Strength characteristics of Mg–Li alloys

We study the mechanical properties of Mg–Li alloys obtained by high-pressure die casting in a cold pressing chamber. A procedure of evaluation of the principal strength characteristics obtained under the conditions of high plastic strains is presented. The fracture toughness of materials under quasistatic and dynamic concentrated loads is investigated. Translated from Problemy Prochnosti, No. 3, pp. 78–88, May–June, 2009.     

Extrusion processing of Al–Li–Mg–Zr alloy

Abstract

The hot deformation behaviour of an Al–Li–Mg–Zr alloy was characterised in hot torsion and extrusion. The alloy was found to have similar hot ductility to existing high strength aluminium alloys, but this could be maintained at higher temperatures. Billets were extruded over a range of process conditions and a limit diagram was constructed for surface cracking. All the extrusions were found to be partially recrystallised after deformation, but the volume fraction of recrystallisation was a strong function of billet temperature and extrusion ratio. In addition, the unrecrystallised areas contained a recovered substructure where the subgrain size was inversely proportional to the temperature compensated strain rate. The as extruded structure was retained during solution treatment and as a result final mechanical properties were strongly dependent on the extrusion conditions. The use of high billet temperatures and low extrusion ratios gave the best combination of strength and toughness.

MST/839     

Compressive creep behaviour of Mg–Li–Al alloy

Abstract

The compressive creep behaviour of as cast Mg–14Li–1·3Al (wt-%) alloy was investigated in the temperature range of 20?85°C and under different compressive stress in the range of 37·3–74·6 MPa with special apparatus. Primary creep deformation and steady creep rate increase with temperature and applied stress. The compressive creep behaviour obeys an empirical equation ln t=C?nln σ + Q/RT, where t is the time to a selected creep strain, σ is the applied stress, T is the absolute temperature, R is the gas constant, and C, n, and Q are constants for the experimental alloy. The average values of the exponent n and the creep activation energy Q are 4·33 and 101·13 kJ mol^?1 respectively. The creep rate controlling mechanism is the dislocation climb and the lattice diffusion of Li in the experimental alloy under the testing conditions.     

High-frequency dielectric behaviour of Li–Mg ferrites

Dielectric properties such as dielectric constant (ε′) and dielectric loss tangent (tanδ) of mixed Li–Mg ferrites have been measured at room temperature in the frequency range 1 to 13 MHz using an HP 4192A impedance analyser. Plots of dielectric constant (ε′) vs. frequency show a normal dielectric behaviour of spinel ferrites. The frequency dependence of dielectric loss tangent (tanδ) is found to be abnormal, giving a peak at certain frequency for all mixed Li–Mg ferrites. A qualitative explanation is given for the composition and frequency dependence of the dielectric constant and dielectric loss tangent.     

Short transverse fracture of Al–Li–Cu–Mg–Zr extrudate

Abstract

The short transverse fracture toughness of an Al–Li–Cu–Mg–Zr extrudate was determined as a function of aging condition and testing temperature. To elucidate the underlying micromechanisms, the short transverse fracture surfaces of the extrudate were characterised via scanning electron microscopy, grain boundary precipitates and precipitation free zones were identified via transmission electron microscopy, and segregation of elements to grain boundaries was analysed using secondary ion mass spectrometry. Three principal observations were made as follows. First, with increasing aging time, the short transverse toughness of the extrudate increased when tested at room temperature, but decreased at liquid N₂ temperature, whereas with decreasing testing temperature, it remained essentially constant for the underaged condition, and decreased sharply for the peak aged and overaged tempers. Second, in addition to regions exhibiting shallow dimples, smooth ‘featureless’ zones were revealed on the short transverse fracture surfaces, which are intergranular in nature for all the specimens tested. The area fraction of the featureless regions decreased noticeably with increasing aging time when tested at room temperature, and increased markedly with decreasing testing temperature for the peak aged and overaged conditions. Third, segregation of Li, Si, Na, and H was detected for both the underaged and overaged specimens, and also of K for the underaged specimens only. In general, the enhancement of the room temperature short transverse toughness with aging and the negative effect of cryogenic temperature on fracture toughness are in obvious contrast to the in plane toughness behaviour reported in the literature, the featureless character of the short transverse fracture and its connection with poor toughness seldom having been emphasised. Based upon the present study, segregation induced brittleness is proposed as the critical micromechanism responsible for grain boundary weakness, and thus for the poor short transverse fracture toughness.

