Structure and Stoichiometry of Mg<Subscript>x</Subscript>Zn<Subscript>y</Subscript> in Hot-Dipped Zn–Mg–Al Coating Layer on Interstitial-Free Steel

Correlations of stoichiometry and phase structure of Mg_xZn_y in hot-dipped Zn–Mg–Al coating layer which were modified by additive element have been established on the bases of diffraction and phase transformation principles. X-ray diffraction (XRD) results showed that Mg_xZn_y in the Zn–Mg–Al coating layers consist of Mg₂Zn₁₁ and MgZn₂. The additive elements had a significant effect on the phase fraction of Mg₂Zn₁₁ while the Mg/Al ratio had a negligible effect. Transmission electron microscope (TEM) assisted selected area electron diffraction (SAED) results of small areas Mg_xZn_y were indexed dominantly as MgZn₂ which have different Mg/Zn stoichiometry between 0.10 and 0.18. It is assumed that the Mg_xZn_y have deviated stoichiometry of the phase structure with additive element. The deviated Mg₂Zn₁₁ phase structure was interpreted as base-centered orthorhombic by applying two theoretical validity: a structure factor rule explained why the base-centered orthorhombic Mg₂Zn₁₁ has less reciprocal lattice reflections in the SAED compared to hexagonal MgZn₂, and a phase transformation model elucidated its reasonable lattice point sharing of the corresponding unit cell during hexagonal MgZn₂ (a, b?=?0.5252 nm, c?=?0.8577 nm) transform to intermediate tetragonal and final base-centered orthorhombic Mg₂Zn₁₁ (a?=?0.8575 nm, b?=?0.8874 nm, c?=?0.8771 nm) in the equilibrium state.     

Effect of Mg composition on sintering behaviors and mechanical properties of Al–Cu–Mg alloy

Al–3Cu–Mg alloy was fabricated by the powder metallurgy (P/M) processes. Air-atomized powders of each alloying element were blended with various Mg contents (0.5%, 1.5%, and 2.5%, mass fraction). The compaction pressure was selected to achieve the elastic deformation, local plastic deformation, and plastic deformation of powders, respectively, and the sintering temperatures for each composition were determined, where the liquid phase sintering of Cu is dominant. The microstructural analysis of sintered materials was performed using optical microscope (OM) and scanning electron microscope (SEM) to investigate the sintering behaviors and fracture characteristics. The transverse rupture strength (TRS) of sintered materials decreased with greater Mg content (Al–3Cu–2.5Mg). However, Al–3Cu–0.5Mg alloy exhibited moderate TRS but higher specific strength than Al–3Cu without Mg addition.     

Effects of fluidised bed quenching on heat treating characteristics of cast Al–Si–Mg and Al–Si–Mg–Cu alloys

Abstract

The quench sensitivity of Al–Si–Mg (D357 unmodified and Sr modified), and Al–Si–Mg–-Cu (354 and 319 Sr modified) cast alloys was investigated using a fluidised bed (FB). The average cooling rate of castings in the fluidised bed is lower than those quenched in water; the cooling rate first increases to a certain maximum and then decreases during quenching. The change in the cooling rate during quenching in water was more drastic, where the cooling rate varied from 0 to ?80 K s^?1 in less than 8 s, as compared with those quenched in FB, where the cooling rate varied from 0 to ?14 K s^?1 in 18 s. The FB quenching resulted in the formation of several metastable phases in Al–Si–Mg–Cu alloys; in contrast, no such transformation was observed during water quenching. The T4 yield strength of the FB quenched alloys was greater than water quenched alloys owing to the formation of a greater volume fraction of metastable phases in the FB quenched alloys. The tensile properties of T6 treated alloys show that Al–Si–Mg alloys (both unmodified and Sr modified) are more quench sensitive than Al–Si–Mg–Cu alloys. The high quench sensitivity of the Al–Si–Mg alloys is because GP zones are not formed, whereas GP zones are formed during quenching of the Al–Si–Mg–Cu alloys as predicted by time temperature transformation and continuous cooling transformation) diagrams.     

