L. GOMID@ELOVI] et al.: THERMODYNAMIC PROPERTIES AND MICROSTRUCTURES ... 47–53 THERMODYNAMIC PROPERTIES AND MICROSTRUCTURES OF DIFFERENT SHAPE-MEMORY ALLOYS TERMODINAMI^NE LASTNOSTI IN MIKROSTRUKTURA RAZLI^NIH ZLITIN Z OBLIKOVNIM SPOMINOM Lidija Gomid`elovi}1, Emina Po`ega1, Ana Kostov1, Nikola Vukovi}2, Dragana @ivkovi}3, Dragan Manasijevi}3 1Mining and Metallurgy Institute, Zeleni bulevar 35, 19210 Bor, Serbia 2University of Belgrade, Faculty of Mining and Geology, \u{ina 7, 11000 Belgrade, Serbia 3University of Belgrade, Technical Faculty, VJ 12, 19210 Bor, Serbia lgomidzelovic@yahoo.com Prejem rokopisa – received: 2014-08-26; sprejem za objavo – accepted for publication: 2015-02-06 doi:10.17222/mit.2014.212 The results of a thermodynamic-properties calculation conducted using a general solution model (GSM) and an experimental investigation of the microstructures of different shape-memory alloys (SMAs) are presented in this paper. The investigated alloys belong to ternary systems Cu-Al-Zn and Cu-Mn-Ni and to quaternary system Ni-Cu-Fe-Mn. The examinations were conducted using light microscopy (LM) and scanning electron microscopy with energy-dispersive X-ray spectrometry (SEM-EDX). Keywords: thermodynamics, shape-memory alloys, microstructure, LM, SEM-EDX V tem ~lanku so predstavljeni rezultati termodinami~nih izra~unov lastnosti, ki so bili izvr{eni z uporabo splo{nega modela re{itev (GSM) in eksperimentalne preiskave mikrostrukture razli~nih zlitin z oblikovnim spominom (SMAs). Preiskovane zlitine pripadajo ternarnim sistemom Cu-Al-Zn in Cu-Mn-Ni in kvaternarnem sistemu Ni-Cu-Fe-Mn. Preiskave so bile izvedene s pomo~jo svetlobne mikroskopije (LM), z vrsti~no elektronsko mikroskopijo (SEM) in z rentgensko energijsko disperzijsko spektrometrijo (EDX). Klju~ne besede: termodinamika, zlitine z oblikovnim spominom, mikrostruktura, LM, SEM-EDX 1 INTRODUCTION Shape-memory materials are able to recover their original shape after being distorted, at the presence of the right stimulus. These materials include: a) shape-me- mory alloys, b) shape-memory polymers, c) shape-me- mory composites and newly developed d) shape-memory hybrids1. The shape-memory effect was first discovered for a gold-cadmium alloy in the 1930s, but this type of behavior of materials did not attracted the attention of the researchers until 1960s, when a significant recover- able strain was observed for a Ni-Ti alloy, enabling commercial applications. Shape-memory alloys (SMAs) are characterized by unique properties (pseudoelasticity and shape-memory effect), which enable them to "remember" their original shapes. These alloys are used as activators, changing their shapes, positions and other mechanical characte- ristics in a response to a variation in the temperature and electromagnetic field. SMAs can be classified, in accordance with the alloying metals, into: 1. Alloys based on nickel (Ti-Ni, Ni-Mn-Ga) 2. Alloys based on copper (Cu-Zn-Al, Cu-Zn-Si, Cu- Zn-Sn, Cu-Zn-Ga, Cu-Zn-Mn, Cu-Zn-Al-Ni, Cu-Zn- Al-Mn, Cu-Al-Ni, Cu-Al-Be, Cu-Al-Mn) 3. Alloys based on iron (Fe-Mn, Fe-Ni-C, Fe-Mn-Cr, Fe-Mn-Si, Fe-Ni-Nb, Fe-Co-Ni-Ti) 4. Alloys based on noble metals (Au-Cd, Au-Ag, Pt-Al, Pt-Ga, Pt-Ti, Pt-Cr) 5. Exotic alloys (In-Te, In-Cd, V-Nb)2. The interest in SMAs is continuously increasing as new areas of application are