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Effect of aging on phase transformation, thermoelastic and fracture behavior of Mn53Ni25Ga22 ferromagnetic shape memory alloy
Materials Science & Engineering A 594 (2014) 1–6
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Review
Effect of aging on phase transformation, thermoelastic and fracture behavior of Mn53Ni25Ga22 ferromagnetic shape memory alloy Z.Y. Gao a, S. Yang b, H.J. Zhang b, G.F. Dong b,c,n, W. Cai a a National Key Laboratory Precision Hot Processing of Metals, School of Materials Science and Engineering, P.O.Box405, Harbin Institute of Technology, Harbin, 150001, China b Department of Physics Dalian University, Dalian 116622, China c Dalian University of Technology, College of Physics and Optoelectronic Engineering, Dalian, China 116024
art ic l e i nf o
a b s t r a c t
Article history: Received 20 June 2013 Received in revised form 11 September 2013 Accepted 15 October 2013 Available online 26 October 2013
The effects of aging on martensitic transformation, thermoelastic and fracture behavior were investigated in the Mn53Ni 25Ga22 ferromagnetic shape memory alloy. The results show that precipitation has obvious effects on the phase transformation behavior and fracture behavior. The transformation temperature gradually decreases and then increases when the samples are aged at increasing temperatures, reaching the minimum value at an aging temperature of 773 K for 3 h, which is mainly attributable to elemental diffusion between the matrix and the precipitate. The enthalpy and entropy changes rapidly increase and then gradually decrease with an increase in the aging temperature, reaching their minimum values at an aging temperature of 573 K. Moreover, the brittleness, low strength and poor processability of Mn–Ni–Ga alloys greatly limit their application. We attempted to improve the mechanical properties without sacrificing the magnetic and thermoelastic properties by using appropriate aging treatments. The highest compressive strength and compression strain of 1594 MPa and 20.3% were obtained in alloys aged at 673 K and 573 K for 3 h, respectively. These values are approximately 750 MPa and 8% higher than the solution-treated Mn53Ni25Ga22 alloy. As a result, the improvements in the mechanical properties of Mn53Ni25Ga22 alloys from the aging treatments are mainly due to changes in the fracture type from intergranular brittleness to a quasi-cleavage ductile fracture. By controlling the number and the size of the precipitates and aging at a temperature no higher than 773 K for no more than 3 h, a significant improvement in the compressive strength and ductility of the alloys can be expected through the finetuning of the small precipitates. & 2013 Elsevier B.V. All rights reserved.
Keywords: Mn53Ni 25Ga22 Alloy Thermoelastic Aging Fracture behavior
Contents 1. Introduction . . . . . . . . . 2. Experimental . . . . . . . . 3. Results and Discussion 4. Conclusions . . . . . . . . . Acknowledgements . . . . . . . References . . . . . . . . . . . . . .
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1. Introduction
n Corresponding author at: Department of Physics Dalian University, Dalian 116622, China. Tel.: þ 86 411 87402712; fax: þ 86 411 87403963. E-mail address: [email protected] (G.F. Dong).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.10.064
During the past few years, considerable attention has been devoted to the Ni-Mn-Ga alloy system due to its large magneticfield-induced strain and high response frequency, which makes it a potential material for new magnetic actuators [1–5]. The well-known NiMn-based FSMAs already showed that the martensitic transformation temperature and ferroelastic behaviors
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Z.Y. Gao et al. / Materials Science & Engineering A 594 (2014) 1–6
are sensitive to composition and can be observed over a wide temperature range [6–8]. However, the extreme brittleness, low strength and low Curie temperature (Tc) hinder its practical application. Although the addition of Co introduces more desirable magnetic properties, it also dramatically lowers the structural transformation temperature [9–11]. Recently, G.D. Liu et al. [12] researched the Mn2NiGa alloy system as a ferromagnetic shape memory alloy (FSMA). This alloy is more interesting than other FSMAs. Early research on Mn2NiGa revealed that it has a high Tc and a large lattice distortion, as observed from the high strain output. Mn2NiGa was discovered as a result of attempts to tailor the composition of Ni2MnGa and exhibits very different physical properties than NiMnGa. In addition, Mn2NiGa is more interesting than other FSMAs due to some distinguishing physical characteristics [12,13]. Nevertheless, Zhang’s research on Mn–Ni–Ga alloys discovered that an appropriate aging process may significantly enhance the compressive strength and ductility [14]. Additionally, a proper aging process may produce a favorable combination of features, such as high magnetization and a large magnetization difference between the two phases [15]. Our previous work demonstrated that aging can provide an alternative way to enhance magnetization and induce Mn-rich (Mn, Ni)4Ga precipitates within a certain temperature range [15,16]. Therefore, it is important to further investigate aging in the Mn53Ni25Ga22 alloys. The purpose of our study was to investigate the effects of aging on the phase transformation, thermoelastic and fracture behavior of polycrystalline Mn53Ni25Ga22. The results show that aging can significantly enhance the compressive strength and improve the ductility of Mn–Ni–Ga alloy.
