Atoms In Solid Solution Research Articles

On the Influence of Precipitation on the Dynamic Strain Aging in Mg-2%Nd

The Mg-2%Nd alloy, thermally treated at 520°C and quenched, exhibits plastic instabilities at intermediate temperatures between 100°C and 300°C when it is compressively tested because of the dynamic strain aging phenomenon caused by the presence of neodymium atoms in solid solution. During compression tests, there is a competition between the precipitation of Mg-Nd particles and the DSA phenomenon, which depends largely on the test temperature. Thus, below 200°C, the formation of neodymium-rich precipitates is slow, and the interaction is insignificant. Under these conditions, the alloy exhibits a normal behavior of the DSA, i.e., the critical strain decreases with the increase in the test temperature. However, for test temperatures > 200°C, the formation of neodymium-rich precipitates is faster and the evolution of the critical strain with temperature is inverted because of the decrease of neodymium in solid solution.

Segregation of Re at the γ/γ′ boundary of Ni-based single crystal superalloys revealed by first-principles calculations based Monte-Carlo simulations

• An accurate cluster expansion derived Monte-Carlo interatomic potential for the Ni-Al-Re system has been developed to clarify the Re distributions and its influence on the disorder-order transition temperatures of superalloys. • Monte-Carlo simulated annealings of Ni-Al-Re alloys show that most of Re atoms stay in the γ phase, while a few of Re atoms stay in the γ ′ phase. • Addition of Re increases the disorder-order transition temperature of the Ni-Al-Re alloys. • Localized supersaturation of Re at the γ / γ ′ boundary is observed. Nickel-based single crystal superalloys have been widely used in aero-engines and gas turbine engines. To improve the creep resistance, rhenium is often added to the alloys. However, it is not yet fully understood how the added Re elements distribute in the alloys and how the microstructure evolves with the addition of Re. Here, we performed extensive first-principles calculations based Monte-Carlo simulated annealing of Ni-Al-Re ternary alloys with different Re concentrations ranging from 0.5 at.% to 6.0 at.%. The results demonstrate that with the decreasing temperature, most of Re atoms stay in the γ phase, while a few of Re atoms stay in the γ′ phase and tend to occupy the Al positions. At low temperatures, the Re atoms segregate at the γ/γ′ boundary, in good agreement with experiment. We find that the disorder-order transition temperature of the Ni-Al-Re ternary alloys increases with the Re concentration due to the Re-enhanced Al-Al ordering tendency. In addition, we observe that at low temperatures the Re segregation at the γ/γ′ boundary promotes the formation of Ni 4 Re- or Ni 8 Re-like local structures as the Re concentration is over 2 at.%. The formation of a large amount of these local structures consumes the Re atoms in solid solutions, and thus from the solid-solution strengthening point of view, this would have a negative influence on the creep resistance of the superalloys. This work provides important atomistic insights on the Re distribution and its effects on the stability of superalloys.

Oxygen stabilized W−Ag films for environmental antibacterial applications

Oxygen was added into sputter deposited tungsten films to reduce residual stress of the films, and silver with content up to 14.3 at.% was further added for antibacterial functionalization. The films were analyzed by SEM-EDX, XPS and XRD. Only X-ray diffraction peaks from β-W or W3O phase were detected for the films. Oxygen and silver existed mainly as solid solution atoms in the films. Silver addition decreased hardness (still ≥10.3 GPa) and elastic modulus of the films. The potentiodynamic polarization demonstrated lower corrosion current density of the WO film than that of the tungsten film, and silver changed corrosion behavior of the films. The Ag-depleted tungsten oxide layer on the film surface suppressed the release of silver ions in water. The WOAg films had good antibacterial activity with E.coli bacteria. The WOAg films would be useful in environmental and other antimicrobial applications.

