Prediction and characterization of growth temperatures in Al–Zn–Mg alloys

Growth temperatures of α-Al, intermetallic τ and eutectic α + τ phases in Al-12 wt.% Zn 6 wt.% Mg alloy has been determined as a function of growth velocity in the range of 3 × 10^? 5 to 1 × 10^? 3 m/s at a temperature gradient of 2500 K/m, using a directional solidification technique. The experimental results are found to be in good agreement with predictions of growth temperatures of competing constituents for multicomponent systems.     

Effect of precipitate free zones on low cycle fatigue life of Al–Zn–Mg alloy

Abstract

The effect of precipitate free zones (PFZs) on the low cycle fatigue (LCF) life of an Al–4·70 wt-%Zn-2·80 wt-%Mg alloy at room temprature was investigated. The alloy used in the present study was uniquely prepared to have different widths of PFZ, whlle keeping the distribution and size of predominant precipitates in the matrix or on grain boundaries almost constant. As the width of the PFZ increased from 0·073 to 0·3 μm,the LCF life was observed to increase to some extent. However, the tensile properties such as yield strength, tensile strength, and elongation remained approximately unchanged. Studies using transmission electron mlcroscopy show that the extent of planar slip decreases with increasing PFZ width throughout cyclic deformation. Thls result suggests that the increase in LCF life is probably due to relaxation of the local stress in the soft PFZ regions.

MST/2093     

Structure of metastable precipitate in some Al–Cu–Mg–Ag alloys

Abstract

The high strength of some Al–Cu–Mg–Ag alloys has been attributed to very thin (~2·5 nm), but broad, hexagonal-shaped precipitates. Previous work has shown that the precipitates have a hexagonal unit cell, but different lattice parameters have been reported. In the present paper, the intensities of X-ray diffraction reflections from the precipitates have been measured on Buerger precession photographs, and it is shown that the crystal structure is monoclinic (space group P2/m) with the parameters a = b = 0·496 nm, c = 0·848 nm, γ = 120°. The special values of these parameters confer a hexagonal symmetry on the lattice. This unusual structure is a slightly distorted form of θ-CuAl₂, to which it appears to change after long aging times at 200°C.     

Nucleation and growth of precipitates in Al–Cu–Mg–Ag alloys

Abstract

Adding small amounts (<1wt-%) of both magnesium and silver to an aluminium alloy containing about 4 wt-% Cu causes precipitates with a hexagonal structure (Ω-phase) to form on {111} planes of the aluminium lattice. Precipitation of θ′ on {100} planes may also occur, the relative proportions of the two types of precipitate being dependent on the levels of magnesium and silver, e.g. ~0·7 wt-% of each element almost entirely suppresses θ′ formation. Even when θ′ does form in parallel with Ω-phase, on prolonged aging it tends to dissolve in favour of Ω growth. Using an X-ray technique to establish foil thickness, the relative amounts of Ω and θ′ precipitate have been measured as a function of aging time, analysis of the data showing that growth is diffusion controlled with an activation energy of 136± 15 kJ mol^?1.

MST/648     

Microstructural characterization of Mg–Al–Sr alloys

The microstructural details of fourteen Mg–Al–Sr alloys were investigated in the as-cast form by a combination of scanning electron microscopy/energy dispersive spectrometer (SEM/EDS) analysis and quantitative electron probe microanalysis (EPMA). The heat transfer method coupled with the DSC measurement has been utilized to determine the solidification curves of the alloys. The morphology and the chemical composition of the phases were characterized. The microstructure of the alloys is primarily dominated by (Mg) and (Al₄Sr). In the present investigation, ternary solid solubility of three binary compounds extended into the ternary system has been reported and denoted as: (Al₄Sr), (Mg₁₇Sr₂) and (Mg₃₈Sr₉). The (Al₄Sr) phase is a substitutional solid solution represented by Mg_xAl_4–xSr and has a plate-like structure. The maximum solubility of Al in Mg₁₇Sr₂ was found to be 21.3 at%. It was also observed that Mg₃₈Sr₉ dissolved 12.5 at% Al.     