2. Experimental The nominal composition of the alloy is Mn53Ni25Ga22. High purity elements (99.97 mol% electrolytic Ni, 99.5 mol% electrolytic Mn, 99.99 mol% Ga and 96.7 mol% Ti) were melted under an argon atmosphere using an electric arc and remelted four times for homogeneity. The melted alloys were cast in a chilled copper mold to obtain a master rod with dimensions of 10 mm in diameter and 70 mm in length. Homogeneity was reached at 1173 K for 12 h followed by ice-water quenching. Subsequently, the solution-treated
samples were aged at temperatures of 573 K, 673 K, 773 K, 873 K, and 973 K for 3 h and then quenched in iced water. The martensitic transformation temperatures were determined by differential scanning calorimetry (DSC, Perkin Elmer Diamond) within the equipment temperature limits of 150–473 K. The crystal structure at room temperature (RT) was determined by X-ray diffraction (XRD) measurements using a Rigaku D/max-rB with Cu Ka radiation. The microstructure was observed by scanning electron microscopy (SEM) using a Hitachi S-4700 equipped with an X-ray energy dispersive spectroscopy (EDS) analysis system. Finally, the compression tests at ambient temperature were conducted using an Instron1186 Model machine that maintained a strain rate of 0.1 mm min 1 by applying compressive stress in the casting direction.
3. Results and Discussion Fig. 1 shows the secondary electron images of Mn53Ni25Ga22 samples with different aging treatments. As shown in Fig. 1(a) and (b), the typical unitary martensitic phase morphology was observed at room temperature in the Mn53Ni25Ga22 alloy aged at 773 K for 3 h and the solution-treated sample. Straight plate twinned martensitic variants are clearly observed at room temperature, whereas other samples contain some (Mn,Ni)4Ga precipitate particles, which is in agreement with the results of our previous study [14]. With increasing aging temperature, the microstructure changed from martensite to a mixture of austenite and (Mn,Ni)4Ga precipitates, then a mixture of martensite and (Mn,Ni)4Ga precipitates, as shown in Fig. 1(c)–(f). The number and size of the second phase first increased, and then, the number of (Mn,Ni)4Ga precipitate particles decreased with increasing aging temperature. At a particular point, etched precipitates nucleated in the matrix, appearing both on the grain boundaries and in the grain interiors. The EDS results (Table 1) for the matrix phases of the solution-treated alloy and the aged Mn53Ni25Ga22 alloys show that, as the aging temperature increased from 573 to 773 K, the precipitation of more Mn-rich (Mn,Ni)4Ga particles resulted in significant Mn depletion in the matrix, while at the same time the concentration of Ni and Ga slightly increased.
5μm
5 μm
5 μm
5 μm
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Fig. 1. SEM microstructures of Mn53Ni25Ga22 alloys aged at different temperatures for 3 h (a) Solution-treated (b) 573 K, (C) 673 K, (d) 773 K, (e) 873 K, and (f) 973 K.
Z.Y. Gao et al. / Materials Science & Engineering A 594 (2014) 1–6
Table 1 EDS results of Mn53Ni25Ga22 alloys aged at different temperatures for 3 h and solution-treated (at.%). Aging process
573 K 673 K 773 K 873 K 973 K solution-treated
Martix
The second phase
Mn
Ni
Ga
Mn
Ni
Ga
52.5 50.9 45.8 47.7 48.5 53.5
24.4 25.9 29.7 28.1 27.2 25.5
23.1 23.2 24.5 24.2 24.3 21.0
– – – 67.0 65.0
– – – 14.6 16.5
– – – 18.4 18.5
Fig. 2. X-ray diffractograms of Mn53Ni25Ga22 alloy aged at different temperatures for 3 h.
