In-situ study of the microstructure evolution during tension of a Mg-Y-Zn-Al alloy processed by rapidly solidified ribbon consolidation technique

Mg-Y-Zn-Al alloys processed by rapidly solidified ribbon consolidation(RSRC)technique exhibit an exceptional mechanical performance indicating promising application potential.This material has a bimodal microstructure consisting of fine recrystallized and coarse non-recrystallized grains with solute-rich stacking faults forming cluster arranged layers(CALs)and nanoplates(CANaPs),or complete long period stacking ordered(LPSO)phase.In order to reveal the deformation mechanisms,in-situ synchrotron X-ray diffraction line profile analysis was employed for a detailed study of the dislocation arrangement created during tension in Mg-0.9%Zn-2.05%Y-0.15%Al(at%)alloy.For uncovering the effect of the initial microstructure on the mechanical performance,additional samples were obtained by annealing of the as-consolidated specimen at 300 and 400℃ for 2 h.The heat treatment at 300℃ had no significant effect on the initial microstructure,its evolution during tension and,thus,the overall deformation behavior under tensile loading.On the other hand,annealing at 400℃ resulted in a significant increase of the recrystallized grains fraction and a decrease of the dislocation density,leading to only minor degradation of the mechanical strength.The maximum dislocation density at the failure of the samples corresponding to the plastic strain of 10-25% was estimated to be about 16-20×10^(14)m^(-2).The diffraction profile analysis indicated that most dislocations formed during tension were of non-basal and pyramidal types,what was also in agreement with the Schmid factor values revealed independently from orientation maps.It was also shown that the dislocation-induced Taylor hardening was much lower below the plastic strain of 3% than above this value,which was explained by a model of the interaction between prismatic dislocations and CANaPs/LPSO plates.

Microstructure characterization and corrosion behavior of Mg-Y-Zn alloys with different long period stacking ordered structures

Mg-Y-Zn alloys with long period stacking ordered(LPSO)structure have received much attention recently and exhibit great potential in applications such as automotive,aerospace and in bio-medical fields.This paper aimed to investigate the effect of different phase constitution of LPSO structures on corrosion rate of bio-medical Mg-Y-Zn alloys.The results showed that as-cast Mg98.5Y1Zn0.5 alloys containing only 18R structure exhibited the highest corrosion resistance with the corrosion rate of 2.78 mm/year.The precipitation of 14H lamellas within a-Mg grains during solid solution treatment introduced the crystallographic orientation corrosion by accelerating micro-galvanic corrosion.The increase of 18R/14H interfaces deteriorated the corrosion resistance,and the grain boundaries also suffered from severe electrochemical dissolution.This work suggested that Mg-Y-Zn alloys with single LPSO structure(either 18R or 14H)exhibited better corrosion resistance than alloys with co-existence 18R and I4H LPSO structures.

Recent developments on corrosion behaviors of Mg alloys with stacking fault or long period stacking ordered structures

Corrosion is one of the most drawbacks which restricts the wide applications of Mg alloys.In the last decade,the corrosion behaviors of Mg alloys with stacking fault(SF)and/or long period stacking ordered(LPSO)structures have obtained increasing attention.However,the corrosion mechanism of the SF–or LPSO–containing Mg alloys has not been well illustrated and even reverse results have been reported.In this paper,we have reviewed recent reports on corrosion behaviors of SF–or LPSO–containing Mg alloys to better clarify and understand the significance and mechanism.Moreover,some deficiencies are presented and advises are proposed for the development of corrosion resistant Mg alloys with SF or LPSO structures.

Effect of Li on formation of long period stacking ordered phases and mechanical properties of Mg-Gd-Zn alloy

Alloys with composition of Mg_(96-x)Gd_3Zn_1Li_x(at.%)(x=0, 2, 4, and 6) were prepared by conventional casting. The microstructures of these alloys under as-cast and solid-solution conditions have been observed, and the mechanical properties were investigated. The results showed that Li is an effective element to refine the grains and break the eutectic networks in as-cast MgGd_3Zn_1 alloy. During solid solution treatment, these broken eutectic networks are spheroidized and highly dispersed. In addition, plentiful lamellar long period stacking ordered(LPSO) phases are precipitated in an α-Mg matrix when the Li addition is not more than 4%. Solid-solution treated Mg_(92)Gd_3Zn_1Li_4 alloy exhibits an optimal ultimate tensile strength(UTS) of 226 MPa and elongation of 5.8%. The strength of MgGd_3Zn_1 alloy is improved significantly, meanwhile, the toughness is apparently increased.

