1.1 General considerations
When a regular metallic alloy is subjected to an external force greater than its elastic limit, it deforms plastically, i.e. the deformation persists after returning to the unloaded state. The Shape Memory Alloys (SMAs) do not follow this behavior. At low temperatures, an SMA specimen may undergo a plastic deformation of about few percent, and then fully recover its initial shape that had at higher temperature by simple heating above a threshold temperature. Their ability to recover their form when the temperature is raised, makes this class of materials unique. This phenomenon has been discovered in 1938 by researchers working on the gold-cadmium alloys [Gilbertson (1994)]. The shape memory effect remained a laboratory curiosity until 1963, when the first industrial and medical applications appeared.
1.2 Martensitic Transformation
The shape memory effect is based on the existence of a reversible phase transformation of thermoelastic martensitic type [Kurdjumov, Khandros (1949), Kumar, Lagoudas (2008)], between a microstructural state at high temperature (austenite phase) and a microstructural state at low temperature (martensite phase) [Patoor et al. (2006), Lagoudas et al. (2006)]. Austenite has in general a cubic crystal lattice, while martensite is of tetragonal, monoclinic, or orthorhombic crystal lattice. The transformation from one crystal lattice to the other occurs by distortion of the shear lattice does and not by atoms diffusion. This type of transformation is called martensitic transformation [Perkins (1975), Funakubo (1987), Otsuka, Wayman (1999)]. In reality, the matrenitic transformation in SMAs is a phase transformation of the first order, where there is co-existence of several phases, and there is presence of interfaces between the phases [Guénin (1986)].
If you need assistance with writing your essay, our professional essay writing service is here to help!Find out more
Historically, the term martensitic transformation describes the transformation of the austenite of steels (iron-carbon alloys) to martensite during a quenching. By extension, this term has been generalized to a large number of alloys whose phase transformations have certain characteristics typical of the transformation of steels [Rosa (2013)].
During martensitic transformation of a SMA, the crystal lattice of the material changes its shape. The microstructure of martensite is characterized by a change in shape and by the difference in volume, which exists between matrensitic and austenitic phase [Duerig et al. (1990)]. Therefore, internal strains arise during the emergence of martensitic areas within the austenite. The internal strains can be partially relaxed by the formation of several areas of self-accommodated martensite crystals that minimize the overall deformation induced. These areas called variants and are oriented in different crystallographic directions [Kumar (2008)].
In the absence of external strains, these variants are equally possible and the distribution of self-accommodated groups allows the material to be transformed in order to retain its original shape. Therefore, the formation of the martensite results in elastic (reversible) deformations [Funakubo (1987)]. At constant temperature, the martensite-austenite interfaces are in steady state. A change in temperature in one direction or the other results in moving these interfaces to the benefit of one or the other phase structure. The interfaces can also move under the action of an imposed strain. A specimen can therefore be distorted not by sliding, which is the usual mechanism of plastic deformation, but by the appearance and disappearance of martensite variants [Kumar (2008)].
Therefore, during martensitic transformation atoms in the structure move on very small distances leading to deformation of the crystal lattice. This causes a small variation in volume with shearing of the structure in a specific direction. During the transformation process, the growth of martensite crystals occur in form of platelets to minimize the energy at the interface.
The martensitic variants can occur in two different types: twinned martensite (formed by combination of self-accommodated martensite variants) and detwinned martensite (reoriented martensite) where a particular variant dominates [Liu, Xie (2007)]. The characteristic behavior of SMAs is based upon the reversible phase transformation from austenitic phase to martensitic phase and the opposite. By cooling under zero loading, the crystal sructure changes from austenitic to martensitic phase (forward transformation to twinned martensitic phase). This transformation is resulting in the development of a number of martensitic variants, which are arranged in a way that the average change in macroscopic shape is insignificant, causing a twinned martensite [Leclercq, Lexcellent (1996)]. When the material is heated at the martensite phase, the crystal structure is transforming to austenite (reverse transformation from detwinned martensitic to austenitic phase), leading to recovery of shape [Saburi, Nenno (1981), Shimizu, Otsuka, Perkins (1975)]. The above process is called Shape Memory Effect (SME) [Schetky (1979), Wayman, Harrison (1989)].
