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Martensitic transformation in Co2NiGa ferromagnetic shape memory alloys
Scripta Materialia 47 (2002) 285–288 www.actamat-journals.com
Martensitic transformation in Co2NiGa ferromagnetic shape memory alloys Corneliu Craciunescu a
a,1
, Yoichi Kishi
a,2
, T.A. Lograsso b, Manfred Wuttig
a,*
Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742-2115, USA b Ames Laboratory, Iowa State University, Ames, IA, USA Received 17 January 2002; received in revised form 29 April 2002; accepted 1 May 2002
Abstract This investigation of the martensitic transformation in ferromagnetic Co2 NiGa shape memory alloys shows that, like in the Ni2 MnGa system, the martensite start temperature is proportional to the valence electron concentration. Al is increasing the transition temperatures when it substitutes Ga and is decreasing them when it substitutes Ni. Ó 2002 Published by Elsevier Science Ltd. on behalf of Acta Materialia Inc. Keywords: Phase transformation; Shape memory; Internal friction; Ferromagnetism
1. Introduction The recent investigation of CoNiGa alloys [1,2] added a new member to the family of ferromagnetic shape memory alloys (FMSMAs), and more specifically to that group associated with the ternary Heusler-type composition. While binary FMSMAs like FePt and FePd are known [3,4], their large-scale commercial future could be significantly affected by the cost of the precious metals. The Ni–Co–Ga FMSMA system is similar to the recently reported Ni–Co–Al alloys [5]. Ternary Ni2 MnGa alloys have been investigated * Corresponding author. Tel.: +1-301-405-5212; fax: +1-301314-9467/9637. E-mail address: [email protected] (M. Wuttig). 1 On leave from ‘‘POLITEHNICA’’ University of Timisoara, Romania. 2 On leave from Kanazawa Institute of Technology, Japan.
thoroughly during the past few years [6–12] and additional Heusler-type FMSMAs have been sought for a long time. A promising path for finding new FMSMAs considers the electron concentration and the saturation magnetization [13] in conjunction with the metallurgical possibilities for the formation of such alloys in the desired electron concentration range. Co2 NiGa FMSMAs resemble Ni2 MnGa alloys in both their electron concentration and saturation magnetization [1]. Therefore they are expected to show a similar trend in transformation temperatures while the presence of Co and the absence of Mn are expected to improve their magneto-mechanical characteristics. This paper reports on the influence of the composition on the martensitic transformation temperature of Co2 NiGa shape memory alloys as well as on some structural characteristics of the alloys.
1359-6462/02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd. on behalf of Acta Materialia Inc. PII: S 1 3 5 9 - 6 4 6 2 ( 0 2 ) 0 0 1 4 8 - 3
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2. Experimental CoNiGa alloys were prepared by arc melting followed by homogenization annealing at 900 °C for 10 h. Ingots of the compositions Cox Niy Ga100ðxþyÞ , 41 < x < 62 and 18 < y < 33, were prepared for this investigation. For two compositions, part of the Ga and Ni respectively were replaced by up to 2% Al. The composition of the ingots was measured using energy dispersive spectroscopy. Strips, 0.3 mm thick, 4 mm wide and 15 mm long, were cut from the ingots and used to determine the martensitic transformation characteristics by measuring the elastic properties in an acoustic elastometer [14]. The temperature of measurement ranged from 180 to þ300 °C.
3. Results and discussion The results of the measurements of the modulus defect DM=M ¼ ½MðT1 Þ MðT2 Þ=MðT1 Þ as a function of temperature for alloys with a Co concentration of XCo ¼ 0:49 0:1 and 18=32 < XNi =XGa < 30=20 are shown in Fig. 1. It can be seen that a martensitic transformation develops in the CoNiGa alloys and that the characteristics of this transformation, i.e. the transition temperature range, are
Fig. 1. Change in the modulus defect vs. temperature of the Co2 NiGa FMSMAs. Higher transitions occur for higher Ni/Ga ratio.