To observe the effect of aging on the phases of the Mn53Ni25Ga22 alloy, X-ray diffraction was performed and is shown in Fig. 2. The martensitic and austenitic peaks are represented as Mhkl and Ahkl, respectively, where h, k and l are the Miller indices. The typical martensitic peaks can be detected in the solution-treated Mn53Ni25Ga22 alloys and the samples aged at 573 K and 973 K for 3 h. The alloy aged at 673 K for 3 h showed diffraction peaks with a dominant martensitic phase and a small fraction of austenite phase. The A220 peak (representing the austenitic phase) has a lower intensity than the M202 peak (representing the martensitic phase) in the alloy aged at 673 K for 3 h. However, with an increase in aging temperature, the peaks showed higher fractions of the austenitic phase (typically A220) and a correspondingly decreased martensitic peak (typically M202). As the aging temperature increased, the M202 peak intensity decreased, whereas that of A220 peak increased, showing the enhancement in the austenitic phase. Aging at 773 K for 3 h yielded very low intensity reflections of martensite in the austenitic phase. Moreover, when the aging temperature was greater than 773 K, the intensity of the M202 diffraction peak gradually increased. In both the martensitic and austenitic phases, some new reflections peaks could be detected as the aging temperature increased, indicating that a new type of secondary phase precipitated in the samples. This result agrees with the previous research on aged Mn50Ni25Ga25 alloys [16] and indicates that the precipitates in the aged Mn53Ni25Ga22 are the same (Mn, Ni)4Ga phases, which agrees with the SEM results. Fig. 3a summarizes the effect of the aging temperature on the transformation temperature and the thermal hysteresis in the aged Mn53Ni25Ga22 alloy. The martensitic transformation start temperatures (Ms), martensitic transformation finish temperature (Mf), reversible martensitic transformation start temperature (As) and reversible martensitic transformation finish temperature (Af) of Mn53Ni25Ga22 are summarized in Fig. 3a. The martensitic transformation temperatures gradually decreased and then increased with an increase in the
3
aging temperature, reaching their minimum values at an aging temperature of 773 K. Moreover, the thermal hysteresis gradually increased and then rapidly decreased with an increase in the aging temperature, reaching their maximum values at an aging temperature of 773 K. The typical DSC curve of alloy aged at 873 K for 3 h shows an exothermic peak and an endothermic peak during the cooling and heating, corresponding to the martensitic and reverse transformations, respectively, as shown in Fig. 3b. The change in transformation temperature can be attributed to the introduction of (Mn,Ni)4Ga precipitates during the aging process. In addition, the change in internal stress and atomic rearrangements also affect the transformation temperature [17]. It is well known that the martensitic transformation temperatures of Ni–Mn–Ga alloys are sensitive to the composition. The decrease in Ni content causes a decrease in the martensitic transformation temperatures [18]. The decrease in the transformation temperatures can be attributed to the precipitation of (Mn,Ni)4Ga precipitates, which initially caused a decrease and then an increase in the Mn content in the matrix. Therefore, the decrease in the transformation temperature is primarily attributed to elemental diffusion between the matrix and the (Mn,Ni)4Ga particles. However, the re-dissolution of (Mn,Ni)4Ga precipitates leads to a decrease in the Ni content in the matrix when the aging temperature is greater than 773 K. The Ni, Mn, and Ga concentrations in the matrix were determined using X-ray energy dispersive spectroscopy measurements in the scanning electron microscopy (SEM). Fig. 3c summarizes the effect of the aging temperature on the enthalpy changes △H and entropy changes △S in the Mn53Ni25Ga22 alloy. The enthalpy and entropy changes (Fig. 3c) have the same trend as the thermal hysteresis. The phase transformation temperatures, thermal hysteresis and △H are obtained by marking the DSC curves using the tangent method in the software of DSC [19] equipment. △S is calculated by △H/Tm, where Tm ¼ (Af þ Ms)/2. With increasing aging temperature, the order of the austenite decreases, leading to an increase in both the internal energy and the latent heat released during phase transformation. Therefore, the enthalpy and entropy increase when the aging temperature is less than 573 K. Additionally, when the volume fraction of the secondary phase increases dramatically, a decrease in the matrix volume that participates in the phase transformation results such that the enthalpy and entropy of the alloy decrease. The two factors mentioned above have a competitive effect on both the enthalpy and entropy changes, As a result, the enthalpy and entropy of the alloy increase and then decrease as the aging temperature increases, reaching their maximum values at an aging temperature of 573 K. To investigate the effect of aging on the mechanical properties, compression tests were carried out at room temperature. All the samples were compressed to fracture. Fig. 4 shows the compression stress–strain curves of the solution-treated Mn53Ni25Ga22 alloy and the Mn53Ni25Ga22 ferromagnetic shape memory alloy aged at 673 K for 3 h. The Mn53Ni25Ga22 alloy aged at 673 K for 3 h exhibits better plasticity during compression than the solution-treated sample because the stress increases when the deformation reaches 15.9%. Fig. 5 summarizes the effect of the aging temperature on the fracture strength and fracture strain under compression in the aged Mn53Ni25Ga22 alloy. As the aging temperature increases, the compressive strength and fracture strain of the alloys initially increase and then decrease, reaching their maximum values at aging temperatures of 673 K and 573 K, respectively. The highest compressive strength of 1594 MPa is obtained in the alloy aged at 673 K for 3 h, which is approximately 750 MPa higher than the solution-treated Mn53Ni25Ga22 alloy. As the aging temperature further increases from 673 to 973 K, the maximum compressive strength gradually decreases. Moreover, the compressive strain initially decreases and then increases, reaching its minimum values at an aging temperature of 773 K. Additionally, the existence of ductile (Mn,Ni)4Ga
Z.Y. Gao et al. / Materials Science & Engineering A 594 (2014) 1–6
90
6
80
4
350
300 70 250 Ms Mf As Af heat hysteresis
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673K aging cooling
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heating
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enthalpy changes H entropy changes S
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Aging temperature (K) Fig. 3. (a) The effect of the aging temperature on the transformation temperatures and thermal hysteresis of Mn53Ni25Ga22 alloys. (b) DSC curve of the Mn53Ni25Ga22 alloy aged at 673 K for 3 h. (c) The effect of the aging temperature on the enthalpy and entropy changes in Mn53Ni25Ga22 alloys. 673K/3h
Compressive stress (MPa)
1600 1400 1200 1000 800
solution-treated
600 400 200 0 0
2
4
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8
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Strain (%) Fig. 4. Room temperature compressive stress-strain curves of the Mn53Ni25Ga22 alloy solution-treated and aged at 673 K for 3 h.
