The ultrahigh mechanical energy-absorption

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Jul 13, 2011 - ($16 MJ/m3) found in the well-known Ni-Ti alloys. In-situ synchrotron x-ray diffraction reveals that a redistribution of stress between the ...
APPLIED PHYSICS LETTERS 99, 024102 (2011)

The ultrahigh mechanical energy-absorption capability evidenced in a high-strength NbTi/NiTi nanocomposite S. J. Hao,1 L. S. Cui,1,a) Y. D. Wang,2 D. Q. Jiang,1 C. Yu,1 J. Jiang,1 D. E. Brown,3 and Y. Ren4

1 Department of Materials Science and Engineering, China University of Petroleum-Beijing, Beijing 102249, China 2 School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China 3 Department of Physics, Northern Illinois University, De Kalb, Illinois 60115, USA 4 X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

(Received 25 February 2011; accepted 22 June 2011; published online 13 July 2011) A nanocomposite composed of NbTi nanowires uniformly embedded in NiTi matrices was fabricated, which exhibits an ultrahigh mechanical-damping capability. The absorption energy measured under an applied 8% strain is up to 54 MJ/m3, which is over three times higher than that (16 MJ/m3) found in the well-known Ni-Ti alloys. In-situ synchrotron x-ray diffraction reveals that a redistribution of stress between the nanowires and matrices was evidenced from an abrupt change in residual lattice strains. The ultrahigh mechanical-damping property is attributed to a combination of the strong interaction of nanowires and matrices and the plastic deformation C 2011 American occurring in NbTi nanowires during deformation causing large energy dissipation. V Institute of Physics. [doi:10.1063/1.3610562] The ability of shape memory alloys (SMAs) to absorb shock energy or damp mechanical vibration is of significant importance due to the broad applications as the functional components of aircraft, architectures, bridges, etc.1–5 Previous investigations have revealed that the amount of energy absorption during the reversible deformation-induced phase transformation of the SMAs is mainly related to two aspects: frictional energy dissipation and elastic strain energy relaxation.6–10 The frictional energy is spent to overcome the frictional resistance to interfacial motion between parent phase/ martensitic phase or twinning interface, and the elastic strain energy dissipation occurs when plastic relaxation occurs at the phase boundaries due to the interaction of transformation interface and structural defects.11,12 We herein report a nanocomposite with a high strength (>1 GPa), composed of NbTi nanowires uniformly embedded in NiTi matrices, exhibits an ultrahigh mechanical-damping capability. Synchrotron x-ray diffraction during in-situ loading/unloading evidenced is an excellent tool to investigate phase transformation behavior and lattice strain evolution in active materials such as Ni-Ti alloys.13 Using this technique, we demonstrate that a redistribution of stress between nanowires and matrices occurs during the reversible deformationinduced transformation in the nanocrystaline NiTi matrices. The ultrahigh mechanical-damping property of the nanocomposite may be explained by the strong interaction of the nanowires and matrices during the reversible deformation-induced transformation causing large energy dissipation. Alloy ingot (7 kg) of Ni41Ti39Nb20 (atomic composition, at. %) was fabricated by vacuum induction melting, then it was hot forged at 850  C into rods with 8 mm in diameter. The hot-forged rod was further hot drawn at 750  C into wires with 1 mm in diameter and then cold drawn at room a)

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temperature into thin wires of 0.58 mm in diameter. The test samples were cut from the thin wire and subsequently annealed at 350  C for 20 min followed by air cooling. In-situ high-energy x-ray diffraction measurements during loading/unloading were performed at room temperature at the 11-ID-C beamline of the advanced photon source. Highenergy x-rays with a beam size of 0.4 mm  0.4 mm and ˚ were used to obtain two-dimenwavelength of 0.10798 A sional (2D) diffraction patterns in the transmission geometry. The transmission electron microscope (TEM) investigations reveal that the nanocomposite is composed of NbTi nanowires evenly dispersed and well aligned in the nanocrystalline NiTi matrices along the wire axis, as shown in Figs. 1(a) and 1(b). The corresponding selected area electron diffraction pattern (SAEDP) inserted in Fig. 1(a) indicates that the [110] direction of NbTi nanowires is well oriented along the wire axial direction. The composition of the NbTi nanowires measured by an energy dispersive x-ray analyzer (EDX) contains 80.43Nb, 16.44Ti, and 3.13Ni (at. %). The volume fractions of NbTi nanowires and NiTi matrices are about 20 % and 80 %, respectively. The average diameter and interspacing of the NbTi nanowires and grain size of the NiTi matrices are 78, 82, and 30 nm, respectively, as shown in Figs. 1(c) and 1(d). The tensile stress-strain curves of the nanocomposite and a commercial Ni-Ti superelastic wire (Ni-49.2 at. % Ti and the average grain size: 20 lm) are shown in Fig. 2(a). The nanocomposite exhibits a high critical stress (1000 MPa) for deformation-induced transformation upon loading and a low critical stress (210 MPa) for reverse martensitic transformation upon unloading, which are much higher and lower than that of the commercial Ni-Ti wire (rMs  600 MPa and rAs  370 MPa), respectively. Plotted in Fig. 2(b) is the absorbed energy by a pseudoelastic cycle per unit volume as a function of applied tensile strain for the nanocomposite, in comparison with that found in other SMAs. The mechanicaldamping energy absorbed by a complete pseudoelastic cycle

