Inhibiting effect of I-phase formation on the plastic instability of the duplex structured Mg-8Li-6Zn-1.2Y(in wt.%)alloy

In this work,the tensile behaviors of Mg-8%Li and Mg-8%Li-6%Zn-1.2%Y alloys at ambient temperature were investigated and compared.It revealed that the plastic instability of Mg-8%Li alloy was quite remarkable and the variation range of serrated flowing stress can reach5 MPa.For the Mg-8%Li-6%Zn-1.2%Y alloy,the formation of I-phase(Mg_(3)Zn_(6)Y)can simultaneously enhance the tensile strength and eliminate the plastic instability phenomenon,whilst its ductility was degraded.The in-situ tensile tests revealed that for the Mg-8%Li alloy,the severity and number of slip traces present in bothα-Mg andβ-Li matrix phases increased remarkably with the applied tensile strain.However,slip traces were quite fine inβ-Li matrix phase and could be clearly observed when the applied tensile strain exceeded 18%.Due to the incompatibility of plastic deformation occurred in two matrix phases,the induced strain concentration atα-Mg/β-Li interfaces caused their subsequent cracking.For the Mg-8%-6%Zn-1.2%Y alloy,the I-phase distributed atα-Mg/β-Li interfaces suppressed the plastic deformation ofα-Mg matrix phase and the tensile strain was dominated by theβ-Li matrix phase,resulting in the disappearance of plastic instability.Moreover,the plastic strain would preferentially concentrate at I-phase/β-Li interfaces and subsequently induced the cracking of I-phase.

Effect of rolling ratios on the microstructural evolution and corrosion performance of an as-rolled Mg-8wt.%Li alloy

In this work,the influence of rolling ratios on microstructural changes and corrosion behavior of an as-rolled Mg-8 wt.%Li alloy in0.1 mol/L Na Cl solution has been investigated.It revealed that with the rolling ratio being increased from 3 to 10,theα-Mg phases were elongated and fragmented,whilst the area fraction of exposedβ-Li phases increased.Meanwhile,the corrosion performance of the alloy decreased with the increased rolling ratios.For all the samples,their corrosion processes were quite similar and can have two stages.At the initial stage with the samples being immersed for less than 6 h,the corrosion mainly occurred inβ-Li phases.When the samples were immersed for longer than 6 h,the corrosion attack transferred toα-Mg phases and the hydrogen evolution rate was accelerated.

Effects of annealing treatment on microstructure and tensile behavior of the Mg-Zn-Y-Nd alloy

In the present study,the Mg-4Zn-0.6Y-0.5Nd alloy was hot extruded and annealed at 200℃,225℃ and 250℃ for different time to optimize microstructure and mechanical properties.The results exhibit that the dual-size grain structure and linearly distributed secondary phase are the main feature of the as-extruded Mg-Zn-Y-Nd alloy,which can be described as the elongated grain is surrounded by the fine equiaxed grain.Moreover,the as-extruded alloy shows strong{011^(-)0}fiber texture feature,especially for the large elongated grains.The annealing treatment results in static recrystallization,which increases fine equiaxed grains but decreases large elongated grains.In addition,the equiaxed grains formed during the annealing treatment demonstrate relative random orientations,which weaken the{011^(-)0}fiber texture of the alloy.Moreover,during the annealing at 225℃and 250℃,the extension twins begin to form in the alloy and weaken the{011^(-)0}fiber texture of the alloy further.The annealing treatment has little influence on the linearly distributed secondary phase but promotes the coarsening of small precipitates at 250℃.The annealing treatment could increase the yield and ultimate strength,but the elongation decreases,especially at higher temperature.Such a variation can be ascribed to the evolution of texture,grain structure,twinning and precipitation during the annealing treatment.

