Ductility Of Mg Research Articles

Energetics and stability of screw dislocations in magnesium: A first-principles study

Magnesium (Mg), as the lightest structural metal, suffers from poor ductility and formability. Although in literature, the cross-slips in Mg have been identified as the key factors that are decisive for ductility, there are still many controversies and ambiguities in these cross-slip processes, e.g., energy, stability, etc. In this study, we propose a force correction method to eliminate the surface effects, which thus effectively enhances the efficiency/accuracy of the first-principles DFT calculations of screw dislocations. Based on this method, both a and c+a screw dislocations are calculated with convergence carefully examined. Our results show that a dislocation on the Basal plane has a lower energy than on the Prism plane, by 62 meV/b. The Prism core is energetically unstable since no barrier appears in the Prism-Basal cross-slip path. However, the energy landscape around the Prism core is extremely flat, where the tiny configurational forces could be below the convergence criterion, making the Prism core obtainable in the atomistic simulations. Then for the c+a dislocation, the ground state is revealed to dissociate on the Pyramidal II plane. Since multiple metastable c+a core structures are found on the Pyramidal planes, the energy difference during the c+a cross-slip is thus within a range of 8–19 meV/b. All these findings here provide the basis and guidance for understanding and enhancing the ductility in Mg and Mg alloys.

The discrepancy in basal slip hardening rates of cast and extruded Mg-Y-Zr alloys

The differences in the deformation mechanisms during uniaxial tension of cast and extruded Mg–2Y-0.5Zr (wt%) alloys have been investigated by using EBSD-based slip trace analysis and VPSC simulation methods, and a novel method for counting the activity of slip/twinning using grain area has been proposed. Slip trace analysis reveals that enhanced ductility and reduced tensile-compressive asymmetry observed in the Mg–2Y-0.5Zr (wt%) alloy can be attributed to the addition of the Y element, which modified the texture, and promoted pyramidal <c + a> slip and suppressed prismatic <a> slip. The extruded alloy exhibits a distinctive pseudo-fiber bimodal microstructure, which contributes to its superior ductility. The VPSC results indicate that the CRSSnon-basal/CRSSbasal ratio of the extruded alloys decreases, whereas the CRSStwinning/CRSSbasal ratio increases, which is attributed to the different effects of the grain refinement on the different deformation modes. In addition, both the VPSC results and the slip trace analysis results indicate that the extruded alloy exhibits reduced non-basal slip activity compared to the cast alloy at similar strain levels. The work-hardening rate curves and VPSC parameters give a reasonable explanation that the key to the increase in the ductility of Mg–Y–Zr alloys after extrusion is the decrease in the basal hardening rate rather than the non-basal slip activity. The refinement of grain size leads to a significant decrease in basal slip hardening rate, primarily due to the decrease in the additional hardening effect caused by tensile twinning and the generation of an additional GBS mechanism in the pseudo fiber microstructure.

Influence of Al and Ca on the Ductility of Mg–Al–Ca Alloys

Magnesium and its alloys offer huge potential for lightweight applications. However, many Mg alloys suffer from limited room‐temperature formability. It has previously been shown that the addition of aluminium and calcium to Mg can improve ductility. Therefore, in the present work, it is aimed to systematically vary the alloying content of Al and Ca and to study their effects on the slip system activity and crystallographic texture after rolling and recrystallization. In the results, it is shown that all investigated ternary alloys in the range of 1–2 wt% Al and 0.005–0.5 wt% Ca have an increased ductility (in the range of 10–17% increase compared to pure Mg), whereas the binary Mg–Al and Mg–Ca alloys suffer from limited ductility (10% tensile elongation) and strength (175 MPa ultimate tensile strength). Non‐basal and especially &lt;c + a&gt;‐slip is active in all compositions (in 15–33% of the grains examined). Basal‐type textures are observed for all compositions, but with significantly weaker basal peak intensities for Ca‐containing samples when compared to pure Mg. The combination of activation of the &lt;c + a&gt; slip system and texture weakening is discussed as being responsible for the improved ductility of the ternary Mg–Al–Ca alloys.

