Nano-scale Precipitates Research Articles

Enhanced precipitation-hardening in Mg–Gd alloys containing Ag and Zn

The maximum hardness achievable in Mg–6 wt.%Gd–0.6 wt.%Zr alloy during isothermal aging at 200 and 250 °C can be increased substantially by controlled additions of Ag or Zn. A further increase in the maximum hardness can be achieved when Ag and Zn are added together. The remarkable improvement in precipitation-hardening in the resultant alloys is associated with a dense distribution of nano-scale basal precipitate plates.

Nanocrystallization kinetics and glass forming ability of theFe65Nb10B25metallic alloy

The crystallization kinetics of glassy ${\mathrm{Fe}}_{65}{\mathrm{Nb}}_{10}{\mathrm{B}}_{25}$ melt-spun ribbons is studied by differential scanning calorimetry in the mode of continuous heating and isothermal annealing and by x-ray diffraction and transmission electron microscopy. Continuous heat treatments of the ribbons show the presence of multiple exothermic peaks before melting. The low-temperature peak corresponds to the precipitation of nanoscale ${\mathrm{Fe}}_{23}{\mathrm{B}}_{6}$-type crystalline metastable phase, and further annealing leads to its transformation into the metastable ${\mathrm{Fe}}_{3}\mathrm{B}$ phase and subsequent formation of $\mathrm{bcc}\text{\ensuremath{-}}\mathrm{Fe}$, ${\mathrm{Fe}}_{2}\mathrm{B}$, and FeNbB stable crystalline phases. The nucleation frequency and the growth rate are determined at selected temperatures from the analysis of the microstructures that emerge during the ${\mathrm{Fe}}_{23}{\mathrm{B}}_{6}$-type nanocrystallization. The master curve method is used to obtain the apparent activation energy and the Avrami exponent at the nanocrystallization onset. The nanocrystallization kinetics is explained in the framework of the Kolmogorov-Johnson-Mehl-Avrami theory. The rejection of insoluble alloy atoms during primary crystallization, the formation of diffusion layers around the crystals, and the decrease in the nucleation frequency caused by alloy enrichment of the residual disordered matrix is modeled through a soft impingement factor. Estimated values for the interfacial energy that provide a satisfactory agreement between experiments and modeling are derived considering that homogeneous nucleation frequency and interface-controlled grain growth are dominant at the onset of the nanocrystallization. Consequently, the time-temperature-transformation diagram is also drawn and the critical cooling rate estimated for this glass forming alloy.

Ostwald ripening of precipitates during two successive heat treatments performed at different temperatures

Two-stage Ostwald ripening of precipitates has been investigated experimentally and theoretically. A nickel-base alloy in which spherical nano-scale precipitates of the γ′-phase form served as a model system. Each specimen was heat treated at two successive temperatures T1 and T2 in the range 949–1119K. Thus, monodal as well as bimodal precipitate dispersions were produced. On the basis of an equation published by Wagner, Lifshitz, and Slyozov for isothermal ageing, the function r(t1,T1,t2,T2)¯ is modeled, where r(t1,T1,t2,T2)¯ is the average precipitate radius after double ageing and ti is the ageing time at Ti. In the case of bimodal γ′-precipitate dispersions, two average radii are considered. The agreement between measured and calculated values for r(t1,T1,t2,T2)¯ is within 7%, which is about the same accuracy as that of experimental data determined by transmission electron microscopy. The quoted agreement holds for monodal γ′-precipitate dispersions as well as for each γ′-population of bimodal dispersions.

Interfacial energy determination of nano-scale precipitates by CALPHAD description of Gibbs–Thomson effect

A theory based on calculation of phase diagrams in the binary systems was developed that describes Gibbs–Thomson effect. In this model effect of both interfacial energy and interface confinement (Laplace–Young pressure) are included in energy shift of alloys and phases. By using the CALPHAD model, interfacial energy of Cu4Ti precipitates in Cu–Ti system was obtained which shows better consistency with experimental results of Gibbs–Thomson effect of 10–20 nm radius precipitates.

