Microstructure modification and precipitation strengthening for Mg−6Zn−1Mn−4Sn−0.5Ca through extrusion and aging treatment

The microstructure revolution and mechanical properties of as-extruded and peak-aged Mg−6Zn−1Mn−4Sn−0.5Ca(ZMT614−0.5Ca)alloy were studied by OM,SEM,TEM,hardness testing and tensile testing.The results showed that the as-cast ZMT614−0.5Ca alloy mainly consisted of α-Mg,Mg−Zn and CaMgSn phase.The hot extrusion process effectively refined the microstructure and led to a completely dynamic recrystallized microstructure.The average grain size of as-extruded alloy was^4.85μm.After solution treatment,remained CaMgSn with high melting point played a significant role in pinning effect and impeding the migration of grain boundary.After aging treatment,peak-aged ZMT614−0.5Ca alloy exhibited a good combination of strength and ductility,with yield strength,ultimate tensile strength and elongation being 338 MPa,383 MPa and 7.5%,respectively.The yield strength of the alloy increased significantly by around 36%compared with that in as-extruded condition,which should be attributed to the precipitation strengthening of β'phase.

Effects of precipitation strengthening heat treatment for Al-Mg alloy

The corrosions resulting from defects in painting layers frequently occur in Al alloys, so the application of corrosion preventing systems is also very important. Optimum conditions in terms of electrochemistry in relation to solution treatment, quenching and artificial aging treatment were established in order to optimize precipitation strengthening conditions intended to enhance the strength of Al alloys. Slow strain rate tests （SSRT） at various applied potentials were conducted in potential range from -1.8 to 0.5 V. The results show that the maximum tensile strengths, elongations and time-to-fracture are shown to be high values. After precipitation strengthening heat treatment, a tendency appear that time-to-fracture increases as elongation increases. In the potential range from -1.3 V to -0.7 V, the specimens show excellent mechanical properties, and thus this range is considered to be a corrosion prevention range.

Simultaneously improved the strength and ductility of laser powder bed fused Al-Cr-Fe-Ni-V high-entropy alloy by hot isostatic pressing:Microcrack closure and precipitation strengthening

Hot isostatic pressing(HIP)is usually applied to reduce the defects including cracks and pores in the materials prepared by laser powder bed fusion(LPBF).In the present research,in order to improve the relative density and mechanical property,HIP was employed on the LPBF-processed Al-Cr-Fe-Ni-V high-entropy alloy(HEA)with microcracks and pores.The microstructure evolution and property improvement induced by HIP were investigated.In the LPBF-processed HEA,the microcracks were caused by residual stress and element segregation,and these microcracks as well as the pores reduced significantly after HIP treatments.Remarkably,HIP temperature has a more critical effect on the microcrack closure than the holding time,thus,microcracks and pores still existed after HIP-1 treatment(1273 K,8 h),while HIP-2 treatment(1473 K,4 h)could close the microcracks significantly.The crack closure was attributed to the interfacial diffusion of the alloying element under high temperature accompanied by high pressure,and the degree of element diffusion at both interfaces of the cracks determined the bonding strength after crack closure.Higher temperatures at high pressure induced more adequate element diffusion and higher bonding strength.The above high temperature and high pressure also induced the growth of the L1_(2) phase and the precipitation of the B2 phase in HEA.Consequently,the tensile strength and elonga-tion of the LPBF-processed HEA after HIP-2 treatment were simultaneously enhanced(80.7%and 222.5%higher than that of LPBF-processed HEA,respectively).This could be attributed to the combined effect of microcrack/pore closure and precipitation strengthening.The strengthening effect of the B2 phase and L1_(2) phase accounted for 53%(dislocation by-pass mechanism)and 47%(dislocation shearing mechanism)of the total precipitation strengthening,respectively.

Investigation of the Micro-Mechanics of an Extruded Precipitation-Strengthened Magnesium Alloy under Cyclic Loading

Precipitation strengthening is a crucial microscopic mechanism for enhancing the strength of magnesium alloys. In order to elucidate the influence of precipitation on the microscopic deformation mechanisms and macroscopic mechanical response of magnesium alloys under cyclic loading conditions, we employed a crystal plasticity model to analyze the stress-strain curves, specific crystal plane diffraction intensities, and the temporal evolution of various microscopic deformation mechanisms and twinning volume fractions for an extruded magnesium alloy, AXM10304, containing coherent precipitates. The research findings indicate that precipitation does not fundamentally alter the microscopic mechanisms of this alloy. However, it hinders twinning during the compression stage, mildly promotes detwinning during the tension stage, and enhances tension secondary hardening by elevating the difficulty of activation of the prismatic slip.