Effects of CuSn on the diamond particle holding force of pressureless sintered FeCuCo-based diamond tools

Gas atomized FeCuCo (Cu 28.7 wt% Cu, 12.1 wt% Co) prealloyed powder was used as matrix of diamond tools, and CuSn (15. wt% Sn) prealloyed powder was used as sintering aids. The content of diamond particles was 0.35 wt%. The alloys were prepared through pressureless sintering. Effects of CuSn addition on mechanical properties, microstructure, and holding force of diamond particle were studied. The results show that the FeCuCo alloy is mainly composed of α-Fe phase and Cu-rich phase. Co and Sn are solid solution atoms of α-Fe phase and Cu-rich phase respectively. The fracture morphology of FeCuCo alloy mainly fine dimple and quasi cleavage fracture, as well as sintered pore surface and crack. With the increase of CuSn addition, the density, hardness, bending strength and diamond particle holding force of the alloy increase first and then decrease. The hardness, bending strength, and relative density of FeCuCo alloy with 8 wt% CuSn reach the peak. The bending strength of this alloy added with diamond particles reaches 871 MPa, and the strength loss rate is only 21.7%. There is an obvious gap between the diamond particles and the matrix, indicating that the holding mechanism is mainly mechanical encapsulation. After the addition of CuSn, there are more attachment on the surface of diamond particles, resulting in metallurgical bonding.

History and temperature dependent cyclic crystal plasticity model with material-invariant parameters

Cyclic deformation of metallic materials depends on the interaction of multiple mechanisms across different length scales. Solid solution atoms, vacancies, grain boundaries, and forest dislocations interfere with dislocation glide and increase the macroscopic strength. In single phase metallic materials under cyclic loading, the localization of dislocation densities in sessile substructures explains a significant fraction of the strain hardening. Upon cycling, these dislocation structures evolve across stable configurations, which depend on the strain accumulation.This work advances substructure-sensitive crystal plasticity models capable of quantifying the cyclic hardening history at various temperatures for single phase FCC materials. The framework predicts the cyclic evolution of dislocation substructure based on the activation of cross slip activation for Al, Cu, and Ni single- and poly-crystals up to 0.5 homologous temperature. The increase in cross slip with temperature and deformation induces a transformation in dislocation structures, which predicts secondary hardening without any additional provision. Moreover, the approach relies on material-invariant mesoscale parameters that are specific to dislocation substructures rather than a material system. Hence, we demonstrate that crystal plasticity predictive power can be augmented by parameterizing the model with single crystal experimental data from multiple materials with common substructures. As a result, the crystal plasticity model shares parameter information across materials without the need for additional single crystal experimental data for calibration.

Microstructural characteristics and deformation behaviors of an Al–Mg–Si alloy with improved strength and conductivity processed by continuous casting and expansion extrusion

The industrial applications of Al–Mg–Si alloys conductors still meet the big challenges of strength and conductivity trade-off. In the present work, a special continuous expansion extrusion forming (CEEF) with violent shearing deformation was performed on continuous cast 6101 Al alloy rods to process the sheets with improved mechanical and electrical properties by combining various artificial aging treatments. The results showed that refined dendrites with few intergranular phases and high supersaturation solid solubility could be obtained in the continuous casted rods with fast cooling rate. In the CEEF sheet, the gradient distribution of grain size from the core to the surface of the sheet was observed, where the grain size of processed sheet was refined remarkably by dynamic recovery (DRV) and dynamic recrystallization (DRX). When artificial aging was conducted on the as-extruded, solid-solution and pre-deformed alloy specimens respectively, different precipitation responses were found. The high-density dislocations in the pre-deformed alloy acted as pipes for rapid diffusion of solid solution atoms to accelerate their precipitation, reducing the matrix solid solution atom content and mitigating electron scattering phenomena. As a result, simultaneous improvements of strength and conductivity could be obtained by combining the above optimized strategies.