Precipitation Hardening in Fe-1.03%Cu Structural Steel

The precipitation of copper during aging at 650oC within ferrite in high-purity Fe-1.03wt%Cu steel was examined by transmission electron microscopy, and the influence of precipitation particles on property of experimental steel was investigated. The microstructure and the corresponding diffraction patterns of different zone axis were analyzed. Nano-scale copper-rich clusters with B2-like structure and high density dislocation around precipitate was observed during either solution treatment or aging. Nano-scale metastable precipitates and high density around them were found to play the most important role for increasing steel strength.

High-strength wrought magnesium alloy with dense nano-scale spherical precipitate

This paper reported the influences of Yb addition on the precipitate and mechanical properties of wrought magnesium alloy ZK60. The ingots of ZK60-1.78Yb (wt%, 0.26 at%) alloys were cast using permanent mould and extruded at 370°C. By means of TEM and HRTEM, it was observed that Yb affected the precipitate and precipitation of ZK60-1.78Yb alloys significantly. Dynamic precipitation occurred in the as-extruded alloy and spherical nano-scale precipitate with high density and homogeneity exhibited in the aged alloys. The precipitate particles were about 5–20 nm in diameter, 10–30 nm in average space length. The tensile test results showed that the ZK60-1.78Yb alloy had excellent precipitation strengthening response with the maximum tensile strength 417.5 MPa at ambient temperature.

Atomic force microscopy (AFM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) of nanoscale plate-shaped second phase particles

The feasibility of accurately measuring the size and the volume fraction of nanoscale plate-shaped precipitates by atomic force microscopy (AFM) has been explored. For quantitative evaluations their unhandy geometry is conveniently described by superellipsoids. The experimental alloy Ni69Co9Al18Ti4 served as a model system: plate-shaped disordered γ-precipitates form in the L12 long-range ordered γ′-matrix. The results obtained by AFM are compared with those derived from transmission (TEM) and from high-resolution scanning electron microscopy (SEM). The agreement between the AFM and the TEM results is good. In spite of the low number of SEM images taken, the same holds for the SEM results. In addition, magnetic force microscopy was applied; its results are acceptable. The main advantages of AFM are (i) the numerical output for all three dimensions, (ii) the simplicity of its operation and (iii) the lower cost of the microscope itself. The first point allows the numerical AFM output data to be directly subjected to automated computer-based evaluations. All present experimental and evaluation procedures are also applicable to cube-shaped particles with rounded edges and corners as found, for example, in γ′-strengthened nickel-based superalloys.

Artificial Nano-Scale Precipitates for Flux Pinning in YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> Thin Films and Coated Conductors

By controlling pulse number of laser ablation, we prepared a series of quasi-multilayers of YBa2Cu3O7-δ/M (M=incomplete oxide layer) namely 70×(40/n) (n = 2, 5, 10 and 20, pulse number of M) which were characterised with nano-scale precipitates. While the texture properties of Y123 inside qusi-multilayer hardly change, its Tc appeared much different depending on the doping level of M. X-ray θ-2θ scanning and pole figures indicated that the different growth-controlled precipitates occured inside Y123 films. In the case of M=YSZ (Yttria Stabilized Zirconia), nanosized perovskite precipitates of BaZrO3 formed as a result of solid state reaction of YSZ with Y123, leading to Tc obviously decreasing with increasing of n. In the case of M=Y2O3, however, Tc did not decrease so much at the studied range of n due to no solid state reaction mentioned above. Magneto-transport and field angular dependence showed that the critical current density in films with lower doping content (such as n = 2) was improved in large ranges of field and temperature, suggesting tailorable enhancement and anisotropy of flux pinning force.

