Editing Shape-memory alloy (section)

== One-way vs. two-way shape memory ==
Shape-memory alloys have different shape-memory effects. The two common effects are one-way SMA and two-way SMA. A schematic of the effects is shown below.
The procedures are very similar: starting from martensite, adding a deformation, heating the sample and cooling it again.

=== One-way memory effect ===
[[File:One way shape memory effect.svg]]

When a shape-memory alloy is in its cold state (below ''M<sub>f</sub>''), the metal can be bent or stretched and will hold those shapes until heated above the transition temperature. Upon heating, the shape changes to its original. When the metal cools again, it will retain the shape, until deformed again.

With the one-way effect, cooling from high temperatures does not cause a macroscopic shape change. A deformation is necessary to create the low-temperature shape. On heating, transformation starts at ''A<sub>s</sub>'' and is completed at ''A<sub>f</sub>'' (typically 2 to 20&nbsp;°C or hotter, depending on the alloy or the loading conditions). ''A<sub>s</sub>'' is determined by the alloy type and composition and can vary between {{val|-150|u=°C}} and {{val|200|u=°C}}.

===Two way effect ===
[[File:Two way shape memory effect.svg]]

The two-way shape-memory effect is the effect that the material remembers two different shapes: one at low temperatures, and one at the high temperature.
A material that shows a shape-memory effect during both heating and cooling is said to have two-way shape memory. This can also be obtained without the application of an external force (intrinsic two-way effect).
The reason the material behaves so differently in these situations lies in training. Training implies that a shape memory can "learn" to behave in a certain way.
Under normal circumstances, a shape-memory alloy "remembers" its low-temperature shape, but upon heating to recover the high-temperature shape, immediately "forgets" the low-temperature shape. However, it can be "trained" to "remember" to leave some reminders of the deformed low-temperature condition in the high-temperature phases. One way of training the SMA consists in applying a cyclic thermal load under constant stress field. During this process, internal defects are introduced into the microstructure which generates internal permanent stresses that facilitate the orientation of the martensitic crystals.<ref>{{Cite book |url=https://www.worldcat.org/oclc/272298744 |title=Shape memory alloys : modeling and engineering applications |date=2008 |publisher=Springer |others=Dimitris C. Lagoudas |isbn=978-0-387-47685-8 |location=New York |oclc=272298744}}</ref> Therefore, while cooling a trained SMA in austenitic phase under no applied stress, the martensite is formed detwinned due to the internal stresses, which leads to the material shape change. And while heating back the SMA into austenite, it recovers its initial shape.

There are several ways of doing this.<ref>[http://www-personal.umich.edu/~btrease/share/SMA-Shape-Training-Tutorial.pdf Shape Memory Alloy Shape Training Tutorial]. (PDF) . Retrieved on 2011-12-04.</ref> A shaped, trained object heated beyond a certain point will lose the two-way memory effect.