Ductility Of Magnesium Alloys Research Articles

Improving the strength and ductility of magnesium alloys by Sc alloying: An experimental and first-principles study

Mg–Sc alloys have great application potential due to the positive effect of Sc on improving corrosion resistance and mechanical properties. In this work, the effects of Sc content on the strength and ductility of Mg-xSc (x=0, 1, 2, 3 at.%) alloys were investigated by experiments and first-principles calculations. The results indicate that, with the increase of Sc content, the hardness, strength, and ductility of the alloys increase significantly. The improvement of strength is attributed to the solid solution strengthening and grain size refinement effect. The addition of Sc reduces the prismatic unstable stacking fault energy of the alloys, thereby promoting the activation of stacking faults and <c+a> dislocations, resulting in the significant improvement of ductility. The mechanism of the enhanced ductility of Mg-xSc solid solution alloys is revealed, which provides guidance for designing high-performance Mg alloys.

Improvement of strength-ductility synergy of Mg–Al–Mn alloy using laser shock peening-induced gradient nanostructure

The trade-off between strength and ductility of magnesium alloy is a general bottleneck. Herein, we presented a method, laser shock peening (LSP), to overcome this problem. The key is the preparation of plastic strain-induced gradient nanostructure along the depth direction. Gradient nanostructure has unique deformation mechanism in the process of tensile deformation. Due to the cooperative effects of deformation twinning, dislocation movement, and dynamic recrystallization, the gradient nanostructured AM50 magnesium alloy contributed to ∼12% improvement of ultimate tensile strength, while ∼3% reduction of tensile ductility. Furthermore, the formation mechanisms of LSP-induced gradient nanostructure at initial plastic deformation stage, severe plastic deformation stage, and surface nanocrystalline stage were explored.

An experimental and crystal plasticity investigation of anisotropic compression behaviour of Mg-Sn-Ca alloy

The ductility of Magnesium alloys is often limited due to the strong basal texture. Initial attempts were made to reduce the intensity of the basal textures by adding rare earth(RE) elements. However, owing to the cost and scarcity of RE elements, alloys with calcium and tin were recently introduced. Among them, Mg-2Sn-2Ca alloys are the most promising, with high strength reported in the extruded condition. In this work, we report, for the first time, the mechanical properties of the alloy in the sheet form. In plane mechanical anisotropy in compressive behaviour is studied along with the detailed characterisation of texture, microstructure and the deformation twins in the material. Unlike strongly basal textured Mg alloys, deformation twinning was also observed in samples loaded along the normal direction. Crystal plasticity simulations were performed to confirm the slip and twin activity. Furthermore, a detailed analysis of deformation twins revealed that the so called Schmid twins accommodate strain by both twin growth and slip, whereas the non-Schmid twins accommodate the strain exclusively by crystallographic slip.

Investigation of crossed-twin structure formation in magnesium and magnesium alloys

In this work, the effect of alloying addition on the propensity for twin-twin interactions to transform into crossed-twin structures in magnesium alloys is investigated. A full-field elasto-viscoplastic fast Fourier transform (EVP-FFT) framework combined with a discrete twin model and dislocation density-based hardening law for slip strengths is used to calculate the micromechanical fields in the crystalline matrix around the interacting twins. AZ31 and MgLi alloys are selected along with pure Mg to study the influence of plastic anisotropy in connection with alloying elements. These alloys were selected since their plastic anisotropy measure, which is defined as the ratio between the critical resolved shear stress for pyramidal 〈c+a〉 and basal 〈a〉 slip modes, spanned a wide range. To quantify the role of twin thicknesses, we probe a range of impinging twin thicknesses while fixing the recipient twin thickness. The analysis reveals that: (i) the local driving stress for crossed twin structure formation generated from the interaction of the two twins is lower in a low plastically anisotropic material, like a MgLi alloy, than a high plastically anisotropic material like pure Mg and (ii) the critical impinging twin thickness needed to form the crossed twin structure in pure Mg, AZ31 and MgLi alloys is ∼0.5, ∼0.75 and ∼1.5 times the recipient twin thickness. We propose a relationship between the tendency for crossed twin structure formation and the experimentally observed higher ductility in MgLi alloys compared to pure Mg. One key implication of the findings is that crossed-twin structure formation can be hindered and the ductility of magnesium alloy thereby improved by properly choosing alloying elements that lower the slip strength for pyramidal c+a slip.

