A.P. Zhilyaev

Orientation imaging microscopy of ultrafine-grained nickel

Orientation imagining microscopy was used to measure the distributions of grain boundary misorientations in ultrafine-grained nickel processed by high-pressure torsion and equal-channel angular pressing. Both procedures give high fractions of high-angle boundaries but also higher fractions of low-angle boundaries than anticipated from a random distribution.

Modelling of grain boundary misorientation spectrum in polycrystals with crystallographic texture

Abstract Modelling of grain boundary misorientation distribution in polycrystals having crystallographic texture has been carried out taking into account the correlation in the orientation of adjacent grains. It is shown that the distribution of misorientations in any plane section is representative for the dependent on texture. However, texture as it is does not unambguously determine the grain boundary spectrum; different grain boundary misorientation distributions xan correspond to one and the same texture depending on the correlation in the orientations of adjacent grains. The correlation depends on two factors: a method of distinguishing “specified” and “calculated” boundaries and a misorientation spectrum of “specified” boundaries. The misorientation distribution is shown to be virtually independent of the former factor and essentially dependent of the latter.

Nanocrystallization Induced by Severe Plastic Deformation of Amorphous Alloys

The paper describes the influence of severe plastic deformation (SPD) on the crystallization of the amorphous rapidly quenched Ti-Ni and Nd-Fe-B alloys. It has been revealed that the SPD by high pressure torsion (HPT) at room temperature leads to formation of nanocrystals in these initially amorphous alloys. The subsequent annealing of these HPT-processed alloys leads to formation of homogeneous nanocrystalline structure. The SPD processing of the amorphous alloys can be used as a novel method for producing bulk nanocrystalline materials.

Calorimetric and X-Ray Measurements in Ultrafine-Grained Nickel

The procedures of X-ray diffractometry and peak profile analysis, together with differential scanning calorimetry (DSC), were used to measure the grain size, dislocation density and corresponding elastic energy stored in ultrafine-grained (UFG) nickel processed by different methods: equal channel angular pressing, high pressure torsion and their combinations. It has been shown that grain growth in these samples begins at temperatures in the range from 500 to 700 K and the measured activation energy for grain growth is close to the activation energy for grain boundary self-diffusion. A difference between the released enthalpy in DSC experiments and the elastic energy evaluated by x-ray diffractometry is attributed to the decrease in surface energy during grain growth in UFG nickel. The grain boundary (GB) surface energy of high angle boundaries was also evaluated.

Microstructural characterization of the crystallization sequence of a severe plastically deformed Al-Ce-Ni-Co amorphous alloy

Discs of Al85Ce8Ni5Co2 amorphous alloy were severely deformed by high pressure torsion. Severe plastic deformation exceeding equivalent strain of 8.2 induces the formation of nanocrystalline fcc-Al in a more stable residual amorphous matrix. Calorimetric and X-ray diffraction measurements revealed that the deformed outer part of the disc crystallizes into a mixture of equilibrium phases during the first thermal event. However, in the amorphous ribbon the same crystalline mixture develops only after the second stage.

Microstructure Evolution and Microstructure-Property Relationships in Friction Stir Processing of NiAl Bronze

Friction stir processing (FSP) has been employed for localized modification and control of microstructures in NiAl bronze materials, which are widely utilized for marine components. The thermomechanical cycle of FSP results in homogenization and refinement and the conversion of microstructures from a cast to a wrought condition within stir zones in the material. However, the direct measurement of stir zone temperatures, strains, strain rates and cooling rates is difficult due to steep gradients and transients in these quantities, and this is an impediment in the assessment of FSP-induced microstructures and properties. Quantitative microstructure analyses following FSP of cast NiAl bronze materials have been used to develop estimates of stir zone thermomechanical cycles. The estimation procedures will be reviewed and the microstructure-based estimates will be compared to results from computational models and embedded thermocouples measurements. Stir zone microstructures comprise a mixture of primary α grains and transformation products of the β that formed during processing. Recrystallization in the primary α occurred due to particle-stimulated nucleation in this low stacking fault energy material. Factors that influence the distribution of strength and ductility in the stir zone appear to include the mixture of microstructure constituents and gradients in microstructure due to gradients in processing conditions.

