MgZn2 Phase Research Articles

Complex internal faults of MgZn2 in Mg[sbnd]Zn binary alloy

Complex internal planar faults in the MgZn2 phase of a binary Mg6Zn alloy were thoroughly studied using transmission electron microscopy. Two staking faults lying on {101¯0}MgZn2 and {213¯0}MgZn2 planes were observed, with the latter one found to be able to be densely arranged in the whole MgZn2 phase, resulting in a new structure with a monoclinic structure, a = 1.71 nm, b = 2.12 nm, c = 4.05 nm, and β = 116°. Subsequently, stacking faults, stacking fault boundary as well as the evolved twin boundaries lead to many additional diffraction spots and even hexagonal diffraction holes in the corresponding SAED patterns.

Evolution of microstructure and hardness during artificial aging of an ultrafine-grained Al-Zn-Mg-Zr alloy processed by high pressure torsion

An ultrafine-grained (UFG) Al-4.8%Zn-1.2%Mg-0.14%Zr (wt%) alloy was processed by high pressure torsion (HPT) technique and then aged at 120 and 170 °C for 2 h. The changes in the microstructure due to this artificial aging were studied by X-ray diffraction and transmission electron microscopy. It was found that the HPT-processed alloy has a small grain size of about 200 nm and a high dislocation density of about 8 × 1014 m−2. The majority of precipitates after HPT are Guinier–Preston (GP) zones with a size of ~ 2 nm, and only a few large particles were formed at the grain boundaries. Annealing at 120 and 170 °C for 2 h resulted in the formation of stable MgZn2 precipitates from a part of the GP zones. It was found that for the higher temperature the fraction of the MgZn2 phase was larger and the dislocation density in the Al matrix was lower. The changes in the precipitates and the dislocation density due to aging were correlated to the hardness evolution. It was found that the majority of hardness reduction during aging was caused by the annihilation of dislocations and some grain growth at 170 °C. The aging effect on the microstructure and the hardness of the HPT-processed specimen was compared to that observed for the UFG sample processed by equal-channel angular pressing. It was revealed that in the HPT sample less secondary phase particles formed in the grain boundaries, and the higher amount of precipitates in the grain interiors resulted in a higher hardness even after aging.

Investigation on microstructure and fracture mechanism of n-SiCp/7075 Al composites

In the present study, the composites have been prepared using vacuum hot pressing process technique. The influence of 2.0 wt.% nanometric SiC particles (n-SiCp) on the microstructure and fracture mechanism of 7075 aluminum alloy was studied by means of metallographic microscope, scanning electron microscope (SEM), energy-dispersive spectrometer (EDS), X-ray diffraction (XRD), laser scanning confocal (LSC). The results indicate that SiCp fill at grain boundaries dispersing evenly and nail to aluminum grains which sizes have been reduced successfully, that making the organization tighter, the gaps fewer and the surface rougher. Meanwhile, phases (MgZn2) precipitated from the 7075 aluminum alloy were been detected. The grain bonding states of matrix alloys are strengthened by the combination of fine grains strengthening and dispersion strengthening and thus affect the fracture morphology. The micro-fracture mechanism of 7075 aluminum alloy is micro-porous aggregation fracture, however, the 7075 aluminum matrix composites (AMCs) are quasi cleavage fracture.

Examination and Analysis of 2 Pct Mg-55 Pct Al-1.6 Pct Si-Zn Coating on Steel

The current study was conducted to examine the microstructure of the 2 pct Mg-55 pct Al-1.6 pct Si-Zn (wt pct) coating on steel in detail and analyse the formation mechanisms of various reaction products in the coating. The coating microstructure was comprised of an α-Al dendrite framework, and an interdendritic network, occupied by a number of solidification products, which were fine Al-Zn mixtures, Mg2Si, Si, MgZn2, and lamellar MgZn2/Zn mixtures. It was found that the Al-Zn mixtures observed in the interdendritic region were the decomposition products of the high Zn α-Al components of various eutectic reactions taking place during solidification. A method was developed to differentiate the Si and Mg2Si phases optically with colour contrasts. It was found that, when Si and Mg2Si formed, they occupied the entire interdendritic spaces at the locations where they formed. Judging from the Al-Mg-Si liquidus projection, the Mg2Si phase should form first during solidification, followed by the simultaneous formation of Mg2Si and Si. On the other hand, the MgZn2 phase formed into two different morphologies, with a large blocky shape and fine precipitates embedded in the lamellar Zn/MgZn2 mixtures, both surrounded by Al-Zn mixtures. It was observed that all eutectic reactions involved the formation of an α-Al component, which formed in a divorced manner via either growing on existing dendrite arms, or sometimes as isolated islands embedded in blocky MgZn2.

