The intelligent structural materials have gained a wide range of attention in recent decade because of their outstanding mechanical properties. Nitinol is one of such alloy, which is capable of showing both superelasticity and shape memory effect under the action of applied stress and/or temperature. These effects are caused by the phase transformation from a highly ordered austenite (BCC cubic structure) to relatively less ordered martensite (monoclinic) when changes occur in stress or temperature.
NITINOL (an acronym for NIckel TItanium Naval Ordinance Laboratory) is a family of intermetallic materials which contain a nearly equal mixture of nickel (55 wt. %) and titanium.
R. Artiaga(2002) et. al The nickel-titanium alloys are usually known as Shape Memory alloys because of their ability to return to some previously defined shape or size when subjected to the appropriate thermal procedure. Mechanical properties of a nickel titanium wire were investigated by DMTA using cylindrical tension mode. The Young's modulus, the maximum strain and residual deformation have been calculated. Recovery of previously deformed samples was observed in constant stress temperature ramp tests. Relaxation stress behavior at temperatures above the austenitic transformation has been studied. The strain and frequency ranges of linear response have been determined by dynamic experiments. Strain amplitude of 0.1% and frequency of 1 Hz have been chosen for the temperature ramp dynamic experiments. A big change between 65 and 95_C is observed in the storage modulus. The values of E´ at temperatures below and above the transition are essentially constant. Finally, the effects of the frequency at different temperatures have been examined.
A near equal-atomic nickel titanium alloy was studied by DMTA in stationary and dynamic modes. The Young's modulus at 25°C was 262.3 MPa, and the maximum strain about 7%. A remaining plastic deformation of 2% was observed after applying a 5%strain. The observed recovery in constant stress temperature ramp tests resulted to be higher than the plastic deformation previously imposed to the samples. The recovery is shifted to higher temperatures as higher constant stress is applied. Slow relaxation of stresses was observed at temperatures above the austenitic transformation. An important increase of the storage modulus is observed between 65 and 95°C. At temperatures enough below and above the transition, the modulus is almost constant in a broad range of frequencies while, at temperatures close to the transition, a strong effect of the frequency is observed in the storage modulus.
R. Artiaga, A. García, L. García, A. Varela, J. L. Mier, S. Naya and
M. Grana. Journal of Thermal Analysis and Calorimetry, Vol. 70 (2002) 199-207
Deformation behavior associated with initial austenite (A), rhombohedral (R) and martensite (M) phase structures was studied in polycrystalline NiTi shape memory alloy tubes by tensile testing at different temperatures. The nominal stress-strain curves of the tubes from room temperature (23 °C) to 70 °C were recorded. The deformation of NiTi tubes with initial structure of R-phase proceeded via R→M martensitic transformation, while the deformation of NiTi tubes with initial structure of M-phase proceeded via martensitic detwinning. It was found that the R→M martensitic type transformation was realized, at the macroscopic level, by nucleation and growth of an inclined cylindrical band, while the detwinning process of the tube was macroscopically homogeneous. Further, two-stage yielding, which is associated with austenite to rhombohedral (A→R) and R→M phase transformations, was observed in the stress-strain curves of NiTi tubes in a certain testing temperature range. With a further increase in temperature, the shape of the nucleated band remained cylindrical until 60 _C (>Af) when the shape of the initial band suddenly became helical which was well observed in the superelastic microtubing.
