Simultaneous refinement of α-Mg grains and β-Mg_(17)Al_(12) in Mg-Al based alloys via heterogeneous nucleation on Al_(8)Mn_(4)Sm

Due to the significant differences in the formation temperature and crystal structure between the primaryα-Mg and eutecticβ-Mg_(17)Al_(12),it is a great challenge to achieve simultaneous refinement of the primary and eutectic phases in Mg-Al based alloys via heterogeneous nucleation.Surprisingly,we found that theα-Mg andβ-Mg_(17)Al_(12) in the AZ80 alloy can be simultaneously refined after 0.2 wt.%Sm addition,with the grain size decreasing from∼217±15μm to∼170±10μm and theβ-Mg_(17)Al_(12) morphology changing from a typical continuous network to a nod-like or spherical structure.The simultaneous refinement mechanism is investigated through solidification simulation,transmission electron microscopy(TEM),and differential thermal analysis(DTA).In the AZ80-0.2Sm alloy,many Al8Mn4Sm particles can be observed near the center of theα-Mg grains or inside theβ-Mg_(17)Al_(12).Crystallographic calculations further reveal that the Al8Mn4Sm has good crystallographic matching with both theα-Mg andβ-Mg_(17)Al_(12),so it possesses the potency to serve as heterogeneous nucleation sites for both phases.The promoted heterogeneous nucleation on the Al8Mn4Sm decreases the undercooling required by the nucleation of the primary and eutectic phases,which enhances the heterogeneous nucleation rate,thus causing the simultaneous refinement of theα-Mg andβ-Mg_(17)Al_(12).The orientation relationships between the Al8Mn4Sm and Mg/Mg_(17)Al_(12) are identified,which are[1210]_(Mg)//[010]_(Al8Mn4Sm),(1010)_(Mg)//(301)_(Al8Mn4Sm) and[112]_(Mg_(17)Al_(12))//[010]_(Al8Mn4Sm),(110)_(Mg_(17)Al_(12))//(301)_(Al8Mn4Sm),respectively.Furthermore,the refinement of theβ-Mg_(17)Al_(12) accelerates its dissolution during the solution treatment,which is beneficial for cost saving in industrial applications.Other Al8Mn4RE compounds such as Al8Mn4Y might have the same positive effect on the simultaneous refinement due to the similar physicochemical properties of rare earth elements.This work not only proves the possibility of simultaneously refining the primary and eutectic phases in Mg-Al based alloys via heterogeneous nucleation,but also provides new insights into the development of refiners for cast Mg alloys.

Recent progress in self-repairing coatings for corrosion protection on magnesium alloys and perspective of porous solids as novel carrier and barrier

Featuring low density and high specific strength, magnesium(Mg) alloys have attracted wide interests in the fields of portable devices and automotive industry. However, the active chemical and electrochemical properties make them susceptible to corrosion in humid, seawater, soil,and chemical medium. Various strategies have revealed certain merits of protecting Mg alloys. Therein, engineering self-repairing coatings is considered as an effective strategy, because they can enable the timely repair for damaged areas, which brings about long-term protection for Mg alloys. In this review, self-repairing coatings on Mg alloys are summarized from two aspects, namely shape restoring coatings and function restoring coatings. Shape restoring coatings benefit for swelling, shrinking, or reassociating reversible chemical bonds to return to the original state and morphology when coatings broken;function self-repairing coatings depend on the release of inhibitors to generate new passive layers on the damaged areas. With the advancement of coating research and to fulfill the demanding requirements of applications, it is an inevitable trend to develop coatings that can integrate multiple functions(such as stimulus response, self-repairing, corrosion warning,and so on). As a novel carrier and barrier, porous solids, especially covalent organic frameworks(COFs), have been respected as the future development of self-repairing coatings on Mg alloys, due to their unique, diverse structures and adjustable functions.

Hardening effect and precipitation evolution of an isothermal aged Mg-Sm based alloy

The age-hardening behavior and precipitation evolution of an isothermal aged Mg-5Sm-0.6Zn-0.5Zr(wt.%) alloy have been systematically investigated by means of transmission electron microscopy(TEM) and atomic-resolution high-angle annular dark field scanning transmission electron microscopy(HAADF-STEM). The Vickers hardness of the present alloy increases first and then decreases with ageing time. The sample aged at 200 ℃ for 10 h exhibits a peak-hardness of 90.5 HV. In addition to the dominant β_(0)’ precipitate(orthorhombic,a = 0.642 nm, b = 3.336 nm and c = 0.521 nm) formed on {11-20}α planes, a certain number of γ’’ precipitate(hexagonal, a = 0.556 nm and c = 0.431 nm) formed on basal planes are also observed in the peak-aged alloy. Significantly, the basal γ’’ precipitate is more thermostable than prismatic β_(0)’ precipitate in the present alloy. β_(0)’ precipitates gradually coarsened and were even likely to transform into β_(1) phase(face centered cubic, a = 0.73 nm) with the increase of ageing time, which accordingly led to a gradual decrease in number density of precipitates and finally resulted in the decreased hardness and mechanical property in the over-aged alloys.