precipitates can effectively hinder crack propagation, which is one of the causes of the enhancement of the strength and ductility. Furthermore, an appropriate aging treatment can improve the mechanical properties of alloys. A dispersed distribution of the secondary phase may partly explain why the alloy is strengthened and the ductility is improved. Therefore, samples aged at a low temperature (673 K) can be strengthened because of the small (Mn, Ni)4Ga particles that initially precipitate. A strong interaction between dislocations and the particles intersecting the slip planes may hinder fracture propagation until the precipitates are torn from the matrix. Additionally, the fact that the austenite is more elastic than martensite may also be responsible for to the increase in
compressive strength. However, as the aging temperature increases, the (Mn,Ni)4Ga precipitates become more coarse, and the compression strain quickly collapses as the precipitates begin to lose coherence with the matrix. This loss of coherence reduces the attraction between the matrix and the precipitates, and the sample can thus be easily torn. Therefore, the most important approach for improving the mechanical properties of Mn53Ni25Ga22 alloys is to optimize the number of small (Mn,Ni)4Ga precipitates by controlling the aging temperature and to achieve a moderate size of precipitates by using a suitable aging time. To clarify the fracture mechanism and investigate the nucleation and propagation of cracks in aged Mn53Ni25Ga22 alloy, SEM observations of fracture surfaces were performed, as shown in Fig. 6. The fracture surfaces of solution-treated Mn53Ni25Ga22 alloys show typical brittle characteristics with an intergranular pattern, and this intergranular pattern becomes weaker after aging, as shown in Fig. 6b–d. In addition, some small holes are observed on the fracture because the secondary phase particles were pulled-out, as shown in Fig.6c. The fracture surfaces of the aged Mn53Ni25Ga22 alloy show characteristics of a ductile fracture, as evidenced by the formation of some tearing edges (indicted by the arrow in Fig. 6b and c). The amount of tearing edges increases, and the shape changes in the presence of the secondary particles in the aged samples, which provides more energy. Therefore, it can be observed that the fracture surfaces of the aged Mn53Ni25Ga22 alloys exhibit typical quasi-cleavage fracture characteristics. The mechanism of the ductility improvement in the aged Mn53Ni25Ga22 alloy is similar to the toughening mechanism of the composite materials. The brittle matrix is connected by the ductile secondary particles, and the improvement of the ductility is partly provided by the plastic deformation of the ductile secondary phase
5
1700 21
1600
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Z.Y. Gao et al. / Materials Science & Engineering A 594 (2014) 1–6
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Aging temperature (K)
20 19 18 17 16 15 14 13
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Fig. 5. The effect of the aging temperature on the mechanical properties of the Mn53Ni25Ga22 alloys (a) Compressive fracture strength and (b) Compressive facture strain.
30 μm
30 μm
30 μm
30 μm
Fig. 6. SEM fractographs of Mn53Ni25Ga22 alloys aged at different temperature (a) solution-treated, (b) 673 K, (c) 773 K, and (d) 973 K.
introduced by the aging treatment. When the alloy is deformed, microcracks are generated, and they propagate along the intergranular boundaries. When the crack meets the secondary particles, the particles can be plastically deformed. Moreover, the crack will stop
propagating or directly traverse the secondary particles while expanding to the boundaries between the matrix and the secondary phase. These effects will increase the energy for crack propagation and increase the ductility. The above results clearly demonstrate that
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the aging treatment can greatly improve the mechanical properties, which can be attributed to the secondary phase strengthening and dispersion strengthening during the aging.
China (Grant No. 21173028, 51271066, 2011CB0129004), Doctor Startup Foundation of Dalian University.
