Bulk Shape Memory NiTi with Refined Grain Size Synthesized by Mechanical Alloying

Transcription

1 Bulk Shape Memory NiTi with Refined Grain Size Synthesized by Mechanical Alloying Wendy C. Crone 1, Alief N. Yahya 1, and John H. Perepezko 2 1 Department of Engineering Physics, University of Wisconsin, Madison, WI 53706, USA 2 Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706, USA Keywords: NiTi, shape memory, pseudoelasticity, metal-metal composite Abstract. Shape memory alloys (SMAs) constitute a unique class of materials that undergo a reversible phase transformation allowing the material to display dramatic stress-induced and temperature-induced deformations which are recoverable. Nickel Titanium (NiTi) SMA has be synthesized with refined grain size by a mechanical alloying process. The SMA is produced through a cold rolling fabrication strategy that yields nanocrystalline product structures. By starting with a stacked sandwich array of individual alloy components and subjecting this sample arrangement to repeated rolling and folding, a metal-metal multilayer composite is created. After the layer thickness is reduced to the nanoscale by multiple cold rolling and folding passes, an alloying and intermediate phase formation reaction is initiated. X-ray diffraction profiles confirm that the primary phase present is intermetallic NiTi phase, which is the phase that displays shape memory behavior. The presence of superelastic behavior and the shape memory effect in the product material is established by mechanical testing. At this refined grain size, the hysteresis stress observed is exceptionally small. Introduction During repeated plastic deformation as in ball milling or cold rolling, a concomitant refinement of grain size is observed. Recently, a new variation of the strategy has been developed which can yield nanocrystalline product structures [1-3]. By starting with a stacked sandwich array of elemental material and subjecting this sample arrangement to repeated rolling and folding, the resulting layers may be readily reduced to the nanometer size scale in bulk form before initiating an alloying and intermediate phase formation reaction [1-4]. One goal of the current research is to assess the suitability of a multilayer cold rolling effort for the preparation of Ni-Ti laminate composites with nanoscale layer thickness. If alloys with suitably refined layered thickness can be produced with this method, the composite material can be further processed to produce an alloyed material with refined grain structure. Similar preparation methods have been employed on elemental foils [5] and composite powders [6] of nickel and titanium, but the compositions investigated were well away from equiatomic NiTi and thus these materials did not display shape memory behavior. Shape memory alloys constitute a unique class of materials that have already proven to have wide ranging applications in industries ranging from aerospace to biomedical. These materials have a crystallographic structure that can change reversibly and reproducibly, allowing the material to display dramatic stress-induced and temperature-induced recoverable deformations. The behavior of NiTi SMA is governed by a phase transformation between austenite and martensite crystal structures. Transformation between the austenite (B2) and martensite (B19') phases can be produced by temperature cycling between the high temperature austenite phase and the low temperature martensite phase (shape memory effect), or loading the material to favor the high strain martensite phase or unloading to favor the low strain austenite phase (superelasticity). 1

