torsion under quasi-hydrostatic pressure abbr., THP (rus. кручение под квазигидростатическим давлением otherwise кручение под давлением abbr., КД; КГД) — method of intensive plastic deformation (IPD), implemented by simultaneous pressing of a thin sample between the two strikers and its torsion by turning one of the strikers to a certain angle.

Description

The main deformation in this method is due to torsion of the sample. The pressure applied coaxially and usually reaching several GPa plays a dual role. First, it creates in the central part of the sample an area of quasi-hydrostatic compression, which prevents destruction of the sample. Second, it increases the friction between the strikers and the sample. Due to the large friction, the torque is transferred from the moving striker to the sample, and it gets deformed by the torsion.

Currently the torsion under quasi-hydrostatic pressure (THP) is used primarily for studying the physics of intense plastic deformation. THP at room temperature or at lower temperatures is used for obtaining nanocrystalline structures in metals, alloys, intermetallics and ceramics. The size of samples before deformation is typically less than 20 mm in diameter and 1 mm in height. After deformation, the height of the samples decreases to 0.2-0.5 mm. Significant refinement of the structure is achieved already by one-half turn of torsional deformation, but the creation of a uniform nanostructure usually requires several turns.

THP of pure metals leads to formation of an equiaxed structure with a mean grain size of 50-100 nm. In alloys the resulting grain size can be much smaller. The mechanism of intense deformation depends on many factors, in particular, the type of crystal lattice and stacking fault energy. The process of nanostructure formation breaks up into clear phases.

In pure FCC metals (metals with face-centred crystal lattices) with high stacking fault energy (Cu, Ni) the sequence of structural transformations is as follows. As torsional deformation increases up to n≈0.1 (where n is the number of turns of the mobile striker) dislocations become concentrated within the boundaries of subgrains (cells), which are dislocation-free grain areas of arbitrary form separated from other areas by low-angle boundaries. As deformation further increases up to n≈1 the sizes of subgrains become smaller, and the degree of their misorientation increases. This is accompanied by gradual transition from subgrain (cell) structure to the grain structure containing, mainly, high-angle grain boundaries.

The intense plastic deformation of alloys, along with the formation of nanostructures, may lead to the formation of metastable states, such as supersaturated solid solutions and metastable phases. In intermetallic compounds the THP can be followed by destruction of long-range order up to complete disordering.

Nanomaterials, obtained by THP are characterised by high internal stresses and significant distortions of the crystal lattice. These nanomaterials may demonstrate abnormalities of some fundamental properties, such as elastic moduli, Curie and Debye temperatures, saturation magnetisation. Nanomaterials produced by IPD are usually characterised by high strength at relatively low temperatures and high plasticity and superplasticity at elevated temperatures. Recently, the THP method has been used at high temperatures in high temperature superconductor ceramics to produce sharp crystallographic texture and high density of defects that serve as pinning centres for the magnetic flux.

Illustrations

a —Structure of Ni (99.99%) after torsion under quasi-hydrostatic pressure at 20 о

a —Structure of Ni (99.99%) after torsion under quasi-hydrostatic pressure at 20 оС, P = 8 GPa, n=5 rotations. Mean grain size ~ 100 nm. Author: G. F. Korznikova. From personal files.

b — Microstructure of high-temperature superconducting ceramic material Bi2Sr2CaCu2O8+δ after torsion under quasi-hydrostatic pressure at 915 °C P~10 MPa, ω=1,5·10–3 rpm, α=90о. Vertical compression axis. Author: Marsel F. Imaev. From personal files.


Author

  • Imaev Marsel F.

Sources

  1. Bridzhmen P.U. Studies of large plastic deformation and rupture The effect of high hydrostatic pressure on the mechanical properties of materials. (in Russian)— Librokom, 2010. — 446 p.
  2. R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Bulk Nanostructured Materials from Severe Plastic Deformation. — Mat. Sci. 2000, pp. 183 p.
  3. Imayev M. F., Daminov R. R., Reissner M. et al. Microstructure, texture and superconducting properties of Bi2212 ceramics, deformed by torsion under pressure // Physica C. 2007. V. 467. P. 14–26.

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