4. Conclusions
References
Aging temperatures significantly modify the morphology, martensitic transformation temperatures, and fracture behavior in Mn53Ni25Ga22 alloys. The results show that the microstructure changes from martensite to a mixture of austenite and (Mn,Ni)4Ga precipitates and then to a mixture of martensite and(Mn,Ni)4Ga precipitates as the aging temperature increase. It is found that the number and size of the secondary phase precipitates initially increase with increasing aging temperature, and then they decrease. The martensitic transformation temperatures gradually decrease and then increase with increasing aging temperature, reaching their minimum values at an aging temperature of 773 K. The enthalpy and entropy changes first rapidly increase and then gradually decrease with increasing aging temperature, reaching their minimum values at the aging temperature of 573 K. The improved mechanical properties of Mn53Ni25Ga22 alloys from the aging treatment are mainly due to the fracture type changing from intergranular brittleness fracture to a quasi-cleavage ductile fracture. By controlling the amount and size of the secondary phase precipitates at an aging temperature no higher than 773 K for no more than 3 h, a significant improvement in the compressive strength and ductility of the alloys can be expected as a result of the fine-tuning of the precipitates. Acknowledgements This study is supported by Postdoctoral Science Foundation of China (Grant No. 20100481218), Natural Science Foundation of
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Review
Effect of aging on phase transformation, thermoelastic and fracture behavior of Mn53Ni25Ga22 ferromagnetic shape memory alloy Z.Y. Gao a, S. Yang b, H.J. Zhang b, G.F. Dong b,c,n, W. Cai a a National Key Laboratory Precision Hot Processing of Metals, School of Materials Science and Engineering, P.O.Box405, Harbin Institute of Technology, Harbin, 150001, China b Department of Physics Dalian University, Dalian 116622, China c Dalian University of Technology, College of Physics and Optoelectronic Engineering, Dalian, China 116024
art ic l e i nf o
a b s t r a c t
Article history: Received 20 June 2013 Received in revised form 11 September 2013 Accepted 15 October 2013 Available online 26 October 2013
The effects of aging on martensitic transformation, thermoelastic and fracture behavior were investigated in the Mn53Ni 25Ga22 ferromagnetic shape memory alloy. The results show that precipitation has obvious effects on the phase transformation behavior and fracture behavior. The transformation temperature gradually decreases and then increases when the samples are aged at increasing temperatures, reaching the minimum value at an aging temperature of 773 K for 3 h, which is mainly attributable to elemental diffusion between the matrix and the precipitate. The enthalpy and entropy changes rapidly increase and then gradually decrease with an increase in the aging temperature, reaching their minimum values at an aging temperature of 573 K. Moreover, the brittleness, low strength and poor processability of Mn–Ni–Ga alloys greatly limit their application. We attempted to improve the mechanical properties without sacrificing the magnetic and thermoelastic properties by using appropriate aging treatments. The highest compressive strength and compression strain of 1594 MPa and 20.3% were obtained in alloys aged at 673 K and 573 K for 3 h, respectively. These values are approximately 750 MPa and 8% higher than the solution-treated Mn53Ni25Ga22 alloy. As a result, the improvements in the mechanical properties of Mn53Ni25Ga22 alloys from the aging treatments are mainly due to changes in the fracture type from intergranular brittleness to a quasi-cleavage ductile fracture. By controlling the number and the size of the precipitates and aging at a temperature no higher than 773 K for no more than 3 h, a significant improvement in the compressive strength and ductility of the alloys can be expected through the finetuning of the small precipitates. & 2013 Elsevier B.V. All rights reserved.
Keywords: Mn53Ni 25Ga22 Alloy Thermoelastic Aging Fracture behavior
Contents 1. Introduction . . . . . . . . . 2. Experimental . . . . . . . . 3. Results and Discussion 4. Conclusions . . . . . . . . . Acknowledgements . . . . . . . References . . . . . . . . . . . . . .
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1 2 2 6 6 6
1. Introduction
n Corresponding author at: Department of Physics Dalian University, Dalian 116622, China. Tel.: þ 86 411 87402712; fax: þ 86 411 87403963. E-mail address: [email protected] (G.F. Dong).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.10.064
During the past few years, considerable attention has been devoted to the Ni-Mn-Ga alloy system due to its large magneticfield-induced strain and high response frequency, which makes it a potential material for new magnetic actuators [1–5]. The well-known NiMn-based FSMAs already showed that the martensitic transformation temperature and ferroelastic behaviors
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are sensitive to composition and can be observed over a wide temperature range [6–8]. However, the extreme brittleness, low strength and low Curie temperature (Tc) hinder its practical application. Although the addition of Co introduces more desirable magnetic properties, it also dramatically lowers the structural transformation temperature [9–11]. Recently, G.D. Liu et al. [12] researched the Mn2NiGa alloy system as a ferromagnetic shape memory alloy (FSMA). This alloy is more interesting than other FSMAs. Early research on Mn2NiGa revealed that it has a high Tc and a large lattice distortion, as observed from the high strain output. Mn2NiGa was discovered as a result of attempts to tailor the composition of Ni2MnGa and exhibits very different physical properties than NiMnGa. In addition, Mn2NiGa is more interesting than other FSMAs due to some distinguishing physical characteristics [12,13]. Nevertheless, Zhang’s research on Mn–Ni–Ga alloys discovered that an appropriate aging process may significantly enhance the compressive strength and ductility [14]. Additionally, a proper aging process may produce a favorable combination of features, such as high magnetization and a large magnetization difference between the two phases [15]. Our previous work demonstrated that aging can provide an alternative way to enhance magnetization and induce Mn-rich (Mn, Ni)4Ga precipitates within a certain temperature range [15,16]. Therefore, it is important to further investigate aging in the Mn53Ni25Ga22 alloys. The purpose of our study was to investigate the effects of aging on the phase transformation, thermoelastic and fracture behavior of polycrystalline Mn53Ni25Ga22. The results show that aging can significantly enhance the compressive strength and improve the ductility of Mn–Ni–Ga alloy.
