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Al-Mg-Si-Cu-Ag-Sc alloy with superior thermal stability has been designed, and a new mechanism for enhancing thermal stability was suggested in this study. The complex addition of Ag and Sc and the increased Cu content significantly enhanced the thermal stability of the Al-Mg-Si-Cu alloy by preventing the growth of over-aged precipitates. Particularly, Ag addition only delayed hardness degradation at the over-aging stage, whereas Sc addition lowered both the precipitation kinetics during the early and over-aging stages. The different roles of Ag and Sc create a synergistic effect so that excellent thermal stability can be obtained by the complex addition of Ag and Sc. The role of Ag in enhancing thermal stability generally depends on the conventional mechanisms of disordering and solute segregation. However, since the amount of Sc detecting was too low to have a sufficient influence on the thermal stability of the precipitate, the presence of Sc solute in the matrix was predicted. Solute analysis of the matrix revealed that Sc enhanced the thermal stability by disturbing Cu diffusion to Cu-containing precipitates. The strong Sc-Cu interaction enabled the temporary catching of Cu atoms by forming an Sc-Cu pair. Therefore, Sc not only enhances thermal stability but also delays age-hardening kinetics because the formation of Cu-containing precipitates is disturbed by the delaying mechanism depending on Sc-Cu pair formation. Additionally, the observation that the complex addition of Ag and Sc accelerated the formation of Mg-rich clusters during early stage aging provides minor evidence for the enhanced thermal stability of the Ag-Sc added Al-Mg-Si-Cu alloy. This is because Mg-rich clusters may be related to precursors of Cu-containing precipitate due to their high Cu contents and high Mg composition of Q′ and L phases. • Complex addition of Ag and Sc and high Cu contents significantly enhance thermal stability of Al-Mg-Si-Cu alloy. • Ag strongly disorders over-aged precipitate structure while the direct effect is small in case of Sc. • Instead, Sc delays overall precipitation kinetic by forming Sc-Cu pairs, so that enhances thermal stability. • Formation of Mg-rich clusters are also accelerated by Ag and Sc, which will form thermally stable precipitates. [ABSTRACT FROM AUTHOR]

2219Al alloys microalloyed with varying Cd contents were processed by casting technique. Differential scanning calorimetry (DSC) was performed from 323 to 823 K at four different heating rates of 283, 289, 293 and 298 K min−1, to investigate the precipitation kinetics of the alloys. The heating cycle of DSC curves exhibited an endothermic peak in the intermediate temperature range from 603 to 713 K corresponding to the precipitation reaction, whereas an endothermic drift at elevated temperatures representing melting of the alloy phases. All the peak temperatures increased with an increase in heating rate, indicating the precipitation reaction to be kinetically controlled. A new methodology was proposed to model the kinetic rate equation, governing the endothermic precipitation peaks. Activation energy and related kinetic parameters were evaluated for the alloys, and the influence of Cd additions was investigated. Activation energy was observed to decrease by around 8.3%, due to microalloying with 0.06 mass% of Cd, thus kinetically favouring the precipitation reaction. Functional forms of mole fraction transformed were optimized, so that the present model of rate equation may achieve a proper fit with the experimental results of the corresponding reaction peaks. The transformation rate predicted from the rate equation and the estimated kinetic parameters, was successfully compared with the experimental data with fairly good accuracy. Such correlation with the experimental results validates the adapted analytical procedure, establishes the confidence and prediction capability of the present kinetic model and the accuracy of the kinetic parameters evaluated. X-ray diffraction (XRD) studies were resorted on the age-hardened alloys, to identify the possible precipitating phase(s). XRD patterns indicated the precipitation of θ-phase (CuAl2) within the Al matrix, which controlled the strengthening mechanism. The DSC and XRD results were further supported with the microstructural characterization of morphology and composition of different phases, as observed under SEM and EDS analysis. Trace contents of Cd exhibited to have significant potential to control the precipitation kinetics, activation energy, precipitating phases and respective microstructural evolution of the investigated 2219Al alloy system, during age-hardening treatment. [ABSTRACT FROM AUTHOR]

