Influence of Processing Techniques on Microstructure and Mechanical Properties of a Biodegradable Mg-3Zn-2Ca Alloy

materials Article Influence of Processing Techniques on Microstructure and Mechanical Properties of a Biodegradable Mg-3Zn-2Ca Alloy Pavel Doležal 1,*, Josef Zapletal 1, Stanislava Fintová 2, Zuzanka Trojanová 3, Miroslav Greger 4, Pavla Roupcová 1,2 and Tomáš Podrábský 1 1Faculty of Mechanical Engineering, Institute of Materials Science and Engineering, Brno University of Technology, Technická 2, Brno 616 69, Czech Republic; [email protected].cz (J.Z.); [email protected].cz (P.R.); [email protected].cz (T.P.) 2Institute of Physics of Materials, Academy of Sciences of the Czech Republic v. v. i., Žižkova 22, Brno 616 62, Czech Republic; [email protected] 3Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, Praha 121 16, Czech Republic; [email protected]f.cuni.cz 4Department of Materials Forming, Faculty of Metallurgy and Materials Engineering, VŠB—Technical University of Ostrava, 17. listopadu 15, Ostrava 725 29, Czech Republic; miroslav.gr[email protected] *Correspondence: [email protected].cz; Tel.: +420-5-4114-3189 Academic Editor: Regine Willumeit-Römer Received: 11 August 2016; Accepted: 25 October 2016; Published: 28 October 2016 Abstract: New Mg-3Zn-2Ca magnesium alloy was prepared using different processing techniques: gravity casting as well as squeeze casting in liquid and semisolid states. Materials were further thermally treated; thermal treatment of the gravity cast alloy was additionally combined with the equal channel angular pressing (ECAP). Alloy processed by the squeeze casting in liquid as well as in semisolid state exhibit improved plasticity; the ECAP processing positively influenced both the tensile and compressive characteristics of the alloy. Applied heat treatment influenced the distribution and chemical composition of present intermetallic phases. Influence of particular processing techniques, heat treatment, and intermetallic phase distribution is thoroughly discussed in relation to mechanical behavior of presented alloys. Keywords: biodegradable magnesium alloy; Mg-Zn-Ca; squeeze casting; ECAP processing; microstructure; mechanical properties 1. Introduction Magnesium and its alloys belong to progressive materials increasingly applied in the transportation, electronic and optical industries. Over the last decades, a new application was found also in the biomedical field. Important advantages predetermining magnesium alloys to be chosen for many usages are their low density, high specific strength and stiffness, and high damping capacity [ 1 – 5 ]. Magnesium alloys have also excellent biomechanical properties for many applications and biocompatibility which allow adopting them as materials convenient for implants in the orthopedics and trauma therapy [ 2 – 4 , 6 – 8 ]. The most restrictive disadvantages of magnesium and its alloys are poor formability and limited ductility at room temperature, which are a consequence of their hexagonal close-packed (hcp) structure with the restricted number of available easy-slip systems and presence of intermetallic phases [ 9 ]. Another disadvantage is the quite poor corrosion resistance; however, this disadvantage may be turned into an advantage for biomedical applications like temporary implants [9]. Materials 2016,9, 880; doi:10.3390/ma9110880 www.mdpi.com/journal/materials Materials 2016,9, 880 2 of 15 Properties of magnesium alloys are dependent on their chemical composition, [ 5 – 7 , 10 ], mainly on the content and combination of alloying elements. Some of these elements ( RE metals, Al, etc. ) improving mechanical properties and corrosion resistance of magnesium alloys are toxic, so their content should to be strictly controlled [ 1 , 5 , 10 ]. Producing technologies and/or additional thermo-mechanical treatment represent another way how to improve the mechanical and corrosion properties of the material [ 1 , 5 , 11 ]. Production technologies influence the microstructure heterogeneity such as casting defects, distribution of intermetallic phases, grain size and further texture or dislocation substructure. Additional thermo-mechanical treatment may homogenize and refine the microstructure (grain size, size of intermetallic compounds typically present in magnesium alloys, dissolution of intermetallic particles). In spite of the several previous studies concerning the mechanical and corrosion properties of Mg-Zn-Ca alloys, there are still problems regarding optimal chemical composition and thermo-mechanical treatment to achieve the best combination of mechanical and corrosion properties. Chemical composition of the newly developed Mg-3Zn-2Ca alloy presented in the study has been chosen analyzing the literature data. Zinc has great potential as the alloying