Shape memory effect is one of the most important characteristics of Nitinol, which can be applied into many applications. Phase transformation temperature is used to characterise this property, which can be probed by DSC.

1. Principle

DSC measures the heat flow difference between the sample and the reference of the sensor in relation to temperature or time. Both the sample and reference are maintained at the same environment in inert gases. The technique provides the phase transformation temperature of the sample.^[1]

2. Data and analysis

The data comes from the experiment data of own material of the group, and it is shown in Fig.1 below.

Fig.1 DSC map of Nitinol. (A_s-Austenite begins to transform, A_f-Austenitic transformation terminates, M_s-Martensitic transition begins, M_f-Martensitic transition ends)

The sample tested by DSC is a piece of alloy wire with 5.9 mg. The sample is cooled to 0 ℃ first, and heated to 200 ℃ at a rate of 10 ℃/min. After that, the sample is cooled to 0 ℃ again (10 ℃/min). The figure shown in the phase transition from Martensite to Austenitic starts around 61℃ and ends at 82 ℃ (irreversible Martensite transition temperature range). Similarly, it is obvious that the endothermic process begins at 46 ℃, and the absorption peak ends at 33 °C (Martensite transition temperature range). The irreversible transition rate, from Martensite to Austenitic, is going to be the fastest around 78 ℃, and the phase transforms sharply at 40 ℃.

The data shows the type of Nitinol needs to finalize the design of shape at above 82 ℃ firstly, and fix the shape until being cooled below 33 ℃ to ensure the phase transition completely. Nitinol is supposed to be stored at a limited ambient temperature below 61 ℃ to otherwise the memory effect comes into play (irreversible Martensite transition begins). Eventually, when heated among transition range and recovered the original shape, Nitinol ought to cool down at phase transition temperature (lower than 33 ℃). This kind of Nitinol applies on thrombus filter, spinal orthopaedic rod and orthopaedic wire, based on its suitable phase transition temperature range.

3. References

[1] Principle of Differential Scanning Calorimetry (DSC). Principle of Differential Scanning Calorimetry (DSC): Hitachi High-Tech GLOBAL. (n.d.). Retrieved from, https://www.hitachi-hightech.com/global/products/science/tech/ana/thermal/descriptions/dsc.html/,  accessed on 6/1/2021.

The components in a material determine its essential properties. For Nitinol, the specific ratio of elements also brings various characteristics. XRF is chosen to assess the composite elements and corresponding contents of Nitinol sample, for its fast and quantitative elemental analysis.

1. Principle

The wavelength of X-ray fluorescence is a characteristic for different elements.^[1]  The intensity of the fluorescence is linearly related to the amount of elements that emit X-ray photos.^[2] Based on this, the spectrum of XRF can be quantitatively analyzed through direct comparison method, in which the elements amount can be obtained by comparing the integral area of the element peak with the standard curve.^[2]

2. Data and analysis

The experimental data presented here is found from literatures due to the limited experimental equipment.

Fig.1 is a XRF result of two types of samples, which are Ni-Ti-coated samples sprayed by HVOF using Ni-rich Ni50.8Ti (at. %) powder and bulk SMA samples with known composition Ni50.8Ti (at. %).^[3]

Fig.1 Ni and Ti X-ray transitions from the Nitinol coating versus the bulk Ni-rich Ni50.8Ti (at. %) sample.^[3]

The positions of peaks infer that there are two main elements, Ni and Ti in the sample. For the overlapping of the peaks of two samples, it is clearly presented that the chemical composition of the Nitinol coating sprayed using HVOF technique is almost identical with that of the bulk sample of known Ni50.8Ti (at. %) composition.

Fig.2 Standard X-ray fluorescence spectrometry curves of (A) Nickel and (B) Titanium.^[4]

In addition, without the bulk sample, the element concentration still can be acquired from the standard X-ray fluorescence spectrometry curves. Fig.2(A) and (B) illustrate the established standard XRF spectrometry curves of Nickel and Titanium, respectively.^[4] The relationships between intensity and XRF concentration are fitted to a regression line with a function shown in the figures. To get the concentration directly, the area of peak (i.e., the intensity of element) in Fig.1 need to be calculated first. Then use Fig.2 to find the corresponding XRF concentration in accordance with the calculated value of intensity. In this way, the concentration can be obtained.

