Aluminum Alloys: Solute Atom Clusters and Their Strengthening

Alloy composition and heat treatment processes have limited possibility to enhance ultra‐high strength of aluminum alloys, which restricts their widespread application in lightweight equipment. Consequently, high‐density dislocations and grain refinement are suggested to strengthen ultra‐high strength aluminum alloys. Herein, a novel nanostructured Al–Zn–Mg–Cu–Zr–Sc (AZMCZS) alloy with homogeneous microstructure is prepared through the synergistic processing of hot extrusion and high‐pressure torsion. Additionally, the microstructures and strengthening mechanisms of the nanostructured Al alloy are analyzed. It is observed that the ultimate tensile strength of the nanostructured Al alloy reaches nearly 1 GPa, and the elongation of the alloy is 1.9%. The nanostructured Al alloy mainly consists of nanoscale grains (≈117.7 nm), high‐density dislocations (2.4 × 1015 m−2), nano‐sized precipitates (the size of 20–51 nm), and solute atom clusters (≈3 nm). The multiple strengthening mechanisms of the nanostructured Al alloy are revealed in terms of grain refinement, dislocations, precipitates, and solute atom clusters. Grain refinement and dislocation strengthening show superior outcomes and are considered to be the predominant strengthening mechanisms. These findings demonstrate that this nanostructural architecture offers a new way to design super‐strength metals and alloys by effectively controlling the processing regime of severe plastic deformation.

Early precipitation stages of aluminum alloys—The role of quenched-in vacancies

The microstructure evolution, atomic ratio(r) of Mg and Si in different precipitates, and precipitation strengthening effects during the ageing process were investigated by HRTEM, atom probe tomography(APT)and hardness testing in LT24 aluminum alloy used for nuclear fuel cladding alternative materials. The results show that the early stage of ageing at 180 ℃ led to a significantly increasing of hardness and the formation of high density of solute clusters and Guinier-Preston(GP) zones in the alloy. The alloy reaches peak hardness after ageing at180 ℃ for 4 h due to a significant increasing density of the β″ precipitates. After the peak hardness, a hardness plateau is maintained for longer ageing time, because of the β″ precipitate is still the main strengthening phase in the specimens. The precipitates grow larger and the r increases with the increasing of ageing time. The r in β″ needles changes from 1.23 to 1.35. β″ needles are the main precipitation strengthening phase of the alloy. The precipitation sequence during the early ageing treatment in alloy can be described as follows: supersaturated solid solution →solute atom clusters→solute atom clusters+GP zone→solute atom clusters+GP zone+ β″.

Excess vacancies play crucial roles in the precipitation of age-hardening precipitates in aluminium alloys, but their spatial evolution across grains during heat treatments is less known. In this work, a numerical model is developed to predict the spatial evolution of non-equilibrium excess vacancies during cooling from solution treatment and during ageing heat treatments of multicomponent aluminium alloys. A finite volume scheme is applied to derive the spatial distribution of vacancy site fraction across grains by solving the diffusion equations of vacancies under the influence of solute elements. Binding energies between solute atoms and vacancies predicted by first-principles calculations has been used to handle the trapping of excess vacancies by solute atoms and atom clusters. The annihilation rates of excess vacancies at grain boundaries and at dislocation jogs have been derived based on a rigorous description of the annihilation mechanisms of vacancies. The evolution of the density of dislocation jogs due to vacancy annihilation has been taken into account. The model is successfully applied to interpret the age hardening behaviors of experimental alloys subjected to different thermomechanical processing conditions. This model will help to reach a deeper understanding of the roles of excess vacancies in precipitation kinetics and therefore is important to further optimize thermomechanical processing parameters and alloy composition to improve the macroscopic mechanical properties of age hardening aluminium alloys.