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FIG. 1. (Color online) (a-b) TEM micrographs of longitudinal-section and crosssection of the nanocomposite wire (NbTi nanowires marked by “NbTi”); The inset is the corresponding SAEDP, (c-d) The statistical distribution in diameter of NbTi nanowires and in grain size of NiTi matrices based on the TEM measurements.

for the nanocomposite with applied to 8% strain is about 54 MJ/m3, which is over three times larger than that found in Ni-Ti alloy (16 MJ/m3 in 8% cycle) and is also almost twice as large as that found in the Fe-28Ni-17Co-11.5Al-2.5Ta0.05B alloy, which shows the largest energy absorption of all SMAs (35 MJ/m3 in 8% cycle).14 We offer the following rationalization for this high strength of the nanocomposite upon loading. As shown in Figs. 1(a) and 1(b), the diameter size (78 nm) and interspacing (82 nm) are small enough for the nanowires to become effective barriers for the shear transformation and the movement of dislocations in NiTi matrices. Another potential reason could be the ultrafine grain size dependence of TiNi matrices. Previous studies indicated that the martensitic transformation was strongly suppressed upon decreasing grain size less than 100 nm in Ni-Ti alloys. In the nanograins, the energy barrier of transformation from austenite to martensite increases with decreasing grain size and causes a decrease of both transformation temperature and transformed volume fraction.15,16 Therefore, it is considered that the ultrafine grain size of NiTi matrices may significantly raise

the critical stress for stress-induced transformation contrasting to the Ni-Ti with micrometer-level grain size. A section of diffraction patterns for the sample at 6% strain, collected in 0.5 mm step-wise along the wire, is shown in Fig. 3(a). The result indicates that the deformationinduced transformation in the nanocomposite proceeds in a localized manner, where martensitic regions are first nucleated near the grips and then propagate through the gage length at a constant stress. The comparison of the 2D diffraction patterns from non-transformation region and martensitic transformation region of the sample at 6% strain is shown in Figs. 3(b) and 3(c). The evolutions of the interplane spacing of NbTi (110) perpendicular to the wire axis in the whole sample for three different tensile states (before loading, at 6% strain and after unloading) are shown in Fig. 3(d). For the sample at 6% strain, the NbTi (110) spacing at both ends (transformation regions) is larger than that of at the center (non-transformation region), which indicates that the NbTi nanowires at both ends subjects to lager elastic lattice strain and stress than that at the center. After unloading, the spacing of NbTi (110) in the whole sample is smaller than that of

FIG. 2. (Color online) (a) Tensile stressstrain curves obtained in the nanocomposite and a commercial Ni-Ti superelastic wire and (b) Energy absorbed by one superelastic cycle as a function of applied tensile strain for the NbTi/NiTi nanocomposite, Fe-Ni-Co-Al-Ta-B, NiTi-Nb, Ni-Ti(1), Ni-Ti(2), and Cu-AlMn-Ni superelastic SMAs.

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FIG. 3. (Color online) (a) Section of diffraction patterns from different positions of the sample at 6% strain, (b-c) The comparison of 2D-diffraction patterns from non-tansformation and transformation regions of the sample at 6% strain, (d-e) The evolutions of the interplane spacing of NbTi (110) and B2-NiTi (110) perpendicular to the wire axis in the whole sample for three different tensile states (before loading, at 6% strain and after unloading marked as “A,” “B,” and “C” points in Fig. 2(a)).

the sample prior to loading, and the spacing at both ends is significantly smaller than that at the center. Meanwhile, the spacing of NiTi (110) perpendicular to the wire axis in the whole sample is bigger than that of the sample prior to loading, and the spacing at both ends is significantly bigger than that at the center, as shown in Fig. 3(e). Based on the above experiments results, it can be speculated that the NbTi nanowires underwent some plastic deformation during loading and then the plastic deformation impeded the complete recovery of NiTi matrices during unloading (as shown “A ! C” in Fig. 2(a)), which causes the internal compressive stress transferred into NbTi nanowires from NiTi matrices during unloading. Meanwhile, the NiTi matrices also subjects to a corresponding internal tensile stress from NbTi nanowires during unloading, which would impede the reverse martensitic transformation of NiTi matrices during unloading. The in-situ high-energy x-ray diffraction experiments reveal that, aside from the deformation-induced transformation in the nanocrystalline NiTi matrices, a redistribution of stress between nanowires and matrices was evidenced from an abrupt change in residual lattice strains of the nanowires and matrices (Figs. 3(d) and 3(e)). This suggests that a loading transfer obviously occurred between the nanowires and martices during deformation. The strong interaction of the NbTi nanowires and the NiTi matrices during the reversible deformation-induced transformation is considered to significantly increase the frictional resistance to interfacial motion resulting in large frictional energy dissipation during the pseudoelastic cycle. Moreover, the plastic deformation occurring in the NbTi nanowires during pseudoelastic cycle would be expected to effectively relax the elastic strains at phase boundary causing large elastic strain energy dissipation. In summary, a NbTi/NiTi nancomposite with high strength was fabricated, which exhibits an ultrahigh mechanical damping capability. The ultrahigh energy-absorption

capability mainly originates from a combination of the strong interaction of the NbTi nanowires and NiTi matrices, and the plastic deformation occurring in the NbTi nanowires during the pseudoelastic cycle, which cause large frictional energy dispassion and elastic strain energy relaxation. We predict that the finding on the high energy-absorption capability found in the nanocomposite would trigger some important applications in many aspects. This work was supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 50971133 and 51001119). The use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, and Office of Basic Energy Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. 1

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