Microstructures and mechanical properties of the age hardened Mg-4.2Y-2.5Nd-1Gd-0.6Zr(WE43)microalloyed with Zn

The effect of trace addition of 0.2 wt.%Zn on the microstructures and mechanical properties of the age-hardening Mg-4.2Y-2.5Nd-1Gd-0.6Zr(wt.%)(WE43)alloy has been investigated.As compared with the WE43 alloy after solid solution treatment at 525 ℃,the block-like Zn-Zr phase was still observed in the WE43-0.2Zn alloy.However,the time for WE43-0.2Zn alloy to get peak hardness at 250 ℃ was two hours,a half earlier than that in WE43 alloy,meaning a accelerated age precipitation kinetics has been achieved due to the addition of 0.2 wt.%Zn.Microalloyed with 0.2 wt.%Zn enhanced the ultimate tensile strength(UTS)slightly and ductility significantly both in the solutionized and peak aged condition.The enhancement in strength and ductility is possible associated with the larger volume fraction of precipitation phases due to a reduction of the solubility of rare earth elements(RE)in theα-Mg matrix,the larger aspect ratio(length to width)of precipitates and a decrease in stacking fault energy by addition of Zn.

Effect of temperature on the mechanical abnormity of the quasicrystal reinforced Mg-4%Li-6%Zn-1.2%Y alloy

The serrated phenomena of the quasicrystalline phase reinforced Mg-4%Li-6%Zn-1.2%Y alloy after the extrusion,solid solution treatment and aged treatment have been investigated at different temperatures.The result shows that when the temperature is above 100℃,the serrated phenomenon becomes weak and all the serrated amplitudes are lower than 1 MPa.Among them,the serrated amplitude of samples in aged condition is the lowest and the value is only 0.1-0.2 MPa.The underneath mechanism for the lower plastic instability at higher temperature(≥100℃)can be ascribed to the weak pining effect of solute atoms on the movement of dislocation and release of the pile-up dislocations.

Quasi-in-situ observing the rare earth texture evolution in an extruded Mg-Zn-Gd alloy with bimodal microstructure

The static recrystallization and associated texture evolution were investigated in an extruded Mg-Zn-Gd alloy with bimodal microstructure based on a quasi-in-situ electron back-scatter diffraction(EBSD)method.The typical rare earth(RE)texture formed during annealing,evolving from the bimodal microstructure with[1010]basal fiber texture that consisted of fine recrystallized(RXed)grains and coarse unrecrystallized(un RXed)grains elongated along the extrusion direction.In both RXed and un RXed regions,the RXed nucleation produced randomized orientations without preferred selection and the RXed grains with RE texture orientation had more intensive growth ability than those with basal fiber orientation,thereby leading to the preferred selection of RE texture orientation during grain growth.The relationships between stored strain energy,solute drag,grain growth and texture evolution are discussed in detail.This study provided direct evidence of the RE texture evolution in an extruded Mg-RE alloy,which assists in understanding the formation mechanisms for RE texture during extrusion and better developing wrought Mg alloys with improved formability.

Stress corrosion cracking susceptibility of a high strength Mg-7%Gd-5%Y-1%Nd-0.5%Zr alloy

Through performing the tensile tests with different strain rates in 3.5 wt.%NaCl solution,the stress corrosion cracking(SCC)behavior and the effect of strain rate on the SCC susceptibility of an extruded Mg-7%Gd-5%Y-1%Nd-0.5%Zr(EW75)alloy have been investigated.Results demonstrate that the alloy is susceptible to SCC when the strain rate is lower than 5×10^(−6) s^(−1).At the strain rate of 1×10^(−6) s^(−1),the SCC susceptibility index(I_(SCC))is 0.96 and the elongation-to-failure(ε_(f))is only 0.11%.Fractography indicates that the brittle quasi-cleavage feature is very obvious and become more pronounced with decreasing the strain rate.Further analysis confirms that the cracking mode is predominantly transgranular,but the partial intergranular cracking at some localized area can also occur.Meanwhile,it seems that the crack propagation path is unrelated to the existing phase particles.

