Severe Plastic Strain Research Articles

Deformation Mechanism of Severely Deformed CP-Titanium by Uniaxial Compression Test

Conventional uniaxial compression test, designed to yield an effective strain of ∼1, was carried out to understand the deformation mechanism of severely deformed commercial pure titanium at various temperatures. TEM analysis showed that only [10\\bar11] twins accompanied with 〈a〉 slip dislocations were activated on non-basal planes in samples compressed at 573 and 623 K. In particular, the whole area of the sample compressed at 623 K were covered with [10\\bar11] twin bands with a width of 200 nm. However, 〈a+c〉 slip instead of [10\\bar11] twinning were found in samples compressed at 673 and 823 K. This indicates that [10\\bar11] twinning plays an important role in accommodating severe plastic strain before the activation temperature of 〈a+c〉 slip.

Dynamic shear rupture of steel plates

Metallic sandwich panels with prismatic cores offer the potential for superior blast resistance relative to monolithic plates of equivalent areal density. However, under sufficiently high impulse, severe plastic strains can occur at the junctions of the face sheets and the core members shortly after arrival of the pressure wave but prior to significant deformation elsewhere. The potential consequence is localized shear rupture with minimal plastic dissipation. To characterize this failure mode, a combined experimentalnumerical protocol has been used to ascertain the plastic strain for dynamic shear rupture of ductile metals. The experimental component involves firing cylindrical projectiles through plates of the targeted materials and monitoring changes in projectile velocity during penetration. With appropriate combinations of plate thickness and projectile velocity, penetration occurs through propagation of an annular shear crack. In parallel, a numerical model of dynamic deformation and rupture has been employed to infer the critical strain through comparisons with projectile velocity change measurements. Experiments and analyses have been performed on both 304 stainless steel and superaustenitic AL6XN. Effects of mesh size on the resolution of the predicted strain distribution and the plastic dissipation associated with penetration are addressed.

Changes in Grain Size Distribution of a Submicron Grained Al-Sc Alloy during High Temperature Annealing

Severe plastic straining is an established method for producing submicron grain (SMG) structures in alloys. However, the development of such a fine grained structure in single-phase alloys is usually futile if they are to be exposed or processed at elevated temperatures. This is a direct consequence of the natural tendency for rapid and substantial grain coarsening which completely removes the benefits obtained by grain refinement. This problem may be avoided by the introduction of nanosized, highly stable particles in the metal matrix. In this work, a SMG structure was generated in an Al-0.3 wt.% Sc alloy by Equal Channel Angular Pressing (ECAP). The alloy was prepared initially to produce a fine grained microstructure exhibiting a large fraction of high angle grain boundaries and a dispersion of nanosized Al3Sc particles. The evolution of microstructure during annealing at temperatures up to 550 °C was examined in detail and grain size distributions generated from the data. It was shown that grain coarsening is rapid at temperatures above 450 °C and the initial log-normal grain size distribution exhibiting low variance and skewness was altered considerably. The statistical information generated from the grain size distributions confirms that discontinuous grain coarsening occurs in this alloy only at temperatures greater than 500 °C.

Stress and strain histories in equal channel angular extrusion/pressing

Severe plastic strains induced in the material via multiple equal channel angular extrusions (ECAEs)/pressings can produce bulk ultrafine grains suitable for structural applications. These severe strains were induced so far with a range of channel angles from 60° to 160°. However, to produce sound ultrafine grain billets and to design an optimized ECAE die, the knowledge of stress–strain histories, and punch force requirements are very important. This can be carried out with the help of finite element analysis, which is an appropriate tool for such studies when real-like conditions can also be considered. Accordingly, the current study was carried by using Abaqus/Explicit for a range of channel angles: 60–150°, with an outer corner angle of 10° and with considerations of realistic parameters like strain hardening of material and the influence of friction. The least difference between the effective strains, at different points, was achieved with channel angle of 90°, and the maximum principal stress values are lower with the same channel angle compared to other channel angles studied. Peak punch force requirements are gradually decreased with the increase in channel angle.

