Ti-Ni-Nb-Alloy-Based Thermomechanical Joints
Development of an Advanced Technology for Safe and Easy-Demountable Thermomechanical Joints Using Ti-Ni-Nb Shape Memory Alloys with a Wide Martensite Hysteresis
Tech Area / Field
- MAN-OTH/Other/Manufacturing Technology
- MAT-ALL/High Performance Metals and Alloys/Materials
8 Project completed
Senior Project Manager
Ryzhova T B
VNIIEF, Russia, N. Novgorod reg., Sarov
- MISIS (Steel and Alloys), Russia, Moscow
- Smart Technology, INC, USA, PA, Pottstown
Project summaryAt present the technical progress is to a great extent defined by development and application in various engineering fields of principally new metallic materials possessing special physical-mechanical properties and ensuring high reliability of joints of structural elements and communications, including those working in the extreme conditions of external physical-chemical and mechanical impact, easiness and safety of their maintenance as well as effective designs and technologies of their manufacture.
The objective of the project is to develop an advanced technology of secure and easy-demountable thermomechanical joints of pipelines and structural elements, basing on the use of Ti-Ni-Nb alloys with a shape memory effect (SME) having a wide martensite hysteresis.
Nowadays, shape memory alloys (SMA) are becoming widely used in engineering. In particular, they are used to produce permanent couplings for pipeline thermomechanical joints (TMJ) and structural elements. Unlike this, in the course of the ISTC project № 294-97 an experimental technology of pipelines demountable thermomechanical joint (DTMJ) by couplings of titanium nickelide – a shape memory alloy was developed. As a result, an experimental technology of pipeline demountable joint by couplings of Fe-alloyed titanium nickelide was elaborated. In so doing high values of bearing capacity of TMJ of the pipeline models of different diameters and different materials were obtained.
For example, for the pipelines of 12X18H10T stainless steel with the outer diameter 12 mm the average value of bearing capacity for a permanent thermomechanical joint was 190 MPa and for a demountable joint – 55 MPa. The developed DTMJ design allows to perform assembling-disassembling repeatedly. In so doing the magnitude of leak-proofness pressure for different pipeline parameters varies from 20 to 80 MPa and bearing capacity – from 20 to 145 MPa.
The technology was tried within diameters of the pipelines to be joined from 6 to 40 mm. But it may be applied for the pipelines of greater diameters. Normal operation of the technology developed was confirmed by results of DTMJ tests within the temperature range from minus 50 to 50 °C. After 20 thermocycles the joint has not lose its leak-proofness.
To achieve TMJ demountability, a reversible shape memory effect (RSME) of Fe-alloyed titanium nickelide alloy was used. This alloy is now widely used in developing new technologies, however it has some disadvantages which limit the possibilities of its practical application. First, the technology of its application is rather complicated as a thermomechanical coupling of Ti-Ni-Fe alloy should be strained, stored and mounted at cryogenic temperatures. This is explained by the fact that the temperature hysteresis of thermoelastic martensite transformation in Ti-Ni-Fe alloys is small and usually does not exceed 30–50 °C. Second, the mechanism of martensite transformations in Ti-Ni-Fe alloys is such that inducing some significant RSME requires a considerable “re-strain” that reduces a reversible strain of the main SME.
At the same time it has been recently discovered that martensite hysteresis can be significantly extended through additional alloying of titanium nickelide by niobium. In this case the alloy properties may be altered so that to make the SME-inducing strain at sub-zero temperatures and to restore the shape (i.e. “work” of the coupling) at slight heating above room temperature. The reactive voltage developed, which ensures reliability of a joint, must relax under cooling, starting from certain temperatures within a sub-zero range. This provides an opportunity to create SMA couplings which are suitable for a long-term storage at room temperatures before being used. The choice of the given alloy also allows to solve the problem of RSME implementation for easy demounting of a joint: peculiar features of structural-phase state of these alloys (particles of easy-deformable structurally-free niobium) may serve as RSME source.
