Structure and mechanical properties of nanocrystalline Ag/MgO composites

Nanocrystalline Ag/MgO composites were prepared by the ultrafine-powder-compaction method. The structure was investigated for the first time by high-resolution electron microscopy. Nanometre-sized Ag grains and MgO grains in the composites bonded directly without any intermediate phase layer. Certain preferred orientation relationships were observed between the Ag and MgO grains. The nanocrystalline Ag/MgO composites retained their grain size during annealing up to 873 K. Vickers microhardness measurements were performed on the as-compacted and annealed specimens. Generation and propagation of cracks were less active in the nanocrystalline Ag/MgO composites than in a single-phase nanocrystalline MgO. The Vickers microhardness of the nanocrystalline Ag/MgO composites remained up to 1073 K. Hot-pressing deformation tests showed that the nanocrystalline Ag/MgO composites deformed plastically at 1073 K.     

Cooling effects on microstructure and mechanical properties of 27Cr–4Mo–2Ni ferritic stainless steel

The effects of cooling manner on the microstructure and mechanical properties of 27Cr–4Mo–2Ni ferritic stainless steel were investigated. It was found that the Laves phase (except for the TiN and Nb(C, N) particles) was distributed both in the grains and at the grain boundaries in the furnace-cooled specimen. The water-quenched and air-cooled specimens showed only TiN and Nb(C, N) particles. After annealing at 1100°C, the furnace-cooled specimen showed significant grain coarsening as compared to the water-quenched and air-cooled specimens. Furthermore, the Vickers hardness of the furnace-cooled specimen increased, while the total elongation decreased because of the formation of the Laves phase. The precipitation of the Laves phase resulted in the brittle fracture of the specimen during the tensile test.     

Microstructural stability of copper with antimony dopants at grain boundaries: experiments and molecular dynamics simulations

This study presents evidence that the microstructural stability of fine-grained and nanocrystalline Cu is improved by alloying with Sb. Experimentally, Cu_100−x Sb _x alloys are cast in three compositions (Cu-0.0, 0.2, and 0.5 at.%Sb) and extruded into fine-grained form (with average grain diameter of 350 nm) by equal channel angular extrusion. Alloying the Cu specimens with Sb causes an increase in the temperature associated with microstructural evolution to 400 °C, compared to 250 °C for pure Cu. This is verified by measurements of microhardness, ultimate tensile strength, and grain size using transmission electron microscopy. Complementary molecular dynamics (MD) simulations are performed on nanocrystalline Cu–Sb alloy models (with average grain diameter of 10 nm). MD simulations show fundamentally that Sb atoms placed at random sites along the grain boundaries can stabilize the nanocrystalline Cu microstructure during an accelerated annealing process.     

Phase separation in immiscible silver–copper alloy thin films

Far from equilibrium, immiscible nanocrystalline Ag–Cu alloy thin films of nominal composition Ag–40 at.% Cu have been deposited by co-sputter deposition. Both X-ray and electron diffraction studies indicate that the as-deposited films largely consist of nanocrystalline grains of a single alloyed face-centered cubic (fcc) phase. However, detailed three-dimensional atom probe tomography studies on the same films give direct evidence of a nanoscale phase separation within the columnar grains of the as-deposited Ag–Cu films. Subsequent annealing of these films at 200 °C leads to two effects; a more pronounced nanoscale separation of the Ag and Cu phases, as well as the early stages of recrystallization leading to the breakdown of the columnar grain morphology. Finally, annealing at a higher temperature of 390 °C for a long period of time leads to complete recrystallization, grain coarsening, and a complete phase separation into fcc Cu and fcc Ag phases.     

