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CuAlPd alloys for sensor and actuator applications
Intermetallics 8 (2000) 605±611
CuAlPd alloys for sensor and actuator applications Z.C. Lin, W. Yu, R.H. Zee, B.A. Chin * Materials Engineering, Auburn University, Auburn, AL 36849, USA
Abstract Currently available shape memory alloys (NiTi, CuAlNi, CuSnAl) lack the high transformation temperatures and long term thermal stability desired in many commercial applications. This paper reports the results of an investigation in which Pd was substituted for Ni to obtain the shape memory ally CuAlPd. The CuAlPd alloys were found to have an austenite transformation temperature range of 115±370 C depending on composition, heat treatment and working process. Optimal shape memory properties were found for a composition of Cu-13.1 wt% A1-2.4 wt% Pd. This alloy has a transformation temperature of 180 C and a recoverable strain of 4.8%. CuAlPd alloys have excellent workability and exhibit fatigue properties comparable to NiTi shape memory alloys. Single crystals of CuAlPd alloys were produced using a modi®ed Bridgeman technique. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Ternary alloy systems; B. Martensitic transformations; B. Phase transformations; B. Twinning; C. Crystal growth; C. Heat treatment; E. Mechanical properties
1. Introduction Shape memory alloys (SMA) have drawn increasing interest from both the academic community and commercial world due to their unique properties. Researchers have reported more than 15 dierent alloys [1] with shape memory eects (SME), but only NiTi, CuAlNi, and CuZnAl alloys are commercially available due to high costs, inadequate mechanical properties or poor reliability of the other SMAs. Sensor and actuator applications, in many cases, require high transformation temperatures (TTR), namely 160±170 C, and good thermal stability. However, of the three commercially available SMAs, none of them meets these requirements. Nitinol (NiTi) works well only below 80 C [2,3]. CuZnAl [2,4] has a low TTR and poor thermal stability. CuAlNi, although it has better thermal stability, has TTRs lower than 100 C [3,5]. This study seeks to develop a new shape memory alloy that has a high TTR and good thermal stability. Pd dissolves well in Cu-Al based alloys readily forming solid solutions and has a similar electron con®guration and atomic radius as Ni. Therefore, Pd may be anticipated to improve the martensitic phase stability by blocking Al migration in the CuAl alloy that causes * Corresponding author. Tel.: +1-334-844-3322; fax: +1-334-8443400. E-mail address: [email protected] (B.A. Chin).
thermal instability. Additionally, CuAlPd is expected to have higher grain boundary adhesion than CuAlNi alloys, thus improving mechanical properties. In light of these hypotheses, CuAlPd alloy was investigated as a possible means of achieving improved TTR and thermal stability of CuAl-based SMAs. 2. Experimental procedures 2.1. Sample fabrication High purity Cu (99.999 wt%), Al (99.99 wt%), Ni (99.99 wt%), Pd (99.95 wt%) and Pt (99.9 wt%) were used to prepare samples. A series of samples with different alloy compositions, shown in Table 1, were induction melted under an argon atmosphere. A Lepel induction furnace (50 kw) was employed to mix and cast the alloys into rods. Samples were mechanically ground to remove the oxide layer on the surface, followed by a 48-hour homogenization treatment at 1000 C in a Centorr vacuum furnace (<10ÿ6 torr). Some of these samples were machined to rods with uniform diameter for thermal and mechanical properties testing and for single crystal growth. These samples underwent a ®nal 15 min heat treatment at 1000 C in air, followed by air cooling, quenching in water, oil or 3% NaOH to obtain dierent cooling rates. Some of the samples were hot-rolled to determine
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workability and eect of hot-working on the SME behavior. Single crystal rods were grown using the Bridgeman technique. 2.2. Microstructure characterization methods Optical specimens were prepared using standard methods. A solution of FeCl3, HCl and H2O was used to etch CuAlPd alloys. A Zeiss Axiovert 10 microscope with a digital image analysis system was employed to study the austenite/martensite morphology, and to characterize the eects of processing on the microstructure. A Rigaku DMAZ-III and Geiger¯ex X-ray Rigaku Laue Camera were used to determine phase distribution and single-crystal/polycrystal state at room temperature.