Fig. 2. Compositional range for the formation of Co2 NiGa FMSMAs (see text for details). The black dots mark the alloys investigated in this study.
dependent on the chemical composition of the alloy. The alloys showing the phase transformation are in the compositional range of the Heusler-type alloys. In the CoNiGa ternary phase diagram favorable conditions exist for the formation of solid solutions of this kind because the Co–Ni binary phase diagram shows a very large solubility of the components [15]. Thus, the overall metallurgical restrictions are not so tight. In addition, both Ga– Ni 3 and Co–Ga 4 binary phase diagrams contain B2 phases that favor the formation of Heusler alloys in the ternary diagram. The shaded region in Fig. 2 shows an estimated composition range in which CoNiGa solid solutions might be obtained. It is likely that this range can be expanded: in both Ga–Ni and Co–Ga phase diagrams, the regions of the B2 phase expand at higher temperatures and can be stabilized by quenching, thereby extending the shaded region where e=a > 7:3. Insight into the potential saturation magnetization of the alloys can be obtained from the known Slater–Pauling curve [16] and a previous study [13] on the occurrence of FMSMAs. It is suggested that finite values of the saturation magnetization can be
3 4
See Ref. [14, vol. 2, p. 1150]. See Ref. [14, p. 766].
C. Craciunescu et al. / Scripta Materialia 47 (2002) 285–288
Fig. 3. The effect of Al addition on the martensitic transformation in Co2 NiGa FMSMAs.
obtained for (d þ s þ p) electron/atom ratios larger than e0 =a 11:5. The structural stability of the alloys is determined by an electron/atom ratio as well [17]. 5 However, only the outer (s þ p) valence electrons of Ga must be counted. The empirical stability diagram of NiMnGa and CoNiGa Heusler-type alloys is shown in Fig. 4. It can be seen from this figure that the limit of the structural stability of both alloy systems occurs at a critical electron/ atom ratio e=a 7:3. The two electron concentrations e=a and e0 =a have been included in Fig. 2. It can be seen that only the metallurgical and structural stability criteria control the formation of potentially unstable ferromagnetic CoNiGa Heusler alloys. To a certain extent, Al can be used to replace Ga or Ni. A slightly higher transition temperature has been observed for alloys in which a part of Ga has been substituted by Al, and a lower transition temperature when Ni was substituted by Al, as shown in Fig. 3.
287
7:3 6 e=a 6 7:8, similar to the case of NiMnGa FMSMAs. The Ms transformation temperatures determined by elastic measurements are in consonance with data reported for NiMnGa and show a similar trend of the martensite start temperature as a function of the valence electron concentration. The contribution of the Co2 Ni ¼ 28 electrons is slightly higher than the contribution of the Ni2 Mn ¼ 27 electrons, and therefore, for the same Ga content, CoNiGa alloys are expected to show higher transition temperatures and higher magnetization. As expected, the ðNi=GaÞ ¼ 10=3 electron ratio is the primary element in adjusting the transition temperature. Fig. 4 demonstrates the similarities in the evolution of the transformation temperatures as a function of the electron concentration of CoNiGa and NiMnGa Heusler-type FMSMAs as discussed by Chernenko 4 and summarized by Schlagel et al. [18]. It can be observed that both systems show a similar trend for which the transition temperatures tend to 0 K when e=a ¼ 7:3 increasing with the same slope as e=a increases. While in the NiMnGa system, higher transition temperatures can be reached by adding Ni at the expense of Ga and Mn, in the CoNiGa system both Co and Ni can be used in the effort to increase the transition temperatures, because their electron concentrations are very close.
4. Summary and conclusions The CoNiGa system has been investigated and for an average electron concentration of about 5
See Ref. [1].
Fig. 4. The influence of the electron/atom ratio on the Ms temperature in Co2 NiGa(Al) and Ni2 MnGa FMSMAs. Ni2 MnGa data (j) compiled after Schlagel et al. [15].
