Q. CHEN et al.: EFFECT OF Y ON THE MECHANICAL PROPERTIES AND CORROSION RESISTANCE OF ... 745–754 EFFECT OF Y ON THE MECHANICAL PROPERTIES AND CORROSION RESISTANCE OF AS-EXTRUDED Mg-4Sn ALLOYS VPLIV Y NA MEHANSKE LASTNOSTI IN KOROZIJSKO ODPORNOST EKSTRUDIRANIH Mg-4Sn ZLITIN Qiuli Chen 1 , Zheng Jia 1,2* , Xiaowei Niu 2 , Ayong Hu 1 , Feida Ji 1 , Chongrui Liu 1 , Qianhe Cui 1 , Yongzhi Yu 1 1 College of Mechanical Engineering, Shenyang University, Shenyang 110044 2 College of Environment, Liaoning Province Environmental Pollution Remediation Professional Technology Innovation Center & Shenyang Key Laboratory of Collaborative Technology Innovation for Industrial Pollution Reduction and Carbon Reduction, Shenyang University, Shenyang 110044, China Prejem rokopisa – received: 2024-07-23; sprejem za objavo – accepted for publication: 2024-10-10 doi:10.17222/mit.2024.1259 In this study, the effects of Y element on the microstructure, mechanical properties and corrosion resistance of an extruded T4 alloy at room temperature were studied by comparing the as-extruded Mg-4Sn (T4) and Mg-4Sn-1Y (TW41) alloys. The results showed that the second phase of the as-extruded T4 alloy is mainly the Mg2Sn phase, while the ternary MgSnY and Mg2Sn phases mainly precipitate after the addition of Y element. Incomplete dynamic recrystallization of the two alloys occurs during the extrusion process, and the addition of Y element can promote dynamic recrystallization and refinement of the grains in T4 alloy. At room temperature, TW41 alloy has higher strength compared to T4 alloy, with tensile and yield strengths of 228 MPa and 165 MPa, respectively, but the elongation and corrosion resistance are reduced. Grain refinement is believed to play the key role in improving the yield strength of TW41 alloy, while the deterioration of the corrosion resistance of TW41 alloy is mainly attributed to the increase in the grain boundary density caused by grain refinement, accelerating the dissolution of the alloy an- ode. Key words: Mg-Sn alloy, alloying, mechanical properties, corrosive properties V ~lanku avtorji opisujejo vpliv dodatka itrija (Y) na mikrostrukturo, mehanske lastnosti in odpornost proti koroziji zlitin na osnovi Mg in Si. Med seboj so pri sobni temperaturi primerjali ekstrudirani zlitini tipa Mg-4Sn (T4) in Mg-4Sn-1Y (TW41). Rezultati raziskave so pokazali, da v prvi ekstrudirani zlitini kot sekundarna faza nastopa Mg2Sn. V zlitini z dodatkom Y pa nastopata kot sekundarni fazi tako Mg2Sn kot tudi MgSnY. Med procesom ekstruzije je pri obeh zlitinah pri{lo do nepopolne dinami~ne rekristalizacije. Vendar je dodatek Y pospe{il dinami~no rekristalizacijo in udrobljenje (zmanj{anje velikosti kristalnih zrn) v zlitini TW41. Pri sobni temperaturi ima ta zlitina vi{jo natezno trdnost (228 MPa) in mejo plasti~nosti (165 MPa) kot zlitina T4, zni`ali pa sta se vrednosti za raztezek in odpornost proti koroziji. Zmanj{anje velikosti zrn v zlitini TW41 je povzro~ilo izbolj{anje meje plasti~nosti. Poslab{anje odpornosti proti koroziji pa je posledica pove~anja {tevila oz. gostote kristalnih mej, kar pospe{uje anodno raztapljanje zlitine. Klju~ne besede: Mg-Sn zlitine, legiranje, mehanske in korozijske lastnosti 1 INTRODUCTION Magnesium alloy is a lightweight metal alloy that has attracted attention for its excellent machinability. Compared to other commonly used metal alloys, magne- sium alloys have a lower density, higher specific strength, and higher specific stiffness, making them ideal for applications that require efficient fuel and weight re- duction, such as aerospace and automotive manufactur- ing. 1–3 In these high-performance applications, the me- chanical properties and corrosion resistance of the material are critical. Among them, improving the me- chanical properties of magnesium alloys is the basic re- quirement for the wide application of magnesium alloys, and the mechanical properties of magnesium alloys can usually be improved by adding alloying elements to them to optimize the alloy composition and plastic deforma- tion. 4–6 For example, Sadeddin et al. 7 studied the effects of different Zr additions (0, 0.5 and 1) w/% and extrusion ratios (6:1 and 12:1) on the microstructure and mechani- cal properties of Mg-5Sn alloys, and found that an addi- tion of Zr could significantly refine the microstructure of the alloy so that the