Achieving gradient heterogeneous structure in Mg alloy for excellent strength-ductility synergy

Most metals including Mg alloys have a longstanding dilemma of strength-ductility trade-off,which is hindering their wider applications.In this study,we propose a gradient heterogeneous grain(GHG)structure for evading this trade-off dilemma and ultrasonic severe surface rolling is attempted to construct this novel structure in ZE41 Mg alloy.Here,the GHG structure combine the benefits of gradient structure and heterogeneous grain structure and introduce large microstructural heterogeneities.Compared to the coarse-grain and heterogeneous-grain structured alloys,the GHG structured one exhibits dramatical enhancement in strength,ductility,and strain hardening capability.To the best of our knowledge,its strength becomes much higher than that of common ZE41 Mg alloys at no reduction in ductility.These unique mechanical properties stem from not only the individual contribution of the heterogeneous structure components including the fine/ultrafine grains and deformed coarse grains but also their synergistic effect via hetero-deformation induced strengthening and hardening effects.In summary,our study provides a feasible way to develop new Mg alloys with high strength and good ductility.

Enhanced strength-ductility synergy in a wire and arc additively manufactured Mg alloy via tuning interlayer dwell time

Strength-ductility trade-off is a common issue in Mg alloys. This work proposed that a synergistic enhancement of strength and ductility could be achieved through tuning interlayer dwell time(IDT) in the wire and arc additive manufacturing(WAAM) process of Mg alloy.The thermal couples were used to monitor the thermal history during the WAAM process. Additionally, the effect of different IDTs on the microstructure characteristics and resultant mechanical properties of WAAM-processed Mg alloy thin-wall were investigated. The results showed that the stable temperature of the thin-wall component could reach 290 ℃ at IDT=0s, indicating that the thermal accumulation effect was remarkable. Consequently, unimodal coarse grains with an average size of 39.6 μm were generated, and the resultant room-temperature tensile property was poor. With the IDT extended to 60s, the thermal input and thermal dissipation reached a balance, and the stable temperature was only 170 ℃, closing to the initial temperature of the substrate. A refined grain structure with bimodal size distribution was obtained. The remelting zone had fine grains with the size of 15.2 μm, while the arc zone owned coarse grains with the size of 24.5 μm.The alternatively distributed coarse and fine grains lead to the elimination of strength-ductility trade-off. The ultimate tensile strength and elongation of the samples at IDT=60s are increased by 20.6 and 75.0% of those samples at IDT=0s, respectively. The findings will facilitate the development of additive manufacturing processes for advanced Mg alloys.

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.

Gradient microstructure and superior strength-ductility synergy of AZ61 magnesium alloy bars processed by radial forging with different deformation temperatures

Gradient microstructure modification is a cost-efficient strategy for high strength without compromising ductility,which is urgently needed in the fundamental science of engineering materials.In this study,heterogeneous structures of AZ61 alloy bars with anisotropic gradients(with different grain size distributions from the surface to the center)were observed to exhibit strong strength-ductility synergies under different deformation tem peratures.The results reveal that the grain refinement process under mediumlow temperature deformation conditions(≤350℃)consists of four transition stages along the radial direction,i.e.,twin activations and deformation band formations,dislocation cells and pile-ups,ultrafine sub-grains,and randomly orientated quasi-micron grains.Different deformation temperatures have a great influence on twin activations and deformation band formations,and the high temperature can easily provoke the initiation of non-basal slip.The deformation bands were determined as a primary nucleation site due to their highly unstable dislocation hindrance ability.Analysis in combination with the Radial forging(RF)deformation process,the differences of dynamic precipitates can be attributed to microstructural difference and solubility limit of Al at different tem peratures.By summarizing the tensile test results,the sample forged at 350℃exhibited the best strength-ductility synergy,exhibiting the highest elongation(EL)of 23.2%with a 251 MPa yield strength(YS)and 394 MPa ultimate tensile strength(UTS)in center region,and combined with the highest strength value of 256 MPa YS and 420 MPa UTS in the center region,while the EL was slightly degraded to 19.8%.