Effect of long period stacking ordered phase on damping capacities of as-cast Mg-Zn-Y alloys

The microstructure and damping capacities of MgZnxYi.33x(x=l-4at.%)alloys were discussed and researched.The main phase composition of the alloys consists of a_Mg and long-period stacking ordered(LPSO)phase.Due to increasedLPSO phase,grain size was refined.LPSO phase was advantageous to the damping properties of the Mg-Zn-Y alloys.Mg-7%Zn-12.8%Y has the highest damping capacity up to0.04.Due to stacking fault probability,the LPSO phase in the Mg-Zn-Yalloys could be new damping source to dissipate energy so as to contribute to the improvement of damping capacities.

Microstructure and mechanical properties of a compound reinforced Mg95Y2.5Zn2.5 alloy with long period stacking ordered phase and W phase

The microstructure evolution of Mg100-2xYxZnx （x=2, 2.5, 3, 3.5） alloys was investigated. Results show that the Mg100-2xYxZnx alloys are composed of a-Mg, long period stacking ordered （LPSO） phase and eutectic structure phase （W phase）, and the Mg95Y2.5Zn2.5 alloy has the best comprehensive mechanical properties. Subsequently, the microstructure evolution of the optimized alloy Mg95Y2.5Zn2.5 during solidification and heat treatment processes was analyzed and discussed by means of OM, SEM, TEM, XRD and DTA. After heat treatment, the lamellar phase 14H-LPSO precipitated in a-Mg and W phase transforms into particle phase （MgyZn2）. Due to the compound reinforcement effect of the particle phase and LPSO phase （18R＋14H）, the mechanical properties of the alloy are enhanced. The tensile strength and elongation of the Mg95Y2.5Zn2.5 alloy is improved by 9.1% and 31.3% to 215 MPa and 10.5%, respectively, after solid-solution treatment.

Dilute long period stacking/order(LPSO)-variant phases along the composition gradient in a Mg-Ho-Cu alloy

We have systematically investigated the microstructures of as-cast Mg_(97.49)Ho_(1.99)Cu_(0.43)Zr_(0.09)alloy by atomic resolution high-angle annular dark field scanning transmission electron microscopy(HAADF-STEM), revealing the coexistence of 18R, 14H and 24R long period stacking/order(LPSO) phases with fully coherent interfaces along step-like composition gradient in a blocky intermetallic compound distributed at grain boundary. The short-range order(SRO) L1_(2)-type Cu_(6)Ho_(8)clusters embedded across AB’C’A-stacking fault layers are directly revealed at atomic scale. Importantly, the order degree of SRO clusters in the present dilute alloy is significant lower than previous 6M and 7M in-plane order reported in ternary Mg-TM(transition metal)-RE(rare earth) alloys, which can be well matched by 9M in-plane order. This directly demonstrates that SRO in-plane L1_(2)-type clusters can be expanded into more dilute composition regions bounded along the definite TM/RE ratio of 3/4. In addition, the estimated chemical compositions of solute enriched stacking fault(SESF) in all LPSO variants are almost identical with the ideal SESF composition of 9M in-plane order, regardless of the type of LPSO phases. The results further support the viewpoint that robust L1_(2)-type TM_(6)RE_(8)clusters play an important role in governing LPSO phase formation.

Periodic Solutions of a Cooperative System with State Feedback Control

In this paper, we propose a semi-continuous dynamical system to study the cooperative system with feedback control. Based on geometrical analysis and the analogue of Poincare criterion, the existence and stability of the positive order one periodic solutions are given. Numerical results are carried out to illustrate the feasibility of our main results.

Achieving high-strain-rate and low-temperature superplasticity in an ECAP-processed Mg-Y-Er-Zn alloy via Ag addition