element for the improvement of mechanical properties such as tensile strength, elongation to fracture and corrosion resistance of Mg alloys [ 4 , 6 , 7 , 9 , 11 – 14 ]. The zinc content positively influences the harmfully corrosive effect of Fe and Ni, which are usually present in a small amount in Mg alloys [ 1 , 15 ]. Addition of other alloying elements such as Mn and Ca may even improve mechanical properties of the Mg-Zn system [ 4 , 7 , 15 – 17 ]. Zn element is also often the main component of various enzymes present in the human body [ 6 ]. This fact predetermines Mg-Zn alloys for biomedical applications. However, higher content of Zn decreases the corrosion resistance of Mg-Zn-Ca alloys due to an increase of the corrosion current owing to the corrosion potential rise [4]. The appropriate content of Ca in Mg alloys may improve both the corrosion resistance and mechanical properties; Ca is also incorporated in the human bones and it can accelerate their growth [ 6 ]. Ca addition into a magnesium alloy improves the oxidation resistance due to formation of protective oxide layers [ 18 ]. It also increases hardness and the creep resistance due to formation of the Mg 2 Ca intermetallic compound with a high melting point [ 18 ]. Mg-Ca alloys containing 0.6–1.0 wt % of Ca provide acceptable mechanical and corrosion properties; therefore, the Mg-Ca binary alloys are considered biodegradable materials [ 6 , 19 ]. Magnesium alloys with 1.0–3.0 wt % of Ca are still non-toxic; however, the higher Ca content deteriorates mechanical characteristics and the alloy corrosion resistance [19,20]. Presence of fine particles in the matrix of Mg-Ca-Zn alloys (mainly Mg 6 Ca 2 Zn 3 and Mg 2 Ca) results in high strength, good creep resistance and acceptable hardness. Mg-Zn-Ca alloys are interesting materials for many applications [ 9 , 15 , 18 , 21 – 24 ]; nevertheless, the quite low fracture toughness should to be taken into account before use. Formation of the Mg 6 Ca 2 Zn 3 secondary phase increases tensile strength in the Mg-Zn-Ca alloys [ 15 ]. The combination of Zn and Ca solute atoms influences not only mechanical characteristics but also corrosion resistivity [ 17 ]. An addition of 2 wt % Zn to the Mg-3Ca alloy improves the mechanical properties (ultimate tensile strength by 23% and ductility by 120%), and also the corrosion resistance may be improved due to the present eutectic phase (Mg + Mg2Ca + Ca2Mg6Zn3) [6]. Both the amount and dispersion of present intermetallic phases influence the alloy mechanical properties. Zhang et al. presented in [ 4 ] mechanical characteristics of Mg-xZn-1Ca alloy depending on Zn content in a wide range of concentrations from 0 to 6 wt % and 1 wt % of Ca. By addition of the different Zn content levels to the Mg-xZn-1Ca alloy, the ultimate tensile strength varies from 105 ±4 to 182 ± 5 MPa ( Mg-0Zn-1Ca vs. Mg-4Zn-1Ca ), the yield stress may vary from 39 to 67 MPa ( Mg-0Zn-1Ca vs. Mg-6Zn-1Ca ) and the values of elongation to fracture exhibited values from 4.1% ±0.5% to 9.1% ±2.5% (Mg-0Zn-1Ca vs. Mg-4Zn-1Ca). Wan et al. studied in [ 19 ] the influence of Ca in Mg-Ca alloys on mechanical and corrosion properties. The bending and compressive strength was increased with the increase of Ca content from 0.6 to 2.0 wt %. However, they also observed the negative influence of the increasing volume fraction of Materials 2016,9, 880 3 of 15 the Mg 2 Ca phase on the corrosion resistance. The Mg-4.0Zn-0.2Ca alloy examined by Sun et al. in [ 17 ] exhibits quite low strength which could be improved by the following hot extrusion. The extruded alloy with fine grains (the average grain size of 3–7 µ m) exhibits the tensile yield stress of 240 ± 5 MPa, ultimate tensile strength of 297 ±5 MPa and elongation to fracture of 21.3% ±3.0%. Many studies (see, for example, [ 6 , 9 , 13 , 16 , 19 , 20 , 23 , 25 ]) are focused on the influence of individual alloying elements on microstructural, mechanical and corrosion characteristics of Mg-Zn-Ca alloys, but only very few of the available sources give any information concerning the synergetic impact of alloying elements and processing technology on the resulting microstructure and mechanical properties. One of the production methods of magnesium alloys is the gravity casting, even though this process is often accompanied by the presence of casting defects [ 26 ]. Squeeze casting technique (SC) is the metal forming process where the solidification is promoted under the high-pressure application within a reusable tool. Molten metal during the SC process is solidified under the applied pressure, which results in the fine-grained microstructure with the excellent surface finish and practically no porosity. Mechanical properties of the SC components can be positively influenced compared to the gravity cast