Dispersoid Size Research Articles

Friction stir welding of PM2000 ODS alloy

Friction stir welding has been used to join sheets of a ferritic, oxide dispersion strengthened alloy, PM2000. A stepped spiral probe, polycrystalline cubic boron nitride tool, with a shoulder diameter of 25 mm, was used to weld 4 mm thick plate in a butt joint configuration. The thermomechanically affected zone underwent dynamic recrystallisation during welding; the resultant microstructure consisted of equiaxed ferritic grains containing a dispersion of yttrium aluminium oxides. Microindentation measurements revealed a significant reduction in hardness within the weld zone, when compared to the parent material. The welding process induced an overall coarsening of the yttrium aluminium oxide particles and depletion in their number density. However, the precipitation of secondary phase particles, likely to be oxides, which took place during the welding process, is indicative that an element of mechanical alloying occurs during the welding process. Annealing the welds for 1 h at 1380°C produced a massive recrystallised grain structure in the weld zone and a uniform hardness across the parent and weld was achieved. Transmission electron microscopy showed that, subsequent to annealing, particles were coarser in the weld zone (33 nm mean diameter) than in the parent alloy (24 nm mean diameter). However, electron diffraction and energy dispersive X-ray spectroscopy confirm that the dominant oxide phase, YAlO3 perovskite (YAP), was the same in both regions. Oxide particle size and number densities were not uniform throughout the weld. Focused ion beam prepared surfaces revealed particles within the size range of 50–600 nm diameter in material beneath the tool shoulder/workpiece contact area; the average size of dispersoids in this region was 130 nm.

Preparation of Mo2C nanoparticles dispersion-strengthened copper-based composite by EB-PVD

In this research, a nano-Mo2C particle dispersion-strengthened copper alloy was prepared by a novel method, i.e., electron beam physical vapor deposition (EB-PVD) which has advantages of simple technical process and low cost compared with the conventional mechanical alloying method. And the microstructure and properties of the material were investigated. The results show that the copper matrix is composed of columnar crystals with the average width of 7 μm, and the size of Mo2C dispersoid is 1–7 nm. The ultimate tensile strength of the material is 486 MPa, and the electrical conductivity is 82 % IACS. As the temperature increases from 293 to 573 K, the material becomes more brittle.

The Effect of Hot Deformation on Dispersoid Evolution in a Model 3xxx Alloy

The influence of hot deformation on the evolution of size, shape, and fraction of dispersoids has been studied in a simple 3xxx aluminium alloy by means of hot torsion testing. It has been shown that at high strain rates, deformation leads to spheroidization of the dispersoids, an increase in number density, and an increase in volume fraction. The increase in number density and volume fraction are associated with precipitation of new particles. The enhancement of manganese diffusion is a key factor in promoting rapid dispersoid evolution during deformation. A model has been developed to estimate the effect of deformation induced vacancies and dislocations on diffusion. This predicts that an order of magnitude increase in diffusion coefficient between may occur under typical hot deformation conditions, consistent with the rapid microstructural changes measured experimentally.

Isothermal and Non-Isothermal Annealing of Cold-Rolled Al-Mn-Fe-Si Alloys with Different Microchemistry States

Investigation of the microstructural evolution of Al-Mn-Fe-Si alloys during annealing after cold rolling has been carried out. The effect of microchemistry state in terms of Mn in solid solution, constituents, size and volume fraction of dispersoids, introduced prior to cold rolling through different homogenization treatments, on microstructural evolution is compared during subsequent isothermal and non-isothermal heating experiments, with focus on the dependency of the amount solute (potential for concurrent precipitation) and pre-existing particles prior to deformation. It is clearly demonstrated that the actual kinetics and final microstructure are the result of a delicate balance between processing conditions and microchemistry state. In general, non-isothermal annealing treatments produce inhomogeneous microstructure, as compared to isothermal annealing. Moreover, more and finer particles (resulting from low-temperature homogenization) tend to hinder boundary motion, leading to even slower recrystallization kinetics and coarse non-equiaxed grains with a strong P texture component.

