Molybdate inhibition of environmental fatigue crack propagation in Al–Zn–Mg–Cu

effectively inhibits environmentally assisted fatigue crack propagation in 7075-T651 stressed during full immersion in low-chloride solution, as understood by hydrogen environment embrittlement and film rupture where -enhanced passivity reduces H production and uptake due to reduced crack hydrolysis, buffered pH, and a diffusion-barrier film. Inhibition is governed by the balance between crack tip strain rate and repassivation kinetics which establish the stability of the passive film. Inhibition is promoted by reduced loading frequency, reduced stress intensity range, increased crack tip concentration, and potentials at or anodic to free corrosion. The inhibiting effect of parallels that of , but molybdate effectiveness is shifted to a lower frequency regime suggesting the Al_xMo_yO_z passive film is less stable against crack tip deformation. For high R loading at sufficiently low frequencies fully inhibits EFCP, quantified by reduced crack growth rate to that typical of ultra-high vacuum, reduction in crack surface facets typical of hydrogen embrittlement, and crack arrest. Chromate did not produce such complete inhibition. Methods exist to incorporate molybdate or Mo in self-healing coating systems, but the complex effects of mechanical and electrochemical variables must be understood for reliable-quantitative fatigue performance enhancement.     

Metallurgical assessment of an emerging Al–Zn–Mg–Cu P/M alloy

The objective of this work was to conduct a detailed assessment of the microstructure and mechanical properties of an emerging Al–Zn–Mg–Cu powder metallurgy (P/M) alloy known as Alumix 431D. A variety of techniques were considered including optical microscopy, X-ray diffraction, electron-probe micro-analysis, thermal dilatometry, and differential scanning calorimetry as well as apparent hardness, tensile testing, and bending fatigue. Alumix 431D exhibited many of the same attributes found in wrought counter parts such as 7075. A sintered density of approximately 99% of theoretical was achieved, indicating that the alloy was highly responsive to sintering. Once heat treated, a T6 hardness of 86 HRB and a room temperature ultimate tensile strength of 448 MPa were noted. Thermal analyses implied that the precipitation behaviour of Alumix 431D closely mimicked comparable 7XXX series wrought alloys and was largely premised on the precipitation of η-phase variants. Tensile properties of the alloy in a T1 temper were found to be relatively stable at temperatures up to 150 °C and 1000 h of exposure time. Those of T6 specimens degraded under the same exposure conditions to the point where equivalency with the T1 product was noted.     

Synthesis of ultra high strength Al–Mg–Si alloy sheets by differential speed rolling

The ultrafine-grained Al–Mg–Si alloy sheets, which were fabricated by severe plastic deformation (SPD) using a high-speed-ratio differential speed rolling (HRDSR) and subsequent low temperature aging, exhibited an ultra high strength (yield stress: 455 MPa, ultimate tensile strength: 489 MPa). The strengthening effect was impressive compared with the results obtained by using other SPD techniques. The achievement could be attributed to formation of very fine grains due to significantly increased dislocation density in solute supersaturated matrix, high Hall-Petch constant and particle strengthening gained by formation nano-scale precipitates during the low temperature aging after the HRDSR process.     

The effect of grain refining and oxide inclusion on the fluidity of Al–4.5Cu–0.6Mn and A356 alloys

The effect of grain refinement and oxide inclusion on the fluidity of Al alloy was investigated with a test casting with eight thin flow channels. Pouring in air increased the amount of oxide in the A356 melt. The fluidity compared between normal A356 melt and contaminated melt. The amount of oxide was evaluated qualitatively by ultrasonic treatment. The flow length varied linearly with the pouring temperature. By adding Ti and Al–5Ti–B, fluidity increased. The grain size decreased by adding grain refiner. The fluidity depended on the degree of grain refining. It was noticed that pouring in air increased the amount of oxides in the melt by ultrasonic treatment. The fluidity of contaminated melt was decreased comparing to the normal one especially in lower temperature.     

Comparing characteristics of serrations in Al–Li and Al–Mg alloys

Characteristics of serrations in the flow stress–strain curves of Al–1Mg and Al–2Li alloys, obtained from tensile tests, are analyzed and compared. The analysis includes stress drop, drop time and reload time at various ageing durations of the alloys. Changes in distributions of the stress drops and the drop time with changing the ageing duration differ markedly in Al–2Li from those in Al–1Mg. The mean values and standard deviations of the stress drops and the reload times increase at large deformation in Al–2Li, while they decrease in Al–1Mg. The influence of precipitates on the characteristics of serrations in the Al–Li alloy is identified and the potential effects are discussed.     