The mechanism for Li-addition induced homogeneous deformation in Mg-4.5wt.% Li alloy

The mechanism by which Li additions induce a more homogeneous deformation was systematically investigated in this study by comparing the deformation behavior of pure Mg and Mg-4.5 wt.% Li using experiments, crystal plasticity simulations, and theoretical calculations. The results of the digital image correlation measurements showed that under tension along the transverse direction (TD), strong localized deformation bands with an angle of approximately 20° with respect to the TD were observed in pure Mg, but were absent in Mg-4.5 wt.% Li. Slip trace analyses indicated that basal slip was the predominant deformation mode for pure Mg, whereas that for Mg-4.5 wt.% Li was prismatic slip. A high relative activity of pyramidal 〈c+a〉 slip, approximately 17 %, was observed in Mg-4.5 wt.% Li at a high strain of 10 %. Contraction or double twins were hardly detected during the entire tension process. Crystal plasticity simulations revealed that the 4.5 wt.% Li addition dramatically reduced the critical resolved shear stress (CRSS) ratio of pyramidal 〈c+a〉 slip: prismatic slip: basal slip, which was 31.5:21:1 for pure Mg and 4.3:1.3:1 for Mg-4.5 wt.% Li. Theoretical calculations using the CRSS ratios determined by simulations revealed that easy and uniform deformation propagation induced by a dramatically reduced CRSS ratio of prismatic slip to basal slip played a more important role in improving the deformation homogeneity, in addition to the enhanced activity of pyramidal 〈c+a〉 slip. The mechanisms for how Li additions improve the ductility of Mg are also revisited in this paper.

Twin nucleation from disconnection-dense sites between stacking fault pairs in a random defect network

Deformation twinning, an important plastic deformation mode, critically influences the strength and ductility of Mg. As such, a fundamental understanding of twin nucleation will provide us with an important insight into the conditions that favor the onset of this deformation mode. To reveal the twin nucleation mechanism, we construct a defect network without making assumptions about the types and relative densities of dislocations present. The generated defect network features a high density of I1 stacking faults as well as disconnections between them. Deformation modeling results suggest that this structure enables a twin nucleation event from disconnection-dense sites between I1 stacking faults pairs. By probing twin nucleation and early-stage growth inside of the random defect network, we propose a geometry-based twin variant selection rule as well as a pure-shuffle twin nucleation and early-stage growth mechanism underpinning twinning in Mg.

Ultra-light Mg–Li alloy by design to achieve unprecedented high stiffness using the CALPHAD approach

Over decades, Mg–Li alloys have been widely used in aerospace industries owing to their low density (<1.65 g/cm3), medium strength (UTS: 130–200 MPa; YS: 100–170 MPa), and exceptional ductility (elongation: 5–30%). However, their stiffness is so poor (Young's Modulus: 45–47 GPa) that cannot meet many engineering design requirements such as space exploration and Lunar/Mars landing. Therefore, increasing modulus without degrading the strength and ductility of Mg–Li alloy has been a tough problem to be solved for many years. In this study, we have successfully made a significant breakthrough in improving the performance of Mg–Li alloys by inventing a new composition and a new processing route using CALPHAD for ultra-light Mg–Li alloys (density∼1.57 g/cm3), achieving high-strength (UTS: 335 MPa and YS: 290 MPa) and high-modulus (62.5 GPa). The origin of modulus improvement has been discovered by using a combination of SEM, TEM, XCT, nanoindentation, and neutron scattering experiments. Thermodynamically, it was found the high strength and modulus are attributed to the enhanced Mg–Mg bonding in the matrix and the increased elastic interaction forces from the lattice mismatch between the solute atoms and the solvent Mg. Meanwhile, the solution strengthening by lithium and precipitation hardening is discovered by inhibiting dislocation motion. Interestingly, age softening in Al–Li has been found to be a result of phase transformation from high-modulus particles into low ones using TEM, SANS, and nanoindentation tests.

Atomistic simulation of the dislocation interactions with the Al2Ca Laves phase in Mg–Al–Ca alloy

The mechanical properties of Mg–Al–Ca alloys are significantly affected by their Laves phases, including the Al2Ca phase. Laves phases are generally considered to be brittle and have a detrimental effect on the ductility of Mg. Recently, the Al2Ca phase was shown to undergo plastic deformation in a dilute Mg-Al-Ca alloy to increase the ductility and work hardening of the alloy. In the present study, we investigated the extent to which the deformation of Al2Ca is driven by dislocations in the Mg matrix by simulating the interactions between the basal edge dislocations and Al2Ca particles. In particular, the effects of the interparticle spacing, particle orientation, and particle size were considered. Shearing of small particles and dislocation cross-slips near large particles were observed. Both events contribute to strengthening, and accommodate to plasticity. The shear resistance of the dislocation to bypass the particles increased as the particle size increased. The critical resolved shear stress (CRSS) for activating dislocations and stacking faults was easier to reach for small Al2Ca particles owing to the higher local shear stress, which is consistent with the experimental observations. Overall, this work elucidates the driving force for Al2Ca particles in Mg–Al–Ca alloys to undergo plastic deformation.