In-situ quasicrystal-reinforced magnesium matrix composite processed by equal channel angular extrusion (ECAE)

Magnesium alloys have a strong potential for applications in structural components because of their low density. Considerable effort is currently being devoted to the development of high performance magnesiumbased materials for aerospace and outer space applications. Apart from conventional alloying practice, there has been a rapid development of particle or fiber reinforced magnesium matrix composites [1, 2]. The use of ceramics as reinforcement in magnesium alloys resulted in significant improvement of strength and stiffness. However, such strength and stiffness benefit is at the expense of a rather poor ductility [1, 2]. Quasicrystals are isotropic and possess quasiperiodic lattice structure [3]. Due to the difficulty of the movement of dislocations in the quasicrystals at room and elevated temperatures, quasicrystals exhibit high hardness and high strength [4]. Therefore, quasicrystals have been successfully used as reinforcements in metal-matrix composites [5–7]. For example, quasicrystal-reinforced aluminum matrix composites with high strength combined with good ductility have been developed by powder-metallurgical method [5, 6] and casting process [7]. Recently, it was reported that a thermal stable icosahedral quaiscrystalline phase (I-phase) with chemical composition of Mg3YZn6 was formed as a coarse eutectic structure in the α-Mg matrix during conventional solidification in Mg-Zn-Y alloy system [8]. The existence of the two-phase (I-phase + α-Mg) region in the Mg-Zn-Y alloy system provides an opportunity to develop quasicrystal-reinforced magnesium matrix composites by thermal processing. Conventional thermomechanical processes, such as hot rolling [9, 10] and extrusion [11], have been employed for the alloy system to separate and distribute the I-phase in the α-Mg matrix, thus in-situ I-phase/magnesium matrix composites with high strength and ductility have been achieved [9–11]. Equal channel angular extrusion (ECAE) is a novel metal forming process to produce severe plastic strains in bulk material without changing the cross-section of the material [12]. This procedure has been recognized as one of the most effective methods in producing bulk ultra-fine grained (UFG) materials with

INFLUENCE OF THE DEGREE OF SEVERE PLASTIC STRAIN AND HEATING ON THE STRUCTURAL EVOLUTION OF A COPPER COMPOSITE

The structure of a Cu + 0.5 wt. % Al2O3 composite subjected to severe plastic torsional strain in different regimes is studied by the methods of electron microscopy and calorimetry and by measuring the microhardness. The parameters of severe plastic strain resulting in the formation of very homogeneous nanostructures in the copper composite at room temperature are determined. Effects of the applied pressure and of the number of revolutions upon torsional strain on the microhardness of the material are investigated. It is demonstrated that the average grain size of the composite structure decreases with increasing degree of strain, and predominantly low-angle disorientations are transformed into large-angle ones. The influence of annealing on changes in the structure of samples after 5 revolutions at a pressure of 6 GPa is studied. Exothermic peaks are established for samples heated with a constant rate, and their activation energies are found.

Superplasticity of SiC whisker reinforced 7075 composite processed by rotary-die equal-channel angular pressing