The effect of Ca addition on the age hardening response and precipitation behavior of a Mg-7Y-0.5Zr (wt%) alloy during isothermal aging at 350 ℃ has been studied by hardness test and high-angle annular dark-field scanning transmission electron microscopy. The addition of 0.6 wt% Ca can effectively enhance the age hardening response of Mg-7Y-0.5Zr alloy at 350 ℃, and the enhanced hardness is mostly attributed to the uniform precipitation of a novel strengthening precipitate phase, designated β p. The β p phase forms as plates on { 10 1 ̅ 0 } α planes. It has a Cd 45 Sm 11 -type structure (space group F 4 ̅ 3m , lattice parameter a = 2.24 nm) and a composition of Mg 5 Y 0.8 Ca 0.2. Its orientation relationship with the α-Mg matrix phase is such that [ 1 ̅ 11 ] β p // [2 1 ̅ 1 ̅ 0] α and (110) β p // (0001) α. After prolonged aging at 350 ℃, the metastable phase β p can transform in-situ to the equilibrium phase β-Mg 24 Y 5 with a cube-to-cube orientation relationship. The β p → β phase transformation is associated with the transition of constituent atomic clusters and the re-distribution of selected atoms within clusters. • The addition of 0.6 wt% Ca to Mg-7Y-0.5Zr alloy can enhance the age hardening response at 350 ℃. • A novel strengthening phase β p is observed in the peak-aged Mg-7Y-0.6Ca-0.5Zr alloy. • The β p phase transforms in-situ to the equilibrium phase β-Mg 24 Y 5 after prolonged aging. [ABSTRACT FROM AUTHOR]

With the rapid development of aerospace technology, the demand for a longer service life of Au-Ag-Cu alloy as conductive slip rings is increasing. Therefore, it is important to improve the hardness of the alloys in order to prolong their service life. Deformation-aging can improve the hardness of many alloys. The effect of the deformation-aging process of Au-20Ag-10Cu (wt%) alloy was investigated on the microstructure and properties, which characterized by DSC(Differential Scanning Calorimeter), XRD(X-ray Diffraction), EBSD(Electron Backscatter Diffraction), TEM(Transmission Electron Microscope) characterization techniques, microhardness and electrical conductivity testing methods. The results indicated that the microhardness increases and the conductivity decreases gradually with the increase of deformation rate. As the aging time increasing, the content of face-centered tetragonal AuCu I ordered phase increases, and thus the hardness increases gradually. The maximum hardness value of 307.44 HV was obtained after undergoing 70 % deformation and then aged at 300 ℃ for 16 h with the electrical conductivity values of 8.40 MS·m−1. This result indicated that both higher hardness and higher electrical conductivity of Au-20Ag-10Cu (wt%) alloy could be obtained, which ensures the requirements in the field of electrical contact materials. • The alloy with 70 % deformation rate and aged at 300 ℃ for 16 h had the maximized hardness value. • The transformation process of the matrix phase into the ordered phase can be significantly facilitated by increasing the aging time. • After aging at 300 ℃, the coherent boundary of {100} α 0 //{100}AuCu Ⅰ occurred because of the small lattice mismatch. [ABSTRACT FROM AUTHOR]

This study examines the precipitation evolution during T5 heat treatment (casting and direct aging) of an Al 7Si 0.35Mg alloy component with varying thicknesses. We employ a step mold casting technique to develop an Al 7Si 0.35Mg alloy component with cavity thickness ranging from 4 to 40 mm. Our experimental investigations unveil varying solidification times (from 5.0 to 152.2 s) and cooling rates (ranging from 2 to 50 K/s) due to the step mold casting. The thermal conductivity of component sections increase from 174.1 to 178.9 W/mK as the cooling rate surged to 50 K/s. Microstructure analysis of 50 K/s alloy reveals a large number density of coarse Mg-Si-rich pre-precipitates near dendritic cell boundaries, alongside fine GPII and β″ pre-precipitates within the cell interior. The pre-precipitation behavior in 50 K/s alloy is attributed to the retention in the mold until thick sections fully solidify, leading to low solute supersaturation and higher conductivity. Contrarily, 2 K/s alloy exhibits the formation of fine dot-like β″ and a few coarse Mg-Si-rich pre-precipitates along the cell boundaries. This alloy demonstrates superior age-hardening behavior and its low thermal conductivity increases during aging. This increment is associated with a small lattice misfit between precipitates and matrix in the as-cast state, inducing significant lattice distortion which lowers the thermal conductivity. Upon peak aging, these distortions are replaced by dislocations at the precipitate-matrix interface due to large lattice misfits. Finally, we offer an insight into how the solidification microstructure affects T5 treatment considering hardness and conductivity synergy. [Display omitted] • Cooling rate during solidification impacts the microstructure of Al 7Si 0.35Mg alloy component with varying thicknesses. • Thinner sections show higher thermal conductivity due to increased pre-precipitation density in the as-cast state. • Thicker section exhibited a better balance between age-hardenability and thermal conductivity after peak aging. • Lattice distortions at interfaces transform into misfit dislocations, enhancing thermal conductivity during peak aging. [ABSTRACT FROM AUTHOR]