Fabrication and characterization of nanocrystalline oxides by crystallization of amorphous precursors

Amorphous zirconia and alumina powders were produced by the electrochemical deposition. Hot-pressing of the zirconia powder at 2.5 GPa and 360°C for 30 min caused simultaneous consolidation of the powder to a dense body and crystallization of a nanocrystalline tetragonal phase. Cold-pressing of the same powder at 2.5 GPa and sintering at 600°C or 800°C for 1 hr resulted in large tetragonal crystallites within the highly porous compacts. Cold-pressing of alumina powder at 2.5 GPa and sintering at 500°C, or 900°C for 2 hrs resulted in partial crystallization of the amorphous phase to a mixture of various polymorphs of alumina. The microstructure was inhomogeneous and composed of both nanometer and submicrometer grains. Microhardness of the hot-pressed partially crystallized nanocrystalline tetragonal zirconia was comparable to that of single crystal monoclinic phase, and decreased after annealing at 450°C for 30 min. Microhardbess of the nanocrystalline alumina was lower by on order of magnitude than that of conventional polycrystalline alumina. The decrease in the microhardness of the sintered specimens was related both to the grain size and the porosity.     

Microstructural evolution of Y123/Ag composites during sintering

The grain-growth kinetics of YBa2Cu3O7–xX (Y123) and the coarsening kinetics of silver inclusions in Y123/Ag composites during sintering were investigated. The sintering was carried out in the temperature range 900–950°C. The addition of silver lowered the formation of a liquid phase and the grain growth of Y123 in Y123/Ag composites was thus enhanced at the beginning of sintering. However, as the effective silver content was increased, more silver inclusions became interconnected, and the grain growth kinetics and coarsening kinetics slowed down significantly. This may be due to the mass transportation paths progressively changing from the grain boundaries to interfacial boundaries as the amount of interconnected silver networks is increased. The grain growth kinetic constant was calculated and compared with other published data. Because the inclusion was ripened with the growth of matrix grains, the coarsening of silver inclusions was deduced to be a coalescence process.     

Effects of annealing and quenching on fatigue behaviour in type 444 ferritic stainless steel

In order to understand the effects of annealing and quenching on fatigue behaviour in type 444 stainless steel, fully reversed axial fatigue tests have been performed using smooth specimens of heat‐treated materials in laboratory air and 3%NaCl aqueous solution. Three materials subjected to different heat treatments, annealing at 960 and 1000 °C, and water‐cooling at 960 °C, were prepared. In laboratory air, the fatigue limit of the annealed specimens was lower than that of the as‐received specimen and decreased with increasing annealing temperature. The subsequent grain coarsening from the heat treatments was primarily responsible for the lower fatigue strength in the annealed specimens. The fatigue strength of the water‐cooled specimen was lower than that of the corresponding annealed specimen. In the annealed specimens, cracks were generated within ferritic grains, while in the water‐cooled specimen, at or near grain boundaries. In 3%NaCl solution, the fatigue strengths of all specimens decreased compared with those in laboratory air. Only in the water‐cooled specimens, crack initiation at grain boundary and intergranular crack growth were observed, indicating the most sensitive to corrosion environment.     

Nanostructured and near defect-free ceramics by low-temperature pressureless sintering of nanosized Y-TZP powders

Nanosized (∼6 nm) Y-TZP (3 mol% Y₂O₃) powders have been produced by chemical co-precipitation (Y-inorganic + Zr-organic precursors) and thorough isopropanol-washing step, after calcining in air at 450 °C. The nanocrystalline Y-TZP powders consisted of spherical soft agglomerates (∼100 nm in size) which were easily broken down during compaction resulting in a very uniform green microstructure with a narrow pore size distribution (average pore size less than 6.5 nm) and no detectable compacting defects. In spite of the relatively low green density (43% theoretical), Y-TZP powder compacts sintered to near theoretical density in the very low-temperature range of 1000 °C for 80–100 h to 1070 °C for 2 h, maintaining a grain size in the nanoscale (< 100 nm) and the sintered bodies were nearly defect-free. Hardly any grain growth took place up to 1000 °C; it was very rapid above this temperature. This revised version was published online in November 2006 with corrections to the Cover Date.     