Nano-scaled iron-carbon precipitates in HSLC and HSLA steels

Abstract: Using mass balance calculate of nano-scaled precipitates measured by chemical phase analysis plus SAXS method, highresolution TEM analyse and thermodynamics calculation, as well as temper rapid cooling treatment of ZJ330, the composition, quantity, particle size distribution of nano-scaled precipitates with size less than 20nm in low carbon high strengh (HSLC) steel and its effect on mechanical properties of HSLC steel have been studied in this paper. It is found that there existed a large quantity of nano-scaled iron-carbon precipitates with size less than

Effect of Aging on Microstructure and Martensitic Transformation in Ti-Zr-Ni Shape Memory Alloys

Effect of isothermal aging on martensitic transformation temperatures, mechanical properties and microstructure was investigated for a Ni-rich Ti-Zr-Ni shape memory alloy at temperatures ranging from 673 K to 773 K. The aging behaviour was two stage process: the first stage associated with an increase in the Vickers hardness and a decrease in martensitic transformation temperatures and the second stage with a decrease in the hardness and increase in the transformation temperatures. Second stage was also characterized by the appearance of nano-scale precipitates, which has never been reported.

Interaction Between Copper Nano Cluster and Edge Dislocation Under Shear Strain

It is well known that nuclear pressure vessel steel shows embrittlement under thermal aging and strong neutron irradiation. We focus on nanoscale copper-rich precipitates and try to clarify the effect of the nanoscale copper-rich precipitates on embrittlement of reactor pressure vessel steels. Our final goal is to evaluate such embrittlement from microscopic viewpoint based on atomistic simulation. In this study, we simulate interaction between motion of an edge dislocation and copper clusters using Molecular Dynamics method with a PBC, where uniform shear strain is applied to the boundaries parallel to the slip plane (1 1 2) in the system. As the results, we clarify the effects of size, distance and pinning of the copper clusters on dislocation motion.

Nanoscale precipitation and mechanical properties of Al-0.06 at.% Sc alloys microalloyed with Yb or Gd

Dilute Al-0.06 at.% Sc alloys with microalloying additions of 50 at. ppm of ytterbium (Yb) or gadolinium (Gd) are studied with 3D local-electrode atom-probe (LEAP) tomography for different aging times at 300 °C. Peak-aged alloys exhibit Al3(Sc1−xYbx) or Al3(Sc1−xGdx) precipitates (L12 structure) with a higher number density (and therefore higher peak hardness) than a binary Al-0.06 at.% Sc alloy. The Al–Sc–Gd alloy exhibits a higher number density of precipitates with a smaller average radius than the Al–Sc–Yb alloy, leading to a higher hardness. In the Al–Sc–Gd alloy, only a small amount of the Sc is replaced by Gd in the Al3(Sc1−xGdx) precipitates, where x = 0.08. By contrast, the hardness incubation time is significantly shorter in the Al–Sc–Yb alloy, due to the formation of Yb-rich Al3(Yb1−xScx) precipitates to which Sc subsequently diffuses, eventually forming Sc-rich Al3(Sc1−xYbx) precipitates. For both alloys, the precipitate radii are found to be almost constant to an aging time of 24 h, although the concentration and distribution of the RE elements in the precipitates continues to evolve temporally. Similar to microhardness at ambient temperature, the creep resistance at 300 °C is significantly improved by RE microalloying of the binary Al-0.06 at.% Sc alloy.

Heterogeneous and homogeneous nanoscale precipitation in dilute Cu–Co alloys

The precipitation behavior of dilute Cu–Co alloys under isothermal annealing has been investigated by means of transmission electron microscopy. The observations show that different mechanisms occur in the coherent and incoherent precipitation stages. Coherent homogeneous Co particles of about 15 nm in size are consistently observed from the early stages of precipitation. The coexistence of coherent nanoscale spherical precipitates together with incoherent microscale precipitates is observed at higher temperatures, above 600 °C, with the latter forming well-developed and faceted Co-rich crystals. In the heterogeneous decomposition mode, the development of discontinuous precipitation colonies consisting of regularly-spaced rod-like Co particles is observed. Upon further growth, these rods break down into chains of spherical particles which are thought to be formed as a consequence of Rayleigh instability. In addition, depending on the thermal conditions, the generation of domains on the order of 100 nm is observed within the matrix, which is constituted of large grains with average size of 100 μm.

