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Formation of Ni2Al and Ni5Al3 superstructures and reversibility of martensitic transformation in NiAl-based β-alloys
Materials Science and Engineering A 438–440 (2006) 312–314
Formation of Ni2Al and Ni5Al3 superstructures and reversibility of martensitic transformation in NiAl-based -alloys N.V. Kataeva ∗ , S.V. Kositsyn, A.I. Valiullin Institute of Metal Physics, Ural Branch RAS, Ekaterinburg, Russian Federation Received 13 June 2005; received in revised form 16 December 2005; accepted 10 February 2006
Abstract Rapidly crystallized NiAl-based -alloys − Ni64 Al36 , Ni65 Al35 , and Ni56 Al34 Co10 (at.%) – were studied by methods of resistometry and electron microscopy. The decomposition of the L10 -martensite, which was accompanied by formation of Ni5 Al3 particles, was observed in alloys whose reverse martensite start temperature was over 250 ◦ C. Formation of nanoparticles with the A5 B3 (Ni5 Al3 ) superstructure in the martensite stabilized it against the reverse shear transformation up to a complete suppression of the thermoelastic (B2) ↔ L10 transformation. The replacement of nickel by 10 at.% cobalt in the Ni66 Al34 alloy decreased the tendency to the formation of Ni5 Al3 and Ni2 Al particles. © 2006 Elsevier B.V. All rights reserved. Keywords: High-temperature shape memory effect; Ni–Al -alloys based; Ni2 Al; Ni5 Al3
1. Introduction The thermoelastic (B2) ↔ L10 martensitic transformation (MT) is observed in nickel-supersaturated Ni–Al -alloys based on nickel aluminide at temperatures, which strongly depend on the degree of supersaturation [1,2]. If the nickel concentration is 65 at.% or larger, Ms and Af can be over 300 ◦ C. Therefore, these alloys are viewed as perspective functional materials possessing the high-temperature shape memory effect (HTSME). The practical use of the alloys is impeded by their high brittleness in the polycrystalline state and the thermal instability of the supersaturated -solid solution, which shows up as its proneness to decomposition at working temperatures of HTSME and precipitation of phases like Ni2 Al and Ni5 Al3 . The last factor limits the use of HTSME to temperatures of 200–220 ◦ C. The solution of this problem requires performing systematic studies into regular features characteristic of the decomposition of the -solid solution both during its heating at different rates and under isothermal holding conditions. The objective of the present study was to analyze how the decomposition of the supersaturated -solid solution, which is
∗
Corresponding author. E-mail address: [email protected] (N.V. Kataeva).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.081
quenched to L10 martensite, influences the solubility and critical temperatures of the martensitic transformation in some quickly crystallized -alloys based on NiAl. 2. Materials and experimental technique Some alloys of the formulas Ni64 Al36 , Ni65 Al35 , and Ni56 Al34 Co10 were prepared for the study. Ingots having the mass of 30 g were prepared by triple electric-arc melting in the helium atmosphere using chemically pure charge materials: 99.99 Ni, 99.98 Co, and 99.995 Al (mass %). The ingots were annealed at 1200 ◦ C for 5 h and quenched in water. Then, the alloy sample was smelted by spinning of the melt in a vacuum chamber on a steel drum rotating at a speed of 30 m s−1 to form ribbon samples 2 mm × 30 m in size. The resistometric study of the ribbon samples was performed on an experimental set-up including potentiometric measurements of the resistivity during heating and cooling at a rate of 5 K/min or isothermal holding at temperatures from 20 to 680 ◦ C. The relative change of the electrical resistance Rrel (%) = [(Rt −Ro )/Ro ] × 100 was chosen as the main characteristic. Here, Rt is the electrical resistance of the sample at the isothermal holding temperature and Ro is the electrical resistance of the sample at room temperature before the test. The transmission electron microscopy examination of thin foils was carried out in a JEM-200CX electron microscope. The
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foils were prepared by electropolishing of the samples in a standard orthophosphoric electrolyte. 3. Results A distinct hysteresis loop, which is characteristic of the thermoelastic transformation, is present usually in the region of the B2 ↔ L10 martensitic transformation in temperature dependences of the relative resistivity (R/R0 ) of Ni–Al-based alloys. The L10 → B2 process is accompanied by a sharp drop of R/R0 during heating, while the forward B2 → L10 martensitic transformation is followed by a similar sharp rise of R/R0 during cooling. The critical temperatures Ms , Mf , As , and Af of this transformation can be determined from extremum points of the formed loop. Experimental R/R0 = f(T) curves for the quickly crystallized alloys under study are shown in Fig. 1. The Ni64 Al36 alloy is a single-phase L10 martensitic compound, for which the measured critical temperatures were equal to Ms ∼ 140 ◦ C, Mf ∼ 90 ◦ C, As ∼ 110 ◦ C, Af ∼ 170 ◦ C (Fig. 1). After the first cycle 20 ◦ C → 600 ◦ C → 20 ◦ C, the resistivity of the sample decreased a little (by ∼5%) owing to the annihilation of quenching vacancies. An almost complete reversibility and a small decrease in the shear transformation temperatures were observed after the second heating-cooling cycle. The L10 martensite was preserved in the structure after annealing at 200 and 250 ◦ C for 2–3 h. The long-period 14M martensite appeared
Fig. 1. Curves showing variation of the relative resistivity during heating and cooling of Ni64 Al36 , Ni65 Al35 , and Ni56 Al34 Co10 alloys, which were quickly quenched from the melt.