and die cast ones [ 26 ]. The SC technique was described in the literature in several modifications (from completely solidified alloys to SC of semisolid metal) [ 27 , 28 ]. With the semisolid melt in the SC process, a positive influence on the tensile properties and elongation to fracture, especially when the SC is followed by some thermal treatment, can be observed. The present paper deals with Mg-3Zn-2Ca alloy developed for biomedical applications processed by various means. It offers the complex study of the microstructure evolution and resulting mechanical properties. Chemical composition of the Mg-3Zn-2Ca alloy was chosen to reach optimum ratio between strength and plasticity. The Mg-3Zn-2Ca alloy was squeeze cast in the liquid and semisolid states. Thermal treatment was applied with the aim of improving the deformation properties of the alloy. The ECAP treatment applied in the combination with a thermal treatment resulted in the significant improvement of the strength of the gravity cast alloy. 2. Experimental Material and Methods Gravity cast Mg-3Zn-2Ca (2.5–3.5 wt % Zn and 1.5–2.5 wt % Ca. balance Mg) alloy was used in this study as the default material. Four material states were studied: gravity cast alloy (GC), squeeze cast liquid alloy (SCL), squeeze cast semisolid alloy (SCS) and equal channel angular pressed alloy (ECAP). Microstructural analysis of the experimental material was performed in the scanning electron microscope (SEM) Philips XL-30 (Brno, Czech Republic) equipped with the EDS EDAX detector. Specimens for the microstructural evaluation were prepared by the standard metallographic procedures including grinding (SiC papers no 1200, 2500 and 4000) and polishing (diamond paste with grains of 3, 1 and 0.25 µ m) using isopropanol as a cooling medium. To reveal the microstructure of the prepared materials, an etchant consisting of 2% Nital was used. The average grain size was estimated by the line method using software for image analysis. Tensile tests were performed to obtain basic mechanical properties of Mg-3Zn-2Ca alloy processed by different methods. Mechanical tests were carried out in a Zwick Z250 PC-controlled testing device (Zwick GmbH & Co.KG in Germany, Ulm, Germany) with an initial strain rate of . ε = 2.5 × 10 −4 s −1 at room temperature (23 ± 2 ◦ C). Tensile specimens were machined and tested according to the EN ISO 6892-1 standard [ 29 ]. The gauge length and diameter of the cylindrical specimens were 25 mm and 5 mm, respectively. Tested samples were cut from the treated semi products so that the longitudinal sample axis was identical with the applied force direction in the manufacturing tool. The specimens for compression tests exhibited diameter and length of 8 mm and 12 mm. For the compression tests, the grip heads were lubricated by MoS 2 . The obtained data are presented in the true stress–true plastic strain curves plot. The yield stress σ02 was estimated as the flow stress at the plastic strain of ε = 0.002; ultimate tensile/compression strength σUTS / σUCS was taken as the true maximum stress achieved in the mechanical test. Materials 2016,9, 880 4 of 15 The microstructural features and the secondary phase composition were characterized by the X-ray powder diffraction (XRD) by the SmartLab (Rigaku, Tokyo, Japan) with the conventional Bragg-Brenato geometry. The CuK α1,2 radiation with the β -filter in a secondary beam and the linear positional sensitive detector D-Tex was used for the microstructure investigations in an angle range of 10 ◦ –125 ◦ , whit step 0.02 ◦ and speed 0.3 s/step. The HighScore Plus program (PANAnalytical, Almelo, The Netherlands) equipped by the JCPDS PDF-4 database (NIST) was used for the qualitative and quantitative analysis of the obtained data. The squeeze casting method in the liquid state (SCL) and modified squeeze casting method in the semisolid state (SCS) were applied to refine the original as-cast alloy microstructure, formed in the gravity cast ingots. The cylindrical specimens with a diameter of 40 mm, covered by a protecting stainless steel shell, were heated to a temperature of 720 ◦C for liquid and to 650 ◦C for the semisolid casting for 30 min. Specimens were subsequently squeezed at a pressure of 150 MPa for 5 min. The shell of the produced specimens was removed after the squeeze casting process. Specimens for mechanical testing were prepared from an SCL and SCS semi-products with a final diameter of 30 mm. The equal channel angular pressing was performed in an NMT 3000 type machine (RAKOVNICKÉ TVÁˇ RECÍ STROJE s. r. o., Rakovník, Czech Republic) with a die angle of 90 ◦ . The tool was preheated to a temperature of 350 ◦ C then the sample was passed through the die with a speed of 4 mm · s −1 . Only one pass through the die channel was applied. Deformation force used for one applied pass through the ECAP die was 3 MN and the cross-section of the final product was of 20 × 20 × 120 mm 3 . To obtain compact products by the ECAP, it was necessary to use backpressure. The pressure of the oil in the system used as the backpressure to the ECAP was 0.5 MPa. The heat treatment (HT) was applied in all four states with the aim of obtaining finer microstructure and improved mechanical properties. The HT consisted of heating to a temperature of 450 ◦C for 24 h followed by quenching in water at an ambient temperature (23 ±2◦C). 