The martensitic transformation is characterized by four temperatures (Figure 2) [Gotthard, Lehnert (2001)]:
- MS: Temperature below which the martensite appears (martensite start)
- MF: Temperature below which the entire sample is transformed into martensite (martensite finish)
- AS: Temperature above which the austenite appears (austenite start)
- AF: Temperature above which the entire sample is transformed into austenite (austenite finish)
The transformation begins at the cooling to the temperature MS. This transformation is completed to the temperature MF. Between these two temperatures, there is coexistence of two phases, which is a characteristic of transformation of the first order. If the cooling is interrupted, the material will not change. To go back to the initial shape, the temperature is increases so that the inverse transformation begins at the temperature AS and finishes to temperature AF, which is higher than MS [Massalski et al. (1990)]. If the trace on a diagram (Figure 1) the volume fraction of material processed as a function of temperature, there is a hysteresis loop, due to the presence of an irreversible energy corresponding to dissipation of mechanical energy transformed into heat [Ortin, Planes, Delaey (2006), Wei,Yang (1988)].
Figure 1 Martensitic transformation temperatures [Gotthard, Lehnert (2001)]
The thermoelastic reversibility of the crystal lattice is certain in the case of an ordered alloy [Otsuka, Shimizu (1977)]. The correlation between the manifestation of martensitic transformation and atomic order was shown experimentally in Fe-Pt SMAs [Dunne, Wayman (1973)]. Nevertheless, in disordered alloys, such as Fe-Pd, Mn-Cu and In-TI, can occur thermoelastic transformation too. The atomic order is, therefore, a sufficient condition for manifestation of thermoelastic transformation, but not necessary [Otsuka, Shimizu (1977)].
1.3 Thermomechanical properties of SMAs
Several effects specific to the SMAs appear through the transformations of the crystal lattice as a function of temperature and of the field of stresses applied on the material [Duerig, Melton, Stöckel (2013)].
1.3.1 Pseudoelastic Effect
In general, by pseudoelasticity we describe both the material’s superelastic behavior, as well as rubble-like behavior. Superelastic behavior is called the reversible phase transformation produced by thermo-mechanical loading. Rubber-like effect refers to the reversible martensitic re-orientation. The stress-strain curve during this process resamples to the superelastic behavior, which is similar to rubber’s nonlinear elastic behavior [Otsuka, Wayman (1999)].
Therefore, a part from inducing phase transformation thermally, martensitic transformation can also be prompt by applying on the material appropriately high mechanical loading, resulting in creating a martensitic phase from austenite. When the temperature of the SMA goes above AF, shape recovery is resulted while unloading. Such behavior of the material is termed pseudoelastic effect [Kumar (2008)].
Stress-induced martensite, is generally forming from austenite when external stress is present. The process of forming stress-induced martensite can occur through different thermomechanical loading routes [Miyazaki, Otsuka (1986)]. One form of stress-induced martensite is the detwinned martensitic phase formed from austenitic after application of external stress. The material, during the stress-induced martensitic transformation and the reversed process, shows nonlinear elastic behavior described by closed Ïƒ-Îµ curves. This nonlinear elastic behavior is called pseudoelastic transformation [Otsuka, K. and K. Shimizu (1981)]. The shape recovery is due to crystallographic reversibility of transformation, like in the shape memory effect. Hence, the two phenomena, transformation pseudoelasticity and shape memory effect are practically the same except the fact that reverse transformation is produced by warming the specimen to temperature above AF. In reality, an alloy that undergoes thermoelastic martensitic transformation exhibits both transformation pseudoelasticity and shape memory effect [Otsuka, K. and K. Shimizu (1981)].
Nevertheless, for occurring transformation pseudoelasticity, the necessary stress for slip should be greater than that for stress-induced martensite transformation. As an example, we can refer to equiatomic Ti-Ni alloys which are exposed to slip and do not exhibit any transformation pseudoelasticity, regardless of their Ni content. It was shown, however, that Ni-rich Ti-Ni alloys subjected to annealing after cold working, causing refining of their grain size, leads in raising critical slip stress, which results in any transformation pseudoelasticity [Miyazaki et al. (1982), Saburi, Tatsumi, Nenno (1982), Saburi, Yoshida, Nenno (1984)]. The existence of transformation pseudoelasticity is affected by crystalline orientation, composition of the alloy, and direction of applied stresses [Miyazaki, Otsuka (1986)].
1.3.2 One-Way Shape Memory Effect
Another property of SMAs is the one-way shape memory effect. It takes place in four steps:
(1) The material is cooled to a temperature lower than MF (the parent austenitic phase) to obtain self-accommodated martensite.
(2) Re-orientation of variants of the martensite is obtained via application of stress.
(3) The stress is released at constant temperature T < MF. The material remains to a shape depending on the stress field.