To investigate the stress-induced phase transformation and detwinning processes in polycrystalline NiTi tubes under tension, displacement controlled uniaxial tensile tests on the tubes with different initial phase structures were performed. Depending on the pre-testing heating/cooling history and the test temperature, different initial phases of the material were obtained and four different types of deformation process (A → M, R → M, A →R→ M and M → M) were realized by the loading. The key experimental findings of this preliminary research are listed as follows:
YONG LIU {, Z. XIE, J. VAN HUMBEECK and L. DELAEY
Dept MTM, Katholieke Universiteit Leuven, de Croylaan 2, 3001 Heverlee, Belgium
(Received 9 January 1998; accepted 27 February 1998)
The stress-strain curves of polycrystalline martensitic NiTi shape memory alloys are often different for loading under tension and compression. Under tension, a flat stress-plateau occurs, while under compression, the material is quickly strain hardened and no flat stress-plateau is observed. Cyclic deformation under tension-compression also shows that it is more difficult to deform the material during compression than during tension, where an asymmetric stress-strain loop is obtained. TEM observations show that, under tension to 4% strain, martensite variants are partially reoriented via migration of variant interfaces with formation of dislocation networks mainly along the junction plane areas, and no significantly plastic deformation has been observed inside the martensite twin bands. While under compression to 4% strain, a high density of dislocations has been generated in both the martensite twin bands and the variant accommodation area, and no significant martensite reorientation via variant interfacial migration has been observed. This shows that the deformation mechanism of martensitic polycrystalline NiTi SMAs under tension is different from that under compression.
The stress-strain curves of binary polycrystalline martensitic NiTi SMAs are generally different for loading under monotonic tension and under monotonic compression. Under tension, a clear stress plateau occurs, while under compression, the material is quickly strain hardened and no clear stress-plateau can be observed. This shows that the deformation mechanisms of martensitic NiTi SMA are different for tension and compression stress modes in the studied materials, and this has been supported by a microstructural study. Under tension, two adjacent martensite plates containing (011) type II twins become (100) compound twin related to each other and, as a result, the interface between the two martensite plates becomes mobile, and the martensite variants have partially reoriented via migration of these interfaces when tensioned to 4% strain. In addition, formation of dislocation networks in the junction plane area has been observed.
Under compression to 4% strain, a high density of dislocations has been generated in both the martensite twin plates and the junction plane areas. However, no formation of a twin relation between two neighbouring martensite plates has been found and the interfacial movements have not been observed. These results reveal that the early stage of martensite deformation under tension may be related mainly to the interfacial migration between two adjacent martensite plates, while under compression it is mainly related to the generation and migration of dislocations.
K. OTSUKA, T. SAWAMURA and), K. SHIMIZU
K. OTSUKA et al.: Crystal Structure of Equiatomic TiNi Martensite 457
phys. stat. sol. (a) 5, 457 (1971)
Subject classification: 3 and 10.1; 1.2; 21.1
Institute of Scientific and Industrial Research,
Osaka University, Yamadakami, Suita, Osaka
The crystal structure and internal defects of equiatomic TiNi martensites have been studied using the techniques of electron microscopy, electron diffraction, and X-ray diffraction. The crystal structure may be described as of distorted B19 type with a monoclinic unit cell, the lattice parameters of which are a = 2.889 A, b = 4.120 A, c = 4.622 A, and β= 96.8". The structure is derived from the matrix B2 in two steps. Namely, the matrix B2 transforms into a B19 first, and then a monoclinic martensite is derived by shearing the B19 in [1oo] B19 direction on (001) B19 plane. As an internal defect of the martensite, transformation twins with (111) twinning plane have been found without ambiguity, whose presence is essential for the understanding of the unique memory effect in this alloy. The orientation relationship between the matrix and martensite, and some characteristics of a pre-martensitic transition are also described.
Deformation behaviour and shape memory effect of near equi-atomic NiTi alloy
H. A. MOHAMED, J. WASHBURN
JOURNAL OF MATERIALS SCIENCE 12 (1977) 469 480
The mechanical shape memory effect associated with the martensitic-type transformation which occurs in polycrystalline Ti-50.3 at. % Ni alloy has been investigated using the techniques of transmission and optical microscopy. Deformation of initially partially transformed material within the recoverable strain range was found to occur by:
(1) stress induced transformation of the most favorably oriented existing martensite variants at the expense of adjacent unfavorably oriented variants and retained high temperature phase
(2) stress-induced re-orientation of favorably oriented martensite by utilizing the most favorably oriented twin system,
(3) stress-induced twin-boundary migration within the martensite.
The reverse transformation during heating restores the original grain structure of the high temperature phase in a highly coherent manner. It was concluded that deformation modes limited to those involved in the transformation process and the reversibility of the transformation give rise to the memory effect.