3. Microstructure Characteristics 3.1. Gravity Cast Alloy (GC) The microstructure of the GC alloy consisted of α grains (solid solution of alloying elements in Mg), decorated by second phase particles at the grain boundaries (Figure 1a,b). The average grain size of the GC alloy was measured to be 40.7 ± 5.9 µ m (Table 1). Two different intermetallic phases were identified in the microstructure presented in Figure 1b. Based on the phase analysis results, introduced in Table 2, Mg 2 Ca intermetallic phase was identified, in the alloy microstructure (see Figure 1b), typical for Mg-Ca systems [ 8 ]. The second intermetallic phase present in the microstructure, identified based on the chemical composition measured by the EDX system (see Table 2and literature data [ 8 ]), was Ca2Mg6Zn3intermetallic compound (light particles in Figure 1b). Optimized HT affected only the intermetallic particles. The average grain size of CG alloy was not affected by the applied HT. The HT partially dissolved particles of both types. The Mg 2 Ca phase was reshaped into spheres surrounded by a thin shell of the Ca 2 Mg 6 Zn 3 intermetallic phase, as it is demonstrated in Figure 1c,d. Applied HT changed also the chemical composition of the matrix (hcp Mg) and present intermetallic phases as it is documented in Table 2. Significant influence of the HT was observed in the case of the Ca 2 Mg 6 Zn 3 phase. Observed decrease of Zn and Ca content in the Ca-Mg-Zn phase, due to its partial dissolution during the HT was observed. Zn atoms from the Ca 2 Mg 6 Zn 3 phase diffused into the α solid solution while the Ca atoms were absorbed into the Mg-Ca phase (Table 2). Observed dissolution of intermetallic phases was manifested by the increase of solute elements in the alloy matrix (Table 2). Materials 2016,9, 880 5 of 15 Materials 2016, 9, 880 5 of 15 Figure 1. Microstructure of the GC alloy: (a) as-cast; (b) secondary phases in the as-cast state; (c) GC + HT; (d) secondary phases in GC + HT state depicted in the SEM–BSE mode. Table 1. Average grain size of the alloy processed by various methods. Processing Method GC SCL SCS ECAP average grain size (μm) 40.7 ± 5.9 9.6 ± 0.7 bimodal 9.6 ± 0.7 up to 200 84.3 ± 0.3 ECAP direction 30.5 ± 2.6 perpendicular d Table 2. Chemical composition of the alloy processed by various methods. Alloy State Phase Element (wt %) Element (at %) Mg Ca Zn Mg Ca Zn GC hcp Mg 98.3 0.4 1.3 99.2 0.3 0.5 Mg2Ca 70.0 24.8 5.1 80.3 17.4 2.2 Ca2Mg6Zn3 62.9 10.5 26.6 79.4 8.1 12.5 GC + HT hcp Mg 97.3 0.5 2.2 98.9 0.3 0.8 Mg2Ca 56.8 37.9 5.3 69.5 28.1 2.4 Ca2Mg6Zn3 71.3 7.2 21.6 85.2 5.2 9.6 SCL hcp Mg 98.7 0.4 1.0 99.4 0.2 0.4 Mg2Ca 80.0 15.2 4.8 87.9 10.1 1.9 Ca2Mg6Zn3 66.2 11.2 22.6 81.3 8.3 10.5 SCL + HT hcp Mg 97.5 0.5 2.0 99.0 0.3 0.7 Mg2Ca 59.1 36.3 4.6 71.4 26.6 2.1 Ca2Mg6Zn3 72.0 8.6 19.4 85.3 6.2 8.5 SCS hcp Mg 99.1 0.2 0.7 99.6 00.1 0.3 Mg2Ca 75.0 19.1 6.0 84.5 13.0 2.5 Ca2Mg6Zn3 62.7 11.4 26.0 79.0 8.7 12.2 SCS + HT hcp Mg 97.7 0.4 1.9 99.0 0.3 0.7 Mg2Ca 57.9 36.9 5.2 70.4 27.3 2.4 Ca2Mg6Zn3 not present in the microstructure not present in the microstructure Figure 1. Microstructure of the GC alloy: ( a ) as-cast; ( b ) secondary phases in the as-cast state; (c) GC + HT; (d) secondary phases in GC + HT state depicted in the SEM–BSE mode. Table 1. Average grain size of the alloy processed by various methods. Processing Method GC SCL SCS ECAP average grain size (µm) 40.7 ±5.9 9.6 ±0.7 bimodal 9.6 ±0.7 up to 200 84.3 ±0.3 ECAP direction 30.5 ±2.6 perpendicular d Table 2. Chemical composition of the alloy processed by various methods. Alloy State Phase Element (wt %) Element (at %) Mg Ca Zn Mg Ca Zn GC hcp Mg 98.3 0.4 1.3 99.2 0.3 0.5 Mg2Ca 70.0 24.8 5.1 80.3 17.4 2.2 Ca2Mg6Zn362.9 10.5 26.6 79.4 8.1 12.5 GC + HT hcp Mg 97.3 0.5 2.2 98.9 0.3 0.8 Mg2Ca 56.8 37.9 5.3 69.5 28.1 2.4 Ca2Mg6Zn371.3 7.2 21.6 85.2 5.2 9.6 SCL hcp Mg 98.7 0.4 1.0 99.4 0.2 0.4 Mg2Ca 80.0 15.2 4.8 87.9 10.1 1.9 Ca2Mg6Zn366.2 11.2 22.6 81.3 8.3 10.5 SCL + HT hcp Mg 97.5 0.5 2.0 99.0 0.3 0.7 Mg2Ca 59.1 36.3 4.6 71.4 26.6 2.1 Ca2Mg6Zn372.0 8.6 19.4 85.3 6.2 8.5 SCS hcp Mg 99.1 0.2 0.7 99.6 00.1 0.3 Mg2Ca 75.0 19.1 6.0 84.5 13.0 2.5 Ca2Mg6Zn362.7 11.4 26.0 79.0 8.7 12.2 SCS + HT hcp Mg 97.7 0.4 1.9 99.0 0.3 0.7 Mg2Ca 57.9 36.9 5.2 