MST/1829     

Superplasticity of Al–Mg–Li alloy prepared by thermomechanical processing

Abstract

Grain refinement of Al–Mg–Li alloys for superplasticity prepared by thermomechanical processing has been a difficult task due to the cracking of these alloys when rolled at low temperatures. Raising the rolling temperature resulted in enhanced rollability of these sheets with no cracks but very coarse grains after recrystallisation. To solve this problem, a cross rolling schedule was developed to hinder fracture and simultaneously provide enough stored energy for following recrystallisaiton coupled by lowering the reheating temperature. Fine, equiaxed grains of ～7 μm was achieved by this new approach and maximum total elongation of about 915% was obtained when deformed at a temperature of 525°C and an initial strain rate of 1×10^?3 s^?1.     

Evaluation of properties and microstructure of non-heat treatable Al–Mg–Li–C–O alloys with variable Li concentration

Abstract

Fine grained Al–Mg–Li–C alloys, with lithium concentrations from 0.7 to 1.5 wt-%, have been produced by a mechanical alloying–powder metallurgy route. An initial range of compositions was chosen for manufacture into 10 kg billets which were uniaxially forged into plate; subsequently two compositions, alloy A (Al–3.7Mg–0.7Li–1.0C (wt-%) and alloy B (Al–4.4Mg–1.4Li–1.0C), were down-selected for a 20 kg scale-up exercise. Billets were forged at 300°C, using an 8:1 reduction ratio, which provided a sufficient level of work to develop properties, while avoiding excessive grain growth. Alloy B exhibited tensile properties (0.2% proof stress 450 MPa; ultimate tensile strength 510 MPa; strain to failure 6%) that exceeded the AECMA specification for AA 5091. Both alloys were confirmed as non-heat treatable and therefore exploitable in the as forged T1 condition. Microstructural analysis has confirmed that a fine grain size (<1 µm) and nanoscale Al₂O₃/Al₄C₃ and MgO dispersoids provided significant Hall–Petch and Orowan strengthening, respectively, capable of increasing the 0.2% proof stress to 450 MPa. Although optimisation of thermomechanical practice is still required, these Al–Mg–Li–C alloys show considerable potential for aerospace, land, and space applications.     

Microstructure and mechanical properties of Mg–8Li–xAl–0.5Ca alloys

The effect of the Al content on the microstructure and mechanical behaviour of Mg–8Li–xAl–0.5Ca alloys is investigated. The experimental results show that an as-cast Mg–8Li–0.5Ca alloy is mainly composed of α-Mg, β-Li and granular Mg₂Ca phases. With the addition of Al, the amount of α-Mg phase first increases and then decreases. In addition, the intermetallic compounds also obviously change. The microstructure of the test alloys is refined due to dynamic recrystallisation that occurs during extrusion. The mechanical properties of extruded alloys are much more desirable than the properties of as-cast alloys. The as-extruded Mg–8Li–6Al–0.5Ca alloy exhibits good comprehensive mechanical properties with an ultimate tensile strength of 251.2?MPa, a yield strength of 220.6?MPa and an elongation of 23.5%.     

Microstructure and mechanical properties of Mg–9Li–3Al–xGd alloys

The influence of gadolinium on the microstructure and mechanical properties of Mg–9Li–3Al alloy was investigated. Results show that the addition of Gd can effectively refine the α-Mg phase and change the morphology of the α-Mg phase. Meanwhile, the Al₃Gd phase is mainly distributed at the boundaries of the α-Mg phase and inside the α-Mg phase. The mechanical property tests reveal that the addition of Gd can effectively improve the mechanical properties of the as cast alloys. When the content of Gd is 2·0%, the tensile strength and yield stress (engineering stress) reach max values of 188 and 174 MPa respectively. When the content of Gd addition is 2·5%, the elongation of the alloy is 15·7%.     

Microstructure and mechanical properties of Mg–6Li–xAl–0.8Sn alloys

As-cast and as-extruded Mg–6Li–xAl–0.8Sn (x?=?0, 1, 3 and 5?wt-%) alloys were prepared. The microstructure and mechanical properties were investigated and discussed. The experimental results show that the Mg–6Li–0.8Sn alloy is composed of three phases: α-Mg, Mg₂Sn and Li₂MgSn. With the addition of Al, the test alloys display typical α-Mg?+?β-Li duplex structures. The new Mg₁₇Al₁₂ and LiMgAl₂ phases were found in the Mg–6Li–1Al–0.8Sn alloy. The lamellar-type AlLi phase was formed whereas the Mg₁₇Al₁₂ phase disappeared in Mg–6Li–3Al–0.8Sn alloy. The LiMgAl₂ phase vanished in the Mg–6Li–5Al–0.8Sn alloy. The mechanical properties of as-extruded alloys were remarkably improved. The as-extruded Mg–6Li–3Al–0.8Sn alloy exhibited the best mechanical properties, with a yield strength, tensile strength and elongation of 209.8?MPa, 242.6?MPa and 15.5%, respectively.     