Fig. 2. Decomposition diagram for the supersaturated -solid solution in the Ni65 Al35 alloy, which was prepared by quick quenching from the melt.
in addition to the L10 martensite after annealing at 350 ◦ C for 2 h. While the martensitic morphology of the structure was preserved, extra-reflections of the Ni2 Al superstructure appeared in the selected area diffraction pattern after annealing at 450 ◦ C for 2 h. The Ni65 Al35 alloy. In distinction to the quenched coarsegrain state, the alloy with the microcrystalline structure (MC) contained one phase. Therefore, the L10 martensite was supersaturated with nickel. This circumstance was reflected in the behavior of the temperature–resistivity curve (Fig. 1). The temperature interval of the reverse martensitic transformation was strongly shifted to the right and was pronounced less (As ∼ 230 ◦ C, Af ∼ 300 ◦ C). The heating curve was much steeper after 300 ◦ C than the similar curve obtained for the previous alloy. The high temperature stability of the L10 martensite, which was prepared by quick quenching from the melt, led to aging (which competed with the reverse L10 → B2 martensitic transformation) of the nickel- and vacancy-supersaturated L10 martensite. The aging process was accompanied by precipitation of Ni5 Al3 nanoparticles having a high resistivity [3]. When Ni5 Al3 particles were formed, reversibility of the martensitic transformation vanished and the residual resistivity increased considerably (by ∼25%). Weak extra-reflections of the Ni5 Al3 superstructure appeared against the background of L10 reflections in selected area diffraction patterns after annealing at 250 ◦ C for 3 h and 300 ◦ C for 3 h. Extra-reflections of the Ni2 Al superstructure were formed after annealing at 450 ◦ C for 3 h, while the martensitic morphology was preserved in general. The decomposition diagram was constructed in the T(◦ C) = f(lg τ(min)) coordinates for the supersaturated -solid solution in the Ni65 Al35 alloy prepared by quick quenching from the melt (Fig. 2). It was constructed on the basis of resistometric measurements, which were made during isothermal annealing
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nanoparticles. The -solid solution decomposed and particles of the stable Ni5 Al3 phase were formed at temperatures above 500 ◦ C. When the samples were cooled after holding under isothermal conditions, the forward martensitic transformation took place. The temperatures Ms and Mf strongly depended on the annealing temperature (Table 1). Preliminary annealing at 450 ◦ C for 1 h stabilized the thermal elasticity MT during four-fold cycling: the critical temperatures of the martensitic transformations changed little (Table 2). The Ni56 Al34 Co10 alloy. When some amount of nickel was replaced by cobalt in Ni65 Al35 , not only the high temperature was preserved, but also the MT reversibility was recovered (Fig. 1). From the analysis of the temperature dependence of the resistivity it follows that the decomposition of the L10 martensite and the formation of Ni5 Al3 particles were less intensive in the Ni56 Al34 Co10 alloy than in the cobalt-free alloy.