The effect of stress state and strain partition mode on the damage behavior of a Mg-Ca alloy

The ductility of magnesium alloys is greatly affected by damage formation and accumulation. Due to its low-symmetry close-packed hexagonal structure, magnesium alloy's damage behaviors are different from most cubic crystal metals. A comprehensive study was conducted to uncover the underlying damage mechanism of a Mg-0.5%wtCa alloy by using mechanical tests with different stress states, microstructure characterization and the crystal plasticity finite element method (CPFEM). It is found that the central hole tension has a greater resistance to damage than uniaxial tension, as confirmed by the results of digital image correlation (DIC) measurement and X-ray micro-computed tomography (XCT) characterization. This unusual phenomenon is related to the variation of fracture mechanisms in the two scenarios. For the uniaxial tension, plentiful grain boundary cracks are formed and the fractography shows an intergranular feature; while for the central hole tension, the propensity of intergranular fracture is less evident. Through combining the SEM observations of the boundary cracks and the strain distribution determined by the CPFEM simulation, it is revealed that the boundary cracking is caused by the intense deformation heterogeneity and the large strain gradient. Uniaxial tension has a diffuse deformation mode with the randomly distributed ″soft″ grains and ″hard″ grains. This strain partition mode results in a number of spots with high local strain gradient; these spots serve as the nucleation sites of the grain boundary cracks. The results of the notched tests with different notch radii also show the fact that the localized deformation mode is beneficial than the diffuse one. Therefore, in addition to the stress state, the strain partition mode also plays a vital role in the fracture of the dilute Mg-Ca alloy. Further corroboration was done by a three-bending test with gradient deformation mode. A large fracture strain was obtained in the test, indicating that the magnesium alloy has a potentially good formability when the forming process is reasonably designed.

Effect of MgO Content on Mechanical Properties of Directionally Solidified Pure Magnesium

The pure magnesium was fabricated by directional solidification, and the effect of the distribution characteristics of magnesium oxide (MgO) on the mechanical properties of pure magnesium was investigated. The statistical results showed that the area fraction, number and size of MgO were decreased gradually from the top region ingot to the bottom region ingot, and these reflected the advantages of directional solidification technology in the controllability of MgO distribution characteristics. The top parts of magnesium ingots have the highest tensile strength (44 MPa), which is mainly due to the presence of a large amount of the coarse MgO. Though the coarse MgO increases the strength obviously, it has harm for the ductility of magnesium. The top parts of magnesium ingots have higher ultimate tensile strength, but lower failure strain (13% and -21% respectively) than the ingot at the center. These results indicate that if the suitable size and amount of MgO existed in magnesium matrix, it could avoid the disadvantages of MgO and provide positive effect for both the strength and ductility of magnesium alloys.

Gradients of Strain to Increase Strength and Ductility of Magnesium Alloys

A strain gradient was produced in an AZ31B magnesium alloy through a plastic deformation of pure torsion at a torsional speed of π/2 per second. Compared with the base material and with the alloy processed by conventional severe plastic deformation, the magnesium alloy provided with a strain gradient possesses high strength preserving its ductility. Microstructural observations show that strain gradient induces the formation of an inhomogeneous microstructure characterized by statistically stored dislocation (SSD) density gradient and geometrically necessary dislocation (GND). GNDs and dislocation density gradient provide extra strain hardening property, which contributes to the improvement of ductility. The combination of SSD density gradient and GND can simultaneously improve the strength and ductility of magnesium alloy.