discovered. Today, SMAs are used in different areas such as civil engineering3,4, the production of microsystems5, medicine6–8, earthquake technologies9–11 and robotics12,13. The first copper-based SMA to be commercially exploited was the Cu-Al-Zn alloy and the shape-memory alloys from this ternary system typically contain mass fractions of w(Zn) = 15–30 % and w(Al) = 3–7 %. Cu-Mn-Ni shape-memory alloys are magnetic, but some of their properties (like the brittleness) limit their applications, so the alloying elements like gallium, iron or aluminum are added to an alloy in order to achieve satisfying characteristics. The objective of this work is to provide some new information about the thermodynamics and microstruc- tures of selected shape-memory alloys. Materiali in tehnologije / Materials and technology 50 (2016) 1, 47–53 47 UDK 536.7:66.017 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(1)47(2016) 2 EXPERIMENTAL WORK The characterization of the selected shape-memory alloys was done using light microscopy and a SEM-EDX analysis. The samples were obtained from the industrial production. The composition, shape and production method of the investigated samples are given in Table 1. The samples were used as prepared (no annealing). The microstructural analysis of the investigated samples was performed with light microscopy (LM), using a Reichert MeF2 microscope (a magnification of up to 500×) and a SEM-EDX analysis performed on a JEOL JSM-6610LV scanning electron microscope (a magnification of up to 300000×) coupled with an Oxford Instruments, X-Max 20 mm2 SDD, energy-dispersive X-ray spectrometer (an accelerating voltage of 20 kV and a beam current of 1.25 nA). Prior to the metallographic analysis, the surfaces of the polished samples were etched with an appropriate etching solution (Table 2) in order to reveal the structures of the investigated alloys. Table 2: Solutions used for sample etching Tabela 2: Raztopine, uporabljene za jedkanje vzorca Sample Etching solution A1 HCl+H2O2+H2O A2 FeCl3+HCl+H2O A3 FeCl3+HCl+H2O A4 FeCl3+HCl+H2O 3 THEORETICAL FUNDAMENTALS Among many available methods for calculating the thermodynamic properties of a ternary system based on the information about the constitutive binary systems, Chou’s general solution model (GSM)14,15 proved to be the most reasonable in all respects, overcoming the inhe- rent defects of the traditional symmetrical and asymme- trical geometric models. This model breaks down the boundaries between symmetrical and asymmetrical sys- tems and generalizes various situations; the accuracy of the calculation was also proven with practical examples16,17. Recently, a new, improved version of the general solution model based on the Redlich-Kister parameters was presented by Zhang and Chou18. As the older version of GSM required a series of integration processes, which significantly complicated the calculation, and a large number of real systems can be approximately fit using a Redlich-Kister polynomial, a new formalism, based on the binary Redlich-Kister-type parameters, was pre- sented. Therefore, this new GSM version is utilized for cal- culating the thermodynamic properties of the Cu-Al-Zn and Cu-Mn-Ni ternary systems. The basic equation of the general solution model for a ternary system is: ΔG x x L x x x x x L i i n i i n E = − + − + + = = ∑1 2 12 0 1 2 12 3 2 3 23 1 0 2 1( ( ) ) ∑ ∑ − + − + + − + − = ( ( ) ) ( ( x x x x x L x x i ik k n 2 3 23 1 3 1 31 0 3 1 31 2 1 2 1 ) )x i2 (1) Similarity coefficient is defined as: 12 = +I I II/ ( ) (2) 23 = +II II III/ ( ) (3) 31 = +III III I/ ( ) (4) and the deviation sum of squares can be calculated using: I = + + + − + + + + = ∑ 12 2 1 2 3 2 5 1 1 12 0 13 2 ( )( )( ) ( ) ( ) i i i L L j k i l n i ( )( ) ( )( ) j k j k L L L L k j m j n i i + + + + − − >= ∑∑ 3 50 12 13 12 1 13 1 (5) II = + + + − + + + + = ∑ 12 2 1 2 3 2 5 1 1 21 0 23 2 ( )( )( ) ( ) ( i i i L L j k i l n i )( )( ) ( )( ) j k j k L L L L k j m j n i i k k + + + + − − >= ∑∑ 3 50 21 23 21 23 (6) III = + + + − + + + + = ∑ 12 2 1 2 3 2 5 1 31 0 32 2 ( )( )( ) ( ) ( i i i L L j k i l n i 1 3 50 31 32 31 32)( )( ) ( )( ) j k j k L L L L k j m j n i i k k + + + + − − >= ∑∑ (7) The basic equation of the general solution model for a quaternary system19 is: L. GOMID@ELOVI] et al.: THERMODYNAMIC PROPERTIES AND MICROSTRUCTURES ... 48 Materiali in tehnologije / Materials and technology 50 (2016) 1, 47–53 Table 1: Composition, shape and production method of investigated samples Tabela 1: Sestava, oblika in na~in izdelave preiskanih vzorcev Sample Alloy Composition (w/%) Shape Production method Al Cu Zn Mn Ni Fe A1 NiCuFeMn / 32 / 1.5 65 1.5 rod, R 1.27 cm casting A2 CuMnNi / 84 / 12 4 / wire, R 1 mm casting, extraction A3 CuAlZn 4.54 68.14 27.31 / / / wire, R 3.5 mm casting A4 CuAlZn 5.7 68.27 26.03 / / / rod, R 8 cm casting ΔG x x L X x x L Xk k n k k k n E = − + = = ∑ ∑1 2 12 0 1 12 1 3 13 0 1 132 1 2( ) (( ) ( ) ( ) ) ( ) ( − + + − + = = ∑ ∑ 1 2 1 21 4 14 0 1 14 2 3 23 0 2 k k k n k k k n x x L X x x L X ( ) ( ) ) ( ) ( 23 2 4 24 0 2 24 3 4 34 0 1 2 1 − + + − + = = ∑ ∑ k k k n k k k n x x L X x x L 2 13 34X k ( ) )− (8) with X x xi ij i i ij k k k i j k( ) ( ) , = + = ≠ ∑  1 4 (9)    i ij k (ij,ik) (ij,ik) (ji,jk)( ) / ( )= + (10) and  (ij,ik) ij l l n ik l l l l L L= + + + − + + = ∑ 12 2 1 2 3 2 5 1 0 2 ( )( )( ) ( ) (l m l m l m L L L L m l m l n ij l ik l ij m ik+ + + + + + − − >= ∑∑ 1 3 50 )( )( ) ( )( m ) (11) In Equation (11) the second part is different from zero only if the sum of m and n is an even number, and it applies to all the Lij parameters that Lkij = (-1)k Lkij. In all the equations given, Lvij is the Redlich-Kister parameter for the binary system ij, independent of the composition and only dependent on the temperature; GE is the integ- ral molar excess Gibbs energy for the ternary or quater- nary system and xi is the mole fraction of component i. Partial thermodynamic quantities are calculated according to the equations: G G x G x RTi i i i E E E= + − =( )( / ) ln1 ∂ ∂  (12) and: a xi i i=  (13) 4 RESULTS AND DISCUSSION The basic thermodynamic data on the constituent binary subsystems, needed for the calculation of the thermodynamic properties of the investigated systems, were taken from the available literature data20–28 and presented in the form of Redlich-Kister parameters in Table 3. The results for the integral molar excess Gibbs energies of the investigated sections at the corresponding temperatures, obtained with the general solution model, are given analytically in polynomial forms (Table 4). The general solution model and Equations (12) and (13) were used for the calculation of