Table 1 Temperatures Ms and Mf for the Ni65 Al35 alloy after annealing for 5 h Annealing temperature (◦ C)
Ms (◦ C)
280 300 320 340 360 380 400 420 440 460 480 500 520 550 580 600 640 680 740 780
They cannot be determined 278 215 288 235 290 240 289 250 266 227 261 222 249 206 143 195 239 195 226 181 225 174 226 176 201 125
<20
Mf (◦ C)
4. Conclusion
<20
Table 2 Martensitic transformation temperatures for the Ni65 Al35 alloy, which was prepared by quick quenching from the melt and pre-annealed at 450 ◦ C for 1 h Number of 20 ◦ C–450 ◦ C–20 ◦ C cycle
Ms (◦ C)
Mf (◦ C)
As (◦ C)
Af (◦ C)
I II III IV
265 265 271 266
224 225 227 227
241 244 243 246
314 310 309 307
treatments: Rrel = f(τ). The curves were plotted taking the time needed for Rrel to change by ±1% as the decomposition start point of the melt-spun Ni65 Al35 alloy. The Rrel = f(τ) curves were used to determine the time required for accomplishment of this change at each annealing temperature. The time was referred to the moment when the sample was heated to the annealing temperature. Three decomposition stages were clearly seen in the diagram. Isothermal holding at temperatures below Af gave rise to the supersat supersat → L10 + Ni5 Al3 nanodecomposition process: L10 particles. Aging of the supersaturated (B2)-phase was observed at temperature of 360–480 ◦ C: B2supersat → B2supersat + Ni2 Al
It was shown that the decomposition of the supersaturated L10 martensite, which inhibits the reversibility of the martensitic transformation in SME alloys, was only observed in alloys with the temperature of the reverse martensitic transformation being over 250 ◦ C. The replacement of 10 at.% nickel by cobalt changed little the interval of thermoelastic MT, but retarded the formation of Ni2 Al and Ni5 Al3 particles. NiAl-based alloys can be used as high-temperature functional alloys possessing the shape memory effect if the thermoelastic L10 ↔ B2 martensitic transformation remains reversible. The transformation reversibility in Ni–Al -alloys can be ensured using a special thermal treatment of the Ni2 Al superstructure nuclei, which impede the L10 → Ni5 Al3 transformation, or by a partial replacement of nickel by cobalt. Acknowledgement This study was financed partly by RFBR (grant 03-0332523). References [1] Y.D. Kim, C.M. Wayman, Scripta Metall. Mater. 24 (1990) 245–250. [2] J.H. Zhu, D.P. Dunne, G.W. Delamore, N.F. Kennon, Monterey Institute for Advanced Studies, 1993, pp. 911–915. [3] Y. Laˇsek, T. Chra´ska, P. Kˇreˇcek, P. Bartuˇska, Scripta Mater. 37 (1997) 897.
Formation of Ni2Al and Ni5Al3 superstructures and reversibility of martensitic transformation in NiAl-based -alloys N.V. Kataeva ∗ , S.V. Kositsyn, A.I. Valiullin Institute of Metal Physics, Ural Branch RAS, Ekaterinburg, Russian Federation Received 13 June 2005; received in revised form 16 December 2005; accepted 10 February 2006
Abstract Rapidly crystallized NiAl-based -alloys − Ni64 Al36 , Ni65 Al35 , and Ni56 Al34 Co10 (at.%) – were studied by methods of resistometry and electron microscopy. The decomposition of the L10 -martensite, which was accompanied by formation of Ni5 Al3 particles, was observed in alloys whose reverse martensite start temperature was over 250 ◦ C. Formation of nanoparticles with the A5 B3 (Ni5 Al3 ) superstructure in the martensite stabilized it against the reverse shear transformation up to a complete suppression of the thermoelastic (B2) ↔ L10 transformation. The replacement of nickel by 10 at.% cobalt in the Ni66 Al34 alloy decreased the tendency to the formation of Ni5 Al3 and Ni2 Al particles. © 2006 Elsevier B.V. All rights reserved. Keywords: High-temperature shape memory effect; Ni–Al -alloys based; Ni2 Al; Ni5 Al3
