Plastic Deformation Of Grains Research Articles

Toward all-dislocation-ceramics for high ionic conductivity produced by dry pressing at relatively low temperatures with and without ultrasound

Numerical simulations of the evolution of mobile and immobile dislocations in ceramics under applied pressure in dry pressing at a relatively low temperature are performed in order to study the possibility of production of all-dislocation-ceramics of solid electrolytes, which are expected to have extremely high ionic conductivity without dendrite formation because the diameter of a dislocation pipe is considerably larger than the distance between neighboring dislocations. The present numerical simulations are only for the densification process by plastic deformation of grains under high pressure under the assumption that the compaction of particles by their rearrangement is completed beforehand. By the plastic deformation, new dislocations are generated inside the grains. The required total dislocation density of about 1017 m−2 seems to be achievable under some conditions of dry pressing according to the present numerical simulations. Very short ultrasound irradiation at the beginning of the dry pressing sometimes considerably increases the dislocation density, while for other cases, it even considerably decreases the dislocation density due to enhanced annihilation of mobile dislocations by ultrasound.

The comparison of densification mechanisms and microstructure evolution among TiB2 ceramics sintered under different levels of high pressure

TiB2 shows significant advantages in high-tech industry but it is hard to fulfill densification. High pressure sintering is an effective method to obtain TiB2 ceramics. This study reports the sintering condition of TiB2 ceramics under different levels of high pressure from 150 MPa to 15 GPa. The results show that the densification temperature reduced with the applied pressure. The densification mechanism of TiB2 ceramics sintered under 150 MPa during the final holding time at 1900℃ was dominated by the mixture of dislocation motion and grain boundary sliding. When the applied pressure achieves at 1.5 GPa, the densification mechanism was transformed into the plastic deformation of grains and grain boundary diffusion, a number of small angle grain boundaries, dislocation cells, substructures and straight grain boundaries were observed. Finally, the densification mechanism was transformed into plastic deformation of grains and grain boundary while the driving force increased to 15 GPa. The dislocation and substructures density increased with pressure, leading to the grain refinement. Our study compared the densification and microstructure evolution of TiB2 ceramics sintered under high pressure, proving the high pressure sintering could regulate the microstructure and refine grains.

Cyclic thermo-mechanical performance of granular beds: Effect of elastoplasticity

Understanding the coupled thermo-mechanical behaviour of compacted granular beds can benefit various industrial applications, such as pebble bed design in fusion reactors. In this study, a thermo-mechanical discrete element method based on our previous work is improved and adapted to investigate the cyclic thermo-mechanical performance of gas-filled granular materials composed of elastoplastic grains. An interparticle contact model is developed considering the plastic deformation of grains. Through the simulation on a representative volume element of beryllium pebble beds, we provide grain-scale insight into the evolution of thermal conductivity and stress. The simulation results suggest that the network of thermal contacts is impeded by plastic deformation leading to a significant drop of thermal conductivity during cooling. This effect can be suppressed by increasing the initial packing factor. Not limited to pebble bed design, the conclusion of this work can also pave the way for optimizing powder-based manufacturing and energy storage, where combined thermo-mechanical loading conditions and elastoplastic deformation of individual particles are involved.

Deformation Mechanisms in Ni-Based Superalloys at Room and Elevated Temperatures Studied by In Situ Neutron Diffraction and Electron Microscopy

Polycrystalline Ni-based superalloys are one of the most frequently used materials for high temperature load-bearing applications due to their superior mechanical strength and chemical resistance. In this paper, we presented an in situ diffraction study on the tensile deformation behavior of the polycrystalline Ni-based superalloy VDM® Alloy 780 at temperatures up to 500 °C performed at the STRESS-SPEC neutron diffractometer at the Heinz Maier-Leibnitz Zentrum. A detailed microstructural investigation was carried out by electron microscopy before and after testing. The results of these studies allowed us to determine the deformation mechanism in the differently orientated grains. It is shown that the deformation behavior, which is mainly dislocation motion and shearing of the γ′-precipitates, does not change at this temperature range. The deformation is strongly anisotropic and depends on the grain orientation. The macroscopic hardening can mainly be attributed to plastic deformation in grains, where the (200) lattice planes were orientated perpendicular to the loading direction. Accordingly, a remaining lattice strain and high dislocation density were detected predominantly in these grains.