The effect of adding a small amount of Ag on the microstructure evolution and superplastic properties of Mg-Y-Er-Zn(WEZ612) alloys was systematically studied.The basal texture of the refined WEZ612 alloy produced by equal channel angular pressing was altered to a non-basal structure upon the addition of Ag.Ag addition also refined the grain size and promoted the formation of a large number of nano-14H-long period stacking ordered phases.Using high-resolution transmission electron microscopy,many nano-precipitated phases were detected on the basal plane of the Mg-Y-Er-Zn-1Ag(WEZ612-1Ag) alloy,The nano-precipitated phases on the basal plane improved the thermal stability of the alloy,lowered the deformation activation energy(Q),and improved the stress sensitivity index(m).At 523 K with a strain rate of 10^(-2) s^(-1),the Q value of WEZ612 was higher than that of WEZ612-1Ag(299.14 and 128.5 kJ mol^(-1),respectively).In contrast,the m value of the WEZ612 alloy(0.16) was lower than that of the WEZ612-1Ag alloy(0.46).At 623 K with a tensile rate of 10^(-2) s^(-1),the WEZ612 and WEZ612-1Ag alloys were elongated by 182% and 495%,respectively,with the latter exhibiting high-strain-rate and low-temperature superplasticity.The improved superplasticity of the WEZ612-1Ag alloy is attributed to the nano-precipitated phases,which effectively limit the cavity extension during superplastic deformation.

Microstructure and tribological behavior of Mg-Gd-Zn-Zr alloy with LPSO structure

A Mg-14.28Gd-2.44Zn-0.54Zr （mass fraction, %） alloy was prepared by conventional ingot metallurgy （I/M）. The microstructure differences in as-cast and solution-treated alloys were investigated. Sliding tribological behaviors of the as-cast and solution-treated alloys were investigated under oil lubricant condition by pin-on-disc configuration. The wear loss and friction coefficients were measured at a load of 40 N and sliding speeds of 30-300 mm/s with a sliding distance of 5000 m at room temperature. The results show that the as-cast alloy is mainly composed ofα-Mg solid solution, the lamellar 14H-type long period stacking ordered （LPSO） structure within matrix, andβ-[（Mg,Zn）3Gd] phase. However, most of theβ-phase transforms to X-phase with 14H-type LPSO structure after solution heat treatment at 773 K for 35 h （T4）. The solution-treated alloy presents low wear-resistance, because the hard β-phase is converted into thermally-stable, ductile and soft X-Mg12GdZn phase with LPSO structure in the alloy.

Dual phases strengthening behavior of Mg-10Gd-1Er-1Zn-0.6Zr alloy

The microstructure evolution and strengthening mechanisms of Mg-10Gd-1Er-1Zn-0.6Zr(wt.%) alloy were focused in the view of the size parameters and volume fraction(fp) of dual phases(long period stacking ordered(LPSO) structures and β’ precipitates).Results show that two types of LPSO phases with different morphologies formed,and the morphology and size of both LPSO phases varied with the solution conditions.However,the volume fraction decreased monotonously with increasing solution temperature,which in turn raised the volume fraction of β’ phase during aging.The alloy exhibited an ultimate tensile strength of 352 MPa,a yield strength of 271 MPa,and an elongation of 3.5% after solution treatment at 500℃ for 12 h and aging at 200℃ for 114 h.In contrast to the LPSO phase,the β’ phase seems to play a more important role in enhancing the yield strength,and consequently,a decreased fLPSO/fβ’,ratio results in an increased yield strength.

Cooling rate controlled basal precipitates and age hardening response of solid-soluted Mg-Gd-Er-Zn-Zr alloy

The precipitation and age hardening response of the solid-soluted Mg–10Gd–1Er–1Zn–0.6Zr(wt.%)alloy performed by water-quenching(QC),air-cooling(AC)and furnace-cooling(FC)in terms of the volume fraction of precipitates and tensile properties were investigated in present paper.Results indicated the solid-soluted alloy contained stacking faults(SFs)and long period stacking ordered(LPSO)phase on the basal planes regardless of the cooling rate,but a larger volume fraction of the LPSO phase was formed with decreasing in the cooling rate.After aging,βandβ1 phases precipitated on the prismatic planes,and their number density decreased but mean particle size increased with decreasing in the cooling rate.The solid-soluted alloys(QC,AC and FC samples)showed no apparent difference in yield strength(YS),but their correspondent peak-aged alloys exhibited sharp difference in hardening response.The strongest hardening response took place in the QC sample and showed 82MPa enhancement in YS,which was much larger than that of AC(+26MPa)and FC samples(+5MPa).The reason lies in that the higher cooling rate promotes the precipitation and reduces the average size ofβprecipitate.A novel cooling-rate controlled precipitation model with respect to the correlation of precipitates on basal and prismatic planes was established.From this model,the basal precipitates showed a restrictive effect on the growth and/or coarsening ofβprecipitate,and composite precipitates containing theβphase with fine size as well as high area-number density and lower volume fraction of the LPSO phase are preferred to strengthen the Mg–10Gd–1Er–1Zn–0.6Zr alloy.

