Shape Memory Alloys: Properties and Engineering Applications
Shape Memory Alloys: Properties and Engineering Applications
A technical paper
Presented
By
L.V.N.SAI KRISHNA
Email ID:
AND
K.PRAVEEN
Email ID :
(IV/IV B.E MECHANICAL ENGINEERING)
R.V.R & J.C COLLEGE OF ENGINEERING
Address for communication:
K.Praveen
S/o k.Basaveswar,
206,Manasa(appt),
6th lane,Chandramouli Nagar,
GUNTUR 522007,
A.P.
Ph: 9985462003
ABSTRACT :
Shape memory alloys (SMAs) are a promising class of advanced materials with interesting properties, which make them suitable for a variety of engineering applications. The uses of SMAs in engineering applications have extended year by year. SMAs are being used in a number of industrial applications such as aerospace, automotive, biomaterial, chemical sensor, construction, electronics, metal forming, storage and retrieval of information, optics etc. This paper highlights some of these engineering applications of SMAs. This also includes the recent research on the possible use of SMAs in smart structures. The important properties of SMAs that provide these materials in innovative applications are (i) shape memory effect. (ii) pseudo elasticity and (iii) high damping capacity. For a better appreciation. Some of the basic principles and properties isolated to the unique behaviour of this class of materials are also discussed.
1. INTRODUCTION :
Shape Memory Alloys (SMAs) refer to a group of metallic materials that have the ability to return to their memorized shape or size by a simple change of temperature. These alloys have two unique properties: the shape memory effect (SME,) and super elasticity (SE), also called pseudo elasticity. SME refers to the ability of the material to be deformed at a low temperature and then revert to its prior shape upon heating above a temperature, characteristic of the particular alloy . SE is the ability of a material to experience large recoverable strains when deformed within a range of’ temperatures, characteristic of the particular alloy. The increasing interest in the research of SMAs continues till date with the in of newer materials with promising applications in frontier areas.
The use of SMAs in engineering applications has been extended year by year. SMAs are being used in a number of industrial applications such as aerospace. Automotive, biomaterial, chemical-sensor, construction, electronics, metal forming. Storage and retrieval of information, optics etc. This paper highlights some of the engineering applications of SMAs. This also includes the recent research on the possible use of SMAs in smart structures. For a better appreciation, the paper has been divided into two sections: the first section describes the basic principles and properties of SMAs, while the second part focuses on applications.
2. BASIC PRINCIPLES AND PROPERTIES:
2.1 General characteristics :
From the metallurgical perspective, the SME and SE realized in SMAs are explained by the solid state phase transformation called martensitic transformation and its reverse transformation. The martensitic transformation that occurs in SMAs yields a thermo elastic marterisite from a high-temperature austenite phase. Usually a long-range ordered phase. The martensitic transformation takes place by a twinning type of deformation. Micro structurally, the result is an arrangement of alternating platelets with a herring bone structure, in which the shape changes of the platelet variants tend to cancel any net-shape change .The martensitic transformation, occurs over a temperature range. and shows a hysteresis effect, which varies with the alloy system. There are four characteristic temperatures defining a thermo elastic martensitic transformation (Fig. I) the martensite start temperature. Ms, at which martensite first appears in the austenite. The transformation proceeds with further cooling and is complete at the martensite finish temperature. Mf Below Mf the entire body is in the martensite phase. And a specimen typically consists of many regions each containing a different variant of martensite. The boundaries between the variants are mobile under small applied loads. While heating the austenite start temperature. As is the temperature at which austenite first appears in the martensite. With further heating. More and more of the body transforms back into austenite and this reverse transformation is complete at the austenite finish temperature. Af . All physical properties also show similar hysteresis loop upon heating and cooling through the transformation temperatures. Plotting one of these properties during forward and reverse transformation allows determining the four characteristics transformation temperatures. Not only is this important to be aware. Of these property changes with temperature. But also in some cases. They can be used to determine the fraction transformed.
Figure 1. Schematic representation of the volume transformed as fuction of temperature.
