Solute Atoms Research Articles

High-throughput study of the effect of indium on the microstructure and properties of copper alloy

In this work, the high throughput sample of Cu-xIn (x = 0–5 wt.%) alloy was prepared by the spark plasma sintering. The effects of In content on the microstructure and properties of Cu alloys were investigated using advanced characterization. The results show that the concentration of In element linearly distributes in Cu alloy after annealing treatment at 450 °C. Most In elements exist in the Cu matrix as the solid solution atoms. Some In elements are precipitated at the grain boundaries as Cu9In4 particles when the In content is higher. The hardness of the alloy increases, while the conductivity decreases with the increase of In content, which is due to the hindrance of the movement of dislocations and electrons by solid solution atoms, grain boundaries, and Cu9In4 particles. The investigation of high throughput samples can provide data support for high strength and high conductivity Cu alloys.

A prediction model for the failure threshold in a CoNiV medium-entropy alloy

Portevin–Le Chatelier effect, associated with dynamic strain aging, is widely found in various alloys, and the maximum serration magnitude from the tensile stress–strain curve can be considered as the point closest to the failure threshold. Due to the difficulty to define solute and solvent atoms in multicomponent alloys, it is a great challenge to explore the serration mechanism. In this study, the atoms that play the role of pinning are defined as solute atoms. Aided by mean-field theory, an in situ pinning model coupled with the twinning effect in a CoNiV alloy with low stacking fault energies is successfully established, which can well predict the failure threshold, i.e., the maximum serration magnitude. The present study paves a new way to correlate the serration dynamics and in situ pinning, and further predicts the failure threshold upon loading for multicomponent high- and medium-entropy alloys.

Ultra-high temperature oxidation resistance of a MoSi2 composite coating with TaB2 diffusion barrier on tantalum alloy

High-temperature elemental inter-diffusion is an important cause of failure for high-temperature antioxidant coatings on tantalum substrate, yet little research has been reported on improving this phenomenon. A new MoSi2 composite coating with TaB2 diffusion barrier was prepared on a tantalum substrate surface by three steps of embedding/slurry sintering/embedding to solve the failure of silicide coating on tantalum substrate due to elemental diffusion under a high-temperature oxidation environment. The composite silicide coating showed excellent oxidation resistance at 1700 ℃, and the protection time was more than 14 h before failure. This may be due to the prevention of diffusion of Si atoms at the initial stage by the TaB2 with many B atoms in a solid solution. The Mo5SiB2 layer with a low diffusion coefficient will be formed with the slow release of B atoms. The diffusion of Si atoms is further hindered by the formation of a new diffusion barrier between the TaB2 phase and the main layer, and thus slowing down the depletion of Si elements in the main coating. In addition, the coating successfully passed 1021 thermal cycles from room temperature to 1700 ℃, which may be because of the alleviation of thermal mismatch between the ceramic coating body and the tantalum substrate by a diffusion layer thus reducing the cracking trend of coating.

Estimation of the reduction potential of transition metal atoms in aqueous solutions

The reduction potentials of the M2+/M0 couples of transition metals Mn, Fe, Co, Ni, Cu, and Zn were calculated by subtracting from their standard electrode potentials in liquid water the standard molar Gibbs energy of formation of atoms in the gas phase ΔfG0(Mgas0) and the Gibbs energy of their hydration in solution ΔfG0(Maq0). The potentials are –2.503 V for Mn0, –2.454 V for Fe0, –2.333 V for Co0, –2.335 V for Ni0, –1.291 V for Cu0, and –1.335 V for Zn0. The disproportionation of ions in the intermediate oxidation state M+ with the formation of the M0 atom is thermodynamically favorable in the solution volume only for zinc.