A novel immiscible high entropy alloy strengthened via L1_(2)-nanoprecipitate

The low-cost Fe-Cu,Fe-Ni,and Cu-based high-entropy alloys exhibit a widespread utilization prospect.However,these potential applications have been limited by their low strength.In this study,a novel Fe_(31)Cu_(31)Ni_(28)Al_(4)Ti_(3)Co_(3) immiscible high-entropy alloy(HEA)was developed.After vacuum arc melting and copper mold suction casting,this HEA exhibits a unique phase separation microstructure,which consists of striped Cu-rich regions and Fe-rich region.Further magnification of the striped Cu-rich region reveals that it is composed of a Cu-rich dot-like phase and a Fe-rich region.The aging alloy is further strengthened by a L1_(2)-Ni_(3)(AlTi)nanoprecipitates,achieving excellent yield strength(1185 MPa)and uniform ductility(~8.8%).The differential distribution of the L1_(2) nanoprecipitate in the striped Cu-rich region and the external Fe-rich region increased the strength difference between these two regions,which increased the strain gradient and thus improved hetero-deformation induced(HDI)hardening.This work provides a new route to improve the HDI hardening of Fe-Cu alloys.

Microstructure and Mechanical Properties of Precipitation Strengthened Fire Resistant Steel Containing High Nb and Low Mo

Through the thermo-mechanical control process （TMCP）, a high Nb low Mo fire resistant steel with the yield strength （YS） of 521 MPa at room temperature （RT） and 360 MPa at elevated temperature （ET） of 600 ℃ was developed based on MX （M=Nb, V, Mo; X=C,N） precipitation strengthening. A series of tensile and con- stant load tests were conducted to study the mechanical properties at ET. The dynamic continuous cooling transfor- mation （CCT） as well as precipitation behavior of microalloy carbonitride was investigated by means of thermal sim- ulator and electron microscopy approaches. Results showed that the failure temperature of tested steel was deter- mined as 653 ℃, and the granular bainite was obtained when the cooling rate was higher than 10 ℃/s. In the rolled state, a certain amount of M/A islands was observed. During heating from RT to ET, M/A islands disappeared, and cementites and high dense compound precipitates （Nb, Mo, V）C with size of less than 10 nm precipitated in ferrite at ET （600 ℃）, which resulted in precipitation strengthening at ET.

Break the strength-ductility trade-off in a transformation-induced plasticity high-entropy alloy reinforced with precipitation strengthening

Transformation-induced plasticity(TRIP)endows material with continuous work hardening ability,which is considered as a powerful weapon to break the strength-ductility tradeoff.However,FCC based alloys with TRIP effect can not get rid of the“soft”feature of the structure entirely,resulting in insufficient yield strength.Here,a Co_(x)Cr_(25)(AlFeNi)_(75-x) high-entropy alloy is designed.NiAl phase is used as strengthening component to improve yield strength,while TRIP effect ensures plasticity.Compared with the previous TRIP high-entropy alloy,its yield strength is nearly doubled,and the uniform elongation is more than 55%at room temperature.Furthermore,the corresponding multiphase microstructure evolution and deformation mechanisms are investigated.Significantly,stacking faults andΣ3 twin boundaries are confirmed to be the nucleation sites of HCP phase by HAADF-STEM.Ingenious composition design and proper heat treatment process make it a perfect combination of precipitation strengthening and transformationinduced plasticity,and thus guide design in the high-performance alloy.

Precipitation Strengthening by Nanometer-sized Carbides in Hot-rolled Ferritic Steels

The mechanical properties of the hot-rolled plates of Ti steel and Ti-Mo steel after isothermal transformation in a temperature range of 600 700 ℃ for 60 min have been tested, and the microstructures of the matrix and the characteristics of precipitated nanometer-sized carbides have also been examined by scanning electron microscopy and transmission electron microscopy. The precipitation regularity of nanometer-sized carbides has been studied by thermodynamic method and the contributions of corresponding strengthening mechanisms to the total yield strength have been calculated. The tensile strength of hot-rolled Ti-Mo ferritie steel can achieve 780 MPa with an elongation of 20.0% after being isothermally treated at 600 ℃ for 60 rain, and the tensile strength of Ti steel is 605 MPa with an elongation of 22.7%, according to the results of tensile tests. The critical nucleation size of （Ti,Mo）C is smaller than that of TiC at a given isothermal temperature, but the nucleation rate of （Ti, Mo）C is larger than that of TiC. The grainrefinement strengthening and precipitation strengthening contribute the main amount of the total yield strength. The major increase in yield strength with the decrease of isothermal temperature results from the contribution of precipi tation strengthening. The contribution of precipitation strengthening to the yield strength of the steels has been esti mated. The ferrite phase can be strengthened by about 400 MPa through precipitation strengthening in Ti-Mo steel isothermally treated at 600 ℃ for 60 rain, which is about 200 MPa higher than that of Ti steel under the same conditions.