2 Another complimentary goal of the research reported here is to explore the impact of small grain size on shape memory properties in NiTi. Various researchers have correlated the reduction in grain size in SMAs to changes and improvements in the shape memory behavior of the material. Smaller grain size have been shown to increase the fatigue life, increase the superelastic and superplastic behavior at high temperatures, and improve workability in Cu-based alloys such as CuZnAl and CuAlNi [7-9]. In NiTi alloys, correlation between the grain size and such factors as the transformation temperatures, transformation stress, transformation modulus, and hysteresis stress has also been observed experimentally [10-11]. The cold rolling fabrication technique described here allows exploration of the effect of highly reduced grain size on the mechanical properties of NiTi SMA. Experimental Technique By starting with a stacked sandwich array of individual alloy components and subjecting this sample arrangement to repeated rolling and folding, the resulting layers are reduced to a nanoscale size in bulk form before initiating an alloying and intermediate phase formation reaction. The level of plastic strain achieved by this technique can not be developed with a simple rolling operation using alloyed NiTi because the material work hardens rapidly. Although up to 40% reduction in thickness is common in cold rolling of alloyed NiTi [12,14], the rolling technique used in this research reduces the elemental layer thickness by greater than 99% without annealing. The significant refinement in the Ni and Ti layer thicknesses achieved with this cold rolling technique is obtained through a fabrication strategy that begins with layered foils of Ni and Ti (purity, 99.0% or greater). Elemental material was weighed out in a ratio of Ni:Ti = 55:45 from foils of mm and 0.032mm thickness, respectively. The stacked foils weighing a total of 0.7 g were folded in quarters and cold rolled directly between the steel rollers of the mill. Each folding and rolling (F&R) pass consisted of folding the foil to double its thickness and rolling it to 50% reduction. The average layer thickness in the samples produced was 600 nm, 90 nm and 60 nm after 20, 30 and 40 passes, respectively. Fig. 1 shows SEM images of a transverse cross-section for representative samples. At this point in the processing an annealing treatment was conducted to initiate a phase formation reaction between the Ni and Ti layers in the metal-metal composite. Based on exothermic peaks observed in differential scanning calorimetry (DSC) measurements conducted on 40 F&R samples, an annealing temperature of 530 C was chosen. After 4 hours, the resulting material was found to be the equiatomic NiTi phase which is the phase that displays the shape memory effect upon aging. Analysis of the STOE X-ray diffractometry (XRD) results confirmed that the only phase present was that of NiTi. No significant levels of elemental Ni, elemental Ti, or other undesirable product phases were identified. In order to tune the A f transformation temperature (the temperature at which complete transformation to the austenitic phase is reached), the annealing was followed by an aging heat treatment at 300 C for 1 hour. The average grain size was determined to be 1-3µm using scanning electron microscopy on etched samples, compared to an average grain size of 20µm in most commercial alloys. The A f temperature was found to be 55 C by DSC. Results and Discussion Factors Influencing Layer Refinement in Ni-Ti Multilayer Composite. One of the keys to grain refinement through this processing technique is the creation of exceptionally refined layers in the 2

3 Ni-Ti multilayer composite prior to heat treatment. However, refinement of multilayer composites created by cold rolling is limited by a number of factors. The most critical of these are the Boudinage effect and excessive work hardening. The Boudinage effect is characterized by necking followed by pinching of layers and island formation during the rolling process which has the effect of joining two previously separate layers of material. This diminishes the efficiency of subsequent rolling passes making further layer thickness refinement difficult to achieve without a substantially larger number of rolling passes. A logarithmic dependence of the average layer thickness with the number of rolling passes is observed with most material combinations. Fig. 2 shows this behavior for several different metal-metal combinations as well as the results for the Ni-Ti composite created. 4 µm 4 µm Figure 1: The backscatter SEM images are shown for transverse cross sections of Ni-Ti multilayer composites at different stages in the rolling process. Dark layers are Ti and light layers are Ni. A composite created by 20 fold and roll passes with an average single layer thickness of 600 nm is shown on the left. With 40 fold and roll passes the average single layer thickness is reduced to 60 nm as shown in the micrograph on the right. For optimal grain refinement, uniform reduction in layer thickness with each rolling pass is desired. In many metal-metal systems this is not observed because of necking instabilities and rupturing of layers. The inefficiency created by these effects is detrimental to the refinement process and only certain metal-metal combinations are well suited for refinement by cold rolling. Bordeaux and Yavari [15] have shown that materials which display multiple necking during processing are well suited for multilayer refinement by cold rolling because fracture of layers or highly localized necking resulting in pinching is reduced. They conclude that a high level of multiple necking occurs when the rupture stress for the constituent materials are close in value to each other. This research has shown that nickel and titanium are amenable to this technique. Furthermore the hypothesis that similar rupture stress of constituent materials promotes layer refinement is supported by this research; pure nickel and pure titanium are both reported to have ultimate tensile strengths near 350 MPa [16]. In addition to uneven layer refinement during processing, work hardening of the material as a result of multiple cold rolling passes also becomes a hindrance. The work hardening effect is not only limiting because of the large forces that must be applied during rolling, but more importantly because of explosions that destroy samples rolled beyond 50F&R passes. Bordeaux et al [5] also 3