In situ high temperature ultrasonic evaluation for on-line characterisation of fine scale precipitation reactions in 8090 Al–Li alloy

Tailoring of microstructure through precise control of thermo-mechanical treatment is essential to obtain the desired properties of 8090 Al–Li Alloy for aerospace applications. Non-destructive diagnosis of nano-scale pre-precipitation and precipitation events, which govern the mechanical behaviour of the alloy, can provide valuable support in controlling the thermal treatment for achieving the peak aged or any alternate desired microstructural state. On-line high temperature evaluation of ultrasonic velocity and attenuation measurements in a series of as-received, solution annealed and prior treated specimens has resulted in differentiating the various phase transitions. The local maximum transition temperatures at the selected heating rate for GP zone formation, precipitation of coherent δ′, dissolution of δ′ and precipitation of stable S′ and δ have been identified to be 368, 470, 532 and 590 K, respectively. It is suggested from this study that the first differential plots of the ultrasonic velocity could be a useful method to differentiate unambiguously all the fine scale precipitation reactions in 8090 Al–Li alloy. Young's modulus and shear modulus have been evaluated from the ultrasonic parameters and their effects on temperature and the microstructural state have been studied. SEM and microhardness results are found to correlate well with the phase transition behaviour observed by ultrasonic measurements.

Study of the developed precipitates in Al–0.63Mg–0.37Si–0.5Cu (wt.%) alloy by using DSC and TEM techniques

Heat treatable Al–Mg–Si containing Cu alloys can be strengthened by the precipitation of the nano-scale metastable precipitates. In order to follow the precipitation sequence in balanced Al–1 mass%Mg 2Si containing 0.5 mass%Cu during continuous heating, differential scanning calorimetry (DSC) was performed. Analysis of non-isothermal DSC scans at various heating rates were carried out to evaluate the overall activation energies associated with the precipitation processes and, therefore, the mechanism of the developed precipitates has been characterized. The most important developed precipitates that assist the strength of the alloy are random, Q′ and β′ precipitates. According to the obtained activation energies, the kinetics of the evolved Q′-precipitates could be controlled by the diffusion of Mg, Si and Cu in the crystal lattice of the alloy. Both conventional and high resolution transmission electron microscopy (HRTEM) were utilized to confirm the obtained results.

The effect of variation of microstructure on fracture mechanics parameters of HSLA-100 steel

The variations in the FCGR curve, the J–R curve, the blunting-line slope and the stretch zone width (SZW) for systematic variation of microstructure in Cu-strengthened HSLA-100 steel has been studied. The microstructural variation in the steel has been introduced through ageing at various temperatures after an initial quenching treatment. This has resulted in progressive tempering of the as-quenched martensitic matrix, accompanied by nanoscale precipitation of coherent Cu particles that gradually coarsen and loose coherency with overageing. It was observed that although there was a systematic trend, the FCGR curves were least sensitive to microstructural changes. The variation of fracture toughness, characterized by JQ in most cases and by KQ for the microstructures displaying highest strengths, correlated well with the inverse relationship between fracture toughness and strength. A systematic trend was also observed for the pre-exponent and exponent of the power-law tearing curve (for cases in which brittle fracture was precluded), the blunting-line slope and the SZW. The effect of coherency of precipitates in restricting plastic flow, as implied from the variation of mechanical properties with ageing temperature is thought to be responsible for the effects observed.