Comparing the role of Zr and Hf atoms on microstructure and mechanical properties optimization of Ti2AlN reinforced Ti48Al 0.5W composites

In order to further optimize the mechanical properties of TiAl composites, Ti2AlN/Ti48Al0.5W composites with 1 at% Zr/Hf addition were prepared by arc melting, which were termed as Base alloy, Zr-containing alloy and Hf-containing alloy. The grain size, element distribution, phase content and compressive properties were investigated and compared in this work. With 1 at% Zr/Hf addition, the lamellae colony size decreased from 33 µm to 25/23 µm, and Zr atoms almost dissolved into the single γ phase in the form of solid solution atoms, while Hf atoms distributed in both α2/γ lamellae and single γ phase, and part Hf atoms also existed in the form of B2 phase, which illustrated the higher solid solution of Zr atoms in TiAl composites comparing with Hf atoms. The γ phase content increased and the α2 phase content decreased, especially with 1 at% Zr addition, which acted as γ-phase stabilizing element. With 1 at% Hf addition, the compressive strength and strain increased from 2010 to 2221 MPa and from 23.9 % to 26.2 %, respectively, which was attribute to the solid solution strengthening of Hf atoms by substituting Ti atoms, refinement of α2/γ lamellae and increased γ phase content. Compared with 1 at% Hf addition, 1 at% Zr addition led to higher compressive strength and strain, which were 2344 MPa and 26.8 %, respectively, and this result was due to the higher concentration of Zr atoms in γ phase and higher content of γ phase. On account of density, cost and performance, Zr atoms are more suitable as an alloying element to further optimize the microstructure and mechanical properties of TiAl composites.

Additive manufactured Mg-Gd-Y-Zr alloys: Effects of Gd content on microstructure evolution and mechanical properties

Laser directed energy deposition (LDED), as one of the most promising additive manufacturing processes, is an excellent technique for preparing large-scale engineering structural components. Herein, a series of Mg- x Gd-3Y-0.2Zr (wt%, x = 9, 12, 15, and 18) alloys was manufactured by LDED, respectively. The microstructure evolution, hardness, tensile properties and fracture behavior were characterized and the effects of Gd addition were systematically explored. With the increment of Gd content, epitaxial growth at the bottom of melting pool transforms to equiaxed growth. The size of equiaxed dendritic grains shows negligible changes, whereas the intermetallic phase increases to a large extent from 1.51% to 15.92% (volume fraction). The Vickers hardness presents a linear growth against Gd content, and peaks at x = 18 wt% (118 Hv). Furthermore, the variation trend of the yield strength (YS) was rationalized by using the superposition law, according to which solute atoms in supersaturated solid solution (i.e . α-Mg matrix) and the intermetallic phase provided the main contribution. The highest YS (230 MPa) appears in Mg-15Gd-3Y-0.2Zr alloy, in contrast, the ultimate tensile strength (UTS) peaks at x = 12 and 15 wt%, at around 260 MPa. Noticeably, the hardness, YS, UTS, and elongation of Mg- x Gd-3Y-0.2Zr alloys manufactured by LDED are generally superior to those of their as-cast counterparts.

Effect of solute Ni on thermal activation process of plastic deformation in Fe–18Cr single-crystal micropillars

The aim of the present study was to identify the effect of solute Ni element on the plastic deformation and its thermal activation process of 18Cr ferritic stainless steels. Single-crystal micropillars with different diameters of approximately 2 and 5 μm were fabricated on the surface of Fe–18%Cr–Ni alloy sheets containing different contents of Ni (0–2 mass%). Compression tests were performed on the micropillars at different initial strain rates to investigate the strain rate sensitivity (m) of stress for the slip initiation and the specimen-size dependence of the m value. In micropillars with a small diameter of 2 μm, the Fe–18%Cr binary alloy specimen exhibited a relatively higher strain-rate sensitivity of initial slip stress (m = 0.12), whereas m was reduced to 0.01 by adding 2% Ni. The same trend was observed in micropillars with a larger size of approximately 5 μm. The experimental results were utilized to evaluate the activation volume of plastic deformation. The activation volumes of the Fe–18%Cr alloy and alloy containing 1% Ni followed the specimen-size dependence trend of activation volume in bcc metals; however, the alloy containing 2% Ni exhibited larger activation volumes, suggesting a different thermal activation process presumably due to the significant interaction that occurred between dislocations and solute Ni atoms in bcc solid-solutions.