Role of heterogenous microstructure and deformation behavior in achieving superior strength-ductility synergy in zinc fabricated via laser powder bed fusion

Zinc(Zn)is considered a promising biodegradable metal for implant applications due to its appropriate degradability and favorable osteogenesis properties.In this work,laser powder bed fusion(LPBF)additive manufacturing was employed to fabricate pure Zn with a heterogeneous microstructure and exceptional strength-ductility synergy.An optimized processing window of LPBF was established for printing Zn samples with relative densities greater than 99%using a laser power range of 80∼90 W and a scanning speed of 900 mm s−1.The Zn sample printed with a power of 80 W at a speed of 900 mm s−1 exhibited a hierarchical heterogeneous microstructure consisting of millimeter-scale molten pool boundaries,micrometer-scale bimodal grains,and nanometer-scale pre-existing dislocations,due to rapid cooling rates and significant thermal gradients formed in the molten pools.The printed sample exhibited the highest ductility of∼12.1%among all reported LPBF-printed pure Zn to date with appreciable ultimate tensile strength(∼128.7 MPa).Such superior strength-ductility synergy can be attributed to the presence of multiple deformation mechanisms that are primarily governed by heterogeneous deformation-induced hardening resulting from the alternative arrangement of bimodal Zn grains with pre-existing dislocations.Additionally,continuous strain hardening was facilitated through the interactions between deformation twins,grains and dislocations as strain accumulated,further contributing to the superior strength-ductility synergy.These findings provide valuable insights into the deformation behavior and mechanisms underlying exceptional mechanical properties of LPBF-printed Zn and its alloys for implant applications.

Heterogeneous fiberous structured Mg-Zn-Zr alloy with superior strength-ductility synergy

Here we reported a heterogeneous fiberous structured Mg-5.6Zn-0.6Zr(wt%)alloy obtained by conventional extrusion method,which exhibited high yield strength of∼345 MPa,ultimate tensile strength of∼370 MPa,and high tensile strain of∼20.5%,superior to most of the Mg-Zn based alloys reported so far.The extraordinarily high mechanical properties were mainly attributed to the heterogeneous fiberous structure consisting of alternating coarse-and fine-grain layers.Grains in the different layers grew into the neighboring layers,ensuring a good layer bonding.A high Schmid factor and geometric compatibility factor for pyramidal slip led to full slip transfer between the neighboring coarse grains and fine grains,which could help to release the stress concentration and avoid early fracture.The profuse acti-vated<c+a>glide dislocations could render the unprecedented high tensile strain.The constraint by the hard fine-grain domains made the soft coarse-grain domains strong like the hard fine-grain domains,as well as the nanoscale precipitates pinning dislocations,contributed to the high strength.The hetero-geneous microstructure design was shown to have synergistic improvement in strength-ductility balance,which could be an inspiring strategy to improve mechanical properties of hexagonal close-packed(hcp)metals.

Enhancing strength-ductility synergy in a Mg-Gd-Y-Zr alloy at sub-zero temperatures via high dislocation density and shearable precipitates

The strength-ductility trade-offdilemma is hard to be evaded in high-strength Mg alloys at sub-zero temperatures,especially in the Mg alloys containing a high volume fraction of precipitates.In this paper,we report an enhanced strength-ductility synergy at sub-zero temperatures in an aged Mg-7.37Gd-3.1Y-0.27Zr alloy.The tensile stress-strain curves at room temperature(RT),−70℃ and−196℃ show that the strength increases monotonically with decreasing temperature,but the elongation increases first from RT to−70℃ then declines from−70℃ to−196℃.After systematic investigation of the microstructure evolutions at different deformation temperatures via synchrotron X-ray diffraction,electron backscattered diffraction(EBSD)and transmission electron microscopy(TEM),it is found that a high dislocation density with sufficient<c+a>dislocations promotes good tensile ductility at−70℃,which is attributed to the minimized critical resolved shear stress(CRSS)ratio of non-basal<c+a>to basaldislocations.In ad-dition,more shearable precipitates can further improve the ductility via lengthening the mean free path of dislocation glide.The present work demonstrates that an excellent strength-ductility synergy at sub-zero temperatures can be achieved by introducing a high dislocation density and shearable precipitates in high-strength Mg alloys.

Lamellar aspect-ratio and thickness-dependent strength-ductility synergy in pure nickel during in-situ micro-tensile loading

In order to overcome the trade-offbetween strength and ductility in traditional metallic materials,the gradient lamellar structure was fabricated through an ultrasound-aided deep rolling technique in pure Ni with high stacking fault energy after heat treatment.The gradient lamellar Ni was successively di-vided into three regions.In-situ micro-tensile tests were performed in different regions to reveal the corresponding microscopic mechanical behaviors.Microscopic characterization techniques were adopted to explore the effects of microstructural parameters and defects on mechanical properties.This work demonstrates that the micro-tensile sample with small lamellar thickness and large aspect ratio possesses excellent strength and ductility when the loading direction is parallel to the long side of lamellar grain boundaries.The finding is helpful to the design of metallic material microstructure with superior com-prehensive properties.On one hand,the reason for high strength is that the strength increases with the decrease of lamellar thickness according to the Hall-Petch effect.Besides,initial dislocation density also participates in the strengthening mechanism.On the other hand,the deformation mechanisms include dislocation slip,grain rotation,and the effects of grain boundaries on dislocations,jointly contributing to good ductility.