Fine-grained microstructures produced by severe plastic straining give rise to superplasticity in composites as well as in monolithic alloys [1]. Conventional deformation processes such as extrusion and rolling have been applied to the production of superplastic alloys and composites [2]. However, producing an ultrafinegrained microstructure by extrusion or rolling that leads to superplasticity in an alloy or composite usually requires a very large area reduction, which, in the case of extrusion, is typically 50 : 1 or higher [3, 4]. Such a high area reduction makes the conventional processes costly. Moreover, the resultant products inevitably have small cross sections that limit the range of application of such conventional deformation processes to metal matrix composites. Equal-channel angular pressing (ECAP) does not suffer from the above limitations and can produce large strains in a bulk material without changing the cross section of the material [5–9]. A drawback, however, is that an ECAP pass can produce a relatively small strain [8]. Therefore, producing a superplastic material with an ultrafine-grained microstructure requires many (typically 8–10) passes. However, since each ECAP pass in current ECAP practice requires removal and reinsertion of the billet for the next pass, repeating many such passes makes the current ECAP process inefficient, difficult to control and costly. To overcome this shortcoming, the authors have developed a more efficient ECAP process that adopts a rotary die design [10–12]. This process, termed rotarydie equal-channel angular pressing (RD-ECAP), has been used to effectively produce a fine-grained Al7mass% Si-0.35mass% Mg alloy having high elevatedtemperature ductility [11]. In this study RD-ECAP was applied to a low-ductility, squeeze-cast SiC whiskerreinforced, 7075 aluminum alloy composite with the objective of producing a superplastic composite. A 20 vol% SiC whisker/7075 composite was prepared by squeeze casting. Preforms of β-SiC whiskers

Apparent grain growth behavior of submicrometer grained Al-3 Pct Mg during static annealing after fabrication by severe plastic straining

Apparent grain growth behavior of submicrometer grained Al-3 Pct Mg during static annealing after fabrication by severe plastic straining

Microstructural stability of ultrafine grained low-carbon steel containing vanadium fabricated by intense plastic straining

Two grades of low-carbon steel, one containing vanadium and the other without vanadium, were subjected to equal channel angular pressing (ECAP) at 623 K up to an effective strain of ∼4. After equal channel angular pressing, a static annealing treatment for 1 hour was undertaken on both pressed steels in the temperature range of 693 to 873 K. By comparing the microstructural evolution during annealing and the tensile properties of the two steels, the effect of the addition of vanadium on the thermal stability of ultrafine-grained (UFG) low-carbon steel fabricated by intense plastic straining was examined. For the steel without vanadium, coarse recrystallized ferrite grains appeared at annealing temperatures above 753 K, and a resultant degradation of the strength was observed. For the steel containing vanadium, submicrometer-order ferrite grain size and ultrahigh strength were preserved up to 813 K. The enhanced thermal and mechanical stabilities of the steel containing vanadium were attributed to its peculiar microstructure, which consisted of ill-defined pearlite colonies and ultrafine ferrite grains with uniformly distributed nanometer-sized cementite particles. This microstructure resulted from the combined effects of (a) the preservation of high dislocation density providing an effective diffusion path, due to the effect of vanadium on increasing the recrystallization temperature of the steel; and (b) precipitation of fine cementite particles at ferrite grain boundaries through the enhanced diffusion of carbon atoms (which were dissolved from pearlitic cementite by severe plastic straining) along ferrite grain boundaries and dislocation cores.

Mechanical alloying of FeNiCuMo powders in a planetary mill

During laser impact welding, severe plastic strains and temperature spikes occurring in less than 1 µs make experimental observation impractical and necessitate computational modeling to characterize in-situ behavior. To understand the effects of microstructure and the associated inhomogeneity/anisotropy in laser impact welding, an Eulerian framework featuring aluminum 1100 flyer and stainless steel 304 target foils is applied to simulate cases with and without microstructure modeling. The transient thermomechanical phenomena revealed by the dynamic simulation provide insights into evolution of the in-situ structure-property relationship, including microstructural variation, phase transformation, and material jetting. In contrast to the homogeneous model, the inhomogeneous model suggests a 10 µm-thick zone of grain refinement at the weld interface establishing new grains 0.1-1 µm in diameter in the flyer, and causing partial martensitic phase transformation in the target, attributable to rapidly induced equivalent plastic strains of up to 10.71 in the flyer and 0.98 in the target.