For the components and corresponding concentration of Ni and Ti are identified, it is appropriate for teeth orthopaedic filament and other novel biomedical applications based on the biophilic metal element Ti and shape memory effect.

3. References

[1] Germanos Karydas. (2014). Introduction to Quantitative XRF analysis. Retrieved from,

http://indico.ictp.it/event/a13226/session/1/contribution/7/material/slides/0.pdf, accessed on 6/1/2021.

[2] Koleske, Joseph V., Paint and Coating Testing Manual - Fifteenth Edition of the Gardner-Sward Handbook: (MNL 17-2nd) - 76.3.6.5 Fundamental Parameter Methods, ASTM International, 2012, p 178-180.

[3] De Crescenzo C, Karatza D, Musmarra D, Chianese, and Baxevanis T., Ni-Ti Shape Memory Alloy Coatings for Structural Applications: Optimization of HVOF Spraying Parameters, Advances in Materials Science and Engineering 2018, 2018, 78, p 67-83.

[4] Bertin, E. P., Principles and Practice of X-ray Spectrometric Analysis, Kluwer Academic, Plenum Publishers, 2017, p 235-237.

Structure is the internal property of materials. It can be used to design the properties for different uses, and XRD is used to observe this characterisation.

1. Principle

When a monochromatic X-ray beam is incident into a crystal, the incident ray will interact with the sample and produce long interference and diffracted ray if the Bragg's law (nλ = 2dsinθ) is satisfied. This technique can provide crystal and lattice structure of the sample.^[1]

2. Data and analysis

Data is found from other research, and the results are shown in the figures below. 

Fig.1 (a) Complete XRD pattern of Case 1 wires; (b) XRD patterns between 30° and 50° 2θ angle.^[2]

Fig. 2 (a) Complete XRD pattern of Case 2 wires; (b) XRD patterns between 30° and 50° 2θ angle.^[2]

• Case 1: Control group.

Samples in four different diameters are detected in XRD and the result can be seen in Fig.1. It is indicated that the outstanding peak is at the 2θ angle of 40° to 45°, and these samples have the same phase state of detwinned martensite.^[2]

It is illustrated that the peak of sample in 0.15 mm diameter has the lowest intensity. This may because that the size of X-ray beam is only 0.12 mm, which will cause lower reflection since the little difference of the diameter for the sample.^[2] In addition, the characteristic peaks of martensite phase for the four samples are at the 2θ angle of 43.5°, which can be proved in other studies.^[3]

• Case 2: Samples are heated to range of 70-80 ℃.

Fig.2 shows that XRD patterns for diameters of 0.15 mm and 0.19 mm are different from that of 0.24 mm and 0.29 mm. The former ones exist another phase called R-phase at the 2θ angle about 42.3°. Then for 0.15 mm, the intensity of (112 R) is higher than the peaks of (002 M). Therefore, the dominant phase for it is R-phase. The difference may due to the insufficient heating for phase transformation of 0.24 mm and 0.29 mm from detwinned martensite to austenite. So, the dominate phase for them is still detwinned martensite.^[2]

3. References

[1] X-ray Powder Diffraction (XRD). Techniques. (2020, February 14). Retrieved from, https://serc.carleton.edu/research_education/geochemsheets/techniques/XRD.html, accessed on 6/1/2021.

[2] Honarvar, M., Konh, B., Podder, T.K. et al. X-ray Diffraction Investigations of Shape Memory NiTi Wire. J. of Materi Eng and Perform 24, 3038–3048 (2015).

[3] M. Iijima, W.A. Brantley, W.H. Guo, W.A.T. Clark, T. Yuasa, and I. Mizoguchi, X-ray Diffraction Study of Low-Temperature Phase Transformations in Nickel-Titanium Orthodontic Wires, Dent. Mater., 2008, 24, p 1454–1460