Sc/Zr ratio-dependent mechanisms of strength evolution and microstructural thermal stability of multi-scale hetero-structured Al-Mg-Sc-Zr alloys

Microstructure and its thermal stability are critical in the development of high-performance Al-Mg alloys.Here,we attempt to tailor Al_(3)(Sc,Zr)precipitates and thus microstructure characteristics to manipulate mechanical properties and microstructural stability of Al-7Mg alloys fabricated by hot extrusion com-bined with two-pass hard-plate rolling via changing Sc/Zr ratio.Increasing Sc/Zr ratio leads to improved strength without any loss of ductility.A strength-ductility synergy,i.e.yield strength of∼548 MPa and ultimate tensile strength of∼605 MPa with an impressive ductility of∼10%elongation was achieved in the Al-7Mg-0.3Sc-0.1Zr alloy.The good strength-ductility synergy is ascribed to the multi-scale het-erogeneous microstructure promoted by the high Sc/Zr ratio,i.e.a bimodal grain structure,profuse low angle grain boundaries,dispersed nano-sized Al_(3)(Sc,Zr)precipitates coexisting with intragranular Mg-Zr co-clusters segregated at dislocations.Upon thermal exposure,the Al-7Mg-0.3Sc-0.1Zr alloy maintained higher hardness at below 250°C,whereas Al-7Mg-0.2Sc-0.2Zr and Al-7Mg-0.1Sc-0.3Zr alloys exhibited higher hardness in moderate-and high-temperature range of 250-350℃and≥400℃,respectively.Atom-probe tomography analysis illustrates that slow-diffusing Zr atoms enhance Al_(3)(Sc,Zr)coarsening resistance through forming a higher-content Zr-enriched protective shell around a Sc-enriched core in Al-7Mg-0.1Sc-0.3Zr.Meanwhile,the high Zr content promotes concurrent Al_(3)(Sc,Zr)precipitation during thermal exposure at high temperatures.The improved microstructural thermal stability in Al-7Mg-0.1Sc-0.3Zr alloy is further discussed in terms of the recrystallization resistance and grain growth behavior.The present study reveals the feasibility for designing high-strength and thermally stable hetero-structured Al-Mg-Sc-Zr alloys via tailoring Sc/Zr ratios for different application temperature ranges.

Effect of large thickness-reduction on microstructure evolution and tensile properties of Mg-9Al-1Zn alloy processed by hard-plate rolling

The effect of large thickness-reduction on microstructure evolution and tensile properties of Mg-9 Al-1 Zn alloy(AZ91)processed by hard-plate rolling(HPR)was investigated.Increasing rolling reduction from55%to 85%increases the volume fraction and refines average size of fine grains(<3μm,FGs),leading to an optimized bimodal-grained structure consisting of coarse grains(CGs)uniformly embedded in FG regions.The sample with 85%reduction exhibits the highest yield strength of~314 MPa,ultimate tensile strength of~381 MPa and elongation of~11%.The high strength is primarily due to the contribution of grain boundaries(GBs)strengthening by FGs(accounting for~65%of strength),meanwhile the improved ductility originates from the optimized bimodal-grained structure and weakened basal texture that favor a higher ductility.The present findings successfully overcome the trade-off dilemma that the largereduction rolling processing on Mg alloys usually enhances strength at expense of ductility.In addition,the intensified heterogeneous deformation and favorable formation of a bimodal-grained microstructure during large-reduct ion HPR was addressed by tracing micro structure evolution details in grains of intere st via quasi-in-situ electron back scattering diffraction(EBSD).The present study can be instructive for further designing novel Mg alloys by tailoring bimodal-grained structures for superior combination of mechanical properties.