Solute clustering and its role in a titanium alloy made by laser powder bed fusion

Thin foils of binary aluminium alloys containing various amounts of copper, silver, zinc and magnesium in solid solution have been prepared from quenched specimens and examined by transmission electron microscopy. In all the alloys closed loops of dislocation line and complex dislocation networks were observed. The dislocation loops did not show any stacking fault contrast so they are glissile dislocations of prismatic 1/2 <110 > type. These loops are formed by the condensation of vacancies into collapsed discs and the results indicate that this often occurs during the quench. The vacancy concentrations required to produce the defects were calculated from the density and size of the loops and varied from 10−3 to 10−6 as the quenching temperature was decreased from 600°c to 500°c and as the solute content was increased when quenching from the same temperature. This result led to the conclusion that most of the vacancies remain 'quenched in' in the concentrated alloys, i.e. the supersaturation of vacancies decreases with increasing alloys content. The magnitude of this effect decreases in the order copper, silver, magnesium, zinc. This is also the order of increasing mobility of these elements in aluminium. At composition above 0.9 at % Cu, 1.3 at % Ag, 8 at % Mg and > 15 at % Zn most of the vacancies may be retained in solution after quenching the alloys from 550°c. The precipitation of vacancies in the Al-Cu, Al-Ag and Al-Zn alloys is thought to be accompanied by clustering of solute atoms. The annealing process in Al-Mg alloys is complicated and there is no evidence for clustering of magnesium atoms. Dislocations and grain boundaries have been shown to be sinks for vacancies and the possibilities that these defects are also collectors for solute atoms is discussed. Helical dislocations were observed in quenched alloys only when the quenching temperature was close to the limit of solid solubility.

Aluminum alloys derive their favorable mechanical properties from heterogeneous microstructures. The heterogeneity of these microstructures leads to localized corrosion. Whilst there has been intense research in localized corrosion in the past, a very important question remains unanswered: "How small is too small for microstructural features to behave as unique electrochemical entities or local corrosion hot-spots?" Our prior work has indicated that precipitates on the order of a few nm in size can indeed serve as unique electrochemical entities. In this work, we investigate the corrosion behavior of a new class of Al alloys based on inhomogeneous solid solutions. Through heat treatment, a spectrum of chemical heterogeneities ranging from about a couple of solute atoms to many tens of atoms (atomic clusters) may be formed without local changes in crystal structure. The mechanical properties of these alloys are strongly affected by the atomic scale clustering of solute atoms - whilst in this work we present some results for the corrosion properties (pitting propensity) of such alloys.

The detailed annealing behaviour of large faulted dislocation loops in aluminium and aluminium-base alloys has been studied by quantitative electron microscopy. Changes in loop shrinkage rates, loop character and dislocation pinning have been observed and used to derive information on solute clustering and vacancy/solute interactions. Although generally small, differences between the measured shrinkage rates of loops in pure Al and A1-base alloys were found to be dependent upon the state of solute dispersion. The results are consistent with a model in which extensive clustering occurs during quenching and ageing, and the differences in apparent activation energies are attributed to binding between vacancies and solute atom clusters, or zones. The apparent discrepancy between the large solute atom/vacancy binding energies measured in quenching studies and the much smaller ones measured in equilibrium studies is thus partially explained.

The effect of a “non-contacting” electric field (Method A) applied during the solution heat treatment of the Al – Mg – Si – Cu alloy AA6111 on subsequent natural aging measured by resistivity was determined and compared with that without field and with a field in which the specimen is connected to the positive terminal of the power supply (Method B(+)). Method A gave an increase in the level of the aging curve compared to that without field, but the increase was less than that with Method B(+). Neither field method had an effect on the general form of the aging curve, which was in accord with the Avrami equation with n = 0.49 and k = 0.02. The maximum resistivity for natural aging increased linearly with the as-quenched resistivity for the range in solutionizing temperatures (475 – 550 °C) without and with an electric field (0.2 and 2.5 kV cm−1) considered. This relationship gave: (a) the magnitude of the contribution of the insoluble constituents to the resistivity and (b) the ratio of the resistivity due to clusters of solutes formed during natural aging to that due to the as-quenched solute distribution. An electric charge on vacancy – solute atom clusters appears to be an important factor in the influence of an applied electric field on solubility.