Role of grain boundaries on ductility in Mg-Y alloys

The impact of the grain size and the chemical composition of an alloying element on mechanical properties in ductility was investigated using wrought processed various Mg-Xat%Y binary alloys (where, X = 0.1, 0.3, 1.0 and 2.0). The average grain sizes of these Mg-Y alloys after a subsequent annealing process varied from around 5 µm to over 100 µm. The elongation-to-failure in tension was influenced by the grain size, irrespective of the yttrium content. The Mg-Y alloys consisting of meso‑grained structures (average grain size of ∼20–30 µm) exhibited good elongation-to-failure of ∼25–30%; however, “further” grain refinement led to a decrease in the properties of ductility. This tendency was confirmed to have no relation to the yttrium content. Deformed microstructural observations revealed that grain boundaries were the source of non-basal dislocation slips, but became crack-propagation sites. Although grain refinement from coarse- to meso‑sizes (up to around ∼20–10 µm) was effective, fine-grained alloys were unlikely to show a large elongation-to-failure due to grain boundary embrittlement. Not only the critical resolved shear stress but also the segregation energy of grain boundary is a considerable characteristic for alloying elements to improve ductility in magnesium alloys. Furthermore, in the case of fine-grained Mg-rare earth alloys, it is necessary to take into consideration that grain boundaries become not only the source of non-basal dislocation slips but also the route for crack-propagation.

Towards high ductility in magnesium alloys - The role of intergranular deformation

This paper investigates the intergranular deformation behavior and its effect on the overall ductility of a Mg-Gd-Y alloy, using in-situ tension in scanning electron microscopy (SEM) combined with electron backscattered diffraction (EBSD) and digital image correlation (DIC) techniques. At regions surrounding grain boundaries, the majority of activated dislocation slip traces were found to form in pairs across grain boundary, and the basal-to-basal (B-B) slip pair is the dominant type. Strain accommodation around the grain boundary interfaces is affected by both Schmid factor in the adjacent grains and the m’ value between these main slip systems. A mk value was proposed in this paper as the maximum value of the product of the Schmid factor and the m’ value of the adjacent grains to measure strain accommodation around the grain boundary interface, where a higher mk value would mean a larger local strain. It is suggested that the overall ductility of a Mg alloy could be evaluated by measuring its average mk value. As an evidence, the Mg-Gd-Y alloy tubes with higher mk value generally show higher elongations during tensile tests in this study.

Designing high ductility in magnesium alloys

The thermally activated pyramidal-to-basal (PB) transition of 〈c+a〉 dislocations, transforming glissile pyramidal dissociated core structures into sessile basal dissociated ones, lies at the origin of low ductility in pure magnesium (Mg). Solute-accelerated cross-slip and double cross-slip of pyramidal 〈c+a〉 dislocations have recently been proposed as a mechanism that can circumvent the deleterious effects of the PB transition by enabling rapid dislocation multiplication and isolating PB-transformed sessile segments. Here, the theory for solute-accelerated cross-slip is revisited with an explicit atomistic derivation, is extended to include multiple very dilute solute concentrations, and various aspects of the theory are demonstrated computationally. DFT inputs to the theory for a wide range of new alloying elements are presented. The theory is validated by comparing predicted ductility to literature experiments for a range of alloys. The theory is then applied to predict composition ranges for ductility in rare-earth free ternary and quaternary dilute alloys. The wide range of new alloys predicted to be ductile can serve as a guide to experimental development of new ductile Mg alloys.