Microstructure and precipitation in Al–Li–Cu–Mg–(Mn,Zr) alloys

Abstract

Hot rolled Al–6Li–1Cu–1Mg–0·2Mn (at.-%) (Al–1·6Li–2·2Cu–0·9Mg–0·4Mn, wt-%) and Al–6Li–1Cu–1Mg–0·03Zr (at.-%) (Al–1·6Li–2·3Cu–1Mg–0·1Zr, wt-%) alloys developed for age forming were studied by tensile testing, electron backscatter diffraction (EBSD), three-dimensional atom probe (3DAP), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). For both alloys, DSC analysis shows that ageing at 150°C leads initially to formation of zones/clusters, which are later gradually replaced by S phase. On ageing at 190°C, S phase formation is completed within 12 h. The precipitates identified by 3DAP and TEM can be classified into (a) Li rich clusters containing Cu and Mg, (b) a plate shaped metastable precipitate (similar to GPB2 zones/S″), (c) S phase and (d) δ′ spherical particles rich in Li. The Zr containing alloy also contains β′ (Al₃Zr) precipitates and composite β′/δ′ particles. The β′ precipitates reduce recrystallisation and grain growth leading to fine grains and subgrains.     

Microstructural evolution and superplastic behavior in friction stir processed Mg–Li–Al–Zn alloy

A Mg–Li–Al–Zn alloy was friction stir processed (FSP) under water, and the microstructures and superplastic behavior in the FSP alloy were investigated. The FSP Mg–Li–Al–Zn alloy consisted of a mixed microstructure with fine, equiaxed, and recrystallized α (hcp) and β (bcc) grains surrounded by high-angle grain boundaries, and the average grain size of the α and β grains was ~1.6 and ~6.8 μm, respectively. The fine α grains played a critical role in providing thermal stability for the β grains. The FSP Mg–Li–Al–Zn alloy exhibited low-temperature superplasticity with a ductility of 330 % at 100 °C and high strain rate superplasticity with ductility of ≥400 % at 225–300 °C. Microstructural examination and superplastic data analysis revealed that the dominant deformation mechanism for the FSPed Mg–Li–Al–Zn alloy is grain boundary sliding, which is controlled by the grain boundary diffusion in the β phase.     

Constitutive Modeling and Hot Deformation Behavior of Duplex Structured Mg–Li–Al–Sr Alloy

Hot deformation behavior of an as-extruded duplex structured Mg–9Li–3Al–2.5Sr alloy is investigated via hot compression tests conducted at 200–350 °C with strain rate of 0.001–1 s-1.The flow behavior of Mg–9Li–3Al–2.5Sr alloy can be described accurately by hyperbolic sine constitutive equation and the average activation energy for deformation is calculated as 143.5 k J/mol.Based on a dynamic materials model,the processing maps of Mg–9Li–3Al–2.5Sr alloy which describe the variation of power dissipation efficiency are constructed as a function of temperature and strain rate.The processing maps exhibit an area of discontinuous dynamic recrystallization occurring at 280–300 °C with strain rate of 0.001–0.01 s-1,which corresponds to the optimum hot working conditions.     

Effects of Li addition on the corrosion behaviour and biocompatibility of Mg(Li)–Zn–Ca metallic glasses

Magnesium alloys with suitable corrosion behaviour and good mechanical properties are desired for biodegradable materials. In the current study, novel Mg–Li-based metallic glasses (MGs) demonstrate potential clinical applications as implantable biodegradable materials. The amorphous structure of MGs provides suitable elastic modulus with human bone. The enhanced corrosion resistance of MGs realises a uniform corrosion process, as well as maintains a stable acid-based environment, and increases cell proliferation. A schematic model is proposed to illustrate the corrosion mechanisms of MGs. Adding Li significantly improves the corrosion resistance of MGs. Both the indirect cytotoxicity and direct cell culture assays are conducted using transfected osteoblasts (hFOB) cells. Results show that the novel Mg–Li–Zn–Ca MGs have good biocompatibility.     