Constitutive analysis of Mg–Al–Zn magnesium alloys during hot deformation

The constitutive behaviors of Mg–Al–Zn magnesium alloys during hot deformation were studied over a wide range of Zener–Hollomon parameters by consideration of physically-based material’s parameters. It was demonstrated that the theoretical exponent of 5 and the lattice self-diffusion activation energy of magnesium (135 kJ/mol) can be used in the hyperbolic sine law to describe the flow stress of AZ31, AZ61, AZ80, and AZ91 alloys. The apparent hyperbolic sine exponents of 5.18, 5.06, 5.17, and 5.12, respectively for the AZ31, AZ61, AZ80, and AZ91 alloys by consideration of deformation activation energy of 135 kJ/mol were consistent with the considered theoretical exponent of 5. The influence of Al upon the hot flow stress of Mg–Al–Zn alloys was characterized by the proposed approach, which can be considered as a versatile tool in comparative hot working and alloy development studies. It was also shown that while the consideration of the apparent material’s parameters may result in a better fit to experimental data, but the possibility of elucidating the effects of alloying elements on the hot working behavior based on the constitutive equations will be lost.     

Thermodynamic analysis of processability of Mg–Al–Zn–Mn alloys for rheocasting

A two-stage thermodynamic calculation procedure was developed to analyse alloy compositions for rheocasting. Based on Kazakov's criteria, the processability of AZ91, AZ61 and AM60B for rheocasting was investigated. The results show that these alloys do not satisfy selected criteria fully, and only AZ91 is possible to be used for rheocasting with semisolid slurry having lower fraction solid (0.2–0.4). In microstructure produced by rheocasting, the β-Mg₁₇Al₁₂ phase forms continuous network along primary α-Mg gain boundary and is concentrated in this limited contiguous space. The routes to modify and optimise alloy composition were proposed.     

Structural and hardness inhomogeneities in Mg–Al–Zn alloys processed by high-pressure torsion

Experiments were conducted to evaluate the evolution of structure and hardness in processing by high-pressure torsion (HPT) of the magnesium AZ91 and AZ31 alloys. Both alloys were processed by HPT at room temperature for 1/4, 1, and 5 turns using a rotation speed of 1 rpm. Structure observations and microhardness measurements were undertaken on vertical cross-sectional planes cut through the HPT disks. The results demonstrate that the deformation is heterogeneous across the vertical cross sections but with a gradual evolution toward homogeneity with increasing numbers of revolutions.     

Fly ash characterization and application in Al–based Mg alloys

Continuing study in metallurgical field calls for growing reinforcements of which fly ash plays an important role. In this study, Al alloys were reinforced with different solid fly ash particles. The X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray fluorescence (XRF) analyses were used to identify the fly ash particles, and they were also applied to the composite alloys. The X-ray diffraction (XRD) results indicated that the crystalline phase of the fly ash was an effective reinforced phase. Meanwhile, the SEM and optical micrographs of the composite samples indicated that fly ash could be reacted or settled in the matrix of the aluminium. The physical, tribological and microhardness analyses were also used to study the Al–fly ash composites. The best wear resistance corresponding to the lowest loss was obtained in the samples with as-received fly ash which were mostly in accordance with the results in the samples containing treated fly ash. Meanwhile, the proportion of the wear results to the hardness of the samples was observed. Finally, the light weight Al alloys was realized, and increasing the strength is a likeness.     

Complete mechanical characterization of nanocrystalline Al–Mg alloy using nanoindentation

In an effort to explore alternate means of mechanical characterization of small material volumes, a nanocrystalline Al–Mg alloy synthesized via cryomilling and consolidated by cold isostatic pressing with subsequent extrusion was subjected to nanoindentation testing. The data collected from these tests was subjected to two different data analysis techniques (one proposed by Dao et al. (2001) and one proposed by Ogasawara et al. (2006)) in an effort to investigate the capabilities of such techniques in full, accurate elastoplastic characterization. A commercially available, coarse-grained sample of this same Al–Mg alloy was also tested to investigate these models’ capabilities of distinguishing between the two types of material. Nanoindentation, as expected, proved to accurately predict the elastic modulus of a tested material. Also, these methods provided evidence that through determination of strain-hardening exponent and yield stress, they could reasonably estimate the plastic properties of a tested material. Both models seemed to slightly overestimate the strength of the nanocrystalline material (according to previously reported values for similar material). In terms of the coarse-grained material, Ogasawara’s model appeared to overestimate the strength while Dao’s model provided estimations closer to values reported in literature. Finite element analysis was used as a verification mechanism for the property values extracted from the nanocrystalline material, and initial results show signs of good accuracy of characterization.     