3. Results and dicsussion 3.1. Eect of Al content on microstructure in CuAlPd alloys Optical microscopy and SEM were employed to examine the microstructure of the CuAlPd alloys with dierent Al contents. Fig. 1 shows the optical microstructures of CAD004q (9.5 wt% Al), CAD005q (11.2 wt% Al), CAD023q (13.1 wt% Al) and CAD006q (14.6 wt% Al) samples. CAD004q is a fcc a phase alloy according to X-ray analysis and does not show a SMA transition over the temperature range of ÿ50 to 300 C according to DSC and DMTA tests. Its microstructure indicates clearly that this is not a typical martensitic morphology, because of the lack of laths. Alloy
2.3. Mechanical properties and SME properties testing Tensile tests were conducted using a SINTECH testing machine. The loading rate was set at the minimum value of 0.002 cm/min. Tensile sample con®gurations followed ASTM standard E8-89b to prevent errors due to slippage. A cantilever beam bending fatigue machine, developed at Auburn University [6], was used to determine alloy fatigue life and fatigue strength. A Perkin-Elmer DSC7 dierential scanning calorimeter (DSC) was employed for the direct quantitative measurement of the endothermic or exothermic behaviors of the materials, as well as for precise transformation temperature measurements. The scanning rate was set at 10 C /min for all tests. A dynamic mechanical thermal analyzer (DMTA) was also used to determine the transformation temperature. Table 1 Sample identi®cation numbers and corresponding nominal compositionsa Sample ID
CAD001 CAD002 CAD003 CAD004 CAD005 CAD006 CAD007 CAD008 CAD009 CAD011
Composition (wt%)
Sample ID
Cu
Al
Pd
84.5 81.2 80.4 88.2 86.5 82.9 86.6 85.8 83.1 84.6
13.1 13.0 12.9 9.5 11.2 14.6 13.2 13.2 13.0 13.0
2.4 5.8 6.7 2.3 2.3 2.4 0.2 1.0 3.9 2.4
CAD013 CAD015 CAD017 CAD018 CAD019 CAD020 CAD021 CAD022 CAD023
Composition (wt%) Cu
Al
Pd
84.6 84.6 84.1 84.3 84.5 84.7 84.9 84.4 84.4
13.0 13.0 13.1 13.1 13.1 13.1 13.1 13.1 13.1
2.4 2.4 2.8 2.6 2.4 2.2 2.0 2.5 2.3
a Note: In later sections, CADXXXs means the CADXXX single crystal sample obtained by heat treating and water quenching; CADXXXn means the CADXXX polycrystal sample heat-treated to a temperature of 1020 C for 15 min followed by water (20 C) quenching.
Fig. 1. Optical micrographs of (a) Cu-11.2 wt% Al-2.3 wt% Pd (CAD005q), martensite lath structure, (b) Cu-13.1 wt% Al-2.3 wt% Pd (CAD023q), b10 twin martensite, and (c) Cu-14.6 wt% Al-2.3 wt% Pd (CAD006q), dark dendrites g20 .
Z.C. Lin et al. / Intermetallics 8 (2000) 605±611
CAD005q shows a martensitic lath structure. Alloy CAD023q is also composed of a martensitic phase, however its lath is much greater than that for CAD005q. Our X-ray results suggest that the CAD005q shows b10 and g10 phases coexisting, while the CAD023q exhibits pure b10 phase at room temperature and should display larger strain recovery during the martensitic transformation. DAD006q's microstructure shows a dendrite structure that has a g2 complex cubic structure according to X-ray diraction results. This indicates that the upper limit of Al content for CuAlPd alloys with a SME is 14.6 wt%. DSC and DMTA tests show that no transition occurred over a temperature range of ÿ50 to 300 C for CAD004q, however an obvious transition occurred in the CAD023q sample. A combination of
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X-ray, microstructure, DSC and DMTA results have indicated that Al content is the dominant factor controlling the SME in CuAlPd alloys, and the optimum Al content for SME in CuAlPd is about 13 wt% Al. 3.2. Eect of Pd Content on microstructure in CuAlPd alloys Optical microscopy and SEM were employed to study the eect of Pd content on the alloy phase structures. Fig. 2 (a)±(f) are micrographs of the Cu-13 wt% Al-x wt% Pd alloys (0
Fig. 2. Optical micrographs of Cu-13 wt% Al-x wt% Pd Alloys showing variation of microstructure with composition: (a) x=0, CA025n; (b) x=0.2, CDA007q; (c) x=2.2, CAD021q; (d) x=2.3, CAD020q; (e) x=2.4, CAD001q; (f) x=5.8, CAD002q.