L1 2 -strengthened high-entropy alloys have become research of interest for their great potential to achieve a superb balance between ductility and strength. While much attention has been paid to strong L1 2 -phase forming elements, little discussion has focused on the role of transition-metals. The concentration of Fe, Co, and Cr in the Al 0.3 CrFeCoNi alloy was decreased to investigate how transition-metals affect the stability and distribution of the L1 2 precipitate. The transmission electron microscope was used to characterize the precipitation behavior in the four alloys. The results show that reducing the level of transition-metals promotes the stability of the L1 2 phase. In the case of the base Al 0.3 CrFeCoNi alloy, B2 phase replaces nanosized L1 2 particles as the main precipitates when aging at 700 °C. In contrast, uniformly dispersed L1 2 nanoparticles remain stable after 700 °C aging, with almost no B2 phase observed. Adjusting the content of these transition-metals can also alter the solid solubility of Al in the fcc matrix and change the site occupancy of Al sites in the L1 2 phase, enabling a shift in the volume percentage of the L1 2 phase. Of the three elements tested, Fe reduction effectively increases the fraction of the L1 2 phase, while Cr reduction even have a little negative influence on the density of the L1 2 particles. As a result, the microhardness and strength decrease in the following sequence: Al 0.3 CrFe 0·6 CoNi 1.4 , Al 0.3 CrFeCo 0·6 Ni 1.4 , Al 0.3 CrFeCoNi and Al 0.3 Cr 0·6 FeCoNi 1.4. This study provides significant insights into the role of transition-metals and facilitates the design of L1 2 -strengthened high-entropy alloys with excellent mechanical properties. • The stability of the L1 2 phase was improved by reducing the content of Fe, Co and Cr. • Fe is an important stabilizer for the B2 phase. • Less Fe can increase the fraction of the L1 2 phase more effectively than Co and Cr. • The Al 0.3 CrFe 0·6 CoNi 1.4 alloy exhibits the highest strength and hardness. [ABSTRACT FROM AUTHOR]

Abstract The synchrotron small-angle X-ray scattering (SAXS) technique was used to reveal the evolution and growth kinetics of nano-precipitates (ppts) of severely cold-rolled CoCrFeMnNi high-entropy alloy aged at 350–500 °C for different times. The nano-ppts in the as cold-rolled alloy have a radius of ∼1.2 nm, grow drastically in the early 30 min of aging at 500 °C, and reach the saturation stage at 60 min aging. The rapid increase in the size/volume fraction of the nano-ppts with the aging time is also quantitatively correlated with significant precipitation hardening, as indicated by measurements of the corresponding hardness and tensile stress-strain curve. TEM observation of the specimen aged at 500 °C for 60 min reveals many homogeneously distributed nano-ppts in the matrix, which have a radius of 2.0–2.5 nm. Graphical abstract Image 1 Highlights • Existence of nano-precipitates with radius of ∼1.2 nm in as severely cold-rolled CoCrFeMnNi HEA. • Evolution of nano-precipitates in terms of the size and volume fraction studied by synchrotron SAXS. • Growth kinetics of nano-precipitates correlated with the alloy's hardness and tensile stress-strain curve, indicating the precipitation hardening. • The nano-precipitates observed by HRTEM exhibit the homogeneous distribution. [ABSTRACT FROM AUTHOR]

Effect of aging temperature on precipitation behavior and mechanical properties of an AA2519 alloy was examined. Long-term natural aging provides the best combination of strength and ductility by the precipitation of dense Guinier–Preston and Guinier–Preston–Bagaryatsky zones. This phenomenon, called “delayed hardening”, has the same origin as the “rapid hardening” in AA2X24 alloys subjected to artificial aging. At 190 °C, high density of θ″-phase provides high strength. Peak aging is characterized by insignificant increase in strength associated with additional precipitation of θ′-phase. The overaging leads to the formation of precipitate structure dominated by θ′-phase. The formation of θ″- and θ′-phases can consume ∼0.7 and ∼3.3%Cu, respectively. Despite this, the number density of θ″-phase precipitates is higher than that of θ′-phase ones by a factor of ∼40. The θ″-phase is effective strengthening agent in the AA2519 alloy. The Ω-phase plates with a very high aspect ratio (AR) > 100 precipitate during artificial aging. [ABSTRACT FROM AUTHOR]