A simultaneous improvement of both strength and ductility by Sn addition in as-extruded Mg-6Al-4Zn alloy

Commercial wrought Mg alloys normally contain low alloying contents to ensure good formability.In the present work,high-alloyed Mg-6 Al-4 Zn-x Sn(x=1,2 and 3 wt.%,respectively)alloys were fabricated by extrusion.Hereinto,Sn was proven to play an effective contribution to simultaneous improvement in strength and ductility that are traditional trade-off features of synthetic materials.It was found that the average grain size of those alloys decreases significantly from^11 to^4μm as a function of Sn contents increasing from 0 to 3 wt.%,while the amounts of Mg2 Sn and Mg17 Al12 particles continuously increase.More importantly,the addition of Sn leads to the transformation of dominated deformation modes from{1012}extension twinning(1 wt.%)to pyramidal slip(3 wt.%)during tensile tests along the extrusion direction at room temperature.The advantageous combination of ultimate tensile strength(~366 MPa)and elongation(~19%)in Mg-6Al-4Zn-3 Sn alloy is mainly attributed to the strong strain hardening ability induced by the enhanced activity of non-basal slip.This work could provide new opportunities for the development of high-alloyed wrought Mg alloys with promising mechanical properties.

Microstructure and mechanical properties of Al-Mg-Si alloy fabricated by a short process based on sub-rapid solidification

Al-Mg-Si(AA6xxx)series alloys have been used widely in automotive industry for lightweight purpose.This work focuses on developing a short process for manufacturing Al-0.5Mg-1.3Si(wt.%)alloy sheets with good mechanical properties.Hereinto,a preparation route without homogenization was proposed on the basis of sub-rapid solidification(SRS)technique.The sample under SRS has fine microstructure and higher average partition coefficients of solute atoms,leading to weaker microsegregation owing to the higher cooling rate(160℃/s)than conventional solidification(CS,30℃/s).Besides,Mg atoms tend to be trapped in Al matrix under SRS,inducing suppression of Mg2Si,and promoting generation of Al Fe Si phase.After being solution heat treated(T4 state),samples following the SRS route have lower yield strength compared with that by CS route,indicating better formability in SRS sample.After undergoing pre-strain and artificial aging(T6 state),the SRS samples have comparable yield strength to CS samples,satisfying the service requirements.This work provides technological support to industrially manufacture high performance AA6xxx series alloys with competitive advantage by a novel,short and low-cost process,and open a door for the further development of twin-roll casting based on SRS technique in industries.

Balancing the strength and ductility of Mg-6Zn-0.2Ca alloy via sub-rapid solidification combined with hard-plate rolling

In this study,we successfully prepared a Mg-6Zn-0.2Ca alloy by utilizing sub-rapid solidification(SRS)combined with hard-plate rolling(HPR),whose elongation-to-failure increases from~17%to~23%without sacrificing tensile strength(~290 MPa)compared with its counterpart processed via conventional solidification(CS)followed by HPR.Notably,both samples feature a similar refined grain structure with an average grain size of~2.1 and~2.5μm,respectively.However,the high cooling rate of~150 K/s introduced by SRS modified both the size and morphology of Ca_(2)Mg_(6)Zn_(3) eutectic phase in comparison to those coarse ones under CS condition.By subsequent HPR,the Ca_(2)Mg_(6)Zn_(3) phase was further refined and dispersed uniformly by severe fragmentation.Specially,the achieved supersaturation containing excessive Ca solute atoms due to high cooling rate was maintained in the SRS-HPR condition.The mechanisms that govern the high ductility of the SRS-HPR sample could be ascribed to following reasons.First,refined Ca_(2)Mg_(6)Zn_(3) eutectic phase could effectively alleviate or avoid the crack initiation.Furthermore,excessive Ca solute atoms inα-Mg matrix result in the yield point phenomenon and enhanced strain-hardening ability during tension.The findings proposed a short-processed strategy towards superior performance of Mg-6Zn-0.2Ca alloy for industrial applications.

Aging Behavior of Nano-SiC/2014Al Composite Fabricated by Powder Metallurgy and Hot Extrusion Techniques

The aging-hardening kinetics of powder metallurgy processed 2014Al alloy and its composite have been studied. The existence of n-SiC particulates leads to the increase of peak hardness. Interestingly, the aginghardening peak of the composite takes place at earlier time than that of the unreinforced alloy. Transmission electron microscopy（TEM） studies indicated that the major precipitation phases are Al_5Cu_2Mn_3 and θ′（Al_2Cu）. Besides, Ω phase appeared in both specimens at peak hardening condition, which has been rarely observed previously in aluminum metal matrix composites without Ag. Accelerated aging kinetics and increased peak hardness may be attributed to the higher dislocation density resulted from the mismatch of coefficients of thermal expansion between n-SiC and 2014Al matrix. The results are beneficial to fabricating high performance composites for the application in automobile field such as pistons, driveshaft tubes, brake rotors, bicycle frames, railroad brakes.

