Austenite shape-memory phase

Fig. 2. The shape-memory process, where Tis temperature, (a) The cycle where the parent phase undergoes a self-accommodating martensite transformation on cooling to the 24 variants of martensite. No macroscopic shape change occurs. The variants coalesce under stress to a single martensite variant, resulting in deformation. Then, upon heating, they revert back to the original austenite crystallographic orientation, and reverse transformation, undergoing complete recovery to complete the cycle, (b) Shape deformation. Strain recovery is typically ca 7%.

Figure 3.6 Shape-memory alloys transform from (a) a partially ordered, high-temperature austenitic phase to (b) a mixed austenite-martensite low-temperature state to (c) an ordered mixed-phase state under deformation.

The behavior of shape memory alloys can be explained on the basis of solid state phase changes that occur within the material. All SMAs exist in one of two phases, known as martensite and austenite, shown in the diagram on page 132. Austenite is the "parent ... 

The shape-memory effect is observed when the temperature of a piece of alloy is cooled to below that required to form the martensite phase Ms (initial martensite formation) until Mf (martensite formation complete), as seen in Figure 3.25. Upon heating the martensitic material, a reformation of austenite begins to occur at Hg... 

Lakhani A, Dash S, Banerjee A, Chaddah P, Chen X, Ramanujan RV. Tuning the austenite and martensite phase fraction in ferromagnetic shape memory alloy ribbons of Ni45Co5Mn38Sn12. Applied Physics Letters. 2011 99 242503(l)-242503(3). DOI 10.1063/1.3669510. 

The structural transformation between austenite and martensite occurs when the mechanical stress attains a certain level, or with an appropriate temperature change, A reversible twinning process takes place at the atomic level, which can result in superelastic behaviour and shape memory [8], The properties of the nickel-titanium endodontic instruments and orthodontic wires depend critically upon the nature and proportions of the NiTi phases in their microstructures, as discussed in the following sections. While X-ray diffraction has been used to study the phases in nickel-titanium endodontic instruments [15,16] and orthodontic wires [7,17,18], this analytical technique is limited to a near-surface region less than 50 pm in depth for metallic materials [19], and study of the phase transformations with temperature is not generally convenient. In contrast, DSC can provide information about the phases present in bulk nickel-titanium endodontic instruments and orthodontic wires with facility, and the effect of temperature changes on the NiTi phase transformations is easily studied. 

The DSC plots for tiie two shape-memory wires were both characterized by a single peak on the heating curve, indicative of direct transformation from martensite to austenite, whereas two peaks were observed on the cooling curves, corresponding to transformation from austenite to R-phase followed by transformation from R-phase to martensite [25]. In contrast to Figures 1 and 2, these two peaks on the cooling curves for tiie shape-memory wires were much more widely separated (by approximately 50 °C). 

Similar results were obtained for TMDSC analyses of the in vivo shape-memory wire, Neo Sentalloy, shown in Figures 9 and 10 [37]. While our previous DSC study [25] suggested that direct transformation of martensite to austenite occurs during heating, the nonreversing heat flow curve in Figure 9 shows that a two-step transformation involving R-phase actually takes place. 

A NiTiNOL shape memory metal alloy can exist in two different temperature-dependent crystal structures or phases called martensite (i.e., lower-temperature phase) and austenite (i.e., higher-temperature or parent phase). Several properties of the austenite and martensite phases are notably different. When martensite is heated, it begins to change into austenite. The temperature at which this phenomenon starts is called the austenite start temperature A). The temperature at which the phenomenon is complete is called the austenite finish temperature (A). When austenite is cooled, it begins to change into martensite. The temperature at which this phenomenon starts is called the martensite start temperature (M ). The temperature at which martensite is again completely reverted is called the martensite finish temperature (Mj). Composition and metallurgical treatments have dramatic impacts on the above transition temperatures. From the point of view of practical applications, NiTiNOL can have three different forms ... 

AH of these materials have at least two phase transitions that can be described in terms of thermodynamic functions with two ordering parameters (see Appendix B). Ferroic materials are operated near an instabiUty to make domain walls with their associated dipoles and strains moveable, as encountered in PZT or Terfenol . On the other hand, a second type of material involves a partially ordered phase, as in PMN or the shape memory alloys. These materials are operated near a diffuse phase transition with two coexisting phases, a high-temperature austenite-like phase and a low-temperature martensite-Uke phase. A third type of smart... 

A typical shape memory alloy starts in its austenitic phase. When the material is strained, it transforms to the martensitic phase, oriented in a way to produce an elongation in the direction of the load. 

The SMA, known as the memory metal or smart wire, was first discovered in an Ag —Cd alloy in 1932. Due to a crystalline transformation between a high temperature austenite phase and a low temperature martensite phase, when materials with this shape memory property... 

The intermetallic Ni-Ti system has the imusual property of after being distorted, returning to its original shape when heated. This was the first of the shape memory alloys (SMAs) and was discovered by accident at the Naval Ordnance Laboratory, hence its name Nitinol. Other SMAs include Cu-Al-Ni, Cu-Zn-Al, and Fe-Mn-Si alloys. The shape memory mechanism depends on a martensitic solid-state phase transition that takes place at a modest temperature (50°C—150°C), depending on the alloy. The high temperature phase is referred to as austenite and the low temperature phase is called martensite (following the terminology of the Fe-FeCa system). 

Recent Advances in Metal-Based Materials The use of shape-memory alloy (SMA) reinforcements is very promising in retrofit and strengthening of existing structures. SMAs have more than one crystal structure. This is called polymorphism. The prevailing crystal structure or phase in polycrystalline metals depends on both temperature and external stress. They are a class of metallic alloys that can remember their initial geometry during transformations (forward and reverse) between two main phases at their atomic level (austenite and martensite). 

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