4 observed explosions after significant rolling of layered Cu-Ti and Ni-Zr samples. This occurs because further processing work hardens the material and promotes the formation of cracks. The oxidation of newly exposed strongly oxidizing elements at cracks (such as Ti), creates local hot spots with temperatures sufficient to induce the exothermic mixing reaction between the constitutive elements leading to an explosion. Often annealing is used after cold rolling to allow for further processing, however this is problematic for the Ni-Ti composite since undesirable phase products can easily be produced that impede creation of the desired shape memory product material Cu 50 Al 50wt% Ni 55 Ti 45wt% Cu 64 Zr 36wt% ln(t o /t n ) Pd 50 Sn 50wt% Al 50 Nb 50wt% Number of Passes, n Figure 2: Number of rolling and folding passes versus change in layer thickness for various metal-metal multilayer composites is shown (where t 0 is the initial average layer thickness and t n is the average layer thickness after n rolling passes). Experimental data points from Ni-Ti samples are shown with a linear best-fit line for Ni-Ti. Results for other material combinations are taken from Bordeaux and Yavari [15]. Mechanical Properties of Alloyed Material. The mechanical properties of the cold rolled samples were characterized by tensile testing using an Instron 5566 testing machine after heat treatment. Tension samples with 1.8 mm width and 6 mm gauge length were machined by electrical discharge machining (EDM). The sample was maintained at a temperature of 60 C during testing using temperature-controlled grips. Typical stress-strain curves for the refined grained NiTi fabricated via cold rolling are shown in Fig. 3. The three cycles shown in the graph were conducted without removal of the samples from the grips. Loading to 6% strain was conducted at a displacement rate of 0.05mm/min, immediately followed by unloading until the stress returned to zero. Between cycles, full recovery of the deformation was achieved after 15 minutes at an elevated temperature while the Instron software actively held the displacement at zero. 4

5 Pseudoelasticity and full deformation recovery after straining to 6% was displayed in repeated testing of tensile samples. Transformation was observed to occur with a distinct nucleation event followed by a flat propagation curve. The forward (austenite to martensite) transformation curve looks typical with the possible exception of the low stress level at which it occurs. Transformation begins at 7 MPa, indicating the onset of martensitic transformation. This is followed by a stress drop to 4.5 MPa at which propagation of the transformation front proceeds. The difference is less than 3 MPa, which is typical for NiTi alloys. The low transformation stress exhibited in Fig. 3 is attributed to the test temperature being very near A f (approximately 5 C difference). Even though Stress (MPa) Strain (mm/mm) the transformation stress is quite low, the entire hysteresis loop can be observed in this material because the hysteresis stress is exceptionally small. Figure 3: Stress/strain diagram of repeated tensile tests on a sample created by mechanical alloying with 40 fold and roll passes and annealed at 530 C. Loading and unloading paths are indicated with arrows. Recovery to zero strain was achieved by heating between each of the three cycles. The most unusual feature of the pseudoelastic behavior observed in this SMA is its hysteresis stress. The difference in transformation stress for the forward (austenite to martensite) and the reverse (martensite to austenite) transformation is less than 1 MPa over the majority of the test. In contrast, the hysteresis stress is typically much greater than 100 MPa in commercially produced NiTi. The magnitude of the hysteresis stress observed here is unusual for a polycrystalline material; this type of behavior is usually only observed for certain crystallographic orientations of single crystal SMA. It is hypothesized that the small hysteresis stress observed in this refined grained alloy occurs because of a decrease in the forward transformation stress combined with an increase in the reverse transformation stress. Studies of TiNiCu alloys have reported that martensites nucleate on dislocations [17]. Grain boundaries seem to favor the initiation of the martensitic transformation. Additionally, grain boundaries appear to obstruct the reverse transformation based on experimental results reported for NiTi and CuZnAl [10-11]. As the grain boundary area increases with decreasing grain size, the combination of the above effects translates to a smaller hysteresis stress. 5