Effect of Dispersoid Size and Volume Fraction on Aging Behavior and Mechanical Properties of TiO2-Dispersed AA7075 Alloy Composites

TiO2-dispersed AA7075 alloy composites were produced by mechanical milling followed by hot uniaxial compaction and sintering. The effects of volume fraction and dispersoid size on precipitation kinetics, densification, and hardness of the composites were studied in detail. While the sinterability of the composites decreases with increasing volume fraction of the particulate reinforcement (dispersoid), the same increases with decreasing particle size of the reinforcement. Microstructural analysis using X-ray diffraction and scanning electron microscopy shows an improvement in the distribution of reinforcement with decreasing particle size. The hardness of the composites increases with increasing volume fraction and decreasing TiO2 particle size. Further, the reinforced composites do not show age hardenability unlike unreinforced AA7075 alloy. Microstructural analysis reveals the formation of MgTiO3 and ZnO near the TiO2-AA7075 interface, which suppresses the formation of Guinier-Preston (GP) zone resulting in no age hardenability of the composites.

Spall properties of Al 5083 plate fabricated using equi-channel angular pressing (ECAP) and rolling

The spall strength and Hugoniot Elastic Limit (HEL) of aluminum alloy 5083 (Al 5083) are compared for plates fabricated using equi-channel angular pressing (ECAP) and rolling. Al 5083 is a light-weight and strain-hardenable aluminum alloy used for armor plating in military transport vehicles, thus requiring the highest achievable spall strength and HEL. Materials that were processed by ECAP displayed a highly refined grain structure with little texture and a large degree of plastic deformation, whereas subsequent rolling resulted in a textured microstructure with both grains and inclusions aligning along the rolling direction. The spall behavior of Al 5083 was determined using plate-impact gas-gun experiments with rear free surface velocity measurements for a variety of processing conditions involving both ECAP and rolling. The spall strength and HEL increased from that of the as-received material after processing with ECAP. Subsequent rolling further increased the HEL but reduced the spall strength. Rolling also resulted in directional dependence of the spall strength, with the lowest spall strength occurring for impact through the plate thickness and highest spall strength in the rolling direction. The trends in the spall behavior correlate with the size and preferential alignment of manganese dispersoids and iron and silicon rich inclusions that are evolved during processing.

Structure and Properties of Nano-Scale Oxide-Dispersed Iron

Bulk samples of pure iron and yttria dispersed iron with and without titanium (i.e., Fe, Fe-Y2O3, and Fe-Y2O3-Ti) were prepared by hot extrusion of high-energy ball-milled powders. An examination of the microstructure using TEM revealed that the addition of titanium resulted in the reduction of the dispersoid size with a concomitant increase in the volume fraction of the dispersoids. As a result, Fe-Y2O3-Ti exhibited a substantial increase in hardness and tensile properties as compared to Fe and Fe-Y2O3. The higher hardness and strength of Fe-Y2O3-Ti is shown to be due to the presence of finer and higher number density of Y-Ti-O complex oxides. Dynamic strain aging in the temperature range of 423 K to 573 K (150 °C to 300 °C) was observed in all the compositions studied.

Mg-based metallic glass matrix composite with in situ porous titanium dispersoids by dealloying in metallic melt

A Mg3Cu3Gd bulk metallic glass matrix composite with porous α-Ti dispersoids was prepared by dealloying in a metallic melt. The in situ formed α-Ti dispersoids had a pore-size of ∼500nm, which was imparted from the Ti2Cu precipitate in the prealloy. Pore size was therefore controllable through the cooling rate of the prealloy preparation. Plasticity was not improved apparently, however the maximum fracture stress under four point bending mode increased from 217MPa for the monolithic counterpart to 387MPa. This was due to the optimal relationship where dispersoid size (L)≈dispersoid interval distance (S)≈process zone size (Rp). This was achieved locally within and around the porous Ti dispersoids, and helped improve the toughness and ductility.

