Zn Segregation Research Articles

Diffusion of elements during reflow ageing of Sn-Zn solder in liquid state on Ni/Cu substrate – theoretical and experimental study

PurposeThis paper aims to study the diffusion of Zn, Ni and Sn in the liquid state during the reflow ageing of the Sn-Zn solder above its melting point on an Ni/Cu substrate in relation to the formation of intermetallic compounds (IMCs).Design/methodology/approachThe Sn-Zn solder is reflowed on Ni/Cu substrates and is aged at 503 K. The formation of IMCs and their composition is characterized using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Diffusion coefficients and diffusion distances of Zn, Ni and Sn in the liquid state during reflow and ageing are theoretically calculated. Both experimental and theoretical behaviours for Ni and Zn diffusions are compared.FindingsCalculations show a linear increment in the liquid-state diffusion coefficients of Ni, Zn and Sn in the solder matrix with a rise in temperature, but they remained constant during ageing. However, diffusion distances increased slowly with temperature but manifold with ageing time. The experimental results revealed segregation of Zn and Ni at the interface in the as-reflow aged specimens. The Zn was concentrated at the solder–substrate interface and it reacted with Ni diffusing from the substrate to form Ni-Sn-Zn IMCs. The rapid diffusion of Zn and Ni with the increase in ageing time increased their atomic concentrations in the IMCs against the reduction in Sn concentration owing to a comparatively slower diffusion.Originality/valueThe novelty of the paper is the detailed study of theoretical diffusion of Zn, Sn and Ni in the liquid state during reflow ageing of Sn-Zn above its melting points on a Ni/Cu substrate. This is compared with values obtained experimentally and related to the mechanisms of IMC formation.

High spatial resolution correlated investigation of Zn segregation to stacking faults in ZnTe/CdSe nanostructures

The correlative use of atom probe tomography (APT) and energy dispersive x-ray spectroscopy in scanning transmission electron microscopy (STEM) allows us to characterize the structure of ZnTe/CdSe superlattices at the nanometre scale. Both techniques reveal the segregation of zinc along [111] stacking faults in CdSe layers, which is interpreted as a manifestation of the Suzuki effect. Quantitative measurements reveal a zinc enrichment around 9 at. % correlated with a depletion of cadmium in the stacking faults. Raw concentration data were corrected so as to account for the limited spatial resolution of both STEM and APT techniques. A simple calculation reveals that the stacking faults are almost saturated in Zn atoms (∼66 at. % of Zn) at the expense of Cd that is depleted.

Heat-treatable Mg-9Al-6Sn-3Zn extrusion alloy

Mg-9Al-6Sn-3Zn (wt%) alloy was extruded and heat treated in T5 and T6 conditions, and its mechanical properties and microstructures were investigated. The extruded product can be slightly strengthened by the T5 treatment as a result of sparse and heterogeneous precipitation. Significant increase in strength is achieved by the T6 treatment, and this is mostly attributed to the formation of lamellar discontinuous Mg17Al12 precipitates. The segregation of Al and Zn at grain boundaries is responsible for the discontinuous Mg17Al12 nucleation. The T6-treated alloy exhibits a tensile yield strength of 341MPa and an ultimate tensile strength of 409MPa, together with an elongation to fracture of 4%.

Thermodynamic stabilization of precipitates through interface segregation: Chemical effects