70.4 27.3 2.4 Ca2Mg6Zn3not present in the microstructure not present in the microstructure Materials 2016,9, 880 6 of 15 3.2. Squeeze Cast Liquid Alloy (SCL) The microstructure of the SCL alloy consisted of fine grains of the α solid solution and intermetallic phases creating a network between the grains (Figure 2a,b). The average grain size of the SCL alloy, 9.6 ±0.7 µm , was much lower compared with the GC material as it can be seen from Table 1. Two types of intermetallic phases were present in the microstructure, as it follows from Figure 2b, namely Mg 2 Ca and Ca 2 Mg 6 Zn 3 , determined by their chemical composition introduced in Table 2, and the phase composition was determined by the X-ray analysis (described in Chapter 3.5). Only a small amount of the Ca 2 Mg 6 Zn 3 phase was observed in the microstructure mainly situated at the interface between the matrix grains and Mg2Ca phase (see Figure 2b). The continuous network of the second phase particles partially dissolved after HT and changed shape into globular particles (Figure 2c,d). A small amount of the remaining Ca 2 Mg 6 Zn 3 phase was present at the interface between spheroid Mg 2 Ca phase and the matrix as it is indicated in Figure 2d. Similar partial dissolution and absorption of Ca 2 Mg 6 Zn 3 phase as well as in the GC sample after the HT was observed. Zn from the Ca 2 Mg 6 Zn 3 phase was dissolved and absorbed by the matrix (see Mg content in Table 2) and, similarly, Ca atoms were absorbed by the Mg 2 Ca phase, which was also proven by the chemical analysis (Table 2). Materials 2016, 9, 880 6 of 15 3.2. Squeeze Cast Liquid Alloy (SCL) The microstructure of the SCL alloy consisted of fine grains of the α solid solution and intermetallic phases creating a network between the grains (Figure 2a,b). The average grain size of the SCL alloy, 9.6 ± 0.7 μm, was much lower compared with the GC material as it can be seen from Table 1. Two types of intermetallic phases were present in the microstructure, as it follows from Figure 2b, namely Mg2Ca and Ca2Mg6Zn3, determined by their chemical composition introduced in Table 2, and the phase composition was determined by the X-ray analysis (described in Chapter 3.5). Only a small amount of the Ca2Mg6Zn3 phase was observed in the microstructure mainly situated at the interface between the matrix grains and Mg2Ca phase (see Figure 2b). The continuous network of the second phase particles partially dissolved after HT and changed shape into globular particles (Figure 2c,d). A small amount of the remaining Ca2Mg6Zn3 phase was present at the interface between spheroid Mg2Ca phase and the matrix as it is indicated in Figure 2d. Similar partial dissolution and absorption of Ca2Mg6Zn3 phase as well as in the GC sample after the HT was observed. Zn from the Ca2Mg6Zn3 phase was dissolved and absorbed by the matrix (see Mg content in Table 2) and, similarly, Ca atoms were absorbed by the Mg2Ca phase, which was also proven by the chemical analysis (Table 2). Figure 2. Microstructure of SCL sample: (a) SCL state; (b) secondary phases in the SCL state; (c) SCL + HT; (d) secondary phases in SCL + HT depicted in the SEM–BSE mode. 3.3. Squeeze Cast Semisolid Alloy (SCS) The resulting microstructure, obtained after modified squeeze casting in the SCS state, is shown in Figure 3a. The network of the secondary phase was observed at the grain boundaries. Two types of intermetallic phases were present in the microstructure, namely Mg2Ca and Ca2Mg6Zn3 (Table 2 and Figure 3b), similar to the GC and SCL case. The grain size distribution is bimodal. The large grains (~200 μm) of the solid solution containing small intermetallic particles were observed in the microstructure as well as fine grains with the grain size of 9.6 ± 0.7 μm (see Figure 3a, Table 1). HT of the alloy in SCS state resulted in the dissolution of the present intermetallic phases (Figure 3c,d). In the microstructure of SCS alloy after the HT, no Ca2Mg6Zn3 phase particles were observed. The Ca2Mg6Zn3 phase was completely dissolved during the HT and absorbed by other structural components. Only rounded Mg2Ca particles, situated in the grain boundaries, were observed in the microstructure (Figure 3d). The remaining Mg2Ca phase particles became larger compared with the Figure 2. Microstructure of SCL sample: ( a ) SCL state; ( b ) secondary phases in the SCL state; (c) SCL + HT; (d) secondary phases in SCL + HT depicted in the SEM–BSE mode. 3.3. Squeeze Cast Semisolid Alloy (SCS) The resulting microstructure, obtained after modified squeeze casting in the SCS state, is shown in Figure 3a. The network of the secondary phase was observed at the grain boundaries. Two types of intermetallic phases were present in the microstructure, namely Mg 2 Ca and Ca 2 Mg 6 Zn 3 (Table 2 and Figure 3b), similar to the