Deformation behavior and constitutive model for dual-phase Mg-Li alloy at elevated temperatures

In order to study the deformation behavior and evaluate the workability of the dual-phase Mg-9Li-3Al-2Sr alloy, isothermal hot compression tests were conducted using the Gleeble-3500 thermal-mechanical simulator, in ranges of elevated temperatures （423-573 K） and strain rates （0.001-1 s^-1）. Plastic instability is evident during the deformation which is in the form of serrated flow; serrated yielding is attributed to the locking of mobile dislocations by the Mg and Li atoms which diffuse during the deformation. The relationships between flow stress, strain rate and deformation temperature were analyzed and the deformation activation energy and some basic material factors at different strains were calculated using the Arrhenius equation. The effects of temperature and strain rate on deformation behavior were represented using the Zener–Hollomon parameter in an exponent-type equation. To verify the validity of the constitutive model, the predicted values and experimental flow curves under different deformation conditions were compared, the correlation coefficient （0.9970） and average absolute relative error （AARE=4.41%） were calculated. The results indicate that the constitutive model can be used to accurately predict the flow behavior of dual-phase Mg-9Li-3Al-2Sr alloy during high temperature deformation.

Rolling texture development in a dual-phase Mg-Li alloy:The role of temperature

Dual-phase Mg-Li alloys sheets were rolled at four different temperatures ranging from liquid nitrogen to 300℃to explore effect of rolling temperature on texture and mechanical properties of the material.Crystal plasticity simulation was utilized to illustrate the influence of slip activity on rolling texture development.The results show that the rolling texture is largely depended on deformation temperature.Unlike commercial Mg alloys,the critical resolved shear stress of basal slip inα-Mg phase of Mg-Li alloy decreased more significantly by increasing temperature compared to that of pyramidal<c+a>slip.Enhancement of basal slip by increasing temperature triggered a decrease of split angle of basal poles for the double-peak texture.Prismaticslip largely enhanced by increasing temperature upon 200℃,which induced a wider orientation spread along the transverse direction.For theβ-Li phase,the promotion of{110}<111>slip system at elevated temperature triggered the enhancement of{211}<110>and{111}<211>texture components.The cryo-rolled sample exhibited the highest strength compared to the others due to a strong hardening behavior at this temperature.A two-stage hardening behavior was observed in these as-rolled dual-phase alloys.Strain transition at phase boundaries could be the reason for appearance of this two-stage hardening.

Tensile strength prediction of dual-phase Al0.6CoCrFeNi high-entropy alloys

The evolution of the microstructure and tensile properties of dual-phase Al0.6CoCrFeNi high-entropy alloys(HEAs)subjected to cold rolling was investigated.The homogenized Al0.6CoCrFeNi alloys consisted of face-centered-cubic and body-centered-cubic phases,presenting similar mechanical behavior as the as-cast state.The yield and tensile strengths of the alloys could be dramatically enhanced to^1205 MPa and^1318 MPa after 50%rolling reduction,respectively.A power-law relationship was discovered between the strain-hardening exponent and rolling reduction.The tensile strengths of this dual-phase HEA with different cold rolling treatments were predicted,mainly based on the Hollomon relationship,by the strain-hardening exponent,and showed good agreement with the experimental results.

Effect of microvoids on microplasticity behavior of dual-phase titanium alloy under high cyclic loading(Ⅰ):Crystal plasticity analysis

A crystal plasticity finite element(CPFE)model was established and 2D simulations were carried out to study the relationship between microvoids and the microplasticity deformation behavior of the dual-phase titanium alloy under high cyclic loading.Results show that geometrically necessary dislocations(GND)tend to accumulate around the microvoids,leading to an increment of average GND density.The influence of curvature in the tip plastic zone(TPZ)on GND density is greater than that of the size of the microvoid.As the curvature in TPZ and the size of the microvoid increase,the cumulative shear strain(CSS)in the primaryα,secondaryα,andβphases increases.Shear deformation in the prismatic slip system is dominant in the primaryαphase.As the distance between the microvoids increases,the interactive influence of the microvoids on the cumulative shear strain decreases.

