Su-min Zeng

Retrogression and re-aging treatment of Al-9.99%Zn-1.72%Cu-2.5%Mg-0.13%Zr aluminum alloy

Abstract The effect of retrogression and re-aging(RRA) heat treatment on the microstructure and mechanical properties of a low frequency electromagnetic casting alloy as Al-9.99%Zn-1.72%Cu-2.5%Mg-0.13%Zr was investigated by tensile properties test, Vickers hardness, electrical conductivity test, DSC analysis, SEM and TEM observation. The results show that RRA heat treatment can improve the stress corrosion cracking(SCC) properties with retention of the high strength of T6 level. After preaging at 100 °C for 24 h, retrogression at 200 °C for 7 min, and then re-aging at 100 °C for 24 h, the alloy obtains tensile strength up to 795 MPa, yield strength up to 767 MPa, maintains 9.1% elongation, and electric conductivity of 35.6%IACS. TEM observation shows that the re-dissolution of GP zone and η′ phase in the early stage of regression leads to the decrease of hardness, then the increase in the volume fraction of η′ and η phases leads to the increase again in the peak value, and finally the general coarsening of all particles results in a softening of the alloy. Meanwhile it is found that the conventional T6 heat treatment as the preaging and re-aging regime is not the optimum regime to the RRA treatment of the high-zinc content super-high strength aluminum alloy.

Enhanced Fatigue Crack Propagation Resistance in an Al-Zn-Mg-Cu Alloy by Retrogression and Reaging Treatment

The microstructures and fatigue crack propagation (FCP) behavior of an Al-Zn-Mg-Cu alloy in T761 and retrogression and reaging (RRA) conditions were characterized by employing differential scanning calorimetry, optical microscopy, scanning electron microscopy, transmission electron microscopy, and electron backscatter diffraction. The results suggested that coarse η′ precipitates were present in T761-treated sample, while fine dispersed η′ precipitates and GP zones were uniformly distributed in RRA-treated ones. Besides, the width of precipitate-free zones (PFZs) in T761-treated sample was found to be much greater than that in RRA-treated ones. Compared with T761-treated sample, the enhanced FCP resistance of RRA-treated sample was attributed to the shearable particles in matrix and narrow PFZs.

Deep-cryogenic-treatment-induced phase transformation in the Al-Zn-Mg-Cu alloy

An aluminum alloy (Al-Zn-Mg-Cu) subjected to deep cryogenic treatment (DCT) was systematically investigated. The results show that a DCT-induced phase transformation varies the microstructures and affects the mechanical properties of the Al alloy. Both Guinier-Preston (GP) zones and a metastable η′ phase were observed by high-resolution transmission electron microscopy. The phenomenon of the second precipitation of the GP zones in samples subjected to DCT after being aged was observed. The viability of this phase transformation was also demonstrated by first-principles calculations.

Redistribution and re-precipitation of solute atom during retrogression and reaging of Al-Zn-Mg-Cu alloys

Abstract The redistribution and re-precipitation of solute atom during retrogression and reaging of three different Al-Zn-Mg-Cu aluminum alloys were investigated. The results of hardness and tensile strength test indicate that after pre-aging at 100 °C or 120 °C and retrogressing at 200 °C for various time and re-aging treatment, the hardness and strength of the alloys are all larger than those under pre-aging condition, some of them even exceed the value under peak aging(T6) condition. TEM observation shows that the PFZ formed during retrogressing in short time becomes narrow and even disappears after re-aging treatment. However, the PFZ formed during retrogressing for a long time does not narrow after re-aging treatment. It is suggested that the redistribution and re-precipitation of solute atom during re-aging treatment result in the narrowing and even disappearance of the PFZ formed during retrogression, which reinforces the grain-boundaries and presents the value of tensile strength exceeding peak-aging strength in the RRA condition, while the precipitates in the matrix of the alloys still keep or even exhibit a more dispersed distribution, and a greater effect of precipitation strengthening is obtained.