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memory capability, is observed, see Fig. 2(a). A Pd addition of 0.2 wt% results in a mixed phase b10 martensite striated region, see Fig. 3(b). With the increase of Pd content to 2.4 wt%, the martensite becomes larger and larger, see Fig. 2(c), (d) and (e). The greatest rough inclination surface relief was observed in sample CAD023q. Samples CAD001 [Fig. 2(e)] and CAD023 [Fig. 2(d)] show large grains and good alignment compared with other samples, therefore these alloys should exhibit better SME. With increased Pd content of 5.8 wt% or more, the twin martensite decreases, as seen in Fig. 2(f), thus their SME should decrease. Based upon microstructural observations, we can predict that the optimal composition for SME in CuAlNi alloy should be around Cu-13 wt%Al-2.3wt % Pd. 3.3. Transformation behavior OF Cu-13 wt%Al-x wt%Pd alloys 3.3.1. Temperature induced martensite transformation CuAlPd alloys, with Pd from 0.0 to 2.8 wt%, were investigated to determine the eect of Pd content on the transformation behavior. Fig. 3 and Table 2 show the changes in the transformation temperatures and transformation hysteresis with respect to changes of Pd content based upon DSC results. Clearly, Cu-13.2 wt% Al-x
wt% Pd alloys undergo a type I martensitic transformation (Ms2.79 wt%. For Pd compositions between 2.0 and 2.6%, the thermoelastic martensitic transformation belongs to a type II (Ms>As) transformation. From Fig. 3, the optimized shape memory alloy (CAD001q: Cu-13.1 wt% Al-2.4 wt% Pd) has a transformation temperature of 180 C. Table 2 also indicates that transformation temperature span (Af-Mf) and hysteresis are a function of Pd content. The hysteresis shows a minimum of 18 C at around Cu-13.1 wt%Al-2.4 wt% Pd. Low hysteresis is desirable when it comes to application of the SMA for temperature regulation. Nitinol which undergoes a type I transformation has a 30 C hysteresis during the transformation, higher than the optimal CuAlPd alloy investigated in this work. Like other SMAs, the properties of CuAlPd shape memory alloys are sensitive to heat treatments. For the optimal composition, the optimal processing condition is 1000 C for 15 min (100 g sample), followed by quenching into 100 C water. This process produced a CuAlPd shape memory alloy with minimum hysteresis and narrow transformation span. Fig. 4 compares the microstructure dierence between an air-cooled sample CAD023n and water quenched sample CAD023q. The results show that the waterquenched process tends to generate lath-shape b10 phase but air-cooling tends to produce a1 dendrites. To optimize the SME in the CuAlPd alloy, two critical factors, the alloy composition and quenching rate of the process, must be carefully considered. 3.4. Mechanical properties
Fig. 3. Transformation temperature as a function of Pd content in Cu-13,2 wt% Al-x wt% Pd alloys.
3.4.1. Tensile test Tensile tests were conducted at temperatures above Af. For alloy CAD019q tested at 230 C with a strain rate of 1.310ÿ4 s, the ultimate tensile strength (UTS) was 607 MPa, upper plateau stress (UP) was 193 MPa, recoverable strain was 4.8% and total elongation was 13%. The less-than-satisfactory mechanical properties are mostly attributed to processing conditions. In particular, inclusions such as oxides and carbides in CuAlPd
Table 2 Transformation temperature and transformation hysteresis Sample
Cu (W/O)
Al (w/O)
Pd (w/O)
Mf
To0
As
Ms
To
Af
T1
Af-Mf
CA025N CAD007q CAD008q CAD021q CAD020q CAD019q CAD018q CAD017q
86.91 86.57 85.82 84.92 84.72 84.52 84.32 84.13
13.09 13.22 13.19 13.08 13.09 13.09 13.08 13.08
0 0.21 0.99 2 2.19 2.39 2.59 2.79
290.2 258 253 87.2 141.4 115.2 115.2 1168
326.1 302.3 294.8 102.7 161.3 138 131.5 215.4
361.9 346.5 336.5 118.1 181.1 160.8 147.7 262.8
317.4 305.5 300 140.6 185.9 182.7 173.1 262.8
364.3 347.25 339.45 151.9 203.9 187.1 179.5 234.3
411.2 389 378.9 163.2 220.3 191.5 185.9 266.8
75 65.9 86.1 21.2 42.7 17.2 17.1 29.3
277.11 244.78 239.81 74.12 128.31 102.11 102.12 154.92
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Fig. 4. Micrograph of CAD023 for (a) air-cooling and (b) water-quenched process.
need to be properly controlled. The obvious advantage of this alloy is its high end-use temperature (250 C), which is much higher than that for Nitinol (100 C). 3.4.2. Fatigue tests Bending fatigue tests were also conducted on CuAlPd alloy for comparison with Nitinol, as shown in Fig. 5. The fatigue characteristics of CuAlPd are comparable with those of Nitinol, but better in the high strain regime. This is signi®cant because CuAlNi alloy is known to have an extremely low fatigue life [7]. 3.4.3. Workability Fig. 6 shows that CuAlPd SMA has excellent hot workability. A 0.6 cm diameter rod (sample CAD001q) was hot rolled at 700 C to the bar shape shown in Fig. 6 without cracking. The cross-section, 1.01.0 cm, corresponds to a 68% reduction. CuAlPd alloys, therefore, show excellent workability that is important in the development of SMAs.
Fig. 5. Strain vs. Nf curves of CuAlPd and Nitinol alloys, at a mean strain of 0.75%.