Laser additive manufacturing of laminated bulk metallic glass composite with desired strength-ductility combination

Introducing ductile crystalline dendrites into a glassy matrix to produce bulk metallic glass composites(BMGCs)is an effective way to improve the poor ductility of bulk metallic glasses(BMGs).However,the presence of soft crystalline phases tends to decrease the strength and causes the strength-ductility tradeoff.Here,relying on the flexible laser additive manufacturing(LAM)technique that allows the composition tailoring of each layer,we successfully fabricate a lamellated Zr-based BMGC constructed by the alternating superimposition of soft and hard layers.The lamellated BMGC shows an exceptional combination of yield strength(∼1.2 GPa)and ductility(∼5%).Such enhanced strength-ductility synergy is attributed to the asynchronous deformation at two scales,i.e.,inter-laminar and intra-laminar,and the unique dual-scale Ta particles that uniformly distribute on the amorphous matrix.The lamellated structure design motif,enabled by the flexible LAM technology,provides a new window for the development of high-performance BMGCs.It is also applicable to the synergistic enhancement of strength and plasticity of other brittle metallic materials.

Achieving excellent strength-ductility synergy in twinned NiCoCr medium-entropy alloy via Al/Ta co-doping

Alloying is an effective strategy to tailor microstructure and mechanical properties of metallic materials to overcome the strength-ductility trade-off dilemma.In this work,we combined a novel alloy design principle,i.e.harvesting pronounced solid solution hardening(SSH)based on the misfit volumes engineering,and simultaneously,architecting the ductile matrix based on the valence electron concentrations(VEC)criterion,to fulfill an excellent strength-ductility synergy for the newly emerging high/medium-entropy alloys(HEAs/MEAs).Based on this strategy,Al/Ta co-doping within NiCoCr MEA leads to an efficient synthetic approach,that is minor Al/Ta co-doping not only renders significantly enhanced strength with notable SSH effect and ultrahigh strain-hardening capability,but also sharply refines grains and induces abnormal twinning behaviors of(NiCoCr)_(92)Al_(6)Ta_(2) MEA.Compared with the partially twinned NiCoCr MEA,the yield strength(σy)and ultimate tensile strength(σUTS)of fully twinned Al/Ta-containing MEA were increased by~102%to~600 MPa and~35%to~1000 MPa,respectively,along with good ductility beyond 50%.Different from the NiCoCr MEA with deformation twins(DTs)/stacking faults(SFs)dominated plasticity,the extraordinary strain-hardening capability of the solute-hardened(NiCoCr)_(92)Al_(6)Ta_(2) MEA,deactivated deformation twinning,originates from the high density of dislocation walls,microbands and abundance of SFs.The abnormal twinning behaviors,i.e.,prevalence of annealing twins(ATs)but absence of DTs in(NiCoCr)_(92)Al_(6)Ta_(2) MEA,are explained in terms of the relaxation of grain boundaries(for ATs)and the twinning mechanism transition(for DTs),respectively.

Excellent strength-ductility synergy properties of gradient nano-grained structural CrCoNi medium-entropy alloy

Tailoring heterogeneities could bring out excellent strength-ductility synergy properties.A gradient nanograined(GNG)structure,i.e.grain size range from nanometer(~50 nm)at topest surface layer to micrometer(~1.3μm)at center layer,was successfully introduced into CrCoNi medium-entropy alloy(MEA)by means of high energy shot peening in this work.Experimental results demonstrated that this GNG CrCoNi MEA shows excellent strength and ductility combination properties,exhibiting high yield strength and ultimate tensile strength of~1215 MPa and~1524 MPa,respectively,while remaining a good ductility of~23.0%.The extraordinary hetero-deformation induced(HDI)hardening origins from heterogeneous structure,i.e.GNG structure,which contributes to the majority strength enhancement.Dynamical reinforced heterogeneous structure during tension process results in the enhanced HDI hardening effect,which facilitates excellent ductility and strain hardening capacity at high-level strength.Our work provide not only a feasible and effective way to strengthen the CrCoNi MEA,and other low stacking faults energy(SFE)materials,but also an useful insight to understanding HDI hardening in heterogeneous structure.