Ion irradiation combined with nanoindentation is a promising tool for studying irradiation-induced hardening of nuclear materials, including reactor pressure vessel (RPV) steels. For RPV steels, the major sources of hardening are nm-sized irradiation-induced dislocation loops and solute atom clusters, both representing barriers for dislocation glide. The dispersed barrier hardening (DBH) model provides a link between the irradiation-induced nanofeatures and hardening. However, a number of details of the DBH model still require consideration. These include the role of the unirradiated microstructure, the proper treatment of the indentation size effect (ISE), and the appropriate superposition rule of individual hardening contributions. In the present study, two well-characterized RPV steels, each ion-irradiated up to two different levels of displacement damage, were investigated. Dislocation loops and solute atom clusters were characterized by transmission electron microscopy and atom probe tomography, respectively. Nanoindentation with a Berkovich indenter was used to measure indentation hardness as a function of the contact depth. In the present paper, the measured hardening profiles are compared with predictions based on different DBH models. Conclusions about the appropriate superposition rule and the consideration of the ISE (in terms of geometrically necessary dislocations) are drawn.

The heat treatment of a number of gas-atomised aluminium alloys prior to cold spraying recently showed that the resultant microstructural modification was accompanied by an improvement in deposition; however, the relationship between the microstructural homogenisation occurring after recrystallisation and the increase in deposition efficiency and particle–particle bonding had not been investigated. In this study, Al 6061 gas-atomised feedstock powder, before and after solution heat treatment, was cold sprayed and these materials were characterised using electron backscatter diffraction and transmission electron microscopy. The solution heat-treated Al 6061 powder showed large stress-free grains as opposed to the as-atomised feedstock powder which exhibited smaller grains with the presence of dislocations. The coating produced from as-received powder exhibited a homogeneous distribution of misorientation and lattice defects throughout the particles, whereas the coating produced from solution heat-treated powder showed a strain concentration in the interfacial zones. This was attributed to the partial dissolution and the clustering of solute atoms, allowing the aluminium matrix to deform around the newly formed precipitates.

The thermoelectric power vs. electrical conductivity plot enables us to separate an isothermal ageing process into distinct stages and to evaluate relative magnitude of the lattice strains caused by clustering of solute atoms or the coherency strains induced along precipitate-matrix interfaces. It is suggested that, under the influence of dislocations, the lattice strains caused by clustering of carbon atoms are relaxed, the coherency strains induced around e-carbide particles remain almost unchanged, and those around cementite particles are slightly enhanced. Prestraining before ageing promotes clustering of carbon atoms in place of the precipitation of e-carbide in the early stages of ageing at medium temperatures.

This paper reports a new structured prismatic platelet, self-assembled by an ellipse-like quasi-unit cell, precipitated in Mg-In-Yb and Mg-In-Ca ternary alloys and aged isothermally at 200°C using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy combined with density functional theory computations. The ordered stacking of solute atoms along the [0001]α direction based on elliptically shaped self-adapted clustering leads to the generation of the quasi-unit cell. The bonding of these ellipse-like quasi-unit-cell rods by the Mg atomic columns along the 〈〉α directions formed a two-dimensional planar structure, which has three variants with a {}α habit plane and full coherence with the α-Mg matrix. This finding is important for understanding the clustering and stacking behaviors of solute atoms in condensed matter, and is expected to guide the future design of novel high-strength Mg alloys strengthened by such high-density prismatic platelets.

Dealloying typically occurs via the chemical dissolution of an alloy component through a corrosion process. In contrast, here we report an atomic-scale nonchemical dealloying process that results in the clustering of solute atoms. We show that the disparity in the adatom–substrate exchange barriers separate Cu adatoms from a Cu–Au mixture, leaving behind a fluid phase enriched with Au adatoms that subsequently aggregate into supported clusters. Using dynamic, atomic-scale electron microscopy observations and theoretical modeling, we delineate the atomic-scale mechanisms associated with the nucleation, rotation and amorphization–crystallization oscillations of the Au clusters. We expect broader applicability of the results because the phase separation process is dictated by the inherent asymmetric adatom-substrate exchange barriers for separating dissimilar atoms in multicomponent materials.

Simulation of the effect of volume size factor of solute atoms and their clusters on one dimensional motion of interstitial clusters in Ni binary alloys