Microstructural characterization and quantification of Zn–Al–Mg surface coatings

The microstructure of Zn–Al–Mg coatings on steel sheets is characterized with the energy dispersive X-ray technique. Optimum parameters on the scanning electron microscope with a field emission gun have been found in order to laterally resolve the fine structure of the individual phases. A quantification of the microstructure is done with a mean shift algorithm that is usually applied for image segmentation in pattern analysis. This nonparametric technique is here based on the chemical composition and the spatial domain. The measured area is partitioned by a quantitative feature space analysis into phases with similar chemistry. A backscattered electron image is compared with the results of the X-ray point map of the same area. As an application the influence of the chemical composition of the melt on the resulting microstructure is compared for two different alloys.     

Geometrical compatibility factor analysis of paired extension twins in extruded Mg–3Al–1Zn alloys

Recently, geometrical compatibility factor (m′) has been used to characterize local strain compatibility between paired twins formed in rolled Mg alloys with basal texture. High m′ and m′ rank were present in most cases, suggesting that m′ plays an important role in twin pair formation. The present study aims to extend the understanding of the effects of m′ and grain boundary (GB) misorientation angle on twin pair formation in an extruded Mg alloy with basal fiber texture. 106 sets of paired twins were extracted from electron backscatter diffraction (EBSD) data and were analyzed. The results show that m′ of paired twins distributes in a wide range (− 0.5 to 1) in extruded Mg alloys due to the presence of a large fraction of high angle GBs. About 60% of twin pairs have the first m′ rank, implying that m′ is still an important factor for variant selection of paired twins in extruded Mg alloys. The distribution of m′ with SF rank was also analyzed, showing that more than half of twin pairs are located in the left-top corner with m′ > 0.6 and SF either rank one or two, which confirms that simultaneously high SF and high m′ is favorable for twin pair formation in extruded Mg alloys.     

Microstructure and mechanical property of high strength Mg–Sn–Zn–Al alloys

This paper aims to reveal the microstructure and mechanical properties of as-cast and hot-rolled Mg–Sn–Zn–Al based alloys. Three alloys, Mg–5Sn–2Zn (TZ52), Mg–5Sn–2Zn–2Al (TAZ522) and Mg–5Sn–5Al–1Zn–0.2Mn (TAZM5510) alloys were studied. The results revealed that the as-cast alloys showed fine dendritic structures. The TAZM5510 alloys exhibited moderate yield strength of 98?MPa with good elongation of ～15%, which was comparable to several commercially used Mg die-castings. Mechanical properties were significantly improved after multi-pass rolling. The TZ52 sheet showed a high yield strength of 277?MPa with excellent ductility exceeding 30%, and the TAZM5510 sheet exhibited the highest tensile strength of 386?MPa while keeping desirable elongation of 16.6%. These sheets are termed as strong and ductile Mg–Sn–Zn–Al wrought alloys.     

Interaction of Zn and Zn–4Al,Zn–15Al (wt-%) solder alloys with aluminium

The interaction of pure zinc and Zn–4Al, Zn–15Al (wt-%) solder alloys with aluminium has been investigated. Two different experimental techniques were used: (i) the aluminium samples soldering and (ii) the holding of liquid solder alloys inside the hollow in an aluminium base. The aluminium content was determined in samples after contact with aluminium at various temperatures and various holding times. It is shown that in soldered seams and solder alloys that had been held inside the aluminium base, the content of the aluminium was higher than at the points on the liquidus line on the Zn–Al equilibrium phase diagram at the corresponding temperatures and increases along with an increase in the holding time. This was apparently due to isothermal solidification.     

Effects of Cu and Al on the crystal structure and composition of η (MgZn2) phase in over-aged Al–Zn–Mg–Cu alloys

Heat treatable Al–Zn–Mg alloys can be strengthened by the precipitation of second phase particles. In this paper, Al–6.57%Zn–2.83%Mg and Al–6.57%Zn–2.83%Mg–3.92%Cu alloys (in wt%) in T7 state (140 °C for 96 h) have been prepared. The effects of Cu and Al on the concentration and structure of equilibrium η (MgZn₂) phase have been investigated by high resolution transmission electron microscopy, aberration-corrected scanning transmission electron microscopy, selected election diffraction pattern simulations, and first-principles calculations. The effects of Cu and Al substitution on the diffraction characteristics of the η phase and the general rule of Cu and Al substitution in the η phase have been discussed.     