Effect of B on the microstructure and mechanical properties of Cu–Cu joints with in-situ Ag–Cu–Zn–Sn filler metal

With the development of advanced manufacturing, traditional Ag-based filler metals cannot reconcile the contradiction between high performance and complex processing. It also unable to meet the requirements of new materials, new structures and new processes for brazing connection performance. In this paper, based on the Ag–Cu–Zn–Sn flux-cored filler metal, the effect of the addition of B on the microstructure and mechanical properties of Cu–Cu brazing joints was investigated using FESEM‒EDS, EPMA, EBSD, TEM, microindenter and universal tensile testing machine. The results indicated that the microstructure in the brazing seam was mainly composed of Ag (s.s), Cu(s.s) and (Ag + Cu) eutectic. With increasing B content, the grain size and texture strength of Ag(s) and Cu(s) gradually decreased. The refinement of Ag-rich dendrites was mainly due to the enrichment of B atoms around the dendrite, which limited the diffusion of solute atoms in the melt and the release of solidification latent heat in front of the liquid‒solid interface, further inhibiting the growth of Ag-rich dendrites. The refinement behavior of Cu-rich dendrites was mainly attributed to the increase in B content, which reduced the tip radius of Cu-rich dendrites. The mass transfer path of B → B2O3 and B → liquid CuSn alloy → mixed melt of CuSn alloy + solder skin alloy → liquid filler metal in the brazed joint → Cu rich phase and grain boundary in the brazed joint → Ag rich phase and grain boundary in the brazing seam was clarified. The addition of B can improve the microhardness of the brazing seam. The shear strength reached a maximum value of 237.2 MPa when B was added at 2%. The fracture models mainly included intergranular fracture, dimple fracture and cleavage fracture. The strength of brazed joints can be improved by solid solution strengthening, process hardening strengthening and fine grain strengthening. The tangle and jog of dislocations within the crystal, the presence of a large number of grain boundaries and B-rich areas can prevent crack tip extension and enhance the toughness of brazing joints.

Mechanisms related to the influence of natural aging on the precipitation behavior of the A357 aluminum alloy during artificial aging

This study investigated the influence of natural aging (NA) on the precipitation behavior of the main precipitate β″ and hardness evolution during the subsequent artificial aging (AA) process of the precipitation strengthened A357 aluminum alloy. The corresponding mechanisms were analyzed. The results demonstrate that natural aging has adverse influence on the precipitation process during artificial aging. The underlying mechanism is that small GP zones are formed during natural aging but cannot become nucleation sites for the β″ phase during artificial aging, which spontaneously consumes the supersaturated solute atoms in the Al matrix, increases the nucleation barrier of the β″ phase, and also consumes the vacancies produced after solid solution and quenching, thereby reducing the nucleation rate of the β″ phase during artificial aging, and leading to an un-expected increase in the size of the precipitated phase, thus reducing the peak aging hardness of the alloy. This effect becomes more pronounced with increasing natural aging time. Increasing the artificial aging temperature can improve the nucleation rate of the β″ phase during artificial aging, thereby to some extent reducing the unfavorable influence of natural aging on the artificial aging by reducing the sizes and increasing the amount of the β″ precipitates.

In Situ Heating Positron Annihilation Lifetime Spectroscopy Experiments on an Al–Mg Alloy

The binding between vacancies and Mg atoms in an aluminum solid solution is not fully understood but essential for understanding its role in age hardening of many Al alloys. After annealing and quenching, Mg prevents the loss of excess vacancies during natural ageing and forms complexes containing one, possibly two, vacancies, and various Mg atoms. By heating the alloy after natural ageing, these complexes are dissolved, i.e., natural ageing is reverted. This reversion process is studied by in situ positron annihilation lifetime spectroscopy utilizing the very high count rate at the accelerator driven facility ELBE. Positron spectra are continuously acquired during heating at rates between 3 and 50 K min−1. After correcting for the contributions of the oxidized surface and decomposing spectra into components, the process can be followed in detail and is found to take place in distinct stages: first, the number of vacancy–Mg complexes is reduced and then the liberated vacancies agglomerate into clusters that eventually dissolve at even higher temperatures.