The thermal stability of high-energy ball-milled nanostructured Cu

The thermal stability of nanostructured (NS) Cu prepared by high-energy ball milling was investigated. The as-prepared samples were isothermal annealed for 1 h in the temperature range of 200–1000 °C. Effects of annealing on NS Cu samples were studied by means of Vickers hardness test, differential scanning calorimetry (DSC) and stress relaxation test. The exceptional high microhardness of as-prepared Cu sample of 1.7 GPa was not detected to decrease after annealing at 500 °C for 1 h with corresponding small value of activation volumes V^* of 22.6b³ and high value of strain rate sensitivity m of 0.0176. A prominent decrease of microhardness was detected after higher temperature annealing with a rapidly increase of activation volume and decrease of strain rate sensitivity. The present investigation demonstrates that the thermal stability of NS Cu prepared by high-energy ball milling is determined by not only the grain size but also the microstructure of grain boundaries, and during annealing process, the strain release process occurred prior to the grain growth process, therefore, the NS Cu has a relatively high thermal stability.     

Evaluation of thermal recovery of neutron-irradiated SA508-3 steel using magnetic property measurements

The Vickers microhardness and magnetic properties have been used to investigate irradiation effects and thermal recovery characteristics of neutron-irradiated SA508-3 reactor pressure vessel steel specimens irradiated to a neutron fluence of 5.5×1017 n cm-2 (E>1 MeV) at 70°C. Two recovery stages were identified from the hardness results during isochronal annealing and the mechanism responsible for the two stages was explained using the results of Barkhausen noise on the basis of the interaction between radiation-induced defects and the magnetic domain wall. The neutron irradiation caused the coercivity to decrease, whereas the maximum magnetic induction increased. Barkhausen noise parameters associated with the domain-wall motion were decreased by neutron irradiation and recovered with subsequent heat treatments. From the sensitive changes in the Barkhausen noise upon annealing heat treatment, it is suggested that the Barkhausen noise measurements may be used as a useful tool for monitoring the early stage of the thermal recovery behaviour of neutron-irradiated reactor pressure vessel steels. This revised version was published online in November 2006 with corrections to the Cover Date.     

Irradiation behavior of nanostructured 316 austenitic stainless steel

In order to get information about radiation resistance of ultrafine grained austenitic stainless steels, a 316 steel was deformed by high pressure torsion. The mean diameter of the grain after deformation was 40 nm. This material was annealed at 350 °C for 24 h or irradiated with 160 keV iron ions at 350 °C. Changes in the microstructure during annealing or irradiation were characterised by transmission electron microscopy (grain size) and laser assisted tomographic atom probe (solute distribution). Results indicate that this annealing has no influence on the grain size whereas the grain diameter increases under irradiation. Concerning the solute distribution, atom probe investigations show evidence of radiation-induced segregation at grain boundaries. Indeed, after irradiation, grain boundaries are enriched in nickel and silicon and depleted in chromium. On the contrary, no intragranular extended defects or precipitation are observed after irradiation.     

Nanocrystalline hydroxyapatite ceramics

The possibility of using high pressures in uniaxial pressing and rolling of nanopowders is investigated. Obtainment of a nanocrystalline hydroxyapatite ceramic intended for use in medicine is studied. The sintering temperature is reduced by approximately 600°C, and a ceramic with a grain size of less than 50 nm and high values of microhardness (up to 5.7 GPa) after annealing at 750°C is obtained. A substantial collective recrystallization of crystallite powder is overcome.     