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The substitution of Ga by Al is effective for the increase of the transition temperatures but this seems to be made at the expense of the transformation elastic properties. Acknowledgements This study was supported by the National Science Foundation, grant DMR0095166, and by the Office of Naval Research, contracts no. MURI N000140110761, N000149910837 and N000140010849. The research also benefitted from the support of the Office of Basic Energy Sciences, Materials Sciences Division, of the U.S. Department of Energy under Contract No. W-7405-ENG82. References [1] Wuttig M, Li J, Craciunescu C. Scripta Mater 2001; 44:2393. [2] Oikawa K, Ota T, Gejima F, Ohmori T, Kainuma R, Ishida K. Mater Trans 2001;42(11):2472.
[3] Kakeshita T, Kuroiwa K, Shimizu K, Ikeda T, Yamagishi A, Date M. Mater Trans JIM 1993;34:415. [4] Dunne DP, Wayman M. Met Trans 1973;4:137. [5] Oikawa K, Wulff L, Iijima T, Gejima F, Ohmori A, Fujita A, et al. Appl Phys Lett 2001;79:3290. [6] Kajiwara S, Owen W. Met Trans 1974;5:2047. [7] Tadaki T, Shimizu K. Scripta Met 1975;9:771. [8] Oshima R. Scripta Met 1981;15:829. [9] Oshima R, Suguyama M, Fujita FE. Met Trans 1988; 19A:803. [10] James RD, Wuttig M. Philos Mag A 1998;77:1273. [11] Kakeshita T, Takeuchi T, Tsujiguchi TM, Saburi T, Oshima R, Muto S. Appl Phys Lett 2000;77:1502. [12] Webster PJ, Ziebeck KRA, Town SL, Peak MS. Philos Mag 1984;49B:295. [13] Wuttig M, Liu L, Tsuchiya K, James RD. J Appl Phys 2000;87:4707. [14] Vibran Technologies Inc. Available from: www.vibran. com. [15] Murray JL, Bennett LH, Baker H, editors. Massalski TB, editor-in-chief. Binary alloy phase diagrams, vol. 1. Metals Park, OH: American Society for Metals; 1986. p. 1064. [16] Bozorth RM, editor. Ferromagnetism. Piscataway, NJ: IEEE; 1993. p. 441. [17] Chernenko VA. Scripta Mater 1999;40(5):523. [18] Schlagel DL, Wu YL, Zhang W, Lograsso TA. J Alloys Compd 2000;312:77.
Martensitic transformation in Co2NiGa ferromagnetic shape memory alloys Corneliu Craciunescu a
a,1
, Yoichi Kishi
a,2
, T.A. Lograsso b, Manfred Wuttig
a,*
Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742-2115, USA b Ames Laboratory, Iowa State University, Ames, IA, USA Received 17 January 2002; received in revised form 29 April 2002; accepted 1 May 2002
Abstract This investigation of the martensitic transformation in ferromagnetic Co2 NiGa shape memory alloys shows that, like in the Ni2 MnGa system, the martensite start temperature is proportional to the valence electron concentration. Al is increasing the transition temperatures when it substitutes Ga and is decreasing them when it substitutes Ni. Ó 2002 Published by Elsevier Science Ltd. on behalf of Acta Materialia Inc. Keywords: Phase transformation; Shape memory; Internal friction; Ferromagnetism
1. Introduction The recent investigation of CoNiGa alloys [1,2] added a new member to the family of ferromagnetic shape memory alloys (FMSMAs), and more specifically to that group associated with the ternary Heusler-type composition. While binary FMSMAs like FePt and FePd are known [3,4], their large-scale commercial future could be significantly affected by the cost of the precious metals. The Ni–Co–Ga FMSMA system is similar to the recently reported Ni–Co–Al alloys [5]. Ternary Ni2 MnGa alloys have been investigated * Corresponding author. Tel.: +1-301-405-5212; fax: +1-301314-9467/9637. E-mail address: [email protected] (M. Wuttig). 1 On leave from ‘‘POLITEHNICA’’ University of Timisoara, Romania. 2 On leave from Kanazawa Institute of Technology, Japan.
thoroughly during the past few years [6–12] and additional Heusler-type FMSMAs have been sought for a long time. A promising path for finding new FMSMAs considers the electron concentration and the saturation magnetization [13] in conjunction with the metallurgical possibilities for the formation of such alloys in the desired electron concentration range. Co2 NiGa FMSMAs resemble Ni2 MnGa alloys in both their electron concentration and saturation magnetization [1]. Therefore they are expected to show a similar trend in transformation temperatures while the presence of Co and the absence of Mn are expected to improve their magneto-mechanical characteristics. This paper reports on the influence of the composition on the martensitic transformation temperature of Co2 NiGa shape memory alloys as well as on some structural characteristics of the alloys.