Microstructure and mechanical properties of TiC nanoparticle-reinforced Mg−Zn−Ca matrix nanocomposites processed by combining multidirectional forging and extrusion

TiC nanoparticle-reinforced Mg−4Zn−0.5Ca matrix nanocomposites were processed by combining multidirectional forging (MDF) and extrusion (EX). The grain size of the nanocomposite after MDF+EX multi-step deformation was significantly decreased compared with that processed only by MDF. The average size of the recrystallized grains gradually increased after EX with increasing the number of MDF passes at 270 °C. However, the grain size significantly decreased by MDF processing at 310 °C. Both fine and coarse MgZn2 phases appeared in the (MDF+EX)-processed nanocomposites, and their volume fractions gradually increased with increasing the number of MDF passes before EX. Ultrahigh tensile properties (yield strength of ~404 MPa, ultimate tensile strength of ~450.3 MPa and elongation of ~5.2 %) were obtained in the nanocomposite after three MDF passes at 310 °C followed by EX. This was attributed to the refinement of the recrystallized grains, together with the improved Orowan strengthening provided by the precipitated MgZn2 particles that were generated by MDF+EX multi-step deformation.

Atomic-scale investigation of the heterogeneous precipitation in the E (Al18Mg3Cr2) dispersoid of 7075 aluminum alloy

The heterogeneous precipitation of the η (MgZn2) phase on the E (Al18Mg3Cr2) dispersoids of the 7075 aluminum alloy was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectrometry (EDX). It is found that coarse η particles are heterogeneously precipitated at the E particle interface after water quenching and isothermal aging at 120 °C. The incoherent E/Al interface is responsible for the high tendency of heterogeneous precipitation of the η phase. Two different orientation relationships (ORs) between the η, E and Al matrix are identified: OR1 [21¯1¯0]η//[011]E//[1¯12]Al, (011¯0)η//(133¯)E//(201)Al, OR2 [1¯12]E//[0001]η//[011]Al, (011¯0)η//(220)E//(344¯)Al. The η phase is preferential to nucleate along the {111}E or the {220}E planes, depending on its OR. The heterogeneous nucleation of η phase on the E particle could stabilize the E/Al interface by introducing a coherent E/η interface, which increases the drive force of heterogeneous precipitation. The reorientation of η phase and mutual diffusion of solute atoms could assist the coherency of the E/η interface. The present results suggest that increasing the coherency of the E/Al interface is a promising method to suppress the heterogeneous precipitation of the η phase.

Improving the corrosion resistance of MgZn1.2GdxZr0.18 (x = 0, 0.8, 1.4, 2.0) alloys via Gd additions

Effects of Gd addition on microstructure, corrosion behavior and mechanism of cast and extruded MgZn1.2GdxZr0.18 alloys are investigated through microstructure observation, weight loss and electrochemical tests. Increasing Gd from 0 to 2.0 at.%, grains are refined, MgZn2 phase, W-phase and X-phase are formed successively, and basal texture intensity is decreased. The significantly decreased grain size by extrusion and Gd addition induces formation of protective Gd2O3 and MgO layer. The extruded MgZn1.2Gd2.0Zr0.18 alloy shows decreased corrosion rate of 3.72 ± 0.36 mm/year, owing to fine and homogeneous microstructure, dual-role (micro-anode and barrier) of X-phase, compact oxidation layer and basal crystallographic texture.