Thus the project proposed will allow creating a more advanced and cheaper technology for TMJ of pipelines and structural elements than done in the ISTC project № 294-97.
The project proposed will considerably promote the progress in this field, as development and use of advanced technologies of thermomechanical joints of structural elements and communications is of interest for all countries with high-level industry. The project implementation will not only allow increasing reliability and improving the design of existing devices, apparatus and their units, but will also create pre-conditions for developing principally new devices, that being the basis of technology of future generations.
In 1997-2000 in the course of the ISTC project № 294-97 a team of specialists has been created in the Russian Federal Nuclear Center (RFNC-VNIIEF), who are involved in studying shape memory alloys and developing the pipeline TMJ and DTMJ by couplings of alloys having SME and RSME. At the same time, to accomplish the project objective, the specialists from Moscow Institute of Steel and Alloys (MISiS) are engaged, who have a practical experience in this field. The first results of investigations that were obtained by the team of the proposed project have shown perspectiveness of using Ti-Ni-Nb alloys having a wide martensite hysteresis to produce secure and easy-demountable TMJ of pipelines and structural elements.
The main objectives of developing DTMJs for specific conditions of their utilization are:
1. Optimizing chemical compositions of Ti-Ni-Nb alloys basing on their functional properties; specifying techniques of their heat- and thermomechanical treatment for specific TMJ operation conditions.
2. Developing an experimental technology and smelting Ti-Ni-Nb alloys of the given composition.
3. Controlling chemical and phase composition, structure, mechanical characteristics, martensite-austenite transformation temperatures of each lot of SMA. Studying the influence of a strain rate and a testing temperature on physical-mechanical properties of Ti-Ni-Nb alloys.
4. Studying the influence of a shape setting rate, as well as a pre-strain and heat treatment, on SME and RSME manifestation in Ti-Ni-Nb alloys.
5. Carrying out computation-experimental simulation of TMJ for assessing their safety and easy demountability; estimating geometrical parameters of couplings possessing SME and RSME. Designing, manufacturing, testing Ti-Ni-Nb thermomechanical couplings for leak-proofness and bearing capacity for different operation conditions.
Scientific value of the Project will lie in specifying regularities of structure formation of Ti-Ni-Nb alloys with a wide martensite hysteresis under intensive strain-thermal effect and its influence on fundamental and special properties of these alloys; developing experimental, computational techniques and obtaining new data on SME and RSME manifestation in different temperature-rate conditions in Ti-Ni-Nb alloys.
Commercial value of the project will lie in developing a cheaper technology of secure and easy-demountable thermomechanical joints of pipelines and structural elements, basing on Ti-Ni-Nb shape memory alloy. The most important operational advantages of such TMJ, as compared to conventional ones based on Ti-Ni-Fe, are: no need of using cryogenic equipment for storing and assembling; easy disassembling of the joint due to RSME, when cooled down below the lowest operating temperature. Easy use and minimum operations, small time expenses when assembling and disassembling TMJ during repair and maintenance of structures of atomic and advanced thermonuclear power engineering systems will considerably reduce an inevitable impact of harmful factors on personnel. The technology developed can be applied in various fields of industry to join both pipelines and structural elements. Demountability of the joint will allow multiple usage of the coupling. The licenses for manufacture of Ti-Ni-Nb couplings can be an object for sale at the market. Also, manufacture of TMJ couplings jointly with foreign companies and selling them at the market are possible.
The project meets the ISTC goals as it will be executed mainly (for 63%) by VNIIEF specialists formerly engaged in the nuclear weapons development. The project implementation will serve for redirecting their scientific and technical activities to fundamental and applied research and technology developments for peaceful purposes. Hence, it is anticipated that long-term perspectives for involving them in the International Cooperation will be established.
Foreign collaborators will have the right to become aware of and to have copies of current scientific and technical reports. It is planned to hold meetings with collaborators for discussing and correcting the progress of efforts. It is also expected that this project will be supported by collaborators for further commercial usage of this development.