microstructure of the as-cast alloy gradually changed from dendritic to nearly spherical. Moreover, the addition of Zr can promote the precipita- tion of Mg 2 Sn phase and improve the thermal stability of the alloy during high-temperature homogenization. In addition, the grains of the alloy are significantly refined after hot extrusion deformation, and the dynamic recrystallization and twinning generated during the ex- trusion process can effectively improve the mechanical properties of Mg-5Sn-xZr alloy. On the other hand, many studies have found that magnesium is chemically active and susceptible to oxi- dation and corrosion, which can lead to degradation of material properties and impairment of structural integ- rity. 8–10 Therefore, corrosion resistance improvement and protection of magnesium alloys is the focus of research Materiali in tehnologije / Materials and technology 58 (2024) 6, 745–754 745 UDK 661.846:52-334.2 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek Mater. Tehnol. *Corresponding author's e-mail: jz140@163.com (Zheng Jia) to further broaden their applications. Some studies have shown that methods such as surface coating and alloying have been used to improve the corrosion resistance of magnesium alloys so that they meet the needs of differ- ent applications. 11,12 For example, Ha et al. 13 studied the corrosion behavior of an as-extruded Mg-5Sn-(1–4 w/%) Zn alloy in a 0.6 M NaCl solution using the hydrogen precipitation weight loss test and electrochemical test. They found that the addition of a small amount of Zn can improve the corrosion resistance of Mg-5Sn alloy, while the addition of too much Zn will rather reduce the corro- sion resistance of the alloy, where Mg-5Sn-2Zn has the lowest corrosion rate and the best corrosion resistance. Therefore, alloying and plastic deformation are com- prehensive and optimized means for improving the me- chanical properties and corrosion resistance of magne- sium alloys, which is crucial for ensuring their long-lasting durability in demanding engineering appli- cations. In order to develop new magnesium alloys, the role of different alloying elements on magnesium alloys should be considered comprehensively. At present, Mg-Sn alloys have become a hot spot in the field of mag- nesium alloy research, which is due to the significant precipitation strengthening effect of the Sn element in Mg alloys. Moreover, the Mg 2 Sn phase precipitated in Mg-Sn alloys has a high hardness, a high melting point, and good thermal stability, which can effectively im- prove the mechanical properties of magnesium alloys. 14 In addition, Sn can also improve the corrosion resistance of magnesium and its alloys, but the mechanical proper- ties and corrosion resistance of Mg-Sn binary alloys at room temperature are still not ideal due to the existence of Mg 2 Sn phases distributed in the network along the grain boundaries. Yang et al. 15 reported that an addition of the Y element to Mg-0.5Sn would refine the grains of the extruded alloy and significantly improve the mechan- ical properties of the alloy, with the ultimate tensile strength and elongation increasing by 21 % and 191 %, respectively. At the same time, the addition of the Y ele- ment forms the Sn 3 Y 5 phase and inhibits the precipitation of the Mg 2 Sn phase. It has been reported that rare earth elements can occupy the pores of the oxide film that forms on the surfaces of rare earths, thus preventing the diffusion of metal atoms. 16 Yu et al. 17 found that the pro- tective properties of the oxide film formed due to the ad- dition of Y element to the Mg-Sn alloy can be improved, providing good protection for the matrix and preventing further oxidation of the matrix. In addition, the oxide film of Mg-Sn-Y has good adhesion to the substrate, thus improving the corrosion resistance of the alloy. In summary, the rare earth element Y can change the type of the second phase in an alloy and realize the re- finement of the alloy microstructure, thereby improving the mechanical properties and corrosion resistance of the alloy. In view of this, in order to introduce Mg-Sn-based alloys into more fields, based on Mg-4Sn alloys, our al- loys were prepared using homogenization heat treatment and back extrusion. The effects of a 1 w/% Y addition on the mechanical properties and corrosion resistance of ex- truded Mg-4Sn alloys were