Response of mechanical properties and corrosion behavior of Al–Zn–Mg alloy treated by aging and annealing: A comparative study

The high strength Al–Zn–Mg plates were processed by combing the hot extrusion and cold rolling. Then, both the artificial aging and annealing were conducted to make a comparative study on the microstructure, mechanical properties, and corrosion resistance. The aged samples were composed of near-equiaxed grains attributed from the static recrystallization during solution process, and owned the main textures of Cube, Goss, Copper, S, and Brass. The precipitate followed the sequence of SSS → GP zone → η′ phase → η phase. With the increase of aging time, the grain boundary precipitations (GBPs) became coarse and discontinuous, and the width of precipitate free zones (PFZs) also increased. In annealed samples, the coarse and elongated grains were finally formed, and the volume fraction of recrystallization textures increased with the extension of annealing time. Moreover, the density of dislocations and the number of MgZn2 phase were both reduced during annealing. The tensile strength along 0° with rolling direction was always the highest in all aged and annealed samples, and the elongation achieved the highest values along 45°. Long-time aging slightly improved the planar anisotropy, while long-time annealing greatly improved the planar anisotropy. The peak aged sample showed a ductility loss of 21.96% during the slow strain rate test in 3.5 wt% NaCl, and a maximum depth of 214.59 μm in intergranular corrosion test. Compared to the annealed sample, the aged one exhibited the worse corrosion resistance due to the wide PFZs and continuously distributed GBPs.

Synthesis and Characterization of Zn-Mg Alloys as Biodegradable Materials

The Zn-Mg alloy is a suitable candidate for the manufacture of biomaterials that can be excessively degraded in the human body without producing a mixture. This study was conducted with the aim to determine variations in the mechanical characteristics of Zn-Mg alloys with Mg ratio of 1%, 3%, 5%, and 7%wt, and to study variations in composition and sintering of the degradation rate of Zn-alloy Mg uses the powder metallurgy method. The synthesis results were characterized by using a defense test and obtained the best value at Zn7% Mg of 117.5 ± 25.37. The presence of MgZn2 and Mg2Zn11 phases that were confirmed by XRD characterization could increase the material hardness. Dynamic degradation test was carried out on samples with the best mechanical properties (Zn7% Mg) with variations in compacting pressure and sintering temperature. The increase in compaction pressure and sintering temperature could reduce the degradation rate of Zn-Mg alloys. The best degradation test was obtained at a pressure of 400 MPa with a sintering temperature of 400°C of 0.70mmpy. The degradation test results were as expected because previous studies stated at the degradation rate (0.40mmpy-1.53mmpy) on statistical testing and the degradation rate (4.9-7.0) mmpy on the change policy supported for bone scaffolding applications. Scanning Electron Microscope (SEM) characterization results showed that samples with compacting pressure and low sintering temperature do not have perfect particle bonding. Samples with high compacting pressure and sintering temperature have good bonding between particles so they do not have a pore composition in the alloy.

Effect of the Zn/Mg Ratio on Microstructures, Mechanical Properties and Corrosion Performances of Al-Zn-Mg Alloys

The effect of the Zn/Mg ratio on microstructures, mechanical properties and corrosion performances of Al-Zn-Mg alloys was studied. Microstructures were characterized using the optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). Tensile tests, intergranular corrosion (IGC) and stress corrosion cracking (SCC) tests were conducted to study the properties. Microstructures results indicated that with the decrease of the Zn/Mg ratio, the recrystallization proportion and the fraction of second phase decreased, while the size of η’ (MgZn2) phases in grain interior also significantly decreased. The number density of η’ phases in grain interior increased and grain boundary precipitates developed discontinuous distribution with the decrease of the Zn/Mg ratio. These microstructures contributed to the significant improvement of the strength and corrosion resistance. The tensile strength and yield strength increased by 34.1% and 47.4%, respectively, with the Zn/Mg ratio decreased from 11.4 to 6.1. Calculating results indicated that the enhancement of strength mainly contributed from the solid-solution strengthening, grain-boundary strengthening and precipitation strengthening. The intergranular corrosion degree was greatly relieved and the stress corrosion sensitivity index decreased from 0.031 to 0.007 with the Zn/Mg ratio decreased from 11.4 to 6.1.

