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NiTi Thin-Film Shape Memory Alloys and Their Industrial Application
Published in Chander Prakash, Sunpreet Singh, J. Paulo Davim, Advanced Manufacturing and Processing Technology, 2020
Ajit Behera, Patitapabana Parida, Aditya Kumar
B19′ is the most common martensite phase observed in NiTi and its alloys. It is known here that the structure of B19 has a lower symmetry than B2. That is, SMA has undergone a transition from a cubic austenite phase with high symmetry (high temperature) to a monoclinic martensite phase with low symmetry (low temperature). B19′ has a space group of P21/m and has a monoclinic crystal structure. For TiNi49.2, its lattice parameters are a = 0.2889 nm, b = 0.4108 nm, and c = 0.4646 nm and β = 97.780. The mesh parameters depend on the composition. The other phase, called R phase, belongs to the space group of P3, and its lattice parameters are a = 0.732 nm and c = 0.532 nm. The R phase extends 0.94% in the direction of the upper [111] B2, which is one order smaller than that of B19″ [1,2]. B19 is an orthogonal crystal phase with a lattice constant of a = 0.29 nm, b = 0.425 nm, and c = 0.45 nm. The B2→R conversion immediately follows and starts from B2 + B19′. Usually, we can write the conversion path as B2→ B2+B19→ R+B19′→ B19′, the second phase of which is narrow, observed in the temperature range (60°C–55°C) [17]. Miyazaki believed that the appearance of the R phase was due to precipitation (such as Ti2Ni in titanium-rich materials or Ni4Ti3 in nickel-rich materials) and fine grain structure, which may hinder martensitic transformation [18,19]. Compared with the martensitic transformation, the hysteresis of the R phase transformation is very small in NiTi SMA. Therefore, the R phase transition plays an effective role in improving the response of high-speed microactuators [20]. Due to the deformation of the B2 battery in the <111> direction, the R phase is formed. The transformation from cubic B2 austenite to the diamond phase is initiated by deformation during cooling at the diamond starting temperature. Thermal cycling under no stress conditions is conducive to the formation of the R phase in NiTi. Another condition is the introduction of dislocations and internal stress through thermal cycling that allows R phase nucleation [21]. Various factors that inhibit the onset temperature of martensite are (i) increasing the percentage of Ni, (ii) aging at intermediate temperature, (iii) annealing at recrystallization temperature after cold working, and (iv) replacing the third element and (v) precipitation and fine grain size. Phase transition temperature will (i) inhibit R phase; (ii) increase grain size; and (iii) reduce oxygen pollution.
Surface integrity analysis of Nitinol-60 shape memory alloy in WEDM
Published in Materials and Manufacturing Processes, 2019
Biplab Kumar Roy, Amitava Mandal
The crystallography and metallurgical characteristics of NiTi alloy are discussed in this section. Laves and Wallbaum assumed that the structure of the equiatomic alloy Ni-Ti was CsCl (B2) type.[22] However, subsequent research works in the phase equilibrium area of Ni-Ti system proved that the structure is a large cubic cell consisting of 27 sub-cells of CsCl structure and is stable at moderately elevated temperatures only. Nitinol exhibits austenite (parent phase) at higher temperatures and martensite (product phase) at a lower temperature. While the austenite phase (B2) is highly symmetric and has a bcc crystal structure, the martensite phase (B19ʹ) has low symmetry and exists as a monoclinic crystal. The conversion of the austenite phase to the martensite phase is based on diffusionless solid phase transformation which results in shape memory effect.[23] During the conversion from austenite to martensite, an intermediate phase may be observed which is termed as the R-phase. The crystal structure of this phase is reported to be rhombohedral distortion. The various temperatures that define this transformation are As, Af, Ms, and Mf. As or austenite start temperature is the temperature where martensite initiates to convert to austenite on heating and Af or austenite finish temperature is the temperature where the conversion to austenite is 100% complete. Similarly, Ms is the temperature where austenite begins to convert to martensite on cooling and Mf is the temperature where the formation of martensite is 100% complete. Apart from these temperatures, there is another temperature Md, which is the highest temperature where martensite cannot be activated by stress and above this temperature, the alloy is permanently deformed.[24] One-way shape memory effect (OWSME) is an important characteristic of Ni-Ti based SMA. In OWSME, Nitinol is in martensite form at lower temperatures, and due to martensitic shear, it can be deformed quite easily.[25] It retains its shape even after the removal of the load due to residual strain and can revert back to its original form by heating above its transformation temperature.[26]