3.5. Thermal stability Martensite is a metastable phase and, therefore, is subject to degradation by both thermal cycling and aging. Thermal instability is a critical factor limiting applications of many SME alloys. We ®nd in CuAlPd alloys, cycling lowers the internal energy of the material leading to a stabilization of the alloy's temperature dependent properties. Fig. 7 shows how cycling aects the transformation temperature of CuAlPd alloy CAD013q (2.2 wt% Pd). A reduction of nearly 30 in the Mf and As temperature occurs after 5 or fewer cycles between Mf and Af. All transformation temperatures stabilize very rapidly with cycling and very little additional change occurs after 5 cycles. Fig. 8 shows a plot of the transformation temperature versus aging time. No signi®cant change in their SME feature was observed with CuAlPd CAD001q at an aging temperature of 150 C for 1000 h. Cycling and aging experiments
Fig. 6. Polycrystalline CAD021 deformed by hot rolling at 700 C. (Note that a 68% reduction in cross-section are was achieved without cracking.).
have demonstrated that the thermal stability of CuAlPd alloy is acceptable. 3.6. Single crystal growth Increasing the yield stress of CuAlPd SMA is a method that may be used to increase the superelasticity range. Single crystals have a higher yield stress compared to polycrystals, in that weak grain boundaries often contribute to lower ultimate and yield stresses. In our research, CuAlPd alloys were obtained in the single crystal form by using a modi®ed Bridgeman technique. A crystal with a diameter of 0.6 cm and length of 10 cm can be readily grown with no seeding required. Crystal
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growth was conducted in a modi®ed RF induction furnace with an Ar atmosphere protection. The optimized parameters for single crystal growth include a growth temperature of 1125±1150 C with a growth rate of 1.5 cm/h. Fig. 9 is the Laue X-ray pattern for polycrystal and single crystal rod formed from alloy CAD019q.
Fig. 7. Transformation temperature versus cycle number for sample CAd13q.
4. Discussion CuAl alloys have high TTRs in the range of 350± 400 C, yet have unacceptable low thermal stability. Ni can be substituted as a replacement for Cu in CuAlNi alloys and the substitution signi®cantly improves the thermal stability. However, doping with Ni lowers the TTR to 100 C and the alloy exhibits poor workability when CuAlNi is in the polycrystalline state. CuAlPd alloys are superior to CuAlNi with a higher TTR, good workability and thermal stability. Pd has good solid solubility in the base Cu and Al alloys. While Pd has a larger atomic radius than Ni, Pd and Cu radii mismatch does not exceed the 15% limit. Therefore, there is no excessive mismatch of atomic size, which can cause large distortion energy and alteration of normal metallic bonds. b1 phase and b10 phase equilibrium can be achieved over a small range of Pd content, hence the shape memory eect is observed only in this range of equilibrium. Pd content therefore greatly aects microstructure and SME. Substitution of Pd or Ni for Cu increases the internal stress in the alloy and depresses the transformation temperature, because both Pd and Ni have a larger atomic radius than Cu. The internal stress is a major factor aecting the transformation temperature. The internal stress is a function of the volume and concentration of the substitutional atom, such as Ni or Pd. For the CuAlPd system, an equation has been developed to estimate the Ms temperature as a function of alloy composition. The equation is:
Ms C 2070 ÿ 134 wt% Al ÿ 11r3x A wt% Pd
Fig. 8. Aging eect on transformation temperature (sample CAD001q, aging temperature 150 C).
where rx is Pd atom radius in units of angstroms. Transformation temperatures of CuAlPd alloys turn out to be higher than CuAlNi alloys. Thermal stability tests suggest that Pd eectively resists the diusion between Cu and Al atoms and suppresses the CuAl eutectoid formation in the CuAlPd alloys. Additionally, Pd has a greater electronegativity relative to Cu than Ni to Cu,
Fig. 9. Laue X-ray pattern in (a) polycrystal CAD019q, and (b) single crystal CAD019sq.
Z.C. Lin et al. / Intermetallics 8 (2000) 605±611
resulting in good thermal stability and better thermal stability in CuAlPd than in CuAlNi. 5. Conclusions 1. CuAlPd alloys have an austenite transformation temperature range of 115±370 C, and hysteresis range from 17 to 85 C, depending on composition, heat treatment and working process. The optimum composition of CuAlPd SMA occurs at Cu-13.1 wt %Al-2.4 wt %Pd (CAD001q). This alloy has a transformation temperature of 180 C with a 25 C hysteresis which is adjustable by heat treatment. 2. Al content in CuAlPd alloy plays a crucial role in determining phase structure and shape memory eects. The optimum Al content for SME is approximately 13 wt% Al. Pd additions aect the shape and size of the martensite. This directly controls the observed shape memory behavior. An addition of Pd between 2.3 and 2.4 wt% produces martensite with large grains and good alignment, and results in optimum shape memory eects. 3. The CuAlPd alloys have excellent workability and acceptable thermal stability. In contrast to poor
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fatigue life of CuAlNi alloys, the CuAlPd alloys exhibit fatigue properties comparable to Nitinol, with better properties under high cyclic strain applications. 4. Single crystal CuAlPd alloys have been produced using a modi®ed Bridgeman technique. A crystal with a diameter of 0.6 cm and length of 10 cm can be readily grown with no seeding required.