GC and SCL case. The grain size distribution is bimodal. The large grains (~200 µ m) of the solid solution containing small intermetallic particles were observed in the microstructure as well as fine grains with the grain size of 9.6 ±0.7 µm (see Figure 3a, Table 1). HT of the alloy in SCS state resulted in the dissolution of the present intermetallic phases (Figure 3c,d). In the microstructure of SCS alloy after the HT, no Ca 2 Mg 6 Zn 3 phase particles were observed. The Ca 2 Mg 6 Zn 3 phase was completely dissolved during the HT and absorbed by other structural components. Only rounded Mg 2 Ca particles, situated in the grain boundaries, were observed Materials 2016,9, 880 7 of 15 in the microstructure (Figure 3d). The remaining Mg 2 Ca phase particles became larger compared with the particles estimated after the heat-treated GC and SCL states as is demonstrated in Figure 1c,d and Figure 4c,d. Also in the case of the SCS alloy after HT, the chemical composition, shown in Table 2, indicates the absorption of Zn and Ca atoms from the dissolved Ca 2 Mg 6 Zn 3 phase into the matrix and the Mg2Ca phase respectively. Materials 2016, 9, 880 7 of 15 particles estimated after the heat-treated GC and SCL states as is demonstrated in Figures 1c,d and 4c,d. Also in the case of the SCS alloy after HT, the chemical composition, shown in Table 2, indicates the absorption of Zn and Ca atoms from the dissolved Ca2Mg6Zn3 phase into the matrix and the Mg2Ca phase respectively. Figure 3. Microstructure of the SCS sample: (a) SCS state; (b) secondary phases in SCS; (c) SCS + HT; (d) secondary phases in SCS + HT state depicted in the SEM–BSE mode. 3.4. ECAPed Mg-3Zn-2Ca (Equal Channel Angular Pressing) ECAP processing of the alloy in the GC state resulted in the microstructure containing elongated grains oriented into extrusion direction surrounded by the intermetallic phases present at the grain boundaries (Figure 4a,b). The average grain size (Table 1) in the extrusion direction was 84.3 ± 0.3 μm. The average grain size in the direction perpendicular to the processing axis was 30.5 ± 2.6 μm. Chemical composition of the microstructural features was not affected by higher temperature in the preheated ECAP tool. The present Ca2Mg6Zn3 phase was observed at the interface between Mg2Ca phase and the matrix (Figure 4b). HT of the GC alloy resulted in the fine microstructure shown in Figure 1c,d. The following ECAP treatment deformed the grain structure toward the extrusion direction (Figure 4c,d). The present Ca2Mg6Zn3 phase was observed in the interface between the Mg2Ca phase and the matrix (Figure 4d). Figure 3. Microstructure of the SCS sample: ( a ) SCS state; ( b ) secondary phases in SCS; ( c ) SCS + HT; (d) secondary phases in SCS + HT state depicted in the SEM–BSE mode. 3.4. ECAPed Mg-3Zn-2Ca (Equal Channel Angular Pressing) ECAP processing of the alloy in the GC state resulted in the microstructure containing elongated grains oriented into extrusion direction surrounded by the intermetallic phases present at the grain boundaries (Figure 4a,b). The average grain size (Table 1) in the extrusion direction was 84.3 ±0.3 µm . The average grain size in the direction perpendicular to the processing axis was 30.5 ± 2.6 µ m. Chemical composition of the microstructural features was not affected by higher temperature in the preheated ECAP tool. The present Ca 2 Mg 6 Zn 3 phase was observed at the interface between Mg 2 Ca phase and the matrix (Figure 4b). HT of the GC alloy resulted in the fine microstructure shown in Figure 1c,d. The following ECAP treatment deformed the grain structure toward the extrusion direction (Figure 4c,d). The present Ca 2 Mg 6 Zn 3 phase was observed in the interface between the Mg 2 Ca phase and the matrix (Figure 4d). Materials 2016, 9, 880 7 of 15 particles estimated after the heat-treated GC and SCL states as is demonstrated in Figures 1c,d and 4c,d. Also in the case of the SCS alloy after HT, the chemical composition, shown in Table 2, indicates the absorption of Zn and Ca atoms from the dissolved Ca2Mg6Zn3 phase into the matrix and the Mg2Ca phase respectively. Figure 3. Microstructure of the SCS sample: (a) SCS state; (b) secondary phases in SCS; (c) SCS + HT; (d) secondary phases in SCS + HT state depicted in the SEM–BSE mode. 3.4. ECAPed Mg-3Zn-2Ca (Equal Channel Angular Pressing) ECAP processing of the alloy in the GC state resulted in the microstructure containing elongated grains oriented into extrusion direction surrounded by the intermetallic phases present at the grain boundaries (Figure 4a,b). The average grain size (Table 1) in the extrusion direction was 84.3 ± 0.3 μm. The average grain size in the direction perpendicular to the processing axis was 30.5 ± 2.6 μm. Chemical composition of the microstructural features was