Towards high strength cast Mg-RE based alloys:Phase diagrams and strengthening mechanisms

Mg-rare earth(RE)based systems provide several important commercial alloys and many alloy development opportunities for high strength applications,especially in aerospace and defense industries.The phase diagrams,microstructure,and strengthening mechanisms of these multicomponent systems are very complex and often not well understood in literature.We have calculated phase diagrams of important binary,ternary,and multicomponent RE-containing alloy systems,using CALPHAD(CALculation of PHAse Diagrams).Based on these phase diagrams,this paper offers a critical overview on phase equilibria and strengthening mechanisms in these alloy systems,including precipitation,long period stacking order(LPSO),and other intermetallic phases.This review also summarized several promising Mg-RE based cast alloys in comparison with commercial WE54 and WE43 alloys;and explored new strategies for future alloy development for high strength applications.It is pointed out that the combination of precipitation and LPSO phases can lead to superior strength and ductility in Mg-RE based cast alloys.The precipitates and LPSO phases can form a complex three-dimensional network that effectively impedes dislocation motion on the basal and non-basal planes.The LPSO phases can also prevent the coarsening of precipitates when they interact,thus providing good thermal stability at elevated temperatures.Future research is needed to determine how the combination of these two types of phases can be used in alloy design and industrial scale applications.

Phase equilibria and microstructure investigation of Mg-Gd-Y-Zn alloy system

In order to develop high strength Mg-Gd-Y-Zn alloys,key experiments coupled with CALPHAD(CALculation of PHAse Diagrams)calculations were carried out in the current work to provide critical understanding of this important alloy system.Three Mg-10 Gd-xY-yZn(x=4 or 5,y=3 or 5,wt.%) were mapped on Mg-Gd-Y-Zn phase diagrams for phase equilibria and microstructure investigation.Electron microscopy was performed for phase identification and phase fraction determination in as-cast and solution treated conditions.In all three alloys,the major phases were Mg-matrix and long period stacking order(LPSO) 14 H phase.With ST at 400 and 500℃,the phase fraction of LPSO 14 H increased,particularly the fine lamellar morphology in the Mg matrix.The as-cast and 400℃ Mg10 Gd5 Y3 Zn samples had Mg(Gd,Y) present.At 500℃,Mg(Gd,Y) is not stable and transforms into LPSO 14 H.The Mg 10 Gd5 Y5 Zn alloy included the WPhase,which showed a reduction in phase fraction with solution treatment.These experimental results were used to validate and improve the thermodynamic database of the Mg-Gd-Y-Zn system.Thermodynamic calculations using the improved database can well describe the available experimental results and make accurate predictions to guide the development of promising high-strength Mg-Gd-Y-Zn alloys.

ON A SYSTEM OF SECOND ORDER DIFFERENTIAL EQUATIONS WITH PERIODIC IMPULSE COEFFICIENTS

A thorough investigation of the systemd^2y(x):dx^2+p(x)y(x)=0with periodic impulse coefficientsp(x)={1,0≤x<x_0(2π>0> -η, x_0≤x<2π(η>p(x)=p(x+2π),-∞<x<∞is given, and the method can be applied to one with other periodic impulse coefficients.

Spherical periodicity as structural homology of crystalline and amorphous states

It has been widely accepted that spherical per- iodicity generally dominates liquid and amorphous structure formation, where atoms tend to gather near spherically peri- odic shells according to Friedel oscillation. Here it is revealed that the same order is just hidden in the atomic global packing modes of the crystalline phases relevant to bulk metallic glasses. Among the multiple nearest-neighbor dusters devel- oped from all the non-equivalent atomic sites in a given phase, there always exists a principal duster, centered by which the spherical periodicity, both topologically and chemically, is the most distinct. Then the principal dusters plus specific glue atoms just constitute the cluster-plus-glue-atom structural units shared by both metallic glasses and the corresponding crystalline phases. It is further pointed out that the spherical periodicity order represents the common structural homology of crystalline and amorphous states in the medium-range through scrutinizing all binary bulk-glass-relevant phases in Cu-（Zr, Hf）, Ni-（Nb, Ta）, Al-Ca, and Pd-Si systems.