References [1] Wayman CM, Duerig TW. In: Duerig TW, Melton KN, Stockel D, Wayman CM, editors. Engineering aspects of shape memory alloys. London: Butterworth-Heinemann, 1990. p. 3±20. [2] Van Humbeeck J. Mat Res Soc Symp Proc 1992;246:377±8. [3] Hodgson DE. In Metals Handbook. 10th ed. Metals Park: ASM, 1990, vol. 2, p. 900. [4] Wu HM. In: Duerig TW, Melton KN, Stockel D, Wayman CM, editors. Engineering aspects of shape memory alloys. London: Butterworth-Heinemann, 1990. p. 79. [5] Johnson AD, Bush JB, Lurtis AR, Sloun C. Mat Res Soc Symp Proc 1993;276:151±9. [6] Gopal Rajan Rao. Thesis of Master Science Auburn University, Auburn, AL, USA, December, 1988. [7] Elst R, Humbeeck JV, Delaey L. Mater Sci and Tech 1988;4:644±8.
CuAlPd alloys for sensor and actuator applications Z.C. Lin, W. Yu, R.H. Zee, B.A. Chin * Materials Engineering, Auburn University, Auburn, AL 36849, USA
Abstract Currently available shape memory alloys (NiTi, CuAlNi, CuSnAl) lack the high transformation temperatures and long term thermal stability desired in many commercial applications. This paper reports the results of an investigation in which Pd was substituted for Ni to obtain the shape memory ally CuAlPd. The CuAlPd alloys were found to have an austenite transformation temperature range of 115±370 C depending on composition, heat treatment and working process. Optimal shape memory properties were found for a composition of Cu-13.1 wt% A1-2.4 wt% Pd. This alloy has a transformation temperature of 180 C and a recoverable strain of 4.8%. CuAlPd alloys have excellent workability and exhibit fatigue properties comparable to NiTi shape memory alloys. Single crystals of CuAlPd alloys were produced using a modi®ed Bridgeman technique. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Ternary alloy systems; B. Martensitic transformations; B. Phase transformations; B. Twinning; C. Crystal growth; C. Heat treatment; E. Mechanical properties
1. Introduction Shape memory alloys (SMA) have drawn increasing interest from both the academic community and commercial world due to their unique properties. Researchers have reported more than 15 dierent alloys [1] with shape memory eects (SME), but only NiTi, CuAlNi, and CuZnAl alloys are commercially available due to high costs, inadequate mechanical properties or poor reliability of the other SMAs. Sensor and actuator applications, in many cases, require high transformation temperatures (TTR), namely 160±170 C, and good thermal stability. However, of the three commercially available SMAs, none of them meets these requirements. Nitinol (NiTi) works well only below 80 C [2,3]. CuZnAl [2,4] has a low TTR and poor thermal stability. CuAlNi, although it has better thermal stability, has TTRs lower than 100 C [3,5]. This study seeks to develop a new shape memory alloy that has a high TTR and good thermal stability. Pd dissolves well in Cu-Al based alloys readily forming solid solutions and has a similar electron con®guration and atomic radius as Ni. Therefore, Pd may be anticipated to improve the martensitic phase stability by blocking Al migration in the CuAl alloy that causes * Corresponding author. Tel.: +1-334-844-3322; fax: +1-334-8443400. E-mail address: [email protected] (B.A. Chin).
thermal instability. Additionally, CuAlPd is expected to have higher grain boundary adhesion than CuAlNi alloys, thus improving mechanical properties. In light of these hypotheses, CuAlPd alloy was investigated as a possible means of achieving improved TTR and thermal stability of CuAl-based SMAs. 2. Experimental procedures 2.1. Sample fabrication High purity Cu (99.999 wt%), Al (99.99 wt%), Ni (99.99 wt%), Pd (99.95 wt%) and Pt (99.9 wt%) were used to prepare samples. A series of samples with different alloy compositions, shown in Table 1, were induction melted under an argon atmosphere. A Lepel induction furnace (50 kw) was employed to mix and cast the alloys into rods. Samples were mechanically ground to remove the oxide layer on the surface, followed by a 48-hour homogenization treatment at 1000 C in a Centorr vacuum furnace (<10ÿ6 torr). Some of these samples were machined to rods with uniform diameter for thermal and mechanical properties testing and for single crystal growth. These samples underwent a ®nal 15 min heat treatment at 1000 C in air, followed by air cooling, quenching in water, oil or 3% NaOH to obtain dierent cooling rates. Some of the samples were hot-rolled to determine
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00133-8
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workability and eect of hot-working on the SME behavior. Single crystal rods were grown using the Bridgeman technique. 2.2. Microstructure characterization methods Optical specimens were prepared using standard methods. A solution of FeCl3, HCl and H2O was used to etch CuAlPd alloys. A Zeiss Axiovert 10 microscope with a digital image analysis system was employed to study the austenite/martensite morphology, and to characterize the eects of processing on the microstructure. A Rigaku DMAZ-III and Geiger¯ex X-ray Rigaku Laue Camera were used to determine phase distribution and single-crystal/polycrystal state at room temperature.