systematically studied to im- prove the comprehensive properties of Mg-Sn-based al- loys. 2 EXPERIMENTAL METHODS In this experiment, the as-cast Mg-4Sn (T4) and Mg-4Sn-1Y (TW41) alloys were prepared in pit resis- tance furnaces (RJ2 series, Hankou Electric Furnace Company, Wuhan, China) using industrially pure Mg (99.9 w/%), pure Sn (99.9 w/%) and Mg-25%Y master alloys as the raw materials. The melting process was as follows: firstly, pure magnesium was placed in a pre- heated crucible and heated to 730 °C. After pure magne- sium was completely melted, the alloying element Sn was added and stirred in thoroughly. After that, the melt temperature was raised to 760 °C and the master alloy Mg-25%Y was added. After the alloy was completely melted, it was thoroughly stirred and cooled to 720 °C for 30 min. The melt was then poured into a mold with a diameter of 65 mm and a height of 240 mm under the protection of a mixed gas (CO 2 :SF 6 =2:1) for natural cooling and molding. In order to reduce or eliminate the intragranular segregation of the alloy, the cast alloy was homogenized at a temperature of 400 °C for 24 h in a box-type resistance furnace (1400 °C manual door box-type furnace) and cooled by water quenching. The obtained homogenized alloy was reverse extruded using a hydraulic press (600-ton vertical hydraulic press) at an extrusion temperature of 300 °C and an extrusion speed of 1 mm/s to obtain a bar with a diameter of 12 mm (an extrusion ratio of 14:1). The actual composition of the alloy was measured by an inductively coupled plasma spectrometer (Plasma 2000), and the results are shown in Table 1. The phase morphology was observed by a S4800 scanning electron microscope (SEM) (Hitachi S-4800), and the elemental composition of the phase was detected with the included energy dispersive spectrome- ter (EDS). Electron backscatter diffraction (EBSD) was used to obtain grain orientation information from the samples, and the data were analyzed by Channel 5 soft- ware. The tensile mechanical properties of the extruded al- loy at room temperature were tested with a Shimadzu AG-X (100 kN) electronic universal tensile testing ma- chine. The sampling standard was in accordance with the national standard (GB/T 228.1-2021), and the tensile speed was 1 mm/min. The polarization curve and imped- ance spectrum were measured with a three-electrode electrochemical workstation (CHI660E) in a 3.5 w/% NaCl solution, in which the saturated calomel electrode was the reference electrode (RE), the platinum electrode was the auxiliary electrode (CE), and the working elec- trode (WE) was the sample to be tested. The hydrogen evolution-weight loss test was organized as follows: a Q. CHEN et al.: EFFECT OF Y ON THE MECHANICAL PROPERTIES AND CORROSION RESISTANCE OF ... 746 Materiali in tehnologije / Materials and technology 58 (2024) 6, 745–754 (10 × 10 × 10) mm sample was immersed in the 3.5 w/% NaCl aqueous solution for 24 h, and the precipitated hy- drogen was collected using a burette and recorded every 2 h. To maintain a stable concentration of the solution, the NaCl solution was replaced every 12 h, and after the corrosion was completed, the sample was removed and placed in the prepared chromate. It was washed under ul- trasonic conditions for 10 min. Equation (1) was used to calculate the loss-in-weight corrosion rate: 18 P g At W = ⋅⋅ ⋅⋅ 87610 4 . Δ (1) Equation (2) was used to calculate the hydrogen evo- lution rate: 19 P V At H = ⋅⋅⋅ ⋅⋅ 87610 4 . ΔΜ (2) where P W – the loss-loss corrosion rate, mm year –1 ; g – the mass loss before and after the alloy corrosion, g; V – the total precipitation of hydrogen during corro- sion, mL. The relationship between the rate of M-hydro- gen production and the rate of alloy mass loss was 0.001083 g·mL –1 . A – The total surface area of the im- mersed sample, cm 2 ; t – the soaking time, h; – the weight loss measurement specimen density, g·cm –3 . 3 RESULTS 3.1 Microstructure XRD patterns of the as-extruded T4 and TW41 alloys are shown in Figure 1. It can be found that the as-ex- truded T4 alloy is mainly composed of two phases,a-Mg and Mg 2 Sn; after adding 1 wt.