High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) are applied to investigate the influence of 2% pre-deformation on precipitation in an Al-Cu alloy. This pre-deformation results in a significant hardness increase and an enhanced age hardening response, which is demonstrated to be associated with the heterogeneous formation of pre-/θ' precipitates on dislocations. Two early-stage pre-θ' phases (pre-θ'−1 and pre-θ'−2) are frequently observed by atomic scale imaging. Pre-θ'− 1 consists of three Cu layers with each separated by two {200} Al planes, while pre-θ'− 2 has a regular structure in between two Cu layers with a distance of 2 a Al (~ 1.5 c θ' ). Pre-existing dislocations have a profound impact on the phase selection during aging, resulting in the unusual occurrence of pre-θ'− 1, pre-θ'− 2 and 1.5 c θ' thick θ'. The 1.5a Al (~ 1 c θ' ) thick hitherto unknown pre-θ'− 3 and the θ'-approximant phase reported before in aged Al-Cu alloy with large pre-deformation also exist but with rare occurrence. The unveiled precipitation scenario leads us to sort out different precipitation reactions at dislocations: (i) direct formation of thick θ'; (ii) formation of pre-θ'− 1 and 2 c θ' thick θ'; (iii) formation of pre-θ'− 2 and 1.5 c θ' thick θ'. There are striking structural similarities between pre-θ' phase and θ' with the same thickness, while the only difference is the atom rearrangement inside the two stable interfacial Cu layers on {002} Al planes. This work provides not only new insights into the effect of pre-deformation on the precipitation behaviors of Al-Cu alloys, but also a guide to tailor the microstructure to achieve desirable mechanical behaviors. [ABSTRACT FROM AUTHOR]

In this study, we investigated the effect of Cu additions (0.1 and 0.3 wt%) on the age-hardening behavior of AA6016 Al–Mg–Si alloys with and without 3 % prestraining by hardness testing and transmission electron microscopy analysis. Our results showed that higher Cu addition slightly increased the peak hardness, but significantly shortened the time to reach peak aging in both alloys with and without prestraining. Prestraining can improve the peak hardness by making the higher Cu addition more pronounced. In alloys without prestraining, the higher Cu addition slowed the hardness reduction in the overaged state, and the 0.3Cu alloy consistently exhibited finer precipitates with higher number density than the 0.1Cu alloy throughout the aging process. In contrast, higher Cu addition in alloys with 3 % prestraining accelerated the hardness decrease in the overaged state and led to a coarser microstructure after 24 h aging. This is primarily because Cu atoms can slow down the reduction in dislocation density during aging, whereas the higher dislocation density promoted the growth of precipitates, resulting in accelerated coarsening of precipitates as the aging time proceeded. Moreover, dislocations accelerated the formation of precursors of the Q′ phase, which was more pronounced in the 0.3Cu alloy. • Higher Cu addition refined the microstructure and shortened the peak aging time. • Prestraining accelerated the hardening rate during initial aging and softening rate during over aging. • During over aging, higher Cu addition slowed the decrease in hardness but accelerated it after applying prestraining. [ABSTRACT FROM AUTHOR]

The ordering transformation occurring in a model Ni-Cr-W superalloy during prolonged exposure to proper temperature has been investigated systematically. It is demonstrated that nanometer-sized precipitates with a DO 22 structure can precipitate in the Ni-Cr-W alloy by means of simple aging treatment at 650–700 °C. The mechanism of transformation to DO 22 superlattice has been determined to be continuous ordering based on the results of high resolution transmission electron microscopy investigation and variation trend in Vickers microhardness. Different variants of DO 22 phase can coexist in the matrix with no signs of overaging as aging time increases, indicating it has a high thermal stability. The precipitates of DO 22 superlattice has been found to be of ellipsoidal shape which results in the greatest reduction of strain energy. The interfaces between DO 22 precipitates and matrix have been revealed to be coherent at the atomic scale, resulting in considerable coherency strain attributing to the lattice misfit between DO 22 particle and matrix. Because of the high-density nanometer-sized DO 22 phase, the microhardness of the alloy has been improved remarkably after aging treatment. [ABSTRACT FROM AUTHOR]

Precipitation evolution and its correspondence to mechanical properties in Mg 4Sm xZn 0.4Zr (x = 0, 0.3, 0.6, 1.3) (wt%) alloys are systematically investigated in this work. Precipitation sequences at 200 °C are identified using transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). A new precipitate β′ Z is found to present with the increase of Zn addition. With higher Zn content (>∼1 wt%), basal γ-series precipitates dominate. The transition in precipitation sequence agrees well with the change of mechanical properties. The microstructure and property evolution in the optimum Mg 4Sm 0.3Zn 0.4Zr alloy is modeled based on obtained experimental data through a combination of classical nucleation and growth model and the CALPHAD (CALculation of PhAse Diagram) simulation. The obtained parameters are then used to predict the properties of other Mg Sm Zn Zr alloys under other heat treatment conditions. Good agreements are found between the calculated and measured data, which validate the accuracy of the current database and suggest a powerful tool for future optimization of this promising alloy system. [ABSTRACT FROM AUTHOR]