Precipitation strengthening of nano-scale TiC in a duplex low-density steel under near-rapid solidification

Precipitation strengthening of nano-scale TiC is a promising method to improve mechanical properties of Fe–16Mn–9Al–0.8C (wt.%) low-density steel. This work attempted to introduce nano-scale TiC precipitates by adding 1 wt.% Ti element. The experimental results show that these precipitates with the total fraction of about 2 vol.% were formed and no coarse precipitates were observed despite the high Ti addition. It was interesting that the polygonal and needle-shaped TiC precipitates were observed in γ-austenite and δ-ferrite, respectively. Ti addition also decreased the volume fraction of γ-austenite significantly. Correspondingly, the yield strength was increased, but the elongation was significantly decreased due to the significant decrease of γ-austenite. Comparing with the Ti-free steel, the formation of TiC precipitates was the main reason for the increase in yield strength of Ti-bearing steel, and TiC precipitates also led to a higher strain hardening index at the first deformation stage. TiC precipitates promoted the Orowan strengthening, resulting in a higher strain hardening capability than Ti-free steel reinforced by shearable κ-carbide.

Abnormal mechanochemical effect in ultraprecision machining of an additively manufactured precipitation-strengthened high-entropy alloy

Recently,researchers have explored the use of precipitation strengthening and finer microstructures with high-density dislocations in additive manufacturing to produce high-entropy alloys(HEAs)with adjustable properties.However,the inherent surface roughness and lack of machinability research in AMed HEAs limit their engineering applications.In this study,we systematically investigated the microstructural characteristics,mechanical properties,and machinability of Fe_(29.3)Co_(28.7)Ni_(28.6)Al_(6.8)Ti_(6.6)(at.%)HEAs with three different structures:single FCC phase cellular(SPC),dual precipitation-strengthened(DPS),and single precipitation-strengthened(SPS).These structures were fabricated by selective laser melting and isothermally annealing at 780 and 940℃.Compared to SPC HEA,DPS HEA exhibits a significant increase in yield strength and ultimate tensile strength but with a dramatic sacrifice in ductility.SPS HEA exhibits similar mechanical properties to SPC HEA due to the pronounced coarsening of L21 precipitates.The ultraprecision machining micro-cutting test showed that SPC HEA had a significant mechanochem-ical effect,as evidenced by a sharp drop in cutting force for inked workpieces,but not DPS HEA.An abnormal finding was that the negligible reflection of cutting force for SPS HEAs suggested a negative mechanochemical effect,even though SPS HEA had equally excellent plasticity like SPC HEA.It was found that nanocrystallization-induced strength enhancement and ductility reduction of SPS HEA lead to chips’deformation dominated by shear avalanche rather than chip folding of SPC HEA,which involves the reduction of surface energy and friction of chips’interfaces.Overall,these results and our research findings may guide the machining of AMed precipitation-strengthened HEAs and accelerate their engineering ap-plication.

Intelligent Design of High Strength and High Conductivity Copper Alloys Using Machine Learning Assisted by Genetic Algor

Metallic alloys for a given application are usually designed to achieve the desired properties by devising experimentsbased on experience, thermodynamic and kinetic principles, and various modeling and simulation exercises.However, the influence of process parameters and material properties is often non-linear and non-colligative. Inrecent years, machine learning (ML) has emerged as a promising tool to dealwith the complex interrelation betweencomposition, properties, and process parameters to facilitate accelerated discovery and development of new alloysand functionalities. In this study, we adopt an ML-based approach, coupled with genetic algorithm (GA) principles,to design novel copper alloys for achieving seemingly contradictory targets of high strength and high electricalconductivity. Initially, we establish a correlation between the alloy composition (binary to multi-component) andthe target properties, namely, electrical conductivity and mechanical strength. Catboost, an ML model coupledwith GA, was used for this task. The accuracy of the model was above 93.5%. Next, for obtaining the optimizedcompositions the outputs fromthe initial model were refined by combining the concepts of data augmentation andPareto front. Finally, the ultimate objective of predicting the target composition that would deliver the desired rangeof properties was achieved by developing an advancedMLmodel through data segregation and data augmentation.To examine the reliability of this model, results were rigorously compared and verified using several independentdata reported in the literature. This comparison substantiates that the results predicted by our model regarding thevariation of conductivity and evolution ofmicrostructure and mechanical properties with composition are in goodagreement with the reports published in the literature.