% Y, in addition to -Mg and Mg 2 Sn, there is another phase in the alloy that can- not be identified with the XRD spectral database. There- fore, in order to further confirm the unknown phase, the composition of the unknown phase is obtained with scanning electron microscopy assisted X-ray spectros- copy (SEM-EDS), as shown in Figure 2. The elements of the D phase are composed of Mg, Sn and Y, and the atomic ratio of Sn and Y is close to 1:1, so it is specu- lated that the phase is MgSnY. Zhao et al. 20 also found that an addition of element Y to the Mg-1Sn alloys re- sults in the formation of ternary phase MgSnY. In addi- tion, it can be seen from Figure 2 that the second phases of the two as-extruded alloys are streamlined along the extrusion direction (ED) on the matrix, and the volume fractions of the second phases of the alloy increase sig- nificantly after the addition of 1 w/% Y. The second phases of the as-extruded T4 alloy are Mg 2 Sn ( shown in Figure 2a) and Mg 2 Sn, which are mainly bulk and gran- ular. The second phases of the TW41 alloy are mainly Mg 2 Sn and MgSnY (shown in Figure 2b), where Mg 2 Sn is massive and granular, and MgSnY is of an irregular polygonal shape. Figure 3 shows the distribution of IPF, PF and orien- tation angles of the as-extruded T4 and TW41 alloys. Both alloys exhibit a typical {0001} substrate ED ex- truded fiber texture. The maximum extreme density of T4 alloy substrate texture is 17.4, and the maximum ex- treme density of TW41 alloy substrate texture is 14.73. In addition, it can be seen that the proportion of Q. CHEN et al.: EFFECT OF Y ON THE MECHANICAL PROPERTIES AND CORROSION RESISTANCE OF ... Materiali in tehnologije / Materials and technology 58 (2024) 6, 745–754 747 Figure 2: Phase distribution of the as-extruded T4 and TW41 alloys Figure 1: XRD patterns of the as-extruded T4 and TW41 alloys small-angle grain boundaries of the two alloys is 9.38 % and 11.7 %, respectively, and the proportion of large-an- gle grain boundaries is 73.37 % and 36.55 %, respec- tively. The results show that the addition of Y can weaken the alloy basal weave strength and reduce the neighboring grain orientation difference. This is due to the fact that the MgSnY ternary phase precipitates after the addition of Y to the T4 alloy, and the precipitated phase particles produce particle-stimulated nucleation (PSN) during the hot extrusion process, resulting in a weak texture strength. It has been reported that coarse particles larger than 1 μm in size can trigger the particle stimulated nucleation (PSN) mechanism, which reduces the orientation correlation between the recrystallized grains and existing grains, thereby weakening the texture strength. 21 Figure 4 shows dynamic recrystallization of the ex- truded T4 and TW41 alloys and the corresponding pro- portional distributions for each region. The blue, yellow, and red grains represent recrystallized, sub-crystalline, and deformed grains, respectively. It has been shown that complete dynamic recrystallization occurs when the ex- trusion temperature of magnesium alloys exceeds 350 °C. 22 It can be seen that the grains of the as-extruded T4 and TW41 alloys are equiaxed and partially recryst- allized, indicating that the alloys underwent partial dy- namic recrystallization after extrusion deformation at 300 °C. Between them, the proportion of dynamically recrystallized grains is 28 %, the proportion of sub-crys- talline grains is 60 %, and the proportion of deformed grains is 12 %. Compared with the T4 alloy, the propor- tion of dynamically recrystallized grains increased sig- nificantly to 85 % after the addition of Y, while the pro- portion of sub-crystalline and deformed grains decreased to 12 % and 3 %, respectively. The results show that the addition of Y affects the dynamic recrystallization be- havior of the alloy during the extrusion process, and can promote the dynamic recrystallization of T4 alloy. In ad- dition, the average grain size of T4 alloy and TW41 alloy is 7.25 μm. The addition of Y can also refine the grain size of T4 alloy. It is concluded that the increase in the dynamic recrystallization fraction of the alloy is caused by the increase in the second phase particles of the alloy after the addition of 1 w/% Y (as shown in Figure 2), and the largest second phase particles can be accelerated by particle stimulation nucleation (PSN) due to a huge amount of energy stored in the deformation region. In addition, the grain refinement of the alloy is due to the fact that Y and Mg have the same close-packed hexago- nal (hcp) crystal structure and similar lattice parameters, which conforms to the principle of "size and structure matching". Thus, Y can be used as a heterogeneous nu- cleation particle for -Mg, making nucleation much easier. In addition, Y is the surfactant element of Mg, enriched at the solid-liquid interface during the solidification pro- cess of the alloy, resulting in a supercooling zone of the composition and thus inhibiting the growth of grains. Q. CHEN et al.: EFFECT OF Y ON THE MECHANICAL PROPERTIES AND CORROSION RESISTANCE OF ... 