Achieving ultra-high strength and ductility in Mg–9Al–1Zn–0.5Mn alloy via selective laser melting

Fabrication of the Mg–9Al–1Zn-0.5Mn alloy with excellent mechanical performance using selective laser melting (SLM) technology is quite difficult owing to the poor weldability and low boiling point. To address these challenges and seek the optimal processing parameters, response surface methodology was systematically utilized to determine the appropriate SLM parameter combinations. Mg–9Al–1Zn-0.5Mn sample with high relative density (99.5 ± 0.28%) and favorable mechanical properties (microhardness = 95.6 ± 5.28 HV0.1, UTS = 370.2 MPa and At = 10.4%) was achieved using SLM parameters (P = 120 W, v = 500 mm/s and h = 45 μm). Microstructure was primarily constituted by quantities of fine equiaxed grains (i.e., α-Mg phase) and a small amount of β-Al12Mg17 structures (i.e., 4.96 vol%). Samples was dominated by a random texture with numerous spherical β-Al12Mg17 ([21¯1¯0]α// [111]β), long lath-like β-Al12Mg17 ([21¯1¯0]α// [11¯5]β and [1¯011]α// [32¯1¯]β), and short rod-shaped Al8Mn5 nanoparticles. Benefiting from grain boundary strengthening, solid solution strengthening, and precipitation hardening of various nanoparticles (β-Al12Mg17 and Al8Mn5), high-performance Mg–9Al–1Zn-0.5Mn alloy biomedical implants can be fabricated. Precipitation hardening dominates the strengthening mechanism of the SLM Mg–9Al–1Zn-0.5Mn alloy.

Improved ductility of Mg–3Gd–0.6Zr alloy by cuboid-shaped phase: experiments and first-principle calculations

A casting Mg–3Gd–0.6Zr alloy was designed to evaluate the effect of cuboid-shaped phase on its mechanical properties. The morphology of cuboid-shaped phase was correlated with the directional covalent bonding between Gd atoms and H atoms along <100 > directions. The cuboid-shaped phase can be eliminated by a designed annealing treatment, without grain coarsening simultaneously. The ductility of as-cast alloy is about 20% higher than that of the annealed one. The improvement of ductility was caused by cuboid-shaped phase in three aspects: first-principle calculations, hydrogen embrittlement and crystal imperfections.

Is Mg17Al12 ductile or brittle? A theoretical insight

The Mg 17 Al 12 -phase, which is common and important in Mg-Al alloy, has long been regarded as a brittle phase in experiments but theoretical calculations report controversial results. To unravel why theoretical calculations report controversial results and determine whether Mg 17 Al 12 is brittle or ductile, density functional theory calculations on atomic level are performed to investigate mechanic properties of Mg 17 Al 12 without containing alloying elements and without taking the size effect. The results showed that the parameter k -point played critical role in the DFT-based elastic calculations. The convergent G/B ratio of Mg 17 Al 12 was about 0.52, suggesting that the Mg 17 Al 12 -phase was theoretically ductile although its ductility was poor. The chemical bonding in Mg 17 Al 12 was the mixture of metallic Mg-Mg bond and covalent Al-Al bond. The advantage of metallic bonding over covalent bonding provided a possible explanation for the ductility of Mg 17 Al 12 . Possible reasons for the brittleness of Mg 17 Al 12 in experiments are also discussed.

Enhancing mechanical properties and degradation performance of Mg−0.8wt.%Ca alloy by directional solidification

The mechanical properties and degradation performance in Hank's solution of the directionally solidified (DSed) Mg−0.8Ca (wt.%) alloys were investigated and compared with those of the as-cast alloy. The microstructure was studied by OM, SEM, TEM, and EBSD. It is found that the columnar grains in DSed alloys have a consistent growth orientation of <1120>. The mechanical strength and ductility of Mg−0.8Ca alloy are significantly improved after directional solidification. Besides, the corrosion resistance of Mg−0.8Ca alloy in Hank's solution is dramatically enhanced after directional solidification. The Mg−0.8Ca alloys in Hank's solution mainly undergo microgalvanic corrosion, and the corrosion products consist of Mg(OH)2, (Ca, Mg)3(PO4)2, and hydroxyapatite (HA). The superior properties of the DSed Mg−Ca alloy are beneficial from the columnar microstructure and the redistribution of the eutectics. The DSed Mg−0.8Ca alloy with columnar dendritic structure is expected to be used as biodegradable materials.