2.2 Crystallography:
The crystallographic transformations associated with the SME are shown schematically in Fig.2. Figure 2a is the original crystal in the austenite phase. Upon cooling the parent phase (austenite below Ms different variants of martensite (generally 24) are formed in a perfectly sell-accommodating group (typically a group of 6). these groups of variants form in a special way (diamond like configuration). such that the net macroscopic change becomes essentially zero by the mutual cancellation of the shape change of individual ‘variants .The thermo elastic martensite thus formed is characterized by their low energy which can be driven by small temperature or stress changes. When heated above Af, the reverse transformation takes place in a crystallographically reversible manner and thus retaining the original shape.
2.3 Shape memory effect (SME) :
When an external force applied to an alloy it first deforms elastically and then plastically. For a conventional alloy plastic deformation is permanent. Which means that it cannot revert to its original shape lien the force is removed. However, SMAs can return to its original shape when heated above a certain temperature. This phenomenon is defined as the SME. Most of the alloys that show this effect undergo thermo elastic martensitic transformation. The shape memory effect is a consequence of a crystallographically reversible martensite phase transformation occurring in the solid state. There are many ways (orientations) by which the martensite can form from austenite during cooling and once the lower symmetry martensite is formed, it has only unique reversion path to the parent austenite phase due to crystallographic restrictions. When the shape memory martensite is deformed, some variants grow at the expense of others, and eventually only one preferred single variant persists. The surviving variant is the one whose shape strain direction is parallel to the tensile axis thus permitting maximum elongation. On heating above A the reverse transformation takes place in a crystallographically reversible manner, with the result that the parent phase of original orientation and shape is regenerated. This has been described schematically in Fig.2.
Figure 2. Schematic of the shape memory effect. Upon cooling the austenite
2.3.1 One-way memory:
The one-way memory effect can be described with the help of a coil spring shown in Fig. 3. It can be seen that there is no change in the shape and size of the spring upon cooling from above Af to below Mf when the spring is deformed Below M, it retains deformed shape until it is heated. Upon heating, the shape recovery egins at As, and is completed at Af. Once the shape is recovered, there is no further change in shape when the spring is cooled to below Mf, to obtain the shape memory effect repeatedly. The spring has to be deformed each time below the Mf, This is generally referred to one way shape memory effect
The ability of shape memory alloys to recover a preset shape upon heating above its transformation temperature and return to an alternate shape upon cooling is known as two-way memory.
Figure 3.Example
2.4 Super elastic effect (SE):
The super elasticity (also refer to as pseudo elasticity) is observed in SMAs when the material is deformed above A, but below a temperature Mi At this temperature. The austenite undergoes stress induced martensitic transformation and remains stable under the applied stress. But becomes unstable when the stress is removed (Fig. 4). This transformation. Usually only one variant of martensite with habit planes favorably oriented the applied stress axis is formed. As a consequence. The shape deformation of this particular variant Produces maximum elongation along the tensile stress. Since there is only one martensite variant, the shape change is fully recoverable upon release of the stress.’’’ The recoverable strain in sonic of the SMAs can be as high as 10%. Shape memory effect and pseudo elasticity are complementary to each other.
2.5 Thermo-mechanical characteristic:
At temperatures sufficiently above A, SMAs in the austenite phase behave as normal metals and alloy with yielding and plastic flows at fairly low strain level. But the mechanical behavior varies with the temperature within the transformation range between Mf, and Af. Small variations in temperature can yield large changes in properties. The martensite is easily deformed to several percent strains at quite low stress. Whereas the anstenite has much higher yield and flow stresses. Above A, and below M, the application of stress results in stress induced martensitic transformation and again the material can be deformed to quite large strain at fairly low stress le el. However, the material returns to its original shape upon releasing the stress.