Technical Approach and Methodology. A peculiar feature of the alloys possessing SME, which is based on thermoelastic martensite transformation, is that their fundamental properties are also considered special (functional), i.e. consumer’s properties. Thus, for example, the critical temperature Ms defines a position of a temperature range of a slight strain for inducing SME while critical temperatures As and Af characterize the interval of shape restoration (“work” interval). The temperature of a start of reactive voltage during cooling (i.e. the lower boundary of TMJ stability) is also associated with the point Ms. Lattice strain at martensite transformation is considered a resource of a reversible strain during SME manifestation.
In doing this work it is necessary to choose the alloys and methods of their treatment, using which will allow to control the position of Ms–Mr and As–Af intervals so that the point Ms would lie below the temperature that is the lowest permissible for TMJ mechanical stability and the point As would be slightly above the room temperature, thus ensuring the most convenient heating conditions for TMJ assembling. At the same time the phase state of the alloy should make it possible to induce RSME in case there is no considerable “re-strain”.
Requirements to the temperature conditions of reliable TMJ operation vary within rather wide range. For structures being employed in climatic conditions of Russia the operation temperature range must be (–50÷50) °C.
The problem of purposeful regulation of special properties of TMJ working element (coupling) materils is supposed to be solved by studying the influence of chemical composition of the alloy and its structural state, on the one hand, and studying the influence of dynamic pre-loading of blanks intended for making couplings and synamic loads at setting a shape to couplings through increasing their inner diameter, on the other hand.
Chemical composition of Ti-Ni-Nb alloys will be chosen basing on literature data and proper investigations. It is planned to vary a ratio of titanium and nickel within small ranges at approximately constant niobium concentration, using also a partial replacement of niobium by zirconium (which, as it is known, gives “natural” high-temperature SME).
For changing a strucutral state Ti-Ni-Nb alloys will undergo heat and thermomechanical treatments (according to specific patterns), which produce a significant effect on SME and RSME manifestation and are considered effective methods of control over these properties. Since the available experimental material related to this type of shape memory alloys is greatly limited and represents fragmentary data, this defines the need of a systematic study within the given field.
Study into a structural-phase state of Ti-Ni-Nb alloys (phase composition, phase grating parameters, phase grating defects, martensite points) is planned to be carried out by use of the x-ray diffractometry methods, transmitting electronic microscopy, dilatometry, special methods of determining SMA functional properties. The functional properties to be determined include characteristic temperatures of martensite transformation interval, temperature range of slight strain for inducing SME, temperature range of shape restoration, reversible strain of a single SME and degree of shape restoration, reversible starin, RSME, temperature of RSME start under cooling, reactive voltage.
The influence of dynamic loads on behavior of Ti-Ni-Nb alloy will be studied using a gas gun and a vertical impact machine.
Based on created Ti-Ni-Nb alloys, the couplings of different size and design will be fabricated. TMJ assemblies with such couplings will be tested for leak-proofness and bearing capacity within the temperature range (–50÷50) °C at different external effects, as well as for long stability at the temperatures up to 300 °C.
To reduce the volume of labor-consuming experiments for optimizing geometrical parameters of couplings, the coupling parameters will be estimated using computational-experimental technique.
For confirming serviceability of produced joints, according to their designation, the techniques of TMJ quality control will be used (with the use of ultrasonic method, gases under high pressure and x-raying the joint).
Based on the study conducted, unlike the existing Fe-alloyed titanium-nickelide couplings, it is planned to develop a more advanced technology of reliable and easy-demountable thermomechanical joints based on the use of Ti-Ni-Nb shape memory alloys with a wide martensite hysteresis. To cut down expenses on titanium nickelide, which is rather expensive alloy, as well as to employ the available equipment in full, the technology development will be performed as applied to the pipelines being 6–40 mm in diameter. However, the technology can be applied for the pipelines of much greater size as well.
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