Microstructure and mechanical properties of Mg-Zn-Ca-Zr alloy fabricated by hot extrusion-shearing process

In this study, ultra-fine grain Mg-3Zn-xCa-0.6Zr (x = 0, 0.6, 1.2, 1.8) alloy was obtained by a novel extrusion-shearing (ES) deformation process combining initial forward extrusion and subsequent equal channel angular extrusion (ECAP) shearing. The purpose of this paper is to improve the strength and plasticity of the alloy by ES process. The results of as-cast and homogenized alloy show that the ternary Ca2Mg6Zn3 phase with excellent thermal stability and refined grain size was formed in the alloy by adding Ca. In the ES process, the ternary phase was broken into ultra-fine particles pinned to the dynamic recrystallization (DRX) grain boundary, which introduced strong particle stimulated nucleation (PSN). Moreover, large number of MgZn phases in Ca-containing alloys were dynamically precipitated at DRX grain boundaries and within grains. In addition, in the early stage of the ES process, Ca2Mg6Zn3 phase promoted the accumulation of dislocations at the grain boundaries, and part of the grain boundary was transformed into low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs). After the die corner shearing, the accumulated LAGBs gradually transformed into HAGBs due to continuous dynamic recrystallization (CDRX). Through the novel ES process and the addition of Ca element, the basal plane texture was significantly weakened and the DRX grain was refined. With the increase in Ca content, the strength and toughness of ES alloy increased obviously, and the yield asymmetry decreased gradually. When the content of Ca was 1.2 wt%, ES alloy exhibited the best combination of strength and toughness (UTS = 342 MPa, δ = 20.9%) due to nano-scale Ca2Mg6Zn3 phase, grain refinement and texture weakening. This property is of great significance for the development of magnesium alloys for biological applications.

Microstructure, tensile and fatigue properties of high strength Al 7075 alloy manufactured via twin-roll strip casting

High strength Al 7075 alloy was manufactured via twin-roll strip casting (TRC) process, and its microstructure, tensile and fatigue properties were investigated. Commercial Al 7075 alloy fabricated via direct-chill (DC) casting process was also used for comparison. T6 and T651 heat treatments were implemented for both the materials to precipitate MgZn2(η) phases. The TRC alloy showed globular grain shape with η phases evenly distributed at the grain boundary and matrix interior, while the DC alloy showed elongated grains due to hot forging process and η phases clumped together at grain boundaries. The TRC alloy’s yield strength was 518.5 MPa, tensile strength was 578.2 MPa, and elongation was 6.9%. Comparing both the alloys’ mechanical properties, the TRC alloy’s strength was at least about 40 MPa higher and elongation was about 4% lower than the DC alloy. The TRC alloy showed about 20 MPa higher fatigue (fatigue limit) than the DC alloy. The DC alloy was observed to have coarse cracks on fatigue fractured surface. By contrast, the TRC alloy showed uniform fractured surface without coarse cracks, and the evenly-distributed η phases improved fatigue resistance efficiently. The present study sought to examine the correlation among TRC process-led microstructure, tensile and high-cycle fatigue properties while discussing the strengthening mechanism.

Influence of particulate B4C size on microstructural evolution and mechanical behavior in nanostructured Al matrix during heat treatment

We reported the influence of particle size (micro-B4C and nano-B4C) on microstructural evolution and mechanical properties in nanostructured Al matrix during heat treatment. The results indicated the incorporation of nano-B4C could significantly inhibit the grain growth of Al during heat treatment. The segregation layer in Al/B4C interface increased after heat treatment for all the composites samples. High density of Guinier-Preston (GP) zones and platelet η' could be observed in grain interior of all samples, while some coarse η phases (MgZn2) could be only detected in grain boundaries of AA7075 alloys and the composites with micro-B4C. The yield strength of the composites with the incorporation of nano-B4C could reach up to 958 ± 28 MPa after heat treatment, which could be ascribed to the Hall-Petch strengthening and dislocation strengthening.