Tailoring phase fractions of T'and η' phases in dual-phase strengthened Al−Zn−Mg−Cu alloy via ageing treatment

Hardness tests and transmission electron microscopy were used to investigate the strategy of tailoring the phase fraction of precipitates in an Al-Zn-Mg-Cu alloy strengthened by T’ and η’ phases. Different phase fractions of T’ and η’ phases are presented in samples subjected to either single or two stages of ageing treatments at 120 and 150 ℃.For both types of ageing, the precipitation of η’ phase is found to be promoted by ageing at lower temperature and its phase fraction increases with prolonging ageing time at 120 ℃;whereas the phase fractions of T’ and η’ phases almost remain constant during ageing at 150 ℃. Besides, the strain fields produced by T’ and η’ phases were analyzed by using the geometric phase analysis technique, and on a macroscale the contributions of T’ and η’ phases to precipitation strengthening have been quantitatively predicted by combining the size, phase fraction and number density of precipitates.

Balancing strength and plasticity of dual-phase amorphous/crystalline nanostructured Mg alloys

The dual-phase amorphous/crystalline nanostructured model proves to be an effective method to improve the plasticity of Mg alloys.The purpose of this paper is to explore an approach to improving the ductility and strength of Mg alloys at the same time.Here,the effect of amorphous phase strength,crystalline phase strength,and amorphous boundary(AB)spacing on the mechanical properties of dual-phase Mg alloys(DPMAs)under tensile loading are investigated by the molecular dynamics simulation method.The results confirm that the strength of DPMA can be significantly improved while its excellent plasticity is maintained by adjusting the strength of the amorphous phase or crystalline phase and optimizing the AB spacing.For the DPMA,when the amorphous phase(or crystalline phase)is strengthened to enhance its strength,the AB spacing should be increased(or reduced)to obtain superior plasticity at the same time.The results also indicate that the DPMA containing high strength amorphous phase exhibits three different deformation modes during plastic deformation with the increase of AB spacing.The research results will present a theoretical basis and early guidance for designing and developing the high-performance dual-phase hexagonal close-packed nanostructured metals.

Electroless Ni-P plating on Mg-Li alloy by two-step method

Pretreated Mg-Li alloy sheets were pre-plated in a NiCO3？2Ni（OH）2？4H2O solution to form a thin Ni-P alloy film and then plating in a NiSO4？6H2O solution was carried out to obtain a protective coating.The surface morphology,structure and corrosion resistance of the coating were studied.The results showed that a flat,bright and compact plating layer,which was integrated into the matrix metal,was obtained.The P content of the Ni-P coating reached 13.56%（mass fraction）.The hardness value of the Ni-P coating was about HV 549.The polarization curve showed that the corrosion potential of the Ni-P coating reached ？0.249 V（vs SCE）.A long passivation region was found on the polarization curve,and this phenomenon indicated that the coating has an excellent anti-corrosion property.

Microstructure and mechanical properties of Mg-Li alloy after TIG welding

Tungsten inert gas weld was carried out on super-light magnesium-lithium alloy plates with a thickness of 2 mm, using argon gas as a protecting atmosphere. The microstructure and mechanical properties of the welded joints were investigated. The results indicate that the microstructure in the fusion zone is fine, and the microstructure in the heat-affected zone is coarser than the parent metal. The tensile strength of the welded joint is about 84% that of the parent metal. The fracture occurs in a mixed type of toughness and brittleness in the heat-affected zone. During the welding process, aluminum and cerium are enriched at grain boundaries in the fusion zone.

Effect of Sr content on microstructure and mechanical properties of Mg-Li-Al-Mn alloy

The as-cast Mg-8Li-3Al-0.5Mn-xSr（LAM830-xSr, x=0-1.0） alloys were designed and prepared in a vacuum induction furnace under controlled argon atmosphere. The alloys were then processed by hot extrusion, and their microstructural evolution and mechanical properties were analyzed. Results indicate that the LAM830 alloy mainly consists of α-Mg, β-Li, Al2Mn3, and LiMgAl2 phases. Sr addition results in the precipitation of Al-Sr. Moreover, Sr addition results in a fact that the secondary dendrite arm spacing（DAS） of the primary α-Mg phase is obvious refined. Microstructure of the investigated alloys is further refined as a result of the hot extrusion treatment. The content and morphology of the secondary phases have important effects on the mechanical properties of the alloys. The as-extruded LAM830-0.5Sr alloy exhibits an optimal elongation of 22.43% and as-extruded LAM830-0.75 Sr alloy shows an optimal tensile strength of 265.46 MPa.

