Papers by Keyword: Severe Plastic Deformation (SPD)

Abstract: Pure palladium (99.95%) was hydrogenated, subsequently deformed by High Pressure Torsion (HPT) and analyzed by differential scanning calorimetry (DSC). For comparison some hydrogen-free HPT processed samples were also investigated. In case of the hydrogenated HPT Pd, the concentration of single / double vacancies is noticeably higher. The importance of hydrogen for the formation and stabilization of vacancy type defects and dislocations is discussed.

Abstract: Equal channel angular pressing (ECAP) is a material processing method for developing an ultrafine-grained (UFG) structure by introducing severe plastic deformation (SPD) in a bulk material with no changes in its cross-section. Numerous analytical and numerical studies on equal channel angular pressing have been performed in recent years. The present work focuses on the effects of die geometry width is defined by the angle between two channels Φ, angle on outer corner of die Ψ (or radius R) and angle within internal corner (or radius r) of die on average effective strain after one pass route. Next, there are analyses of strength properties, plastic properties, fracture mechanism, as well as analyses of Cu structure evolution after SPD by ECAP technology, in the paper. The sixteen passes through the ECAP matrix were realized using route C. The following experimental results and their analyses, the biggest increase of strength and microhardness was proved already after 4th pass. Valuation of fracture surfaces shows that after 12th pass plastic fracture is transformed from transcrystalline ductile mixed fracture. After 4th pass, the avarage grain size decreased from initial approximate size 7 µm to 200 nm, whereby the average grain size was changeless after subsequent deformations. Possible mechanism of high-angle boundary nanograins evolution consists of formation of cell structure, subgrains that transform with the increase of deformation into nanograins with big-angle misorientation.

Abstract: The microstructure evolution during high pressure torsion and its influence on the mechanical properties of AZ80 magnesium alloy is presented in this study. Significant grain refinement was observed after high pressure torsion, while the homogeneity of the grain structure increases with the number of revolutions. Grain size decreases to about 50 nm after 15 revolutions. The microhardness profiles measured at through-thickness and through-width directions show no significant variation at different positions of the sample. Moreover, the negligible effect of the revolution number on the microhardness value was observed.

Abstract: Microstructure evolution with equal channel angular pressing (ECAP) using route Bc, that is a 90° axial rotation of the billet between passes, up to 8 passes, was investigated by transmission electron microscopy. The study has been focused on the induced development of boundary misorientation and spacing toward microstructure refinement. Cell (low-angle) and grain (high-angle) misorientation and spacing were determined from about 250 boundaries per pass of ECAP, systematically using whether Kikuchi patterns or Moiré fringes, these latter where possible. The average cell size and misorientation saturate within the first two passes. Misorientation and spacing of high-angle boundaries decrease with the number of passes. After 8 passes, mean cell size is ≈ 1.3 µm and the fraction of high-angle boundaries is ≈ 0.7. Differences in rate of grain structure evolution per pass are linked to differences in ability of dislocations introduced in new passes to recombine with the existing ones. As ECAP strain rises, the misorientation distribution develops strong deviations from the MacKenzie distribution for statistical grain orientation. This is interpreted as a result of the tendency to form equiaxed grains in a textured grain structure.

Abstract: Differential Scanning Calorimetry (DSC) is a thermal analysis technique that measures the energy absorbed or released by a sample as a function of temperature or time. DSC has wide application for analysis of solid state reactions and solid-liquid reactions in many different materials. In recent years, DSC has been applied to analyze materials and alloys processed through Severe Plastic Deformation (SPD). The basic principle of SPD processing is that a very high strain is introduced into materials which achieve significant grain refinement and improve properties of materials. This review paper presents some recent examples of the applications of DSC for materials subjected to SPD, especially by Equal-Channel Angular Pressing and High-Pressure Torsion.