Precipitation hardening, which relies on a high density of intermetallic precipitates, is a commonly utilized technique for strengthening structural alloys. Structural alloys are commonly strengthened through a high density of small size intermetallic precipitates. At high temperatures, however, the precipitates coarsen to reduce the excess energy of the interface, resulting in a significant reduction in the strengthening provided by the precipitates. In certain ternary alloys, the secondary solute segregates to the interface and results in the formation of a high density of nanosize precipitates that provide enhanced strength and are resistant to coarsening. To understand the chemical effects involved, and to identify such systems, we develop a thermodynamic model using the framework of the regular nanocrystalline solution model. For various global compositions, temperatures and thermodynamic parameters, equilibrium configuration of Mg-Sn-Zn alloy is evaluated by minimizing the Gibbs free energy function with respect to the region-specific (bulk solid-solution, interface and precipitate) concentrations and sizes. The results show that Mg$_2$Sn precipitates can be stabilized to nanoscale sizes through Zn segregation to Mg/Mg$_2$Sn interface, and the precipitates can be stabilized against coarsening at high-temperatures by providing a larger Zn concentration in the system. Together with the inclusion of elastic strain energy effects and the input of computationally informed interface thermodynamic parameters in the future, the model is expected to provide a more realistic prediction of segregation and precipitate stabilization in ternary alloys of structural importance.

Stress corrosion cracking and corrosion fatigue cracking behavior of A7N01P‐T4 aluminum alloy

A modified single edge notch tension (SENT) specimen exposed to saline environment was used to investigate the stress corrosion cracking and corrosion fatigue cracking behavior of A7N01P‐T4 aluminum alloy. The crack propagation rate was determined, the microstructure, crack growth path, and the fracture surfaces of specimens were examined by optical microscopy (OM) and scanning electron microscopy (SEM). The results showed that the stress corrosion cracking rate of A7N01P‐T4 alloy in 3.5% NaCl was three orders lower than that of corrosion fatigue crack propagation rate. Mg and Zn segregation at the grain boundaries increased the stress corrosion crack susceptibility. The grain orientation had a significant effect on the crack propagation performance, and the high angle grain boundaries were susceptible to cracking. Hydrogen embrittlement was the primary cause of stress corrosion crack and corrosion fatigue crack propagation.

The corrosion behaviour of machined AA7150-T651 aluminium alloy

The present work investigates the corrosion behaviour of machined AA7150-T651 aluminium alloy. It is found that machining operation introduces a near-surface deformed layer, characterized by ultrafine grains and segregation of Mg and Zn at grain boundaries, to the alloy. The near-surface deformed layer is electrochemically more active compared with the bulk alloy, therefore, is preferentially corroded during immersion in an 3.5wt.% sodium chloride solution acidified to pH 3.2. The open circuit potential of the machined alloy shows a transient at the beginning of immersion in the solution, due to the preferential dissolution of the near-surface deformed layer.

The effects of Zn segregation and microstructure length scale on the corrosion behavior of a directionally solidified Mg-25 wt.%Zn alloy

Biodegradable implants can be used in order to avoid removal surgery. Mg-Zn alloys are considered interesting alternatives for biomedical applications, however, studies concerning the effects of microstructural features in the as-solidified condition and segregation aspects on the resulting electrochemical behavior are scarce. This investigation is focused on the evaluation of the electrochemical corrosion of an as-solidified Mg-25 wt.% Zn alloy in a 0.15 M NaCl solution at 25 °C. EIS plots, potentiodynamic polarization curves and equivalent circuits are used. It is shown that Zn segregation affects both the galvanic couple and the cathode-to-anode area ratio. It was found that finer and homogeneously distributed Mg-rich six-fold branched equiaxed dendritic grains induce lower corrosion current density and higher polarization resistance when compared with equivalent results of coarser ones.

Numerical simulation of the multicomponent mass transfer during Bridgman growth of CdZnTe crystal using Maxwell-Stefan diffusion model

To reveal the complicated mechanism of the multicomponent mass transfer during the growth of ternary compound semiconductors, a numerical model based on Maxwell-Stefan equations was developed to simulate the Bridgman growth of CdZnTe crystal. The Maxwell-Stefan diffusion coefficients in the melt were estimated. Distributions of Zn, Cd, and Te were calculated with variable ampoule traveling rate and diffusion coefficients. The experimental results show that Zn in melt near the growth interface decreases and diffuses from the bulk melt to the growth interface. For Cd, the situation is just the opposite. The coupling effects of Zn and Cd diffusions result in an uphill diffusion of Te at the beginning of the growth. Throughout the growth, the concentration of Te in the melt keeps low near the growth interface but high far from the growth interface. Increasing the ampoule traveling rate will aggravate the segregation of Zn and Cd, and hence deteriorate the uniformity of Te. We also find that not only the diffusion coefficients but also the ratios between them have significant influence on the species diffusions.