(4) The sample is heated at a temperature T > AF making re-appear the austenitic phase and the material gets its original shape, as shown in Figure 2.
Figure 2 One-way shape memory effect [Miyazaki, Otsuka (1986)]
Two conditions are necessary for occurring shape recovery by shape memory effect. Firstly, the transformation should be reversible, and second, slip should not occur during the entire deformation process. Martensitic transformations in ordered alloys are reversible in nature [Miyazaki, Otsuka (1986), Arbuzova, Khandros (1964)], so the entire shape memory effect mainly occurs in this type of alloys. The second condition is necessary because in the case of high stress and every type of deformation mode (stress-induced martensitic transformation in parent phase, twinning in the martensitic phase) slip can be induced, resulting in plastic strain and, not completed recovery of shape.
In the one-way shape memory effect, the shape in memory by the SMA is the one of the parent phase.
1.3.3 Two-Way Shape Memory Effect
The two-way shape memory effect is the reversible passage of a shape at a high temperature to another shape at low temperature under stress.
The two-way shape memory effect should precede the SMA training [Nagasawa, et al. (1974]. Training of SMAs consists of temperature cycling at constant stress or stress cycling at constant temperature. During training, microstructural defects (i.e. dislocations) lead to internal stresses and therefore promote oriented martensite. A SMA subjected to training can then move from austenitic phase to oriented martensite under zero load by simple change of temperature [Schroeder, Wayman (1977)]. It has then a shape in memory for each of the two phases.
Various methods that cause two-way shape memory effect have been suggested, such as, large deformation in stress-induced martensite transformation at temperatures > MS [Delaey et al. (1974)], shape memory effect training [Schroeder, Wayman (1977)], stress-induced martensite training [Schroeder, Wayman (1977)], training involving both of shape memory effect as well as stress-induced martensite [Perkins, Sponholz (1984)] remaining in martensite state while heating at a temperature > AF [Takezawa, Shindo, Sato (1976)], as well as using precipitates [Tadaki, Otsuka, Shimizu (1988)].
1.4 Transformation Induced Plasticity (TRIP)
Several experimental studies have shown the development of nonlinear plastic (irreversible) strain when phase transformations occur [Greenwood, Johnson (1965), Abrassart (1972), Magee (1966), Desalos (1981), Olson, Cohen (1986), Denis et al. (1982)]. This mechanism of deformation is termed Transformation Induced Plasticity (TRIP), resulting from internal stress rising from the change in volume related to the transformation, as well as from the associated change in shape [Marketz, Fischer (1994)]. TRIP differs from classical plasticity. Although plasticity is caused from the applied stress or variation in temperature, TRIP is triggered by phase variations, and occurs even at low and constant stress levels [Gautier et al. (1989), Leblond et al. (1989), Gautier (1998), Tanaka, Sato (1985), Fischer et al. (2000, 1996)]. TRIP takes place because of two separate mechanisms. The first, refers to a process of accommodation of micro-plasticity related to volume change [Greenwood, Johnson (1965)]. The other, refers to an orientation caused by shear internal stresses, favoring the direction of preferred orientation for the formation of martensite when and external stress is present, which involves change in shape [Magee (1966)]. TRIP is caused by the difference in compactness of the lattice structure between the austenite (parent) and the martensite (product) phase [Greenwood, Johnson (1965)]. During martensitic transformation, this difference has produces a change in volume as well as internal stresses causing plasticity in the phase with less yield stress, which is weakerÂ [Paiva, Savi, Pacheco (2005)].
Abrassart, F., Influence des Transformations Martensitiques sur les Propriétés Mécaniques des Alliages du Système Fe-Ni-Cr-C, Thèse d’État, Université de Nancy I (Trance), 1972.
Arbuzova, I. and L. Khandros, Abnormal elongation and reduced resistance to plastic deformation due to martensitic transformation in the alloy CU-AL-NI. Phys. Metals Metallogr., 17(3), pp. 68-74, 1964.
Delaey, L., et al., Thermoelasticity, pseudoelasticity and the memory effects associated with martensitic transformations. Journal of Materials Science, 9(9), pp. 1521-1535, 1974.
Denis, S., Simon, A. and Beck, G., Estimation of the Effect of Stress/Phase Transformation Interaction when Calculating Internal Stress during Martensitic Quenching of Steel, Trans. Iron Steel Inst. Jap., Vol. 22, pp. 505, 1982.
Desalos, Y., Comportement dilatométrique et mécanique de l’Austénite Métastabled’un Acier A 533, IRSID Report n. 95.34.94.01 MET 44, 1981.