Effect of Cu and Sn on liquid–liquid interfacial energy in ternary and quaternary Al–Bi-based monotectic alloys

The influence of Cu and Sn on the interfacial energy between Al-rich and Bi-rich liquids has been studied. The liquid–liquid interfacial tension is increased when Cu is added to the Al_34.5Bi_65.5 (numbers indicate wt.%) binary and it is decreased when Sn is added. Simultaneous addition of Cu and Sn in equal quantity to the binary Al–Bi alloy results in a decrease of the interfacial tension at low temperature and in its increase at high temperature. Temperature dependences of the interfacial tension in the alloys studied are well described by the function σ_αβ = σ₀ (1 − T/T_C)^μ with a constant σ₀ and the critical-point exponent μ = 1.3.     

Effect of the extrusion conditions on microstructure evolution of the extruded Al–Mg–Si–Cu alloy rods

Hot extrusion experiment was conducted using an Al–Mg–Si–Cu alloy and the effect of the extrusion conditions on microstructure and texture changes through the radial direction was investigated by using SEM/EBSP analysis method. In the surface layer where severe frictional shear deformation is predominant, the recrystallized 1 1 0//ED grains surrounded by high angle grain boundaries are formed in spite of the existence of some peripheral overcoarse grains. Strong 1 0 0//ED and 1 1 1//ED fiber textures evolve in the center where axisymmetric deformation along the extrusion direction is intensive. As the extrusion ratio increases, number of 1 1 1//ED grains remarkably decreases while the number of 1 0 0//ED grains apparently increases. It is also found that the 1 0 0//ED grains surrounded by low angle grain boundaries form orientation colonies in the center of the extruded rods.     

Effects of strain rate on mechanical properties and failure mechanism of structural Al–Mg alloys

The effects of strain rate on the mechanical properties and failure mechanism of AA5754 and AA5182 sheets were investigated. Tensile tests were conducted at quasi-static (less than 10⁻¹ s⁻¹) and dynamic (600, 1100 and 1500 s⁻¹) strain rates at room and elevated temperature using an INSTRON machine and Tensile Split Hopkinson Bar (TSHB) apparatus, respectively. Shear band decoration, interrupted tensile tests, electron microscopy, and image analysis techniques were also utilized. The results obtained show that the studied alloys exhibit negative strain rate sensitivity at quasi-static rates, but mild positive sensitivity at dynamic rates. Different failure mechanisms were also observed. Strain localization and shear band formation was found to be a necessary pre-requisite for the development of damage and final failure under quasi-static conditions. In the dynamic strain rate regime however, less shear banding was observed. Void nucleation, growth and coalescence that is characteristic of dynamic tensile fracture appears to be the dominant mode for failure under dynamic conditions.     

Microstructure and properties of a new super-high-strength Al–Zn–Mg–Cu alloy C912

Using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the microstructure of a new super-high-strength Al–Zn–Mg–Cu alloy (C912) has been investigated. Compared with some other high-strength aluminum alloys, the C912 alloy exhibits higher strength and good stress-corrosion resistance and its specific strength is even higher than some Al–Li alloys. Its potential for use in the Chinese AE100 airplane is discussed.     

Microstructure and mechanical property developments in Al–12Si gravity die castings after Ti and/or Sr additions

This study evaluates the influence that modification and refinement have on the mechanical properties of the eutectic Al–Si alloy and on the quantitative and qualitative correlations with the microstructure. A general improvement in the mechanical properties of the alloys was observed after the additions. The best mechanical values were obtained when both Ti and Sr were present, in particular for the experimental composition with the higher Ti/Sr ratio. The results also indicate the development of a ductility trough for the alloy with lower Ti/Sr ratio, indicating a poisoning effect of Ti over Sr.     

Fatigue behaviour of monotectoid-based Zn–Al–Cu alloys in 3.5% NaCl and 1% HCl solutions

One binary Zn–40Al and three Zn–40Al-based ternary alloys containing 1%, 3%, and 5% Cu were produced by permanent mold casting. Their fatigue behavior was investigated in 3.5% NaCl and 1% HCl solutions by a rotary bending fatigue test machine at a frequency of 33.3 Hz. The stress amplitude versus number of cycles to failure (S–N) curves of the alloys was plotted for both environments. Corrosion degradation factors of the alloys were determined. The corrosion environments reduced the fatigue strength and fatigue life of the alloys considerably. However, acid solution was found to be more detrimental for these alloys than the salt water. In addition, copper content was found to be less effective on the fatigue strength and fatigue life of the alloys in both salt water and acid solution than it was in air. Corrosion degradation factor of the alloys increased with increasing copper content up to approximately 3%, above which it decreased as the copper content increased. It was also shown that the fatigue data obtained from the monotectoid-based Zn–Al–Cu alloys in the corrosive environments obey the Basquin's law.     