Abstract: The structure and mechanical properties of amorphous alloy Ni44Fe29Co15Si2B10 after severe plastic deformation (SPD) in Bridgman chamber at the different temperatures (77 and 298 K) have been studied. It is shown that the early stages of the SPD of amorphous alloy cause a noticeable decrease in microhardness HV and significant changes in the physical properties. With increasing the value of SPD the transition from inhomogeneous to homogeneous (or to qualitatively different) mode of plastic flow is observed, which is accompanied by the effects of homogeneous nanocrystallization. The nanoparticle size does not exceed 10 nm. It is established that the thermally activated nanocrystallization processes can occur at very low temperatures (77 K).

Abstract: A positron annihilation study of free volumes was performed on Cu-Zr and Al-Sm alloys in the course of repeated cold rolling (RCR) in order to contribute to the microscopic understanding of the complex processes of solid-state amorphization and nanocrystallization. In addition to positron lifetime spectroscopy, which yields information on the size of free volumes, twodimensional Doppler broadening technique was applied in order to study the local chemical environment of free volumes on an atomistic scale. Both in Cu60Zr40 and Al92Sm8 a characteristic variation of the chemical environment of free volumes with the number of folding and rolling (F&R) cycles could be observed. In the first (Al-Sm) and intermediate state (Cu-Zr) of F&R, free volumes with an enhanced amount of Sm or Zr content in the local environment occur, indicating interfacial segregation or the formation of solute-vacancy complexes. Upon further F&R cycling, a complete (Cu-Zr) or partially amorphous structure (Al-Sm) is obtained with the free volumes exhibiting a chemical environment characteristic of the average chemical composition. In contrast to melt-spun or ball-milled amorphous alloys, free volumes of the size of a few missing atoms are found in amorphous alloys prepared by RCR presumably due to strong athermal conditions of F&R cycling.

Abstract: Possibilities of X-ray methods, using diffractometric measurements of X-ray line profiles and crystallographic texture, are compared as applied to study of SPD metal materials. The texture analysis gives the most direct data on the degree of grain fragmentation under conditions of deformation. The consideration is illustrated by results, obtained by X-ray studies of ECAP rods from Cu and Ti.

Abstract: All the SPD techniques introduce reversal straining principally, but effects of the reversal deformation on structure evolution were not studied directly yet. In the present work, an attempt was made to manage structure in pure (99.99%) Al by strain reversal through high pressure torsion (HPT). Total accumulated deformation up to equivalent strain ~8 was used. General trend of the grain refinement is similar for both deformation modes; and it is typical with all other SPD processed FCC metals. At the same time, the difference in microstructure evolution at the vicinity of the specimen axis and with increasing distance in the radial direction introduces microstructural heterogeneities which are specific features of the reversal straining. In the monotonic deformation process the A ({111}<011>) fiber is gradually substituted by the C component ({ 0 0 1}< 1 1 0>) with increasing strain before it is found to weaken. In the reverse straining process the A fiber is found to dominate the deformation texture in the low strain region. In the reverse straining process at high strain level, a {001}<100> component appear.

Abstract: Severe plastic deformation (SPD) has received considerable attention for its capability to produce ultrafine and nano structured materials. On the one hand, SPD, especially in the forms of equal channel angular pressing (ECAP) and high pressure torsion (HPT) is able to refine bulk materials with coarse grain structures. On the other hand, SPD has been used to synthesise bulk materials from particles. It enables particles from nano to micro scales to be consolidated into fully dense materials at much lower temperatures and shorter times, compared to the conventional sintering processing. It is particularly relevant to consolidating particles with non-equilibrium microstructures and to producing complex multiphase alloys. In this summary, ECAP as an effective process to synthesise a range of light metal based materials from particles with various sizes and structures, including aluminium and aluminium composites, titanium and magnesium, will be demonstrated. Full density and good bonding are achieved easily with the application of a back pressure. Microstructures from nano to ultrafine scales have been produced, resulting in significantly enhanced strength. Simultaneous increase in ductility has also been achieved in some alloys by virtue of multi-scale structures.