Solute atom mediated crystallization of amorphous alloys

We propose a new mechanism of crystallization of amorphous alloy which is mediated by additional solute atoms produced by electronic excitation. For a freestanding Pd-19at%Si amorphous alloy film, electron irradiation causes subtle structural change. By contrast, extensive crystallization of Pd-19at%Si amorphous alloy sandwiched by amorphous (a-) SiOx films, i.e., a-SiOx/a-Pd-Si/a-SiOx, occurred by 75 keV and 200 keV electron irradiation. This crystallization was induced by irradiation not only at room temperature but also even at 90 K. It should be emphasized that the crystallization can be realized by 75 keV irradiation at 90 K via the electronic excitation; where both knock-on damage and a possible thermal crystallization can be excluded. A resultant product of the crystallization after electronic excitation (i.e., electron irradiation), hexagonal Pd2Si, differs from that of thermal annealing (orthorhombic Pd3Si). Evidence for decomposition of a-SiOx by electronic excitation contributes essentially to the crystallization of the a-Pd-Si in the composite film has been obtained in this study. Supply of dissociated Si to the a-Pd-Si layer may cause instability of the amorphous phase, which serves as the trigger for the remarkable structural change; i.e., additional solute atom mediated crystallization.

Revealing the influence of solute segregation on the stability and strength of Cu Σ11 [110](113) symmetrical tilt grain boundary via first-principles investigation

The stability and strength of the grain boundary (GB) structure are critical for the practical application of nanocrystals. This study systematically calculates the impact of the segregation of 26 transitional alloy elements on Cu Σ11 [110](113) GB using the first-principle theoretical calculation method. The research findings demonstrate that there is a parabolic connection between atomic number and segregation energy of TM elements in the same period. In addition to Fe, Co, and Ni, all other 23 elements have the potential to segregate to Cu GB, with Zr exhibiting the strongest segregation tendency. The difference in atomic radius and electronegativity between solute atoms and copper atoms strongly affects the segregation tendency, the greater the difference, the greater the tendency for segregation. The segregation energy of solute segregation has strong linear correlation with its GB energy, and the stronger the segregation ability, the more stable the GB. Except for Zn, Ag, Cd, and Au, other 19 alloying elements can increase the strength of GB. Electronic analysis indicates that the strengthening and embrittlement of solutes are primarily related to the accumulation and consumption of electrons near GB and the bonding between atoms.

Strengthening and thermal conductivity behaviors of cast Cu-La-Zn alloys

A new Cu-La-Zn alloy system with high thermal conductivity was developed for die casting, and the thermal conductivity of this alloy system was 200-300 W/(m·K), which was twice as that of ordinary brass. The effects of Cu6La intermetallic compounds and Zn solute atoms on the strengthening and thermal conductivity behaviors were quantitatively studied in as-cast binary Cu-La (2.0-4.5 wt.% La) and ternary Cu-2La-xZn (0-3.0 wt.% Zn) alloys, respectively. The results showed that for the increase of per 1 wt.% La or Zn, the thermal conductivity decreased by about 34 W/(m·K). In Cu-2La-xZn alloys, the lattice constant of α-Cu matrix increased from 3.6163 to 3.6239 Å. Due to the solid solution strengthening of Zn atoms, the hardness of the α-Cu matrix showed a parabolically increasing tendency, from 1.495 to 1.597 GPa. According to the calculations of the Maxwell-Eucken model based on the microstructure, the thermal conductivity of Cu6La phase was determined to be about 35.37 W/(m·K), and the reduction in thermal conductivity of α-Cu matrix caused by Zn solute atoms was quantified to be 51.38 W/(m·K) per 1.0 at.% Zn increase.