Effects of Zn concentration and heat treatment on the microstructure, mechanical properties and corrosion behavior of as-extruded Mg-Zn alloys produced by powder metallurgy

Magnesium alloys have been regarded as potential degradable biomedical materials due to their suitable mechanical properties and excellent biocompatibility. In this study, Mg-Zn alloys were fabricated by powder metallurgy and then hot extruded. The effects of Zn concentration and heat treatment on their microstructure, mechanical properties and corrosion behavior were investigated. The compression test results showed that their mechanical properties were suitable for bone tissue engineering. The corrosion behavior of Mg-Zn alloys was analyzed by immersion tests and electrochemical tests in Ringer's solution. The results revealed that the increasing Zn concentration reduced the corrosion potential but increased corrosion current because severe micro-galvanic corrosion was caused by more large-size intermetallic phases. Although solid solution treatment could reduce intermetallic phase amounts, it caused micropores on the surface. And these micropores could resulted in the pitting corrosion and reduce the corrosion resistance. Notably, aging treatment could reduce the segregation of Zn and promote the compactness and uniformity of corrosion product layer on the surface, leading to the improvement of corrosion resistance. In addition, the as-extruded Mg-6%Zn alloy (wt.%) aged for 72 h was harmless to L-929 cells in cytotoxicity test. Consequently, the as-extruded Mg-Zn alloys prepared by powder metallurgy are promising to develop to be ideal biodegradable implants for bone tissue engineering.

A journey from bulk brass to nanobrass: A comprehensive study showing structural evolution of various Cu/Zn bimetallic nanophases from the vaporization of brass

This paper presents the structural and morphological analysis of various Cu/Zn based nanoparticles evolved from bulk brass under different experimental conditions. Vaporization of bulk brass in a solvated metal atom dispersion (SMAD) method results in dealloying of brass into Zn and Cu atoms. Subsequent in situ nucleation and growth of dealloyed components yields Cu/Zn@Cu core-shell nanocomposites. The behavior of these nanocomposites under different heating conditions is studied in detail. Solid state annealing generates brass nanoparticles going through several phases of Cu-Zn phase diagram such as η-CuZn5, γ-Cu5Zn8, β-CuZn (observed as martensite) and α-CuZn in sequence. Whereas, a digestive ripening process which involves refluxing of nanocomposites prepared with capping agents imparts spherical morphology and significantly brings down the size and size distribution of the particles. A detailed characterization of refluxed nanocomposites demonstrates certain processes such as diffusion of Zn atoms and segregation of Cu and Zn phases.

Interfacial segregation induced by severe plastic deformation in a Mg–Zn–Y alloy

For the first time, we report here segregation of Zn atoms to twin boundaries formed under 9% compression at room temperature (RT) in a Mg-3Zn-0.5Y (in at%) alloy. The wrought processed (textured) alloy was severely plastically deformed (SPD) through high pressure torsion (HPT) at RT. Zn segregation (2 to 4at%) was observed on newly formed boundaries, such as twin boundaries (formed on compression only in HPT), low angle boundaries formed by deformation and grain boundaries due to recrystallization. First principles calculations performed on low angle boundaries shows that Zn atom prefers to segregate to the dislocations in the boundary.