Two high-purity aluminium alloys based on composition Al–3.0Mg–1.0Cu (wt%), one with added 0.4 wt% Ag, were compared up to 11 days ageing at 443 K by means of transmission electron microscopy and hardness measurements. The base alloy exhibits an inhomogeneous precipitate microstructure with a high density of fine needle-shaped Guinier-Preston-Bagaryatsky (GPB) zones together with coarser precipitates of S′-Al 2 CuMg and rods of the structurally unknown Z-phase. The S′ phase is preferably formed on dislocations. The addition of Ag has a strong effect, leading to a homogeneous distribution with fine Ag-containing icosahedral quasi-crystalline precipitates (iQC). Both the GPB zones in the base alloy and the iQC phase in the Ag added alloy survive even after long term ageing. Ag is found to suppress the formation of the S′ phase. It is suggested that the Z phase is an approximation phase to the quasi-crystalline phase as is the case for the T-phase, implying they are based on similar (Bergman) clusters. [ABSTRACT FROM AUTHOR]

Age-hardened aluminium alloys have various degrees of precipitate-free zones (PFZs) along grain boundaries (GBs). The PFZs are weak zones and their existence promotes combined transgranular and intergranular fracture, thus reducing the ductility of the alloy. In this study, transmission electron microscopy (TEM) is used to display the geometrical properties and the chemical composition of the PFZs in the AA7075-T651 aluminium alloy. PFZs are found along grain and sub-GBs and their widths are about 40 and 20 nm, respectively. The PFZs are depleted of alloying elements compared with the nominal composition due to GB precipitation, but still they contain a certain amount of such elements in solid solution which will contribute to increase the yield strength and the work hardening compared to pure aluminium. Based on the results from the TEM study, a micromechanical finite element model of an idealized microstructure including grains and soft zones along the GBs is established. The Gurson model was used to represent the behaviour of the material in the grains and in the soft zones, using different initial void volume fractions to account for GB precipitation. Several loading conditions were applied to the micromechanical model in order to evaluate the localization of strains inside the soft zones and thus to get a better understanding of the role of the PFZs in ductile fracture of age-hardened aluminium alloys. It was found that the global failure strain varies non-monotonically with the global stress triaxiality due to the heterogeneity of the idealized microstructure. [ABSTRACT FROM PUBLISHER]

Effects of thermal history on precipitation behavior and precipitation kinetics in Mg–3Nd–0.2Zn (wt.%) alloy were studied. Microstructure analysis of the aged alloys revealed the presence of new phases in the microstructure in addition to the old description: Super saturated solid solution (SSSS) → G. P. zones → β ″ → β ′ /β 1 → β , reported for Mg–Nd system. In this regard, the sequence of precipitation for the studied alloy was identified as: Super saturated solid solution (SSSS) → Clusters of atoms → G. P. zones (I, II, III) → β ′ → β 2 → β 1 /γ ′ → β. The formation of clusters of solute atoms in this system was confirmed by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and atom probe tomography analysis. In addition, new metastable phases designated as G. P. zones (I, II, III) and β 2 were identified in the microstructure of the aged alloys by means of high resolution HAADF-STEM. The β 2 phase was found to be a transition phase in transformation of β ′ to β 1 . Furthermore, the microstructure analysis revealed that the structure of β 1 phase is indeed a body centered tetragonal structure in spite of face centered cubic structure that has been reported for this phase in the literature. [ABSTRACT FROM AUTHOR]

The processing challenges and microstructural inhomogeneity associated with ceramic (nano)composites render it important to develop hardened/toughened oxide-oxide ceramic alloys, possessing good mechanical properties, via a facile solid-state precipitation-based approach. However, detailed understanding of processing-microstructure-property correlations in age-hardened/toughened polycrystalline ceramics is still lacking. The present research, therefore, focuses on understanding the possible effects of variation of dimension/stoichiometry of coherent second-phase precipitate particles in such ceramic alloys on the coherency strains, associated residual stresses and mechanical properties. Polycrystalline MgO-based ceramic alloys have been developed via controlled aging treatments of partially supersaturated Fe(oxide)-containing MgO solid solution. Air-quenching post sintering-cum-solution treatment results in the formation of ultra-fine (∼5–20 nm) coherent intragranular Mg-ferrite precipitates; with subsequent aging treatments causing increases in their size and amount. The intragranular precipitate particles remain coherent with the MgO matrix even upon aging at 1000 °C for 20 h, despite coarsening to ∼130 nm. Detailed compositional analysis using EDS (with HAADF-STEM) and 3D APT reveal progressive increment in Fe-content of the precipitates with aging duration, such that the ultra-fine particles obtained right after solution treatment correspond to non-stoichiometric Fe-deficient Mg-ferrite (with ∼6 at.% Fe), whereas the relatively coarser particles obtained after 20 h of aging correspond to near-stoichiometric MgFe 2 O 4 (with ∼27 at.% Fe). Such increment in Fe-content results in increase in lattice misfit/coherency strains, which dominates the hardening response and improves the hardness despite loss of solution hardening. Furthermore, increase in compressive residual stress in the matrix due to increment in misfit/coherency strains results in improved resistance to crack propagation. [Display omitted] [ABSTRACT FROM AUTHOR]