Effect of solute atom gradient segregation structure and twin spacing on mechanical properties of nanotwin Cu-Ag alloy

Effect of solute atom gradient segregation structure and twin spacing on mechanical properties of nanotwin Cu-Ag alloy

First-principles study of the mixing effects of rare earth solute atoms on the physical properties of Al2RE phases in Al-based alloys at finite temperatures

The structural stability, mechanical properties, internal nucleation and growth mechanisms, and surface energies of Al2RE (RE = Sc, Y, La-Lu) second phases in Al alloys have been investigated by combining first-principles calculations with the Debye model. The results show that the lattice mismatch of Al2RE precipitates with Al matrix, which determines the strength of materials associated with dislocation slips, are closely related to the transferred electron e t between Al and RE atoms. Furthermore, the methods of cluster expansion (CE) and special quasi-random structures (SQS) have been adopted to calculate the mixing enthalpy H x of Al2RE11−xRE2x (RE1 = La, Ce, Pr, Nd; RE2 is the rest RE atoms) for studying the possibility of replacing in the Al2RE (RE = La, Ce, Pr, Nd) compounds, which is explored in experiments, and it is found that the mixing enthalpy H x increases with the substitute concentration. The calculated elastic constant C ij, bulk modulus B, shear modulus G, Young’s modulus E decrease with increasing atomic number from Sc to Y at first, and then increases to Lu; while the Cauchy pressure C 12-C 44 and pugh’s ratio B/G show the opposite trends. Moreover, the critical nucleation radius R* and coarsening rate K LSW of Al2RE are obtained from the classical nucleation theory (CNT) and LSW model, respectively. The results show that, the R* for all interfaces decreases from Y to La at first, and then increases linearly to Lu; whereas the K LSW of all models increases from Y to Ce firstly, and then changes very little. Finally, the calculated surface energy E sur of all Al2RE compounds is much higher than that of Al, and with the increase of atomic number, E sur of Al2RE intermetallic compounds decreases firstly from Sc to La, and then increases to Lu. The result is helpful for the further optimal designing high performance of Al alloys.

Multi-scale analyses of phase transformation mechanisms and hardness in linear friction welded Ti17(α + β)/Ti17(β) dissimilar titanium alloy joint

The Ti17(α + β)-Ti17(β) dual alloy-dual property blisk produced using Linear Friction Welding (LFW) is considered as high-performance component in advanced aeroengine. However, up to now, microstructure evolution and relationship between microstructure and micro mechanical properties of LFWed Ti17(α + β)/Ti17(β) dissimilar joint have not been thoroughly revealed. In this work, complex analyses of the phase transformation mechanisms of the joint are conducted, and phase transformations in individual zones are correlated to their microhardness and nanohardness. Results reveal that α dissolution occurs under high temperatures encountered during LFW, which reduces microhardness of the joint to that of Ti17(α + β) and Ti17(β). In Thermo-Mechanically Affected Zone of Ti17(α + β) (TMAZ-(α + β)) side joint, a large number of nanocrystalline α phases form with different orientations. This microstructure strengthens significantly by fine grains which balances partial softening effect of α dissolution, and increases nanohardness of α phase and microhardness of TMAZ-(α + β). Superlattice metastable β phase precipitates from metastable β in Weld Zone (WZ) during quick cooling following welding, because of short-range diffusion migration of solute atoms, especially β stabilizing elements Mo and Cr. The precipitation of the superlattice metastable β phase results in precipitation strengthening, which in turn increases nanohardness of metastable β and microhardness in WZ.

Density functional study of atomic arrangements in CrMnFeCoNi high-entropy alloy and their impact on vacancy formation energy and segregation

Using the density functional theory-coupled Monte Carlo approach, we explored the chemical short-range order (SRO) and element segregation in equimolar CrMnFeCoNi alloy. We found that state-of-the-art approximation of random element distribution is only applicable at > 1100 K close to the melting temperature, while the Cr-Cr repulsion driving the system stabilization and accompanying the formation of cubic Cr sublattice, and mild Ni-Ni attraction are the most prominent pair interactions at lower temperatures. Chemical potential and vacancy formation energy calculations indicate that Cr is most sensitive to the local chemical environment, making Cr atoms most stabilized when the preferred SRO is introduced. While the vacancy formation is predicted equally probable among five constituting elements in the random solid solution, Cr and Ni atoms show the lowest vacancy formation energies in the structure with SRO. Furthermore, distinct element segregation was predicted in the vicinity of planar defects, including symmetric tilt grain boundary and stacking fault, which we correlated to the site- and chemistry-dependent atomic volume and bond lengths. It suggests that the local mechanical strain and bond energy induce the SRO development and element segregation: Namely, the system takes advantage of segregation of Ni atoms having large atomic volume or Cr-Cr pairs having elongated bond lengths to fill in the excess volume at defects that relaxes the mechanical strain field and optimizes bond energy distribution. The correlation between the SRO and properties of CrMnFeCoNi alloy needs further investigations, which is expected to greatly help understand and control the properties of high-entropy alloys.