Comparative Study of Two Aging Treatments on Microstructure and Mechanical Properties of an Ultra-Fine Grained Mg-10Y-6Gd-1.5Zn-0.5Zr Alloy

Developing high strength and high ductility magnesium alloys is an important issue for weight-reduction applications. In this work, we explored the feasibility of manipulating nanosized precipitates on LPSO-contained (long period stacking ordered phase) ultra-fine grained (UFG) magnesium alloy to obtain simultaneously improved strength and ductility. The effect of two aging treatments on microstructures and mechanical properties of an UFG Mg-10Y-6Gd-1.5Zn-0.5Zr alloy was systematically investigated and compared by a series of microstructure characterization techniques and tensile test. The results showed that nano γ’’ precipitates were successfully introduced in T5 peak aged alloy with no obvious increase in grain size. While T6 peak aging treatment stimulated the growth of α-Mg grains to 4.3 μm (fine grained, FG), together with the precipitation of γ’’ precipitates. Tensile tests revealed that both aging treatments remarkably improved the strengths but impaired the ductility slightly. The T5 peak aged alloy exhibited the optimum mechanical properties with ultimate strength of 431 MPa and elongation of 13.5%. This work provided a novel strategy to simultaneously improve the strength and ductility of magnesium alloys by integrating the intense precipitation strengthening with ductile LPSO-contained UFG/FG microstructure.

Mechanistic origin and prediction of enhanced ductility in magnesium alloys.

Pure magnesium exhibits poor ductility owing to pyramidal [Formula: see text] dislocation transformations to immobile structures, making this lowest-density structural metal unusable for many applications where it could enhance energy efficiency. We show why magnesium can be made ductile by specific dilute solute additions, which increase the [Formula: see text] cross-slip and multiplication rates to levels much faster than the deleterious [Formula: see text] transformation, enabling both favorable texture during processing and continued plastic straining during deformation. A quantitative theory establishes the conditions for ductility as a function of alloy composition in very good agreement with experiments on many existing magnesium alloys, and the solute-enhanced cross-slip mechanism is confirmed by transmission electron microscopy observations in magnesium-yttrium. The mechanistic theory can quickly screen for alloy compositions favoring conditions for high ductility and may help in the development of high-formability magnesium alloys.

Achieving High Strength and Ductility in Magnesium Alloys via Densely Hierarchical Double Contraction Nanotwins.

Light-weight magnesium alloys with high strength are especially desirable for the applications in transportation, aerospace, electronic components, and implants owing to their high stiffness, abundant raw materials, and environmental friendliness. Unfortunately, conventional strengthening methods mainly involve the formation of internal defects, in which particles and grain boundaries prohibit dislocation motion as well as compromise ductility invariably. Herein, we report a novel strategy for simultaneously achieving high specific yield strength (∼160 kN m kg-1) and good elongation (∼23.6%) in a duplex magnesium alloy containing 8 wt % lithium at room temperature, based on the introduction of densely hierarchical {101̅1}-{101̅1} double contraction nanotwins (DCTWs) and full-coherent hexagonal close-packed (hcp) particles in twin boundaries by ultrahigh pressure technique. These hierarchical nanoscaled DCTWs with stable interface characteristics not only bestow a large fraction of twin interface but also form interlaced continuous grids, hindering possible dislocation motions. Meanwhile, orderly aggregated particles offer supplemental pinning effect for overcoming latent softening roles of twin interface movement and detwinning process. The processes lead to a concomitant but unusual situation where double contraction twinning strengthens rather than weakens magnesium alloys. Those cutting-edge results provide underlying insights toward designing alternative and more innovative hcp-type structural materials with superior mechanical properties.