Strong pinning enhancement in MgB2 using very small Dy2O3 additions

0.5 – 5.0 wt % Dy2O3 was in situ reacted with Mg+B to form pinned MgB2. While Tc remained largely unchanged, Jc was strongly enhanced. The best sample (only 0.5wt% Dy2O3) had a Jc∼6.5×105Acm−2 at 6K, 1T and 3.5×105Acm−2 at 20K, 1T, around a factor of 4 higher compared to the pure sample, and equivalent to hot-pressed or nano-Si-added MgB2 at ⩽1T. Even distributions of nanoscale precipitates of DyB4 and MgO were observed within the grains. The room temperature resistivity decreased with Dy2O3 indicative of improved grain connectivity.

Effect of Nano-Scale Copper Sulfide Precipitation on Mechanical Properties and Microstructure of Rapidly Solidified Steel with Tramp Copper Element

Copper is one of the main residual elements in steel, especially in recycled scrap steel, whereas sulfur is one of the main impurities in steel. A large quantity of slag and CO2 is produced during the process of removing the sulfur from the steel. However, copper and sulfur may combine to form copper sulfide, especially during the rapid cooling process. In this paper samples containing and not containing fine copper sulfide are prepared by the rapid solidification process. The microstructure, sulfide precipitates, and the mechanical properties of the samples are investigated by optical microscopy, Scanning Electron Microscope (SEM), Transmission Electron Microscopy (TEM), and the tensile testing. A large difference in the yield strength between the samples containing and not containing copper sulfides is observed. Each contribution of solid solution strengthening, grain refining strengthening, and sulfide precipitates strengthening in the samples with and without copper sulfide has been discussed. Particular attention has been paid to effect of the nano-scale copper sulfides, that is, main factor contributing to the alloy strengthening. Contribution of reduction of the copper sulfide particle size has also been discussed by comparing the results of two rapid solidification processes with different cooling rates. [doi:10.2320/matertrans.47.2312]

Ａｌ‐３％Ｍｇ‐１％Ｃｕ合金の時効硬化およびナノ析出組織に及ぼすＡｇ添加の影響

In this work, the bake-hardening (BH) response of Al–3.0%Mg–1.0%Cu alloys with and without Ag has been investigated by a hardness test, and related to the nano-scale precipitate microstructures observed by high resolution transmission electron microscopy (HRTEM) and a three-dimensional atom probe (3DAP). The small addition of Ag increased hardness under a BH condition (e.g. 443 K for 1.2 ks) due to the rapidly formed nanoclusters containing Mg, Ag and/or Cu. The formation of the equiaxed Z phase was also accelerated by the Mg/Cu/Ag clusters, resulting in the more increased second-stage hardening. This strongly suggests that Mg/Cu/Ag clusters provide effective nucleation sites for the Z phase, whereas Mg/Cu clusters formed in the ternary alloy do less.

Effect of nano-scaled precipitates on shape memory behavior of Ti-50.9at.%Ni alloy

In order to clarify the effect of low temperature aging (<600K) on the shape memory behavior of a Ti-50.9at.%Ni alloy, the deformation behavior and microstructures were investigated using tensile tests, X-ray diffractometry and transmission electron microscopy (TEM). The specimens were solution-treated at 1073K for 3.6ks followed by aging at 373, 473 and 573K for various times ranging between 1.2 and 10,800ks. TEM observation revealed that after aging at low temperature (<600K) the size of Ti3Ni4 precipitates hardly increases, but their density increases with increasing aging time in the early stage of formation of Ti3Ni4 precipitates. The critical stress for slip deformation abruptly increases with increasing density of fine Ti3Ni4 precipitates (<10nm) in the specimens aged at the low temperature region. This is attributed to the increase in the number of pinning points to hinder the movement of dislocations. With increasing aging time and temperature the size of Ti3Ni4 precipitates increases, resulting in decreasing the critical stress for slip.