Ca Segregation Research Articles

Achieving high strength-ductility synergy via Guinier-Preston zone formation in a new Mg–Zn–Al–Ca–Mn–Ce sheet alloy

ZAXME11100 (Mg–1Zn–1Al-0.5Ca-0.4Mn-0.2Ce, wt.%) is a recently developed sheet alloy with excellent formability at room temperature. The yield strength of this alloy is significantly improved by a short 1-h aging treatment at 210 °C (T6), from 159 MPa in solution-treated state (T4) to 270 MPa in the T6 condition, with a slight reduction in tensile elongation (from 31 % in T4 to 26 % in T6). In this work, precipitation microstructure was characterized to explain the aging hardening and high strength-ductility synergy of the new alloy. A high density (8 × 1023/m3) of ordered monolayer Guinier-Preston (GP) zones enriched in Al, Ca and Zn uniformly form on the Mg basal planes after T6 treatment. Grain boundary segregation of Al, Ca and Zn is also observed. Deformation microstructure of T4 and T6 samples tested to various strain levels was examined. At small strain levels, basal and non-basal <a> dislocation slip dominates the deformation process, and cross slip of <a> dislocations between basal and non-basal planes is observed in T6 samples. The GP zones are gradually destroyed by plastic deformation. Based on those results, the strengthening contribution of ordered GP zones was modeled to be 116 MPa. Therefore, formation of GP zones is an effective way to achieve high strength-ductility synergy in wrought Mg alloys.

Exploring solute behavior and texture selection in magnesium alloys at the atomistic level

This study advances our understanding of how chemical binding and solute distribution impact grain boundary segregation behavior and subsequent annealing texture modification in lean Mg-X-Zn alloys (X = RE or Ca). Notably, differences in Ca and Gd solute behavior at grain boundaries were revealed, where Ca exhibited stronger binding to vacancy sites than Gd, resulting in elevated Ca segregation and an RD-TD-type texture. The introduction of Zn showed significant synergistic effects on solute clustering, with Gd-Zn pairs forming more favorably than Ca-Zn pairs, leading to a strong synergy between Zn and Gd. This promoted their co-segregation and high concentration at the grain boundary, generating a unique TD-spread texture. In contrast, weaker binding in Ca-Zn pairs did not affect Ca segregation but influenced Zn segregation, which underscores the importance of solute binding behavior in alloy design concepts. Additionally, the combined atomic-scale experiments and ab initio predictions provide strong evidence that selective texture development in Mg alloys is tied to heterogeneous solute-boundary interactions, where the sensitivity of the binding energy to volumetric strain affects solute segregation at grain boundaries, resulting in varying grain boundary mobilities and specific texture component growth. It also emphasizes that solute behavior in clustering and segregation is influenced not only by atomic size but also by chemical binding strength with vacancies or co-added Zn.

Construction and analysis of surface phase diagrams to describe segregation and dissolution behavior of Al and Ca in Mg alloys

Construction and analysis of surface phase diagrams to describe segregation and dissolution behavior of Al and Ca in Mg alloys

Achieving high strength near 400 MPa in an ultra-fine grained Mg–Bi–Ca ternary alloy via low-temperature extrusion

Significantly enhancing the mechanical properties of Mg–Bi base series alloys is of great significance for their wider application prospects. Here, a new Mg–3Bi–1Ca (BX31, wt.%) alloy showing high tensile yield strength of ∼397.1 MPa, high ultimate tensile strength of ∼416.2 MPa and acceptable elongation of ∼8% was fabricated by low-temperature extrusion. The analysis of microstructure showed that the enhanced yield strength was attributed to the ultra-fine grain structure (∼0.84 μm), nano-scale second phases including Mg2Bi2Ca and Mg2Ca, and Ca segregation at grain boundaries. In addition, a weak basal fiber texture to some extent provided more basal slip activities, guaranteeing the suitable ductility. The dynamic recrystallization and dislocation behavior of the BX31 billet during extrusion were also systematically observed and discussed to reveal the formation mechanisms of the ultra-fine grain structure with the weak basal fiber texture in the as-extruded BX31 alloy. As mentioned above, this work offers a new insight to obtain high-performance Mg–Bi base series alloys.