Effect of alloying elements on room temperature tensile ductility in magnesium alloys

The impact of alloying elements on the room temperature tensile behaviour was investigated for a wide range of strain rates using eight types of extruded Mg-0.3 at.% X (X = Ag, Al, Li, Mn, Pb, Sn, Y and Zn) binary alloys with an average grain size of 2–3 μm. The solid solution alloying element affected not only tensile plasticity but also rate-controlling mechanism for these fine-grained magnesium alloys. Most of the alloys exhibited an elongation-to-failure of 20–50% , while the alloys with a high m-value exhibited large tensile plasticity, such as an elongation-to-failure of 140% in a strain rate of 1 × 10−5 s−1 for the Mg–Mn alloy. This elongation-to-failure is more than two times larger than that for pure magnesium. This is due to the major contribution of grain boundary sliding (GBS) on the deformation. Microstructural observations reveal that grain boundary segregation, which is likely to affect gain boundary energy, plays a role in the prevention or enhancement of GBS. The present results are clearly expected to open doors to the development of magnesium alloys with good secondary formability at room temperature through the control of alloying elements.

Strength and ductility with {10͞11} - {10͞12} double twinning in a magnesium alloy.

Based on their high specific strength and stiffness, magnesium alloys are attractive for lightweight applications in aerospace and transportation, where weight saving is crucial for the reduction of carbon dioxide emissions. Unfortunately, the ductility of magnesium alloys is usually limited. It is thought that one reason for the lack of ductility is that the development of — double twins (DTW) cause premature failure of magnesium alloys. Here we show with a magnesium alloy containing 4 wt% lithium, that the same impressively large compression failure strains can be achieved with DTWs as without. The DTWs form stably across the microstructure and continuously throughout straining, forming three-dimensional intra-granular networks, a potential strengthening mechanism. We rationalize that relatively easier <c+a> slip characteristic of this alloy plastically relaxed the localized stress concentrations that DTWs can generate. This result may provide key insight and an alternative perspective towards designing formable and strong magnesium alloys.

Strength and ductility optimization of Mg–Y–Nd–Zr alloy by microstructural design

Understanding of essential microstructural features necessary to develop an optimum combination of strength and ductility in magnesium alloys is needed for materials by the microstructural design approach. Microstructural features required to optimize both strength and ductility of Mg–Y–Nd–Zr alloy were investigated by examining different thermo-mechanical processing conditions. Different microstructural states were obtained by either isothermally aging the hot-rolled base alloy or by friction stir processing (FSP) the base alloy and then aging the FSP microstructure. The plastic deformation behavior of different microstructural states was characterized by uniaxial tensile testing, whereas microstructural features were probed using scanning electron microscope, electron backscatter diffraction, and transmission electron microscope. As expected, precipitate formation within α-Mg matrix increased the alloy strength irrespective of processing condition. Concomitantly, the base alloy experienced a drastic drop in ductility. The presence of twins, coupled with a loss of toughness due to aging heat treatment, resulted in very poor ductility of the alloy. A good combination of strength and ductility was achieved by altering the base Mg–Y–Nd–Zr microstructure through FSP and subsequent aging. In addition to producing refined grains, FSP resulted in twin-free microstructure. The refined microstructure had a near-random texture, where a large fraction of grains was favorably oriented for plastic deformation. In this condition, intragranular precipitation increased the strength while retaining good ductility by activating additional slip systems. These results indicate that an optimum balance of strength and ductility is achieved by engineering a microstructure containing randomly-oriented fine grains, intragranular precipitation and minimal twins within α-Mg matrix. An empirical correlation among ductility, work hardening exponent, and slope of work hardening vs. flow stress curve was established, and will serve as a useful guideline for designing microstructure for the optimization of strength and ductility in magnesium alloys.

Improved strength and ductility of magnesium alloy below micro-twin lamellar structure

The effect of {10−12} micro-twin lamellar structure on the mechanical properties of magnesium alloy has been examined. It has been found that the detwinning behavior effectively increases the flow stress of Mg alloy, and this strengthening effect becomes more significant with the volume fraction of twin lamellae increasing for the annealed materials. Meanwhile, the refined microstructure caused by the micro-twin lamellae plays an important role in improving the ductility of materials. But a latent Hall–Petch hardening correlated to grain refinement by twin lamellae was not substantiated by experiments, indicating strengthening from twin boundaries is relatively less pronounced below twin lamellae of micrometer scale. Here, the high density of twin boundaries and the high volume fraction of twins are considered as the structural characteristics of the critical {10−12} twin lamellae, which can effectively optimize strength and ductility of magnesium alloy.