High-Temperature Tensile Properties of Nano-Oxide Dispersion Strengthened Ferritic Steels Produced by Mechanical Alloying and Spark Plasma Sintering

Oxide-dispersion strengthened (ODS) ferritic steels were produced by mechanical alloying and subsequent spark plasma sintering. Very fast heating rates were used to minimize porosity when controlling grain size and precipitation of dispersoids within a compacted material. Sintering cycles performed at 1373 K (1100 °C) induced heterogeneous, but fine grain size distribution and high density of nano-oxides. Yield strengths at room temperature and at 923 K (650 °C) are 975 MPa and 298 MPa, respectively. Furthermore, high-temperature ductility is much increased: total strain of 28 pct at 923 K (650 °C).

Effect of one-step and two-step homogenization treatments on distribution of Al3Zr dispersoids in commercial AA7150 aluminium alloy

Semi-quantitative electron probe microanalysis (EPMA) mapping, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to study the effect of one-step and two-step treatments on the Zr distribution and Al3Zr dispersoid characteristics in as-cast commercial AA7150 aluminum alloy. It is shown that the Zr concentration in the dendrite centre regions is higher than that near the dendrite edges in the as-cast condition, and that homogenization at 460 °C for 20 h is insufficient to remove these concentration gradients. After homogenizing at 460–480 °C, a high number density of larger dispersoids can be observed in dendrite centre regions but not near dendrite edges. Furthermore, the dispersoid size increases with increasing the temperature during both one-step and two-step homogenization treatments.

Dispersion of C.I. Pigment Violet 23 in acetone using supercritical carbon dioxide

This study investigates the dispersion of 1 wt.% of C.I. Pigment Violet 23 particles in acetone using a supercritical fluid-assisted dispersion process (SFAD) with commercial dispersants. The effects of several key factors on the mean size of pigment dispersoids were investigated. These factors included the formulations of dispersants, the amount of adsorbed dispersants on the surface of pigment particles, and the temperature and pressure during the holding stage in the SFAD process. As evidenced by the experimental results, the isotherm adsorption of dispersoids was well correlated with the Langmuir isotherm model. Increasing the amount of Disperbyk-2150 dispersant adsorbed on the surface of pigment particles and holding the SFAD process at a higher temperature and pressure are both good for improving pigment dispersion in acetone.

Compressive yield strength of the nanocrystalline Cu with Al 2O 3 dispersoid

Nanocrystalline (nc) Cu with Al 2O 3 dispersoid (∼4 vol.%) was successfully synthesized by simple cryo-milling at 210 K with a mixture of Cu 2O, Al, and Cu elemental powders. The milled powder was consolidated by hot pressing (HP) at 1123 K and 50 MPa for 2 h. TEM (Transmission Electron Microscopy) work revealed that both of the milled powder and the hot pressed (HPed) materials were comprised with a mixture of the nc-Cu and homogeneous distribution of Al 2O 3 dispersoids. The compressive and micro Vickers hardness tests were performed on the HPed materials (nanocrystalline Cu with 4 vol.% of Al 2O 3 dispersoid) at room temperature to characterize the mechanical properties of the materials. The compressive yield strength of the materials was as high as 863 MPa; the micro Vickers hardness 2600 MPa. The results of the mechanical tests apparently show that the relationship between the yield strength and the micro hardness of the HPed materials is in well agreement with Tabor's rule, H v = 3 σ y in MPa. The grain size of the nc-Cu was estimated by XRD using Scherrer's formula and TEM observation; the Al 2O 3 dispersoid size was measured from element mapping by STEM–EDS (Scanning Transmission Electron Microscopy–Energy Dispersive Spectroscopy) works. An attempt was made to quantify the possible strengthening effects of the nc-Cu materials with Al 2O 3 dispersoid. Two strengthening mechanisms were proposed for high hardness and yield strength of the materials, i.e., grain size and dispersion hardening effects.

