Editing Shape-memory alloy (section)

== Overview ==
The two most prevalent shape-memory alloys are [[copper]]-[[aluminium]]-[[nickel]] and [[nickel]]-[[titanium]] ([[Nickel titanium|NiTi]]), but SMAs can also be created by alloying [[zinc]], [[copper]], [[gold]] and [[iron]].
Although iron-based and copper-based SMAs, such as [[Iron|Fe]]-Mn-Si, Cu-Zn-Al and Cu-Al-Ni, are commercially available and cheaper than NiTi, NiTi-based SMAs are preferable for most applications due to their stability and practicability<ref>{{cite journal |last1=Wilkes |first1=Kenneth E. |last2=Liaw |first2=Peter K. |last3=Wilkes |first3=Kenneth E. |title=The fatigue behavior of shape-memory alloys |journal=JOM |date=October 2000 |volume=52 |issue=10 |pages=45–51 |doi=10.1007/s11837-000-0083-3 |bibcode=2000JOM....52j..45W |s2cid=137826371 }}</ref><ref>{{cite journal |last1=Cederström |first1=J. |last2=Van Humbeeck |first2=J. |title=Relationship Between Shape Memory Material Properties and Applications |journal=Le Journal de Physique IV |date=February 1995 |volume=05 |issue=C2 |pages=C2-335–C2-341 |doi=10.1051/jp4:1995251 |doi-access=free }}</ref><ref>{{cite book |doi=10.31399/asm.hb.v02.a0001100 |chapter=Shape Memory Alloys |title=Properties and Selection: Nonferrous Alloys and Special-Purpose Materials |pages=897–902 |year=1990 |isbn=978-1-62708-162-7 |last1=Hodgson |first1=Darel E. |last2=Wu |first2=Ming H. |last3=Biermann |first3=Robert J. }}</ref> as well as their superior thermo-mechanical performance.<ref>{{cite journal |last1=Huang |first1=W. |title=On the selection of shape memory alloys for actuators |journal=Materials & Design |date=February 2002 |volume=23 |issue=1 |pages=11–19 |doi=10.1016/S0261-3069(01)00039-5 }}</ref> SMAs can exist in two different phases, with three different crystal structures (i.e. twinned martensite, detwinned martensite, and austenite) and six possible transformations.<ref>{{cite journal |last1=Sun |first1=L. |last2=Huang |first2=W. M. |title=Nature of the multistage transformation in shape memory alloys upon heating |journal=Metal Science and Heat Treatment |date=21 May 2010 |volume=51 |issue=11–12 |pages=573–578 |doi=10.1007/s11041-010-9213-x |bibcode=2009MSHT...51..573S |s2cid=135892973 }}</ref><ref>{{cite journal |last1=Mihálcz |first1=István |title=Fundamental characteristics and design method for nickel-titanium shape memory alloy |journal=Periodica Polytechnica Mechanical Engineering |date=2001 |volume=45 |issue=1 |pages=75–86 |url=https://pp.bme.hu/me/article/view/1410 }}</ref> The thermo-mechanic behavior of the SMAs is governed by a phase transformation between the austenite and the martensite.

<!-- Austenite phase is low temperature phase, so by cooling we go from martensitic phase to austenite phase and vice-versa. -->NiTi alloys change from [[austenite]] to [[martensite]] upon cooling starting from a temperature below ''M<sub>s</sub>''; ''M<sub>f</sub>'' is the temperature at which the transition to martensite completes upon cooling. Accordingly, during heating ''A<sub>s</sub>'' and ''A<sub>f</sub>'' are the temperatures at which the transformation from martensite to austenite starts and finishes. 

Applying a mechanical load to the martensite leads to a re-orientation of the crystals, referred to as “de-twinning”, which results in a deformation which is not recovered (remembered) after releasing the mechanical load. De-twinning starts at a certain stress ''σ<sub>s</sub>'' and ends at ''σ<sub>f</sub>'' above which martensite continue exhibiting only elastic behavior (as long as the load is below the yield stress). The memorized deformation from detwinning is recovered after heating to austenite.

The phase transformation from austenite to martensite can also occur at constant temperature by applying a mechanical load above a certain level. The transformation is reversed when the load is released.

The transition from the martensite phase to the austenite phase is only dependent on temperature and stress, not time, as most phase changes are, as there is no diffusion involved. Similarly, the austenite structure receives its name from steel alloys of a similar structure. It is the reversible diffusionless transition between these two phases that results in special properties. While martensite can be formed from austenite by rapidly cooling [[carbon]]-[[steel]], this process is not reversible, so steel does not have shape-memory properties.

[[File:Martensite to Austenite transition graph.svg]]

In this figure the vertical axis represents the martensite fraction. The difference between the heating transition and the cooling transition gives rise to [[hysteresis]] where some of the mechanical energy is lost in the process. The shape of the curve depends on the material properties of the shape-memory alloy, such as the alloy's composition<ref>{{cite journal|last1=Wu|first1=S|last2=Wayman|first2=C|title=Martensitic transformations and the shape-memory effect in Ti50Ni10Au40 and Ti50Au50 alloys|journal=Metallography|volume=20|page=359|year=1987|doi=10.1016/0026-0800(87)90045-0|issue=3}}</ref> and [[work hardening]].<ref>{{cite journal |last1=Filip |first1=Peter |last2=Mazanec |first2=Karel |title=Influence of work hardening and heat treatment on the substructure and deformation behaviour of TiNi shape memory alloys |journal=Scripta Metallurgica et Materialia |date=May 1995 |volume=32 |issue=9 |pages=1375–1380 |doi=10.1016/0956-716X(95)00174-T }}</ref>