The deformation behavior and shape memory effect in polycrystalline Ti-50.3 at. %Ni alloy have been investigated. From this work the following conclusions could be drawn:
(1) Deformation within the recoverable strain range of a partially transformed material occurs by modes limited to those involved in the transformation process. These are: (a) stress-induced transformation of the most favorably oriented existing martensite variant at the expense of adjacent unfavorably oriented variant and retained high temperature phase;
(b) stress-induced re-orientation of martensite by utilizing the most favorably oriented twin system;
(c) stress-induced twin boundary migration within a martensite variant.
(2) The reverse transformation during heating occurs in a highly coherent manner that restores the original grain structure of the high temperature phase.
(3) The memory effect arises from:
(a) Reversibility of the transformation,
(b) The limitation of deformation modes within the recoverable strain range to those involved in the transformation process itself.
STRENGTH DIFFERENTIAL EFFECT IN PSEUDOELASTIC
NiTi SHAPE MEMORY ALLOYS
R. PLIETSCH"f and K. EHRLICH
Received 28 June 1996; accepted 20 September 1996
Pseudoelastic shape memory alloys of NiTi-type can be reversibly deformed up to maximum strains of 8%. In strain-controlled tension/compression testing of pseudoelastic NiTi shape memory wires. compression recovery forces were found to be markedly higher than tension forces. An explanation for this "strength differential effect" is proposed: The analysis of the generation process of stress-induced martensite (SIM) variants in a single crystal reveals that for a given load axis, the largest possible transformation strains in tension loading can be more than twice as high as respective compression strains. Thus, the formation of martensite in tension is facilitated, and high force levels are generated in compression loading. Despite this asymmetric behavior of transformational SIM strains being most pronounced near (11 I)BZ load directions, even for untextured polycrystalline samples the averaged SIM strains in tension exceed compression strains by 46%. Consequently, the strength differential effect must not only be considered in single crystals, but also in commercial shape memory materials and applications.
J. Koike, D. M. Parkin, M. Nastasi
A NiTi intermetallic compound was cold rolled at room temperature by 30% and 60% thickness reductions, and microstructures were studied by means of transmission electron microscopy (TEM). In the cold-rolled samples we observed both a phase of nanometer-sized crystals and an amorphous phase. A substantially high dislocation density, 1013 to 1014/cm2, was evident in the transition region between crystalline and amorphous phases. A simple estimate of the elastic energy arising from this dislocation density is of the same order as the crystallization energy, suggesting that dislocation accumulation is a major driving force for amorphization in cold-rolled NiTi.
Ken Gall , Jeff Tyber , Valerie Brice , Carl P. Frick , Hans J. Maier , Neil Morgan
Journal of Biomedical Materials Research Part A,Volume 75A Issue 4, Pages 810 - 823
We examine the structure and properties of cold drawn Ti-50.1 at % Ni and Ti-50.9 at % Ni shape memory alloy wires. Wires with both compositions possess a strong <111> fiber texture in the wire drawing direction, a grain size on the order of micrometers, and a high dislocation density. The more Ni rich wires contain fine second phase precipitates, while the wires with lower Ni content are relatively free of precipitates. The wire stress-strain response depends strongly on composition through operant deformation mechanisms, and cannot be explained based solely on measured differences in the transformation temperatures. We provide fundamental connections between the material structure, deformation mechanisms, and resulting stress-strain responses. The results help clarify some inconsistencies and common misconceptions in the literature. Ramifications on materials selection and design for emerging biomedical applications of NiTi shape memory alloys are discussed.
X.D. Wu, G.J. Sun, J.S. Wu, Materials Letters 57 (2003) 1334-1338
In the present paper, constant stress tests were conducted, and it was found that the relationship between transformation strain and external stress for NiTi is nonlinear because of the variation of the microstructure of martensite with external stress. Thermodynamical calculation shows that the nonlinear external stress dependence of transformation temperatures was caused by the variation of transformation strain with applied stress.