Structural, textural, surface chemistry and sensing properties of mesoporous Pr, Zn modified SnO2–TiO2 powder composites

Mesoporous Zn and Pr modified SnO2-TiO2 mixed powders (Sn:Ti:Zn:Pr contents 60:20:15:5) have been prepared by a modified sol–gel method involving Tripropylamine (TPA) as chelating agent, TritonX100 as template and Polyvinylpyrrolidone as dispersant and stabilizer, respectively. The obtained gels have been dried at different temperatures and calcined in air at 600 and 800°C, respectively. Phase identification of the synthesized samples and their evolution with the calcination temperature has been performed by X-ray diffraction. N2 adsorption/desorption isotherms were found to be characteristic for mesoporous materials, showing relatively low values for the specific surface area (15–32m2g−1) and nanometric sized pores. In case of the sample calcined at 800°C, a bimodal pore size distribution can be observed, with maxima at 20 and 60nm. SEM results demonstrate a porous nanocrystalline morphology stable up to 800°C. The surface chemistry investigated by XPS reveals the presence of the elements on the surface as well as the oxidation states for the detected elements. At 800°C a diffusion process of Sn from surface to the subsurface/bulk region accompanied by a segregation of Ti and Zn to the surface is noticed, while Pr content is unchanged. The sensing properties of the prepared powders for CO detection have been tested in the range of 250–2000ppm and working temperatures of 227–477°C.

Interphase boundary segregation of Zn in Mg-Sn-Zn alloys

Additions of Zn can significantly improve the age-hardening response of binary Mg-Sn alloys by refining the distribution of Mg2Sn precipitates, but its role in the precipitation is still unclear. In this study, the Mg2Sn precipitates with different morphologies and orientations in a Mg-9.8Sn-1.2Zn (wt.%) alloy are characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and STEM X-ray mapping. It is found that Zn atoms invariably segregate to the Mg2Sn-Mg interface, irrespective of the interfacial structures and orientation relationships of Mg2Sn. This finding provides insight to the understanding of the enhanced nucleation of Mg2Sn precipitates in Mg-Sn-Zn alloys.

Overcoming Zn segregation in CdZnTe with the temperature gradient annealing

The availability of large volume crystals with the same energy gap in melt-grown CdZnTe (CZT) is restricted due to the Zn segregation in CdTe hosts. We observed the migration of Zn in the solid phase along the positive temperature gradient direction both in situ and post-growth temperature gradient annealing (TGA) of CZT. Diffusivity of Zn obtained from the in situ TGA was approximately 10−5cm2/s order and completely different mechanism with that of post-growth. The CZT ingots obtained through in situ TGA have uniform Zn and resistivity of 1010Ωcm orders. The CZT detectors fabricated from in situ TGA applied ingots exhibit 10% of energy resolution for 59.5keV peak of 241Am.

Structural and electronic properties of the heterointerfaces for Cu2ZnSnS4 photovoltaic cells: a density-functional theory study.

The quaternary compound semiconductor Cu2ZnSnS4 (CZTS) is a promising photovoltaic absorber material for thin-film solar cell applications. Density-functional theory calculations have been performed to investigate the structural and electronic properties of the CdS/CZTS heterointerfaces in CZTS-based cells. We find that CdS favors epitaxial growth on the Cu-Zn plane of CZTS along the direction of [100], which can eliminate the effects of the wrong bonds at the interfaces and enhance the energetic barrier for charge carrier recombination across the interfaces with an increased band gap. The band alignment is calculated for the epitaxial CZTS/CdS heterointerface by employing the HSE06 functional and the results show a type-II band alignment with VBO and CBO values of 0.95 eV and -0.05 eV, respectively. Also, the experimental phenomenon of Zn segregation at CdS/CZTS interfaces is corroborated. Zn segregation can enhance the stability of the heterointerfaces, but damage the solar cell performance by decreasing the band gap when the Zn concentration is sufficiently high. We show that besides the defects and undesired phases in CZTS, the heterointerfaces between the absorption layers (CZTS) and the buffer layer (CdS) can also be an important factor that affects the performance of CZTS cells. The present work provides a theoretical base for engineering the heterointerfaces and achieving better performance of CZTS-based solar cells.