Modeling the strength and ductility of magnesium alloys containing nanotwins

ABSTRACTMagnesium alloys have been receiving much attention recently as potential lightweight alternatives to steel for automotive and other applications, but the poor formability of these alloys at low temperatures has limited their widespread adoption for automotive applications. Recent work with face centered cubic (FCC) materials has shown that introduction of twins at the nanometer scale in ultra-fine grained FCC polycrystals can provide significant increase in strength with a simultaneous improvement in ductility. This objective of this work is to explore the feasibility of extending this concept to hexagonal close packed (HCP) materials, with particular focus on using this approach to increase both strength and ductility of magnesium alloys. A crystal plasticity based finite element (CPFE) model is used to study the effect of varying the crystallographic texture and the spacing between the nanoscale twins on the strength and ductility of HCP polycrystals. Deformation of the material is assumed to occur by crystallographic slip, and in addition to the basal and prismatic slip systems, slip is also assumed to occur on the {1 0 $\bar 1$ 1} planes that are associated with compression twins in these materials. The slip system strength of the pyramidal systems containing the nanotwins is assumed to be much lower than the strength of the other systems, which is assumed to scale with the spacing between the nanotwins. The CPFE model is used to compute the stress-strain response for different microstrucrutral parameters, and a criterion based on a critical slip system shear strain and a critical hydrostatic stress is used to compute the limiting strength and ductility, with the ultimate goal of identifying the texture and nanotwin spacing that can lead to the optimum values for these parameters.

High temperature tensile response of nano-Al2O3 reinforced AZ31 nanocomposites

Nano-Al2O3 reinforcement's capability to simultaneously enhance the room temperature (25°C) strength and ductility of magnesium alloys has effectively been exploited in ingot metallurgy processed AZ31/1.5Al2O3 nanocomposite in this study. Tensile characterization revealed that at high temperature (150–250°C), instead of strengthening, the thermally stable nano-Al2O3 reinforcement ironically exacerbated the softening of AZ31 alloy. However, an incredible increment in AZ31 alloy (with grain size of ∼2.3μm) ductility (up to 184%) has been achieved in the nanocomposite with increasing temperature due to the incorporation of nano-Al2O3 as reinforcement. Microstructural characterization of the nanocomposite revealed that the dynamic recrystallization process has induced a complete recrystallization in AZ31 alloy matrix at a relatively much lower temperature (150°C) with tremendous grain growth near the fracture surface at higher temperature (250°C). Fractography of the nanocomposite revealed that the room temperature mixed ductile mode fracture behavior of AZ31 alloy transformed to a complete ductile mode at high temperature due to the presence of nano-Al2O3 particulates.

Anisotropy of Deformation Behavior of AZ31 Magnesium Alloy Sheets

In order to develop magnesium alloy sheets with high formability at room temperature, the anisotropy of deformation behavior of AZ31 magnesium alloy sheets produced by equal channel angular rolling were examined, which were compared with that of the sheets produced by the unidirectional hot rolling. The differences in the deformation behavior of the sheets at the rolling direction (0°), 45° and the transverse direction (90°) were discussed in term of the texture and microstructure. Compared with the as-received specimens, the anisotropy of deformation behavior of AZ31 magnesium alloy sheet produced by equal channel angular rolling was enhanced, which was following with an improved ductility and a large work hardening phenomenon. These could be due to the non-basal texture, which was induced by the continuous shearing deformation during equal channel angular rolling procedure. The fracture mechanism transferred from the cleavage fracture for the unidirectional rolling to the quasi-cleavage fracture for the sheets produced by equal channel angular rolling, which proved that the non-basal texture was in favor of the ductility of magnesium alloy.