FAST Observations of an Extremely Active Episode of FRB 20201124A:Ⅰ.Burst Morphology

We report the properties of more than 600 bursts(including cluster-bursts)detected from the repeating fast radio burst(FRB)source FRB 20201124A with the Five-hundred-meter Aperture Spherical radio Telescope during an extremely active episode on UTC 2021 September 25–28,in a series of four papers.The observations were carried out in the band of 1.0–1.5 GHz by using the center beam of the L-band 19-beam receiver.We monitored the source in sixteen1 hr sessions and one 3 hr session spanning 23 days.All the bursts were detected during the first four days.In this first paper of the series,we perform a detailed morphological study of 624 bursts using the two-dimensional frequencytime“waterfall”plots,with a burst(or cluster-burst)defined as an emission episode during which the adjacent emission peaks have a separation shorter than 400 ms.The duration of a burst is therefore always longer than 1 ms,with the longest up to more than 120 ms.The emission spectra of the sub-bursts are typically narrow within the observing band with a characteristic width of~277 MHz.The center frequency distribution has a dominant peak at about 1091.9 MHz and a secondary weak peak around 1327.9 MHz.Most bursts show a frequencydownwarddrifting pattern.Based on the drifting patterns,we classify the bursts into five main categories:downward drifting(263)bursts,upward drifting(3)bursts,complex(203),no drifting(35)bursts,and no evidence for drifting(121)bursts.Subtypes are introduced based on the emission frequency range in the band(low,middle,high and wide)as well as the number of components in one burst(1,2,or multiple).We measured a varying scintillation bandwidth from about 0.5 MHz at 1.0 GHz to 1.4 MHz at 1.5 GHz with a spectral index of 3.0.

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.

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.

Microstructural characteristics and mechanical properties of the hot extruded Mg-Zn-Y-Nd alloys

A new type of Mg-Zn-Y-Nd alloy for degradable orthopedic implants was developed.In the present study,the Zn and Y content was adjusted and their influences on the microstructures and mechanical behaviors were discussed in depth.The results showed that the as-extruded Mg-Zn-Y-Nd alloys are mainly composed of fine dynamic recrystallized grains(DRXed grains),la rge unDRXed grains and linearly distributed secondary phases.The cha nge of Zn content exerts little influence on the grain structure of the extruded Mg-Zn-Y-Nd alloy,while the increase of Y content would hinder the dynamic recrystallization process and the growth of the DRXed grains,thus the size and volume fraction of the equiaxed DRXed grains decrease.The tensile and compressive properties are very little affected by Zn content because of the similar grain structure.As Y content increases,the tensile yield strength(TYS) and ultimate strength(TUS) increase while the elongation decreases,this is caused by a combined strengthening effect of grain refinement,texture,precipitation and twinning.The compressive yield strength(CYS) and ultimate strength(CUS) of Mg-Zn-Y-Nd alloy with diffe rent Y content exhibit a similar tendency as the tensile test.

Effect of corrosion product films on the in vitro degradation behavior of Mg-3%Al-1%Zn(in wt%) alloy in Hank's solution

Through investigating the corrosion behavior of an as-extruded Mg-3wt%gAl-lwt%Zn （AZ31） alloy in a simulated physiological fluid of Hank＇s solution, it demonstrates that the corrosion process was depen- dent on the immersion time. Further analyses revealed that the highest corrosion resistance could be obtained at 24 h due to the formation of a compact layer of corrosion products on the sample surface. With increasing the immersion time to up to 48 h, the thickness of surface films increased gradually but obvious de-bonding of such film from the substrate could take place, resulting in a certain resilience of the overall corrosion resistance.

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.

Structural evolution of Al–8%Si hypoeutectic alloy by ultrasonic processing

High intensity power ultrasound was respectively introduced into three different solidification stages of Al–8%Si hypoeutectic alloy, including the fully liquid state before nucleation, the nucleation and growth process of primary α（Al） phase and L →（Al） ＋（Si） eutectic transformation period. It is found that both the primary α（Al） phase and（Al ＋ Si） eutectic structure were refined by different degrees with various growth morphologies depending on the ultrasonic treatment stage. Based on the experimental results,the cavitation-induced nucleation due to the high undercooling caused by the collapse of tiny cavities was proposed as the major reason for refining the primary α（Al） phase. Meanwhile, obvious eutectic morphological change was observed only when ultrasound was directly introduced in the eutectic transformation stage, in which typical divorced eutectics and（Al ＋ Si） eutectic cells with symmetrical flower shape were formed at the top of the alloy sample. The introduction of ultrasound in each solidification stage also improves the yield strength of Al–8% Si alloy to a diverse extent.