1359-6462/02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd. on behalf of Acta Materialia Inc. PII: S 1 3 5 9 - 6 4 6 2 ( 0 2 ) 0 0 1 4 8 - 3
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2. Experimental CoNiGa alloys were prepared by arc melting followed by homogenization annealing at 900 °C for 10 h. Ingots of the compositions Cox Niy Ga100ðxþyÞ , 41 < x < 62 and 18 < y < 33, were prepared for this investigation. For two compositions, part of the Ga and Ni respectively were replaced by up to 2% Al. The composition of the ingots was measured using energy dispersive spectroscopy. Strips, 0.3 mm thick, 4 mm wide and 15 mm long, were cut from the ingots and used to determine the martensitic transformation characteristics by measuring the elastic properties in an acoustic elastometer [14]. The temperature of measurement ranged from 180 to þ300 °C.
3. Results and discussion The results of the measurements of the modulus defect DM=M ¼ ½MðT1 Þ MðT2 Þ=MðT1 Þ as a function of temperature for alloys with a Co concentration of XCo ¼ 0:49 0:1 and 18=32 < XNi =XGa < 30=20 are shown in Fig. 1. It can be seen that a martensitic transformation develops in the CoNiGa alloys and that the characteristics of this transformation, i.e. the transition temperature range, are
Fig. 1. Change in the modulus defect vs. temperature of the Co2 NiGa FMSMAs. Higher transitions occur for higher Ni/Ga ratio.
Fig. 2. Compositional range for the formation of Co2 NiGa FMSMAs (see text for details). The black dots mark the alloys investigated in this study.
dependent on the chemical composition of the alloy. The alloys showing the phase transformation are in the compositional range of the Heusler-type alloys. In the CoNiGa ternary phase diagram favorable conditions exist for the formation of solid solutions of this kind because the Co–Ni binary phase diagram shows a very large solubility of the components [15]. Thus, the overall metallurgical restrictions are not so tight. In addition, both Ga– Ni 3 and Co–Ga 4 binary phase diagrams contain B2 phases that favor the formation of Heusler alloys in the ternary diagram. The shaded region in Fig. 2 shows an estimated composition range in which CoNiGa solid solutions might be obtained. It is likely that this range can be expanded: in both Ga–Ni and Co–Ga phase diagrams, the regions of the B2 phase expand at higher temperatures and can be stabilized by quenching, thereby extending the shaded region where e=a > 7:3. Insight into the potential saturation magnetization of the alloys can be obtained from the known Slater–Pauling curve [16] and a previous study [13] on the occurrence of FMSMAs. It is suggested that finite values of the saturation magnetization can be
3 4
See Ref. [14, vol. 2, p. 1150]. See Ref. [14, p. 766].
C. Craciunescu et al. / Scripta Materialia 47 (2002) 285–288
Fig. 3. The effect of Al addition on the martensitic transformation in Co2 NiGa FMSMAs.
obtained for (d þ s þ p) electron/atom ratios larger than e0 =a 11:5. The structural stability of the alloys is determined by an electron/atom ratio as well [17]. 5 However, only the outer (s þ p) valence electrons of Ga must be counted. The empirical stability diagram of NiMnGa and CoNiGa Heusler-type alloys is shown in Fig. 4. It can be seen from this figure that the limit of the structural stability of both alloy systems occurs at a critical electron/ atom ratio e=a 7:3. The two electron concentrations e=a and e0 =a have been included in Fig. 2. It can be seen that only the metallurgical and structural stability criteria control the formation of potentially unstable ferromagnetic CoNiGa Heusler alloys. To a certain extent, Al can be used to replace Ga or Ni. A slightly higher transition temperature has been observed for alloys in which a part of Ga has been substituted by Al, and a lower transition temperature when Ni was substituted by Al, as shown in Fig. 3.