Design related aspects in advanced nuclear fission plants

Important issues to be considered for design of future reactors are: extrapolation of stress rupture data, creep–fatigue, negligible creep, damage monitoring. The paper highlights some new developments taking examples from a martensitic steel (mod 9% Cr), oxide dispersion strengthened (ODS) steels and nickel-base superalloys. Traditional approaches to extrapolation of (thermal) stress rupture data like Larson–Miller Parameter or Monkman–Grant rule seem to be valid concepts also for advanced reactors. However, a significant influence of cyclic softening on creep rates and stress rupture data can be expected as shown for grade 91. This is particularly true for creep–fatigue interactions. Based on cyclic stress–strain behaviour it is also possible to get very good life-time predictions under creep–fatigue with a strain range separation (inelastic fatigue and creep ranges) technique which could replace the currently used linear life fraction rule. Results from in-beam irradiation creep reveal no significant influence of dispersoid size. It can be assumed that irradiation creep is a matrix property. Finally it is shown that micro-sample testing of exposed material could be used as an advanced method for damage assessment in future nuclear power plants.

Caracterización microestructural de aleaciones base cobre obtenidas mediante molienda reactiva

The micro and nanostructure of Cu-Al, Cu-V and Cu-Ti alloys produced by reactive milling were analyzed using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Samples with different milling times (t= 0, 10, 20 and 30 h) were considered. The grain size, dislocation density and residual microstrain were evaluated form the XRD data using the Williamson-Hall and Klug-Alexander methods. The evolution of texture as a function of milling time was also studied using XRD. It was found, using TEM, that the grain size and dispersoid size were nanometric in all three alloys considered.

Dispersoid Formation and Recrystallization Behavior in an Al-Mg-Si-Mn Alloy

The nucleation and precipitation of Mn-containing dispersoids in an Al-Mg-Si-Mn alloy (6082) have been studied by optical microscopy, EPMA (electron probe microanalysis) and TEM (transmission electron microscopy). The in∞uence of Mn-containing dispersoids on the recrystallization behavior was also investigated. The size and distribution of dispersoids were strongly afiected by both the homogenization process and the alloying element distribution formed in the direct chill cast procedure. The Mn-containing dispersoids were observed to nucleate preferentially on the fl 0 -Mg2Si phase and to be aligned along the direction of the matrix. After cold deformation, the morphology of dispersoids greatly in∞uences the recrystallization and grain growth behavior in the annealing process.

Damage assessment in structural metallic materials for advanced nuclear plants

Future advanced nuclear plants are considered to operate as cogeneration plants for electricity and heat. Metals and alloys will be the main portion of structural materials employed (including fuel claddings). Due to the operating conditions these materials are exposed to damaging conditions like creep, fatigue, irradiation and its combinations. The paper uses the most important alloys: ferritic-martensitic steels, superalloys, oxide dispersion strengthened steels and to some extent titanium aluminides to discuss its responses to these exposure conditions. Extrapolation of stress rupture data, creep strain, swelling, irradiation creep and creep–fatigue interactions are considered. Although the stress rupture- and the creep behavior seem to meet expectations, the long design lives of 60 years are really challenging for extrapolations and particularly questions like negligible creep or occurrence of diffusion creep need special attention. Ferritic matrices (including oxide dispersion strengthened (ODS), steels) have better irradiation swelling behavior than austenites. Presence and size of dispersoids having a strong influence on high-temperature strength bring only insignificant improvements in irradiation creep. A strain-range-separation based approach for creep–fatigue interactions is presented which allows a real prediction of creep–fatigue lives. An assessment of capabilities and limitations of advanced materials modeling tools with respect to damage development is given.