Strength and ductility optimization of HPDC AlSi8MgCuZn2 alloys by modifying pre-aging treatment

Considering the components produced by high pressure die casting(HPDC)process usually with ultra-large sizes and complex morphologies,high temperature solid solution treatment is not a suitable method to further improve their mechanical properties.In this study,two-stage aging treatment with different pre-aging times was designed and employed to further improve the mechanical properties of HPDC Al8SiMgCuZn alloy.The characteristics of precipitates were evaluated by a transmission electron microscope(TEM),and the precipitation strengthening mechanism was discussed.The results reveal that the strengthening is mainly contributed by the precipitation ofβ″phase after two-stage aging,and the number density and size of the precipitates are significantly depended on the pre-aging time.The number density of precipitates is increased with the pre-aging time prolonged from 0 h to 4 h,and then decreases with the further increase of pre-aging time from 4 h to 6 h.The precipitates with the highest density and smallest size are observed after pre-aging for 4 h.After pre-aged at 100℃for 4 h and then artificial aged at 200℃for 30 min,the yield strength of 207 MPa,ultimate tensile strength of 325 MPa and elongation of 7.6%are achieved.

Improved mechanical properties and strengthening mechanism with the altered precipitate orientation in magnesium alloys

Aging prior to twinning deformation was proposed to alter the precipitate orientation of the plate-shapedβ-MgAlfrom(0002)basal planes(named basal plates)to■prismatic planes(named prismatic plates)in AZ31 Mg alloy.The experimental results showed that the compressive yield strength(CYS)of the sample containing prismatic plates increased 40 MPa and the compression ratio raised by 22%compared to that containing basal plates.The underlying strengthening mechanism was analyzed via a yield strengthen(YS)model with a function of grain size,precipitate characters(size,oritention,fraction)and Schmid factor(SF).It revealed that the improvement of CYS was mainly attributed to the altered precipitate orientation and refined grain size produced by twinning deformation.Particularly,the prismatic plates always have a stronger hardening effect on basal slip than basal plates under the same varites of precipitate diameter and SF.Besides,the decreased CRSS ratio of prismatic slip to basal slip revealed that the activity of non-basal slip in Mg alloy might be enhanced.More activated slip systems provided more mobile dislocations,contributing to the large compression ratio of the Mg rolled sheet with prismatic plates.

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.

Effects of trace Ag on precipitation behavior and mechanical properties of extruded Mg−Gd−Y−Zr alloys

The effects of trace Ag element on the precipitation behaviors and mechanical properties of the Mg−7.5Gd−1.5Y−0.4Zr(wt.%)alloy by means of tensile test,X-ray diffractometry,scanning electron microscopy,electron backscattered diffractometry,and scanning transmission electron microscopy.There is an unusual texture(á0001ñ//extrusion direction)in the extruded Mg−Gd−Y−Zr alloys containing 0.5 wt.%Ag.During the aging periods at 225℃,the addition of the trace Ag does not form new precipitates,just accelerates aging kinetics,and refinesβ′precipitates,thereby increasing the number density of theβ′precipitates by Ag-clusters.Moreover,the Mg−Gd−Y−Zr alloy containing 0.5 wt.%Ag shows the most excellent synergy of strength and plasticity(408 MPa of ultimate tensile strength,265 MPa of yield strength,and 12.9%of elongation to failure)after peak-aging.

Nanoscale precipitates and comprehensive strengthening mechanism in AISI H13 steel

The effects of heat treatment on the precipitates and strengthening mechanism in AISI H13 steel were investigated. The results showed that the presence of nanoscale precipitates favorably affected grain refinement and improved the yield strength. The volume fraction of precipitates increased from 1.05% to 2.85% during tempering, whereas the average precipitate size first decreased then increased during tempering. Contributions to the yield strength arising from the various mechanisms were calculated quantificationally, and the results demonstrated that grain refinement and dislocation density most strongly influenced the yield strength. In addition, under the interaction of average size and volume fraction, precipitates＇ contribution to the yield strength ranged from 247.9 to 378.5 MPa. Finally, a root-mean-square summation law of σ = σg ＋ σs ＋（σd^2 ＋ σp^2）^1/2, where σg, σs, σd, and σp represent the contributions of fine-grain strengthening, solid-solution strengthening, dislocation strengthening, and precipitation strengthening, respectively, was confirmed as the most applicable for AISI H13 steel, which indicates a strong link between precipitates and dislocations in AISI H13 steel.

