Biomedical Engineering Reference
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Figure 8.12 Different states of structure in SMA: (a) austenite to martensite
transformation, (b) austenite phase, (c) twinned (self-
accommodated) martensite, and (d) detwinned martensite
possible coniguration [41].
8.4.1
Thermally Induced Transformation
In the situation without external stress applied, the shape memory
material reacts with temperature decrease by a phase transition from
austenite to martensite and reversely with temperature increase.
No macroscopic shape change associated with that transformation
occurs; the atoms remain on their lattice position in the absence of
diffusion process. From the energetic point of view in the situation
of temperature reduction and lack of stress, the formation of self-
accommodated twins in structure during austenite to martensite
transformation, where microscopic strain is negligible, is favorable.
Figure 8.13 Thermally induced phase transformation in state of absence
of external load [22].
The characteristic temperature at which martensite trans-
formation begins is marked by M s , and the temperature at which
 
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