Vacancy-solute Complexes Research Articles

Vanadium-Induced Structural Effects on the Corrosion and Tribological Properties of an Al-Li Binary Alloy

The influence of vanadium transition metal alloying on the structure, corrosion resistance and tribological properties of a binary Al-Li alloy containing 6wt% Li was investigated in this research. Varying vanadium contents of 0.05wt%, 0.1wt%, 0.5wt%, and 1.0wt% were examined. The results demonstrate that the addition of 1.0wt% vanadium optimally enhanced the corrosion resistance and wear resistance of the alloy. With 1.0wt% V, the wear rate decreased from 3.6 ± 0.50 ×10-2 m3/m to 1.5 ± 0.07 ×10-2 m3/m, the hardness increased from 51.7 ± 7.3 HV to 77.6 ± 4.1 HV, and the corrosion current density decreased from 43.6µA/cm2 to 3.9µA/cm2. Microstructural analyses revealed significant grain refinement induced by the addition of vanadium. The grain size decreased by 36.7%, and the dendrite arm spacing (DAS) decreased from 35µm to 8µm. Adding vanadium reduced the overall porosity of the alloy resulting from a peritectic reaction leading to the formation of vacancy-solute complexes at the grain boundaries. Phase and chemical composition analyses confirmed the formation and presence of δ (AlLi) strengthening phase within the interdendritic regions, particularly with the addition of 1.0wt% V. The presence of this phase strongly contributed to the observed improvements in corrosion resistance and tribological properties of the vanadium-modified alloy.

Micro-structural analysis of oxygen irradiated V-4Cr-4Ti by X-ray and electron back scattered diffraction

The effect of O6+ion irradiation on the microstructure of pre-deformed V-4Cr-4Ti alloy has been reported in this study. Changes in the microstructure has been assessed using detail X-ray diffraction line profile analysis in terms of the coherent domain size, rms micro-strain, dislocation density, their nature and their arrangement etc., revealing the importance of pre-deformation in controlling the material response to irradiation. The results are supported by Electron back scattered diffraction and Transmission electron microscopy. Energy Dispersive X-ray analysis has been used to map the micro-chemical changes caused in the sample due to the ion irradiation. Vicker's micro-hardness values were determined to understand the effect of irradiation on the mechanical property of the alloy. The results are explained in terms of the occurrence of vacancy-solute complexes, the formation of which isestablished using Density Functional Theory calculations..

The effect of quenching on the hinderance of the NbC growth process in a model microalloyed steel

ABSTRACT The effect of vacancies on the evolution of strain-induced NbC precipitates (SIP) was studied in a model austenitic Fe–30Ni–0.1Nb-0.1C steel deformed at 925 °C. Precipitation growth and coarsening characteristics have been investigated in detail by employing high resolution transmission electron microscopy. In general, the SIP preferentially nucleated on the nodes of the periodic dislocation networks constituting microband walls and grew over the varying holding time following the conventional precipitation growth and coarsening theory. Nevertheless, the current results demonstrated unexpected sluggishness in the process of growth and coarsening of the precipitates on the introduction of intermediary population of vacancies by a quenching step added in the thermomechanical processing schedules. It has been suggested that the formation of stable vacancy-solute complexes may reduce the effective availability of solutes for the precipitate growth.

Novel data analysis tool for the evaluation of Coincidence Doppler Broadening Spectra of the positron–electron annihilation line

Coincidence Doppler Broadening Spectroscopy (CDBS) of the 511keV annihilation line reveals the elemental signature at the annihilation site in matter. For this reason, CDBS enables the analysis of foreign atoms in the host matrix, vacancy-solute complexes and precipitates in solids. Due to the lack of a comprehensive program for the analysis of CDBS data we developed a modular python-based open source software solution for the reliable extraction of the annihilation line to provide so-called ratio curves highlighting the elemental signature in the spectra. In particular, the new program allows the readout and spectra averaging of multiple detector pairs. Moreover, we implemented an improved algorithm to further reduce the background by about one order of magnitude at high Doppler shifts.