287
7:3 6 e=a 6 7:8, similar to the case of NiMnGa FMSMAs. The Ms transformation temperatures determined by elastic measurements are in consonance with data reported for NiMnGa and show a similar trend of the martensite start temperature as a function of the valence electron concentration. The contribution of the Co2 Ni ¼ 28 electrons is slightly higher than the contribution of the Ni2 Mn ¼ 27 electrons, and therefore, for the same Ga content, CoNiGa alloys are expected to show higher transition temperatures and higher magnetization. As expected, the ðNi=GaÞ ¼ 10=3 electron ratio is the primary element in adjusting the transition temperature. Fig. 4 demonstrates the similarities in the evolution of the transformation temperatures as a function of the electron concentration of CoNiGa and NiMnGa Heusler-type FMSMAs as discussed by Chernenko 4 and summarized by Schlagel et al. [18]. It can be observed that both systems show a similar trend for which the transition temperatures tend to 0 K when e=a ¼ 7:3 increasing with the same slope as e=a increases. While in the NiMnGa system, higher transition temperatures can be reached by adding Ni at the expense of Ga and Mn, in the CoNiGa system both Co and Ni can be used in the effort to increase the transition temperatures, because their electron concentrations are very close.
4. Summary and conclusions The CoNiGa system has been investigated and for an average electron concentration of about 5
See Ref. [1].
Fig. 4. The influence of the electron/atom ratio on the Ms temperature in Co2 NiGa(Al) and Ni2 MnGa FMSMAs. Ni2 MnGa data (j) compiled after Schlagel et al. [15].
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The substitution of Ga by Al is effective for the increase of the transition temperatures but this seems to be made at the expense of the transformation elastic properties. Acknowledgements This study was supported by the National Science Foundation, grant DMR0095166, and by the Office of Naval Research, contracts no. MURI N000140110761, N000149910837 and N000140010849. The research also benefitted from the support of the Office of Basic Energy Sciences, Materials Sciences Division, of the U.S. Department of Energy under Contract No. W-7405-ENG82. References [1] Wuttig M, Li J, Craciunescu C. Scripta Mater 2001; 44:2393. [2] Oikawa K, Ota T, Gejima F, Ohmori T, Kainuma R, Ishida K. Mater Trans 2001;42(11):2472.
[3] Kakeshita T, Kuroiwa K, Shimizu K, Ikeda T, Yamagishi A, Date M. Mater Trans JIM 1993;34:415. [4] Dunne DP, Wayman M. Met Trans 1973;4:137. [5] Oikawa K, Wulff L, Iijima T, Gejima F, Ohmori A, Fujita A, et al. Appl Phys Lett 2001;79:3290. [6] Kajiwara S, Owen W. Met Trans 1974;5:2047. [7] Tadaki T, Shimizu K. Scripta Met 1975;9:771. [8] Oshima R. Scripta Met 1981;15:829. [9] Oshima R, Suguyama M, Fujita FE. Met Trans 1988; 19A:803. [10] James RD, Wuttig M. Philos Mag A 1998;77:1273. [11] Kakeshita T, Takeuchi T, Tsujiguchi TM, Saburi T, Oshima R, Muto S. Appl Phys Lett 2000;77:1502. [12] Webster PJ, Ziebeck KRA, Town SL, Peak MS. Philos Mag 1984;49B:295. [13] Wuttig M, Liu L, Tsuchiya K, James RD. J Appl Phys 2000;87:4707. [14] Vibran Technologies Inc. Available from: www.vibran. com. [15] Murray JL, Bennett LH, Baker H, editors. Massalski TB, editor-in-chief. Binary alloy phase diagrams, vol. 1. Metals Park, OH: American Society for Metals; 1986. p. 1064. [16] Bozorth RM, editor. Ferromagnetism. Piscataway, NJ: IEEE; 1993. p. 441. [17] Chernenko VA. Scripta Mater 1999;40(5):523. [18] Schlagel DL, Wu YL, Zhang W, Lograsso TA. J Alloys Compd 2000;312:77.