2. Experimental The nominal composition of the alloy is Mn53Ni25Ga22. High purity elements (99.97 mol% electrolytic Ni, 99.5 mol% electrolytic Mn, 99.99 mol% Ga and 96.7 mol% Ti) were melted under an argon atmosphere using an electric arc and remelted four times for homogeneity. The melted alloys were cast in a chilled copper mold to obtain a master rod with dimensions of 10 mm in diameter and 70 mm in length. Homogeneity was reached at 1173 K for 12 h followed by ice-water quenching. Subsequently, the solution-treated
samples were aged at temperatures of 573 K, 673 K, 773 K, 873 K, and 973 K for 3 h and then quenched in iced water. The martensitic transformation temperatures were determined by differential scanning calorimetry (DSC, Perkin Elmer Diamond) within the equipment temperature limits of 150–473 K. The crystal structure at room temperature (RT) was determined by X-ray diffraction (XRD) measurements using a Rigaku D/max-rB with Cu Ka radiation. The microstructure was observed by scanning electron microscopy (SEM) using a Hitachi S-4700 equipped with an X-ray energy dispersive spectroscopy (EDS) analysis system. Finally, the compression tests at ambient temperature were conducted using an Instron1186 Model machine that maintained a strain rate of 0.1 mm min 1 by applying compressive stress in the casting direction.
3. Results and Discussion Fig. 1 shows the secondary electron images of Mn53Ni25Ga22 samples with different aging treatments. As shown in Fig. 1(a) and (b), the typical unitary martensitic phase morphology was observed at room temperature in the Mn53Ni25Ga22 alloy aged at 773 K for 3 h and the solution-treated sample. Straight plate twinned martensitic variants are clearly observed at room temperature, whereas other samples contain some (Mn,Ni)4Ga precipitate particles, which is in agreement with the results of our previous study [14]. With increasing aging temperature, the microstructure changed from martensite to a mixture of austenite and (Mn,Ni)4Ga precipitates, then a mixture of martensite and (Mn,Ni)4Ga precipitates, as shown in Fig. 1(c)–(f). The number and size of the second phase first increased, and then, the number of (Mn,Ni)4Ga precipitate particles decreased with increasing aging temperature. At a particular point, etched precipitates nucleated in the matrix, appearing both on the grain boundaries and in the grain interiors. The EDS results (Table 1) for the matrix phases of the solution-treated alloy and the aged Mn53Ni25Ga22 alloys show that, as the aging temperature increased from 573 to 773 K, the precipitation of more Mn-rich (Mn,Ni)4Ga particles resulted in significant Mn depletion in the matrix, while at the same time the concentration of Ni and Ga slightly increased.
5μm
5 μm
5 μm
5 μm
5 μm
5 μm
Fig. 1. SEM microstructures of Mn53Ni25Ga22 alloys aged at different temperatures for 3 h (a) Solution-treated (b) 573 K, (C) 673 K, (d) 773 K, (e) 873 K, and (f) 973 K.
Z.Y. Gao et al. / Materials Science & Engineering A 594 (2014) 1–6
Table 1 EDS results of Mn53Ni25Ga22 alloys aged at different temperatures for 3 h and solution-treated (at.%). Aging process
573 K 673 K 773 K 873 K 973 K solution-treated
Martix
The second phase
Mn
Ni
Ga
Mn
Ni
Ga
52.5 50.9 45.8 47.7 48.5 53.5
24.4 25.9 29.7 28.1 27.2 25.5
23.1 23.2 24.5 24.2 24.3 21.0
– – – 67.0 65.0
– – – 14.6 16.5
– – – 18.4 18.5
Fig. 2. X-ray diffractograms of Mn53Ni25Ga22 alloy aged at different temperatures for 3 h.
To observe the effect of aging on the phases of the Mn53Ni25Ga22 alloy, X-ray diffraction was performed and is shown in Fig. 2. The martensitic and austenitic peaks are represented as Mhkl and Ahkl, respectively, where h, k and l are the Miller indices. The typical martensitic peaks can be detected in the solution-treated Mn53Ni25Ga22 alloys and the samples aged at 573 K and 973 K for 3 h. The alloy aged at 673 K for 3 h showed diffraction peaks with a dominant martensitic phase and a small fraction of austenite phase. The A220 peak (representing the austenitic phase) has a lower intensity than the M202 peak (representing the martensitic phase) in the alloy aged at 673 K for 3 h. However, with an increase in aging temperature, the peaks showed higher fractions of the austenitic phase (typically A220) and a correspondingly decreased martensitic peak (typically M202). As the aging temperature increased, the M202 peak intensity decreased, whereas that of A220 peak increased, showing the enhancement in the austenitic phase. Aging at 773 K for 3 h yielded very low intensity reflections of martensite in the austenitic phase. Moreover, when the aging temperature was greater than 773 K, the intensity of the M202 diffraction peak gradually increased. In both the martensitic and austenitic phases, some new reflections peaks could be detected as the aging temperature increased, indicating that a new type of secondary phase precipitated in the samples. This result agrees with the previous research on aged Mn50Ni25Ga25 alloys [16] and indicates that the precipitates in the aged Mn53Ni25Ga22 are the same (Mn, Ni)4Ga phases, which agrees with the SEM results. Fig. 3a summarizes the effect of the aging temperature on the transformation temperature and the thermal hysteresis in the aged Mn53Ni25Ga22 alloy. The martensitic transformation start temperatures (Ms), martensitic transformation finish temperature (Mf), reversible martensitic transformation start temperature (As) and reversible martensitic transformation finish temperature (Af) of Mn53Ni25Ga22 are summarized in Fig. 3a. The martensitic transformation temperatures gradually decreased and then increased with an increase in the