Microalloying-modulated strength-ductility trade-offs in as-cast Al–Mg–Si–Cu alloys

This work presents detailed and quantitative strengthening effects from various kinds of secondary phases in as-cast Al–Mg–Si–Cu alloys for the first time, through changing Ge and Sb content. Three typical kinds of strength-ductility trade-off relations are elaborated by deformation, embrittling and strengthening mechanisms, as reflected by solidification experiments, multi-mechanistic models and first-principles calculations. Microalloying-modulated strength-ductility trade-offs are strongly associated with their special microstructures, where the size and volume fraction of coarse primary phases increase and fine precipitates are modified. As compared to the secondary phases contributing up to 81.9% of the total strength, strengthening effects from solid-solution atoms (6.5–8.7%), grain boundaries (1.8–2.5%) and coarse primary phases are negligible. Therefore, the increased strength of modified alloys is modulated by modifying the fine precipitates, including Mg2Si (β) plates, Al4Cu2Mg8Si7 (Q) needles, Al2Cu (θ) dispersoids, (Ge, Si) rods/particles and nano-sized Si particles. On the other hand, the decreased ductility of modified alloys is determined by the secondary phases, especially the coarse primary phases like β, Q, Mg3Sb2, Mg2(Ge, Si) and (Ge, Si) structures, which also implies that the ductility cannot be predicted by the Peierls’ ductility D derived from simulating α-Al solid solutions by first-principles methods.

Roles of the heat treatment on the Mg-Nd phases and corrosion mechanisms of Mg-4Nd alloy under sulfuric acid type acid rain

Mg alloys are widely used in the fields of aerospace, automotive, and electronic communication equipment. Understanding the corrosion behavior of Mg alloys is critical to prevent their catastrophic failures from the acid rain corrosion. The influences of heat treatment on corrosion behaviors of Mg-4Nd alloy in acid rains were investigated. The corrosion mechanisms of Mg-4Nd alloy are dominated by electrochemical and chemical corrosion, and the corrosion behaviors of Mg-4Nd alloy are affected by the concentrations of acid rain and heat-treated temperatures. Furthermore, Nd2O3 are derived from the oxidation of exposed Nd atoms at the edge of precipitates and solid-solution atoms.

Regulation of primary phase in Cu-Cr-Zr alloy and its effect on nano-structure and properties

In this study, Cu-0.8Cr-0.1Zr alloy was prepared by vacuum horizontal continuous casting method. The effect of different thermo-mechanical treatment processes on the properties was studied, and the microstructure evolution at different stages of thermo-mechanical treatment were observed by SEM, EBSD and (HR)TEM. The results showed that the optimal comprehensive performance of tensile strength (803 MPa) and conductivity (76% IACS) was obtained by three-stage drawing and thermal treatment (primary 43.75% drawing and solution treatment at 950 °C for 60 min + secondary 97.2% drawing and aging at 500 °C for 30 min + tertiary 75% drawing). Through the microstructure characterization, it was found that the morphology and distribution of the Cr-rich primary phase can be regulated by solution treatment, and inherited during the thermo-mechanical treatment. It acts as a nucleation point and hard point, thereby refining the grain and improving the strength of the alloy. Additionally, it reduces the number of solid solution atoms in the matrix and prevents the loss of conductivity. Furthermore, drawing with a large amount of deformation can produce a nano-lamellar structure with high temperature stability, and drawing after high temperature aging can induce substructure formation. The combination of nano-lamellar structure and substructure further improves the strength.

Microstructure and mechanical properties of caliber rolled AlN/AZ91 composite

The multi-pass caliber rolling was applied to in-situ AlN/AZ91 composite rods, and the yield strength of 295.3 MPa, ultimate tensile strength of 384.7 MPa, fracture elongation of 12.3% and a good surface morphology were obtained under the reduction ratio of 78%. The results showed that the grains of the caliber rolled AlN/AZ91 composite rods were remarkably refined due to the occurrence of dynamic recrystallization (DRX), while the in-situ AlN reinforcements induced the nucleation of {101‾2} extension twins, and preferentially promoted the formation of extension twin-induced DRX before discontinuous DRX. Especially, twinning-induced DRX weakened the texture more effectively compared with discontinuous DRX. In addition, it was also found that the increased yield strength was mainly attributed to solid solution atoms, twinning-induced DRX, discontinuous DRX, high density dislocations and nano-sized γ-Mg17Al12 precipitates in the in-situ AlN/AZ91 composite rods, while the high ductility was mainly ascribed to grain size refinement, weakened texture and the activation of non-basal slip systems at room temperature.