Microstructures and Mechanical Properties of an Al-Zn-Mg-Cu Alloy Processed by Two-Step Aging Treatment

In this paper, the mechanical properties and microstructures of an Al-Zn-Mg-Cu alloy subject to two-step aging treatment were systematically investigated. The results indicate that the mechanical strength and tensile ductility of the alloy can be significantly improved by pre-aging treatment at 120 °C for 6 h and then by second-step aging treatment at 165 °C for 12 h. After second-step aging treatment at 165 °C for 12 h, the tensile strength and yield strength of the alloy reach 718 and 654 MPa, increased by ~ 23 and ~ 35%, respectively, compared to the alloy without aging treatment. The increment in the strength is mainly attributed to the formation of ultrafine metastable η′ phase dispersed in the matrix. The fracture mode of the aging treated alloy includes both intergranular fracture and dimpled transgranular fracture. The coarse grain boundary precipitates (MgZn2 phase) and the wide precipitation free zones are beneficial to the tensile ductility of the alloy.

Modified friction stir clinching of 2024-T3 to 6061-T6 aluminium alloy: Effect of dwell time and precipitation-hardening heat treatment

This paper investigates the effect of dwell time and heat treatment on the modified friction stir clinched (MFSC) joint of AA2024-T3/AA6061-T6 Al alloys. The precipitation-hardening heat treatment method involves the combination of solution heat treatment (at 520 °C for 1 h) and aging (at 165 °C for 18 h) processes. The microstructure, failure load, hardness, and fracture behavior of the as-welded and heat-treated MFSC joints were investigated. TEM images show that re-precipitation of strengthening Al2CuMg and MgZn2 phases, dislocation density, and tangles are more pronounced in the heat-treated MFSC joint. A rise in dwell time increases the average grain sizes (1.39–6.65 μm), tensile-shear strength (101–133 MPa), and cross tension strength (59–88 MPa) of the MFSC welded 2024-T3/6061-T6 joints due to an upsurge in the in-process exposure time-induced heat input and inter-material flow. An increase in dwell time beyond 15 s is undesirable. It induces the formation of nugget cracks and micro-voids in the joints and an impaired joint failure load consequently ensues. Heat treatment processing further causes grain coarsening (2.48–9.15 μm) and improves the hardness (at the weld center), tensile-shear (146 MPa), and cross tension (102 MPa) failure strengths of the MFSC joints due to the re-precipitated strengthening phases.

Formation of a Three-Phase Spiral Structure Due to Competitive Growth of a Peritectic Phase with a Metastable Eutectic

Metastable spiral MgZn2/Zn eutectics grow in binary Zn-3 wt.% Mg alloys at small liquid undercooling. In this article, we report growth of a new class of three-phase spiral structures in metastable solidification of Ni-doped Zn-3Mg alloys. Thermodynamic calculations demonstrated that a minor Ni substitution for Zn allows for peritectic growth of the Mg2Zn11 phase following secondary MgZn2 growth. In situ synchrotron x-ray diffraction confirmed such metastable solidification reactions and suggested concomitant growth of MgZn2, Mg2Zn11 and Zn close to the end of solidification. Synchrotron x-ray nano-tomography revealed a localized three-phase spiral structure, wherein Mg2Zn11 grows at the phase-interface of the metastable eutectic. Such a novel spiral structure can be attributed to competitive growth of a metastable binary eutectic and a peritectic phase in metastable solidification. Ni doping increases the nucleation rate of MgZn2 and the driving force for peritectic growth of Mg2Zn11, thus leading to the formation of the three-phase spiral structure.

Correlative characterization of Zn-Al-Mg coatings by electron microscopy and FIB tomography

The microstructure of a hot-dip Zn-Al-Mg anti-corrosion coating on steel was investigated upon quenching during solidification. As expected, at least two different eutectic microstructures were generated and subsequently characterized using scanning and transmission electron microscopy, elemental analysis, and selected area diffraction. The difference was found in the eutectic intermetallic MgZn phase, which was identified as MgZn2 and Mg2Zn11, respectively. Based on the experimental results, we developed an image processing algorithm for electron micrographs, which automatically detects the altered eutectic microstructure. Furthermore, a 3D focused ion beam tomography was performed for an in-depth analysis of the phase morphologies at the interface between the different eutectic structures.