The enhanced Zn and Ca co-segregation and mechanical properties of Mg–Zn–Ce alloy with micro Ca addition

The micro addition of Ca is demonstrated to noticeably increase the strength and ductility of Mg–Zn–Ce alloy, which is closely correlated with the enhanced alloying segregation at grain boundary. Addition of Ca is found to yield the enhanced Zn segregation and co-segregation of Zn, Ce, and Ca. The mechanism behind is that Ca decreases the segregation energy while promoting the diffusion process of Zn, ultimately accelerating the thermodynamic and kinetic process of segregation. The enhanced alloying segregation induced by Ca effectively stabilizes grain boundary, which contributes to the refined grain size, retardant mobility of dislocation and ultimately improved strength. Furthermore, the intergranular fracture energy can be noticeably increased by Ca owing to the promoted alloying element segregation at grain boundary, which contributes to the improved grain boundary cohesion and ductility. The controllable segregation at grain boundary via alloying is intended to contribute to the development of advanced magnesium alloys.

Designing Mg alloys with high strength and ductility by reducing the strength difference between the basal and non-basal slips

The additions of alloying elements can significantly improve the mechanical properties of magnesium (Mg) alloys, mainly due to the fact that their additions change the critical shear stresses (CRSS) for dislocation slips. In this work, experimental and computational methods were used to explore the mechanisms responsible for the roles of Sm element addition in affecting the mechanical properties of Mg. The results showed that the addition of Sm obviously improves the microstructure and mechanical properties. It promotes the formation of twins and beneficially activated the non-basal <a> slip at the initial stage of plastic deformation, resulting in a high ductility. The Visco-Plastic Self-Consistent (VPSC) and the two-beam diffraction results confirmed that the pyramidal <c + a> slip and prismatic <a> slip were activated during tensile testing. The quantitative analysis of slip traces verified that the volume of non-basal slips reached 35 % after Sm addition. The additions of Sm with solid solution increased the activities of pyramidal <c + a> dislocation during deformation, which was beneficial to accommodate the c-axis strain, and finally improved the room temperature ductility of Mg. First-principle calculations demonstrate that the solute Sm atoms would reduce the stacking fault energy for basal and prismatic slips.

Achieving superior combination of yield strength and ductility in Mg–Mn–Al alloys via ultrafine grain structure

Ternary magnesium-based alloys with an aluminum content of 0.5 wt.% and a varying manganese content of x = 1, 2, and 3 wt.% (Mg-xMn-0.5Al) were prepared and characterized. The study focused on the effect of Mn addition on the formation of ultrafine grains (UFGs, average grain size <1 μm) and the mechanical properties of the alloys. After extrusion at a relatively low temperature of 250 °C, the average grain sizes of Mg-2Mn-0.5Al and Mg-3Mn-0.5Al samples were successfully refined to values of 0.9 ± 0.39 μm and 0.8 ± 0.34 μm, respectively, of submicron scale. The Mg-3Mn-0.5Al sample simultaneously demonstrated superior yield stress (YS, 274 MPa) and fracture elongation (FE, 48.9%). The high YS was associated with the UFG structure, residual dislocations, and nanosized precipitates, while the high FE was attributed to the low dislocation storage rate and UFGs with random orientations. The formation of UFGs is mainly caused by a recrystallization process promoted by particle-stimulated nucleation. The grain growth is suppressed by dynamic precipitated α-Mn particles.

The role of Ca, Al and Zn on room temperature ductility and grain boundary cohesion of magnesium