Microstructure and properties of oxalate conversion coating on AZ91D magnesium alloy

Abstract The oxalate coating formed on AZ91D magnesium alloy by chemical conversion treatment methods in oxalate salt solutions was investigated. The surface morphologies and chemical composition of coating were examined using scanning electron microscopy (SEM) equipped with energy dispersive analysis of X-ray (EDX). Electrochemical impedance spectroscopy (EIS), potentiodynamic polarization curves and salt spray tests were employed to evaluate corrosion protection of the coating to substrate in 5% NaCl solution. The mechanism of coating formations was also considered in details. The results indicate that a compact and dense surface morphology with fine particle clusters of the oxalate coating on magnesium alloy is presented, which mainly consists of oxide or/and organic of Mg, Al and Zn. And the anti-corrosion of the magnesium after oxalate conversion treatment is better than that of the magnesium substrate. The results of salt spray test for oxalate coating is evaluated as Grade 9 according to ASTM B117. The electric resistance of oxalate chemical conversion coating to substrate is below 0.1 Ω.

Transition of crack propagation from a transgranular to an intergranular path in an overaged Al-Zn-Mg-Cu alloy during cyclic loading

The fatigue crack propagation behavior in the overaged Al-Zn-Mg-Cu alloy was characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The results revealed that a fatigue crack tended to transgranularly propagate in the near-threshold regime, whereas intergranular crack propagation was dominant at the high ΔK regime. The transition of crack propagation from a transgranular to an intergranular path that occurred in the Paris regime was strongly influenced by the misorientation of adjacent grains and precipitate free zones. In addition, a crystallographic model of crack propagation was proposed to interpret the transition. The fatigue short crack propagation on a single slip plane was responsible for the formation of a transgranular propagation path in the near-threshold regime. The fatigue long crack propagation, which was conducted by a duplex slip mechanism in the Paris regime, led to the formation of fatigue striations. The formation of a zigzag crack in the near-threshold regime was ascribed to the high misorientation of adjacent grains.

Effects of electrolyte components on properties of Al alloy anode

Abstract The effects of Na 2 SnO 3 , In(OH) 3 and Na 2 SnO 3 +In(OH) 3 on the properties of Al alloy anode were studied in alkaline medium at 25 °C. The self-corrosion rate of Al alloy anode was studied by method of H 2 immersed in aqueous medium, and the electrochemical properties of Al alloy anode were tested via galvanostatic discharge and dynamic potential methods. The results show that the self-corrosion rate of Al alloy anode in 4 mol/L NaOH+3 mol/L NaAlO 2 medium can be minimized by adding Na 2 SnO 3 , In(OH) 3 and Na 2 SnO 3 + In(OH) 3 . Na 2 SnO 3 , In(OH) 3 and Na 2 SnO 3 +In(OH) 3 make Al anodic potential shift positively in galvanostatic discharging. The most effective additive of Al alloy anode in 4 mol/L NaOH+3 mol/L NaAlO 2 medium is 0.075 mol/L Na 2 SnO 3 +0.005 mol/L In(OH) 3 , integrating self-corrosion rate and electrochemical properties.

Deformation behavior of SiC particle reinforced Al matrix composites based on EMA model

Abstract Effects of the matrix properties, particle size distribution and interfacial matrix failure on the elastoplastic deformation behavior in Al matrix composites reinforced by SiC particles with an average size of 5 βm and volume fraction of 12% were quantitatively calculated by using the expanded effective assumption(EMA) model. The particle size distribution naturally brings about the variation of matrix properties and the interfacial matrix failure due to the presence of SiC particles. The theoretical results coincide well with those of the experiment. The current research indicates that the load transfer between matrix and reinforcements, grain refinement in matrix, and enhanced dislocation density originated from the thermal mismatch between SiC particles and Al matrix increase the flow stress of the composites, but the interfacial matrix failure is opposite. It also proves that the load transfer, grain refinement and dislocation strengthening are the main strengthening mechanisms, and the interfacial matrix failure and ductile fracture of matrix are the dominating fracture modes in the composites. The mechanical properties of the composites strongly depend on the metal matrix.