The Stretch, Limit and Path Forward for Particle Reinforced Metal Matrix Composites of 7075 Al-Alloys

Al-based metal matrix composites [MMCs] have been the research interest of a wide spectrum of material scientists throughout the world for some over two decades now. The present paper has chosen one alloy system namely the 7xxx series and from an extensive literature review concluded that since the beginning of the new millennium nothing note worthy has been added to the knowledge already gained in the last quarter of the last century except confirm the earlier findings that MMCs if properly fabricated by choosing the processing route and with appropriate size and volume fraction of dispersoids can improve most of the mechanical, corrosion and wear resistant properties of the base alloy. The author’s own research activities using this alloy system for making MMCs that include attempts to improve upon the properties by making composites, ageing and also secondary processing have been included. An attempt has been made to establish the stretch to which improvement is possible in the alloy system by making composites and trying all other routes known for meaningful improvement in properties. Further, the way forward for such particulate composites has been drawn to realise the material scientists’ dream of seeing such MMCs as engineering components. For this, the areas which now need research include mass production of composites, focus on its machining, joining, processing as also reduction in the size of dispersoids are some of the areas that have been identified and discussed in the paper.

Irradiation creep of oxide dispersion strengthened (ODS) steels for advanced nuclear applications

Ferritic oxide dispersion strengthened steels with different microstructure were in-beam creep tested in a temperature range from 300 to 500 °C. Irradiation was by He-ions. Elongation was determined as a function of stress and irradiation damage rate. Damage was investigated by transmission electron microscopy. A thorough analysis of the loops developing during irradiation creep did not show any dependence of orientation or size on the direction of the applied stress. At 400 °C radiation induced segregation was found (most probably an iron aluminide) which had no effect on irradiation creep. No pronounced influence of microstructure or dispersoid size on the irradiation creep behavior was detected. Irradiation creep compliance of PM2000 with dispersoids of about 30 nm diameter were found to differ little from material with dispersoids of only 2–3 nm diameter. This is in contrast to thermal creep where dislocation–obstacle interactions are extremely important. An assessment of the technical relevance of irradiation creep in advanced nuclear systems is presented.

Estimate of the Hall-Petch and Orowan effects in the nanocrystalline Cu with Al 2O 3 dispersoid

Nanocrystalline (nc) Cu materials with Al 2O 3 dispersoid (Cu-4 vol. % Al 2O 3) were successfully produced by a simple cryo-milling at 210 K with a mixture of Cu 2O, Al, and Cu ingredient powders, followed by hot pressing at 1123 K. The results of microstructural analysis showed that the hot pressed material was comprised of a mixture of Cu matrix (25.1 nm) with a homogeneous size distribution of the Al 2O 3 dispersoid (∼4 nm in radius). Grain size of Cu was measured by XRD (Scherrer's formula); dispersoid size of Al 2O 3 was confirmed by STEM-EDS (Scanning Transmission Electron Microscopy-Energy Dispersive Spectroscopy) and TEM (Transmission Electron Microscopy). Result of micro hardness tests indicated that the hot pressed materials have a significantly high hardness (265 ± 8, VHN; 2.6 GPa, SI units) at room temperature. Two strengthening mechanisms are considered for high hardness: strengthening by grain refinement and dispersion hardening. An estimate of the individual strengthening effect to total strength of the material at room temperature, based on the Hall-Petch and Orowan equations, is presented.

Simulation and control of dispersoids and dispersoid-free zones during homogenizing an AlMgSi alloy

Dispersoids and dispersoid-free zones (DFZs) near grain boundaries are produced when homogenizing as-cast alloys. The distribution and size of dispersoid precipitates and the width of DFZs affect subsequent alloy extrudability performance. A multicomponent Scheil model, coupled to a thermodynamic databank package (MTDATA) was used to calculate the concentration profiles of solutes and predict the width of the DFZs. In an attempt to eliminate the DFZs and refine the microstructure, equal channel angular extrusion was performed prior to homogenization. This had the effect of both reducing the size and improving the distribution of dispersoids, minimizing the extent of the DFZs and refining the grain size owing to recrystallization of the deformed structure during homogenization.