3. Results and dicsussion 3.1. Eect of Al content on microstructure in CuAlPd alloys Optical microscopy and SEM were employed to examine the microstructure of the CuAlPd alloys with dierent Al contents. Fig. 1 shows the optical microstructures of CAD004q (9.5 wt% Al), CAD005q (11.2 wt% Al), CAD023q (13.1 wt% Al) and CAD006q (14.6 wt% Al) samples. CAD004q is a fcc a phase alloy according to X-ray analysis and does not show a SMA transition over the temperature range of ÿ50 to 300 C according to DSC and DMTA tests. Its microstructure indicates clearly that this is not a typical martensitic morphology, because of the lack of laths. Alloy
2.3. Mechanical properties and SME properties testing Tensile tests were conducted using a SINTECH testing machine. The loading rate was set at the minimum value of 0.002 cm/min. Tensile sample con®gurations followed ASTM standard E8-89b to prevent errors due to slippage. A cantilever beam bending fatigue machine, developed at Auburn University [6], was used to determine alloy fatigue life and fatigue strength. A Perkin-Elmer DSC7 dierential scanning calorimeter (DSC) was employed for the direct quantitative measurement of the endothermic or exothermic behaviors of the materials, as well as for precise transformation temperature measurements. The scanning rate was set at 10 C /min for all tests. A dynamic mechanical thermal analyzer (DMTA) was also used to determine the transformation temperature. Table 1 Sample identi®cation numbers and corresponding nominal compositionsa Sample ID
CAD001 CAD002 CAD003 CAD004 CAD005 CAD006 CAD007 CAD008 CAD009 CAD011
Composition (wt%)
Sample ID
Cu
Al
Pd
84.5 81.2 80.4 88.2 86.5 82.9 86.6 85.8 83.1 84.6
13.1 13.0 12.9 9.5 11.2 14.6 13.2 13.2 13.0 13.0
2.4 5.8 6.7 2.3 2.3 2.4 0.2 1.0 3.9 2.4
CAD013 CAD015 CAD017 CAD018 CAD019 CAD020 CAD021 CAD022 CAD023
Composition (wt%) Cu
Al
Pd
84.6 84.6 84.1 84.3 84.5 84.7 84.9 84.4 84.4
13.0 13.0 13.1 13.1 13.1 13.1 13.1 13.1 13.1
2.4 2.4 2.8 2.6 2.4 2.2 2.0 2.5 2.3
a Note: In later sections, CADXXXs means the CADXXX single crystal sample obtained by heat treating and water quenching; CADXXXn means the CADXXX polycrystal sample heat-treated to a temperature of 1020 C for 15 min followed by water (20 C) quenching.
Fig. 1. Optical micrographs of (a) Cu-11.2 wt% Al-2.3 wt% Pd (CAD005q), martensite lath structure, (b) Cu-13.1 wt% Al-2.3 wt% Pd (CAD023q), b10 twin martensite, and (c) Cu-14.6 wt% Al-2.3 wt% Pd (CAD006q), dark dendrites g20 .
Z.C. Lin et al. / Intermetallics 8 (2000) 605±611
CAD005q shows a martensitic lath structure. Alloy CAD023q is also composed of a martensitic phase, however its lath is much greater than that for CAD005q. Our X-ray results suggest that the CAD005q shows b10 and g10 phases coexisting, while the CAD023q exhibits pure b10 phase at room temperature and should display larger strain recovery during the martensitic transformation. DAD006q's microstructure shows a dendrite structure that has a g2 complex cubic structure according to X-ray diraction results. This indicates that the upper limit of Al content for CuAlPd alloys with a SME is 14.6 wt%. DSC and DMTA tests show that no transition occurred over a temperature range of ÿ50 to 300 C for CAD004q, however an obvious transition occurred in the CAD023q sample. A combination of
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X-ray, microstructure, DSC and DMTA results have indicated that Al content is the dominant factor controlling the SME in CuAlPd alloys, and the optimum Al content for SME in CuAlPd is about 13 wt% Al. 3.2. Eect of Pd Content on microstructure in CuAlPd alloys Optical microscopy and SEM were employed to study the eect of Pd content on the alloy phase structures. Fig. 2 (a)±(f) are micrographs of the Cu-13 wt% Al-x wt% Pd alloys (0
Fig. 2. Optical micrographs of Cu-13 wt% Al-x wt% Pd Alloys showing variation of microstructure with composition: (a) x=0, CA025n; (b) x=0.2, CDA007q; (c) x=2.2, CAD021q; (d) x=2.3, CAD020q; (e) x=2.4, CAD001q; (f) x=5.8, CAD002q.