748 Materiali in tehnologije / Materials and technology 58 (2024) 6, 745–754 Figure 3: IPF and PF diagrams of extruded alloys: (a) T4, (b) TW41, (c)–(d) misorientation angle distribution of extruded alloys 3.2 Mechanical properties The tensile mechanical properties of the as-extruded T4 and TW41 alloys at room temperature are shown in Figure 5, where it can be seen that Y has an effect on the yield strength (YS), ultimate tensile strength (UTS) and elongation of the alloys. The UTS of T4 and TW41 al- loys was 219 MPa and 228 MPa; the YS was 135 MPa and 165 MPa, respectively, while the elongation was 14.7 % and 8.4 %, respectively. The results show that the as-extruded TW41 alloy had the highest UTS and YS, which were significantly higher than those of T4 alloy, but the elongation of TW41 alloy was reduced by 42 %. It is believed that the increase in the strength of TW41 alloy was due to the addition of Y, which refined the microstructure of the alloy, and the grain refinement led to the increase in the grain boundaries, making the trans- port of dislocations difficult, thereby improving the yield strength of the alloy. The reason for the decrease in the alloy elongation may have been due to the dislocation plugging caused by the largest second-phase particles precipitated in the alloy matrix, resulting in stress con- Q. CHEN et al.: EFFECT OF Y ON THE MECHANICAL PROPERTIES AND CORROSION RESISTANCE OF ... Materiali in tehnologije / Materials and technology 58 (2024) 6, 745–754 749 Figure 4: Dynamic recrystallization distribution map of as-extruded T4 and TW41 alloys: (a) T4, (b) TW41 Figure 5: Mechanical properties of as-extruded alloys T4 and TW41 centration and becoming the source of cracks, resulting in a significant decrease in the alloy elongation. Figure 6 shows the room-temperature tensile fracture morphology of the extruded T4 and TW41 alloys. It can be seen that the fractures of T4 and TW41 alloys have dimples of different depths, showing ductile fracture characteristics. The size and depth of a dimple of the as-extruded T4 alloy are large and deep, and there is a massive Mg 2 Sn phase (as shown with the EDS analysis in Figure 7) at the bottom of the dimple. On the con- trary, compared with T4 alloy, the dimple size of TW41 alloy is shallow, and the second phase particles at the bottom of dimples are mainly the Mg 2 Sn phase and MgSnY phase. Under tensile stress, stress concentration occurs at the interface between the Mg matrix and MgSnY phase particles, promoting the formation of cracks. 3.3 Corrosion properties The changes in the hydrogen volume and average corrosion rate of the two alloys after soaking for 24 h are shown in Figure 8. It can be seen that the hydrogen pre- cipitation of the two alloys has a linear relationship with the immersion time, indicating that the amount of hydro- gen produced in each time period is almost equal, while the amount of hydrogen precipitated by TW41 alloy is much higher than that of T4 alloy, indicating that its cor- rosion resistance is relatively poor. Figure 9 shows the polarization curves and Nyquist plots of the as-extruded T4 and TW41 alloys. In general, the polarization curve reflects the corrosion thermody- namic tendency, and the impedance spectrum reflects the corrosion kinetic tendency. This is typically supple- mented by EIS, reflecting the actual corrosion resistance of the material and the polarization curve. 23 As can be seen on Figure 9a, the corrosion potential of TW41 al- loy is slightly higher than that of T4 alloy, indicating that Q. CHEN et al.: EFFECT OF Y ON THE MECHANICAL PROPERTIES AND CORROSION RESISTANCE OF ... 