Abstract :
Shape memory alloys (SMAs) are metallic
alloys which can recover permanent strains when they are heated above a certain
temperature. The key characteristic of all SMAs is the occurrence of a
martensitic phase transformation. The martensitic transformation is a shear-dominant diffusionless
solid-state phase transformation occurring by nucleation and growth of the
martensitic phase from a parent austenitic phase. When an SMA undergoes a
martensitic phase transformation, it transforms from its high-symmetry, usually
cubic, austenitic phase to a low-symmetry martensitic phase, such as the
monoclinic variants of the martensitic phase in a NiTi SMA.
The martensitic transformation that
occurs in the shape memory alloys yields a thermoelastic martensite and develops from a
high-temperature austenite phase with long-range order. The martensite
typically occurs as alternately sheared platelets, which are seen as a herringbone structure
when viewed metallographically. The transformation, although a first-order phase change,
does not occur at a single temperature but over a range of temperatures that
varies with each alloy system.
An interesting feature of the
stress-strain behavior is seen in Fig. 3c, where the material is tested
slightly above its transformation temperature. At this temperature, martensite can be stress-induced. It then immediately
strains and exhibits the increasing strain at constant stress behavior, seen in AB.
Upon unloading, though, the material reverts to austenite at a lower stress, as seen in line
CD, and shape recovery occurs, not upon the application of heat but upon a
reduction of stress. This effect, which causes the material to be extremely
elastic, is known as pseudoelasticity. Pseudoelasticity is nonlinear. The Young's modulus is therefore difficult to define in this
temperature range as it exhibits both temperature and strain dependence.
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