Rinat K. Islamgaliev

Bulk nanostructured materials from severe plastic deformation

2. Methods of severe plastic deformation and formation of nanostructures . . . . . . . 105 2.1. SPD techniques and regimes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 2.1.1. Torsion straining under high pressure . . . . . . . . . . . . . . . . . . . . . 106 2.1.2. ECA pressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 2.1.3. Multiple forging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 2.2. Typical nanostructures and their formation . . . . . . . . . . . . . . . . . . . . . . . 115

A two step SPD processing of ultrafine-grained titanium

Abstract Equal channel angular pressing (ECAP) and high pressure torsion (HPT) are two severe plastic deformation (SPD) processes that have been used to process ultrafine-grained (UFG) materials. In this investigation, we have attempted to combine these two processes to refine the grain size of coarse-grained pure titanium. ECAP processing was first carried out at 500–450 °C to refine the grain size to about 300 nm. Further processing by HPT resulted in finer grain size and higher dislocation density. The second step, HPT processing, also increased the microhardness, ultimate strength, yielding strength and ductility of the UFG pure titanium.

Processing Nanocrystalline Ti and Its Nanocomposites from Micrometer-sized Ti Powder Using High Pressure Torsion

Abstract Nanocrystalline Ti and Ti–TiO 2 nanocomposites were produced by high pressure torsion (HPT) of precompacts of Ti powder (21 μm) and its mixture with TiO 2 powder (36 nm). Effects of processing temperature and pressure on material density and microhardness were systematically studied. The HPT process simultaneously consolidated the Ti and Ti–TiO 2 powders and refined the grains to nanometer size. The microstructure of as-processed samples contained high dislocation density, high internal stress, high angle, non-equilibrium grain boundaries, and texture. Mechanical properties such as microhardness increased with increasing density. Tensile testing showed that the as-processed materials were very brittle. High pressure torsion was found to be a promising technique for producing nanocrystalline materials from micrometer-sized metallic powders.

Nanostructure and related mechanical properties of an Al-Mg-Si alloy processed by severe plastic deformation.

Microstructural features and mechanical properties of an Al–Mg–Si alloy processed by high-pressure torsion (HPT) have been investigated using transmission electron microscopy, X-ray diffraction, three-dimensional atom probe, tensile tests and micro-hardness measurements. It is shown that HPT processing of the Al–Mg–Si alloy leads to a much stronger grain size refinement than of pure aluminium (down to 100 nm). Moreover, massive segregation of alloying elements along grain boundaries is observed. This nanostructure exhibits a yield stress even two times higher than that after a standard T6 heat treatment of the coarse-grained alloy.

High-strain-rate Superplasticity from Nanocrystalline Al Alloy 1420 at Low Temperatures

Abstract Superplasticity was investigated in nanocrystalline Al alloy 1420 to evaluate the scalability of conventional constitutive relationships in the nanocrystalline range. The parametric dependences of superplastic flow were obtained by constant-strain-rate tensile tests in the temperature range 200–300°C and a strain rate range of 3 × 10−4-5 × 10−1 s−1. The nanocrystalline alloy exhibits superplasticity at low temperatures and higher strain rates compared with the microcrystalline alloy. The observation of high-strain-rate superplasticity coincided with the temperature range for microstructural instability. A comparison with the theoretical models for superplasticity and a constitutive relationship for superplasticity in microcrystalline alloys shows a transition to slower deformation kinetics on a normalized basis. The transmission electron microscopy of deformed specimens supports the slip accommodation models for superplasticity. It also shows a change in the intragranular dislocation density and dislocation configuration with grain size.

Thermal stability of submicron grained copper and nickel

The features of structure and thermal stability of submicron grained copper and nickel processed by severe plastic deformation are considered. The results of studies by various techniques: transmission electron microscopy, X-ray diffraction, differential scanning calorimetry, electrical resistance and microhardness are presented. The investigations have shown that thermal stability of submicron grained materials is determined not only by a mean grain size but also by the density and distribution of the grain boundary dislocations. The relaxation of the grain boundary dislocations precedes the grain growth starting at 175°C and influences on thermal stability.

Consolidation of nanometer sized powders using severe plastic torsional straining

Severe Plastic Deformation (SPD) via torsional straining was used to consolidate nanometer-sized metallic powders and metal-ceramic nanocomposites at room temperature. Materials processed using this technique included copper, Al and nanocomposites based on these metallic nanopowders and SiO2, SiC and Al2O3 nanopowders. The as-processed materials were nearly fully dense. Transmission electron microscopy (TEM) and microhardness measurements were used to characterize the materials. The as-consolidated materials had a grain size smaller than the average particle size of starting powder, indicating grain refinement during severe plastic torsional straining. Due to the oxidation of the initial nanometer sized powders, the as-processed samples were very brittle. Thermal stability was investigated by annealing the as-consolidated samples at various temperatures and then measuring their microhardness.

Strengthening and grain refinement in an Al-6061 metal matrix composite through intense plastic straining

Intense plastic straining techniques such as torsion straining and equal channel angular (ECA) pressing are processing procedures which may be used to make beneficial changes in the properties of materials through a substantial refinement in the microstructure. Although intense plastic straining procedures have been used for grain refinement in numerous experiments reported over the last decade, there appears to have been no investigations in which these procedures were used with metal matrix composites. The present paper describes a series of experiments in which torsion straining and ECA pressing were applied to an Al-6061 metal matrix composite reinforced with 10 volume % of Al{sub 2}O{sub 3} particulates. As will be demonstrated, intense plastic straining has the potential for both reducing the grain size of the composite to the submicrometer level and increasing the strength at room temperature by a factor in the range of {approximately}2 to {approximately}3.

Characteristics of superplasticity in an ultrafine-grained aluminum alloy processed by ECA pressing

Abstract Equal-channel angular pressing was used to process a commercial 1421 aluminum alloy at temperatures from 340 to 410 °C. An optimum ultrafine microstructure was achieved after pressing at 370 °C with an equiaxed grain size of ∼0.3–0.4 μm. Tensile testing showed this material is superplastic at 400 °C with elongations up to ∼1500%. Offsets in surface marker lines demonstrate grain boundary sliding is the dominant flow process.

High-pressure torsion-induced grain growth and detwinning in cryomilled Cu powders

Two mechanisms for deformation-induced grain growth in nanostructured metals have been proposed, including grain rotation-induced grain coalescence and stress-coupled grain boundary (GB) migration. A study is reported in which significant grain growth occurred from an average grain size of 46 nm to 90 nm during high pressure torsion (HPT) of cryomilled nanocrystalline Cu powders. Careful microstructural examination ascertained that grain rotation-induced grain coalescence is mainly responsible for the grain growth during HPT. Furthermore, a grain size dependence of the grain growth mechanisms was uncovered: grain rotation and grain coalescence dominate at nanocrystalline grain sizes, whereas stress-coupled GB migration prevails at ultrafine grain sizes. In addition, detwinning of the preexisting deformation twins was observed during HPT of the cryomilled Cu powders. The mechanism of detwinning for deformation twins was proposed to be similar to that for growth twins.