EELS Spectrum Imaging of Ca Segregation at Grain Boundaries in Magnesium Aluminate Spinel.

EELS Spectrum Imaging of Ca Segregation at Grain Boundaries in Magnesium Aluminate Spinel.

Influence of aluminum content on microstructure and performance of Mg-Zn-Ca-Al-Mn magnesium alloys

The influences of Al content on microstructure, mechanical properties, stretch formability and thermal conductivity of Mg-1.5Zn-0.3Ca-xAl-0.2Mn (x: 0.2, 0.5, 1, 1.25, 1.5, 2, wt%) alloys were investigated. With increasing the Al content from 0.2 wt% to 2 wt%, the texture changed from the TD-split texture to the quadrupolar texture at the Al content of 1.25 wt%, and finally became the RD-split texture with an increased texture intensity. The change of texture can be attributed to the formation of Al2Ca phase, which weakens the segregation of Ca at grain boundaries due to the decreased amount of Ca solute atoms in the α-Mg matrix. In addition, Al-Mn precipitates changed from a spherical shape to a rod shape accompanied by a largely increased Al/Mn ratio and a decreased amount of precipitates, when the Al content was increased to be larger than 1 wt%. The 0.2–1.5 wt% Al added alloys exhibited excellent stretch formability with the Erichsen values of 8.4–9.1 mm, which were much larger than that (6.8 mm) of the 2 wt% Al added alloy due to texture effect. The 1 wt% Al added alloy had the largest Erichsen value of 9.1 mm, while the 0.2 wt% and 2 wt% Al added alloys exhibited much higher yield strength (165 and 171 vs. 144 MPa). The thermal conductivity continuously decreased with increasing the Al content. Compared to the 2 wt% Al added alloy, the 0.2 wt% Al added alloy exhibited a much higher thermal conductivity (140 vs. 105 W/(m·K)) benefiting from the low concentration of Al solute atoms in the α-Mg matrix. Consequently, a combination of strong mechanical strength, excellent stretch formability and high thermal conductivity was achieved in the low Al-containing alloys with 0.2–0.5 wt% Al.

Novel Ultrafine-Grain Mg-Gd/Nd-Y-Ca Alloys with an Increased Ignition Temperature.

Two novel ignition-resistant magnesium alloys, Mg-2Gd-2Y-1Ca and Mg-2Nd-1Y-1Ca, were prepared in the ultrafine-grain condition by equal channel angular pressing (ECAP). In addition, four commercial alloys-AZ31, AX41, AE42 and WE43-were prepared similarly as a reference. The microstructure, mechanical properties and ignition temperature were thoroughly investigated. Both novel alloys exhibited a mean grain size of ~1 µm and dense distribution of small secondary phase particles. The mechanical strength measured by the tensile deformation test showed that the novel alloys are much stronger (~290 MPa) than all commercial alloys except WE43. However, Ca segregation into the grain boundaries caused a significant decrease in ductility (<6%). The ignition temperature of the novel alloys (~950 °C) was considerably improved by the presence of Gd/Nd, Y and Ca. This study showed that both novel alloys exhibit high strength and high ignition temperature in the ultrafine-grain condition.

The enhanced Zn and Ca co-segregation and mechanical properties of Mg–Zn–Ce alloy with micro Ca addition

The micro addition of Ca is demonstrated to noticeably increase the strength and ductility of Mg–Zn–Ce alloy, which is closely correlated with the enhanced alloying segregation at grain boundary. Addition of Ca is found to yield the enhanced Zn segregation and co-segregation of Zn, Ce, and Ca. The mechanism behind is that Ca decreases the segregation energy while promoting the diffusion process of Zn, ultimately accelerating the thermodynamic and kinetic process of segregation. The enhanced alloying segregation induced by Ca effectively stabilizes grain boundary, which contributes to the refined grain size, retardant mobility of dislocation and ultimately improved strength. Furthermore, the intergranular fracture energy can be noticeably increased by Ca owing to the promoted alloying element segregation at grain boundary, which contributes to the improved grain boundary cohesion and ductility. The controllable segregation at grain boundary via alloying is intended to contribute to the development of advanced magnesium alloys.