the copper activities in the selected sections of ternary systems Cu-Al-Zn and Cu-Mn-Ni and for the calculation of the nickel activity in the selected cross-section of quaternary system Ni-Cu- Fe-Mn. The results of these calculations are presented in a graphic form (Figures 1 to 3). The thermodynamic L. GOMID@ELOVI] et al.: THERMODYNAMIC PROPERTIES AND MICROSTRUCTURES ... Materiali in tehnologije / Materials and technology 50 (2016) 1, 47–53 49 Table 3: Redlich-Kister parameters for constitutive binary systems Tabela 3: Redlich-Kister parametri za konstitutivne binarne sisteme System ij Loij (T) L1ij (T) L2ij (T) L3ij (T) Al-Cu20 –67094 + 8.555*T 32148 – 7.118*T 5915 – 5.889*T –8175 + 6.049*T Cu-Zn21 –40695.54 + 12.65269*T 4402.72 – 6.55425*T 7818.1 – 3.25416*T 0 Al-Zn22 10465.55-3.39259T 0 0 0 Cu-Mn23 1118.55 – 5.6225T –10915.375 0 0 Cu-Ni24 11760 + 1.084T –1672 0 0 Mn-Ni25 –85853 + 22.715*T –1620 + 4.902*T 0 0 Fe-Ni26 –18782 + 3.7011*T 12308 – 2.7599*T 4457 – 4.1536*T 0 Cu-Fe27 +35625.8 – 2.19045*T –1529.8 + 1.15291*T +12714.4 – 5.18624*T +1177.1 Fe-Mn28 –3950 + 0.489*T +1145 0 0 Table 4: Polynomial form of integral molar excess Gibbs energies calculated using general solution model Tabela 4: Oblika polinoma integralnih molskih odve~nih Gibbsovih energij, izra~unanih z uporabo splo{nega modela re{itev System Cross-section T/K Gxs/J mol–1 R2 Ni-Cu-Fe-Mn Cu:Fe:Mn=20:1:1 1873 –2180.5*xNi3 – 6229*xNi2 + 7760.2*xNi + 643.38 1 Cu-Mn-Ni Mn:Ni=3:1 1773 13831*xCu3 – 24352*xCu2 + 18408*xCu – 7883.2 1 Cu-Al-Zn Al:Zn=1:2 1373 11751*xCu2 – 7231*xCu – 5241.8 0.9895 Figure 1: Dependence of nickel activity on the composition, for cross-section Cu : Fe : Mn = 20 : 1 : 1 from quaternary Ni-Cu-Fe-Mn system, calculated with GSM, at 1873 K Slika 1: Odvisnost aktivnosti niklja od sestave, za presek Cu : Fe : Mn = 20 : 1 : 1 v kvaternarnem sistemu Ni-Cu-Fe-Mn, izra~unana z upo- rabo GSM, pri 1873 K properties calculated with the general solution model are related to the liquid phase of the system, so the tem- perature, at which the calculation was carried out, was selected according to that rule, taking into account the melting points of all the metals in the investigated system. From Figure 1, it can be seen that the nickel activity in section Cu : Fe : Mn = 20 : 1 : 1 and at T = 1873 K shows a variable character of the deviation from Raoult’s law, where up to xNi = 0.4 the deviation is positive, but with a higher content of nickel in the alloy the deviation becomes negative, indicating that a higher amount of nickel in the alloy leads to a better miscibility of the alloy components. The copper activity in cross-section Mn : Ni = 3 : 1 and at T = 1773 K (Figure 2) shows a clear positive deviation from Raoult’s law, which can even result in an occurrence of layering. The copper activity in cross-section Al : Zn = 1 : 2 and at T = 1373 K (Figure 3) exhibits an apparent negative deviation from Raoult’s law, indicating that the L. GOMID@ELOVI] et al.: THERMODYNAMIC PROPERTIES AND MICROSTRUCTURES ... 