6 Conclusion Ni-Ti metal-metal multilayer composite material with nanoscale layer thickness has been successfully produced by mechanical alloying via cold rolling. Layer refinement to an average thickness of 60 nm has been demonstrated. Further refinement of the layer thickness prior to heat treatment is diminished in efficiency by the Boudinage effect and limited by excessive work hardening. The Ni-Ti composite material has been used to obtain NiTi SMA, providing an alternative method for fabricating SMAs with refined grain structure. Through subsequent heat treatment of the Ni-Ti composite, an average grain diameter under 3µm can be consistently produced. This method holds promise for further refining the grain structure to the nanoscale regime, however this depends almost exclusively on the prevention of grain growth during annealing. Mechanical testing confirms the presence of superelastic behavior and the shape memory effect in this material. At the highly refined grain sizes produced in this research, exceptionally low hysteresis stress is observed. Acknowledgements This research was supported by a Seed Grant from the Materials Research Science and Engineering Center (MRSEC) on Nanostructured Materials and Interfaces (DMR ) at the University of Wisconsin - Madison and by the University of Wisconsin Graduate School. Continued research on refined grain SMA has been funded by the Energy Efficiency Science Initiative of the Department of Energy (DE-FC36-01GO11055). References [1] J.H. Perepezko, J.S. Park, K. Landry, H. Sieber, M.H. da Silva Bassani and A.S. Edelstein: Mater. Res. Soc. Symp. Proc. Vol. 481 (1998), p [2] H. Sieber and J.H. Perepezko: Mater. Res. Soc. Symp. Proc. Vol. 481 (1998), p [3] H. Sieber and J.H. Perepezko: Materials Processing Fundamentals, B. Mishra editor, (The Minerals, Metals & Materials Society, Warrendale, PA, 1998) p [4] A. Sagel, H. Sieber, H.J. Fecht and J. H. Perepezko: Acta Mater. Vol. 46, No. 12 (1998), p [5] F. Bordeaux, A.R. Yavari and P. Desre: Mater. Sci. Eng. Vol. 97 (1988), p [6] T.D. Shen, M.X. Quan and J.T. Wang: J. Mater. Sci. Vol. 28 (1993), p [7] H. Funakubo editor: Shape Memory Alloys (Gordon and Breach Science Publishers, New York, J. B. Kennedy, translator, 1986). [8] D.W. Roh, J. W. Kim, T. J. Cho and Y. G. Kim: Mater. Sci. Eng. A: Struct. Mater. Prop. Microstruct. Process. Vol. A136 (1991), p. 17. [9] J.W. Kim, E.S. Lee, T.J. Cho and Y.G. Kim: J. Mater. Sci. Lett. Vol. 9, No. 4 (1990), p [10] F.J. Gil and J. M. Guilemany: J. Mater. Sci. Lett. Vol. 12, No. 1 (1993), p. 6. [11] F.J. Gil, J. M. Manero and J. A. Planell: J. Mater. Sci. Vol. 30, No. 10 (1995), p [12] D. Hodgson and S. Russell: Min. Invas. Ther. & Allied Technol. Vol. 9, No. 2 (2000), p. 61. [13] A.R. Pelton, J. DiCello and S. Miyazaki: Min. Invas. Ther. & Allied Technol. Vol. 9, No. 2 (2000), p [14] H.C. Lin, S.K. Wu, T.S. Chou and H.P. Kao: Acta Metall. Mater. Vol. 39, No. 9 (1991), p [15] F. Bordeaux and R. Yavari: Z. Metallkde. Vol. 81, No. 2 (1990), p [16] E.A. Brandes and G.B. Brook, editors: Smithells Metals Reference Book (Butterworth- Heinemann, Jordan Hill, Oxford, England 1998). [17] Tokarev, V. N. and Y. F. Dudarev, Phys. Metal. Metallography, Vol. 68, No. 2 (1989), p