Role of electronic binding energy on the stability of B, C, N, and O in Fe

Using first-principles density functional theory (DFT), we separate the distortion energy (DE) on Fe due to the introduction of solute atoms like B, C, N, and O at different sites from the electronic binding energy (EBE) of a solute atom with Fe. Contrary to the belief that distortion energy alone dictates the preference of solute atoms for a site in bulk, we show that EBE dictates the preference of the O for the octahedral site in Fe, with DE being the highest at the site. The site preference for C and N in bulk Fe is dictated by both DE and EBE. However, DE alone dictates site preference for B. The DE of solute atoms cannot be predicted by calculated radius, which is highest for B and lowest for O. We find that O and B have similar distortion energy due to large charge transfer to the O atom from Fe.

Quantifying the solute-induced additional repulsive force between two partials of pure screw dislocations

The interactions between two partials of a pure screw dislocation are closely associated with numerous deformation mechanisms in face-centered cubic metals. However, the classical method for quantifying the repulsive force between two partials fails for some binary alloys or the newly emerging medium/high entropy alloys due to the presence of additional forces arising from the randomly distributed solute atoms. This paper presents a study on the interaction between two curved partial dislocations, taking into account the effects of solute-induced dislocation perturbations through the use of elastic mechanics and atomistic simulations. The theoretical analysis reveals the existence of an additional repulsive force between two curved partials, which monotonically increases with the average power spectrum of perturbation amplitudes, i.e., Δf∝⟨(Cn)2⟩. Atomistic simulations of CuAg and CuTa alloys further demonstrate that the average power spectrum exhibits a linear dependence on the solute fractions, denoted as ⟨(Cn)2⟩∝χ, which bears resemblance to the relationship between power spectrum and temperature, i.e., ⟨(Cn)2⟩∝T in the case of temperature-induced fluctuations. Therefore, it can be concluded that the additional repulsive force is linearly proportional to the solute fractions in binary alloys. The methodology presented in this study can be easily applied to other face-centered cubic (FCC) alloys as well as multicomponent solute-strengthened alloys.

Atomistic analysis of temperature-dependent dislocation dynamics in Ni3Al-based intermetallic alloys

In this study, we investigate the dynamics of superlattice edge dislocations composed of partial dislocations, complex stacking faults, and antiphase boundaries in binary and ternary Ni-based alloys (γ’ phase) with the L12 structure. In the case of Ni3Al, the dislocation velocity decreases with increasing temperature at low applied shear stress, mainly owing to the phonon drag effect. Meanwhile, in the case of ternary alloys constructed by exchanging Ni atoms with Co atoms in Ni3Al, the effects of temperature on dislocation motion are more complex. At low applied shear stress, the dislocation motion is disturbed by solute Co (i.e., solid-solution strengthening). In this case, the temperature promotes dislocation motion because the potential energy barrier dominates the thermally activated dislocation dynamics. In contrast, when the applied shear stress increases, the temperature suppresses dislocation motion owing to the phonon drag effect. The drag effect also increases with increasing Co concentration. The origin of the energy barrier due to solute atoms at low applied shear stress can be explained by the interaction between the partial dislocation and solute Co. Meanwhile, the origin of the drag effect on the superlattice dislocation dynamics is discussed on the basis of atomic vibrational motions and wavy dislocation behaviors. In this study, we propose and construct a map showing the dominant effects on complex superlattice dislocation dynamics. The map has energy-barrier-dominated, phonon-drag-dominated, and sound velocity-dominated regions, suggesting that the dominant factor changes depending on the applied shear stress, temperature, and strength of the solute–dislocation interaction.