50 Materiali in tehnologije / Materials and technology 50 (2016) 1, 47–53 Figure 4: Microstructure of sample A1: a) LM (magnification of 500×), b) SEM (magnification of 4000×) and c) positions of EDX analysis Slika 4: Mikrostruktura vzorca A1: a) LM (pove~ava 500×), b) SEM (pove~ava 4000×) in c) polo`aj EDX-analiz Table 5: Results of EDX analysis of sample A1 in amount fractions, (x/%) Tabela 5: Rezultati EDX-analiz vzorca A1 v mno`inskih dele`ih, (x/%) Position A1 Mn Fe Ni Cu Spectrum 1 1.23 1.52 65.49 31.75 Spectrum 2 1.20 1.69 66.63 30.48 Spectrum 3 1.26 1.67 67.95 29.12 Spectrum 4 1.31 1.27 64.97 32.46 Figure 2: Dependence of copper activity on the composition, for cross-section Mn : Ni = 3 : 1 from ternary Cu-Mn-Ni system, calcul ated with GSM, at 1773 K Slika 2: Odvisnost aktivnosti bakra od sestave, za presek Mn : Ni = 3 : 1 v ternarnem sistemu Cu-Mn-Ni, izra~unana z uporabo GSM, pri 1773 K Figure 3: Dependence of copper activity on the composition, for cross-section Al : Zn = 1 : 2 from ternary Cu-Al-Zn system, calculated with GSM, at 1373 K Slika 3: Odvisnost aktivnosti bakra od sestave, za presek Al : Zn = 1 : 2 v ternarnem sistemu Cu-Al-Zn, izra~unana z uporabo GSM, pri 1373 K miscibility of the metals in the ternary Cu-Al-Zn system is quite good. The results of the microstructural analysis with light optical microscopy and SEM-EDX for sample A1 are given in Figure 4, with the chemical composition deter- mined with EDX presented in Table 5. The microphotograph obtained with LM (Figure 4a) shows that the alloy structure consists of sharp-edged polygonal grains. The SEM image on Figure 4b reveals the structure of sample A1 as a gray matrix with imbedded triangular grains, but the EDX analysis shows that the grains and the matrix have almost identical chemical compositions. These findings are in agreement with the fact that copper and nickel, two components that together account for over 90 % of the alloy’s mass, form a continuous series of solid solutions29. L. GOMID@ELOVI] et al.: THERMODYNAMIC PROPERTIES AND MICROSTRUCTURES ... Materiali in tehnologije / Materials and technology 50 (2016) 1, 47–53 51 Figure 5: Microstructure of sample A2: a) SEM (magnification of 2000×) and b) positions of EDX analysis Slika 5: Mikrostruktura vzorca A2: a) SEM (pove~ava 2000×) in b) polo`aj EDX-analiz Table 6: Results of EDX analysis of sample A2 in amount fractions, (x/%) Tabela 6: Rezultati EDX-analiz vzorca A2 v mno`inskih dele`ih, (x/%) Position A2 Mn Ni Cu Spectrum 1 15.04 4.70 80.25 Spectrum 2 14.92 4.50 80.58 Spectrum 3 15.11 4.48 80.41 Spectrum 4 15.35 4.41 80.24 Figure 6: Microstructure of sample A3: a) LM (magnification of 500×), b) SEM-EDX (magnification of 1000×) and c) positions of EDX analysis Slika 6: Mikrostruktura vzorca A3: a) LM (pove~ava 500×), b) SEM-EDX (pove~ava 1000×) in c) polo`aj EDX-analiz Table 7: Results of EDX analysis of sample A3 in amount fractions, (x/%) Tabela 7: Rezultati EDX-analiz vzorca A3 v mno`inskih dele`ih, (x/%) Position A3 Al Cu Zn Spectrum 1 8.83 69.75 21.42 Spectrum 2 7.93 71.11 20.96 Spectrum 3 1.11 78.03 20.87 The results of the microstructural analysis with light microscopy and SEM for sample A2 are given in Figure 5 and the chemical compositions determined with the EDX analysis are presented in Table 6. Technical difficulties like the fact that the maximal magnification of the LM apparatus is just 500× and a very small diameter (1 mm) of sample A2 prevented us from getting a LM photograph. The microstructure of sample A2 (Figure 5b) is characterized by the grains irregular in the shape and size, and the results of the EDX analysis presented