Impact of austenitizing temperature on the wear behaviour of AISI H13 steel

Austenitizing temperature is of great importance to achieve the desired properties of die steel. It governs the number of carbides dissolved in the austenitic matrix, which later transforms to martensite. This paper intends to find out the impact of austenitizing temperature on the wear behaviour of AISI H13 die steel. Austenitizing of H13 steel is done at different temperatures, i.e., 1000 °C, 1020 °C, 1040 °C, 1060 °C and then tempering is done twice at 560 °C for two hours. H13 die steel when tempered after austenitizing at 1020 °C lath martensite of large size is produced. Whereas, quite smaller lath martensitic structure has been observed in H13 die steel tempered after austenitizing at 1060 °C. Wear test investigation carried out using a pin on disc tribometer for H13 steel pins austenitized at different temperatures against D2 steel disc having 61 HRC. It is observed that the wear volume of H13 die steel exhibits an inverse linear relationship with its austenitizing temperature due to an increase in hardness. It is seen that small protective layer like patches of oxidized debris formed on the worn surface of H13 steel austenitized at 1060 °C. Whereas, no such protective layer formation is found on H13 die steel austenitized at a lower temperature. Post wear test, subsurface cross-section study shows plastic deformation of grains just beneath the worn surface along the direction of wear tracks. H13 die steel austenitized at 1060 °C with larger grains shows plastic deformation of grains up to a greater depth. Whereas, H13 die steel austenitized at 1000 °C with finer grain exhibits plastic deformation up to a lesser depth. An increase in grain boundaries of nearly twice is also found below the worn subsurface up to 80 to 100 µm depth. The present study will help to select the austenitizing temperature for H13 die steel to have better wear resistance.

Strengthening of alumina ceramics under cold compression

High pressure obviously decreased the sintering temperature of high melting point ceramics and produced bulks with excellent mechanical and thermal properties. In this study, the effect of pressure alone on the densification process of alumina ceramics was demonstrated. The translucent alumina bulks with unexpected hardness were obtained by compressing α-Al2O3 powder under ultra-high pressure at room temperature. Through analyzing microstructure and hardness of cold compressed samples, the results reveal that ultra-high pressure could crush alumina grains under appropriate pressure, and initiate plastic deformation of grains when the applied pressure exceeds the yield strength of alumina. Thus to enhance the hardness of the samples by grain boundary strengthening and plastic deformation at higher pressure.

Zirconia ceramics consolidated by oscillatory pressure sintering and subsequent carburization

We proposed a novel method consisting in the combination of oscillatory pressure sintering (OPS) followed by carburization in the vacuum, to prepare zirconia ceramics with high fracture strength and Weibull modulus. In this study, the zirconia particles were densified by an oscillatory pressure ranging from 27.5 to 32.5 MPa at 1450 °C, and then were carburized in the vacuum at 1300 °C. The result showed that the oscillatory pressure achieved several densification mechanisms, such as rearrangement of particles, plastic deformation of grains and sliding of grain boundaries. Consequently, the OPS specimen exhibited an increase in flexural strength from 998 MPa to 1556 MPa, and grain size reduction from 460 nm to 283 nm. The subsequent carburization of OPS zirconia induced high concentration of oxygen ion vacancies, and then high content and stability of the tetragonal phase. Thus, the carburized zirconia exhibited excellent comprehensive behaviors, demonstrating a flexural strength of 1646 MPa, fracture toughness of 8.8 MPa m1/2 and Weibull modulus of 25.8. The combination of OPS and consequent carburization in the vacuum is considered as an effective technique for preparing high-quality zirconia ceramics.

Microscopic Elastic and Plastic Inhomogeneous Deformations and Height Changes on the Surface of a Polycrystalline Pure-Titanium Plate Specimen under Cyclic Tension

A cyclic tensile test was carried out using a plate specimen of commercial pure titanium on a digital holographic microscope stage. Microscopic deformation of the grains was observed, and their height distribution was measured on the specimen surface. Each grain showed nanoscopic movement up and down, as well as reverse movement corresponding to specimen loading and unloading. We suggest that the different grain-specific changes in height were caused by microscopic inhomogeneities in the material, such as differences in the crystal orientation and geometries of both the surface and subsurface grains. Changes in grain height increased with tensile load, and a strong relationship was found between the height changes that occurred under elastic and plastic conditions. This suggests that microscopic plastic deformation is predictable from microscopic elastic deformation. In order to investigate the plastic deformation of grains in more detail, slip-line angles were measured after the tensile test. We found slip lines with similar angles in neighboring grains, suggesting that the plastic deformation of grains was not independent, but rather was related to that of surrounding grains and influenced by the deformation of subsurface grains.

