I am a PhD Researcher that focuses on characterising the microstructural behaviour of superalloys.
Follow this board to keep updated with all the exciting research in shape memory alloys
In 10 Seconds? Shape memory alloys are a relatively new development in the field of material science – and over the past few years, they have seen a significant amount research to develop its capabilities.
Therefore, in simple terms, a shape memory alloy is a metallic material that can revert to its original shape. However, it can only revert if a thermal load is applied. The main fields of applications lie in aerospace, structural and biomedical. The most commonly used shape memory alloy is NITINOL, which is a combination of nickel and titanium.
Don’t believe it? Review the selected pinned articles to observe just how big an impact SMA’s have had on biomedical applications in particular. They have been very successful in the biomedical field due to their super elastic material properties and being biocompatible.
The science behind the shape memory phenomenon
It is all to do with the alloys crystal structure i.e. the arrangement of atoms – more specifically, the phase change from martensite to austenite and vice versa. Therefore, when the alloy is in its martensite phase it can be easily deformed, note that this deformation only occurs at low temperatures (i.e. below the alloys transformation temperature). Consequntly, as you heat the alloy above its transformation temperature it will automatically revert to its original shape because the atoms within the crystal structure revert to their preferred positions.
What is the opportunity?
Medical – As stated earlier, the medical industry realised the potential of SMA’s. SMA’s have allowed certain surgical procedures to be less invasive. This is because the SMA can be inserted into the body then a particular stimuli will cause the SMA to change size or shape – for example self-expanding stents.
Problem – The drawback from SMA’s for medical applications within the body is that it they cannot be identified through x-rays. Therefore, research is ongoing to identify the most cost effective technique to allow x-rays to pick up SMA components within the body.
For all you entrepreneurial scientists out there, this could be an opportunity to make some $$$’s– do you have a big idea to solve the issue?
Non-technical factors that could hinder innovations within this field
• The main hurdle that would restrict the potential of SMA’s within the biomedical field would be government funding!
• Global political issues can cause the price of raw materials to rise
Abstract: Shape memory behavior of porous NiTi alloy is dependent on the phases, and mechanical or thermal background. The phases change with solution heat treatment and aging. Fully reversible shape memory behavior was observed during thermal cycling, and recoverable strains increased with the increasing stress from 2 to 50 MPa. The porous NiTi sample shows recoverable transformation strain response under lower constant load.
Pub.: 19 Jan '16, Pinned: 20 Apr '17
Abstract: Nanoactuators made from nanoparticulate NiTi shape memory alloy show potential in the mechanical stimulation of bone tissue formation from stem cells. We demonstrate the fabrication of Ni, Ti, and NiTi shape memory alloy nanoparticles and their biocompatibility to human adipose-derived stem cells. The stoichiometry and phase transformation property of the bulk alloy is preserved during attrition by femtosecond laser ablation in liquid, giving access to colloidal nanoactuators. No adverse effect on cell growth and attachment is observed in proliferation assay and environmental electron scanning microscopy, making this material attractive for mechanical stimulation of stem cells.
Pub.: 12 Mar '10, Pinned: 20 Apr '17
Abstract: Substituting Ni with Au in NiTi leads to dramatic increases in transformation temperatures, meeting one of the requirements for a viable high temperature actuator material. Consequently, four alloys containing between 49 and 51 at.% Ti, a fixed 40 at.% Au, and balance Ni, were prepared and investigated in detail using load-biased thermal cycling (LBTC), scanning electron microscopy (SEM), aberration corrected scanning transmission electron microscopy (STEM), and X-ray energy dispersive spectroscopy (XEDS). LBTC experiments demonstrated work output well above 400 °C, with full recovery up to 100 MPa. The alloys exhibit minimal variation in shape memory properties despite the relatively large composition range from Ti-lean to Ti-rich, in stark contrast to most other NiTi-based systems, which demonstrate extreme compositional sensitivity. Electron beam analysis revealed the presence of two types of secondary phases present in all compositions, which are subsequently characterized. Differences in secondary phase content as a function of alloy composition is shown to have a moderating effect on the transforming matrix composition - an important asset for this alloy system - potentially easing processing requirements and opening up shape memory alloys to new fabrication techniques. Unrecovered strain during cycling at higher loads is analyzed from a theoretical perspective to gain insight into the mechanisms of defect formation responsible for functional fatigue.
Pub.: 21 Mar '17, Pinned: 20 Apr '17
Abstract: Modern Physics Letters B, Ahead of Print. The effect of rare earth element neodymium (Nd) addition on the microstructure and martensitic transformation behavior of Ni[math]Ti[math]Nd[math] ([math] = 0, 0.1, 0.3, 0.5 and 0.7 at.%) shape memory alloy was investigated by scanning electronic microscope, X-ray diffraction and differential scanning calorimetry. The results show that the microstructure of Ni–Ti–Nd ternary alloy consists of NiNd phase, NiTi2 and the NiTi matrix. A one-step martensitic transformation is observed in the alloys. The martensitic transformation temperature Ms increases sharply increasing 0.1–0.7 at.% Nd content is added.