Phase transformations in commercial cold-rolled Al–Zn–Mg–Cu alloys with Sc and Zr addition

The influence of cold rolling on thermal and mechanical properties together with microstructure observation of cast AlZnMgCu(ScZr) alloys has been investigated. Differential scanning calorimetry measurements and microhardness were compared to microstructure that was observed by microscopy (scanning electron and transmission), electron backscatter and X-ray diffractions. Microstructure observation of all studied alloys proved eutectic phase at (sub)grain boundaries. The eutectic phase at grain boundary in the AlZnMgCuScZr has a disordered quasicrystalline structure (known as the T phase or Mg32(Al,Cu,Zn)49). In the AlZnMgCu alloy, the eutectic phase consists of two phases—predominant MgZn2 phase and minor quasicrystalline T phase. During casting and subsequent cooling, multilayer primary Al3(Sc,Zr) particles also precipitated in the alloy with Sc,Zr addition. Solute clusters and/or Guinier–Preston zones were dissolved during the annealing up to ~ 170 °C in the alloys. The highest hardening is caused by particle formation of metastable η′ and stable η phase in AlZnMgCu system observed at ~ 200 °C. Precipitation of the secondary Al3(Sc,Zr) particles is probably the reason of hardening after annealing above 300 °C in the Sc,Zr-containing alloys. Melting of eutectic phases was observed in DSC curves at temperatures ~ 481 and ~ 493 °C in the studied alloys. Activation energies of the Guinier–Preston zones dissolution and/or solute clusters were calculated using Kissinger and Starink method as QA ≈ 100 kJ mol−1 and the formation of the particles of Al–Zn–Mg–Cu system as QB ≈ 150 kJ mol−1. No significant effect on the calculation of activation energy values of thermal processes was observed in deformed alloys. Sc,Zr addition in the alloys stabilizes grains, and there is no recrystallization in the AlZnMgCuScZr alloy at temperature 450 °C/10 h.

Effects of alloying elements on the amounts of MgZn2 and S-Al2CuMg phase in 7075 aluminum alloy

Thermodynamic equilibria of 27 compositions of 7075 aluminum alloy are computed using JMatPro. The maximum amount of MgZn2 phase (MP[M]) and the maximum amount of S-Al2CuMg phase (MP[S]) in each composition are taken as the objective functions. Effects of the contents of Zn, Mg and Cu on MP[M] and MP[S] are studied using analysis of variance (ANOVA) with second-order interactions. Analysis results suggest that Zn, Mg, Cu, Zn[Formula: see text]Mg and Mg[Formula: see text]Cu have a significant influence on the MP[M] with a sequence of Zn[Formula: see text]Mg[Formula: see text]Mg[Formula: see text]Cu[Formula: see text]Cu[Formula: see text]Zn[Formula: see text]Mg, whereas Zn, Mg and Cu have a significant influence on the MP[S] with a sequence of Cu[Formula: see text]Zn[Formula: see text]Mg. Predictive equations for calculating the amounts of MP[M] and MP[S] are obtained using regression analyses. With the requirement of maximum amount of MP[M] and minimum amount of MP[S] within the alloy, the optimized composition of 7075 aluminum alloy is predicted and it contains 6.1%Zn, 2.27%Mg and 1.2%Cu.

Effect of post-weld heat treatment on microstructure and mechanical properties of 7055 aluminum alloy electron beam welded joint

The spray forming 7055 aluminum alloy is welded by electron beam welding (EBW). After welding, the single solution treatments under different temperatures and then aging treatment (STA) and the stepped solution treatment and then aging treatment (SSTA) are carried out for the welded joints respectively. The effect of different post-weld heat treatment (PWHT) procedures on the microstructure and mechanical properties of welded joints is systematically investigated. Results show that, in as-welded condition, the fusion zone mainly consists of α-Al phase, and there are some precipitated phases such as Mg32(Al,Zn)49, Al7Cu2Fe, Al2CuMg, and a small amount of phase MgZn2 in weld metal. After PWHT, the continuous network structures at grain boundary disappear. There are only a few isolated particle phases at grain boundary, and many ellipsoidal η′ phases and rod-like η phases are precipitated within grains. The mechanical property of welded joint is improved after PWHT. Compared with that of in STA condition, the strengthening effect to welded joint is more obvious after SSTA. In SSTA condition, the hardness of weld zone is close to that of base metal (BM), and the tensile strength of welded joint reaches 486.2 MPa, which is 85.6% of that of BM. There are many equiaxed dimples on the tensile fracture surface, and the joint mainly presents the characteristic of ductile fracture.