High cycle fatigue behavior of the forged Mg-7Gd-5Y-1Nd-0.5Zr alloy

This paper investigated the high cycle fatigue behavior of a forged Mg-7Gd-5Y-1Nd-0.5Zr alloy with different stress concentration factor(Kt),under different stress ratio(R),and along different loading direction.The smooth specimen(Kt=1),under R=0.1 and along longitude direction,shows a high fatigue strength of 162 MPa at 107 cycles.The fatigue behavior of the forged Mg-7Gd-5Y-1Nd-0.5Zr alloy exhibits a high sensitive to the notch.Moreover,change of stress ratio from 0.1 to−1 may also result in a bad fatigue property.The flux inclusions were elongated along longitude direction and/or transverse direction during the forging process of the Mg-7Gd-5Y-1Nd-0.5Zr alloy.The interface between the flux inclusion and the matrix may debond and serve as the crack initiation site during the fatigue loading process,leading to the deterioration of the fatigue property along thickness direction and a high anisotropic fatigue behavior between longitude direction and thickness direction.

Fatigue crack initiation of magnesium alloys under elastic stress amplitudes:A review

The most advantageous property of magnesium(Mg)alloys is their density,which is lower compared with traditional metallic materials.Mg alloys,considered the lightest metallic structural material among others,have great potential for applications as secondary load components in the transportation and aerospace industries.The fatigue evaluation of Mg alloys under elastic stress amplitudes is very important in ensuring their service safety and reliability.Given their hexagonal close packed structure,the fatigue crack initiation of Mg and its alloys is closely related to the deformation mechanisms of twiuning and basal slips.However,for Mg alloys with shrinkage porosities and inclusions,fatigue cracks will preferentially initiate at these defects,remarkably reducing the fatigue lifetime.In this paper,some fundamental aspects about the fatigue crack initiation mechanisms of Mg alloys are reviewed,in cluding the 3 followings:1)Fatigue crack initiation of as-cast Mg alloys,2)influence of microstructure on fatigue crack initiation of wrought Mg alloys,and 3)the effect of heat treatment on fatigue initiation mechanisms.Moreover,some unresolved issues and future target on the fatigue crack initiation mechanism of Mg alloys are also described.

Recent progress in fatigue behavior of Mg alloys in air and aqueous media: A review

Due to the interactions between mechanics and chemistry, Mg alloys are inevitably subjected to the combined effect of corrosion attack and cyclic loading, which eventually leads to corrosion fatigue failure. In this paper, fundamental aspects regarding the fatigue behavior of Mg alloys have been reviewed,including：（1） fatigue behavior of Mg alloys in air and aqueous media; and（2） the influence of microstructure, anodic dissolution, hydrogen embrittlement（HE）, heat treatment and surface protection on fatigue behavior of Mg alloys. Moreover, some remaining unresolved issues and future targets to deeply understand the failure mechanism of corrosion fatigue have been described.

Short-time Oxidation of Alloy 690 in High-temperature and High-pressure Steam and Water

The oxidation behavior of alloy 690 exposed to high-temperature and high-pressure steam and water at 280℃ for 1 h was investigated by scanning electron microscopy （SEM）, atomic force microscopy （AFM） and X-ray photoelectron spectroscopy （XPS）. In high-temperature and high-pressure steam, the oxide film is composed of an outermost Ni-rich hydroxides layer, an intermediate layer of hydroxides and oxides enriched in Cr, an inner oxide layer. The film formed in high-temperature water is similar to that in steam, except for missing the Ni-rich hydroxides layer. Samples with different surface finishes （electropolished, mechanically polished, ground, and as-received） were prepared for comparison. A general increase of the oxide thickness with the degree of surface roughness is observed. The equivalent oxide thicknesses lie in the range of 100-200 nm for the as-received samples, 150-250 nm for the samples ground to 400# and 10-20 nm for the samples ground to 1500#, mechanically polished, and electropolished.