Oxidation of α-brass: A photoelectron spectroscopy study

The oxidation of Cu0.7Zn0.3(111) exposed to O2 at low pressure (7.0×10−7mbar) and higher pressure (1mbar) both at 400K has been investigated by photoelectron spectroscopy. The results evidenced a preferential oxidation of Zn at the surface in agreement with the literature and Zn segregation to the surface. Moreover, a systematic fitting procedure of high-resolution spectra enabled us to decompose the Zn 2p core-level peak into metallic and oxide components in order to follow the growth of the oxide. By combining these results with careful analysis of the Zn L3M45M45 Auger transition, we evidenced the formation of a discontinuous ZnO layer (3D ZnO islands) in both pressure conditions. Measurements of the O 1s peak for two photon energies using synchrotron radiation allowed us to identify interface and bulk components. Changes in the Zn 3d level and valence band during the initial steps of oxidation were also followed. We show that not only thermodynamics but also kinetic effects have to be considered to describe this complex oxidation process. The importance of the role played by the structural quality of the sample on the composition, growth kinetic and structure of the surface oxide layer is evidenced.

Ultrafine Grained Structures Resulting from SPD‐Induced Phase Transformation in Al–Zn Alloys

Three different Al–Zn alloys (2, 10, 30 wt% Zn) have been processed by severe plastic deformation (SPD) in the solutionized state to investigate the concomitant mechanisms of grain refinement and phase separation. The data of high resolution analytical transmission electron microscopy clearly reveal the key role of grain boundaries that promote Zn segregation and fast diffusion. Surprisingly, it is also shown that SPD carried out at 150 °C could give rise to a finer multiphase structure comparing to room temperature processing. Our observations seem to indicate that it resulted from a competition between the classical discontinuous precipitation of Zn and SPD‐induced dynamic precipitation.

Origin of the Zn-induced Al intergranular corrosion of the outermost surface layer of the aluminium grain boundary: An ab initio study

Abstract The effects of zinc segregation on intergranular corrosion of the outermost surface layer of the AlΣ5 (0 1 2)/[1 0 0] grain boundary (GB) in an aluminium alloy have been investigated based on first-principles density functional calculations. It is demonstrated that the outermost surface of an Al GB with Zn atom segregation is less stable than that of the pure Al GB, and the first or second nearest-neighbour Al atom of a segregated Zn atom is more prone to corrosion dissolution than those in other sites. The electronic structure analysis shows that a charge reduction region forms along the grain boundary, which causes a more negative electrode potential shift than the pure Al GB. The Zn–Al bond has a metallic character that is similar to the Al–Al bond regardless of the degree of Zn segregation. It is confirmed that the corrosion of the outermost surface of the Al grain boundary with Zn segregation is enhanced because of the charge depletion among Al atoms and Zn atoms. This work contributes to our understanding of the comprehensive effects of Zn segregation on stress corrosion cracking of aluminium alloys.

Macroscopic Segregation and Stress Corrosion Cracking in 7xxx Series Aluminum Alloy Arc Welds

Arc welds of Al-Zn-Mg alloy with Al-Mg filler wire have shown a preferential macroscopic segregation of Mg and Zn to the weld toes. Islands of large precipitates, which are observed in those solute-enriched weld toes, are identified as T phase (Mg32(Al,Zn)49) using diffraction pattern analysis. The location of T precipitates consistently coincides with the initiation site for stress corrosion cracking. Therefore, it is hypothesized that they induce the crack initiation due to preferential dissolution.

Effect of Shoulder Surface Dimension and Geometries on FSW of AA7039

AbstractIn the present study, friction stir welding of AA7039 butt plates were investigated with respect to threaded probed tools having different shoulder diameters and shoulder surface geometries. The ultimate tensile properties, micro hardness and grain size of the welds were investigated with respect to the varying tool geometries. The mechanical properties of the welded joints were observed to be influenced by the shoulder diameter and shoulder surface geometry. With the increase in the shoulder diameter, the effect of shoulder concavity on refining the weld grain size diminished. Good quality welds were produced by using tools with semi-concave shoulder surface having shoulder and probe diameters of 19 mm and 7 mm respectively. The fracture surfaces of AA7039 weld samples were dimpled and the crack front energy-dispersive X-ray spectroscopy analysis exhibited absence of segregation of Zn.