Impact of solute-vacancy complex defects on ideal shear strength, stacking fault energy and elastic constants of perfect Ni crystals

Impact of solute-vacancy complex defects on ideal shear strength, stacking fault energy and elastic constants of perfect Ni crystals

Mapping defects during phase transformation in high Cr content NiCr solid-solution through positron trapping

Understanding the role of point defects towards hardening –and influence on secondary phase evolution demands a systematic investigation of defects during aging. In the present work, we investigate defects present/evolved during thermal aging in high Cr content NiCr solid-solution using positron annihilation lifetime and coincidence Doppler broadening spectroscopy. Thermal aging at the studied temperature results in phase transformation of the matrix primarily by discontinuous precipitation leading to Cr precipitation as revealed from field-effect scanning electron microscopy. Positron annihilation results reveal that non-equilibrium thermal (excess) vacancies are predominant defects in the studied alloy. These vacancies are retained in the alloy due to the formation of vacancy-solute complex. During solutionization treatment, the excess vacancies are largely recovered attaining an equilibrium vacancy concentration. On the other hand, excess vacancies are not recovered during thermal aging and stable up to 24 h aging period. In addition to vacancy-solute complex, dislocations are evolved at higher aging times and suggested to be present at the interface of Cr precipitate and matrix. Under the consideration of positron trapping model, the estimated atomic concentration of vacancies (vacancy-solute complex) in the phase transformed NiCr is ~ 10−5 and the dislocation density is ~ 4 × 109 cm−2.

Freezing solute atoms in nanograined aluminum alloys via high-density vacancies

Low-temperature decomposition of supersaturated solid solution into unfavorable intergranular precipitates is a long-standing bottleneck limiting the practical applications of nanograined aluminum alloys that are prepared by severe plastic deformation. Minimizing the vacancy concentration is generally regarded as an effective approach in suppressing the decomposition process. Here we report a counterintuitive strategy to stabilize supersaturated solid solution in nanograined Al-Cu alloys via high-density vacancies in combination with Sc microalloying. By generating a two orders of magnitude higher concentration of vacancies bonded in strong (Cu, Sc, vacancy)-rich atomic complexes, a high thermal stability is achieved in an Al-Cu-Sc alloy that precipitation is nearly suppressed up to ~230 °C. The solute-vacancy complexes also enable the nanograined Al-Cu alloys with higher strength, greater strain hardening capability and ductility. These findings provide perspectives towards the great potentials of solute-vacancy interaction and the development of nanograined alloys with high stability and well-performed mechanical properties.

Impact of Solute-Vacancy Complex Defects on Ideal Shear Strength, Stacking Fault Energy and Elastic Constants of Ni Crystals

Impact of Solute-Vacancy Complex Defects on Ideal Shear Strength, Stacking Fault Energy and Elastic Constants of Ni Crystals

Structural Transition of Vacancy–Solute Complexes in Al–Mg–Si Alloys

To theoretically examine the structural transition of vacancy–solute complexes in Al–Mg–Si alloys, we performed first-principles calculations for layered vacancy–solute complexes with additional Mg atoms. The central Mg atom in the additional Mg layer shifted to the Si layer with the increase in the number of Mg atoms to weaken the repulsive Mg–Mg interaction and to form Mg–Si bonds. When five Mg atoms were added to the layered vacancy–solute complex, the central Mg atom completely shifted to the Si layer, and a Mg vacancy was formed in the Mg layer, which indicated that the β″-eye is formed upon the addition of Mg atoms. We reproduced β″-eye formation from a solid solution with a vacancy using first-principles-based Monte Carlo simulations. Once the β″-eye was formed on the layered vacancy–solute complex, the process can be repeated by the formation of alternate Mg and Si layers along (010) β″. These results clearly indicate that the layered vacancy–solute complex plays an important role in β″-eye formation.