Duerig, T., K. Melton, D. Stockel, C. Wayman (Eds.), Engineering Aspects of Shape Memory Alloys, Butterworth-Heinemann, London, 1990.
Duerig, T.W., K. Melton, and D. Stöckel, Engineering aspects of shape memory alloys,Â Butterworth-Heinemann, 2013.
Dunne, D. and C. Wayman, The effect of austenite ordering on the martensite transformation in Fe-Pt alloys near the composition Fe3Pt: I. Morphology and transformation characteristics. Metallurgical Transactions, 4(1), pp. 137-145, 1973.
Fischer, F.D., Reisner, G., Werner, E., Tanaka, K., Cailletaud, G. and Antretter, T., A New View on Transformation Induced Plasticity, International Journal of Plasticity, vol. 16, pp. 723-748, 2000.
Fischer, F.D., Sun, Q.P. and Tanaka, K., Transformation induced plasticity (TRIP), Applied Mechanics Review, Vol. 49, pp. 317-364, 1996.
Funakubo, H. (Ed.), Shape Memory Alloys, Gordon and Breach Science Publishers, 1987.
Funakubo, H., Shape Memory Alloys, Gordon and Breach Sci. Publ, New York, p. 275, 1987.
Gautier, E., Déformation de transformation et plasticité de transformation, École d’été MH2M, Méthodes d’Homogénéisation en Mécanique des Matériaux, La Londe Les Maures (Var, France), 1998.
Gautier, E., Zhang, X.M. and Simon, A., Role of Internal Stress State on Transformation Induced Plasticity and Transformation Mechanisms during the Progress of Stress Induced Phase Transformation, International Conference on Residual Stresses- ICRS2, (Ed: G. Beck, S. Denis and A. Simon), Elsevier Applied Science, London, pp. 777-783, 1989.
Gilbertson, R. G. , “Muscle Wires Project Book”, Mondotronics, p. 2-1/2-8, 1994.
Gotthard R. and T. Lehnert, “Alliages à mémoire de forme”, Traité des matériaux nÂ°19: Matériaux émergents, p. 81-105, 2001.
Greenwood, G.W. & Johnson, R.H., The Deformation of Metals under Small Stresses during Phase Transformation, Proceedings of the Royal Society A 283, pp. 403-422, 1965.
Greenwood, G.W. and Johnson, R.H., The Deformation of Metals under Small Stresses during Phase Transformation, Proceedings of the Royal Society A 283, pp. 403-422, 1965.
Guénin, G., Alliages à mémoire de forme, Techniques de l’Ingénieur, vol. 10, p. 1-11, 1986.
Kumar P., Introduction to Shape Memory Alloys, Shape Memory Alloys, 2008
Kumar, P. and D. Lagoudas, Shape Memory Alloys – Modeling and Engineering Applications. 2008, Springer Science, New York, NY.
Kurdjumov, G. V., L. G. Khandros, First reports of the thermoelastic behaviour of the martensitic phase of Au-Cd alloys, Doklady Akademii Nauk SSSR 66 (1949) 211-213.
Lagoudas, D. C., P. B. Entchev, P. Popov, et al., “Shape memory alloys, Part II: Modeling of polycrystals”, Mechanics of Materials, vol. 38, p. 430-462, 2006.
Leblond, J., Devaux, J. and Devaux, J.C., Mathematical Modeling of Transformation Plasticity in Steels I: Case of Ideal-plastic Phases, International Journal of Plasticity, Vol. 5, pp. 551-572, 1989.
Leblond, J., Mathematical Modeling of Transformation Plasticity in Steels II: Coupling with Strain Hardening Phenomena, International Journal of Plasticity, Vol. 5, pp. 573-591, 1989.
Leclercq S., and C. Lexcellent, A general macroscopic description of the thermomechanical behavior of shape memory alloys, Journal of the Mechanics and Physics of solids, 44, 953-980, 1996.
Liu Y., and Z. Xie, Detwinning in shape memory alloy, In: Progress in Smart Materials and Structures, Ed. Peter L. Reece, pp. 29-65, 2007.
Magee, C.L., Transformation Kinetics, Microplasticity and Aging of Martensite in Fe-31 Ni, Ph.D. thesis, Carnegie Institute of Technology, Pittsburg, PA, 1966.
Marketz, F.Â andÂ Fischer,Â F.D.,Â AÂ MicromechanicalÂ StudyÂ onÂ theÂ CouplingÂ EffectÂ BetweenÂ Microplastic Deformation and Martensitic Transformation, Computational Materials Science, Vol. 3, pp. 307-325, 1994.