High pressure deformation induced precipitation in Al–Zn–Mg–Cu alloy (Al7075)

Precipitate-matrix interactions govern the mechanical behavior of precipitate strengthened Al-based alloys. These alloys find a wide range of applications ranging from aerospace to automobile and naval industries due to their low cost and high strength to weight ratio. Structures made from Al-based alloys undergo complex loading conditions such as impact and shock. Understanding the microstructural changes in Al-alloys under extreme conditions is challenging due to the fast time scale of the experiments. Here, we study the high-pressure behavior of Al7075 alloy separately from the high strain-rate behavior using quasi-static experiment in a diamond anvil cell. We combine in situ X-ray diffraction (XRD) and pre- and post-compression transmission electron microscopy (TEM) imaging to analyze microstructural changes and estimate high pressure strength. XRD showed that non-hydrostatic pressure leads to a significant increase in defect density and peak broadening with pressure cycling. XRD mapping under non-hydrostatic pressure revealed that the region with the highest local pressure had the greatest increase in defect nucleation, whereas the region with the largest local pressure gradient underwent texturing and had larger grains. TEM analysis showed that pressure cycling leads to the nucleation, growth and significant increase in density of precipitates. The significant increase in precipitate density was due to the destabilization of solid solution atoms under high pressure and led to an increase in strength for Al7075 alloy at high pressures.

Microstructure and damping properties of LPSO phase dominant Mg-Ni-Y and Mg-Zn-Ni-Y alloys

This work studied the microstructure, mechanical properties and damping properties of Mg95.34Ni2Y2.66 and Mg95.34Zn1Ni1Y2.66 alloys systematically. The difference in the evolution of the long-period stacked ordered (LPSO) phase in the two alloys during heat treatment was the focus. The morphology of the as-cast Mg95.34Ni2Y2.66 presented a disordered network. After heat treatment at 773 K for 2 hours, the eutectic phase was integrated into the matrix, and the LPSO phase maintained the 18R structure. As Zn partially replaced Ni, the crystal grains became rounded in the cast alloy, and lamellar LPSO phases and more solid solution atoms were contained in the matrix after heat treatment of the Mg95.34Zn1Ni1Y2.66 alloy. Both Zn and the heat treatment had a significant effect on damping. Obvious dislocation internal friction peaks and grain boundary internal friction peaks were found after temperature-dependent damping of the Mg95.34Ni2Y2.66 and Mg95.34Zn1Ni1Y2.66 alloys. After heat treatment, the dislocation peak was significantly increased, especially in the alloy Mg95.34Ni2Y2.66. The annealed Mg95.34Ni2Y2.66 alloy with a rod-shaped LPSO phase exhibited a good damping performance of 0.14 at ε=10−3, which was due to the difference between the second phase and solid solution atom content. These factors also affected the dynamic modulus of the alloy. The results of this study will help in further development of high-damping magnesium alloys.

Molecular dynamics simulations of ultralow hysteretic behavior in super-elastic shape memory alloys

Shape memory alloys (SMAs) that exhibit superelasticity with large recoverable strain and small hysteresis are in demand for practical applications, although their synthesis remains a challenge. We introduce metastable engineering to dope conventional SMA solid-solution atoms of relatively high concentration with "weak" local lattice distortion to realize ultralow hysteretic superelasticity. Large-scale molecular dynamic (MD) simulations of NiTi-based SMAs are performed to demonstrate how the presence of 2 ∼ 4 at.% Nb dopants lead to a stress-induced transition from a metastable pretransitional state to a strain-glass state. This is facilitated by a macroscopically homogeneous and continuous phase transformation in the course of superelastic loading and unloading. This spinodal decomposition-like phase transformation process endows SMAs with anhysteretic superelasticity that is insensitive to loading direction and grain size (below 15 nm). These findings show promise of achieving ultralow hysteretic superelasticity with large recoverable strain for SMAs.