The structure evolution of titanium–vacancy complex in a vanadium-based alloy

Under irradiation conditions, excess point defects interact strongly with the solute atoms in fusion reactor materials. The solutes can bind with point defects and form clusters such as solute–vacancy complexes. The evolution of these complexes at long timescale significantly influences the mechanical properties of the irradiated materials. In this study, we mainly use climbing image nudged elastic band method to investigate the evolution and migration of titanium–vacancy complexes in a vanadium-based alloy. The transition pathways associated with atomic trajectories of complex evolution in bulk and in grain boundary vicinity are calculated. Results show that both basin energies and transition barriers are affected when a complex is in grain boundary vicinity. The evolution is revealed to be driven by multiple titanium–vacancy interactions synergistically. The migration is supposed to be induced by two mechanisms: the vacancy reorientation jumps and complex dissociation–reformation which are both based on titanium–vacancy interactions. This research contributes to the understanding of solute diffusion and early-stage radiation defects in vanadium-based alloys.

Structure and stability of vacancy–solute complexes in Al–Mg–Si alloys

This study investigates the structure and stability of vacancy–solute complexes in Al–Mg–Si alloys using first-principles calculations along with the prediction of initial structures using multiple linear regression analysis. In Al–Mg and Al–Si binary alloys, vacancy–solute complexes become unstable when more than five solute atoms are combined with a vacancy, due to the repulsive interactions between adjacent atom pairs. In Al–Mg–Si alloys, the formation energy of the vacancy–solute complexes continues to decrease as the number of solute atoms increase. The stability of vacancy–solute complexes mainly arises from the Mg–Si bonds formed with the increasing number of solute atoms. The combination of inward displacement of the Mg atoms and outward displacement of the Si atoms accommodates the increase in the number of Mg–Si bonds. Owing to short range repulsive interatomic interactions, Mg–Mg bonds cannot be formed in vacancy–solute complexes of Al–Mg–Si alloys. However, Si–Si bonds are a preferable alternative to enhance stability in vacancy–solute complexes, despite their decreased strength compared with Mg–Si, Al–Mg, and Al–Si bonds. The concentration of Si atoms in the vacancy–solute complexes is always larger than that of Mg atoms at finite temperatures. The stability of the Si-rich composition originates from the difference between the Si–Si and Mg–Mg interactions in the vacancy–solute complexes.

Solute-vacancy clustering in aluminum

We present an extensive first-principles database of solute-vacancy, homoatomic, heteroatomic solute-solute, and solute-solute-vacancy binding energies of relevant alloying elements in aluminum. We particularly focus on the systems with major alloying elements in aluminum, i.e., Cu, Mg, and Si. The computed binding energies of solute-vacancy, solute-solute pairs, and solute-solute-vacancy triplets agree with available experiments and theoretical results in literature. We consider physical factors such as solute size and formation energies of intermetallic compounds to correlate with binding energies. Systematic studies of the homoatomic solute-solute-vacancy and heteroatomic (Cu, Mg, or Si)-solute-vacancy complexes reveal the overarching effect of the vacancy in stabilizing solute-solute pairs. The binding energy database presented here elucidates the interaction between solute cluster and vacancy in aluminum, and it is expected to provide insight into the design of advanced Al alloys with tailored properties.

