Shape memory alloys Flashcards
(7 cards)
What are shape memory alloys?
Shape memory alloys are particular alloys that exhibit the ability to recover the original shape when a stress was applied to deform the material. We must operate before the yield point, that is, we need elastic deformation. To exploit this behavior, we go from one state to another (austenite to martensite and back, for instance). Changing the composition of the alloy slightly will allow it to actuate at lower or higher temperatures, according to the needs of the engineer.
A common example is NiTiNOL (nickel titanium naval ordinance laboratory)
Explain the austenite and martensite phases:
In shape memory alloys (SMAs), the two primary phases involved are austenite and martensite. Austenite is the high-temperature, parent phase that is typically more symmetric and has a relatively stable crystal structure. It exists at higher temperatures and has a more ordered arrangement of atoms, which gives the alloy its original shape.
Martensite, on the other hand, forms at lower temperatures through a diffusionless, solid-state phase transformation. It has a less symmetric crystal structure and can exist in two different forms: twinned and detwinned. The twinned martensite consists of variants (24) arranged in a self-accommodated manner, meaning the internal structure is balanced so that no macroscopic shape change occurs.
When the martensite is subjected to stress, it can transform into detwinned martensite. This detwinned state involves the reorientation of the variants so that the alloy deforms macroscopically, leading to a shape change. This reorientation is reversible, which allows the alloy to recover its original shape when heated back to the austenite phase.
The transformation between phases relies of 4 different temperatures: austenite start, austenite finish, martensite start, and martensite finish. To obtain these we can perform a calorimetry analysis with no stress applied. Alternatively, we can perform iso-stress heating and measure the strain. The result will allow us to identify the transition period for each stress level.
The driving force between these changes is the Gibbs free energy difference between each configuration. As always, the system tries to minimize Gibbs free energy.
What are the Clausius-Clapeyron equations?
A series of equations that shows the dependence of all four relevant temperatures on stress. Given that we perform calorimetry with no stress applied, this allows us to adjust for our use cases.
See equations on doc.
Explain the phase change diagram of a shape memory alloy
See doc
Explain the one way memory shape functionality
In what is functionally a 3d graph, we see that as temperature rises (from a zero stress point) initially nothing changes, but upon exceeding the austenite start temperature we relax strain until the initial configuration is reached.
See doc for figure.
Explain two-way memory shape functionality
Two-way memory shape functions via fixing the configuration in a partially detwinned state. As such, cool a material will result in it reverting to this partially detwinned state, heating it will revert it to an austenitic state, and a variable third configuration is always availible via inclusion of stress.
The partially detwinned state is a result of the material’s history, internal microstructural stresses, and crystallographic constraints, which together prevent full twinning and allow the two-way shape memory effect to manifest. As such, upon cooling and releasing stress there will still be a strong enough bias to have a fixed shape/configuration.
Explain the superelasticity regime
When starting as austenite and operating under the critical yield stress (that is, we avoid plastic deformation), and exceedind the stress that will send us to martensite (stress that will induce martensite by shifting the martensite start temperature beyond the current temperature), we can stay in a regime where all deformations are elastic but the material will revert to its default shape after the stress is removed, and the minimum Gibbs Free energy configuration is once again austenite. This is superelasticity.
See figure in doc.