Influence of Processing Parameters on Texture and Microstructure in Aluminum after ECAP

The influence of strain path during equal-channel angular pressing (ECAP) has been evaluated in pure aluminum by orientation imaging microscopy (OIM) and transmission electron microscopy (TEM). The material was examined after four pressing operations by route BC in a 90° die, or eight pressing operations by route BC in a 135° die. The von Mises equivalent strains were essentially the same for these two ECAP procedures. The microtexture data indicate that the distortion during ECAP corresponds to a simple shear in a direction approximately parallel to diechannel exit and on a plane perpendicular to the flow plane. For both procedures the OIM data reveal prominent meso-scale band-like features. Lattice orientations in each band correspond to a texture orientation but the particular combinations of orientations depend upon ECAP die angle. High-angle boundaries in the structure correspond to interfaces between the bands.

Enhancement of Ductility and Strength through Microstructural Refinement by FSP of Nickel Aluminum Bronze

Friction stir processing (FSP) is a severe plastic deformation (SPD) method that has been applied to as-cast NiAl bronze (NAB) materials, which are widely used for marine components. The thermomechanical cycle of FSP results in homogenization and refinement, and the selective conversion of microstructures from a cast to a wrought condition. The physical metallurgy of NAB is complex and interpretation of the effects of FSP on microstructure has required detailed analysis by optical and electron microscopy methods. Annealing and isothermal hot rolling have been employed to confirm microstructure-based estimates of stir-zone peak temperatures. The variation of mechanical properties was assessed by use of miniature tensile samples and correlated with microstructure for samples from stir zones of single and multi-pass FSP. Exceptional improvement in strength – ductility combinations may be achieved by FSP of NAB materials.

A computer simulation study of the relation between grain boundary misorientation distribution and crystallographic texture

Modelling of two types of crystallographic texture corresponding to cold-rolled material (so-called copper type texture) and to completely recrystallized material (cubic texture) has been performed for hypothetical FCC metals. Orientation distribution functions (ODFs) of these two states differ considerably in both the position and intensity of texture maxima. These model ODFs have been used as input parameters for simulation of basic grain boundary (GB) misorientation spectra. It has been established that the texture of copper causes little difference in basic GB misorientation spectra as compared with a textureless polycrystal. Apparently, in materials with such a texture, the GB spectrum is primarily determined by correlations in orientations of adjacent grains. In contrast, the cubic texture has a great influence on basic spectra, and they are significantly different from spectra in a textureless polycrystal. For this type of texture, the GB spectrum is determined by both the texture and the correlation between orientations of nearest-neighbour crystallites.

Microstructure and Texture Evolution in Metals and Alloys during Intense Plastic Deformation

ntense plastic deformation is generally effective in producing grain refinement. IPD methods include equal channel angular pressing/extrusion (ECAP/ECAE), high-pressure torsion (HPT), accumulative roll bonding (ARB), and friction stir processing (FSP), among others. In this work, we summarize the main results on grain refinement by these processing methods and present our own data on microstructure and texture evolution in metals and alloys during ECAP, HPT and FSP. Whereas ECAP and HPT are usually performed with the work piece material initially at room temperature or even at liquid nitrogen temperature to enhance refinement, FSP involves a brief but complex thermomechanical cycle with peak temperatures up to 0.7 0.9 TMelt. Apparently, materials undergo dynamic recrystallization (DRX) during FSP. DRX also occurs also in metals and alloys of low TMelt due to adiabatic heating during HPT performed at room temperature. The paper is devoted to revisiting of previous as well as new results and a comparative analysis of microstructure and texture evolution in commercially pure aluminum and selected pure metals and alloys during ECAP, HPT and FSP in order to illustrate the limits of grain refinement.