Effects of grain refinement on age hardening behavior and precipitation kinetics of Al–Si–Cu–Mg alloys

The influences of grain refinement on precipitation kinetics were investigated for an Al–11 wt% Si–1.5 wt% Cu–0.3 wt% Mg casting alloy doped with B and and with La–B respectively by microstructure observation, hardness test and Johnson–Mehl–Avrami (JMA) equation. Co‐alloying of La–B facilitates the faster hardening response with higher hardness value for the alloy. The calculated Avarmi exponent indicates that the nucleation of θ′‐Al₂Cu precipitates occurs on grain boundaries for the refined alloys. The activation energies for the precipitation are of 42 kJ/mol and 30 kJ/mol for B‐doped and La–B co‐doped alloys, respectively.     

Development and characterization of Al–Li alloys

Increased strength to weight ratio of aluminium–lithium alloys has attracted material scientists to develop these for aerospace applications. But commercial scale production of these alloys has always been slow in view of difficulties encountered during addition of lithium and in ensuring homogeneous billet composition. A new technique of Li addition has been adapted, which gives maximum recovery of Li in the billet. Using this technique, aluminium–lithium alloys of two different grades for aerospace application were cast. Billets were hot forged and rolled to the thickness range of 3–4 mm and heat-treated for different temper conditions. Mechanical properties were evaluated in T6 (solution treated and artificial aged), T8 (solution treated, cold worked and artificial aged) and T4 (solution treated and natural aged) temper conditions. Both alloys exhibit a strong natural aging response. Reversion for short periods at 180 °C results in decrease of strength. With artificial reaging strength reaches above the T4 temper condition level. Characterization was carried out using optical microscope (OM) and scanning electron microscope (SEM). Experimental investigation shows that addition of lithium at high melt temperature gives lower recovery of Li, and use of impure aluminium adversely affects the mechanical properties of the alloy in all temper conditions.     

Formation of vacancy clusters in deformed thin films of Al–Mg and Al–Cu dilute alloys

In the present study, vacancy clusters in elongated Al–Mg and Al–Cu thin films (Mg/Cu CONCENTRATION=0.05–1.70 at.%) were examined by electron microscopy. No dislocations were observed in these films. In Al–Mg thin films deformed at room temperature, a large number of stacking fault tetrahedra (sft) were observed alongside a few vacancy loops. The opposite was true for Al–Cu thin films, where well-grown loops predominated, and only a few sft were observed. The Al–Cu film results show that the majority of vacancies form loops larger than sft. We also deformed Al–0.05at.% (Mg or Cu) alloys in liquid nitrogen and cold-transferred to an electron microscope. In Al–Mg, a large number of dotted defects (possibly sft) were observed, while very few such defects were observed in Al–Cu. This indicates that loops observed in Al–Cu thin films deformed at room temperature, grew during/after deformation. The likely contribution of strain-induced vacancies in deformed Al thin films to the voiding in VLSI interconnect wires due to electro-migration were discussed.     

Microstructure and superplasticity of Mg–Al–Ca electromagnetic casting alloys after hot extrusion

Microstructure and superplastic properties of the plates extruded from the Ca containing Mg alloy (1 wt.% Ca–AZ31) billets fabricated by electromagnetic casting (EMC) without and with electromagnetic stirring (EMS) were examined. The linear intercept grain sizes of the extruded materials were 3.7 μm and 2.1 μm, respectively. The material extruded from the EMC + EMS billet exhibited good superplasticity at low temperatures as well as at high strain rates, including the tensile elongations of 370% at 1 × 10⁻³ s⁻¹, −523 K and 550% at 1 × 10⁻² s⁻¹, −673 K. These values largely exceeded those of the AZ31 alloys with the similar grain sizes. The superior superplasticity of the extruded EMC + EMS billet could be attributed to fine grains and high grain stability at elevated temperatures by the presence of finely dispersed particles of thermally stable (Al,Mg)₂Ca phase. The constitutive equations were developed for describing the high-temperature deformation behavior of the fine-grained 1 wt.% Ca–AZ31 alloys with different grain sizes in wide range of temperature and strain rate.     

Thermal analysis and solidification pathways of Mg–Al–Ca system alloys

Mg–Al–Ca alloys are creep resistant magnesium alloys with high application potentials. The solidification pathways and microstructure formation in this alloy system are still under discussion. In this paper, the solidification behavior of AZ91 and AM50 with Ca addition (AZC91x and AMC50x alloys) was investigated by a computer-aided cooling curve analysis (CA-CCA) system. Microstructure and phase identification were carried out by SEM and EDX analysis. The results show that the Ca-containing phase formation mainly depends on Ca content and Ca/Al ratio. With increasing the Ca/Al ratio these phases transform from Al₂Ca to (Mg, Al)₂Ca and Mg₂Ca. Moreover, Ca addition decreases the liquidus temperature of Mg–Al alloys, but influences the solidus temperature in a more complex way. Increasing the Ca content also decreases the solid fraction at which dendrite coherency occurs. The relationship between solidification interval, dendrite coherency point, formation of Ca-containing phases and hot tearing is also discussed.