Influence of Rare Earth Elements on Microstructure and Mechanical Properties of Mg-Li Alloys

A series of a-based Mg-Li-A1-Zn-xRE alloys were prepared. These alloys have low density ranging from 1.5 to 1.7 g·cm^-3 and high strength properties. The influence of RE element on the microstructure and the mechanical properties of these alloys were studied. The results indicate that the addition of RE （La, Pr, Ce） leads to the formation of rodshaped intermetallic compound Al2Zn2La distributed in the matrix. Al2Zn2La induces reduction of the laminar spacing and causes refinement of the microstructure. Therefore, this compound improves the strength of alloys at a high temperature.

Corrosion behavior and surface treatment of superlight Mg-Li alloys

Mg-Li alloy,as a superlight metallic engineering material,shows great potential in the fields of aerospace and militarydue to its high specific strength,better formability,and excellent electromagnetic shielding performance.The research process ofMg-Li alloys is reviewed and three main problems are pointed out.Aimed at the poor corrosion resistance of Mg-Li alloys,thecorrosion behavior is mainly summarized.The surface treatment technologies,including electroplating,electroless plating,plasmaspraying,molten salt replacement,conversion coating,anodizing,micro-arc oxidation,organic coating,and organic-inorganic hybridcoating,are introduced in detail.Finally,the future development of corrosion and protection of Mg?Li alloys is discussed.

Micro-arc oxidation coatings on Mg-Li alloys

Micro-arc oxidation (MAO) method was used for the surface modification of an Mg-5wt.%Li alloy. Ceramic coatings were in-situ fabricated on the Mg-Li alloy. The morphology feature,phase composition,and corrosion-resistance of the formed ceramic coatings were studied by SEM,XRD,and electrochemical methods,respectively. The results showed that the coatings produced in a sodium silicate solution system were composed of MgO and Mg2SiO4. The ceramic coating became thicker and the content of Mg2SiO4 phase increase...

Microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels

The microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels were studied by scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, and tensile test. The results show that Si can promote the transformation of austenite （γ） to ferrite （α）, enlarge the （α＋γ） region, and increase the aging stability of martensite by inhibiting carbide precipitation. Adding Cr leads to the formation of retained austenite and martensite/austenite （M/A） constituents, as well as the decomposi- tion of martensite during the overaging stage. Both of the steels show higher initial strain-hardening rates and two-stage strain-hardening characteristics. The C-Mn-Si-Nb steel shows the higher strain-hardening rate than the C-Mn-Cr-Nb steel in the first stage; however, there is no significant difference in the second stage. Although the tensile strength and elongation of the two steels both exceed 1000 MPa and 15%, respectively, the comprehensive mechanical properties of the C-Mn-Si-Nb steel are superior.

Rolling route for refining grains of super light Mg-Li alloys containing Sc and Be

Pioneering work on Sc or/and Be added Mg-Li alloys with refined grains was initiated. Various rolling-based thermo-mechanical treatments on these Mg-Li alloys were carried out. Four Mg-Li alloys were prepared by vacuum melting process. A unique route for producing fine grains was applied which concluded solution treatment at 350 ℃, cold rolling with 60% thickness reduction and 250 ℃ annealing, successively.

Cerium Chemical Conversion Coating on a Novel Mg-Li Alloy

A novel Mg-Li alloy was treated in a cerium nitrate solution and cerium chemical conversion coating was obtained on the alloy. Then the forming process, structure and corrosion resistance of the coating were investigated. The influential factors of cerium conversion coating were discussed through orthogonal experiments, and the optimum processing parameters were confirmed. XPS spectra displayed that the conversion coating consisted of cerium compounds, and the major component of the protective layer was a mixture of Ce （IV） oxide and Ce （IV） hydroxide. In addition, XRD pattern illustrated that there was crystalline CeO2 in the conversion coating. Analysis by SEM showed that the cerium conversion coating was uniform with a fiber-like morphology. The thickness of the conversion coating was 12 μm. The results of electrochemical potentiodynamic polarization and hydrogen evolution measurement indicated that the cerium conversion coating provided effective protection to the novel Mg-Li alloy.