Enhanced nucleation and precipitation hardening in Al–Mg–Si(–Cu) alloys with minor Cd additions

This work reports a novel effect of impurity element Cd on enhancing the precipitation kinetics and increasing the peak hardness of Al–Mg–Si(–Cu) alloys during artificial ageing. It is found that the number density of age hardening Mg–Si(–Cu) precipitates is greatly increased by Cd addition (~0.06 at.%) at both the under-aged and peak-aged stages. A systematic study on the precipitation behaviour by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) shows that most Mg–Si(–Cu) precipitates in the Cd-containing Al–Mg–Si alloys are associated with Cd-rich precipitates and have highly disordered structures. It is also found that the formation of Q'/C-like sub-units in Mg–Si(–Cu) precipitates is significantly promoted by Cd additions. To explore the nucleation mechanism under the influence of Cd addition, atom probe tomography (APT) is applied to study the solute clustering behaviour in the early stages of artificial ageing, and density functional theory (DFT) calculations are used to evaluate the binding energies of different solute-vacancy complexes and therefore the formation kinetics of Mg–Si–Cd clusters.

The interstitial emission mechanism in a vanadium-based alloy

Under nuclear fusion environments, displacement cascades in the potential first protective wall material vanadium (V) and its alloys lead to a large number of point defects. As sink of point defects, grain boundaries are observed to significantly affect the radiation resistance of structured metals. By using potential energy surface searching tools, the interaction between the interstitial-loaded ∑3<110>{111} symmetric tilt grain boundary (STGB) and the point defects in V-based alloy is investigated. For the self-interstitial atoms (SIA)-loaded STGB, the vacancy located within certain distances from the STGB can be effectively annihilated within several nanoseconds via the interstitial emission (IE) mechanism. If an alloy doping interstitial as a Cr atom substitute the SIA, or the vacancy is stabilized by alloy solutes as Ti atoms forming a solute-vacancy complex near the STGB, IE works more effectively with lower activation energy barriers and less thermal activation time. The results indicate that the STGB-induced IE mechanism raises the radiation resistance of the V-based alloy. The results contribute to the micro-structure and constituent optimizations in designing an excellent V-based first protective wall material.

Solute-Vacancy Clustering in Aluminum

We present an extensive first-principles database of solute-vacancy, homoatomic, heteroatomic solute-solute, and solute-solute-vacancy binding energies of relevant alloying elements in aluminum. We particularly focus on the systems with major alloying elements in aluminum, i.e., Cu, Mg, and Si. We consider physical factors such as solute size and formation energies of intermetallic compounds to correlate with binding energies. Systematic studies of the homoatomic solute-solute-vacancy and heteroatomic (Cu, Mg, Si)-solute-vacancy complexes reveal the overarching effect of the vacancy in stabilizing solute-solute pairs. The computed binding energies of the solute-solute-vacancy triplet successfully explain several experimental observations that remained unexplained by the reported pair binding energies in literature. The binding energy database presented here elucidates the interaction between solute cluster and vacancy in aluminum, and it is expected to provide insight into the design of advanced Al alloys with tailored properties.

Self healing of radiation-induced damage in Fe–Au and Fe–Cu alloys: Combining positron annihilation spectroscopy with TEM and ab initio calculations

Self healing of early stage radiation damage by site selective solute segregation is a promising approach to extend the lifetime of nuclear reactor components. In the present study, the creation and autonomous healing of irradiation-induced damage is investigated in pure Fe and high purity Fe–Au and Fe–Cu model alloys. To create radiation damage samples are irradiated at 550 °C by 120 keV He+ ions with fluences of 5.0 × 1015, 1.0 × 1016 and 5.0 × 1016 ions/cm2. The observed increase in the S and W parameters determined in the variable energy positron annihilation spectroscopy measurements indicates the formation of vacancy-like defects, precipitates and vacancy-solute complexes. The presence of substitutionally dissolved Au is found to reduce the formation of radiation defects more efficiently than solute Cu. Site-specific Au precipitation at defect sites is indicated, which results in damage healing with a reduced swelling, whereas Cu precipitates and radiation damage only show weak interaction. Ab initio calculations show that the binding energies of Au solutes to vacancy clusters (Au-Vn) are significantly larger than those of Cu solutes (Cu-Vn) whereas the binding energies of helium filled vacancy clusters Au-HenVn and Cu-HenVn are comparable.