D0_(22) precipitates strengthened W-Ta-Fe-Ni refractory high-entropy alloy

Refractory high-entropy alloys have recently emerged as promising candidates for high-temperature structural applications.However,their performance is compromised by the trade-off required between strength and ductility.Here,a novel W30Ta5(FeNi)65 refractory high-entropy alloy with an outstanding combination of strength and plasticity at both room and elevated temperatures is designed,based on the multi-phase transitions design strategy.The alloy comprises a body-centered cubic dendrite phase,a topologically close-packed μ rhombohedral phase,and a high-density coherent nano-precipitate γ"phase with the D0_(22)structure(Ni3Ta type)embedded in a continuous face-centered cubic matrix.Owing to pre-cipitation strengthening of D0_(22),the yield stress of the alloy is determined as high as 1450 MPa,which is a significant improvement(~100%)in comparison with the D0_(22)-free alloy,without a loss of ductil-ity.This alloy exhibits an excellent high-temperature strength,with the yield strengths of 1300 MPa at 600 ℃ and 320 MPa at 1000 ℃.Detailed microstructural characterization using transmission electron mi-croscopy,high-angle annular dark-field imaging,and three-dimensional atom probe tomography analyses indicated that this superior strength-plasticity combination stems from the synergy of a multiple-phase structure.These results provide a new insight into the design of RHEAs and other advanced alloys.

Effect of minor Sc and Zr addition on microstructure and properties of ultra-high strength aluminum alloy

The Al-9Zn-2.8Mg-2.5Cu-xZr-ySc alloys （x=0, 0.15%, 0.15%; y=0, 0.05%, 0.15%）, produced by low-frequent electromagnetic casting technology, were subjected to homogenization treatment, hot extrusion, solution and aging treatment. The effects of minor Sc and Zr addition on microstructure, recrystallization and properties of alloys were studied by optical microscopy （OM）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. The results show that Sc and Zr addition can refine grains of the as-cast alloy by precipitation of primary Al3（Sc,Zr） particles formed during solidification as heterogeneous nuclei. Secondary Al3（Sc,Zr） precipitates formed during homogenization treatment strongly pin the movement of dislocation and subgrain boundaries, which can effectively inhibit the alloys recrystallization. Compared with the alloy without Sc and Zr addition, the Al-Zn-Mg-Cu-Zr alloy with 0.05%Sc and 0.15%Zr shows the increase in tensile strength and yield strength by 172 MPa and 218 MPa, respectively. Strengthening comes from the contributions of precipitation, substructure and grain refining.

T6处理对Mg-10Gd-3Y-0.6Zr合金组织及拉伸性能的影响(英文)

The effects of solution and ageing treatment (T6) on microstructure and tensile properties of as-extruded Mg-10Gd-3Y-0.6Zr (mass fraction. %) alloy were investigated. The results show that after T6 treatment, the diameter of grain increases to 20 μm. As the second phases dissolve into the matrix, the smaller and denser β′ phases precipitate inside the grains. After T6-treatment, both yield strength (TYS) and ultimate tensile strength (UTS) are increased. Comparing with that in only ageing condition (T5), the UTS and TYS increased from 365 MPa,285 MPa to 400 MPa,310 MPa, respectively, but the elongation decreased from 7.0% to 3.5%. It has been found that the effects of precipitates on the strength are stronger than that of the growth of grain size.

Enhancement of mechanical properties of duplex Mg-9Li-3Al alloy by Sn and Y addition

For enhancement of mechanical properties in Mg-9Li-3Al alloys,Mg-9Li-3Al duplex alloys were alloyed by addition of Sn and Y.Microstructure evolution and mechanical property response of as-cast Mg-9Li-3Al alloys by alloying with Sn and Y were investigated by optical microscopy,scanning electron microscopy,X-ray diffractometry and tensile tests.The results indicate that considerable blocky dendrites of primaryαphase in Mg-9Li-3Al alloys become lath-like due to the addition of Sn.With addition of Y,Mg-9Li-3Al alloy consists of both block-like and lath-likeα-Mg dendrites.The as-cast Mg-9Li-3Al-1Sn-1Y alloy shows a yield strength of118MPa,ultimate tensile strength of148MPa and the elongation to failure of21%.Improvement in both strength and elongation of Mg-9Li-3Al alloys with Sn and Y addition is attributed to the combined action of MgLi2Sn and Al2Y intermetallic compounds.