Nanotechnology of Functional and Structured Intellect Shape Memory Materials at the Institute of Metal Physics, Ural Division of RAS

 

 

V.V. Ustinov, V.G. Pushkin, V.V. Sagaradze

Russia, Ekaterinburg, Institute of Metal Physics, Ural Division of RAS

 

 

NANOTECHNOLOGY OF FUNCTIONAL AND STRUCTURED INTELLECT SHAPE MEMORY MATERIALS IN INSTITUTE OF METAL PHYSICS, URAL DIVISION OF RAS

 

V.V. Ustinov, V.G. Pushin, V.V. Sagaradze

 

 

Beginning with the discovery of thermoelastic martensitic transformations (TMT) and the phenomenon of thermoelastic equilibrium by the Soviet scientists, G.V. Kurdyumov and L.G. Khandros, has begun a new milestone in the development of martensitic transformations. Alloys with TMT and related physical and mechanical properties and shape memory effects (SME) have unusual and often unique characteristics. An important feature of this class of materials is that a number of their fundamental properties is at the same time practically attractive. But only alloys based on titanium nickelin with the shape memory found practical use and, above all in critical equipment and products for engineering and medicine. Among the functional and structural materials that undergo TMT, nickelin titanium alloys have the highest strength and plastic properties. At the same time they demonstrate the unique as to magnitude and reproducibility of the shape memory effect, high reliability and durability of their implementation (mechanotermical, mechanociclical, thermocyclical). Having a good weldability, high corrosion resistance, biological compatibility and relatively simple chemical composition, they also differ in adaptability of metallurgical process and subsequent production limits. However, these alloys in normal polycrystalline cast state and after one or another traditional thermal and thermomechanical treatments often do not provide the required by practice physical-mechanical and operational characteristics of the products, which significantly limits their broad and diverse applications.

 

Development and improvement of items of special equipment for various purposes, and modern medical equipment, increasing requirements for reliability and miniaturization of components and assemblies of their design requires an increase in the yield even for alloys based on nickel-titanium in 1.5-2 times, while maintaining the shape memory effect (SME) and superelasticity (SE). This is particularly important to increase the magnitude of reversible deformation in the implementation of these effects in alloys. In a conventional microcrystalline (MC) state the
TiNi-based alloys have low voltages martensite shift (less than 200 MPa), but they still are high yielding even under heavier loads. Therefore, too intense relaxation processes take place in them during the formation of strain induced martensitic phase during loading of the samples near the beginning of the direct TMT. Negative consequence of these processes is significant non-return of shape and a small value of reversible deformation during discharge (1.5-2%). Studies show that the resources to increase physical and mechanical properties of these alloys by conventional methods (alloying, thermal and thermomechanical treatment without significant changes in grain size) are mainly exhausted. This encourages the development of methods for obtaining materials in bulk nanostructured (NS) submicrocrystalline (SMC) or nanocrystalline (NC) and nanophase (NF) conditions.

 

This report provides an overview of works performed at the Institute of Metal Physics, UB RAS, for the development of nanotechnology of bulk shape memory alloys of different structural and functional purpose. It is known that methods of rapid quenching from the melt (RQM) and severe plastic deformation (SPD) allows to obtain metallic materials with unusual properties in the nanostructured state. These approaches were used bu us for the first time to create high-endurance NS alloys based on nickel-titanium with SME. Their use has opened up new and unique possibilities of changing the microstructure, influence on phase transitions and related physical and mechanical properties of these alloys.

 

 

Extremely useful in the fundamental aspect was the use of SPD torsion under high pressure (HPT). It is established that the HPT, as RQM, provides the extreme grain refinement, down to the metastable amorphous alloys based on nickelid-titanium. For the first time it was shown that while their amorphous matrix contains a large number of nanoregions several nanometers in size with a highly distorted, but close to the B2-lattice atomic-crystalline structure, which become the centers of subsequent nanocrystallization during low-temperature tempering. Parameters of the nanostructure (dз from 10 to 100 nm) can be easily controlled by choosing the temperature and duration. Such alloys exhibit record values of tensile strength (up to 3 GPa), syield (2 GPa), reactive voltage (up to 1.5 GPa) for the plasticity of 15-20%, high thermal stability of the structure and properties.

 

Using the SPD by equal-channel angular pressing (ECAP) at different conditions has allowed for the first time to create three-dimensional NS-shape memory alloys. The main mechanism of formation of the grain structure (dз of 200 nm) in this case is a combination of dynamic processes of polygonization and recrystallization of fragmentation along with nanophase hardening. Compared with MK-prototypes (dз (30-100) microns, depending on the methods of synthesis), volume (ECAP) and long (after RQM) NS-nickel-titanium alloys have significant advantages: high strength properties over a wide temperature range, the limit set narrow gisterezis SME and SE (temperature, strain, force). At the same time the alloys are characterized by high reliability in operation, wear resistance, corrosion resistance and biocompatibility, and favorable ductility, toughness.

 

In a number of important technical and socially significant areas of industrial applications, special equipment and medicine nitinol is needed as rods, bars, wires, tapes of various sizes. Therefore, as a way to further improve the mechanical characteristics of the NS-TiNi shape-memory alloys, there have been developed combined treatments combining ECAP with other stress-thermal effects, including the form-building, such as multipass cold or warm rolling or drawing. This subsequent deformation at large accumulated degree translates TiNi alloys in highly fragmented martensitic or austenitic state, respectively. After total reduction of 90% alloys in the form of strips and wires it was obtained in them the high-strength volume amorphous state in which the inside of the amorphous matrix contains isolated nanograin smaller than 5-10 nm. Therefore, subsequent controlled low-temperature annealing provided their total transition to the nanocrystalline state.

 

An important new step was taken when creating a high metastable nanophase austenitic steels with FME. Formation of nanocarbides VC in the aging process ensures the stabilization or destabilization of the austenite in relation to the formation of e-martensite deformation, which allows to adjust the value of FME and the degree of hardening. The proposed steel have significant advantages over the known shape-memory alloys based on Fe-28Mn-6Si: they are more technological, contain significantly less manganese (14-20% instead of 28%) and silica (2% instead of 6%), have high strength characteristics (sВ up to 1500 MPa) s0.2 up to 1000-
1100 MPa), there were first carried out the possibility of controlling the shape memory effect. In the factories of the Urals of the proposed SME-melting of steel was conducted and it was obtained rolled steel sheet of 1000 mm wide. There were produced covers for heated cylindrical shells to seal the defective casing in oil wells.

 

Thus, up to date, there are developed and comprehensively researched and are being produced high-volume and multi-purpose plastic NS-alloys with thermoelastic MT and associated EMF with the desired characteristics for practical use. The main ways of obtaining such materials along with traditional methods of doping, thermal and thermomechanical treatments are, on one hand, the combined methods based on extreme conditions (which include rapid quenching from the melt, severe plastic deformation, welding etc.), on the other hand, the combined methods of making various composite structures (spherulitic, multilayered, multi-strand) of the type of "metal – intermetallic", "intermetallic – intermetallic", "intermetallic compound – the phase of implementation." Examples of the latter are mikrocomposite hard alloys based on nickelin-titanium and titanium carbonitride (cermets), composite materials such as TiNi-Ni2MnGa, TiNi-Ti, TiNi-Ni.

 

In conclusion, we woul like to note that the creation of high-functional materials with shape memory effect in the nanostructured state significantly extends the capabilities of their diverse practical applications. The report will present examples of specific applications of these materials in engineering and medicine, as well as the methods of investigations of alloys.

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