not affected by higher temperature in the preheated ECAP tool. The present Ca2Mg6Zn3 phase was observed at the interface between Mg2Ca phase and the matrix (Figure 4b). HT of the GC alloy resulted in the fine microstructure shown in Figure 1c,d. The following ECAP treatment deformed the grain structure toward the extrusion direction (Figure 4c,d). The present Ca2Mg6Zn3 phase was observed in the interface between the Mg2Ca phase and the matrix (Figure 4d). Figure 4. Cont. Materials 2016,9, 880 8 of 15 Materials 2016, 9, 880 8 of 15 Figure 4. Microstructure of the ECAPed alloy: (a) ECAPed state; (b) secondary phases in the ECAPed state; (c) HT + ECAP; (d) secondary phases in HT + ECAP state depicted in the SEM–BSE mode. 3.5. Chemical and Phase Composition of Mg-3Zn-2Ca Alloy Processed by Various Techniques Chemical composition of the observed intermetallic phases in the alloy processed by different technologies was modified due to the heat treatment in all the cases (Table 2). The X-ray pattern of Mg-3Zn-2Ca alloy processed by different methods is shown in Figure 5. The phase composition of the matrix (α grains) and the present intermetallic phases was similar for all the alloy states after applied HT. Only in the case of the SCS + HT alloy state was the Ca2Mg6Zn3 phase completely dissolved and absorbed by other structural components during the heat treatment (matrix and the Mg2Ca phase) (Table 2, Figure 3c,d). The phase composition of the alloy was not changed by ECAP treatment compared to the GC state. Figure 5. X-ray pattern of Mg-3Zn-2Ca magnesium alloy processed by different methods. 4. Deformation Characteristics True stress–strain curves obtained at room temperature in tension and compression for the GC alloy are introduced in Figure 6. Characteristic values—the yield stress, σ02, the ultimate tensile/compression strength σUTS/σUCS, strain to fracture, ε, were evaluated and are presented in Tables 3 and 4. All mechanical characteristics were estimated with the standard deviation of ±5%. 35 40 45 50 55 Position 2 ( ° )  Intensity (a.u.) (100) o (002) o (101) o (102) o (110) o K o - Mg + - Mg Ca 2 (103) + (200) + (112) + (104) + (103) + (213) + (302) + (115) + (203) + K Figure 4. Microstructure of the ECAPed alloy: ( a ) ECAPed state; ( b ) secondary phases in the ECAPed state; (c) HT + ECAP; (d) secondary phases in HT + ECAP state depicted in the SEM–BSE mode. 3.5. Chemical and Phase Composition of Mg-3Zn-2Ca Alloy Processed by Various Techniques Chemical composition of the observed intermetallic phases in the alloy processed by different technologies was modified due to the heat treatment in all the cases (Table 2). The X-ray pattern of Mg-3Zn-2Ca alloy processed by different methods is shown in Figure 5. The phase composition of the matrix ( α grains) and the present intermetallic phases was similar for all the alloy states after applied HT. Only in the case of the SCS + HT alloy state was the Ca 2 Mg 6 Zn 3 phase completely dissolved and absorbed by other structural components during the heat treatment (matrix and the Mg 2 Ca phase) (Table 2, Figure 3c,d). The phase composition of the alloy was not changed by ECAP treatment compared to the GC state. Materials 2016, 9, 880 8 of 15 Figure 4. Microstructure of the ECAPed alloy: (a) ECAPed state; (b) secondary phases in the ECAPed state; (c) HT + ECAP; (d) secondary phases in HT + ECAP state depicted in the SEM–BSE mode. 3.5. Chemical and Phase Composition of Mg-3Zn-2Ca Alloy Processed by Various Techniques Chemical composition of the observed intermetallic phases in the alloy processed by different technologies was modified due to the heat treatment in all the cases (Table 2). The X-ray pattern of Mg-3Zn-2Ca alloy processed by different methods is shown in Figure 5. The phase composition of the matrix (α grains) and the present intermetallic phases was similar for all the alloy states after applied HT. Only in the case of the SCS + HT alloy state was the Ca2Mg6Zn3 phase completely dissolved and absorbed by other structural components during the heat treatment (matrix and the Mg2Ca phase) (Table 2, Figure 3c,d). The phase composition of the alloy was not changed by ECAP treatment compared to the GC state. Figure 5. X-ray pattern of Mg-3Zn-2Ca magnesium alloy processed by different methods. 4. Deformation Characteristics True stress–strain curves obtained at room temperature in tension and compression for the GC alloy are introduced in Figure 6. Characteristic values—the yield stress, σ02, the ultimate tensile/compression strength σUTS/σUCS, strain to fracture, ε, were evaluated and are presented in Tables 3 and 4. All mechanical characteristics were estimated with the standard deviation of ±5%. 