Metastable-Stable Phase Transformation Behavior of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Scale Formed on Fe–Ni–Al Alloys

The oxidation behavior of Ni–Fe–41.5at.%Al alloys with different Fe/Ni ratios was investigated in air at 1000 °C in order to clarify the effect of Fe on the phase transformation of Al₂O₃ scale, using in-situ high-temperature X-ray diffraction by means of synchrotron radiation. The oxidation mass gain of alloys after 25 h of oxidation generally decreased with increasing Fe content; however, the initial oxidation mass gain was significantly decreased by increasing alloy Fe content. In-situ X-ray diffraction analysis indicated that higher alloy Fe contents promoted rapid formation of the stable α-Al₂O₃, while lower Fe in the alloy maintained the metastable Al₂O₃ for longer time oxidation. The effect of Fe on promoting α-Al₂O₃ formation can be explained by the initial formation of α-Fe₂O₃, whose structure is isomorphous with α-Al₂O₃. The additional effect of Fe on the growth rate of α-Al₂O₃ is also discussed.     

Effect of Al on Expansion Behavior of Mg–Al Alloys During Solidification

The cooling curves and the change of contraction/expansion during solidification and cooling were tested by using a selfmade device which could achieve the one-dimensional contraction instead of three-dimensional contraction of the casting.Then, the effects of Al content(0, 1.1, 3, 5, 10, 12.9, 15, 17, 19, 22, 24 and 30 wt%) on the thermal contraction/expansion of the binary Mg-Al as-cast alloys during solidification were obtained. The results showed that expanding instead of contraction was present in Mg-Al alloys with the addition of 0-30 wt% Al during solidification. The values of expansion significantly increased at first and then decreased with the increase in Al content. And the maximum expansion ratio of 0.44%(maximum expansion value: 0.841 mm) was present in the Mg-15 wt% Al alloy. Contraction instead of expansion occurred once the temperature drops to the temperature corresponding to the expansion value in total, indicating the occurrence of a continuous expansion during the solidification process in mushy zone for the Mg alloys with Al addition of 5-30 wt%. The expansion value in total consisted of two parts: the expansions occurring in the liquid-phase zone and mushy zone. The expansion in liquid zone was present in every Mg-Al alloy, and it contributed to the most proportion of the total expansion value when the Al content in Mg-Al alloy was lower than 10 wt% or higher than 22 wt%. However, the total expansion value was mainly determined by the solidification behavior in mushy zone when the Al content was among 10-22 wt% in Mg-Al alloys.     

The Microstructure and Tensile Properties of a Newly Developed Mg–Al/Mg<Subscript>3</Subscript>Sb<Subscript>2</Subscript> In Situ Composite in As-Cast and Extruded Conditions

The microstructures and tensile properties of Mg–x wt%Al–y wt%Sb alloys have been studied where x/y ratio was 1 and Sb(Al) contents were 5, 10, 15 and 20 wt%, respectively. The results indicated that by increasing Sb(Al) content, not only the crystals of primary Mg₃Sb₂ alter from small flake-like particles to polygonal or needle-like morphology, but also the eutectic structure changes from semi-continuous network in Mg–5Al–5Sb to continuous network in Mg–20Sb–20Al alloy. The results obtained from thermal analysis revealed different peaks related to the formation of Mg₃Sb₂ as primary phase and eutectic structure containing Mg₁₇Al₁₂?+?Al₃Mg₂ intermetallic phases. Further results also revealed that Sb(Al) additions change the solidification performance of the material by depressing the Mg₃Sb₂ nucleation temperature, reducing solidification range and widening eutectic area. Tensile testing results showed that with the increase in Sb (Al) content, ultimate tensile strength (UTS) and elongation values of the alloys are decreased in as-cast condition. But, significant improvement in the UTS and elongation values of the extruded specimens was attributed to the severe fragmentation of intermetallic phases and well distributed fine particles in the matrix which provided proper obstacles for dislocation motion. It was interesting to note that the fracture behavior of intermetallic particles was found to be different, while Mg₃Sb₂ was ductile, intermetallic compounds in eutectic regions were brittle.     

Effect of Mg addition on mechanical and thermoelectrical properties of Al–Al2O3 nanocomposite

The effects of Mg addition on mechanical thermo-electrical properties of Al–Mg/5%Al₂O₃ nanocomposite with different Mg contents (0, 5%, 10% and 20%) produced by mechanical alloying were studied. Scanning electron microscopy analysis (SEM), X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM) were used to characterize the produced powder. The results show that addition of Mg forms a predominant phase (Al–Mg solid solution). By increasing the mass fraction of Mg, the crystallite size decreases and the lattice strain increases which results from the atomic penetration of Mg atoms into the substitutional sites of Al lattice. The microhardness of the composite increases with the increase of the Mg content. The thermal and electrical conductivities increase linearly with the temperature increase in the inspected temperature range. Moreover, the thermal conductivity increases with the increase of Mg content.     