in Table 6 are consistent with the fact that copper forms solid solutions with nickel and manganese29. The results of the microstructural analysis with light optical microscopy and SEM for sample A3 are given in Figure 6, with the chemical compositions determined with the EDX analysis presented in Table 7. The microstructure of alloy A3, obtained with a LM microphotograph (Figure 6a), consists of polygonal grains with a significant variation in size. The results of the microstructural analysis with light optical microscopy and SEM-EDX for sample A4 are given in Figure 7 and the chemical compositions deter- mined with the EDX analysis are presented in Table 8. The microstructure of sample A4 consists of polygonal grains, which vary in size. According to the phase diagram of the binary Cu-Zn and Cu-Al systems29, the solid solubility of aluminum in copper is approximately 18 % of amount fractions, and for zinc it goes up to 30 % of amount fractions. Consi- dering that the base material for samples A3 and A4 is copper (w(Cu) = 68 %), it is reasonable to expect that aluminum and zinc will dissolve in copper, creating solid solutions. This was confirmed with the results of the EDX analysis presented in Tables 7 and 8. In addition, the EDX results indicate that the homogeneity of sample A4 is quite good because there is no significant difference in the chemical composition analyzed at various measuring points. 5 CONCLUSION Different shape-memory alloys belonging to ternary systems Cu-Al-Zn and Cu-Mn-Ni and to quaternary sys- tem Ni-Cu-Fe-Mn were investigated. The termodynamic properties of these alloys were investigated analytically, using the general solution model (GSM) and the known Redlich-Kister parameters for the constitutive binary systems. The thermodynamic analysis showed that the alloys with high copper amounts from systems Cu-Al-Zn and Ni-Cu-Fe-Mn display a good miscibility, while the alloys from the Cu-Mn-Ni system tend to display positive deviations from Raoult’s law, which can even lead to layering. The microstructures of the selected alloys were inve- stigated experimentally by means of light optic micro- scopy (LM) and scanning electron microscopy with energy-dispersive X-ray spectrometry (SEM-EDX). The microstructure analysis of the investigated alloy samples L. GOMID@ELOVI] et al.: THERMODYNAMIC PROPERTIES AND MICROSTRUCTURES ... 52 Materiali in tehnologije / Materials and technology 50 (2016) 1, 47–53 Figure 7: Microstructure of sample A4: a) LM (magnification of 80×), b) SEM-EDX (magnification of 1000×) and c) positions of EDX analysis Slika 7: Mikrostruktura vzorca A4: a) LM (pove~ava 80×), b) SEM-EDX (pove~ava 1000×) in c) polo`aj EDX-analiz Table 8: Results of EDX analysis of sample A4 in amount fractions, (x/%) Tabela 8: Rezultati EDX-analiz vzorca A4 v mno`inskih dele`ih, (x/%) Position A4 Al Cu Zn Spectrum 1 12.14 63.45 24.41 Spectrum 2 11.95 64.22 23.83 Spectrum 3 12.03 63.60 24.37 Spectrum 4 12.09 63.66 24.25 Spectrum 5 12.10 63.15 24.76 revealed that the microstructure is built of polygonal grains that can significantly vary in size. The EDX analysis results provided the information about the alloy chemical compositions and were, overall, in agreement with the known facts about the investigated systems. 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