Pseudoelasticity

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Pseudoelasticity, sometimes called superelasticity, is an elastic (reversible) response to an applied stress, caused by a phase transformation between the austenitic and martensitic phases of a crystal. It is exhibited in shape-memory alloys.

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Overview

Pseudoelasticity is from the reversible motion of domain boundaries during the phase transformation, rather than just bond stretching or the introduction of defects in the crystal lattice (thus it is not true super elasticity but rather pseudo elasticity). Even if the domain boundaries do become pinned, they may be reversed through heating. Thus, a pseudoelastic material may return to its previous shape (hence, shape memory) after the removal of even relatively high applied strains. One special case of pseudoelasticity is called the Bain Correspondence. This involves the austenite/martensite phase transformation between a face-centered crystal lattice (FCC) and a body-centered tetragonal crystal structure (BCT). [1]

Superelastic alloys belong to the larger family of shape-memory alloys. When mechanically loaded, a superelastic alloy deforms reversibly to very high strains (up to 10%) by the creation of a stress-induced phase. When the load is removed, the new phase becomes unstable and the material regains its original shape. Unlike shape-memory alloys, no change in temperature is needed for the alloy to recover its initial shape.

Superelastic devices take advantage of their large, reversible deformation and include antennas, eyeglass frames, and biomedical stents.

Nickel titanium (Nitinol) is an example of an alloy exhibiting superelasticity.

Size effects

Recently, there have been interests of discovering materials exhibiting superelasticity in nanoscale for MEMS (Microelectromechanical systems) application. The ability to control the martensitic phase transformation has already been reported. [2] But the behavior of superelasticity has been observed to have size effects in nanoscale.

Qualitatively speaking, superelasticity is the reversible deformation by phase transformation. Therefore, it competes with the irreversible plastic deformation by dislocation motion. At nanoscale, the dislocation density and possible Frank–Read source sites are greatly reduced, so the yield stress is increased with reduced size. Therefore, for materials exhibiting superelasticity behavior in nanoscale, it has been found that they can operate in long-term cycling with little detrimental evolution. [3] On the other hand, the critical stress for martensitic phase transformation to occur is also increased because of the reduced possible sites for nucleation to begin. Nucleation usually begins near dislocation or at surface defects. But for nanoscale materials, the dislocation density is greatly reduced, and the surface is usually atomically smooth. Therefore, the phase transformation of nanoscale materials exhibiting superelasticity is usually found to be homogeneous, resulting in much higher critical stress. [4] Specifically, for Zirconia, where it has three phases, the competition between phase transformation and plastic deformation has been found to be orientation dependent, [5] indicating the orientation dependence of activation energy of dislocation and nucleation. Therefore, for nanoscale materials suitable for superelasticity, one should research on the optimized crystal orientation and surface roughness for most enhanced superelasticity effect.

See also

Related Research Articles

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Martensite

Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation.

Plasticity (physics) Deformation of a solid material undergoing non-reversible changes of shape in response to applied forces

In physics and materials science, plasticity, also known as plastic deformation, is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. For example, a solid piece of metal being bent or pounded into a new shape displays plasticity as permanent changes occur within the material itself. In engineering, the transition from elastic behavior to plastic behavior is known as yielding.

Bainite

Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C. First described by E. S. Davenport and Edgar Bain, it is one of the products that may form when austenite is cooled past a temperature where it is no longer thermodynamically stable with respect to ferrite, cementite, or ferrite and cementite. Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite.

A shape-memory alloy(SMA) is an alloy that can be deformed when cold but returns to its pre-deformed ("remembered") shape when heated. It may also be called memory metal, memory alloy, smart metal, smart alloy, or muscle wire.

Magnetic shape memory alloys (MSMAs), also called ferromagnetic shape memory alloys (FSMA), are particular shape memory alloys which produce forces and deformations in response to a magnetic field. The thermal shape memory effect has been obtained in these materials, too.

Dislocation Linear crystallographic defect or irregularity

In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to slide over each other at low stress levels and is known as glide or slip. The crystalline order is restored on either side of a glide dislocation but the atoms on one side have moved by one position. The crystalline order is not fully restored with a partial dislocation. A dislocation defines the boundary between slipped and unslipped regions of material and as a result, must either form a complete loop, intersect other dislocations or defects, or extend to the edges of the crystal. A dislocation can be characterised by the distance and direction of movement it causes to atoms which is defined by the Burgers vector. Plastic deformation of a material occurs by the creation and movement of many dislocations. The number and arrangement of dislocations influences many of the properties of materials.

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Work hardening

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Recrystallization (metallurgy)

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In metallurgy, materials science and structural geology, subgrain rotation recrystallization is recognized as an important mechanism for dynamic recrystallisation. It involves the rotation of initially low-angle sub-grain boundaries until the mismatch between the crystal lattices across the boundary is sufficient for them to be regarded as grain boundaries. This mechanism has been recognized in many minerals and in metals.

Nickel titanium High-durability alloy

Nickel titanium, also known as Nitinol, is a metal alloy of nickel and titanium, where the two elements are present in roughly equal atomic percentages. Different alloys are named according to the weight percentage of nickel; e.g., Nitinol 55 and Nitinol 60. It exhibits the shape memory effect and superelasticity at different temperatures.

Ferroelasticity is a phenomenon in which a material may exhibit a spontaneous strain. Usually, a crystal has two or more stable orientational states in the absence of mechanical stress or electric field, i.e. remanent states, and can be reproducibly switched between states by the application of mechanical stress. In ferroics, ferroelasticity is the mechanical equivalent of ferroelectricity and ferromagnetism. When stress is applied to a ferroelastic material, a phase change will occur in the material from one phase to an equally stable phase, either of different crystal structure, or of different orientation. This stress-induced phase change results in a spontaneous strain in the material.

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The R-phase is a phase found in nitinol, a shape-memory alloy. It is a martensitic phase in nature, but is not the martensite that is responsible for the shape memory and superelastic effect.

In materials science, the yield strength anomaly refers to materials wherein the yield strength increases with temperature. For the majority of materials, the yield strength decreases with increasing temperature. In metals, this decrease in yield strength is due to the thermal activation of dislocation motion, resulting in easier plastic deformation at higher temperatures.

Toughening is the improvement of the fracture resistance of a given material. The material's toughness is described by irreversible work accompanying crack propagation. Designing against this crack propagation leads to toughening the material.

References

  1. ↑ Bhadeshia, H. K. D. H. "The Bain Correspondence" (PDF). Materials Science and Metallurgy. University of Cambridge.
  2. ↑ Thorsten Krenke; et al. (2007). "Magnetic superelasticity and inverse magnetocaloric effect in Ni-Mn-In". Physical Review B. 75 (10): 104414. arXiv: 0704.1243 . doi:10.1103/PhysRevB.75.104414.
  3. ↑ J. San Juan; et al. (2014). "Long-term superelastic cycling at nano-scale in Cu-Al-Ni shape memory alloy micropillars". Applied Physics Letters. AIP. 104: 011901. doi:10.1063/1.4860951.
  4. ↑ J. San Juan; et al. (2013). "superelasticity and shape memory at nano-scale: size effects on the martensitic transformation". Journal of Alloys and Compounds. Elsevier. 577: S25–S29. doi:10.1016/j.jallcom.2011.10.110.
  5. ↑ Ning Zhang; et al. (2016). "Competing mechanisms between dislocation and phase transformation in plastic deformation of single crystalline yttria-stabilized tetragonal zirconia nanopillars". Acta Materialia. 120: 337–347. arXiv: 1607.03141 . doi:10.1016/j.actamat.2016.08.075.
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