The influence of liner material on the dynamic response of the finite steel target subjected to high velocity impact by explosively formed projectile

Subjected to high velocity impact (collision pressure over 30 GPa) of copper and iron liner EFP, the finite steel targets experience the failure modes of plug and spall. Dynamic deformation and microstructure evolution of the finite steel targets are investigated in detail through ANSYS/LS-DYNA software and experiment methods including optical, scanning and transmission electron microscopy. Morphology analysis of fracture surfaces indicates that the copper EFP remainder plated to the crater wall shows extremely plastic deformation which consists of elongated parabolic dimples on the crater wall and the mild carbon steel target exhibits excellent brittle features that material fails mainly along the cleavage facets on the rear surface of target. The whole part of copper EFP remainder and partial material of steel target undergoes completely dynamic recrystallization, which displays an extreme plastic flow. The thickness of irregular DRX zone (30.4 µm) formed by the interaction between iron EFP and steel target is wider than the thickness of DRX zone (21.3 µm) formed by the penetration of copper EFP. Numerical simulation results indicate that the highest plastic strains at the crater wall can reach about 2 which can fully accommodate the grains deformation of steel target during the dynamic recrystallization process at strain rates of the order of 104 s−1, and the average size of the refined DRX grains significantly decreases from 53.17 µm to approximately 200 nm. The refined grains in the DRX zone and elongated grains in the SPD zone distribute along EFP impact direction. Irreversible plastic deformation of grains extending from the crater wall to the matrix of steel target is consistent with the microhardness changes.

Preparation of Zirconia Ceramics with High Density and Fine Grains by Oscillatory Pressure Sintering

To fulfill the demands of ceramic components with high strength, high toughness and high reliability in key projects, a novel oscillatory pressure sintering (OPS) method was proposed in this study. With this new method, zirconia ceramics with high relative density, fine grains, high fracture strength, and excellent reliability, were prepared. As a comparison, zirconia ceramics were sintered by pressure-less sintering (PS) method and hot pressing (HP) method at same temperature. SEM analysis shows that the oscillatory pressure inhibits the rapid grain growth of zirconia at high temperature. Statistic results indicate that the OPS zirconia presents fine grain size at 251 nm, while the average sizes of PS and HP zirconia are 451 and 298 nm, respectively. In addition, the oscillatory pressure induces plastic deformation of grains and sliding of grain boundaries, facilitating elimination of closed pores at grain boundaries. As a result, the relative density of OPS zirconia reaches approximately 99.7%. Due to the refinement of grains and the increase in relative density, the OPS zirconia presents an increase from 1003 MPa to 1572 MPa in flexural strength, and an increase from 13 to 32 in Weibull modulus. Furthermore, its resistance to low temperature degradation is also improved because of the 208 无 机 材 料 学 报 第 31卷 microstructure evolution.

Impact-induced brittle deformation, porosity loss, and aqueous alteration in the Murchison CM chondrite

X-ray computed tomographic scanning of a 44g Murchison stone (USNM 5487) reveals a preferred alignment of deformed, partially altered chondrules, which define a prominent foliation and weak lineation in 3D. The presence of a lineation and evidence for a component of rotational, noncoaxial shear suggest that the deformation was caused by impact. Olivine optical extinction indicates that the sample can be classified as shock stage S1, and electron backscatter diffraction (EBSD) and electron microscopy reveal that plastic deformation within the chondrules was minimal and that brittle deformation in the form of fracturing, cataclasis, and grain boundary sliding was the dominant microstructural strain-accommodating mechanism. Textural evidence such as serpentine veins parallel to the foliation fabric and crosscutting alteration veins strongly suggest that some aqueous alteration post-dated or was contemporaneous with the deformation and that multiple episodes of fracturing and mineralization occurred. Finally, using the deformed shape of the chondrules we estimate that the strain experienced by Murchison was 17–43%. This combined with the current measured porosity of Murchison suggests that the original bulk porosity of Murchison prior to its deformation was 32.2–53.4% and likely at the upper end of this range due to chondrule compressibility, providing a unique estimate of pre-deformation porosity for a carbonaceous chondrite. Our findings suggest that significant porosity loss, deformation, and compaction from impact can occur on chondrite parent bodies whose samples may record only a low level of shock, and that significant chondrule deformation resulting in a chondrite foliation fabric can occur primarily through brittle processes and does not require plastic deformation of grains.