Alloying design and microstructural control strategies towards developing Mg alloys with enhanced ductility

Nowadays,magnesium(Mg)alloys are promising lightweight structural materials,especially in transportation and aerospace fields,due to their inherent low density and high specific strength.Most of the high-strength Mg alloys exhibit poor formability and ductility at room temperature,which limit their wide applications.However,by proper alloying design and/or delicate microstructural control,some newly developed Mg alloys,including rare-earth(RE)and RE-free ones,show enhanced ductility without significant loss of strength.To identify the critical reasons,recent researches on ductile Mg alloys have been reviewed from the aspects of alloying design strategies and microstructural control via advanced processing technologies.Moreover,some outlooks on enhanced ductility of Mg alloys are suggested,e.g.enhancing the beneficial effect of solute atoms,introducing second phase particles,tailoring bimodal-grained structures,introducing pre-twinning structures,etc.The current research progresses in alloying design and/or novel microstructural control have shed some lights on designing and producing Mg alloys with enhanced ductility.

A rolled Mg-8Al-0.5Zn-0.8Ce alloy with high strength-ductility synergy via engineering high-density low angle boundaries

Developing low-cost rolled Mg alloys with both high strength and ductility is desirable,while the improved strength is generally accompanied with decreased ductility.Here,by using rotated hard-plate rolling(RHPR)with a total thickness reduction of~85%,we obtained a Mg-8Al-0.5Zn-0.8Ce(wt.%,AZ80-0.8Ce)alloy with a high strength-ductility synergy,i.e.,the yield strength(YS),ultimate tensile strength(UTS)and elongation-to-failure(EF)are~308 MPa,~360 MPa and~13.8%,respectively.It reveals that the high YS is mainly originated from grain boundary strengthening(~212 MPa),followed by dislocation strengthening(~43 MPa)and precipitation hardening(~25 MPa).It is found that a relatively homogeneous fine grain structure containing a large fraction(~62%)of low angle boundaries(LABs)is achieved in the RHPRed alloy,which is benefit for the high tensile EF value.It demonstrates that LABs have important contributions to strengthening and homogenizing tensile deformation process,leading to the simultaneous high strength and high EF.Our work provides a new insight for fabrication of low-cost high performance Mg alloys with an excellent strength-ductility synergy.

High-capacity potassium ion storage mechanisms in a soft carbon revealed by solid-state NMR spectroscopy

Carbon materials are crucially important for the realization of potassium-ion batteries.However,the potassium storage mechanisms in various carbon materials are incompletely understood.Herein,solid-state ^(13)C nuclear magnetic resonance(NMR) spectroscopy coupled with Raman and X-ray diffraction(XRD) techniques are employed to study the reaction mechanism in a soft carbon quantitatively.It is revealed that the insertion of potassium ions into the soft carbon firstly induces a transformation of the disordered region to short-range ordered stacking,involving both the pristine local unorganized and organized carbon layers.Subsequently,potassium ions intercalate into the rearranged carbon structure,finally producing the nano-sized KC_(8).Moreover,a remarkable c apacity of 322 mAh·g^(-1) with a low mid potassiation voltage of <0.3 V is present for the prepared soft carbon,which is on account of the underlying potassium storage sites,including the disordered stacking carbon as a main component of the soft carbon.These results suggest that regulating the disordered stacking region in the turbostratic structure of soft carbon is a critical issue for further improving the potassium storage performance.

A corrosion-resistant and age-hardenable Mg-Al-Mn-Ca-Ce dilute alloy with fine-grained structure processed by controlled rolling

Low strength and poor corrosion resistance are two long-standing bottlenecks with dilute Mg alloys.Here,we report that the corrosion resistance of Mg-0.6Al-0.5Mn-0.2Ca(wt.%)alloy sheet can be significantly improved by micro-alloying with 0.3 wt.%Ce,and the strength can be considerably augmented after aging treatment.Simultaneous optimization of strength,ductility,and corrosion resistance is difficult due to the inherent trade-off in Mg alloys.Surprisingly,our aged Mg-0.6Al-0.5Mn-0.2Ca-0.3Ce alloy features with yield strength(YS)of∼194 MPa,ultimate tensile strength(UTS)of∼265 MPa,elongation to failure(EF)of∼17.2%,and corrosion rate(CR)of 2.2 mm y^(−1),the combination of which exhibits competitive advantage over other comparative dilute Mg alloys.It is found that Ce addition decreases the activity of cathodic phases,inhibits detrimental effects of Fe impurities,and forms a protective Ce-containing surface film.The high strength stems from the precipitation of ordered Guinier-Preston(G.P.)zones dispersed in uniform fine grains with an average grain size of∼8.2μm.The atomic-scale G.P.zones result in a noticeable age hardening response but do not act as crack initiation and propagation site,so our alloy also performs satisfactory ductility.This work sheds light on the design and fabrication of strong,ductile,and corrosion-resistant dilute magnesium alloys to be used in electronic products and automotive bodies.