Chemical Mechanical Polishing of MgO Substrate and Its Effect on Fabrication of Atomic Step-Terrace Structures on MgO Surface by Subsequent High-Temperature Annealing

Single-crystalline MgO is used as a substrate for the deposition of various functional thin films. The present study focused on the development of a complete sequence of fabricating atomic step-terrace structures on the MgO substrate via a method that includes grinding, precise mechanical polishing, chemical mechanical polishing (CMP), and high-temperature annealing. The effect of a damage-free surface pretreatment on the subsequent high-temperature annealing was investigated. An atomically smooth and damage-free MgO substrate surface with an average surface roughness of 0.05 nm was obtained via a CMP process using a colloidal silica slurry. Atomic step-terrace structures were formed on the substrate after the high-temperature annealing process at 1000 °C for 20 h under atmospheric air. The obtained step height was 0.20 nm, which corresponds to one-half the unit cell of an MgO crystal (0.21 nm). By contrast, when a mechanically damaged MgO substrate was subjected to the annealing process, Ca segregation was observed on the annealed surface, without the formation of an atomic step-terrace structure. CMP was found to be necessary prior to high-temperature annealing to attain atomic step-terrace structures and to avoid the out-diffusion of impurities in the MgO bulk crystals.

Fly ash waste-derived Fe@Fe3O4 core-shell nanoparticles for acetic acid ketonization

Iron oxide is a cost-effective catalyst for the ketonization of carboxylic acid components of bio-oil crude derived from the pyrolysis of lignocellulosic biomass. Fly ash (FA) waste from coal-fired power plants is an abundant source of iron oxide precursors. Here we report the reductive thermal processing of hematite derived from acid-leached fly ash. Annealing of the hydroxide precipitated leachate produces nanoparticulate α-Fe2O3 containing ∼10 wt% metal impurities (principally Al, Mg, Ca and Ti). Subsequent extended (5 h) reduction under H2 at 400 °C generates a Fe@Fe3O4 core-shell structure, and is accompanied by surface segregation of Al, Mg, Ca and Ti as their oxides, a decrease in particle size, and the concomitant formation of surface oxygen vacancies. These physicochemical changes increase the surface area and introduce Brønsted basicity alongside the intrinsic Lewis acidity of the non-stoichiometric magnetite, promoting ketonization through synergistic activation of two acetic acid molecules. The specific activity of the 5 h reduced FA derived catalyst (FA-5 h) was 3.9 mmol·g−1·min−1 at 400 °C, with a corresponding turnover frequency per acid site of 14.3 min−1. In-situ DRIFTS reveals acetic acid dissociatively adsorbs over FA-5 h, adopting a bidentate acetate binding mode identical to that for commercial magnetite, albeit the fly ash derived catalyst stabilises acetate to a higher temperature (∼300 °C) facilitating C-C coupling versus desorption. Successive doping of commercial magnetite by low concentrations of Al, Mg, Ca and Ti, and subsequent reduction, generated a model catalyst whose acid-base properties and ketonization activity reproduced those of FA-5 h. Surface segregated dopants, notably Ti, are largely responsible for the high activity of FA-5 h through the introduction of additional acid and base sites for acetic acid ketonization.

Proton transfer in La2-xCaxZr2O7-δ pyrochlores: Reasons for limited water uptake and high grain boundary conductivity

Proton-conducting electrolytes based on zirconates with a pyrochlore-like structure are known for their excellent thermal and chemical stability, low grain boundaries (GB) resistance, and low contribution of electronic conductivity. However, the values of proton conductivity seem to be undermined by insufficient incorporation of protons into the structure. The current study aims to identify the causes of these barriers from standpoints of crystal and defect structure, water uptake, and proton transfer. The thermogravimetric analysis (TGA) results show limited water uptake, which is ∼60% of the theoretical value. We show that several oxygen vacancies are not involved in the hydration, due to a significant structural nonequivalence of oxygen sites in the pyrochlore. Additionally, the proton positions themselves are energetically nonequivalent due to the different electrostatic repulsion energy of protons from the nearest cations, which ultimately leads to a weak proton transfer in the bulk of the material. At the same time, we observe the segregation of Ca on the grain boundaries without any impurity formation. This leads to the high apparent GB conductivity according to electrochemical impedance spectroscopy. These findings suggest that the proton conductivity of pyrochlore-like zirconates could be improved by tailoring their GBs instead of perfecting bulk properties.