Pub.: 13 Oct '16, Pinned: 20 Apr '17
Abstract: Effects of Fe addition on martensitic transformation and shape memory behavior of Ti-V-Al-Fe alloys were investigated. A little amount of Fe addition reduces martensitic transformation temperature rapidly and enhances shape memory effect of Ti-V-Al alloy. Alloy with an optimal Fe content, which is Ti-13V-3Al-1Fe, shows the largest recoverable strain of 5.8% and highest recovery stress of 250 MPa when pre-strain is 6%. Meanwhile, Ti-13V-3Al-1Fe alloy has a large elongation of 30%.
Pub.: 20 Apr '16, Pinned: 20 Apr '17
Abstract: The microstructures, phase transformations, mechanical properties and shape memory effect of Ti-20Zr-10Nb-xAl (x=1, 2, 3, 4 at.%) alloys were investigated. The X-ray diffraction results show that the alloys are composed of a single martensitic α″-phase and that the corresponding unit cell volume decreases with increasing Al content. The reverse martensitic transformation start temperature (As) of the Ti-20Zr-10Nb-Al alloy is 534 K and decreases with increasing Al content. The addition of Al results in solid solution strengthening and grain refinement strengthening, thus improving the mechanical properties and the shape memory effect of the Ti-20Zr-10Nb-xAlalloys. The Ti-20Zr-10Nb-3Al alloy shows the greatest shape memory strain (3.2%) and the largest tensile strain (17.6%) as well as a very high tensile strength (886 MPa).
Pub.: 08 Oct '16, Pinned: 20 Apr '17
Abstract: Publication date: December 2016 Source:Materials Characterization, Volume 122 Author(s): Wentao Qu, Xuguang Sun, Bifei Yuan, Chengyang Xiong, Fei Zhang, Yan Li, Baohui Sun The microstructures, phase transformations and shape memory properties of Ti-30Zr-xNb (x=5, 7, 9, 13at.%) alloys were investigated. The X-ray diffraction and transmission electron microscopy observations showed that the Ti-30Zr-5Nb, Ti-30Zr-7/9Nb and Ti-30Zr-13Nb alloys were composed of the hcp α′-martensite, orthorhombic α″-martensite and β phases, respectively. The results indicated the enhanced β-stabilizing effect of Nb in Ti-30Zr-xNb alloys than that in Ti-Nb alloys due to the high content of Zr. The differential scanning calorimetry test indicated that the Ti-30Zr-5Nb alloy displayed a reversible transformation with a high martensitic transformation start temperature of 776K and a reverse martensitic transformation start temperature (A s) of 790K. For the Ti-30Zr-7Nb and Ti-30Zr-9Nb alloys, the martensitic transformation temperatures decreased with the increasing Nb content. Moreover, an ω phase transformation occurred in the both alloys upon heating at a temperature lower than the corresponding A s , which is prompted by more addition of Nb. Although the critical stress in tension of the three martensitic alloys decreased with increasing Nb content, the Ti-30Zr-9Nb alloy showed a critical stress of as high as 300MPa. Among all the alloys, the Ti-30Zr-9Nb alloy exhibited the maximum shape memory effect of 1.61%, due to the lowest critical stress for the martensite reorientation.
Pub.: 23 Oct '16, Pinned: 20 Apr '17
Abstract: Extensive research on the effect of spark-plasma sintering and free forging on the microstructure, transformation temperature, and superelasticity of Ti-23 at% Nb shape-memory alloys (SMAs) was undertaken. Based on the obtained results, it was found that the specimen showed significant results after deformation than the homogenization step, from which it can be concluded that the deformation has considerable effects on the mechanical properties and shape-memory characteristics of Ti-based SMAs. The microstructure of homogenized and deformed Ti-23 at% Nb SMAs consists of β, α, α′ and ω phases, while the sintered sample shows only β and α phases. Endothermic and exothermic peaks of the austenite ↔ martensite transformation are seen in the homogenized sample, then after being deformed, the exothermic peak disappeared. The highest austenite start and finish temperatures were observed with the deformed sample due to disappearance of TiNb4 intermetallic compounds and a high percentage of β phase. The ductility of the deformed sample shows the highest value compared with the homogenized sample due the elimination of coarse α precipitates and brittle intermetallic compounds of TiNb4, along with the suppression of the brittle ω phase and retaining β matrix.
Pub.: 11 Jan '17, Pinned: 20 Apr '17
Abstract: Nickel-titanium (NiTi) shape memory alloys are widely used for medical components, as they can accommodate large strains in their superelastic state. In order to further improve the mechanical properties of NiTi, grain refinement by severe plastic deformation is applied to generate an ultrafine-grained microstructure with increased strength. In this work comprehensive fracture and fatigue crack growth experiments were performed on ultrafine-grained NiTi to assess its damage tolerance, which is essential for the safe use of this material in medical applications. It was found, that equal channel angular pressing of NiTi for 8 passes route BC increases the transformation stress by a factor of 1.5 and the yield stress of the martensite by a factor of 2.6, without significantly deteriorating its fracture and fatigue crack growth behavior. The fatigue crack growth behavior at high mean stresses is even improved, with lower fatigue crack growth rates and higher threshold stress intensity factor ranges, however, beneficial contributions from crack closure are slightly reduced.
Pub.: 12 Apr '17, Pinned: 20 Apr '17