Dynamic competition between coarsening mechanisms of Ti2Cu phase during thermal cycling

The dynamic competition between the coarsening mechanisms activates various coarsening behavior of the precipitate in multi-phase metallic materials, leading to different mechanical performances of alloys. In this work, the morphological evolution and coarsening kinetic of Ti2Cu phase in Ti–14Cu alloy were investigated comprehensively during thermal cycling at 200–600 °C. Our results suggest that the coarsening kinetic showed a gradual transition from the Lifshitz-Slyozov-Wagner (LSW) mechanism to the trans-interface diffusion-controlled (TIDC) mechanism. Ti2Cu phases precipitated at the triple junctions of α-Ti matrix by discontinuous precipitation under the control of LSW mechanism at lower temperatures (200–400 °C), and grew into lamellae along the high-energy grain boundaries. While the low-angle grain boundaries within Ti2Cu phase accelerated solute atom diffusion during the coarsening governed by TIDC mechanism at higher temperatures (400–600 °C), which leads to the boundary splitting of lamellae. In addition, it was also found that the blocky Ti2Cu phase improved the yield strength of the alloy as its high solid solution strengthening. This work provides a clear understanding for the coarsening of Ti2Cu phase, it could provide theoretical evidence for the hot working process design to improve the damage tolerance of Ti–Cu alloys.

Physically Based Constitutive Modeling of Dynamic Strain Aging in C45 Steel

Abstract Dynamic strain aging (DSA) is a sudden increase in the strength of a material under certain combinations of temperatures and strain rates. Despite the phenomenon being reported in several other studies, the literature still lacks a specific constitutive model that can physically interpret its effect. Therefore, this work proposes a modification based on physical parameters to the Voyiadjis and Abed (VA) model to account for the effect of DSA in C45 steel. The resulting modified model is then coupled with an energy-based damage model to further capture the effect of material softening. Previously, in VA model, it was assumed that the total activation energy for overcoming the obstacles without external work remains the same which works well in the absence of DSA. However, during DSA, the mobile dislocations are pinned by the diffusing solute atoms. This results in an increase in the total activation free energy needed by the dislocations to overcome the obstacle. Thus, an increase in strength is observed. It is shown in the current work that utilizing the concept of increased solute concentrations at local obstacles, in conjunction with the physical description that the VA model is based upon, successfully captures the phenomenon of DSA in C45 steel. In addition, the metal experiencing softening after reaching its ultimate strength is due to the significant growth of voids and cracks within the microstructure. To capture this behavior, an energy-based damage parameter is incorporated into the proposed model. The coupled plasticity-damage model shows a good comparison with the experimental results.

Effect of solute atoms segregation on Al grain boundary energy and mechanical properties by first-principles study

First-principles calculations were carried out to study the segregation behavior of Mg, and Cu and their effect on the energy and mechanical properties of different Al grain boundaries (GBs). Four symmetrical tilt GBs were selected for study, namely Σ5[001](210) GB, Σ5[001](310) GB, Σ9[110](221) GB, and Σ11[110](332) GB. The results show that both Mg and Cu have a segregation tendency at the GBs, and the segregation tendency of Cu is stronger than Mg. Mg is prone to form substitutional segregation at the GBs, but Cu is more likely to segregate at the interstitial sites. The segregation of Mg and Cu can reduce GB energy, and the GB energy continues to decrease with the increase of the segregation concentration. First-principles calculation tensile test shows that the segregation of Mg has a negative effect on the strength of GBs, and the GB strength decreases with the increase of the Mg concentration, while the GB strength was gradually enhanced with the increase of the Cu concentration. The strength of Σ5(210) GB and Σ9(221) GB are more sensitive to the segregation of solute atoms than the other two GBs. By calculating the charge density and the density of states of the pristine and the segregated GBs, it was found that the segregation of Mg caused charge depletion and structure expansion at the GBs, while the segregation of Cu increases the charge density of GBs and form new bonds with the surrounding Al atoms. The results provide useful information for improving the mechanical properties of materials by using the concept of GB segregation engineering.