Dislocation behavior in a polycrystalline Mg-Y alloy using multi-scale characterization and VPSC simulation

In this study,the dislocation behavior of a polycrystalline Mg-5Y alloy during tensile deformation was quantitatively studied by an in-situ tensile test,visco-plastic self-consistent(VPSC)modeling,and transmission electron microscopy(TEM).The results of the in-situ tensile test show that<a>dislocations contribute to most of the deformation,while a small fraction of<c+a>dislocations are also activated near grain boundaries(GBs).The critical resolved shear stresses(CRSSs)of different dislocation slip systems were estimated.The CRSS ratio between prismatic and basal<a>dislocation slip in the Mg-Y alloy(~13)is lower than that of pure Mg(~80),which is considered as a major reason for the high ductility of the alloy.TEM study shows that the<c+a>dislocations in the alloy have high mobility,which also helps to accommodate the deformation near GBs.

Towards relieving center segregation in twin-roll cast Al-Mg-Si-Cu strips by controlling the thermal-mechanical process

Serious center segregation greatly limits the application of twin-roll casting(TRC)technology for produc-ing 6xxx alloy strips.Herein,Al-0.9Mg-0.6Si-0.2Cu-0.1Fe(wt.%,6061)strips with different thicknesses were fabricated by TRC,and we found that the center segregation was well relieved with the thick-ness increased from 3 mm to 4 mm.To reveal the mechanisms of mitigation of center segregation in the 4 mm strip,various techniques including solidification simulation,crystallographic calculation,elec-tron backscatter diffraction(EBSD),and electron probe micro-analyzer(EPMA)were utilized.The re-sults disclosed that the Fe-containing phase in the 3 mm strip wasπ-AlFeMgSi,while the counterpart in the 4 mm strip wasα-AlFeSi.Theα-AlFeSi could serve as nucleation substrates for Mg_(2)Si and Q-AlCuMgSi phases,thus promoting the uniform distribution of elements and preventing the accumulation of phases in the center region.Three matching planes between theα-AlFeSi and Q/Mg_(2)Si were exam-ined as:(1120)_(α-AlFeSi)//(0001)_(Q),(0001)_(α-AlFeSi)//(110)_(Mg2Si),and(1120)_(α-AlFeSi)//(110)_(Mg2Si).Meanwhile,the smaller roll separating force during the TRC process in the 4 mm strip could weaken the force-induced liquid flow behavior in the semi-solid region,which is the other reason for the alleviation of center seg-regation.Owing to the elimination of the center segregation,a more excellent fracture elongation was achieved in the as-homogenized 4 mm strip(∼29%)compared with the counterpart of the 3 mm strip(∼20%).This work may provide a strategy to eliminate the center segregation,thus further promoting the application of TRC process and producing high-performance Al alloy strips efficiently.

Stabilizing a severely deformed Al-7Mg alloy with a multimodal grain structure via Mg solute segregation

Single-phase Al-Mg alloys processed by severe plastic deformation(SPD)usually suffer from unsatisfactory thermal stability at moderate to high temperatures with recrystallization occurring and obvious grain coarsening.In the present work,an Al-7Mg alloy prepared by equal-channel angular pressing(ECAP)possessed markedly enhanced thermal stability upon annealing at moderate to high temperatures(200-275℃),compared with those ultrafine-grained dilute Al-Mg alloys with a uniform microstructure.The enhanced thermal stability is due primarily to the multimodal grain structure consisting of nano-,ultrafine-and micron-sized grains,strong segregation and/or clusters of Mg solute along grain boundaries(GBs),and Al_(3)Mg_(2)precipitates formed during annealing.First,extensive recovery predominates over recrystallization and consumes most of the stored energy in the ECAPed Al-7Mg alloy annealed at≤275℃,leading to the recrystallization and growth of nano/ultrafine grains being retarded or postponed.Moreover,Mg solute segregation and/or clusters along GBs of nano/ultrafine grains could further suppress grain growth via diminishing GB energy and dragging GBs efficiently.In addition,Al_(3)Mg_(2)precipitates formed with increasing annealing time could inhibit grain growth by pinning GBs.The present multimodal-grained Al-7Mg alloy with enhanced thermal stability is believed to be particularly attractive in potential engineering applications at moderate to high temperatures.