Modeling the crystallographic changes in processing of Al alloys

The evolution of deformation and recrystallization (RX) textures in 6016 Al alloy is analyzed in the current study by means of experimental measurements and numerical simulations. The deformation texture is modeled with various Taylor-type homogenization models whereas the development of RX texture is analyzed by evaluation of energy stored during the plastic deformation in grains of various orientations employing crystal plasticity (CP) calculations. It is shown that the main features of texture which evolve during discontinuous RX could be reproduced by taking into consideration both a microgrowth selection criterion and orientation selection based on crystallographically resolved stored energy of deformation. The influence of the strain heterogeneities on the development of RX texture is analyzed on the basis of CP and results derived from finite element calculations.

Phase transformation and subsurface damage in 3Y-TZP after sandblasting

ObjectiveThe goal of this work is to investigate t–m phase transformation, and subsurface damage in 3Y-TZP after sandblasting. MethodsCommercial grade 3Y-TZP powder was conventionally sintered and fully dense specimens were obtained. Specimens were sandblasted using different particle sizes (110 and 250μm) and pressures (2 and 4bar) for 10s. Phase transformation was measured on the surface and in the cross-section using X-ray diffraction and micro Raman spectroscopy, respectively. Subsurface damage was investigated on cross-sections using SEM and in shallow cross-sections machined by focused ion beam. ResultsSandblasting induced monoclinic volume fraction is in the range of 12–15% on the surface. In the cross-section, a non-homogeneous phase transformation gradient is found up to the depth of 12±1μm. The subsurface damage observed was plastic deformation in grains with the presence of martensite plates, and this effect is found to be larger in specimens sandblasted with large particles. SignificanceThe extent of subsurface tetragonal–monoclinic transformation and damage induced by sandblasting are reported for different sandblasting conditions. This knowledge is critical in order to understand the effect of sandblasting on mechanical properties of zirconia used to fabricate dental crowns and frameworks.

Study on the effect of temperature rise on grain refining during fabrication of nanocrystalline copper under explosive loading

Nanocrystalline (NC) copper was fabricated by severe plastic deformation of coarse-grained copper at a high strain rate under explosive loading. The feasibility of grain refinement under different explosive loading and the influence of overall temperature rise on grain refinement under impact compression were studied in this paper. The calculation model for the macroscopic temperature rise was established according to the adiabatic shock compression theory. The calculation model for coarse-grained copper was established by the Voronoi method and the microscopic temperature rise resulted from severe plastic deformation of grains was calculated by ANSYS/ls-dyna finite element software. The results show that it is feasible to fabricate NC copper by explosively dynamic deformation of coarse-grained copper and the average grain size of the NC copper can be controlled between 200∼400 nm. The whole temperature rise would increase with the increasing explosive thickness. Ammonium nitrate fuel oil explosive was adopted and five different thicknesses of the explosive, which are 20 mm, 25 mm, 30 mm, 35 mm, 45 mm, respectively, with the same diameter using 20 mm to the fly plate were adopted. The maximum macro and micro temperature rise is up to 532.4 K, 143.4 K, respectively, which has no great effect on grain refinement due to the whole temperature rise that is lower than grain growth temperature according to the high pressure melting theory.

In situ observation of crack growth in a WC-Co hardmetal and characterisation of crack growth morphologies by EBSD

Controlled crack growth in a WC-Co hardmetal has been observed in-situ in the SEM using an ultra-stiff test rig to allow controlled application and removal of loads. Cracks were observed to propagate in jumps of between 20μm and 70μm with each jump occurring in less than 200ms. High resolution SEM images and EBSD mapping have been used to characterise the effects of microstructure and WC grain orientation on the crack path and the degree of plastic deformation in both binder and carbide phases. The fracture in the hardmetal grade examined is dominated by transgranular fracture of the carbide phase with increased plastic deformation in grains fracturing on {10–10} planes observed by EBSD. At variance with some published research, plastic deformation in the Co binder phase ahead of the crack tip has been observed over distances of several times the Co mean free path.

