Papers by Keyword: Mg-Li Alloys

Abstract: Lithium is an attractive element for Mg alloys for several reasons. It can improve room temperature ductility by transforming the single-phase hcp microstructure of Mg to a duplex phase followed by a single-phase bcc structure. With a solubility of ~5 wt.% Li, α-Mg is less prone to localized corrosion due to the absence of intermetallics. Furthermore, the strength of Mg-Li based alloys can be enhanced by alloying and thermomechanical processing. However, grain refinement has proven to be an effective mechanism in offsetting a compromise in ductility. It is for these reasons that Mg-Li based alloys have been the focus of great interest as a biomaterial where high strength, appreciable ductility and uniform corrosion behavior are required.

Abstract: Mg-9Li-3Al magnesium alloys with varied level of Nd added (i.e., 0-2.0wt. %) alloys were prepared electric resistance stir casting furnace under controlled inert gas atmosphere. Impact of Nd addition on metallographical changes and mechanical characteristics of as-cast alloys were analyzed by using an OM, XRD, SEM with EDS. The metallography evaluation revealed that adding Nd refines the microstructures due to formation of Al₂Nd and Al₁₁Nd₃ precipitates, these precipitates exists together with α-Mg, β-Li, AlLi, MgLi₂Al phases in the alloy. Furthermore, adding Nd improves the mechanical strength and elongation. 1.5wt.% Nd added alloy displayed the better mechanical characteristics; UTS is 166MPa; Elongation 16.87%.

Abstract: Relationship between microstructure and exfoliation corrosion in Mg-14mass%Li-3mass%Al cold rolled material were investigated by controlling microstructure using heat treatment. The exfoliation corrosion was exhibited on the specimens with elongated microstructure by cold-rolling. Heat treatment at 200 C only recovery occurred. Recovery and recrystallization were caused by heat treatment at 300 C However, heat treated at 300 C decreased precipitate fraction. Corrosion rate of heat-treated at 200 C decreased by release the residual strain. However, heat treatment at 300 C has lower corrosion resistance than heat-treated at 200 C because it has less precipitate density than as-rolled. Exfoliation corrosion was shown in as-rolled and heat-treated at 200 C. In as-rolled exfoliation corrosion was shown more significant when reduction rate increase. In as-rolled, exfoliation corrosion was shown more significant than heat-treated at 200 C. After heat treatment to change the grain shape from elongated to equiatial, exfoliation was suppressed by the distribution of AlLi phase.

Abstract: Super-high pressure (SHP) changes crystal structure and electronic distribution of metallic materials, which plays an important role in properties. Herein, a duplex Mg-7%wt.Li alloy was heat-treated under SHP (2 GPa) by cubic-anvil large-volume press with six rams for 2 h in the temperature range of 450~1350 °C. Microstructure, phase transformation behavior and mechanical properties were examined. Compared with the as-cast sample, the SHP samples after heat-treating from 450 °C to 750 °C under 2 GPa were composed of twinning in addition to duplex structure. Comparatively, the samples treated between 1050 °C and 1350 °C exhibit typical dendritic morphology. Phase transformation from Li₃Mg₇ phase or Li_0.92Mg_4.08 phase to Li₃Mg₁₇ phase occurred during the whole investigated temperature range, in which only the Li₃Mg₁₇ phase maintained when the temperature exceeds 1050 °C. The microhardness of the sample prepared at 750 °C under 2 GPa was 73.15HV, which is 1.5 times higher than that of the as-cast one. The improved microhardness is mainly attributed to the formation of nanosized twins during SHP treatment. These fine twins effectively prohibit the dislocation movement during deformation. It reveals the SHP is an effective approach to prepare high performation Mg alloys.