The influence of SiO<Subscript>2</Subscript> and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles on corrosion resistance enhancement of Ni–P on magnesium

In this study, the influence of adding SiO₂ and Al₂O₃ to Ni–P coated on magnesium substrate and the related corrosion resistance behavior were evaluated. The surface morphology of Ni–P–SiO₂–Al₂O₃ composite coating was investigated by field emission scanning electron microscopy (FESEM). The amount of Al₂O₃ and SiO₂ in the coating was measured by energy dispersive analysis of X-ray (EDX) and the corrosion behavior of coating was monitored by electrochemical impedance spectroscopy (EIS) and polarization techniques, showing the corrosion resistance of Ni–P–SiO₂–Al₂O₃ increases compare to Ni–P–SiO₂ and Ni–P–Al₂O₃. Furthermore, the microhardness of the coating was examined and the final hardness of Ni–P–SiO₂–Al₂O₃ reached 461 VH.     

A comparative study on corrosion of Mg–Al–Si alloys

Corrosion behavior of various Mg–Al–Si alloys (AS11, AS21, AS41, AS61 and AS91 series), cast under the same cooling conditions and controlled alloying composition, was investigated systematically. Optical microscopy and scanning electron microscopy were used for microstructural examinations. The corrosion behavior was evaluated by immersion tests and potentiodynamic polarization measurements in 3.5% NaCl solution. The results from both immersion tests and the potentiodynamic polarization measurements showed that marginal improvement in corrosion resistance was observed with 2.0% Al (mass fraction) containing alloy (AS21) whereas Al addition above 2.0% (AS41, AS61 and AS91) deteriorated the corrosion resistance which was attributed to β phase, acting as cathode, and the interruption of continuity of the oxide film on the surface of the alloys owing to coarsened β and Mg₂Si phases.     

Effect of pre-straining on structure and formation mechanism of precipitates in Al–Mg–Si–Cu alloy

The effect of pre-straining on the structure and formation mechanism of precipitates in an Al–Mg–Si–Cu alloy was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Elongated and string-like precipitates are formed along the dislocations in the pre-strained Al–Mg–Si–Cu alloy. The precipitates formed along the dislocations exhibit three features: non-periodic atomic arrangement within the precipitate; Cu segregation occurring at the precipitate/α(Al) interface; different orientations presented in one individual precipitate. Four different formation mechanisms of these heterogeneous precipitates were proposed as follows: elongated precipitates are formed independently in the dislocation; string-like precipitates are formed directly along the dislocations; different precipitates encounter to form string-like precipitates; precipitates are connected by other phases or solute enrichment regions. These different formation mechanisms are responsible for forming different atomic structures and morphologies of precipitates.     

Effect of neutron irradiation and postradiation annealing on the microstructure and properties of an Al–Mg–Si alloy

The effect of long-term neutron irradiation and postradiation thermal-induced aging on the microstructure and mechanical properties of an aluminum-based reactor Al–Mg–Si alloy grade SAV-1 has been studied. The material under study is the shell of an automatic fine-control rod used to control the reactivity of the core of a VVR-K research reactor. Successive 1-h annealings of specimens of the SAV-1 alloy irradiated to doses of 0.001 and 5 dpa in the temperature range of 100–550°C have been carried out. The evolution of the fine structure of the material and changes in its mechanical characteristics have been studied. The phenomenon of the acceleration of the aging of the SAV-1 alloy under the effect of a high neutron fluence at an irradiation temperature of 80°C has been observed, which involves the formation of numerous lineage (stitch) Guinier–Preston zones in the alloy. It has been shown that the strength characteristics of the SAV-1 alloy depend significantly on the degree of its radiation- and thermal-induced aging.     

The effect of iron on liquid film migration and sintering of an Al–Cu–Mg alloy

Analytical transmission electron microscopy indicates that liquid film migration occurs during sintering of an Al–Cu–Mg alloy, that intragranular liquid pools develop from migrating films and that iron segregates to these pools. It is suggested that a high localised iron concentration retards the liquid film migration rate by reducing the coherency strain in the retreating grain, causing a region of the film to detach from the boundary, thus forming an intragranular pool in the advancing grain. Alloys with low iron levels develop few intragranular pools and have high sintered densities.     