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aging temperature, reaching their minimum values at an aging temperature of 773 K. Moreover, the thermal hysteresis gradually increased and then rapidly decreased with an increase in the aging temperature, reaching their maximum values at an aging temperature of 773 K. The typical DSC curve of alloy aged at 873 K for 3 h shows an exothermic peak and an endothermic peak during the cooling and heating, corresponding to the martensitic and reverse transformations, respectively, as shown in Fig. 3b. The change in transformation temperature can be attributed to the introduction of (Mn,Ni)4Ga precipitates during the aging process. In addition, the change in internal stress and atomic rearrangements also affect the transformation temperature [17]. It is well known that the martensitic transformation temperatures of Ni–Mn–Ga alloys are sensitive to the composition. The decrease in Ni content causes a decrease in the martensitic transformation temperatures [18]. The decrease in the transformation temperatures can be attributed to the precipitation of (Mn,Ni)4Ga precipitates, which initially caused a decrease and then an increase in the Mn content in the matrix. Therefore, the decrease in the transformation temperature is primarily attributed to elemental diffusion between the matrix and the (Mn,Ni)4Ga particles. However, the re-dissolution of (Mn,Ni)4Ga precipitates leads to a decrease in the Ni content in the matrix when the aging temperature is greater than 773 K. The Ni, Mn, and Ga concentrations in the matrix were determined using X-ray energy dispersive spectroscopy measurements in the scanning electron microscopy (SEM). Fig. 3c summarizes the effect of the aging temperature on the enthalpy changes △H and entropy changes △S in the Mn53Ni25Ga22 alloy. The enthalpy and entropy changes (Fig. 3c) have the same trend as the thermal hysteresis. The phase transformation temperatures, thermal hysteresis and △H are obtained by marking the DSC curves using the tangent method in the software of DSC [19] equipment. △S is calculated by △H/Tm, where Tm ¼ (Af þ Ms)/2. With increasing aging temperature, the order of the austenite decreases, leading to an increase in both the internal energy and the latent heat released during phase transformation. Therefore, the enthalpy and entropy increase when the aging temperature is less than 573 K. Additionally, when the volume fraction of the secondary phase increases dramatically, a decrease in the matrix volume that participates in the phase transformation results such that the enthalpy and entropy of the alloy decrease. The two factors mentioned above have a competitive effect on both the enthalpy and entropy changes, As a result, the enthalpy and entropy of the alloy increase and then decrease as the aging temperature increases, reaching their maximum values at an aging temperature of 573 K. To investigate the effect of aging on the mechanical properties, compression tests were carried out at room temperature. All the samples were compressed to fracture. Fig. 4 shows the compression stress–strain curves of the solution-treated Mn53Ni25Ga22 alloy and the Mn53Ni25Ga22 ferromagnetic shape memory alloy aged at 673 K for 3 h. The Mn53Ni25Ga22 alloy aged at 673 K for 3 h exhibits better plasticity during compression than the solution-treated sample because the stress increases when the deformation reaches 15.9%. Fig. 5 summarizes the effect of the aging temperature on the fracture strength and fracture strain under compression in the aged Mn53Ni25Ga22 alloy. As the aging temperature increases, the compressive strength and fracture strain of the alloys initially increase and then decrease, reaching their maximum values at aging temperatures of 673 K and 573 K, respectively. The highest compressive strength of 1594 MPa is obtained in the alloy aged at 673 K for 3 h, which is approximately 750 MPa higher than the solution-treated Mn53Ni25Ga22 alloy. As the aging temperature further increases from 673 to 973 K, the maximum compressive strength gradually decreases. Moreover, the compressive strain initially decreases and then increases, reaching its minimum values at an aging temperature of 773 K. Additionally, the existence of ductile (Mn,Ni)4Ga
Z.Y. Gao et al. / Materials Science & Engineering A 594 (2014) 1–6
90
6
80
4
350
300 70 250 Ms Mf As Af heat hysteresis
500
600
60
700
800
900
2
673K aging cooling
0
-2
1000
heating
200
250
H (J/g)
Aging temperature (K)
300
350
400
Temperature(K)
14
40
12
35
10
30
8
25 20
6
S (mJ/g.K)
200
Heat Flow(mW/g)
Transformation temperature (K)
4
15
4
enthalpy changes H entropy changes S
10
2 0
200
400
600
800
1000
5
Aging temperature (K) Fig. 3. (a) The effect of the aging temperature on the transformation temperatures and thermal hysteresis of Mn53Ni25Ga22 alloys. (b) DSC curve of the Mn53Ni25Ga22 alloy aged at 673 K for 3 h. (c) The effect of the aging temperature on the enthalpy and entropy changes in Mn53Ni25Ga22 alloys. 673K/3h
Compressive stress (MPa)
1600 1400 1200 1000 800
solution-treated
600 400 200 0 0
2
4
6
8
10
12
14
16
18
20
22
Strain (%) Fig. 4. Room temperature compressive stress-strain curves of the Mn53Ni25Ga22 alloy solution-treated and aged at 673 K for 3 h.