Exceptional strength-ductility synergy of additively manufactured CoCrNi medium-entropy alloy achieved by lattice defects in heterogeneous microstructures

The selective laser melting(SLM)with subsequent cold rolling and annealing is used to produce high-density lattice defects and grain refinement in the CoCrNi medium-entropy alloys(MEAs).The superior comprehensive mechanical properties have been achieved in the as-SLMed CoCrNi alloy after rolling and annealing.The as-SLMed alloys delivered the yield strength of 693.4 MPa,the ultimate tensile strength of 912.7 MPa and the fracture strain of 54.4%.After rolling with 70%reduction in thickness and annealing at 700℃for 2 h.the yield strength,ultimate tensile strength and fracture strain reached 1161.6 MPa,1390.8 MPa and 31.5%,respectively.The exceptional strength-ductility synergy is mainly attributed to the refined hierarchical microstructures with coarsening grains at a level of 30μm and ultrafine grains at a level of 1μm,and the heritage of dislocation-formed sub-grains and other lattice defects.This investigation demonstrates that the SLM with subsequent rolling and annealing is beneficial to fabricate high strength and ductile MEAs with single face-centered cubic(fcc)structure.

Enhanced strength-ductility of CoCrFeMnNi high-entropy alloy with inverse gradient-grained structure prepared by laser surface heat-treatment technique

The inverse gradient-grained CoCrFeMnNi high-entropy alloy with a desirable mechanical property that evades the strength-ductility trade-off is fabricated by the process of cold rolling and subsequent laser surface heat-treatment.Due to the gradually decayed thermal effect along with the thickness,the grain size increases from the hard core to the soft surface in terms of the inverse gradient-grained sample,which is in good consistent with the microhardness profiles.The hetero-deformation induced strengthening and strain hardening caused by the inverse gradient-grained structure improve the strength-ductility combination,as well as the high-order hierarchal nanotwins due to the enhanced interaction with dislocations.For the laser surface heat-treatment technique,the strength and ductility are significantly increased by enlarging the microhardness difference and decreasing the thermal stress.Considering the high volume fraction of gradient-grained layer and a great deal of high-order hierarchal nanotwins in the central region,the laser surface heat-treatment technique is a promising way to produce the gradientstructured materials without thickness limitation.

Effect of Short-Range Ordering on the Strength-Ductility Synergy of Fine-Grained Cu–Mn Alloys at Different Temperatures

Uniaxial tensile tests were carried out at room temperature(RT)and 250℃,respectively,to investigate the effect of shortrange ordering(SRO)on the mechanical properties and deformation micromechanism of fine-grained(FG)Cu–Mn alloys with high stacking fault energy.The results show that at RT,with the increase in SRO degree,the strength of FG Cu–Mn alloys is improved without a loss of ductility,and corresponding deformation micromechanism is mainly manifested by a decrease in the size of dislocation cells.In contrast,at a high temperature of 250℃,the SRO degree becomes violently enhanced with increasing Mn content,and the deformation microstructures thus transform from dislocation cells to planar slip bands and even to deformation twins,significantly enhancing the work hardening capacity of the alloys and thus achieving a better strength-ductility synergy of FG Cu–Mn alloys.

Gradient structure induced simultaneous enhancement of strength and ductility in AZ31 Mg alloy with twin-twin interactions

Gradient nanostructure was introduced to enhance the strength and ductility via deformation incompatibility accommodated by geometrical necessary dislocations for most metallic materials recently.However,few intensive researches were carried out to investigate the effect of gradient structure on the deformation twin evolution and resulting performance improvements.In the present paper,we produced gradient-structured AZ31 Mg alloy with fine-grain layers,parallel twin laminates and a coarse-grain core from two upmost surfaces to the center of plate.Surprisingly,this architected Mg alloy exhibited simultaneous enhancement of strength and ductility.Subsequent microstructural observations demonstrated that abundant twin-twin interactions resulting from higher strength and multi-axial stress state could make great contributions to the increase of work-hardening capability.This was further proved by the measurement of full-field strain evolution during the plastic deformation.Such a design strategy may provide a new path for producing advanced structure materials in which the deformation twinning works as one of the dominant plasticity mechanisms.

