Shape Memory Alloys and Their Applications in Power Generation and Refrigeration
The shape memory effect is closely related to the reversible martensitic phase transformation, which is diffusionless and involves shear deformation. The recoverable transformation between the two phases with different crystalline symmetry results in reve
- PDF / 1,051,888 Bytes
- 19 Pages / 439.37 x 666.142 pts Page_size
- 5 Downloads / 177 Views
Shape Memory Alloys and Their Applications in Power Generation and Refrigeration Jun Cui
Abstract The shape memory effect is closely related to the reversible martensitic phase transformation, which is diffusionless and involves shear deformation. The recoverable transformation between the two phases with different crystalline symmetry results in reversible changes in physical properties such as electrical conductivity, magnetization, and elasticity. Accompanying the transformation is a change of entropy. Fascinating applications are developed based on these changes. In this chapter, the history, fundamentals and technical challenges of both thermoelastic and ferromagnetic shape memory alloys are briefly reviewed; applications related to energy conversion such as power generation and refrigeration as well as recent developments will be discussed.
12.1 Introduction A martensitic phase transformation is diffusionless and involves crystallographic shearing deformation. It reduces the symmetry of the parent phase and results in formation of crystallographic domains with several possible geometric arrangements. These possible domains are referred to as martensitic variants. They can be described by the transformation stretch tensor. The exact forms of the variants can be derived if lattice parameters of both the austenite and the martensite phases are known [1]. For example, there are six martensitic variants for a transformation from cubic to orthorhombic. In the case of transformation through face-diagonal stretch, the exact forms of the variants are
J. Cui (&) Pacific Northwest National Laboratory, Richland, WA 99352, USA e-mail: [email protected]
A. Saxena and A. Planes (eds.), Mesoscopic Phenomena in Multifunctional Materials, Springer Series in Materials Science 198, DOI: 10.1007/978-3-642-55375-2_12, Springer-Verlag Berlin Heidelberg 2014
289
290
J. Cui
0
b B0 U1 ¼ @ 0 0 b B U1 ¼ @ 0 0
0 aþc 2 ac 2
0 aþc 2 ca 2
0
1
ac C 2 A; aþc 2
0
1
ca C 2 A; aþc 2
0 aþc 2
B U1 ¼ @ 0
ac
2 0 aþc
0 b 0
2
0
ca 2
b 0
B U1 ¼ @ 0
ac 1 2
0 aþc
aþc 2 ca 1 2
0 0 aþc
C 0 A ; U1 ¼
2 B ac @ 2
2
C B 0 A; U1 ¼ @ ca 2 aþc 0 2
ac 2 aþc 2
0 ca 2 aþc 2
0
0
1
C 0 A; b 1 0 C 0 A; b ð12:1Þ
where a, b, c are proportional to a/a0, b/a0, c/a0, and a, b, c, and a0 are the lattice parameters of the orthorhombic and cubic lattices, respectively. Note that the middle eigenvalue, k2, of the stretch tensor U describes the geometric compatibility of the martensite variant with the austenite phase. It has been established that the value of k2 has direct impact on the transformation hysteresis and fatigue life [2].
12.2 Thermoelastic and Ferromagnetic Shape Memory Effects When a reversible martensitic phase transformation is thermally induced, the material has no preference for any of the variants because they all have the same free energy. The randomly distributed variants leave the material with little change in its overall shape. However, when a stress is applied to this mixture of variants, certain v
Data Loading...