Thermal properties of Mg–Li and Mg–Li–Al alloys

Abstract

The present work is a study of the thermal properties of Mg–xLi–y Al with x= 4, 8 and 12 wt-% and y= 0, 3 and 5 wt-% as a function of temperature in the range 20–375°C. The thermal diffusivity and coefficient of thermal expansion (CTE) have been measured and the thermal conductivity calculated. The thermal diffusivity of all alloys decreases with an increasing content of lithium. The CTE of the single phase alloys Mg–4Li and Mg–12Li has a linear character, and the CTE of Mg–12Li is higher than that of Mg–4Li. The influence of thermal stresses in the two phase alloy Mg–8Li is perceptible in terms of temperature dependence of the CTE. In Mg–4Li–3Al and Mg–4Li–5Al, an influence of the solution of AlLi phase on all the studied thermal properties has been found.     

The paint–bake response of three Al–Mg–Zn alloys

The aging behaviors of three Al–Mg–Zn alloys have been investigated under conditions similar to the paint–bake cycle currently used in automotive manufacturing. The three alloys contain Mg in atomic concentrations from one to two times those of Zn. Natural aging at 25 °C after solutionizing is found to produce a linear increase in hardness with logarithmic time for times of up to 1 year. Hardnesses in naturally and artificially aged conditions are found to increase with Mg content. Artificial aging at 175 °C for 30 min, which simulates the automotive paint–bake cycle, produces increases in hardness of 15–36% over the solution-treated conditions. Peak hardness from artificial aging at 175 °C is produced in all alloys after approximately 8 h. Natural aging for 10 days prior to artificial aging at 175 °C does not produce significant changes in hardness compared with artificial aging alone. Natural aging for 1 year after simulated paint–bake aging increases hardnesses by 41–78% over those after simulated paint–bake aging alone. The precipitation strengthening mechanism in these alloys is consistent with η′ formation. Increases in hardness and strength with increasing Mg content are consistent with increased solid–solution strengthening, which is retained even after artificial aging.     

Investigation of mechanical and corrosion properties of an Al–Zn–Mg–Cu alloy under various ageing conditions and interface analysis of η′ precipitate

The mechanical and corrosion properties under various ageing treatment conditions were investigated in an Al–6.0Zn–2.3Mg–1.8Cu–0.1Zr (wt.%) alloy. The results showed that the retrogression and re-ageing (RRA) were capable of providing higher strength and improved corrosion resistance in comparison with the conventional T6 and T74 ageing. The optimised ageing process had been found to be 120 °C/24 h + 180 °C/60 min + 120 °C/24 h for the experimental alloy. The results obtained from the high resolution transmission electron microscopy (HRTEM) interface analysis revealed that a semi-coherent stress field between the η′ precipitate and the Al matrix was critical in controlling the strength of the Al–Zn–Mg–Cu alloy heat-treated under different conditions. Furthermore, Transition Matrix calculation showed that the η′ phases had only two zone axes: [1̅21̅3]_η′ and [108̅2̅3]_η′, which were parallel to the [112]_Al zone axis, when being precipitated from the Al matrix. Therefore, the orientation relationships between the η′ precipitates and the Al matrix under the [112]_Al zone axis could be described as: [1̅21̅3]_η′//[112]_Al;(12̅12)_η′//(11̅)_Al and [108̅2̅3]_η′//[112]_Al;(12̅12)_η′//(111̅)_Al. Consequently, a new diffraction pattern model from η′ precipitates in two variants under the [112]_Al zone axis had been established, which was in a good agreement with the experimental data.     

Structural characteristics and corrosion behavior of biodegradable Mg–Zn, Mg–Zn–Gd alloys

In this research, binary Mg–Zn (up to 3 wt% Zn) and ternary Mg–Zn–Gd (up to 3 wt% Gd, 3 wt% Zn) alloys were prepared by induction melting in an argon atmosphere. The structures of these alloys were characterized using light and scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction and X-ray fluorescence. In addition, Brinell hardness measurements were taken to supplement these studies. Corrosion behavior was evaluated by immersion tests and potentiodynamic measurements in a physiological solution (9 g/l NaCl). Depending on the composition, structures of the as-cast alloys contained α-Mg dendrites, MgZn, Mg₅Gd and Mg₃Gd₂Zn₃ phases. Compared to pure Mg, zinc improved the corrosion resistance of binary Mg–Zn. Gadolinium also improved the corrosion resistance in the case of Mg–1Zn–3Gd alloy. The highest corrosion rate was observed for Mg–3Zn–3Gd alloy. Our results improve the understanding of the relationships between the structure and corrosion behavior of our studied alloy systems.