Influence of interrupted quenching on artificial aging of Al–Mg–Si alloys

In this study the influence of interrupted quenching (IQ) in the temperature range 150–250 °C for periods of 15–1080 s on artificial aging after long-term natural pre-aging was studied for the Al–Mg–Si alloy AA6061 by atom probe tomography, transmission electron microscopy, electrical resistivity and hardness measurements, and differential scanning calorimetry. Compared with a standard quenching procedure, the results showed that hardening kinetics and the age hardening response were enhanced for IQ at low temperatures but reduced at high temperatures. Quenched-in vacancies were shown to be of particular importance for the nucleation of precipitates occurring during IQ at the lower end of the temperature range, finally leading to the formation of a dense distribution of β″ during artificial aging. For standard water quenching and subsequent natural aging, nucleation is hindered by a low concentration of quenched-in vacancies in the matrix. IQ at high temperatures affects subsequent artificial aging via the formation of precipitates which do not contribute to hardening but consume a significant amount of solute.     

Effect of flame rectification on corrosion property of Al–Zn–Mg alloy

The corrosion resistance of Al–Zn–Mg alloy subjected to different times in flame rectification was investigated based on the exfoliation corrosion test. The results indicate that the flame rectification deteriorates the exfoliation corrosion resistance of Al–Zn–Mg alloy. The corrosion resistance of Al–Zn–Mg alloy is ranked in the following order: base metal>two times>three times>one time of flame rectification. The exfoliation corrosion behavior was discussed based on the transformation of precipitates at grain boundaries and matrix. With increasing the number of times in flame rectification, the precipitate-free zones disappeared and the precipitates experienced dissolution and re-precipitation. The sample was seriously corroded after one time of flame rectification, because the precipitates at grain boundaries are more continuous than those in other samples.     

Effect of minor Zr and Sc on microstructures and mechanical properties of Al–Mg–Si–Cu–Cr–V alloys

The effects of minor contents of Zr and Sc on the microstructures and mechanical properties of Al–Mg–Si–Cu–Cr–V alloy were studied. The results show that the effects of minor Zr and Sc on the as-cast grain refinement in the ingots, the improvement in the strength of the as-extruded alloys and the restriction of high angle grain boundaries in the aged alloys can be sorted as Al₃Sc>Al₃(Zr,Sc)>Al₃Zr. None of them could stop the nucleation of recrystallization, but Al₃(Zr,Sc) phase is a more effective inhibitor of dislocation movement compared to Al₃Sc in the aged alloys. Compared with the mechanical properties of the aged alloy added only 0.15% Sc, the joint addition of Zr and Sc to the alloy leads to a very slight decrease in strength with even no cost of ductility. Taking both the production cost and the little bad influence on mechanical properties into consideration, an optimal content of Zr and Sc in the Al–Mg–Si–Cu–Cr–V alloy to substitute 0.15% Sc is 0.13% Zr+0.03% Sc.     

Study of precipitation in aged binary Mg–Al and ternary Mg–Al–Zn alloys using 27Al NMR spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy of ²⁷Al was used to study the development of precipitation in aged Mg–6 wt%Al, Mg–9 wt%Al and Mg–9 wt%Al–(x) wt%Zn alloys. The ²⁷Al spectra for the aged alloys consist of two peaks; one from the aluminium in solid solution and the other from aluminium in the precipitate phase. The proportion of aluminium atoms in the matrix and precipitate phases was measured, as a function of time at temperature, using the relative intensities of peaks. The nucleation of the continuous precipitates was found to be highly dependent on the initial supersaturation and it is proposed that it is a homogeneous process. The Austin–Rickett relation successfully models the amount of continuous precipitation with aging time; the kinetics is consistent with one-dimensional and interface-controlled growth. Changes in composition of the matrix and precipitate phases were correlated with the ²⁷Al Knight shift characterising these phases. The Knight shift data from a series of ternary Mg–9 wt%Al–(x)wt%Zn alloys indicates that the Zn segregates to the precipitate phase during precipitation.     

The effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles on tribological and corrosion behavior of electroless Ni–B–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite coating

In this study, the Ni–B–Al₂O₃ composite was successfully coated on the surface of Ck45 steel by elecroless method. X-Ray diffraction analysis (XRD) and scanning electron microscopy (SEM) were utilized in order to investigate and identify the coating properties. Wear behavior of the coating was studied by the pinon- disk test. Corrosion behavior of the Ni–B and Ni–B–Al₂O₃ coatings was investigated by using Tafel polarization diagrams in the 3.5% NaCl solution at room temperature. The obtained data demonstrate that the addition of Al₂O₃ nanoparticles to the coating has resulted in improving the tribological behavior of the coating due to the presence of the composite nanoparticles. Also, the results of electrochemical testing show that corrosion resistance of the electroless Ni–B coating with Al₂O₃ nanoparticles has dramatically increased.     

Dependence of Protein Adsorption on Wetting Behavior of UHMWPE–HA–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–CNT Hybrid Biocomposites

Ultrahigh-molecular-weight polyethylene (UHMWPE) is used as an articulating surface in total hip and knee joint replacement. In order to enhance long-term durability/wear resistance properties, UHMWPE-based polymer–ceramic hybrid composites are being developed. Surface properties such as wettability and protein adsorption alter with reinforcement or with change in surface chemistry. From this perspective, the wettability and protein adsorption behavior of compression-molded UHMWPE–hydroxyapatite (HA)–aluminum oxide (Al₂O₃)–carbon nanotube (CNT) composites were analyzed in conjunction with surface roughness. The combined effect of Al₂O₃ and CNT shows enhancement of the contact angle by ~37° compared with the surface of the UHMWPE matrix reinforced with HA. In reference to unreinforced UHMWPE, protein adsorption density also increased by ~230% for 2 wt.%HA–5 wt.%Al₂O₃–2 wt.%CNT addition to UHMWPE. An important conclusion is that the polar and dispersion components of the surface free energy play a significant role in wetting and protein adsorption than do the total free energy or chemistry of the surface. The results of this study have major implications for the biocompatibility of these newly developed biocomposites.     

Crystal structure and stability of complex precipitate phases in Al–Cu–Mg–(Si) and Al–Zn–Mg alloys

We demonstrate how first-principles total energy calculations may be used to elucidate both the crystal structures and formation enthalpies of complex precipitates in multicomponent Al alloys. For the precipitates, S(Al–Cu–Mg), η′ (Al–Zn–Mg), and Q(Al–Cu–Mg–Si), energetics were computed for each of the models of the crystal structures available in the literature allowing a critical assessment of the validity of the models. In all three systems, energetics were also calculated for solid solution phases as well as other key phases (e.g., equilibrium phases, GP zones) in each precipitation sequence. For both the S and η′ phases, we find that recently proposed structures (based on electron microscopy) produce unreasonably high energies, and thus we suggest that these models should be re-evaluated. However, for all three precipitates, we find that structures based on X-ray diffraction refinements provide both reasonable energetics and structural parameters, and therefore the first-principles results lend support to these structural refinements. Further, we predict energy-lowering site occupations and stoichiometries of the precipitate phases, where experimental information is incomplete. This work suggests that first-principles total energy calculations can be used in the future as a complementary technique with diffraction or microscopy for studying precipitate structures and stabilities.     

Effect of Mg addition on the creep and yield behavior of an Al–Sc alloy

The relationships between microstructure and strength were studied at room temperature and 300 °C in an Al–2 wt% Mg–0.2 wt% Sc alloy, containing Mg in solid-solution and Al₃Sc (L1₂ structure) as nanosize precipitates. At room temperature, the yield strength is controlled by the superposition of solid-solution and precipitation strengthening. At 300 °C and at large applied stresses, the creep strength, which is characterized by a stress exponent of ~5, is significantly improved compared to binary Al–Sc alloys, and is independent of the size of the Al₃Sc precipitates. At small applied stress, a threshold stress exists, increasing from 9% to 70% of the Orowan stress with increasing Al₃Sc precipitate radius from 2 to 25 nm. An existing model based on a climb-controlled bypass mechanism is in semi-quantitative agreement with the precipitate radius dependence of the threshold stress. The model is, however, only valid for coherent precipitates, and the Al₃Sc precipitates lose coherency for radii larger than 11 nm. For semi-coherent precipitates with radii greater than 15 nm, the threshold stress remains high, most likely because of the presence of interfacial misfit dislocations.