Metastable states in R 2Fe 14B-based alloys processed by severe plastic deformation

The influence of severe plastic torsion straining (SPTS) under high pressure on microstructure, phase composition and coercivity of cast sub-, super- and stoichiometric R 2Fe 14B (R = Nd, Pr)-based alloys was investigated. Deformation results in the formation of a metastable state characterised by ultrafine-grained (UFG) structure and non-equilibrium phase composition. Annealing at 600°C leads to complete or partial restoration of the initial phase composition depending on the alloy composition. Due to SPTS and subsequent annealing of the Pr 20Fe 73.5B 5Cu 1.5 alloy, the formation of high coercivity UFG state with jH c up to 2240 kA/m is observed.

Processing by Equal-Channel Angular Pressing: Potential for Achieving Superplasticity

AbstractEqual-channel angular (ECA) pressing is a processing procedure whereby a very severe plastic strain is imposed on a sample without any change in the cross-sectional dimensions of the material. This processing method leads to a substantial grain refinement, producing grains which are within the submicrometer or even the nanometer scale. This paper discusses the potential for using this method to prepare materials for superplasticity. The results demonstrate that it is possible to achieve superplastic deformation in selected materials subjected to ECA pressing and, in addition, there is the possibility of extending the superplastic region so that it occurs at very rapid strain rates.

Formation of metastable states in nanostructured Al- and Ti-based alloys by the SPTS technique

Nanostructured Al and Ti-based alloys have been processed by severe plastic torsion straining (SPTS). It is shown that SPTS under high pressure of these quenched alloys results in refinement of their microstructures to grain sizes of 70–100nm and leads to formation of supersaturated solid solutions. Structural changes during heating of the obtained nanostructured alloys are investigated.

Microstructures and properties of nanocomposites obtained through SPTS consolidation of powders

The microstructures and properties of copper- and aluminum-based nanocomposites processed through severe plastic torsional straining (SPTS) consolidation of metallic micrometer powders and ceramic nanopowders were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), microhardness and electrical resistivity measurements, and mechanical tests. It was shown that the SPTS consolidation of powders is an effective technique for fabricating metal-ceramic nanocomposites with a high density, ultrafine grain size, and high strength. Copper samples processed under a high pressure of 6 GPa exhibited high failure strength and strain as well as unusual strain hardening. Superplastic-like behavior was found in Al-Al2O3 nanocomposite samples.

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 SiO 2, SiC and Al 2O 3 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.

Severe Plastic Deformation: New Technique for Powder Consolidation and Grain Size Refinement

Severe plastic deformation (SPD) processing is widely used to control and refine the grain size of monolithic metals and alloys, with grain sizes of less than 100 nm produced in recent years. One particular SPD technique, severe plastic torsional straining, has been found to effectively consolidate both micrometre and nanometre sized metallic powders and metalceramic powder mixtures, with resultant grains refined to submicrometre or nanometre size.

Contact mechanics and the wear of metals

It is commonly observed that metallic wear debris takes the form of thin platelets, leading to the term ‘delamination wear’. Modelling this phenomenon has proved a stiff challenge in Contact Mechanics since the fractures which give rise to wear particles lie parallel, or nearly so, to the surface; i.e. on planes of maximum compressive stress. Sectioning the surface layer beneath a wear track has revealed it to have acquired severe plastic strains, which suggests that the cracks are ductile fractures, driven by plastic strain rather than elastic stress intensity. The paper reviews recent research into the progressive plastic deformation of surfaces in repeated sliding: the process known as ‘ratchetting’. Included is an analysis of ‘running-in’ of rough surfaces by repeated sliding and a discussion of the criterion of rupture under cyclic plastic strain.