Effects of solution treatment on grain coarsening and hardness of laser welds in UNS N10003 alloy contained different carbon content

Microstructure and microhardness evolution of laser welds in low carbon UNS N10003 alloy (LC alloy) and high carbon UNS N10003 alloy (HC alloy) before and after solution treatment have been characterized and investigated in this work. The eutectic M₆C-γ carbides have been transformed into spherical M₆C carbides in fusion zone of HC alloy, while it can be found that the spherical M₆C carbides were precipitated in fusion zone of LC alloy after solution treatment. The grain coarsening of fusion zone in HC alloy was slight because the migration of grain boundaries were impeded by the eutectic M₆C-γ carbides. However, the columnar grains of fusion zone in LC alloy were transformed into the coarse equiaxed grains due to the migration of grain boundaries were not impeded. The activation energy of grain growth between 1093 °C and 1177 °C for 20 min in LC fusion zone was 144.3 kJ mol⁻¹, while that of HC fusion zone was 309.5 kJ mol⁻¹ calculated according to the classical Arrhenius equation. The microhardness of fusion zone in LC alloy was lower than that of fusion zone in HC alloy after solution treatment because of no dispersion strengthening and grain coarsening.     

Influence of diffusion-annealing temperature on physical and mechanical properties of Cu-diffused bulk MgB2 superconductor

This study reports not only the effect of Cu diffusion on physical and mechanical properties of bulk MgB₂ superconductors with the aid of Vickers microhardness (H_v) measurements but also the diffusion coefficient and the activation energy of copper (Cu) in the MgB₂ system using the resistivity measurements for the first time. Cu diffusion is examined over the different annealing temperature such as 650, 700, 750, 800 and 850 °C via the successive removal of thin layers and resistivity measurement of the sample. Further, Vickers microhardness, elastic modulus, yield strength, fracture toughness and brittleness index values of the samples studied are evaluated from microhardness measurements. It is found that all the results obtained depend strongly on the diffusion annealing temperature and applied load. The microhardness values increase with ascending the annealing temperature up to 850 °C owing to the increment in the strength of the bonds between grains but decreasing with the enhancement in the applied load due to Indentation Size Effect behaviour of the bulk samples. Moreover, the diffusion coefficient is observed to enhance from 2.84 × 10^?8 to 3.22 × 10^?7 cm² s^?1 with the increase of the diffusion-annealing temperature, confirming that the Cu diffusion is more dominant at higher temperatures compared to lower ones. Besides, temperature dependence of the Cu diffusion coefficient is described by the Arrhenius relation D = 2.66 × 10^?3 exp(?1.09 ± 0.05 eV/k_BT) and the related activation energy of the Cu ions in the MgB₂ system is obtained to be about 1.09 eV. Based on the relatively low value of activation energy, the migration of the Cu ions primarily proceeds through defects such as pore surfaces and grain boundaries in the polycrystalline structure, resulting in the improvement of the physical and mechanical properties of the bulk MgB₂ samples.     

Effect of annealing treatment on the microstructure and mechanical properties of ultrafine-grained aluminum

Microstructural and property evolution of commercial pure Al subjected to multi-axil compression (MAC) and subsequent annealing treatment were investigated. After series of MAC pressings up to 15 passes, the samples were annealed at different temperatures. The deformed and deformed with sequent annealing treatment samples were characterized by X-ray diffraction, electron back scatter diffraction (EBSD), transmission electron microscopy (TEM) and tensile tests. The present results showed that on annealing the grain structures coarsen and transform from lamellar to equiaxed ones. Remarkably, the fraction of high angle grain boundaries drastically increases from 29.3% to 76.3% after annealing at 60 °C. Meanwhile, a significant decrease of lattice microstrain is observed after annealing, from 0.0839% to 0.0731% at 130 °C. A controlled 30 min annealing treatment on ultrafine-grained (UFG) Al at 60 °C can result obviously in a higher strength and a lower elongation, which may be associated with the nucleation and subsequent motion of dislocations in grain boundaries. As the annealing temperature is above 60 °C, the yield strength decreases and elongation increases gradually, which is attributed to the grain coarsening and microstructural enhancement.     