The deformation in the martensite phase which gives rise to the shape memory effect is quite interesting (Fig.4). The stress strain curve can be divided into three regions (Fig. 4b). In region I. linear elastic deformation of martensite takes place. Upon further deformation into region II the martensite structure deforms by the movement of twin boundaries. Which is quite mobile? Hence the strength of the martensite is quite low compared to that of the austenite which deforms by slip. This gives rise to a flat region in the stress—strain curve and is commonly known as “martensitic plateau”. The deformation in this region is accommodated by twin movement process refers to as detwinning of martensite. At higher stresses, there is region Ill, which is linear but not necessarily purely elastic. The deformation mechanism in this region is a mixture of elastic deformation of detwinned martensite together with the formation of new orientation of martensites which intersect with those that are already present and provide additional recoverable strain. After region III, the plastic deformation as in the case of yielding of all conventional metals takes place. Thus the maximum amount of recoverable strain is obtained up to the deformation at the end of region III. Beyond this point, the plastic deformation of the material sets in by irreversible slip mechanism and the memory strain decreases.
Figure 4. Schematic diagram showing (a) variation of Stress—strain characteristics of shape memory alloy.
2.6 Fatigue characteristic: -
Most of the SMA applications require the material to undergo numerous deformation cycles and hence the fatigue properties of SMAs in such applications are very important. The three different type of fatigue that are important in SMAs are (i) usual failure due to fracture caused b cyclic loading stresses or strains at a constant temperature. ( changes in material properties. such as the transformation temperatures and transformation hysteresis because of thermal c cling through the transformation temperatures and (iii) degradation of SME because of mechanical or thermal cling.’ In general. NM i alloys have good fatigue characteristics. It is reported’ that the fatigue lit of these alto s for c’ die loading induced b thermal cycling is more than 283K which is quite good for most of the applications. But. The fatigue life for cyclic super elastic deformation is insufficient for many applications.
2.8 Commercially important shape memory alloys:
Though a large number of alloy systems exhibit SME and SE. the only two alloy systems that have achieved a level of commercial exploitation are the NiTi alloys and copper-base alloys. Properties of these two systems are quite different. Among these two alloys, the Ni alloys have better shape memory strain (up to about 8% versus 4 - 5% for the copper base alloys), higher ductility, better corrosion resistance and better thermal stability. On the other hand. copper-base alloys are less expensive, can be processed with ease and have a wide transformation range. The two alloy systems thus have advantages and disadvantages that must be considered in a particular application. Among the new alloy systems. The stainless steel shape memory alloys appear to have potentials in many engineering applications. Although the martensitic transformation is not completely elastically accommodated and its hysteresis is little wide, a complete 3-5% deformation can be obtained with strain induced martensite. The advantage of these alloys is that they are cheap and processing technology is similar to that of stainless steel.
3. APPLICATIONS:
The unique functions such as shape memory effect and super elasticity possessed by SMAs make them amenable to a variety of applications in science and technology. The first commercial application of SMAs was as couplings for titanium alloy hydraulic tubing on the U.S. Navy’s F-14 Tomcat fighter aircraft in 1971. This was a classic example of technology being adapted to real market need. After this successful application, the use of SMAs as couplings expanded to many industrial applications and as of 1998, couplings accounted for the largest tonnage usage of SMAs. But, medical applications such as teeth-root prosthesis. Partial dentures, plates for repairing broken bones, actuators for artificial heart and kidney pumps. And flexible guide wires still account for the largest dollar value.
Based on the primary function of memory element, applications of SMAs are generally classified into four categories
1. Free recovery includes applications in which the function of memory element is to cause motion or strain or to return to its original shape. A prime application of this is the blood-clot filter. There are very few engineering applications of free recovery.
2. Constrained recover;’ includes applications in which the memory element is prevented from changing shape and thereby generate a stress. This is the most successful and exploited application of SMAs.
3. Actuator or work production applications are those in which the motion against a stress and thus work is being done by the memory element.
4. Superelasticorpseudoelastic applications are isothermal in nature and involve the storage of potential energy.
Some of the applications, which gained engineering importance, are discussed in detail. The areas where SMAs can be used as smart materials in the context of smart or adaptive structures, with emphasis on control of the stiffness or shape of an aerospace structure are also reviewed.