The role of Ca, Al and Zn on room temperature ductility and grain boundary cohesion of magnesium

It is know from literature that small additions (<1 wt%) of Ca, Al and Zn significantly improve the intrinsic ductility of Mg. The exact role of each element, both qualitatively and quantitatively, and their combined effects, however, are poorly understood. Here we achieved a much clearer view on the quantitative role of each element with respect to ductility improvement and on the collaborative effect, particularly of Ca and Zn in Mg. Some of our findings and conclusions are in disagreement with data and interpretation found in literature. Four different alloys, namely, Mg-0.1 Ca, Mg-0.1 Ca-1 Al, Mg-0.05 Ca-1 Al, Mg-0.1 Ca-2 Al-1 Zn (all are in wt%) were selected for this investigation. All alloys were treated such that approx. similar grain sizes and textures were obtained. This largely excludes the effect of extrinsic factors on ductility. EBSD-guided slip trace analyses reveal that the addition of Ca eases activation of prismatic and pyramidal II slip systems. Using in-situ deformation experiments in SEM and atom probe tomography observations of grain boundaries direct evidence is given for the individual and synergetic effects of Ca and Zn on grain boundary cohesion as an important contribution to improve the ductility of these alloys. We conclude that Ca reduces the slip anisotropy and ameliorates ductility, however, the weak grain boundary cohesion in the Mg-0.1 wt% Ca alloy limits the material's tensile ductility. The addition of Zn alters the Ca segregation at the grain boundaries and helps to retain their cohesive strength, an effect which thus enables higher ductility and strength. The further addition of Al primarily improves the strength. The results show that the balanced influence of reduced slip anisotropy on the one hand and increased grain boundary cohesion on the other hand allow to design a high strength high ductility rare-earth free Mg alloy.

Influence of strain-driven segregation in low-angle grain boundaries on critical current density of Y0.9Nd0.1Ba2Cu3O7-d

Low-angle grain boundaries (GBs) constitute the most important current-limiting mechanism in the operation of biaxially textured YBa2Cu3O7−d (YBCO)-coated conductors. Ca doping of YBCO is known to improve the critical current density J c across the GB because of carrier doping by anisovalent Ca2+ substitution for Y3+ and the strain relief induced by Ca segregation at the GB cores; however, the reduction of the superconducting critical temperature T c accompanying such doping is a marked drawback. Here we study the substitution of isovalent Nd3+ for Y3+ again using strain-driven segregation, in this case Nd3+, to improve J c without incurring significant T c reduction. Transport characteristics of low-angle GBs of 10% Nd-doped YBCO, Y0.9Nd0.1Ba2Cu3O7−d, grown on single crystal and 6° and 9° [001] tilt symmetric bicrystal SrTiO3 substrates are reported. It was found that J c across the 6° GB recovers to the intra-grain J c value in the 10% Nd-doped YBCO, while the 9° GB shows a modest J c enhancement compared to the pure YBCO 9° GB without a significant T c reduction. It is shown that the transparency of the GB could be enhanced without a large T c reduction by the isovalent substitution of rare-earth ions, suggesting new opportunities for cation segregation engineering in YBCO by isovalent rare-earth substitution.

Understanding the role of Ca segregation on thermal stability, electrical resistivity and mechanical strength of nanostructured aluminum

Achieving a combination of high mechanical strength and high electrical conductivity in low-weight Al alloys requires a full understanding of the relationships between nanoscaled features and physical properties. Grain boundary strengthening through grain size reduction offers some interesting possibilities but is limited by thermal stability issues. Zener pinning by stable nanoscaled particles or grain boundary segregation are well-known strategies for stabilizing grain boundaries. In this study, the Al–Ca system has been selected to investigate the way segregation affects the combination of mechanical strength and electrical resistivity. For this purpose, an Al–Ca composite material was severely deformed by high-pressure torsion to achieve a nanoscaled structure with a mean grain size of only 25 nm. X-ray diffraction, transmission electron microscopy and atom probe tomography data revealed that the fcc Ca phase was dissolved for large levels of plastic deformation leading mainly to Ca segregations along crystalline defects. The resulting microhardness of about 300 HV is much higher than predictions based on Hall and Petch Law and is attributed to limited grain boundary mediated plasticity due to Ca segregation. The electrical resistivity is also much higher than that expected for nanostructured Al. The main contribution comes from Ca segregations that lead to a fraction of electrons reflected or trapped by grain boundaries twice larger than in pure Al. The two-phase state was investigated by in-situ and ex-situ microscopy after annealing at 200 °C for 30 min, where precipitation of nanoscaled Al4Ca particles occurred and the mean grain size reached 35 nm. Annealing also significantly decreased electrical resistivity, but it remained much higher than that of nanostructured pure Al, due to Al/Al4Ca interfaces that reflect or trap more than 85% of electrons.

Synergistic refining mechanism of Mg-3%Al alloy refining by carbon inoculation combining with Ca addition

Abstract Mg-3%Al alloy was refined by carbon inoculation combining with 0.2%Ca addition. High grain refining efficiency was obtained and the synergistic refining mechanism was deeply discussed in the present study. Al–C–O particles, actually Al4C3 particles, were formed in the carbon-inoculated Mg-3%Al alloy acting as nuclei for α-Mg grains. Ca addition had no obvious effect on size distribution of the nucleating particles. Ca segregation was proved on Al4C3 particles, which should reduce the interfacial energy of nuclei/Mg. The constitutional undercooling in front of nucleus/liquid was increased from 0.12 °C to 0.15 °C induced by 0.2%Ca addition. The synergistic grain refining efficiency can be attributed to the higher constitutional undercooling and lower the interface energy of nucleus/Mg induced by Ca addition. More nucleating particles with small size could be activated acting as potent nuclei of α-Mg grains. Consequently, Mg-3%Al alloy could be effectively refined due to the synergistic effect induced by carbon inoculation combining with Ca.

Study on Ca Segregation toward an Epitaxial Interface between Bismuth Ferrite and Strontium Titanate.

Segregation is a crucial phenomenon, which has to be considered in functional material design. Segregation processes in perovskite oxides have been the subject of ongoing scientific interest, since they can lead to a modification of properties and a loss of functionality. Many studies in oxide thin films have focused on segregation toward the surface using a variety of surface-sensitive analysis techniques. In contrast, here we report a Ca segregation toward an in-plane compressively strained heterostructure interface in a Ca- and Mn-codoped bismuth ferrite film. We are using advanced transmission electron microscopy techniques, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. Ca segregation is found to trigger atomic and electronic structure changes at the interface. This includes the reduction of the interface strain according to the Ca concentration gradient, interplanar spacing variations, and oxygen vacancies at the interface. The experimental results are supported by DFT calculations, which explore two segregation scenarios, i.e., one without oxygen vacancies and Fe oxidation from 3+ to 4+ and one with vacancies for charge compensation. Comparison with electron energy loss spectroscopy (EELS) measurements confirms the second segregation scenario with vacancy formation. The findings contribute to the understanding of segregation and indicate promising effects of a Ca-rich buffer layer in this heterostructure system.

Interphase boundary segregation induced phase transformation in a high-pressure die casting Mg-Al-La-Ca-Mn alloy

The well-known Al11La3 structure in a die casting Mg-4Al-2La-2Ca-0.3Mn alloy was decomposed when heat treated at 400 °C. Transmission electron microscopy characterizations reveal the decomposition process as: Ca firstly segregated at Mg-Al11La3 interphase, then Al and Ca simultaneously segregated at the interphase boundaries and the radial Al2Ca phase formed, finally the Al11La3 phase gradually decomposed into Al2(Ca,La) structure. First-principles calculations indicate that Ca atoms substituting for La atoms would decrease the stability of Al11La3 and result in that Al11La3 become metastable even at 0 K. Therefore, the decomposition of Al11La3 is attributed to that the interphase boundary segregation of Ca and the diffusion of Ca into Al11La3 change its thermodynamic stability.