This study examined the relationship between the lamellar-forming grain boundary reaction and the change in hardness during the aging process of an Au-Pt-based metal-ceramic alloy composed of 76.6 Au-9.9 Pt-9.3 Pd-1.7 In-1.2 Ag-0.56 Sn (wt%) with minor ingredients. The phase decomposition of the parent Au-rich α phase occurred within a very short time (30 s) by aging at 550 °C after a solution-treatment at 950 °C due to the solubility limits of Au and Pt in each other, which initiated a lamellar-forming grain boundary reaction. The observed hardening was attributed to both the grain interior and grain boundary precipitate. On the other hand, the grain boundary precipitate comprising the fine lamellar structure caused more powerful hardening from the early stage of the aging process. The alternate layer of the grain boundary lamellar structure was composed of Pt-, In- and Sn-depleted Au-rich α phase as well as precipitated Pt-, In- and Sn-concentrated β phase. The extremely fine nature of the grain boundary lamellar structure supplied large amounts of inter-phase boundaries, which contained lattice strain by the difference in the lattice parameter between the α and β phases, resulting in hardening. [ABSTRACT FROM AUTHOR]

Mg–Sn-based alloys are considered as a promising precipitation-hardening system for applications at elevated temperatures, but the hardening effect is not satisfactory owing to sluggish nucleation and rapid coarsening of the major Mg2Sn lath precipitates. In this study, Cu and Al are added to a Mg–6Sn–1Mn base alloy. The age-hardening response and the microstructures of these modified alloys have been investigated and are compared to that of the base alloy. The additional elements are found to bring several beneficial effects to the alloys for applications at elevated temperatures. Firstly, a eutectic structure consisting of strong intermetallic phases, i.e. Mg2Cu in the Mg–6Sn–1Mn–2Cu alloy and Al0.93Cu1.07Mg in the Mg–6Sn–1Mn–2Cu–2Al alloy, remains stable along the grain boundaries after solution and ageing heat treatments. Secondly, the precipitate density has been increased significantly and the precipitate size has been refined remarkably during ageing at 200?°C. Moreover, the growth of the precipitates is inhibited remarkably during the over-ageing period. Therefore, the age-hardening response and over-ageing resistance are notably improved. [ABSTRACT FROM AUTHOR]

Precipitates in a lean Al–Mg–Si alloy with low Cu addition (~0.10 wt.%) were investigated by aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Most precipitates were found to be disordered on the generally ordered network of Si atomic columns which is common for the metastable precipitate structures. Fragments of known metastable precipitates in the Al–Mg–Si–(Cu) alloy system are found in the disordered precipitates. It was revealed that the disordered precipitates arise as a consequence of coexistence of the Si-network. Cu atomic columns are observed to eitherin-between the Si-networkorreplacing a Si-network column. In both cases, Cu is the center in a three-fold rotational symmetry on the Si-network. Parts of unit cells of Q? phase were observed in the ends of a string-type precipitates known to extend along dislocation lines. It is suggested that the string-types form by a growth as extension of the B?/Q? precipitates initially nucleated along dislocation lines. Alternating Mg and Si columns form a well-ordered interface structure in the disordered Q? precipitate. It is identical to the interface of the Q? parts in the string-type precipitate. [ABSTRACT FROM AUTHOR]