Influence of the lithium content on the negative difference effect of Mg-Li alloys

The so-called 'negative difference effect'(NDE) was often defined by the increasing rate of hydrogen evolution from magnesium(Mg) surface under anodic polarization.In this work,a series of electrochemical tests and microstructure observations were performed to provide an evidence that the NDE of Mg-Li alloys can be retarded by increasing lithium content.Potentiostatic,galvanostatic and potentiodynamic polarization experiments using Mg-xLi(x=4,7.5 and 14 wt%) alloys electrodes indicated that Mg-4 Li alloy maintained the enhancing NDE prior to anodic dissolution as that of conventional Mg alloys.However,the emergence of β-Li phase weakened the NDE of duplex Mg-7.5 Li alloy at a low anodic current density,but it was still enhanced apparently after a high applied anodic value(more than 2 mA/cm^2).The surface observations,including the plane and cross-sectional morphologies,confirmed that the cracked surface film derived from the anodic dissolution resulted in the catalytic activity of NDE for Mg-4 Li and Mg-7.5 Li alloys.Furthermore,the NDE of Mg-14 Li alloy was suppressed obviously after a prior applied anodic polarization,which was attributed to the persistent and integrated surface film which endured a higher level of applied anodic potential and current.

Suppressing Effect of Heat Treatment on the Portevin–Le Chatelier Phenomenon of Mg–4%Li–6%Zn–1.2%Y Alloy

Microstructural evolution and Portevin-Le Chatelier（PLC） phenomenon of the as-extruded Mg-4%Li-6%Zn-1.2%Y alloy before and after heat treatment have been investigated.It has been demonstrated that for the as-extruded and solid solution treated（T4） samples,the PLC phenomenon could be obviously observed on tensile stress-strain curves.Moreover,the PLC phenomenon in T4 samples was more salient than that in the as-extruded condition,suggesting that the occurrence of PLC phenomenon was closely related to the super-saturation degree of solute atoms in the matrix.Since most of solute atoms were consumed for the formation of Mg Zn precipitates（β1′ and a little of β2′） during the subsequent ageing treatment（T6）,the PLC phenomenon of T6 samples was eliminated.Meanwhile,due to the pinning effect of the formed Mg Zn precipitates on mobile dislocations,the tensile strength of T6 samples was relatively higher than those of the other two conditions.

Effect of Icosahedral Phase on Crystallographic Texture and Mechanical Anisotropy of Mg–4%Li Based Alloys

Through investigating and comparing the microstructure and mechanical properties of the as-extruded Mg alloys Mg-4%Li and Mg-4%Li-6%Zn-l.2%Y （in wt%）, it demonstrates that although the formation of I-phase （Mg3Zn6Y, icosahedral structure） could weaken the crystallographic texture and improve the me- chanical strength, the mechanical anisotropy in terms of strength remains in Mg-4%Li-6%Zn-1.2%Y alloy. Failure analysis indicates that for the Mg-4%Li alloy, the fracture surfaces of the tensile samples tested along transverse direction （TD） contain a large number of plastic dimples, whereas the fracture surface exhibits quasi-cleavage characteristic when tensile samples were tested along extrusion direction （ED）. For the Mg-4%Li-6%Zn-I.2%Y alloy, typical ductile fracture surfaces can be observed in both ＂TD＂ and ＂ED＂ samples. Moreover, due to the zonal distribution of broken l-phase particles, the fracture surface of ＂TD＂ samples is characterized by the typical ＂woody fracture＂.

Relationship between the fatigue behavior and grain structures of an as-extruded Mg-6.2%Zn-0.6%Zr(in wt.%)alloy

Through investigating the tension-tension fatigue behavior of an as-extruded Mg-6.2 wt.%Zn-0.6 wt.%Zr(ZK60)alloy,it revealed that the determined fatigue strength at 107 cycles was quite sensitive to the grain structure.Among them,the fine grain structure had the highest fatigue strength of 130 MPa,whereas the typical“bi-modal”grain structure had the lowest fatigue strength of 110 MPa.Failure analysis demon-strated that for the fine grain structure,fatigue cracks preferentially nucleated at grain boundaries.For the“bi-modal”and coarse grain structures,the fatigue crack initiation was dominated by the cracking along slip bands.