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memory capability, is observed, see Fig. 2(a). A Pd addition of 0.2 wt% results in a mixed phase b10 martensite striated region, see Fig. 3(b). With the increase of Pd content to 2.4 wt%, the martensite becomes larger and larger, see Fig. 2(c), (d) and (e). The greatest rough inclination surface relief was observed in sample CAD023q. Samples CAD001 [Fig. 2(e)] and CAD023 [Fig. 2(d)] show large grains and good alignment compared with other samples, therefore these alloys should exhibit better SME. With increased Pd content of 5.8 wt% or more, the twin martensite decreases, as seen in Fig. 2(f), thus their SME should decrease. Based upon microstructural observations, we can predict that the optimal composition for SME in CuAlNi alloy should be around Cu-13 wt%Al-2.3wt % Pd. 3.3. Transformation behavior OF Cu-13 wt%Al-x wt%Pd alloys 3.3.1. Temperature induced martensite transformation CuAlPd alloys, with Pd from 0.0 to 2.8 wt%, were investigated to determine the eect of Pd content on the transformation behavior. Fig. 3 and Table 2 show the changes in the transformation temperatures and transformation hysteresis with respect to changes of Pd content based upon DSC results. Clearly, Cu-13.2 wt% Al-x
wt% Pd alloys undergo a type I martensitic transformation (Ms
Fig. 3. Transformation temperature as a function of Pd content in Cu-13,2 wt% Al-x wt% Pd alloys.
3.4.1. Tensile test Tensile tests were conducted at temperatures above Af. For alloy CAD019q tested at 230 C with a strain rate of 1.310ÿ4 s, the ultimate tensile strength (UTS) was 607 MPa, upper plateau stress (UP) was 193 MPa, recoverable strain was 4.8% and total elongation was 13%. The less-than-satisfactory mechanical properties are mostly attributed to processing conditions. In particular, inclusions such as oxides and carbides in CuAlPd
Table 2 Transformation temperature and transformation hysteresis Sample
Cu (W/O)
Al (w/O)
Pd (w/O)
Mf
To0
As
Ms
To
Af
T1
Af-Mf
CA025N CAD007q CAD008q CAD021q CAD020q CAD019q CAD018q CAD017q
86.91 86.57 85.82 84.92 84.72 84.52 84.32 84.13
13.09 13.22 13.19 13.08 13.09 13.09 13.08 13.08
0 0.21 0.99 2 2.19 2.39 2.59 2.79
290.2 258 253 87.2 141.4 115.2 115.2 1168
326.1 302.3 294.8 102.7 161.3 138 131.5 215.4
361.9 346.5 336.5 118.1 181.1 160.8 147.7 262.8
317.4 305.5 300 140.6 185.9 182.7 173.1 262.8
364.3 347.25 339.45 151.9 203.9 187.1 179.5 234.3
411.2 389 378.9 163.2 220.3 191.5 185.9 266.8
75 65.9 86.1 21.2 42.7 17.2 17.1 29.3
277.11 244.78 239.81 74.12 128.31 102.11 102.12 154.92
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Fig. 4. Micrograph of CAD023 for (a) air-cooling and (b) water-quenched process.
need to be properly controlled. The obvious advantage of this alloy is its high end-use temperature (250 C), which is much higher than that for Nitinol (100 C). 3.4.2. Fatigue tests Bending fatigue tests were also conducted on CuAlPd alloy for comparison with Nitinol, as shown in Fig. 5. The fatigue characteristics of CuAlPd are comparable with those of Nitinol, but better in the high strain regime. This is signi®cant because CuAlNi alloy is known to have an extremely low fatigue life [7]. 3.4.3. Workability Fig. 6 shows that CuAlPd SMA has excellent hot workability. A 0.6 cm diameter rod (sample CAD001q) was hot rolled at 700 C to the bar shape shown in Fig. 6 without cracking. The cross-section, 1.01.0 cm, corresponds to a 68% reduction. CuAlPd alloys, therefore, show excellent workability that is important in the development of SMAs.
Fig. 5. Strain vs. Nf curves of CuAlPd and Nitinol alloys, at a mean strain of 0.75%.
3.5. Thermal stability Martensite is a metastable phase and, therefore, is subject to degradation by both thermal cycling and aging. Thermal instability is a critical factor limiting applications of many SME alloys. We ®nd in CuAlPd alloys, cycling lowers the internal energy of the material leading to a stabilization of the alloy's temperature dependent properties. Fig. 7 shows how cycling aects the transformation temperature of CuAlPd alloy CAD013q (2.2 wt% Pd). A reduction of nearly 30 in the Mf and As temperature occurs after 5 or fewer cycles between Mf and Af. All transformation temperatures stabilize very rapidly with cycling and very little additional change occurs after 5 cycles. Fig. 8 shows a plot of the transformation temperature versus aging time. No signi®cant change in their SME feature was observed with CuAlPd CAD001q at an aging temperature of 150 C for 1000 h. Cycling and aging experiments
Fig. 6. Polycrystalline CAD021 deformed by hot rolling at 700 C. (Note that a 68% reduction in cross-section are was achieved without cracking.).
have demonstrated that the thermal stability of CuAlPd alloy is acceptable. 3.6. Single crystal growth Increasing the yield stress of CuAlPd SMA is a method that may be used to increase the superelasticity range. Single crystals have a higher yield stress compared to polycrystals, in that weak grain boundaries often contribute to lower ultimate and yield stresses. In our research, CuAlPd alloys were obtained in the single crystal form by using a modi®ed Bridgeman technique. A crystal with a diameter of 0.6 cm and length of 10 cm can be readily grown with no seeding required. Crystal
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growth was conducted in a modi®ed RF induction furnace with an Ar atmosphere protection. The optimized parameters for single crystal growth include a growth temperature of 1125±1150 C with a growth rate of 1.5 cm/h. Fig. 9 is the Laue X-ray pattern for polycrystal and single crystal rod formed from alloy CAD019q.