Abstract: The effects of reinforcing particle type (SiC and TiC) on morphology and precipitation hardening behavior of Al–4.5%Cu based nanocomposites synthesized via mechanical milling were investigated in the current work. In order to study the microstructure and morphology of mechanically milled powder, X-ray diffraction technique, scanning electron microscopy and high resolution transmission electron microscopy were utilized. Results revealed that at the early stages of mechanical milling, when reinforcing particles are polycrystal, the alloying process is enhanced more in the case of using the TiC particles as reinforcement. But, at the final stages of mechanical milling, when reinforcing particles are single crystal, the alloying process is enhanced more in the case of using the SiC ones. Transmission electron microscopy results demonstrated that Al–4.5wt.%Cu based nanocomposite powders were synthesized and confirmed that the mutual diffusion of aluminum and copper occurs through the interfacial plane of (200). The hardness results showed that not only does introducing 4vol.% of reinforcing particles (SiC or TiC) considerably decrease the porosity of the bulk composite samples, but also it approximately doubles the hardness of Al–4.5wt.%Cu alloy (53.4 HB). Finally, apart from TEM and scanning electron microscopy observation which are localized, a decline in hardness in the TiC and SiC contained samples, respectively, after 1.5 and 2h aging time at 473K proves the fact that the size of SiC particles is smaller than the size of the TiC ones. [Copyright &y& Elsevier]

Abstract: In this work, the precipitation behaviour of an Mg–Ca–Zn alloy with high content of Ca relative to Zn was studied. Differential scanning calorimetry (DSC) was combined with transmission electron microscopy (TEM) and hardness measurement to examine the precipitate evolution in this alloy. A non-isothermal age-hardening heat treatment was performed to allow for a direct comparison to DSC results. Thermodynamic analysis aided in identifying ageing heat treatments to further de-convolute the overlapping DSC data. Results suggested the precipitation events followed sequential evolution towards the formation of two types of equilibrium phases (i.e. Mg2Ca and Mg6Ca2Zn3). Early stage decomposition of the solid solution had no observable effect on hardness, but was followed by the formation of hardening GP zones. Fine basal plates, as well as large coarse basal plates of Mg2Ca-type formed on further ageing. Contrast due to fine unidentified nanoscale precipitates was also observed in the over-aged state. These precipitates tended to disappear, while blocky Mg2Ca equilibrium phase precipitates formed as the heat treatment progressed to the highest temperature studied (i.e. 300°C). However, the equilibrium Mg6Ca2Zn3phase was not observed when this final test temperature was achieved at the end of the non-isothermal heat treatment process. [Copyright &y& Elsevier]

Abstract: Multinary Ti-Al-N coatings are used for various applications where hard, wear and oxidation resistant materials are needed. Here, we prepare TiAlN/TiN nano-multilayer coatings with modulation period of ~20nm in order to further improve the properties of Ti-Al-N coating. Annealing of both coatings up to 700°C results in an increase in hardness due to the precipitation of cubic Al-rich domains by spinodal decomposition. Multilayer structure results in an increase in adhesion with substrates from ~72N for Ti-Al-N single layer coating to 98N for TiAlN/TiN nano-multilayer coating. Additionally, the interfaces of TiAlN/TiN nano-multilayer coating retard the outward diffusion of metal atoms (Al and Ti) and inward diffusion of O while exposing coatings in air atmosphere with elevated temperature, and thus improve its oxidation resistance. An improved machining performance regardless of continuous cutting and milling is obtained by TiAlN/TiN nano-multilayer coated inserts, which can be attributed to the combined effects of higher adhesion with substrates and better oxidation resistance. [Copyright &y& Elsevier]

Abstract: The microstructures and mechanical properties of the new quaternary Mg–6Zn–xCu–0.6Zr alloys (x =0, 0.5, 1.0 and 2.0wt.%) have been investigated. The results show that the Cu content has a significant effect on the age-hardening response, tensile performance and fracture behavior of the alloys. The addition of 0.5wt.% Cu resulted in a remarkable age-hardening response and a striking improvement of the room temperature tensile properties after an isothermal ageing at 180°C. However, an excessive Cu addition of 2.0wt.% caused the formation of continuous brittle grain-boundary intermetallic particles, thus degrading the age-hardening response and the mechanical properties of the alloy. The microstructural factors associated with the improvements in the mechanical properties are discussed in detail. [Copyright &y& Elsevier]

The present paper is focused on fabrication, characterization, and aging behavior of nanostructured Al6061 alloy prepared by mechanical milling. The milling was performed on Al6061 in a planetary ball mill under argon atmosphere for various times up to 30 h. Isothermal heat treatment was then carried out on 30 h milled powder at different temperatures up to 550°C. The structural evolution of milled and annealed powders was studied by X-ray diffraction, SEM observation, and hardness measurements. The results showed that the final milled product was an Al-Mg-Si supersaturated solid solution having a grain size of 35 nm and a microhardness of 157 HV. Additionally, Mg2Si precipitation between 250 and 300°C led to an increase in hardness with the peak-aging hardness value of 207 HV after heat treatment at 300°C for 2 h. Above this temperature, due to the dissolution of the precipitates and grain growth, the hardness values decreased to 130 HV after heat treatment at 550°C for 3 h. [ABSTRACT FROM AUTHOR]