Optimizing mechanical properties of magnesium alloys by philosophy of thermo-kinetic synergy:Review and outlook

Although several strategies(including grain refinement,texture adjustment,precipitation hardening,etc.)have been verified to effectively improve the mechanical properties of lightweight magnesium(Mg)alloys,considerable efforts are still needed to be made to comprehensively understand the potential mechanisms controlling complex microstructures and deformation behaviors exhibited by the hexagonal close-packed host lattice of Mg,thus assisting the rational design of materials at a more physical level.As the cornerstone of this review,a universal rule,the so-called synergy of thermodynamics and kinetics(i.e.,thermo-kinetic diversity,correlation and connectivity),including a recently proposed theory of generalized stability(GS),is introduced to deepen our understanding on common behaviors in Mg alloys(i.e.,deformations(slip and twining modes),phase transformations(especially for precipitations)and interactions in between)at a new perspective.Guided by the GS theory,typical cases for Mg alloys design are qualitatively evaluated to reemphasize the traditional strengthening and toughening strategies mentioned above and to illuminate their exquisite coordination for breaking through the trade-off relationship between strength and ductility,corresponding to a typical thermo-kinetic pair(i.e.,high driving force(ΔG)-high GS).To produce the Mg alloys with superior strength-ductility balances,the potential capacity of this GS theory for guiding processing path design is discussed,finally。

Effect of inter-critical quenching on mechanical properties of casting low-alloy steel

For some casting low-alloy steels,traditional quenching and tempering heat treatments can improve the strength;however,sometimes the ductility is not satisf ied.Therefore,some kind of effective heat treatment method seems necessary;one which could improve the ductility,but not seriously affect the strength.In this paper,the effect of inter-critical quenching(IQ)on the mechanical properties of casting low-alloy steel was studied.IQ was added between quenching and tempering heat treatment;and the microstructure and mechanical properties were compared to the same steel with the traditional quenching and tempering treatments.The experimental results show that the microstructure comprises small-size ferrite and martensite when the IQ is adopted;and that different temperatures can control the ferrite quantity and distribution,and,as a result,infl uence the mechanical properties.In the case of IQ,the tensile strength decreases just a little,but the ductility increases a lot;and the strength-ductility product(its value is the arithmetic product of elongation and tensile strength)increases by between 6%and 10%,which means the IQ heat treatment can improve comprehensive mechanical properties.

Nitride-reinforced HfNbTaTiV high-entropy alloy with excellent room and elevated-temperature mechanical properties

Nitride-reinforced(HfNbTaTiV)_(90)N_(10) high-entropy alloy aiming at high-temperature applications is de-signed in this paper.Abundant FCC nitride phases are formed in situ in theBCC matrix by arc melt-ing technique,without complex deformation or heat treatment.The(HfNbTaTiV)_(90)N_(10) alloy exhibits a remarkable yield strength of 2716 MPa and ultimate compressive strength of 2833 MPa with a plas-tic strain of 10%at room temperature.Besides,the alloy remains a high yield strength of 279 MPa at 1400℃.The nitride phases play an essential role in maintaining the excellent strength-ductility com-bination at room temperature and enhancing the high-temperature softening resistance.Alternating BCC and FCC phases possess the semi-coherent interface,which not only strengthens the BCC matrix but also promotes the compatible deformation of the duplex microstructure.The lattice coherency structure of the semi-coherent interface is conducive to the slip transfer of partial dislocations through the interface,which facilitates the accommodation of plastic deformation.The cross-slip of the screw dislocations ef-fectively eliminates stress concentration and leads to good ductility of the dual-phase alloy.The results demonstrate that the nitride phases achieve coordinate deformation with the matrix without deteriorat-ing the ductility of the(HfNbTaTiV)_(90)N_(10) alloy.

High strength and ductility Mg-8Gd-3Y-0.5Zr alloy with bimodal structure and nano-precipitates

To resolve the strength-ductility trade-off problem for high-strength Mg alloys,we prepared a high performance Mg-8Gd-3Y-0.5 Zr(wt%)alloy with yield strength of 371 MPa,ultimate tensile strength of 419MPa and elongation of 15.8%.The processing route involves extrusion,pre-deformation and aging,which leads to a bimodal structure and nano-precipitates.Back-stress originated from the deformationincompatibility in the bimodal-structure alloy can improve ductility.In addition,dislocation density in coarse grains increased during the pre-deformation strain of 2%,and the dislocations in coarse grains can promote the formation of chain-like nano-precipitates during aging treatment.The chain-like nanoprecipitates can act as barriers for dislocations slip and the existing mobile dislocations enable good ductility.

316L Austenite Stainless Steels Strengthened by Means of Nano-scale Twins

By means of dynamic plastic deformation （DPD） followed by thermal annealing, a mixed structure of micro-sized austenite grains embedded with nano-scale twin bundles （of about 20% in volume） has been synthesized in a 316L stainless steel （SS）. Such a 316L SS sample exhibits a tensile strength as high as 1001 MPa and an elongation-to-failure of about 23%. The much elevated strength originates from the presence of a considerable number of strengthening nano-twin bundles, while the ductility from the recrystallized grains. The superior strength-ductility combination achieved in the nano-twins-strengthened austenite steel demonstrates a novel approach for optimizing the mechanical properties in engineering materials.