Microstructure Evolution in Ultrafine-grained Magnesium Alloy AZ31 Processed by Severe Plastic Deformation

Commercial MgAlZn alloy AZ31 was processed by two techniques of severe plastic deformation (SPD): equal channel angular pressing and high pressure torsion. Several microscopic techniques, namely light, scanning and transmission electron microscopy, electron backscatter diffraction, and automated crystallographic orientation mapping were employed to characterize the details of microstructure evolution and grain fragmentation of the alloy as a function of strain imposed to the material using these SPD techniques. The advantages and drawbacks of these techniques, as well as the limits of their resolution, are discussed in detail. The results of microstructure observations indicate the effectiveness of grain refinement by severe plastic deformation in this alloy. The thermal stability of ultrafine-grained structure that is important for practical applications is also discussed

Thermal Stability of Ultra-Fine Grained Microstructure in Mg and Ti Alloys

This chapter reviews the thermal stability of ultra-fine grained (UFG) microstructure in selected magnesium and titanium-based materials prepared by severe plastic deformation (SPD). The focus is on the wide palette of experimental methods applicable for investigation of microstructural stability. These methods include scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), microhardness measurement, positron annihilation spectroscopy (PAS), and electrical resistance measurement. Microstructural stability of UFG commercially pure (CP) Ti and Ti-6Al-7Nb alloy produced by equal-channel angular pressing (ECAP) is studied ex situ after annealing by SEM, by microhardness measurements, and in situ during heating, by high precision electrical resistance measurements. Both materials show stable UFG structure up to 440°C. Further annealing causes recovery and recrystallization of the microstructure. At 650°C, the microstructure is completely recrystallized. Magnesium alloy AZ31 is prepared by hot extrusion followed by ECAP. UFG microstructure recovers and continuously recrystallizes during annealing. The microstructure of UFG AZ31 alloy is stable up to 170°C and subsequent grain growth is analyzed. Special attention is paid to interpret the activation energy of the grain growth. The superplastic properties of UFG AZ31 alloy are investigated in the temperature range of 170–250°C

Mechanical Properties and Microstructure Development in Ultrafine‐grained Materials Processed by Equal‐channel Angular Pressing

In this chapter, the detailed characterization of processes of grain fragmentation and refinement resulting from gradual imposition of strain by individual equal‐channel angular pressing (ECAP) passes is reported. A great emphasis is placed on the processing of materials with different crystal structure, particularly the face‐centred cubic (FCC), the body‐centred cubic (BCC) and the hexagonal close‐packed (HCP). Advanced techniques of electron microscopy, electron and X‐ray diffraction and positron annihilation spectroscopy have been employed to characterize microstructure, texture and defect evolution in the material as a function of strain imposed by ECAP. Microstructure development was correlated with mechanical properties obtained by both mechanical tests and microhardness measurements. Processes controlling the microstructure refinement and texture development were identified and discussed in detail

Nanomaterials by severe plastic deformation: review of historical developments and recent advances

International audienceSevere plastic deformation (SPD) is effective in producing bulk ultrafine-grained and nanostructured materials with large densities of lattice defects. This field, also known as NanoSPD, experienced a significant progress within the past two decades. Beside classic SPD methods such as high-pressure torsion, equal-channel angular pressing, accumulative roll-bonding, twist extrusion, and multi-directional forging, various continuous techniques were introduced to produce upscaled samples. Moreover, numerous alloys, glasses, semiconductors, ceramics, polymers, and their composites were processed. The SPD methods were used to synthesize new materials or to stabilize metastable phases with advanced mechanical and functional properties. High strength combined with high ductility, low/room-temperature superplasticity, creep resistance, hydrogen storage, photocatalytic hydrogen production, photocatalytic CO2 conversion, superconductivity, thermoelectric performance, radiation resistance, corrosion resistance, and biocompatibility are some highlighted properties of SPD-processed materials. This article reviews recent advances in the NanoSPD field and provides a brief history regarding its progress from the ancient times to modernity

Composition of ω-Phase Particles in Ti(Mo) Alloys Studied by Anomalous X-ray Diffraction

Nanoparticles of hexagonal ω phase in bcc-Ti(Mo) single crystals ( β phase) occur due to a diffusionless athermal β to ω transformation and they grow during follow-up ageing at elevated temperatures, while the alloying atoms (Mo in our case) are expelled from the nanoparticle volumes. We investigated the Mo content in growing ω nanoparticles by anomalous X-ray diffraction and demonstrate that the Mo expulsion from the ω phase is not full; a thin shell of a nanoparticles where the β to ω transformation is not complete still contains a considerable amount of Mo atoms

Microstructure and texture in cryomilled and spark plasma sintered Ti Grade 2

Titanium (Grade 2) was processed by cryogenic milling and subsequently sintered by spark plasma sintering (SPS) method with the aim of creating and preserving the ultra-fine grained (UFG, < 1 μm) microstructure. Microstructural investigation was performed after both cryogenic milling and spark plasma sintering. An advanced technique of transmission Kikuchi diffraction (TKD) was used to characterize the individual milled powder particles. Investigations of milled powders showed significant grain refinement down to 50 nm after milling in liquid argon with tungsten carbide balls. We assume that this is the equilibrium grain size resulting from the balance of deformation, recovery and dynamic recrystallization. A texture, resembling the rolling texture in Ti, was also found in the milled particles, which can be explained by the nature of deformation during milling. UFG microstructure was not maintained after sintering, with the mean grain size of 2.6 μm. Although the grains are completely recrystallized, a texture, similar to the powder texture, was also found in these samples as a result of packing of the powder particles and the nature of the recrystallization process (continuous static recrystallization)