Fig. 7. Transformation temperature versus cycle number for sample CAd13q.
4. Discussion CuAl alloys have high TTRs in the range of 350± 400 C, yet have unacceptable low thermal stability. Ni can be substituted as a replacement for Cu in CuAlNi alloys and the substitution signi®cantly improves the thermal stability. However, doping with Ni lowers the TTR to 100 C and the alloy exhibits poor workability when CuAlNi is in the polycrystalline state. CuAlPd alloys are superior to CuAlNi with a higher TTR, good workability and thermal stability. Pd has good solid solubility in the base Cu and Al alloys. While Pd has a larger atomic radius than Ni, Pd and Cu radii mismatch does not exceed the 15% limit. Therefore, there is no excessive mismatch of atomic size, which can cause large distortion energy and alteration of normal metallic bonds. b1 phase and b10 phase equilibrium can be achieved over a small range of Pd content, hence the shape memory eect is observed only in this range of equilibrium. Pd content therefore greatly aects microstructure and SME. Substitution of Pd or Ni for Cu increases the internal stress in the alloy and depresses the transformation temperature, because both Pd and Ni have a larger atomic radius than Cu. The internal stress is a major factor aecting the transformation temperature. The internal stress is a function of the volume and concentration of the substitutional atom, such as Ni or Pd. For the CuAlPd system, an equation has been developed to estimate the Ms temperature as a function of alloy composition. The equation is:
Ms C 2070 ÿ 134 wt% Al ÿ 11r3x A wt% Pd
Fig. 8. Aging eect on transformation temperature (sample CAD001q, aging temperature 150 C).
where rx is Pd atom radius in units of angstroms. Transformation temperatures of CuAlPd alloys turn out to be higher than CuAlNi alloys. Thermal stability tests suggest that Pd eectively resists the diusion between Cu and Al atoms and suppresses the CuAl eutectoid formation in the CuAlPd alloys. Additionally, Pd has a greater electronegativity relative to Cu than Ni to Cu,
Fig. 9. Laue X-ray pattern in (a) polycrystal CAD019q, and (b) single crystal CAD019sq.
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resulting in good thermal stability and better thermal stability in CuAlPd than in CuAlNi. 5. Conclusions 1. CuAlPd alloys have an austenite transformation temperature range of 115±370 C, and hysteresis range from 17 to 85 C, depending on composition, heat treatment and working process. The optimum composition of CuAlPd SMA occurs at Cu-13.1 wt %Al-2.4 wt %Pd (CAD001q). This alloy has a transformation temperature of 180 C with a 25 C hysteresis which is adjustable by heat treatment. 2. Al content in CuAlPd alloy plays a crucial role in determining phase structure and shape memory eects. The optimum Al content for SME is approximately 13 wt% Al. Pd additions aect the shape and size of the martensite. This directly controls the observed shape memory behavior. An addition of Pd between 2.3 and 2.4 wt% produces martensite with large grains and good alignment, and results in optimum shape memory eects. 3. The CuAlPd alloys have excellent workability and acceptable thermal stability. In contrast to poor
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fatigue life of CuAlNi alloys, the CuAlPd alloys exhibit fatigue properties comparable to Nitinol, with better properties under high cyclic strain applications. 4. Single crystal CuAlPd alloys have been produced using a modi®ed Bridgeman technique. A crystal with a diameter of 0.6 cm and length of 10 cm can be readily grown with no seeding required.
References [1] Wayman CM, Duerig TW. In: Duerig TW, Melton KN, Stockel D, Wayman CM, editors. Engineering aspects of shape memory alloys. London: Butterworth-Heinemann, 1990. p. 3±20. [2] Van Humbeeck J. Mat Res Soc Symp Proc 1992;246:377±8. [3] Hodgson DE. In Metals Handbook. 10th ed. Metals Park: ASM, 1990, vol. 2, p. 900. [4] Wu HM. In: Duerig TW, Melton KN, Stockel D, Wayman CM, editors. Engineering aspects of shape memory alloys. London: Butterworth-Heinemann, 1990. p. 79. [5] Johnson AD, Bush JB, Lurtis AR, Sloun C. Mat Res Soc Symp Proc 1993;276:151±9. [6] Gopal Rajan Rao. Thesis of Master Science Auburn University, Auburn, AL, USA, December, 1988. [7] Elst R, Humbeeck JV, Delaey L. Mater Sci and Tech 1988;4:644±8.