Nariman A. Enikeev

On the origin of the extremely high strength of ultrafine-grained Al alloys produced by severe plastic deformation

Ultrafine-grained Al alloys produced by high-pressure torsion are found to exhibit a very high strength, considerably exceeding the Hall–Petch predictions for ultrafine grains. This phenomenon can be attributed to the unique combination of ultrafine structure and deformation-induced segregations of solute elements along grain boundaries, which may affect the emission and mobility of intragranular dislocations.

Grain Boundary Segregation in UFG Alloys Processed by Severe Plastic Deformation

Grain boundary (GB) segregations were investigated by atom probe tomography in an Al–Mg alloy, a carbon steel and Armco® Fe processed by severe plastic deformation (SPD). In the non-deformed state, the GBs of the aluminum alloy are Mg depleted, but after SPD some local enrichment up to 20 at% was detected. In the Fe-based alloys, large carbon concentrations were also exhibited along GBs after SPD. These experimental observations are attributed to the specific structure of GBs often described as “non-equilibrum” in ultra fine grained materials processed by SPD. The GB segregation mechanisms are discussed and compared in the case of substitutional (Mg in fcc Al) and interstitial (C in bcc Fe) solute atoms.

Grain size refinement due to relaxation of disclination junction configurations in the course of plastic deformation of polycrystals

A model is proposed for the formation of the substructure in polycrystals during plastic deformation. According to this model, fragmentation of a grain occurs through the formation of a system of diagonal low-angle boundaries, which originate at the edges of a rectangular grain. Misorientation boundaries form through relaxation of a nonsymmetric junction quadrupole disclination configuration accumulated at the grain corners under severe deformation when the disclination strength reaches a certain critical value. The energetics of this process is analyzed. A general case is considered where the disclinations at the junctions of the chosen grain differ in strength. The energetic approach used makes it possible to determine the misorientation angle ωx of the resulting boundaries corresponding to the maximum energy gain and to find the dependence of this angle on the degree of asymmetry of the quadrupole configuration of junction disclinations. According to the proposed model, the splitting of a grain with a short edge greater than 0.5 μm is energetically favorable and decreases the latent energy of the grain for any ratio between the junction disclination strengths if the grain length-to-width ratio is less than 30. It is shown that the minimum possible grain size in the proposed model does not exceed 0.1 μm.

Enhanced Mechanical Properties and Electrical Conductivity in Ultrafine-Grained Al 6101 Alloy Processed via ECAP-Conform

This paper studies the effect of equal channel angular pressing-Conform (ECAP-C) and further artificial aging (AA) on microstructure, mechanical, and electrical properties of Al 6101 alloy. As is shown, ECAP-C at 130 °C with six cycles resulted in the formation of an ultrafine-grained (UFG) structure with a grain size of 400–600 nm containing nanoscale spherical metastable β′ and stable β second-phase precipitates. As a result, processed wire rods demonstrated the ultimate tensile strength (UTS) of 308 MPa and electrical conductivity of 53.1% IACS. Electrical conductivity can be increased without any notable degradation in mechanical strength of the UFG alloy by further AA at 170 °C and considerably enhanced by additional decomposition of solid solution accompanied by the formation of rod-shaped metastable β′ precipitates mainly in the ultrafine grain interior and by the decrease of the alloying element content in the Al matrix. It is demonstrated that ECAP-C can be used to process Al-Mg-Si wire rods with the specified UFG microstructure. The mechanical strength and electrical conductivity in this case are shown to be much higher than those in the industrial semi-finished products made of similar material processed by the conventional T6 or T81 treatment.

Superstrength of nanostructured metals and alloys produced by severe plastic deformation

Metals and alloys produced by severe plastic deformation (SPD) are characterized by not only an ultrafine grain size, but also other structural features, such as nonequilibrium grain boundaries, nanotwins, grain-boundary segregations, and nanoparticles. The present work deals with the study of the effect of these features on the strength of SPD metals and alloys. In particular, it has been shown that, with segregations on grain boundaries and nonequilibrium boundaries, the yield stress of the material can exceed considerably the values extrapolated to the range of ultrafine grains using the Hall-Petch relationship.

Observations of Texture in Large Scale HPT-Processed Cu

Bulk nanostructured Cu samples with 20 mm in diameter and 1 mm in height were processed by HPT at a pressure of 6 GPa by 10 rotations. Measurements of texture by means of state-of-the-art XRD technique have been achieved in directions parallel and normal to the torsion axis. Local texture measurements were performed by a spot size of 500 -m in steps of 3.5 mm in order to examine the homogeneity of deformation. The texture data were resolved to shear plane and analyzed in terms of ideal shear orientations. At both inner and outer areas of the disc plane typical shear textures are observed. However, the intensity of components of textures at inner areas is higher than that of outer one. These results can be interpreted in terms of dynamic lattice relaxations rather than by heterogeneities in deformation.

Low-temperature plasticity in nanocrystalline titanium and copper

The stress-strain compressive curves, temperature dependences of the yield stress, and small-inelastic-strain rate spectra of coarse-grained and ultrafine-grained (produced by equal-channel angular pressing) titanium and copper are compared in the temperature range 4.2–300 K. As the temperature decreases, copper undergoes mainly strain hardening and titanium undergoes thermal hardening. The temperature dependences of the yield stress of titanium and copper have specific features which correlate with the behavior of their small-inelastic-strain rate spectra. Under the same loading conditions, the rate of microplastic deformation of ultrafine-grained titanium is lower than that of coarse-grained titanium and the rate peaks shift toward high temperatures. The deformation activation volumes of titanium samples differing in terms of their grain size are (10–35)b3, where b is the Burgers vector magnitude. The dependences of the yield stress on the grain size at various temperatures are satisfactorily described by the Hall-Petch relation.

A mechanism of grain nucleation during relaxation of the latent energy of junction disclinations in the course of plastic deformation

A model describing the nucleation of a new grain at the boundary of an existing grain during relaxation of the latent energy of junction disclinations formed in the course of plastic deformation is considered. This nucleation mechanism is shown to be energetically favorable. The dependence of the equilibrium size and misorientation of a nucleus on its shape is analyzed.

Submicrocrystalline Austenitic Stainless Steel Processed by Cold or Warm High Pressure Torsion

The formation of submicrocrystalline structure during severe plastic deformation and its effect on mechanical properties of an S304H austenitic stainless steel with chemical composition of Fe – 0.1C – 0.12N – 0.1Si – 0.95Mn – 18.4Cr – 7.85Ni – 3.2Cu – 0.5Nb – 0.01P – 0.006S (all in mass%) were studied. The severe plastic deformation was carried out by high pressure torsion (HPT) at two different temperatures, i.e., room temperature or 400°C. HPT at room temperature or 400°C led to the formation of a fully austenitic submicrocrystalline structure. The grain size and strength of the steels with ultrafine-grained structures produced by cold or warm HPT were almost the same. The ultimate tensile strengths were 1950 MPa and 1828 MPa after HPT at room temperature and 400°C, respectively.

Using intensive plastic deformations for manufacturing bulk nanostructure metallic materials

The results of studies concerned with new trabds in the development of intensive plastic deformation methods for manufacturing nanostructure metals and alloys are presented. Much attention is paid to the mechanical properties of bulk nanomaterials. Keywords: intensive plastic deformation, nanostructure material, gain boundary, mechanical property, microstructure, segregation.