Massalski, T.B., et al., Binary alloy phase diagrams. vol. 3. ASM International, pp. 1485, 1990.
Miyazaki S, Ohmi Y, Otsuka K, Suzuki Y. Characteristics of deformation and transformation pseudoelasticity in Ti-Ni alloys. Le Journal de Physique Colloques, 43, 1982.
Miyazaki, S. and K. Otsuka, Deformation and transition behavior associated with theR-phase in Ti-Ni alloys. Metallurgical Transactions A, 17(1), pp. 53-63, 1986.
Nagasawa, A., et al., Reversible shape memory effect. Scripta Metallurgica, 8(9), pp. 1055-1060, 1974.
Olson, G.B. and Cohen, M., Mechanical Properties and Phase Transformation in Engineering Materials, TMS-AIME, Warrendale, Pa (Ed: S. D. Antolovich, R. O. Ritchie and W. W. Gerberich), pp.367, 1986.
Ortin, J., A. Planes and L. Delaey , Hysteresis in Shape-Memory Materials in The Science of Hysteresis, (2006), pp. 467-553.
Otsuka and K, Shimizu, K., Ser. Metall. 1, pp. 757-60, 1977.
Otsuka, K. and K. Shimizu, Pseudoelasticity, In: Metals Forum, 1981.
Otsuka, K., C. M.Wayman (Eds.), Shape Memory Materials, Cambridge University Press, Cambridge, 1999.
Paiva, A., M.A. Savi, P.M. Pacheco, Modeling transformation induced plasticity in shape memory alloys, Proceedings of COBEM, 18th International Congress of Mechanical Engineering, Nov. 6-11, 2005, Ouro Preto, MG, 2005.
Patoor, E., D. C. Lagoudas, P. B. Entchev, et al., “Shape memory alloys, Part I: General properties and modeling of single crystals”, Mechanics of Materials, vol. 38, p. 391-429, 2006.
Perkins, J. and R. Sponholz, Stress-induced martensitic transformation cycling and two-way shape memory training in Cu-Zn-Al alloys. Metallurgical transactions A, 15(2), pp. 313-321, 1984.
Perkins, J., Shape Memory Effects in Alloys, Plenum Press, New York, 1975.
Rosa M., Phase Transformations in Steels, Volume 1: Fundamentals and Diffusion-Controlled Transformations, International Journal of Environmental Studies, vol. 70(2), pp. 337-338, 2013.
Saburi, T. and S. Nenno, The shape memory effect and related phenomena. Solid to Solid Phase Transformations, pp. 1455-1479, 1981.
Saburi, T., M. Yoshida, and S. Nenno, Deformation behavior of shape memory TiNi alloy crystals. Scripta metallurgica, 18(4), pp. 363-366, 1984.
Saburi, T., T. Tatsumi, and S. Nenno, Effects of heat treatment on mechanical behavior of Ti-Ni alloys. Le Journal de Physique Colloques, 43(C4), pp. C4-261-C4-266, 1982.
Schetky, L., Shape-memory alloys, Scientific American 241 (74-82), 1979.
Schroeder, T. and C. Wayman, The two-way shape memory effect and other “training” phenomena in Cu Zn single crystals. Scripta Metallurgica, 11(3), pp. 225-230, 1977.
Shimizu, K., K. Otsuka, and J. Perkins, Shape Memory Effects in Alloys. Perkins, J., Ed.(New York: Plenum), pp. 60-87, 1975.
Tadaki, T., K. Otsuka, and K. Shimizu, Shape memory alloys. Annual Review of Materials Science, 18(1), pp. 25-45, 1988.
Takezawa, K., T. Shindo, and S.I. Sato, Shape memory effect in 1-CuZnAl alloys. Scripta Metallurgica, 10(1), pp. 13-18, 1976.
Tanaka, K. and Sato, Y., A Mechanical View of Transformation-Induced Plasticity, Ingenieur Archiv 55, pp. 147-155, 1985.
Wayman, M., J. Harrison, The origins of the shape memory effect, Journal of Minerals, Metals, and Materials 41 (99) pp. 26-28, 1989.
Wei, Z., D. Yang, On the hysteresis loops and characteristic temperatures of thermoelastic martensitic transformations, Scripta Metallurgica, Volume 22, Issue 8, 1988, pp. 1245-1249.
Cite This Work
To export a reference to this article please select a referencing stye below:
Related ServicesView all
DMCA / Removal Request
If you are the original writer of this essay and no longer wish to have your work published on UKEssays.com then please: