of 6675.32mm/a,which is almost 20 times higher than the Alloy 3. It should be mentioned that the degradation rate of present Alloy 1 is compared favorably with the fastest degrading alloy (SoluMag Magnesium Alloy)of the Luxfer company. Fig. 6 shows the corrosion rate and the ultimate compression strength of the Mg97.5Ni1Y1.5alloy and the other comparatively protruding fracturing materials,such as the SoluMag Magnesium Alloy from the Luxfer Mel technologies, Mg–17Al–3Zn–1Gd–7Cu alloy [32], Mg–3Zn–1Y–4Cu alloy [30], Mg–17Al–3Zn–5Cu alloy [29] and so on.The current Mg97.5Ni1Y1.5alloy exhibits an excellent overall performance with ultimate compression strength of 410MPa and corrosion rate of 6675.32mm/a, which is almost three times of that in SoluMag Magnesium Alloy.

Fig. 7. Potentiodynamic polarization curves of Mg–Ni–Y alloys and pure magnesium in 3.0wt.% KCl solution at 93 °C.

Table 4 Parameters derived from the polarization curves by Tafel region extrapolation.

Fig. 7 shows the electrochemical polarization curves of the as-cast Mg–Ni–Y alloys in 3wt.% KCl solution. In order to make our results clear, we chose the commercial pure magnesium (99.99wt.%) tested in the same condition as a comparison. They both exhibit asymmetrical curves but clearly differ from the other two. Compared with the pure magnesium, both the anode and cathode current density of the Mg–Ni–Y alloys increase significantly. It means that the addition of Ni and Y elements accelerates the cathode hydrogen evolution and also advances the anode dissolving.Table 4 shows the corrosion potential(Ecorr)and the corrosion current density (icorr) derived directly from the polarization curves by Tafel region extrapolation. The corrosion potentials of pure magnesium, the Alloy 1, the Alloy 2 and the Alloy 3 are ?1.806V, ?1.28917V, ?1.2504V and ?1.0241V respectively. As we mentioned before, the increasing alloying element leads to more atoms dissolved into the matrix. The potentials of nickel and yttrium are higher than the magnesium which leads to a certain increase in the potential of the alloy. However, the corrosion potentials increase for a large certain,which implies the potential of LPSO phase was higher than that of magnesium matrix. Therefore, with the increase of LPSO phase, the corrosion potential moves towards a positive direction. Moreover, the icorrof the Mg97.5Ni1Y1.5alloy(8.318mA/cm2) is about 250 times of that in pure magnesium (0.03327mA/cm2), which indicates that the addition of Ni and Y can increase corrosion rate of the alloys tremendously. Obviously, the formed LPSO phase as major second phase in Mg97.5Ni1Y1.5can act as active cathode and provide more galvanic couplings and then accelerate the galvanic corrosion process.Whereas in the Mg92.5Ni3Y4.5and the Mg87.5Ni5Y7.5, this pattern seems extremely different. Compared with the Mg97.5Ni1Y1.5, the content of LPSO phase increases a lot, while the corrosion current densities (icorr)of the Mg92.5Ni3Y4.5and the Mg87.5Ni5Y7.5alloys are 5.495mA/cm2and 0.3085mA/cm2respectively that are much lower than that of Mg97.5Ni1Y1.5alloy. It has been reported that LPSO phase can act as corrosion barrier and prevent penetration of corrosive ions in Mg–Gd–Zn alloys [34]. Therefore,combined with the previous SEM images of the samples,it can be inferred that a large amount of LPSO phase may act as a corrosion barrier which hinders the further dissolution of Mg matrix.So,the order of corrosion rate is:Mg97.5Ni1Y1.5>Mg92.5Ni3Y4.5> Mg87.5Ni5Y7.5, which is in good agreement with the immersion test.

Fig. 8. Corrosion morphologies of (a) Alloy 1, (b) Alloy 3 after immersion in 3wt.% KCl solution for 30s and removal of the corrosion products; (c)EDS analysis results of point A and (d) point B.

To further clarify the corrosion mechanism of the Mg–Ni–Y alloys, short-term immersion tests for 30 s were carried out. Corrosion morphologies of the Mg97.5Ni1Y1.5and the Mg87.5Ni5Y7.5are shown in Fig. 8. As we can see, plenty of corrosion etching pits are found on the surface of the two alloys, while the Mg97.5Ni1Y1.5seems much more serious.Furthermore, the corrosion surface for the Mg97.5Ni1Y1.5alloy is covered with bright grays and whites marked with A and B, which are mainly made up of 89.6 at.% Mg, 6.2 at.%Ni, 4.1 at.% Y and 40.8 at.% Mg, 43.2 at.% Ni, 16 at.% Y by EDS analysis respectively, as shown in Fig. 6c and d. So,the composition of bright grays is closed to the Mg12NiY and the whites is confirmed as nikel-rich and yttrium-rich regions.So, the composition of bright grays is closed to the Mg12NiY and the whites is confirmed as nikel-rich and yttrium-rich regions. As previously reported [26,27], the cathode activity of the Mg matrix is more negative than the Ni and Y particles. So, the areas around nikel-rich and yttrium-rich regions would become the initiating sites of the galvanic corrosion between those particles and the Mg matrix. With the corrosion proceeding, the areas around nikel-rich and yttriumrich regions were basically dissolved. Then these particles would gradually peel from the matrix, leaving corrosion pits on the surface. In the case of Alloy 3 covered with plentiful strip LPSO phases, corrosion pits were initiated in junction of LPSO phases and the matrix or between the LPSO and some other (Mg2Ni, Mg24Y5) phases, resulting in corrosion proceeded along the orientation of the LPSO phase (see Fig. 8b). Therefore, the morphologies of the corroded area were transformed from dot shapes into strip shapes. A strip structure instead of a network structure was formed by the strip shape LPSO phase in the Mg87.5Ni5Y7.5(as indicated by the arrows in Fig. 8b). From the results of electrochemical polarization tests, it can be concluded that the corrosion potential of LPSO phase was higher than that of magnesium matrix. Due to the presence of high-potential LPSO phases,the areas covered by them show high corrosion resistance.There is a large area covered by the LPSO phase in the Mg92.5Ni3Y4.5and the Mg87.5Ni5Y7.5, resulting much lower degradation rate. Our findings provide some evidences for the degree of alloying in the development of fracturing ball materials.

4. Conclusions

The outstanding performance of Mg97.5Ni1Y1.5 alloy makes it a promising candidate for fracturing ball by introducing particular Mg12NiY (LPSO) phase. The as-cast Mg100-2.5xNixY1.5xalloys are mainly composed of a-Mg and LPSO phase. With the increase of LPSO phase, the compressive strengths have been improved significantly, while the plasticity of the alloys decreased. The degradability of the as-cast Mg–Ni–Y alloys were improved notably by introducing a certain amount of Mg12NiY phase (LPSO phase),However, with plentiful LPSO phase in Mg92.5Ni3Y4.5and Mg87.5Ni5Y7.5alloys,the corrosion rates are much more lower than that of Mg97.5Ni1Y1.5. These results can be attributed to the higher corrosion potential of LPSO phase and its inhibition to the corrosion process. Mg97.5Ni1Y1.5alloy shows an extremely high average corrosion rate, up to 6675.32mm/a at 366K (93 °C) and the ultimate compressive strength of the alloy are 410MPa, with favorable compressive strain(?) of 18.2% maintained. The outstanding performance of Mg97.5Ni1Y1.5alloy makes it a promising candidate for fracturing balls.

Declaration of Competing Interest

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

Acknowledgments

This work is financially supported by the National Key Research and Development Program of China (Grant No.2016YFB0301100), the Chongqing Foundation and Advanced Research Project (Grant No. cstc2019jcyj-zdxmX0010), the Natural Science Foundation Commission of China (Grant No.51571044 and 51874062) and Fundamental Research Funds for the Central Universities (Grant No. 2018CDGFCL0005 and 2019CDXYCL0031).