35 40 45 50 55 Position 2 ( ° )  Intensity (a.u.) (100) o (002) o (101) o (102) o (110) o K o - Mg + - Mg Ca 2 (103) + (200) + (112) + (104) + (103) + (213) + (302) + (115) + (203) + K Figure 5. X-ray pattern of Mg-3Zn-2Ca magnesium alloy processed by different methods. 4. Deformation Characteristics True stress–strain curves obtained at room temperature in tension and compression for the GC alloy are introduced in Figure 6. Characteristic values—the yield stress, σ02 , the ultimate tensile/compression strength σUTS / σUCS , strain to fracture, ε , were evaluated and are presented in Tables 3and 4. All mechanical characteristics were estimated with the standard deviation of ±5%. Materials 2016,9, 880 9 of 15 Materials 2016, 9, 880 9 of 15 GC+HT_C true strain  0.00 0.04 0.08 0.12 0.16 true stress  (MPa) 0 100 200 300 GC_T true strain  0.000 0.002 0.004 0.006 true stress  (MPa) 0 40 80 120 GC+HT_T GC_C Figure 6. True stress–true strain curves obtained for GC alloy in tension (T) and compression (C). Due to very low ductility of the GC alloy in tension, the curve is introduced in the insert. Table 3. Mechanical characteristics of the Mg-3Zn-2Ca alloy processed by different methods obtained in tension: yield stress, σ02, ultimate tensile strength, σUTS, and strain to fracture, e. Alloy State σ02 (MPa) σUTS (MPa) e (%) GC 89.8 101.0 0.4 GC + HT 87.8 126.0 2.0 SCL 79.7 134.8 0.9 SCL + HT 74.2 144.1 3.3 SCS 67.0 110.6 1.2 SCS + HT 67.6 133.5 4.4 ECAP 166.1 206.4 1.1 HT + ECAP 173.5 223.0 2.4 Table 4. Mechanical characteristics of Mg-3Zn-2Ca alloy processed by different methods obtained in compression: yield stress, σ02, ultimate compression strength, σUCS, and strain to fracture, εB. Alloy State σ02 (MPa) σUCS (MPa) εB(%) GC 83.4 234.6 15.5 GC + HT 80.1 334.9 13.7 SCL 84.5 243.2 15.2 SCL + HT 85.3 273.2 15.0 SCS 71.2 219.5 18.4 SCS + HT 73.4 220.2 18.3 ECAP 119.6 296.8 10.8 HT + ECAP 131.6 299.6 9.7 From Figure 6 it is obvious that the ductility of the GC alloy is very low in tension, lower than 1% (see insert in the Figure 6). Applied heat treatment increased the plasticity in tension up to 2%, while in the compression test it remained approximately the same. The yield stress obtained in compression is slightly lower than that found in tension. Similar curves were found for SCL alloy and SCL + HT alloy, Figure 7. SCL alloy showed lower yield stress and slightly higher ductility than measured in the GC alloy. The pressure application during the squeeze casting process of the semisolid alloy (SCS) decreased the tensile yield stress (see Table 3). Corresponding values obtained in compression are slightly higher (see Table 4). True stress–strain curves obtained in tension and compression are introduced in Figure 8 for SCS and SCS + HT alloys. Small serrations were observed on curves received in compression. These serrations are more pronounced in the alloy after heat treatment. Figure 6. True stress–true strain curves obtained for GC alloy in tension (T) and compression (C). Due to very low ductility of the GC alloy in tension, the curve is introduced in the insert. Table 3. Mechanical characteristics of the Mg-3Zn-2Ca alloy processed by different methods obtained in tension: yield stress, σ02, ultimate tensile strength, σUTS, and strain to fracture, e. Alloy State σ02 (MPa) σUTS (MPa) e (%) GC 89.8 101.0 0.4 GC + HT 87.8 126.0 2.0 SCL 79.7 134.8 0.9 SCL + HT 74.2 144.1 3.3 SCS 67.0 110.6 1.2 SCS + HT 67.6 133.5 4.4 ECAP 166.1 206.4 1.1 HT + ECAP 173.5 223.0 2.4 Table 4. Mechanical characteristics of Mg-3Zn-2Ca alloy processed by different methods obtained in compression: yield stress, σ02, ultimate compression strength, σUCS, and strain to fracture, εB. Alloy State σ02 (MPa) σUCS (MPa) εB(%) GC 83.4 234.6 15.5 GC + HT 80.1 334.9 13.7 SCL 84.5 243.2 15.2 SCL + HT 85.3 273.2 15.0 SCS 71.2 219.5 18.4 SCS + HT 73.4 220.2 18.3 ECAP 119.6 296.8 10.8 HT + ECAP 131.6 299.6 9.7 From Figure 6it is obvious that the ductility of the GC alloy is very low in tension, lower than 1% (see insert in the Figure 6). Applied heat treatment increased the plasticity in tension up to 2%, while in the compression test it remained approximately the same. The yield stress obtained in compression is slightly lower than that found in tension. Similar curves were found for SCL alloy and SCL + HT alloy, Figure 7. SCL alloy showed lower yield stress and slightly higher ductility than measured in the GC alloy. The pressure application during the squeeze casting process of the semisolid alloy (SCS) decreased the tensile yield stress (see Table 3). Corresponding values obtained in compression are slightly higher (see Table 4). True stress–strain curves obtained in tension and compression are introduced in Figure 8for SCS and SCS + HT alloys. Small serrations were observed on curves received in compression. These serrations are more pronounced in the alloy after heat treatment.