1. Introduction The thermoelastic (B2) ↔ L10 martensitic transformation (MT) is observed in nickel-supersaturated Ni–Al -alloys based on nickel aluminide at temperatures, which strongly depend on the degree of supersaturation [1,2]. If the nickel concentration is 65 at.% or larger, Ms and Af can be over 300 ◦ C. Therefore, these alloys are viewed as perspective functional materials possessing the high-temperature shape memory effect (HTSME). The practical use of the alloys is impeded by their high brittleness in the polycrystalline state and the thermal instability of the supersaturated -solid solution, which shows up as its proneness to decomposition at working temperatures of HTSME and precipitation of phases like Ni2 Al and Ni5 Al3 . The last factor limits the use of HTSME to temperatures of 200–220 ◦ C. The solution of this problem requires performing systematic studies into regular features characteristic of the decomposition of the -solid solution both during its heating at different rates and under isothermal holding conditions. The objective of the present study was to analyze how the decomposition of the supersaturated -solid solution, which is
∗
Corresponding author. E-mail address: [email protected] (N.V. Kataeva).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.081
quenched to L10 martensite, influences the solubility and critical temperatures of the martensitic transformation in some quickly crystallized -alloys based on NiAl. 2. Materials and experimental technique Some alloys of the formulas Ni64 Al36 , Ni65 Al35 , and Ni56 Al34 Co10 were prepared for the study. Ingots having the mass of 30 g were prepared by triple electric-arc melting in the helium atmosphere using chemically pure charge materials: 99.99 Ni, 99.98 Co, and 99.995 Al (mass %). The ingots were annealed at 1200 ◦ C for 5 h and quenched in water. Then, the alloy sample was smelted by spinning of the melt in a vacuum chamber on a steel drum rotating at a speed of 30 m s−1 to form ribbon samples 2 mm × 30 m in size. The resistometric study of the ribbon samples was performed on an experimental set-up including potentiometric measurements of the resistivity during heating and cooling at a rate of 5 K/min or isothermal holding at temperatures from 20 to 680 ◦ C. The relative change of the electrical resistance Rrel (%) = [(Rt −Ro )/Ro ] × 100 was chosen as the main characteristic. Here, Rt is the electrical resistance of the sample at the isothermal holding temperature and Ro is the electrical resistance of the sample at room temperature before the test. The transmission electron microscopy examination of thin foils was carried out in a JEM-200CX electron microscope. The
N.V. Kataeva et al. / Materials Science and Engineering A 438–440 (2006) 312–314
313
foils were prepared by electropolishing of the samples in a standard orthophosphoric electrolyte. 3. Results A distinct hysteresis loop, which is characteristic of the thermoelastic transformation, is present usually in the region of the B2 ↔ L10 martensitic transformation in temperature dependences of the relative resistivity (R/R0 ) of Ni–Al-based alloys. The L10 → B2 process is accompanied by a sharp drop of R/R0 during heating, while the forward B2 → L10 martensitic transformation is followed by a similar sharp rise of R/R0 during cooling. The critical temperatures Ms , Mf , As , and Af of this transformation can be determined from extremum points of the formed loop. Experimental R/R0 = f(T) curves for the quickly crystallized alloys under study are shown in Fig. 1. The Ni64 Al36 alloy is a single-phase L10 martensitic compound, for which the measured critical temperatures were equal to Ms ∼ 140 ◦ C, Mf ∼ 90 ◦ C, As ∼ 110 ◦ C, Af ∼ 170 ◦ C (Fig. 1). After the first cycle 20 ◦ C → 600 ◦ C → 20 ◦ C, the resistivity of the sample decreased a little (by ∼5%) owing to the annihilation of quenching vacancies. An almost complete reversibility and a small decrease in the shear transformation temperatures were observed after the second heating-cooling cycle. The L10 martensite was preserved in the structure after annealing at 200 and 250 ◦ C for 2–3 h. The long-period 14M martensite appeared
Fig. 1. Curves showing variation of the relative resistivity during heating and cooling of Ni64 Al36 , Ni65 Al35 , and Ni56 Al34 Co10 alloys, which were quickly quenched from the melt.
Fig. 2. Decomposition diagram for the supersaturated -solid solution in the Ni65 Al35 alloy, which was prepared by quick quenching from the melt.