The nanocrystalline structure formation in germanium subjected to severe plastic deformation

The peculiarities of nanocrystalline (NC) structure formation in germanium subjected to severe plastic deformation (SPD) are examined in this paper. Transmission electron microscopy (TEM), X-ray analysis and differential scanning calorimetry were employed in the structural study of germanium specimens. The crystal-to-amorphous transition induced by SPD in germanium is observed. The NC structure formation is the result of annealing at 850°C. Crystallites in the NC state have non-equilibrium grain boundaries (GB) and a particular “spread” diffraction contrast, observed by TEM, testifies to this.     

Investigation of annealing behavior of nanocrystalline NiAl

Nanocrystalline NiAl prepared by mechanical alloying and the vacuum heat pressing method has been studied to investigate its thermal stability. Isothermal annealing was conducted at 1000°C. The results show that a significant grain growth takes place from 30 nm at the initial state to 56 nm by annealing for 30 h, then maintains this value regardless of annealing time up to 100 h. Microhardness test results show that the microhardness of the annealed specimens reduces significantly with annealing time up to 5 h and then increases slightly with further annealing due to off-stoichiometry. In addition, positron annihilation spectroscopy has been used to investigate the interfacial defects in the nanocrystalline NiAl specimens. The results show that there are three kinds of defects in the interfacial region of the nano-NiAl alloy, which are monovacancy-sized free volumes, microvoids and large voids. During the present annealing process, the sizes of the three defects have no significant change, but the amount of the defects, especially for monovacancy-sized free volumes and microvoids varies clearly with annealing time.     

Abnormal Hall–Petch behavior in nanocrystalline MgO ceramic

Pure and dense nanocrystalline MgO with grain size ranging between 25 and 500 nm were prepared by hot-pressing. Vickers microhardness was found to increase with decrease in the grain size down to 130 nm, following the Hall–Petch relation. Further decrease in the grain size was followed by continuous decrease in microhardness. A composite model was used to describe the microhardness behavior in terms of plastic yield of the nanocrystalline grains accompanied by strain accommodation and nanocracking at the grain boundaries (gb’s). Good agreement between the experimental and the calculated values indicates that gb’s may have significant effect on strengthening and ductility of nanocrystalline-MgO ceramics in the nanometer size range. Critical grain size exists below which limited plastic deformation within the grains and nanocracking at gb’s enhance the brittleness of the ceramic.     

Thermal stability of MgO nanoparticles

Thermal stability of nanocrystalline MgO particles with average diameter of 11 nm was investigated by annealing of the cold isostatically pressed compacts between 600 °C and 900 °C for various durations. Sintering time versus grain radius at 800 °C demonstrated a linear line with the slope of ∼ 4 similar to that expected for surface diffusion. High resolution scanning electron microscope images from different specimens showed a porous microstructure of interconnected particles typical for initial sintering. Arrhenius plot of the grain size data revealed the activation energy of 161 ± 11 kJ mol^− 1 for the growth process in agreement with those reported for grain boundary grooving experiments. It was found that MgO particles undergo coarsening already at temperatures as low as 0.31 of the MgO melting point (3125 K). Increase in the particle diameter and decrease in the surface area were associated with surface diffusion mechanism that leads to initial sintering between the particles.     

Effect of annealing on tensile properties of electrodeposited nanocrystalline Ni with broad grain size distribution

Abstract

The microstructures and tensile properties of electrodeposited nanocrystalline Ni (nc-Ni) with a broad grain size distribution after annealing at 150, 200 and 300°C for 500 s were investigated. The as deposited broad grain size distribution nc-Ni sample exhibited a moderate strength σ_UTS of ～1107 MPa but a markedly enhanced ductility ?_TEF of ～10%, compared with electrodeposited nc-Ni with a narrow grain size distribution. Annealing below 200°C increased the strength but caused a considerably reduction in tensile elongation. This behaviour is attributed to the grain boundary relaxation and the increased order of grain boundaries after annealing, which can make the grain boundary activities, such as the grain boundary sliding and grain rotations, more difficult. Further annealing at 300°C decreased both the yield strength and tensile elongation significantly due to significant grain growth.