Influence of Sn on the age hardening behavior of Al–Mg–Si alloys at different temperatures

Addition of minute amounts of Sn to Al–Mg–Si alloys is known to have a pronounced effect on their age hardening characteristics. In this study, the influence of Sn addition on the ageing behavior of lean and concentrated alloys at five different temperatures was studied. Hardness, positron annihilation lifetime spectroscopy and transmission electron microscopy measurements complemented by differential scanning calorimetry yield information that allow one to assess the microscopic mechanisms that govern ageing. It is found that Sn slows down the ageing kinetics at 100 °C and 140 °C but accelerates the kinetics and enhances the hardening response at 210 °C and 250 °C. At the standard artificial ageing temperature of 180 °C, the effect of Sn on ageing varies depending on the alloy composition. The observed different ageing kinetics can be explained by the different vacancy behaviors in the presence of Sn. Moreover, the activation energy analysis reveals that the ageing process in Sn-added alloys is controlled by both the separation of Sn-vacancy complexes and the migration of solute-vacancy complexes.

Influence of Sn on the Age Hardening Behavior of Al-Mg-Si Alloys at Different Temperatures

Addition of minute amounts of Sn to Al-Mg-Si alloys is known to have a pronounced effect on their age-hardening characteristics. In this study, the influence of Sn addition on the ageing behavior of lean and concentrated alloys at five different temperatures was studied. Hardness, positron annihilation lifetime spectroscopy and transmission electron microscopy measurements complemented by differential scanning calorimetry yield information that allow one to assess the microscopic mechanisms that govern ageing. It is found that Sn slows down the ageing kinetics at 100 °C and 140 °C but accelerates the kinetics and enhances the hardening response at 210 °C and 250 °C. At the standard artificial ageing temperature of 180 °C, the effect of Sn on ageing varies depending on the alloy composition. The observed different ageing kinetics can be explained by the different vacancy behaviors in the presence of Sn. Moreover, the activation energy analysis reveals that the ageing process in Sn-added alloys is controlled by both the separation of Sn-vacancy complexes and the migration of solute-vacancy complexes.

Muon Spin Relaxation Study of Solute–Vacancy Interactions During Natural Aging of Al-Mg-Si-Cu Alloys

Muon spin relaxation has the unique ability to detect very low concentrations of vacancies and vacancy–solute complexes in solids. In this work, we investigate quaternary Al-Mg-Si-Cu alloys and show that after quenching to room temperature from 848 K (575 °C), vacancies gradually become incorporated into clusters in the Al matrix. The total amount of vacancies in the material increases as small vacancy-rich clusters are formed, which is the opposite of the behavior in Cu-free Al-Mg-Si alloys.

Vacancy effect on the generalized stacking fault energy of alloyed γ-Ni system: A first-principles study

It is of great importance to explore alternative alloying elements to replace rhenium (Re) for the design of low-cost Ni-based single crystal (SC) superalloys. Due to the partitioning of Re to γ matrix, the individual and the combined effects of the single (vacancy, Mo, W and Re) and solute-vacancy complex (Mo-Va, W-Va and Re-Va) defects on the plasticity of γ-Ni have been studied by the first-principles calculations of generalized stacking fault energies (GSFE). It is found that a single vacancy and an alloying solute Mo/W/Re at the fault plane decreases the intrinsic stacking fault energy of Ni from 131 mJ/m2 to 106 mJ/m2 and ∼97 mJ/m2 respectively. Further reductions are made by the introduction of a solute-vacancy complex defect (∼80 mJ/m2). Furthermore, the unstable planar fault energies tuned by the Mo-Va and W-Va complex defects are almost identical, which are both slightly smaller than those by Re-Va. Finally, the calculated values of three twinnability measures elucidate that W and Mo provide similar enhancements in the twinning propensity of γ-Ni to that by Re. This result suggests a possibility of partly replacing Re by Mo or W in Ni-based SC superalloys.