Characteristics of abrasive waterjet cut-edges and the affect on formability and fatigue performance of high strength steels

The characteristic surface properties and internal workpiece transformations formed during the Abrasive Waterjet (AWJ) cutting process influences the formability and fatigue performance of steel cut-edges. This relatively established cutting technology is used in industry to generate specialist low production components, which may undergo continual loading cycles under operational service conditions. The fatigue performance of AWJ cut-edges can be critical since individual notch defects produced by the cutting process can act as initiation sites from where fatigue cracks can propagate. Due to the increased sensitivity of high strength structural steels to cut-edge fatigue, AWJ cut-edge defects have an ever more significant influence on fatigue performance. The relationship between the traverse cutting speed and the influence on the resulting properties of the cut-edge has been the critical area of investigation. The affects of traverse cutting speed on the surface roughness properties and cut-edge hardening through a process of plastic deformation of grains in the near edge region were observed to be influenced by the traverse cutting speed. It is these characteristic factors that were determined to influence the cut-edge ductility and fatigue performance of steel components. It is a combination of the AWJ properties that produces cut-edges, which are positive for the Hole Expansion Capacity (HEC) but negative for stress life and cyclic stress strain life fatigue performance of AWJ cut high strength steels.

The results of the so far performed investigations of Al-Cu butt cold pressure welding by the method of upsetting

The paper presents the results of the author's investigation obtained so far concerning the relationships between the properties of the Al-Cu weld and the surface condition prior to welding, the properties of the welded material and the process of plastic deformation of the materials being welded. The investigation performed has shown evolution modification of the geometrical structure of the surface and the layers below it during the approximation of the welded surfaces to each other. After the first physical contacts of the surface irregularities and their squashing, new irregularities of larger sizes arise as a result of the mechanism of plastic deformation of grains in the surface layer. The new geometrical structure of the surface thus formed has no features of similarity to the primary structure. It has been found that modelling of the approximation of the welded surfaces to each other, from the first physical contact of the surface irregularities to the distance of atomic force interaction in the methods of metal forming, should incorporate evolution change of the surface geometrical structure including the phenomenon of closing the voids. Initial computer simulations of the Al-Cu cold pressure welding process confirm the possibility of applying the finite element method (FEM) for the determination strains and stresses present in the welded materials.

Effects of Grain-Boundary Sliding and Plastic Deformation of Grains on the Formation of Grain-Boundary Cracks in a Double Cylindrical Bicrystal of a Cu-SiO<SUB>2</SUB> Alloy

A double cylindrical bicrystal is a bicrystal consisting of a cylindrical inside grain with a second grain wrapped around its curved side surface. Using a double cylindrical bicrystal of a Cu-SiO 2 alloy, tensile tests were performed at 800 K under 2 x 10 -4 s -1 . Three specimens cut from the same bicrystal were deformed to three different strains. Preferential formation of grain-boundary cracks around the cylindrical inside grain was observed after the tensile tests. In addition to the occurrence of grain-boundary sliding, the misfit of slip deformation at the grain boundary affects the formation of the grain-boundary cracks.

Coupling Between Precipitation and Plastic Deformation During Electromigration in a Passivated Al (0.5wt%Cu) Interconnect

AbstractIn the present paper the evolution of the dislocation structure during electromigration in different regions along the Al(Cu) interconnect line is considered. It is shown that plastic deformation increases in the regions close to cathode end of the interconnect line. A coupling between the dissolution, growth and re-precipitation of Al2Cu precipitates and the electromigration-induced plastic deformation of grains in interconnects is observed. Possible mechanism of the Cu doping effect on the improved electromigration resistance of the Al(Cu) interconnects is discussed.

Orientation Change Measured by the EBSD Method and the Plastic Deformation of Grains in Polycrystalline Copper

Orientation Change Measured by the EBSD Method and the Plastic Deformation of Grains in Polycrystalline Copper