07-04-2011, 12:27 PM
SUBMITTED BY:
ROHIT KUMAR DAS
[attachment=11844]
INTRODUCTION
Shape memory alloys: As apart of Smart materials
SHAPE MEMORY ALLOYS
Shape memory alloys (SMA's) are alloys like NiTi alloy, which exhibit two very unique properties, pseudo-elasticity, and the shape memory effect. Pseudo-elasticity is the rubber like flexibility shown by these alloys and shape memory effect is ability of the alloys to regain the original shape by heating after severe deformation. Due to these unique properties these alloys are known as Shape Memory alloys.
Arne Olander first bserved these unusual properties in 1938 (Oksuta and Wayman 1998), but not until the 1960's were any serious research advances made in the field of shape memory alloys. The most effective and widely used alloys include NiTi (Nickel - Titanium), CuZnAl, and CuAlNi.
PROPERTIES OF SHAPE MEMORY ALLOYS
General Properties
A NiTi shape memory metal alloy can exist in two different temperature-dependent crystal structures (phases) called martensite (lower temperature) and austenite (higher temperature or parent phase). Several properties of austenite NiTi and martensite NiTi are notably different. When martensite NiTi is heated, it begins to change into austenite (Fig. 1). The temperature at which this phenomenon starts is called austenite start temperature (As ). The temperature at which this phenomenon is complete is called austenite finish temperature (Af ). When austenite NiTi is cooled, it begins to change into martensite. The temperature at which this phenomenon starts is called martensite start temperature (Ms ). The temperature at which martensite is again completely reverted is called martensite finish temperature (Mf ) [BUEHLER et al., 1967].
The composition and metallurgical treatments have dramatic impacts on the above transition temperatures. From the point of view of practical applications, NiTi can have three different forms: martensite, stress-induced martensite (superelastic), and austenite. When the material is in its martensite form, it is soft and ductile and can be easy deformed. Superelastic NiTi is highly elastic (rubber-like), while austenitic NiTi is quite strong and hard (similar to titanium) (Fig.2). The NiTi material has all these properties, their specific expression depending on the temperature in which it is used. In Fig. 1 Md represents the highest temperature to straininduced martensite and the grey area represents the area of optimal superelasticity.
Hysteresis
The temperature range for the martensite-to-austenite transformation, i.e. soft-to-hard transition that takes place upon heating is somewhat higher than that for the reverse transformation upon cooling (Fig.1). The difference between the transition temperatures upon heating and cooling is called hysteresis. Hysteresis is generally defined as the difference between the temperatures at which the material is in 50% transformed to austenite upon heating and in 50% transformed to martensite upon cooling. This difference can be up to 20–30 .
Thermoelastic Martensitic Transformation
The unique behavior of NiTi is based on the temperature-dependent austenite-to-martensite phase transformation on an atomic scale, which is also called thermoelastic martensitic transformation. The thermoelastic martensitic transformation causing the shape recovery is a result of the need of the crystal lattice structure to accommodate to the minimum energy state for a given temperature [OTSUKA etal., 1998].
In NiTi, the relative symmetries between the two phases lead to a highly ordered transformation, where the displacements of individual atoms can be accurately predicted and eventually lead to a shape change on a macroscopic scale. The crystal structure of martensite is relatively less symmetric compared to that of the parent phase. If a single crystal of the parent phase is cooled below Mf , then martensite variants with a total of 24 crystallographically equivalent habit planes are generally created. There is, however, only one possible parent phase (austenite) orientation, and all martensitic configurations revert to that single defined structure and shape upon heating above Af. The mechanism by which single martensite variants deform is called twinning, and it can be described as a mirror symmetry displacement of atoms across a particular atom-plane, the twinning plane.
ROHIT KUMAR DAS
[attachment=11844]
INTRODUCTION
Shape memory alloys: As apart of Smart materials
SHAPE MEMORY ALLOYS
Shape memory alloys (SMA's) are alloys like NiTi alloy, which exhibit two very unique properties, pseudo-elasticity, and the shape memory effect. Pseudo-elasticity is the rubber like flexibility shown by these alloys and shape memory effect is ability of the alloys to regain the original shape by heating after severe deformation. Due to these unique properties these alloys are known as Shape Memory alloys.
Arne Olander first bserved these unusual properties in 1938 (Oksuta and Wayman 1998), but not until the 1960's were any serious research advances made in the field of shape memory alloys. The most effective and widely used alloys include NiTi (Nickel - Titanium), CuZnAl, and CuAlNi.
PROPERTIES OF SHAPE MEMORY ALLOYS
General Properties
A NiTi shape memory metal alloy can exist in two different temperature-dependent crystal structures (phases) called martensite (lower temperature) and austenite (higher temperature or parent phase). Several properties of austenite NiTi and martensite NiTi are notably different. When martensite NiTi is heated, it begins to change into austenite (Fig. 1). The temperature at which this phenomenon starts is called austenite start temperature (As ). The temperature at which this phenomenon is complete is called austenite finish temperature (Af ). When austenite NiTi is cooled, it begins to change into martensite. The temperature at which this phenomenon starts is called martensite start temperature (Ms ). The temperature at which martensite is again completely reverted is called martensite finish temperature (Mf ) [BUEHLER et al., 1967].
The composition and metallurgical treatments have dramatic impacts on the above transition temperatures. From the point of view of practical applications, NiTi can have three different forms: martensite, stress-induced martensite (superelastic), and austenite. When the material is in its martensite form, it is soft and ductile and can be easy deformed. Superelastic NiTi is highly elastic (rubber-like), while austenitic NiTi is quite strong and hard (similar to titanium) (Fig.2). The NiTi material has all these properties, their specific expression depending on the temperature in which it is used. In Fig. 1 Md represents the highest temperature to straininduced martensite and the grey area represents the area of optimal superelasticity.
Hysteresis
The temperature range for the martensite-to-austenite transformation, i.e. soft-to-hard transition that takes place upon heating is somewhat higher than that for the reverse transformation upon cooling (Fig.1). The difference between the transition temperatures upon heating and cooling is called hysteresis. Hysteresis is generally defined as the difference between the temperatures at which the material is in 50% transformed to austenite upon heating and in 50% transformed to martensite upon cooling. This difference can be up to 20–30 .
Thermoelastic Martensitic Transformation
The unique behavior of NiTi is based on the temperature-dependent austenite-to-martensite phase transformation on an atomic scale, which is also called thermoelastic martensitic transformation. The thermoelastic martensitic transformation causing the shape recovery is a result of the need of the crystal lattice structure to accommodate to the minimum energy state for a given temperature [OTSUKA etal., 1998].
In NiTi, the relative symmetries between the two phases lead to a highly ordered transformation, where the displacements of individual atoms can be accurately predicted and eventually lead to a shape change on a macroscopic scale. The crystal structure of martensite is relatively less symmetric compared to that of the parent phase. If a single crystal of the parent phase is cooled below Mf , then martensite variants with a total of 24 crystallographically equivalent habit planes are generally created. There is, however, only one possible parent phase (austenite) orientation, and all martensitic configurations revert to that single defined structure and shape upon heating above Af. The mechanism by which single martensite variants deform is called twinning, and it can be described as a mirror symmetry displacement of atoms across a particular atom-plane, the twinning plane.