750 Materiali in tehnologije / Materials and technology 58 (2024) 6, 745–754 Figure 6: SEM fracture morphologies of the as-extruded alloy tensile fracture specimens : a)–b)T4, c)–d)TW41 Figure 7: EDS analysis of fracture phases in as-extruded T4 and TW41 alloys the self-corrosion tendency of TW41 alloy is lower than that of T4 alloy from a thermodynamic point of view. In addition, the corrosion potential (Ecorr) and corrosion current density (Icorr) were fitted in combination with the polarization curves in Table 1. It can be seen that there is little difference between the two alloys in terms of corrosion potential; in terms of corrosion current den- sity, TW41 alloy is slightly higher than T4 alloy, indicat- ing that the corrosion resistance of TW41 alloy is poorer than that of T4 alloy. It is believed that the decrease in the corrosion tendency of the alloy may be caused by the precipitation of the MgSnY ternary phase due to Y ele- ment and the inhibition of the precipitation of Mg 2 Sn phase, thereby reducing the galvanic corrosion tendency caused by the Mg 2 Sn phase. From the Nyquist curves in Figure 9b, it can be seen that there are a high-frequency capacitive arc and a low-frequency inductive arc resistance in the Nyquist curves of T4 and TW41 alloys, indicating that the addi- tion of Y does not change the corrosion mechanism of T4 alloy. In addition, an inductive arc is present in the extruded T4 and TW41 alloys, indicating that pitting cor- rosion occurs during the corrosion process. Therefore, the impedance spectra of the as-extruded T4 and TW41 alloys are fitted to an equivalent circuit, shown in Fig- ure 9c, and the specific parameters are shown in Table 2. Among them, Rs is the resistance of the solution, R t is the resistance of the corrosion product film, R L and L are connected in series to indicate pitting areas on the sur- face of the alloy, and the constant phase angle element Q. CHEN et al.: EFFECT OF Y ON THE MECHANICAL PROPERTIES AND CORROSION RESISTANCE OF ... Materiali in tehnologije / Materials and technology 58 (2024) 6, 745–754 751 Figure 8: Corrosion of as-extruded T4 and TW41 alloys immersed for 24 h: (a) hydrogen content; (b) corrosion rate Figure 9: a) Polarization curves, b) Nyquist diagrams and fitting circuit of as-extruded T4 and TW41 alloys CPE dl and resistor R ct are connected in parallel to fit the high-frequency capacitive arc in the equivalent circuit. It is found that the charge transfer resistance R t on the sur- face of T4 alloy is greater than that of TW41 alloy, indi- cating that the electrochemical corrosion rate of T4 alloy is relatively slow and the corrosion resistance is better. Table 1: As-extruded T4 and TW41 alloys in 3.5 w/% NaCl Alloy E corr /V I corr / A·cm 2 T4 –1.461 1.5×10 –5 TW41 –1.414 2.5×10 –5 Table 2: Equivalent circuit fitting results of T4 and TW41 alloys Alloy R s Ù·cm 2 CPE R t Ù·cm 2 R L Ù·cm 2 L H·cm 2 Y1 n1 T4 6.102 8.90×10 –5 0.86 149.8 38.17 223 TW41 6.315 12.97×10 –5 0.90 71.93 17.63 104.5 Figure 10 shows macroscopic and SEM mor- phologies of the extruded T4 and TW41 alloys after 24 h of corrosion, with the corrosion products removed. Com- parisons revealed that the as-extruded T4 alloy shows less surface corrosion than the TW41 alloy, and a metal- lic luster was observed on its surface. On the SEM im- ages, we can see corrosion pits that occurred when the corrosion products were peeled off the surface of the as-extruded T4 and TW41 alloys. This is mainly due to the fact that the corrosion medium can penetrate the sur- face film and directly contact the Mg matrix after the oc- currence of pitting, which causes the corrosion and dis- solution of the -Mg matrix, resulting in the second phase falling off and producing pits. In addition, it can be observed on Figure 10 that the pits of TW41 alloy are deeper than those of T4 alloy, indicating that the corro- sion resistance of T4 alloy is higher than that of TW41 alloy; it is believed that this is due to the addition of Y, which precipitates the ternary MgSnY phase and in- creases the volume fraction of the second phase. This in- creases galvanic corrosion between the second phase as the microcathode and the magnesium matrix, resulting in a rapid corrosion of the magnesium matrix around the second phase. 