Cheng, J.; Ardell, A. J., Nuclear Instruments and Methods in Physics Research Section B, Volume 44, Issue 3, p. 336-343, 01/1990
The crystalline to amorphous (C-A) transition induced by 2 MeV proton irradiation of a NiTi alloy containing 50.95 Ni was investigated at - 45°C and - 153°C using transmission electron microscopy and scanning electron fractography. The fractographic studies indicate that the irradiated region contains two layers, the one closer to the free surface consisting of a mixture of amorphous and crystalline phases, and the other having transformed completely to the amorphous phase. The progress of the C-A transition was dependent on both dose and temperature. The critical dose for complete amorphization was estimated as 0.25 displacements per atom (dpa) at - 153°C and 0.32 dpa at - 45°C, as determined from measurements of the distance from the surface of the sample to the interface between the two layers in the irradiated region. Amorphization commenced well before the crystalline matrix was completely disordered, and the amorphous phase was generally observed to nucleate homogeneously throughout the crystalline matrix. The features of the C-A transition induced by proton irradiation more closely resemble those of ion irradiation than electron irradiation. The spatially nonuniform buildup of point defects during proton and heavy-ion irradiation is invoked to explain the different responses of intermetallic compounds to ion and electron irradiation.
Z. G. Wang, X. T. Zu, L. J. Liu, S. Zhu, Y. Huo, L. B. Lin, X. D. Feng and L. M. Wang, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Volume 211, Issue 2, October 2003, Pages 239-243
Ti-50.6at.%Ni shape memory alloy specimens were irradiated in the parent phase using a tandem accelerator by 18 MeV protons at a dose rate of 4.27 Ã- 109 cm−2 s−1 for 1 h and 10 h, respectively. The total doses were 1.5 Ã- 1013 and 1.5 Ã- 1014 H+/cm2. Microstructures and phase transformation characteristics of the specimens before and after irradiations were evaluated by transmission electron microscopy and differential scanning calorimetry. There is no observable change of the microstructures after the irradiation. The parent phase (austenitic phase) was stabilized by the irradiation, since the R-phase transformation start temperature Rs and the reverse martensitic transformation finish temperature Af had decreased with increasing proton dose, about 6 K for Rs and 13 K for Af at a dose of 1.5 Ã- 1014 H+/cm2. As and Rf were not affected by the proton irradiation. The variation of the transformation temperatures was assigned to local stress fields and changes in the degree of the lattice order produced by the proton irradiation.
Z. H. Dughaish, Materials Letters, Volume 32, Issue 1, July 1997, Pages 29-32
Specimens of shape memory nitinol alloy (Tc = 323 K) were irradiated with 1.5−2 MeV protons with a fluence of 6.6 Ã- 1015 protons cm−2 over a period of 6 h. The electrical resistivity of the specimens was measured under similar conditions before and after proton irradiation in the temperature range 233-350 K. In addition to the sharp rise in the electrical resistivity of the specimens at the martensitic transformation temperature Tc after proton irradiation, a considerable gradual decrease in Tc was seen as the energy of protons increased over 1.75 MeV. Tc decreased from 323 K for the virgin sample to 302 K when the proton energy was 2 MeV. This is an indication that increasing the proton energy enhances the creation of defects and the production of lattice disorder by proton irradiation, which consequently increases the anti-phase boundaries brought about by the particular co-operative shear movement of atoms that takes place, causing Tc to be lower than that of the virgin specimen. This shows that proton beam energy plays an important role in lowering the martensitic transformation temperature Tc. This result provides a new method of producing SMA materials of low Tc and seems to have an important application in the robots industry, controls, sensors etc. In addition, it appears that the results of this work will extend the applications of the shape memory alloys (SMA) of low martensitic transition temperatures.
A.A. Al-Aql, Physica B 239 (1997) 345-349
Study of proton-irradiation effect on the transformation temperature of four nitinol specimens of different transformation temperatures As was carried out with protons of 2 MeV energy and a fluence of 1.4 x 1018 protons cm- 2 for 6 h, by electrical resistivity measurements, in the temperature range -40-80°C. The result indicates that irradiation with 2 MeV protons lowers the transformation temperature of all specimens under investigation. The drop in As-values was 7°C for the specimen of lowest As and 16°C for the highest As.