Dynamics of Edge Dislocation in Ti–O Single Crystal Alloys at the Atomic Scale

The addition of oxygen atoms in solid solution to α‐Ti alloy can increase not only its hardness but also its brittleness. Herein, the edge dislocation mobility of Ti–O single crystals at oxygen concentrations of 0–2 at% and temperatures of 100–900 K is investigated by molecular dynamics simulations. The results show that the interstitial oxygen atoms have a strong hindering effect on the a/3 [110] edge dislocations on the {100} prismatic planes, which is attributed to the pinning effect of the vacancy clusters on the dislocation movement due to the interstitial oxygen atoms, thus leading to pronounced dislocation strengthening and hardening. The critical resolved shear stress (CRSS) for dislocation movement increases with increasing oxygen content and decreases with increasing temperature. Moreover, the effect of oxygen concentration on dislocation mobility also diminishes with increasing temperature.

Effect of Mo and Nb on mechanical properties and hydrogen embrittlement of hot-rolled medium-Mn steels

The effects of Mo and Nb on the mechanical properties and hydrogen embrittlement (HE) of hot-rolled medium-Mn steel were investigated. The diffusion behavior of hydrogen was examined using an electrochemical hydrogen permeation test. The mechanical properties and HE were studied using a slow strain rate test. The results show that most Mo and Nb served as solid-solution atoms in the hot-rolled steels, and little carbide content was observed. Both Mo and Nb increased the volume fraction of retained austenite (VA), decreased the starting temperature of martensitic transformation (MS), and retarded the martensite transformation. The effect of Nb was more pronounced. Simultaneous addition of Mo and Nb had a synergistic effect on increasing VA, decreasing MS, and retarding martensite transformation. The elongation of the samples increased with an increase in VA. The strength exhibited the opposite trend. A small amount of Nb (0.04 wt%) significantly improved the HE resistance of steels to a much greater extent than Mo (1 wt%). Filmy retained austenite formed along the primary austenite grain boundaries because of the Nb addition; it inhibited hydrogen-induced cracks, leading to better HE resistance. Blocky retained austenite, formed by the simultaneous addition of Mo and Nb, deteriorated the HE resistance.

Effect of Natural Aging on Precipitation Strengthening Behaviors in Al-Mg-Si Alloy

Natural aging (NA) is unavoidable in Al-Mg-Si alloys during actual manufacturing. Revealing the reasons for the alloy strength reduction caused by the negative NA effect is of great significance to research and practice. In this work, atom probe tomography (APT) and transmission electron microscopy (TEM) were employed simultaneously to study the effect of NA on precipitates and their contributions to the strength of Al-Mg-Si alloys. It was found that the numerous clusters formed after significant NA are unstable and will dissolve into the matrix during initial AA. However, a few stable clusters are undissolved and will result in abnormal growth of β″, which then transforms to β′. Due to the decrease of nucleation sites resulting from the abnormal growth of precipitates, more solute atoms remain in the matrix. The calculation results show that the strengthening of the solid solution atoms is much smaller than that of large-sized β′, which is much less than that of fine β″. Therefore, lengthy NA causes a significant reduction in AA strength of the Al-Mg-Si alloy.

Spin and charge fluctuations in solid solutions of titanium substitution by iron group metals

Topological electronic transitions from omega- to alpha-phase and from alpha- to beta-phase that occur with an increase in the concentration of substitution atoms in solid solutions of titanium with iron group metals are considered. It is found that due to the difference in the potentials of the Hubbard repulsion of titanium and substitution atoms, as well as temperature Bose excitations, spin and charge fluctuations of the electron density increase in the topological alpha phase. Accounting for fluctuations makes it possible to describe the concentration increase in the entropy of the phases of the titanium alloys under consideration at different temperatures. The amplification of electronic fluctuations with increasing temperature and concentration shifts the chemical potential beyond the topological area of the electronic spectrum, thereby suppressing the alpha phase and inducing an electronic transition to the beta phase. Using the example of calculations of the entropy of titanium substitution alloys with iron at different temperatures, a diagram of their structural phases is constructed, which is consistent with the available experimental data. Keywords: solid solutions, topological electronic transitions, spin and charge fluctuations, entropy, structural phases.