Effect of Ca addition on microstructures of aged Mg-15Gd-0.5Zr alloy

Effect of Ca addition on the microstructures of Mg-15Gd-0.5Zr (wt%) alloy during isothermal ageing at 250 °C have been investigated using high-angle annular dark-field scanning transmission electron microscopy and X-ray mapping. The results show that precipitation in the Ca-containing alloy involves the formation of ordered solute clusters, G.P. zones, β′, β1, β, Mg2Ca, and a novel ternary phase containing Mg, Gd and Ca. The novel precipitate phase has a plate morphology with its habit plane parallel to the basal plane of the matrix. It has a hexagonal structure (with lattice parameters a = 0.59 nm and c = 1.01 nm) and an orientation relationship with the matrix phase: (0001)P//(0001)α, 1¯21¯0P//011¯0α. Ca segregation inside β1 and β phases is observed and interpreted based on density functional theory calculations.

Effects of Calcium on Strength and Microstructural Evolution of Extruded Alloys Based on Mg-3Al-1Zn-0.3Mn

A family of alloys based on the Mg-Al-Zn-Ca-Mn system (Mg-3Al-1Zn-1Ca-0.3Mn, Mg-3Al-1.5Zn-0.5Ca-0.3Mn, and Mg-3Al-1Ca-0.3Mn, wt pct) was developed for extrusion. Their mechanical properties were evaluated by tensile testing at room temperature, and compared to those of the benchmark Mg-alloy Mg-3Al-1Zn-0.3Mn (AZ31). The microstructures of the extruded alloys were characterized in detail in order to reveal the effect of Ca on microstructural evolution, and consequently the alloy strength. The addition of Ca to the AZ31 stifles dynamic recrystallization and grain growth, with only ~30 pct recrystallization and a recrystallized grain size of ~480 nm. In contrast, the benchmark alloy is essentially completely recrystallized with an average grain size of ~2.3 μm. A high density of low-angle grain boundaries (LAGBs) and dislocations were observed in Ca-containing alloys, and were identified as a major factor in the observed strengthening. Such LAGBs form cellular subgrains predominantly along initial grain boundaries, or newly formed boundaries that are closely spaced (~ 600 nm) and nearly parallel to the extrusion direction. The subgrains have an ultrafine size of 100 to 400 nm, and difficult to convert to recrystallized grains. Solute segregation to grain boundaries was also observed. It is hypothesized that it is the Ca segregation to dislocation cores along LAGBs that decreases the dislocation mobility and stabilizes LAGBs, by thermodynamically decreasing the dislocation energy and/or kinetically imposing a solute drag effect.

Bulk structures of silica-rich calcium aluminosilicate (CAS) glasses along the molar CaO/Al2O3 = 1 join via molecular dynamics (MD) simulation

In order to understand how surface charge accumulation and other reactivity-related phenomena are controlled by the surface structure, it is necessary to have a detailed appreciation of the bulk glass structures. Here, we report a detailed MD study of the bulk structures of silica-rich calcium aluminosilicate (CAS) glasses along the tecto-silicate join.It is confirmed that the structures of these glasses contain small concentrations of over- and under-coordinated defect species, such as NBO, TBO and [5]-coordinated aluminum, AlC5, in contrast to the simple idea that these fully charge-compensated compositions, with Ca/Al2O3 ratios of unity, should have fully polymerized structures. The Si,Al distribution was found to be largely random (i.e. Lowenstein's rule is not obeyed) and no significant segregation of Ca to Al was observed.Interactions between TBO and AlC5 were manifest both in the presence of an additional small peak in the Al-O-Al BAD and in the formation of structural complexes involving small-membered rings, particularly 2-rings. These defect structural complexes contributed to the structural inhomogeneity of these glasses.