Electrochemical formation and phase control of Mg-Li alloys

The electrochemical formation of Mg-Li alloys was investigated in a molten LiCl-KCl (58-42 mol%) eutectic melt at 723 K. The cyclic voltammogram. for a Mo electrode showed that the electroreduction of Li+ proceeds in a single step and the deposition potential of Li metal was -2.40 V (vs. Ag/AgCl). For Mg electrode, the electroreduction of Li+ takes place at less cathodic potential than that at the Mo electrode which was caused by the formation of Mg-Li alloys. Phase of the deposited Mg-Li alloys could be controlled by the electrolysis potential, and the samples were characterized by X-ray diffraction and scanning electron microscopy. The results showed that alpha-Mg and beta-Li phases were obtained at -2.35 and -2.55 V, respectively. (c) 2007 Mi Lin Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Preparation of Mg-Li alloys by electrolysis in molten salt at low temperature

A new technology for preparation of low cost Mg-Li alloys was studied. The alloys were prepared by electrolysis in molten LiCl-KCl （weight ratio is 1：1） electrolyte with Mg rod severing as the consumed cathode. Main factors that affect current efficiency were investigated, and optimal electrolysis parameters were obtained. Mg-Li alloys with low lithium Content （about 25%） were prepared by the unique method of a higher post-thermal treatment temperature after electrolysis at low temperature. The results showed that the electrolysis can be carried out at low temperature, which resulted in reducing preparation cost due to energy saving. The new technology for the oreoaration of Mg-Li alloy by electrolysis in molten salt was laroved to be feasible.

Microstructure and tensile properties of Mg-Li-Al-Zn based alloys with Ce addition

Mg-5Li-3Al-2Zn-xCe（x=0-2.5;mass fraction,%） alloys were prepared by casting,and heat treatments of homogenization at 300 °C and solid solution at 370 °C were carried out.The microstructure and tensile properties of as-cast alloys and their evolutions after solid solution were investigated.The results show that with the increase of Ce content,Al2Ce/Al3Ce precipitates are formed and the alloys mainly consist of α-Mg,Al2Ce,Al3Ce and AlLi phases,and the amount of AlLi and Al-Ce intermetallics decreases after solid solution.The content and morphology of the second phases have important effects on the mechanical properties of the alloys;the alloy with 1.0%Ce content exhibits excellent tensile strength.The tensile strength and elongation of Mg-5Li-3Al-2Zn-0.5Ce alloy is remarkably improved by the solution strengthening effect because of the addition of Ce.

Quasi-in-situ study on{10-12}twinning-detwinning behavior of rolled Mg-Li alloy in two-step compression(RD)-compression(ND)process

Twinning-detwinning(TDT)behavior in a strongly basal-textured Mg-Li alloy during two-step compression(RD)-compression(ND)process was investigated using quasi-in-situ EBSD.TDT behavior and TDT variants selection were statistically discussed with the loading path for the first time.Non-Schmid twinning behavior was observed in the first step compression,owing to the local stress fluctuations by neighboring twins;in contrast,Schmid’s law well predicted the detwinning variants selection.This asymmetrical TDT behavior was first investigated to date related with the strong basal texture and loading path.Besides,with the progress of compression,Schmid factors for twinning demonstrated a decreasing tendency;however,those for detwinning during the second step displayed an abnormally increasing trend,fundamentally stemming from prior twinning behavior.

Microstructure, cold rolling, heat treatment, and mechanical properties of Mg-Li alloys

The magnesium-lithium （Mg-Li） alloy exhibits two phase structures between 5.7wt% and 10.3wt% Li contents, consisting of the a （hcp） Mg-rich and the β （bcc） Li-rich phases, at room temperature. In the experiment, Mg-5Li-2Zn, Mg-9Li-2Zn, Mg-16Li-2Zn, Mg-22Li-2Zn, Mg-5Li-2Zn-2Ca, Mg-9Li-2Zn-2Ca, Mg-16Li-2Zn-2Ca, and Mg-22Li-2Zn-2Ca （wt%） were melted. During the melting process, the flux, which was composed of lithium chloride （LiCl） and lithium fluoride （LiF） in the proportion of 3：1 （mass ratio） and argon gas were used to protect the alloys from oxidation. The microstructure, mechanical properties, and cold-rolling workability of the wrought alloys were studied. The crystal grain of the alloys （adding Ga） is fine . The hardness of the studied alloys decreases with an increase in element Li. The density of the studied alloys is in the range of 1.187 to 1.617 g/cm^3. The reduction of the Mg-16Li-2Zn and Mg-22Li-2Zn alloys can exceed 85% at room temperature. The Mg-9Li-2Zn-2Ca alloy was heat treated at 300℃ for 8, 12, 16, and 24 h, respectively. The optimum heat treatment of the Mg-9Li-2Zn-2Ca alloy is 300℃×12h by metallographic observation and by studying the mechanical properties of the alloys.