Titanium phase transformation

Phase Transformations in Titanium- and Zirconium-Based Alloys... 

It is rather likely that pressure-induced phase transformations can also occur in hydrogenated multicomponent industrial titanium alloys. However, there were no available data on the high-pressure behavior of such alloys. 

Vol. 12 Phase Transformations Examples from Titanium and Zirconium Alloys by... 

Non-equilibrium Processing of Materials edited by C. Suryanarayana Phase Transformations in Titanium- and Zirconium-based Alloys... 

Shape-memory alloys (e.g. Cu-Zn-Al, Fe-Ni-Al, Ti-Ni alloys) are already in use in biomedical applications such as cardiovascular stents, guidewires and orthodontic wires. The shape-memory effect of these materials is based on a martensitic phase transformation. Shape memory alloys, such as nickel-titanium, are used to provide increased protection against sources of (extreme) heat. A shape-memory alloy possesses different properties below and above the temperature at which it is activated. Below this temperature, the shape of the alloy is easily deformed due to its flexible structure. At the activation temperature, the alloy can be changed by applying a force, but the structure resists this deformation and returns back to its initial shape. The activation temperature is a function of the ratio of nickel to titanium in the alloy. In contrast with Ni-Ti, copper-zinc alloys are capable of a two-way activation, and therefore a reversible variation of the shape is possible, which is a necessary condition for protection purposes in textiles used to resist changeable weather conditions. 

Khidirov, I., Karimov, I., Em, V.T. et al. (1981) Neutron diffraction study of phase transformations of disordered-ordered in nitridohydride titanium, Izv Akad Nauk SSSR, Neorg Mater, 17 (8), 1416-1420. 

Bohne et al. (2004) Molybdenum oxides, titanium oxides Phase transformation upon oxidation + + n.a. Ion implantation, effect of oxygen diffusivity on subsurface phase formation... 

Anatase, brookite and rutile are three polymorphs of titanium dioxide. Anatase is a kind of thermodynamically metastable form while rutile is a kind of stable one. Anatase can transform irreversibly to rutile at elevated temperatures ranged from 400 to 1200 °C according to particle size, morphology and additives. The solid-state phase transformation behavior has been widely investigated while the phase evolution between anatase and rutile under hydrothermal condition has been little paid attention to so far [5]. In this work, the structural evolution from anatase to rutile under milder hydrothermal conditions is proposed as well [7, 10]. 

We have studied the phase transformation from nanometer-sized amorphous titania (Ti02) to nanocrystalline anatase at 300 - 400° C (unpublished). Amorphous titania samples were prepared by fast hydrolysis of titanium ethoxide in water at 0° C (Zhang et al. 2001). The extent of transformation was monitored using XRD determination of the phase mixture as a function of time. We also found that the transformation kinetics do not follow the widely employed JMAK equation. 

During calcination, the precursor undergoes a continuous crystallization and recrystallization process during which thermodynamically unstable or metastable phases are converted to more stable ones. X-ray diffraction patterns in Fig. 7 show phase transformation of hydrous titanium oxide from 100°C to 550°C.P l... 

The V-Ti-0 system has been extensively studied in connection with catalytic oxidation and ammoxidation reactions of aromatic hydrocarbons. Two principally different types of catalysts can be distinguished. One type of catalyst is prepared by impregnation, precipitation or mixing of the vanadium and titanium phases followed by calcination in air below the melting point of V. (1-4). The simultaneous reduction of V 0- and transformation of iiO (anatase) into rutile when heating below the V O melting point has been demonstrated to be due to topotactic reactions ( ). The formation of lower vanadium oxides can be of importance, because it has been found that reduced phases determine the activity and selectivity of catalysts (6,7). 

Past year development of new titanium alloys was based on possibility of using phase transformations and on explorations to study alloying effect on these transformations. A wide range of a-, (3- and (a+P)-alloys for various applications was created. The most widely known alloy is Ti-6A1-4V. The... 

It is shown here that alpha, beta and alpha-beta titanium alloys can be friction stir welded. The phase transformations that occur in the nugget region and the heat affected zone are discussed. Residual stresses were found to be quite high in the present work. Tensile properties in transverse direction were... 

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. 

Since our pioneering study on as-received [22] nickel-titanium endodontic instruments, other DSC studies have confirmed that as-received instruments are in the superelastic condition, which persists after numerous sterilization cycles [28,29]. As would be expected, mechanical properties of these instruments are related to the phase transformation behaviour of the nickel-titanium alloy [30]. Accordingly, suitable elevated-temperature heat treatment may favourably alter the mechanical properties of these instruments [31,32], as was previously found for nickel-titanium orthodontic wires [21,33]. 

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