Topic 6: Severe plastic deformation of materials with conventional grain size (4:00)

Plan of the lecture

1. Methods intensivonoy plasichskoy strain.

2. Torsion under high pressure.

3. Equal channel elastic compression.

In this regard, much interest in the production of nanostructured materials by severe plastic deformation, ie large deformations at high applied pressures. The methods SDI is a strong refinement of the microstructure of metals and alloys to nano-scale due to large deformations. In developing these methods, there are several requirements for the production of bulk nanomaterials. First, the importance of formation of ultrafine-grained (UFG) structures with of large-grain boundaries, as in this case, qualitatively change the properties of materials. Second, the formation of nanostructures, uniform throughout the volume of the sample, which is necessary for the stability properties of the materials. Third, the samples do not have to have mechanical damage or destruction, despite intense deformation.

The first work to demonstrate the possibility of bulk nanostructured metals and alloys were made in the late 80's - early 90's RZ Valiev and co-workers, who used two methods to SDI - torsion under high pressure and raznokanalnoe elastic pressing (ECAP).

The design of the torsional deformation under high pressure is based on the principle of Bridgman anvils, where the sample is placed between the strikers and compressed under applied pressure (P) in a few GPa, then attached to the deformation of a very large extent (10 and over). Lower firing pin rotates, and the forces of surface friction force to deform the sample shift. The geometrical shape of the sample is such that the bulk of the material is deformed under hydrostatic pressure, resulting in the samples were not destroyed. The thus-obtained samples are discs of diameter 10 ... 20 mm and a thickness of 0.2 ... 0.5 mm. Although their structure is ground deformation after a half-turn, but for the formation of ultrafine uniform strain in the structure is needed several times. The average grain size in the obtained samples, usually of 100 ... 200. However, the final size of the grains in the structure depends on the conditions of intense deformation - the applied pressure, temperature, strain rate, and the type of the material.

For the production of bulk nanostructured materials using a different method, also based on shear strain - equal-channel angular pressing. In implementing the ECAP billet repeatedly pressed in a special snap in two overlapping channels with the same cross section at room or elevated temperatures, depending on the deformable material. In the most commonly used angles F = 900, each pass corresponds to a high degree of deformation is approximately equal to 1. For the formation of nanostructures is very important is not only the number of passes, but also the direction of passage through channels, depending on which there are several routes of ECAP. An important factor for this method is the preservation of the integrity of samples even maloplastichnyh materials. The implementation of this method can ensure the formation of UFG structure of an average grain size of 200 nm to 500 nm.

In recent years, equal channel angular pressing was the object of numerous investigations in connection with the possible practical applications of bulk nanostructured metals and alloys. Latest developments designed to increase the geometric dimensions of the samples and blanks with a diameter of 60 mm and a length of 200 mm, obtaining long workpieces, the formation of nanostructures in hard-and maloplastichnyh metals and alloys. Much attention is paid to the development of other methods of severe plastic deformation - comprehensive forging, rolling, special, etc., to improve the efficiency of the process.