Abstract: The corrosion behavior of Mg-9Li-3Al-2.5Sr alloy, solution treated at 400°C for 3h with various aging temperature (T_a; T_a=75,100,125,150,175°C), was investigated in 3.5% NaCl solution by immersion test and polarization curve measurement. Experimental results suggest that the alloy aged at150°Cfor 6h exhibits the highest corrosion resistance while the alloy treated at 75°C for 6h presents lowermost corrosion resistance. The corrosion resistance of the alloy is related to the grain size and the distribution of Al₄Sr compound. The segregation of intermetallic compounds can accelerate the corrosion process. It was found that duo to the proper solid-solution and aging treatment, intermetallic compounds were distributed evenly along the grain boundaries, which can reduce the corrosion rate and corrosion current. In addition,with the growth of grain, the decrease of the grain boundary area per unit volume lead to improvement of corrosion resistance of the alloys.

Abstract: The room-temperature mechanical properties of the extruded Mg-5Li-3Al-2Zn-2Cu alloy (LAZ532-2Cu) were researched previously. In this paper, the creep behavior of the extruded LAZ532-2Cu alloy was studied at the temperature range from 398K to 448K, with the stress 60MPa, 80MPa and 100MPa. The microstructure of the alloy consists of the matrix α-Mg solid solution and the intermetallic compounds on the grain boundary or in the grain. The microstructural analysis of the alloy reveals the correlation between microstructure and creep properties. The stress exponent n 3.72, 4.8, 6.1, the activation energy Qc 94.8kJ/mol, 123.9kJ/mol, 128 kJ/mol were calculated at the test condition. The creep test samples were combined with detailed transmission electron microscopy and X-ray diffractometry in order to characterize the precipitated AlLi phase, which contributes to the creep resistance by obstructing the dislocation movement in dislocation creep.

Abstract: In this paper, a new biomedical Mg-Li alloy for the improvement of the comprehensive mechanical properties by micro-alloying and processing to meet the need of mechanical properties of biomedical materials. And the Mg-Li (Mg-Li-Al-Zn-Ca-Sr) alloy's processing and heat treatment were investigated in detail. The crystal texture of cast state, forged state and rolled state were observed and analyzed by OM, XRD and SEM. The mechanical properties of every stage were tested as well. The results showed that the grain size was refined obviously by the concentrating of Ca and Sr in the grain boundary. With the increase of rolling lane, the second phase's distribution was changed to a scattered state gradually from the reunion state. The tensile strength of the forged alloy was improved as well as its elongation after cold rolling and with rolled heat treatment process. The tensile strength reached 220MPa and the elongation reach 22%, which might meet the demand of cardiovascular stents mechanics.

Abstract: The as-cast Mg-14Li-3Al-(0-0.9)RE alloys were prepared with vacuum melting method, then processed by hot extrusion. The microstructure and tensile properties were investigated. The results show that both addition of RE and extrusion deformation can refine the grain size. Al₃La compounds are formed with addition of La-rich misch metal. The as-extruded Mg-14Li-3Al-0.6RE alloy obtains the finest grain size (4.28 μm) and the highest mechanical properties (σ_b =222.75 MPa, δ=23.8%), which is related to the grain refinement and the formation of Al₃La.

Abstract: A number of studies have shown that TiN film is correlated with corrosion resistance. In this study, we used a reactive direct current magnetron sputtering system to prepare TiN coating on Mg-Li alloys at low temperature. The intermediate TiN layer of thickness was about 1.6 μm from Ti target (99.99% purity). The structures of the resulting nanocatalysts were investigated, using X-ray diffraction analysis (XRD). The surface morphology of the coating was observed by Atomic Force Microscope (AFM). In the corrosive compare experiment the samples of Mg-Li alloys with and without titanium nitride film were put in solution with 5% NaCl respectively. We fund that the Mg-Li alloys with TiN coating has a lower rate of liberation of hydrogen, and the eroded surface morphology was examined by Scanning Electron Microscopy (SEM), it prove that the TiN deposition on the surface of Mg-Li alloys has improved the corrosion resistance performance.