Mg—Li基复合材料研究进展

Mg-Li基复合材料具有很高的比强度和比刚度，是宇航、兵器等行业的理想结构材料之一，本文综述了Mg-Li基复合材料常用的基体合金和增强体，制备方法以及复合材料的组织与性能，对目前存在的问题进行了探讨，并给出了可能的解决方法。     

Blood compatibility of zinc–calcium phosphate conversion coating on Mg–1.33Li–0.6Ca alloy

Magnesium alloys as a new class of biomaterials possess biodegradability and biocompatibility in comparison with currently used metal implants. However, their rapid corrosion rates are necessary to be manipulated by appropriate coatings. In this paper, a new attempt was used to develop a zinc-calcium phosphate (Zn-Ca-P) conversion coating on Mg-1.33Li-0.6Ca alloys to increase the biocompatibility and improve the corrosion resistance. In vitro blood biocompatibility of the alloy with and without the Zn-Ca-P coating was investigated to determine its suitability as a degradable medical biomaterial. Blood biocompatibility was assessed from the hemolysis test, the dynamic cruor time test, blood cell count and SEM observation of the platelet adhesion to membrane surface. The results showed that the Zn-Ca-P coating on Mg-1.33Li-0.6Ca alloys had good blood compatibility, which is in accordance with the requirements for medical biomaterials.     

New Al–Mg–Li–C dispersion strengthened alloy Part 1 – Composition and process development

Abstract

New fine grained aluminium–magnesium–lithium–carbon alloys, produced by mechanical alloying/powder metallurgy techniques, have been developed. The alloys are suitable for use in the as forged (T1) condition. A preferred chemistry (Al–5.2Mg–1.3Li–0.35%C, wt-%) has been defined, and methods for production of bulk (50 kg) billets and open die uniaxial forgings have been examined. These have produced attractive mechanical properties (0.2%PS=441 MPa, UTS=501 MPa and K_1C=23 MPa m^1/2) without recourse to cold compression. The new alloy is differentiated from AA 5091 by its much lower C content, and by being enriched in magnesium, which provides improved fracture toughness and superior powder handling ability. The balance of properties, achievable in a lightweight, non-heat-treatable alloy, make the alloy a candidate for aerospace, and other applications.     

Self-growth of micro- and nano-structured Mg(OH)2 on electrochemically anodised Mg–Li alloy surface

Formation of Mg(OH)₂ nanowires and their assemblies with sphere-shaped, echinus-shaped and nest-like micro-structures on a Mg-Li alloy surface was successfully demonstrated. Mg(OH)₂ nanowires were formed on the alloy surface via a self-growth process, after the alloy was electrooxidised in NaCl solution and the oxidation products were removed. The nanowires were characterised by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Infrared spectroscopy. The nanowires have hexagonal rod-like or brush-like morphology and are poorly crystallised. Pre-anodisation and the presence of Li in the alloy are key factors for the nanowire growth, which likely proceeds via a dissolution–precipitation mechanism. The roles of Li can be attributed to the prohibition of the formation of passivation film on the alloy surface and the acceleration of the formation of OH^? anions through hydrogen-evolution reaction.     

Intergranular fracture of Al–Li–Cu–Mg alloy resulting from non-equilibrium eutectic melting during solution treatment

Abstract

This investigation has examined intergranular fracture during heat treatment and deformation of an Al–Li–Cu–Mg alloy and of an Al–Li–Cu alloy. When solution treatment of the Al–Li–Cu–Mg alloy was initiated by rapid heating to temperatures ≥ 545°C, non-equilibrium eutectic melting of a grain boundary precipitate phase occurred and the liquid spread along grain boundaries as a thin film. On quenching, intergranular cracks were observed at grain boundaries into which a liquid film had penetrated during solution treatment. For less rapid heating rates, non-equilibrium eutectic melting did not occur and no intergranular cracks were observed after quenching. No evidence of non-equilibrium eutectic melting was observed in the Al–Li–Cu alloy irrespective of the rate of heating to 550°C. During tensile testing of as quenched and quenched and aged specimens of the two alloys, intergranular fracture occurred in most specimens, whether or not non-equilibrium eutectic melting had taken place during solution treatment, indicating that at least one additional mechanism of intergranular fracture was initiated by deformation.

MST/947     

Influence of yttrium addition on the corrosion behaviour of as-cast Mg–8Li–3Al–2Zn alloy

The influence of yttrium on the microstructure and corrosion behaviour of as-cast Mg–8Li–3Al–2Zn–xY alloys was investigated. The results show that Y addition leads to the formation of Al₂Y particles and the transformation of α-Mg from the long needle-like to the round-like. The noble Al₂Y particles on the grain boundary inhibit the galvanic corrosion between β-Li phase and α-Mg phase, and also weaken the corrosion occurrence inside β-Li phase due to the decrease of the AlLi phase. With Y addition, the corrosion resistance is improved gradually, especially when the content of Y is up to 1.5 wt-%. Moreover, Y addition makes the corrosion film become more compact, which can prevent the base materials from being attacked continuously.