precipitates can effectively hinder crack propagation, which is one of the causes of the enhancement of the strength and ductility. Furthermore, an appropriate aging treatment can improve the mechanical properties of alloys. A dispersed distribution of the secondary phase may partly explain why the alloy is strengthened and the ductility is improved. Therefore, samples aged at a low temperature (673 K) can be strengthened because of the small (Mn, Ni)4Ga particles that initially precipitate. A strong interaction between dislocations and the particles intersecting the slip planes may hinder fracture propagation until the precipitates are torn from the matrix. Additionally, the fact that the austenite is more elastic than martensite may also be responsible for to the increase in
compressive strength. However, as the aging temperature increases, the (Mn,Ni)4Ga precipitates become more coarse, and the compression strain quickly collapses as the precipitates begin to lose coherence with the matrix. This loss of coherence reduces the attraction between the matrix and the precipitates, and the sample can thus be easily torn. Therefore, the most important approach for improving the mechanical properties of Mn53Ni25Ga22 alloys is to optimize the number of small (Mn,Ni)4Ga precipitates by controlling the aging temperature and to achieve a moderate size of precipitates by using a suitable aging time. To clarify the fracture mechanism and investigate the nucleation and propagation of cracks in aged Mn53Ni25Ga22 alloy, SEM observations of fracture surfaces were performed, as shown in Fig. 6. The fracture surfaces of solution-treated Mn53Ni25Ga22 alloys show typical brittle characteristics with an intergranular pattern, and this intergranular pattern becomes weaker after aging, as shown in Fig. 6b–d. In addition, some small holes are observed on the fracture because the secondary phase particles were pulled-out, as shown in Fig.6c. The fracture surfaces of the aged Mn53Ni25Ga22 alloy show characteristics of a ductile fracture, as evidenced by the formation of some tearing edges (indicted by the arrow in Fig. 6b and c). The amount of tearing edges increases, and the shape changes in the presence of the secondary particles in the aged samples, which provides more energy. Therefore, it can be observed that the fracture surfaces of the aged Mn53Ni25Ga22 alloys exhibit typical quasi-cleavage fracture characteristics. The mechanism of the ductility improvement in the aged Mn53Ni25Ga22 alloy is similar to the toughening mechanism of the composite materials. The brittle matrix is connected by the ductile secondary particles, and the improvement of the ductility is partly provided by the plastic deformation of the ductile secondary phase
5
1700 21
1600
Compressive fracture strain (%)
Compressive fracture strength (MPa)
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1500 1400 1300 1200 1100 1000 900
prior to aging
800 300
400
500
600
700
800
900 1000
Aging temperature (K)
20 19 18 17 16 15 14 13
prior to aging 300
400
500
600
700
800
900 1000
Aging temperature (K)
Fig. 5. The effect of the aging temperature on the mechanical properties of the Mn53Ni25Ga22 alloys (a) Compressive fracture strength and (b) Compressive facture strain.
30 μm
30 μm
30 μm
30 μm
Fig. 6. SEM fractographs of Mn53Ni25Ga22 alloys aged at different temperature (a) solution-treated, (b) 673 K, (c) 773 K, and (d) 973 K.
introduced by the aging treatment. When the alloy is deformed, microcracks are generated, and they propagate along the intergranular boundaries. When the crack meets the secondary particles, the particles can be plastically deformed. Moreover, the crack will stop
propagating or directly traverse the secondary particles while expanding to the boundaries between the matrix and the secondary phase. These effects will increase the energy for crack propagation and increase the ductility. The above results clearly demonstrate that
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the aging treatment can greatly improve the mechanical properties, which can be attributed to the secondary phase strengthening and dispersion strengthening during the aging.
China (Grant No. 21173028, 51271066, 2011CB0129004), Doctor Startup Foundation of Dalian University.
4. Conclusions
References
Aging temperatures significantly modify the morphology, martensitic transformation temperatures, and fracture behavior in Mn53Ni25Ga22 alloys. The results show that the microstructure changes from martensite to a mixture of austenite and (Mn,Ni)4Ga precipitates and then to a mixture of martensite and(Mn,Ni)4Ga precipitates as the aging temperature increase. It is found that the number and size of the secondary phase precipitates initially increase with increasing aging temperature, and then they decrease. The martensitic transformation temperatures gradually decrease and then increase with increasing aging temperature, reaching their minimum values at an aging temperature of 773 K. The enthalpy and entropy changes first rapidly increase and then gradually decrease with increasing aging temperature, reaching their minimum values at the aging temperature of 573 K. The improved mechanical properties of Mn53Ni25Ga22 alloys from the aging treatment are mainly due to the fracture type changing from intergranular brittleness fracture to a quasi-cleavage ductile fracture. By controlling the amount and size of the secondary phase precipitates at an aging temperature no higher than 773 K for no more than 3 h, a significant improvement in the compressive strength and ductility of the alloys can be expected as a result of the fine-tuning of the precipitates. Acknowledgements This study is supported by Postdoctoral Science Foundation of China (Grant No. 20100481218), Natural Science Foundation of
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