チタン／ＡＩＳＩ３０４Ｌステンレス鋼の摩擦圧接で生じる残留応力・ひずみ分布の特徴とそれらの生成機構

In order to elucidate the characteristics of residual stress and plastic strain produced by the dissimilar friction welding operation, detailed FEM modelling was carried out. The obtained main results are as follows :(1) During numerical modelling of residual stress and plastic strain formation in dissimilar joints, the distribution of δz (axial component) in the radial direction produced by uniform cooling from 800°C, in material adjacent to the bondline in both substrates, provides an effective means for evaluating the accuracy of the computational procedure in dissimilar materials welds.(2) Heat transfer into a narrow zone, z=±20 mm on either side of the bondline promotes residual stress formation. The peak temperature occurs in the AISI304L substrate.(3) Adjacent to the bondline, σr (radial component) is tensile in the higher thermal expansion substrate (AISI304L), and is compressive in the substrate (titanium) with the lowest thermal expansion value. σz is tensile in the central region of the component (where the stiffness is large) and is compressive at the periphery of the component.(4) The σz distribution in the axial direction, in material far from the bondline, indicates that σz is compressive in the central region of the component and is tensile at the component periphery. Specially, σz is tensile in titanium substrate, except in regions extremely close to the bondline at the periphery of the component.(5) Considerable plastic strain is produced perpendicular to the bondline and in the radial direction. Limited plastic strain occurs in the circumferential direction.(6) Severe plastic strain is produced in titanium substrate close to the bondline. This region in titanium close to the bondline has a hardness 1.5 times higher than in the base metal.

Structural Evolution and Deformation in an Aluminum-Based Solid Solution Alloy with Submicron Grain Size

AbstractSuperplastic ductilities may be achieved in tension using polycrystalline materials with small grain sizes (typically <10 μm). Experiments were conducted to evaluate the significance of the grain boundaries in an Al-3% Mg solid solution alloy with a submicron grain size (∼0.2 μm). The material was produced by subjecting the alloy to severe plastic strain using a pressing technique. This paper describes the nature of the microstructure in the as-fabricated condition, the evolution of the microstructure as a function of time and/or temperature, and the effect of testing in tension at a temperature of 403 K.

Effect of Superelastically Deforming NiTi Surface Microalloys on Fatigue Crack Nucleation in Copper.

AbstractAlloys based on near-equiatomic NiTi are capable of thermoelastic martensite transformations which give rise to shape memory and superelasticity (pseudoplasticity) effects. In particular, at temperatures above Ms and Af but below Md, NiTi alloys can deform by mechanisms of stressinduced martensite transformation and may display large anelastic strains which involve little or no deformation by slip. Under these conditions, the strain-controlled fatigue resistance of NiTi alloys may be exceptionally high [1]. In contrast, fcc metals like copper will, during strain controlled fatigue, eventuate severe plastic strain localization in the form of persistent slip bands (PSBs), whose behavior at free surfaces is intimately associated with fatigue crack initiation [2]. It is well known that fatigue crack initiation in fcc metals can be delayed by modifying surfacebreaking PSB structure or environment [3,4]. However, any attempt to strengthen the surface region in order to impede surface expression of persistent slip bands must contend with the large subsurface strain localizations enforced by the PSBs, which subject the film to strain cycling at roughly the characteristic PSB shear strain amplitude. Thermoelastic nickel-titanium alloys, capable of pseudoplastic straining, may be uniquely suited to the role of surface protection in situations where fatigue crack initiation is associated with PSB interaction with the external environment. In the present work, thin surface microalloys of NiTi, produced by ion sputtering, have been applied to polycrystalline copper fatigue specimens that were subsequently subjected to both monotonic loading, and fatigue under plastic strain control to produce mature PSB structures in the bulk material. Optical and scanning electron microscopy studies are described which assess the effect of the NiTi surface microalloy on the behavior of PSBs in the near surface region, and the attendant effect of the films on surface behavior of persistent slip bands. Although the results are incomplete with respect to the effects of the film on fatigue crack initiation, it has been shown for the first time that martensite transformation can be stress-induced (at temperatures above Ms) in a thin NiTi film on a plastically deforming substrate. This finding augurs well for the potential of such films to effectively suppress slip band penetration during low cycle fatigue.