4 DISCUSSION Studies have shown that the grain size, second phase, texture, or grain orientation of magnesium alloys are im- portant factors affecting the properties of magnesium al- loys. 24,25 The properties of an alloy are the results of a combination of factors such as grain size, second phase, microstructure uniformity, defect density, etc. In terms of mechanical properties, grain refinement leads to an in- crease in grain boundaries, making the transport of dislo- cations difficult, thereby increasing the strength of the al- loy. In this study, the yield strength and tensile strength of the as-extruded alloy increased and the elongation de- creased after the addition of Y to T4 alloy, which was mainly due to the effect of the addition of Y on the grain Q. CHEN et al.: EFFECT OF Y ON THE MECHANICAL PROPERTIES AND CORROSION RESISTANCE OF ... 752 Materiali in tehnologije / Materials and technology 58 (2024) 6, 745–754 Figure 10: Macroscopic and SEM images of as-extruded T4 and TW41 alloys: (a), (c) T4; (b), (d) TW41 refinement of the alloy and the effect of fine-grain strengthening. According to the Hall-Petch formula, the yield strength is inversely proportional to the grain size, and the yield strength of the alloy can be improved by fine grains, so the strength of TW41 alloy is increased. 26 In order to further investigate the main deformation mechanisms of the two as-extruded alloys in this test, the Schmid factor diagram shown in Figure 11 was deter- mined. It was shown that the Schmid factor can qualita- tively analyze the initiation of various dislocation slip mechanisms during plastic deformation of metals, and the initiation of a slip usually follows Schmid’s law. 27 As can be seen on Figure 11, with the addition of 1 w/% Y, the basal, cylindrical and cone slip SF of the alloy in- creases significantly, resulting in an increase in the duc- tility of the alloy. In terms of corrosion performance, there is a similar Hall-Petch relationship between the corrosion current density and grain size of magnesium alloys. The grain boundary density increases due to the grain refinement, while the grain boundary, as a highly active region, increases the dissolution rate of the anode, thereby reducing the corrosion resistance of the alloy. 28 In this study, the corrosion resistance of the as-extruded TW41 alloy was better than that of the as-extruded T4 al- loy, which was due to the increase in the grain boundary density caused by the addition of Y. The grain boundary, as a highly active region, accelerated the dissolution of the anode, thereby increasing the corrosion rate of TW41 alloy. In addition, the volume fraction of second phase of TW41 alloy is higher than that of T4 alloy, increasing the initiation sites for galvanic corrosion between the second phase and the magnesium matrix. This results in a rapid corrosion of the magnesium matrix around the second phase particles. Thus, the corrosion rate of TW41 alloy is higher than that of T4 alloy. 5 CONCLUSION In this paper, the effects ofaYa d d i tion on the microstructure, mechanical properties and corrosion re- sistance of Mg-4Sn alloy are systematically studied, and the following conclusions are drawn: • The addition of 1 wt.% Y element can refine the grain size of the extruded T4 alloy. The volume frac- tion of the second phase of TW41 alloy increases, and the precipitated second phase is mainly com- posed of the ternary MgSnY and the Mg 2 Sn phases. • The addition of 1 wt.% Y failed to change the texture type of the alloy, but it could weaken the texture strength of the alloy. The precipitated MgSnY phase in TW41 causes grain refinement in the alloy, leading to an increase in the grain boundaries. This increase makes dislocation motion more difficult, thereby im- proving the yield strength of the alloy. • The addition of 1 w/% Y can increase the self-corro- sion potential of T4 alloy, but an excessive second Q. CHEN et al.: EFFECT OF Y ON THE MECHANICAL PROPERTIES AND CORROSION RESISTANCE OF ... Materiali in tehnologije / Materials and technology 58 (2024) 6, 745–754 753 Figure 11: Distribution of Schmid factors: a), c) T4, b), d) TW41 phase increases the initiation sites for galvanic corro- sion, thus increasing the corrosion current of the al- loy and reducing the corrosion resistance. Acknowledgements The authors acknowledge the National College Stu- dents’ Entrepreneurship Project (202311035033), the fi- nancial support from the Liaoning Province Natural Sci- ence Foundation Project of China (2023-MS-321) and the Liaoning Province International Cooperation Project (Project Number: 2023030491-JH2/107). 6 REFERENCES 1 J. Song, J. Chen, X. 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