in addition to the L10 martensite after annealing at 350 ◦ C for 2 h. While the martensitic morphology of the structure was preserved, extra-reflections of the Ni2 Al superstructure appeared in the selected area diffraction pattern after annealing at 450 ◦ C for 2 h. The Ni65 Al35 alloy. In distinction to the quenched coarsegrain state, the alloy with the microcrystalline structure (MC) contained one phase. Therefore, the L10 martensite was supersaturated with nickel. This circumstance was reflected in the behavior of the temperature–resistivity curve (Fig. 1). The temperature interval of the reverse martensitic transformation was strongly shifted to the right and was pronounced less (As ∼ 230 ◦ C, Af ∼ 300 ◦ C). The heating curve was much steeper after 300 ◦ C than the similar curve obtained for the previous alloy. The high temperature stability of the L10 martensite, which was prepared by quick quenching from the melt, led to aging (which competed with the reverse L10 → B2 martensitic transformation) of the nickel- and vacancy-supersaturated L10 martensite. The aging process was accompanied by precipitation of Ni5 Al3 nanoparticles having a high resistivity [3]. When Ni5 Al3 particles were formed, reversibility of the martensitic transformation vanished and the residual resistivity increased considerably (by ∼25%). Weak extra-reflections of the Ni5 Al3 superstructure appeared against the background of L10 reflections in selected area diffraction patterns after annealing at 250 ◦ C for 3 h and 300 ◦ C for 3 h. Extra-reflections of the Ni2 Al superstructure were formed after annealing at 450 ◦ C for 3 h, while the martensitic morphology was preserved in general. The decomposition diagram was constructed in the T(◦ C) = f(lg τ(min)) coordinates for the supersaturated -solid solution in the Ni65 Al35 alloy prepared by quick quenching from the melt (Fig. 2). It was constructed on the basis of resistometric measurements, which were made during isothermal annealing
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nanoparticles. The -solid solution decomposed and particles of the stable Ni5 Al3 phase were formed at temperatures above 500 ◦ C. When the samples were cooled after holding under isothermal conditions, the forward martensitic transformation took place. The temperatures Ms and Mf strongly depended on the annealing temperature (Table 1). Preliminary annealing at 450 ◦ C for 1 h stabilized the thermal elasticity MT during four-fold cycling: the critical temperatures of the martensitic transformations changed little (Table 2). The Ni56 Al34 Co10 alloy. When some amount of nickel was replaced by cobalt in Ni65 Al35 , not only the high temperature was preserved, but also the MT reversibility was recovered (Fig. 1). From the analysis of the temperature dependence of the resistivity it follows that the decomposition of the L10 martensite and the formation of Ni5 Al3 particles were less intensive in the Ni56 Al34 Co10 alloy than in the cobalt-free alloy.
Table 1 Temperatures Ms and Mf for the Ni65 Al35 alloy after annealing for 5 h Annealing temperature (◦ C)
Ms (◦ C)
280 300 320 340 360 380 400 420 440 460 480 500 520 550 580 600 640 680 740 780
They cannot be determined 278 215 288 235 290 240 289 250 266 227 261 222 249 206 143 195 239 195 226 181 225 174 226 176 201 125
<20
Mf (◦ C)
4. Conclusion
<20
Table 2 Martensitic transformation temperatures for the Ni65 Al35 alloy, which was prepared by quick quenching from the melt and pre-annealed at 450 ◦ C for 1 h Number of 20 ◦ C–450 ◦ C–20 ◦ C cycle
Ms (◦ C)
Mf (◦ C)
As (◦ C)
Af (◦ C)
I II III IV
265 265 271 266
224 225 227 227
241 244 243 246
314 310 309 307
treatments: Rrel = f(τ). The curves were plotted taking the time needed for Rrel to change by ±1% as the decomposition start point of the melt-spun Ni65 Al35 alloy. The Rrel = f(τ) curves were used to determine the time required for accomplishment of this change at each annealing temperature. The time was referred to the moment when the sample was heated to the annealing temperature. Three decomposition stages were clearly seen in the diagram. Isothermal holding at temperatures below Af gave rise to the supersat supersat → L10 + Ni5 Al3 nanodecomposition process: L10 particles. Aging of the supersaturated (B2)-phase was observed at temperature of 360–480 ◦ C: B2supersat → B2supersat + Ni2 Al
It was shown that the decomposition of the supersaturated L10 martensite, which inhibits the reversibility of the martensitic transformation in SME alloys, was only observed in alloys with the temperature of the reverse martensitic transformation being over 250 ◦ C. The replacement of 10 at.% nickel by cobalt changed little the interval of thermoelastic MT, but retarded the formation of Ni2 Al and Ni5 Al3 particles. NiAl-based alloys can be used as high-temperature functional alloys possessing the shape memory effect if the thermoelastic L10 ↔ B2 martensitic transformation remains reversible. The transformation reversibility in Ni–Al -alloys can be ensured using a special thermal treatment of the Ni2 Al superstructure nuclei, which impede the L10 → Ni5 Al3 transformation, or by a partial replacement of nickel by cobalt. Acknowledgement This study was financed partly by RFBR (grant 03-0332523). References [1] Y.D. Kim, C.M. Wayman, Scripta Metall. Mater. 24 (1990) 245–250. [2] J.H. Zhu, D.P. Dunne, G.W. Delamore, N.F. Kennon, Monterey Institute for Advanced Studies, 1993, pp. 911–915. [3] Y. Laˇsek, T. Chra´ska, P. Kˇreˇcek, P. Bartuˇska, Scripta Mater. 37 (1997) 897.