Formation Behavior of 14H Long Period Stacking Ordered Structure in Mg–Y–Zn Cast Alloys with Different α-Mg Fractions

Phase compositions and microstructure evolutions of three Mg-Y-Zn cast alloys during isothermal annealing at 773 K have been systematically investigated to clarify the formation behavior of 14 H long period stacking ordered（LPSO） structure from α-Mg grains.The annealed microstructure characteristics indicate that the 18 R phase is thermal stable in Mg86Y8Zn6 alloy where 18 R serves as matrix,and 14 H lamellar phase only forms within tiny α-Mg slices（less than 1% for volume fraction）.The α-Mg grains in Mg88Y8Zn4 and Mg89Y8Zn3 alloys exhibit cellular shape,and 14 H phase forms and develops into lamellar shape in these cellular grains after annealing.The results suggest that the presence of α-Mg grains is a requirement for the generation of 14 H phase.The nucleation and growth rates of 14 H lamellas are accelerated in α-Mg grains with higher concentrations of stacking faults and solute atoms.Moreover,the 14 H lamellas are parallel to adjacent 18 R plates in Mg86Y8Zn6 alloy,but the 14 H phase precipitated in cellularα-Mg grains of Mg88Y8Zn4 and Mg89Y8Zn3 alloys exhibits random orientation relationship with surrounding 18 R phase,indicating that the orientation relationship between 14 H and 18 R phases depends on the relationship between α-Mg grains and 18 R phase.

EFFICIENT SIXTH ORDER P-STABLE METHODS WITH MINIMAL LOCAL TRUNCATION ERROR FOR y"=f(x,y)

A family of symmetric (hybrid) two step sixth P-stable methods for the accurate numerical integration of second order periodic initial value problems have been considered in this paper. These methods, which require only three (new) function evaluation per iteration and per step integration. These methods have minimal local truncation error (LTE) and smaller phase-lag of sixth order than some sixth orders P-stable methods in [1-3,10-11]. The theoretical and numerical results show that these methods in this paper are more accurate and efficient than some methods proposed in [1-3,10].

Microstructure and yield phenomenon of an extruded Mg-Y-Cu alloy with LPSO phase

A yield phenomenon was firstly reported in an extruded Mg-6.8Y-2.5Cu alloy and the corresponding microstructure was also investigated in this work,The cast alloy is mainly composed ofα-Mg,18R long period stacking order(LPSO)phase,eutectic phase(Mg_(20)Cu_(4)Y_(1)),and Mg_(2)Cu phase.The 18R LPSO phase at the dendritic grain boundary transforms into the 14H LPSO phase in the grain interior during homogenization.After extrusion,the grain size of the homogenized alloy is remarkably refined to-3.69μm and the second phase is significantly broken and distributed in the extrusion direction.Tensile testing curves of the extrude alloy at room temperature indicate that the yield strength and ultimate tensile strength increase while the elongation of the alloy decreases with increasing strain rate.Interestingly,a yield plateau fo rms and gradually decreases with increasing strain rate.The yield phenomenon is related to the dislocation multiplication and the interaction between the movable dislocations and solute atoms.

Hot Compression Behavior of Mg-Gd-Y-Zn-Zr Alloy Containing LPSO with Different Morphologies

In this study,hot compression behavior of as-homogenized Mg-9Gd-4Y-2Zn-0.5Zr alloys containing long period ordered(LPSO)phases with different initial morphologies was investigated under a strain rate of 0.01 s−1 at 450°C.The microstructure,texture evolution and dynamic recrystallization(DRX)mechanism under different strains(0.1,0.3,0.5,0.7,and 1.4)were studied.Under compression conditions,the block-shape phase and deformedα-Mg matrix were regularly arranged vertically in the compression direction(CD).The particle phase(Mg5RE)and amount of kink bands were produced.The degree of kink increased initially and decreased significantly when the deformation strain reached 1.4.Three DRX mechanisms were produced during the compression process:(i)The block LPSO,Mg5RE and Zr-Zn phase activated DRX through the particle stimulated nucleation(PSN)mechanism.(ii)Several DRX activated on the serrated grain boundary between the two parent grains through discontinuous DRX(DDRX)mechanism.(iii)A large number of kink bands and larger spacing of lamellar LPSO increased the nucleation of DRX through continuous DRX(CDRX)mechanism.The combination of different recrystallization mechanisms has evident effects on grain refinement.DRX increased as compression proceeds,and the texture was gradually weakened.