Abstract: This study investigated the effects of cooling rate, heat treatment as well as additions of Mn and Sr on hardness and hardening characteristics in Al–11Si–2.5Cu–Mg alloys. The results of scanning electron microscopy reveal that the age-hardening behaviour is related to the precipitation sequence of alloy. An energy dispersive spectroscopy analysis was used to identify the precipitated phases. The results also show that the hardness of the solution heat-treated samples is higher in air-cooled alloys than in furnace-cooled ones. Furthermore, the hardness observed in solution heat-treated samples is higher than in as-cast samples for air-cooled alloys, with the highest hardness level in the non-modified alloys. The highest hardness levels among the artificially aged samples were observed in the non-modified, air-cooled alloys. These levels occur after aging for longer times at lower temperatures (e.g. 30h at 155°C). The alloys studied did not display any softening after 44h at 155°C, whereas at 180°C, softening was noted to occur after 10–15h. At short aging times of 5–10h, high hardness values may be obtained by aging at 180°C. At aging temperatures of 200°C, 220°C and 240°C, softening began after 2h had elapsed. The cooling rate during solidification does not appear to have any significant effect on the precipitation characteristics and hardness of the Sr-modified alloys at certain aging temperatures. On the other hand, the effects of cooling rate may be clearly observed in the non-modified alloys. Manganese has a minimal effect on the hardness of the aged samples as it diminishes the potential action of age-hardening, while strontium lessens the hardness of the artificially aged samples. The effect of strontium, however, is more pronounced in the air-cooled alloys than in the furnace-cooled alloys. Strontium also has a noticeable effect on the reduction of hardness in aged Mg-containing Al–Si–Cu alloys, in that it affects the precipitates containing Cu and Mg. [Copyright &y& Elsevier]

This study examined the age-hardening by miscibility limit in a multi-purpose dental gold alloy containing platinum. The hardness increased rapidly in the initial stage of the aging process, reached the maximum value, then decreased continuously with aging time. The significant hardness increase resulted from the heterogeneous precipitation of the Pt-rich β phase from the grain boundary of the Au-rich α1 matrix due to the miscibility limit of Au-Pt system. With increasing aging time, the fine Pt-rich β precipitates covered almost the whole matrix, and by further aging, the precipitates grew coarse. The microstructural coarsening reduced the interface between the Au-rich α1 matrix and the Pt-rich β precipitates, which released the lattice strains between the two phases, resulting in a softening effect. In the later stage of aging process, the Au-containing Pt3In particle-like structure was transformed into the Au-depleted particle-like structure containing relatively large amounts of Cu resulting from the overlapping miscibility limit of both Au-Pt and Ag-Cu systems, which was responsible for the slow decreasing rate in hardness in the later stage of aging. [ABSTRACT FROM AUTHOR]

Abstract: The microstructures and age-hardening response of a Mg–5wt.%Gd alloy and a Mg–5wt.%Gd–1.1wt.%Mn–0.8wt.%Sc alloy have been examined under different solution treatment conditions. Both alloys exhibit little or no age-hardening response during isothermal ageing at temperatures above 200°C when the alloys are solution treated at 520°C for 24h. Solution treatment at 610°C for 24h does not improve the age-hardening response of the binary alloy. However, it leads to a significant increase in hardness during ageing for the quaternary alloy. This improved age-hardening response in the quaternary alloy is ascribed to the dense precipitation of two fine-scale precipitate phases during ageing. [Copyright &y& Elsevier]

Abstract: The present study examines the aging behavior of Al–Mg–Cu alloys based on the elastic effects on the matrix due to coherent precipitates; these effects were followed by X-ray diffraction peak broadening analysis. We conclude that the growing of matrix distortion zones around the precipitates is well described by the 2M factors established by Houska. In terms of mechanisms, in the first stages of ageing the rapid hardening seems to not be related with the interaction of dislocations with the stress field around the precipitates. The incremental microhardness observed in this alloy can be attributed to the formation of clusters or to solute-dislocation interactions. [Copyright &y& Elsevier]

New Mg–Y–Gd–Mn alloys, strengthened by a dense fine scale precipitation, have been recently designed for structural applications up to 523 K. The qualitative small-angle X-ray scattering study presented here enlightens the mechanical properties at high temperature of these alloys. Small Guinier–Preston zones have been evidenced in alloys maintained at room temperature after quenching. The metastable phases, studied in parallel using high-resolution transmission electron microscopy and responsible for the good mechanical properties, are difficult to quantify completely owing to their complex microstructure of interconnected globular and faceted precipitates. [ABSTRACT FROM AUTHOR]