It is know from literature that small additions (<1 wt%) of Ca, Al and Zn significantly improve the intrinsic ductility of Mg. The exact role of each element, both qualitatively and quantitatively, and their combined effects, however, are poorly understood. Here we achieved a much clearer view on the quantitative role of each element with respect to ductility improvement and on the collaborative effect, particularly of Ca and Zn in Mg. Some of our findings and conclusions are in disagreement with data and interpretation found in literature. Four different alloys, namely, Mg-0.1 Ca, Mg-0.1 Ca-1 Al, Mg-0.05 Ca-1 Al, Mg-0.1 Ca-2 Al-1 Zn (all are in wt%) were selected for this investigation. All alloys were treated such that approx. similar grain sizes and textures were obtained. This largely excludes the effect of extrinsic factors on ductility. EBSD-guided slip trace analyses reveal that the addition of Ca eases activation of prismatic and pyramidal II slip systems. Using in-situ deformation experiments in SEM and atom probe tomography observations of grain boundaries direct evidence is given for the individual and synergetic effects of Ca and Zn on grain boundary cohesion as an important contribution to improve the ductility of these alloys. We conclude that Ca reduces the slip anisotropy and ameliorates ductility, however, the weak grain boundary cohesion in the Mg-0.1 wt% Ca alloy limits the material's tensile ductility. The addition of Zn alters the Ca segregation at the grain boundaries and helps to retain their cohesive strength, an effect which thus enables higher ductility and strength. The further addition of Al primarily improves the strength. The results show that the balanced influence of reduced slip anisotropy on the one hand and increased grain boundary cohesion on the other hand allow to design a high strength high ductility rare-earth free Mg alloy.

Simultaneously Improving Strength and Ductility of Mg−3Li−8Gd−2Y−1.5Ag Alloy by Solution Treatment and Hot‐Rolling Process

The microstructure and mechanical properties of Mg−3Li−8Gd−2Y−1.5Ag (wt%) alloy at as‐cast state, as‐solution state, and as‐rolled state (three rolling reductions of 30, 50, and 70%) are investigated. With the increase in solution time at 520 °C, the dissolution of Mg24(Gd,Y)5 phase along the grain boundaries and the coarsening of grain size occur. The solution parameters of 520 °C/4 h are chosen in combination with microstructural evolution, grain size, and microhardness. The dynamic precipitation of irregular spherical‐like β1, needle‐like β′, fine β, and twins is observed after 30% hot rolling at 450 °C. With increasing the reduction to 70%, the number of β1, β′, and twins increases and massive stacking faults (SFs) appear in the twins. The strength and ductility of Mg−3Li−8Gd−2Y−1.5Ag alloy are simultaneously improved by the combination of solution treatment and hot‐rolling process due to the precipitation strengthening of β1 and β′, the formation of twins and SFs, and the fragmentation of Mg24(Gd,Y)5 phase along the grain boundaries.

Effect of Alloying Elements on the Stacking Fault Energy and Ductility in Mg2Si Intermetallic Compounds

Alloying elements can pronouncedly change the mechanical properties of intermetallic compounds. However, the effect mechanism of this in Mg2Si alloys is not clear yet. In this paper, systematic first-principles calculations were performed to investigate the effect of alloying elements on the ductility of Mg–Si alloys. It was found that some alloying elements such as In, Cu, Pd, etc. could improve the ductility of Mg2Si alloys. Moreover, the interatomic bonding mechanisms were analyzed through the electron localization functional. Simultaneously, the machine-learning method was employed to help identify the most important features associated with the toughening mechanisms. It shows that the ground state atomic volume (VGS) is strongly related to the stacking fault energy (γus) of Mg2Si alloys. Interestingly, the alloying elements with appropriate VGS and higher Allred–Rochow electronegativity (En) would reduce the γus in the Mg–Si–X system and yield a better ductility. This work demonstrates how a fundamental theoretical understanding at the atomic and electronic levels can rationalize the mechanical properties of Mg2Si alloys at a macroscopic scale.

A novel method for improving the strength and ductility of Mg–Y–Er–Zn alloy using rotary-die equal-channel angular pressing

Abstract A novel method was developed via the continuous refinement of grains to prepare an Mg–6.5Y–1.2Er–1.6Zn (wt.%) alloy with a ductility of 35% and an ultimate tensile strength of 377 MPa. It involved the use of a 32-pass equal-channel angular pressing process. The strength and ductility of the obtained magnesium alloy after the rapid processing were improved by 188.5% and 700%, respectively, compared with an as-cast alloy. These improvements are attributed to the refined 14H-LPSO, uniform distribution of the spheroidized second phase, and a large number of dynamic recrystallized grains.

Enhanced ductility of Mg–1Zn–0.2Zr alloy with dilute Ca addition achieved by activation of non-basal slip and twinning

Wrought Mg–Zn–Zr alloys with a strong basal texture usually exhibit highly anisotropic plastic behavior, resulting in premature fracture during deformation at room temperature. In the present study, we prepared a Mg–1Zn–0.2Zr (ZK10) alloy sheet by multi-pass hot rolling and annealing, tailoring the basal texture and reducing the plastic anisotropy with the addition of dilute Ca. Microstructural characterizations of the tensile-deformed alloys show that pyramidal <c+a> slip and 101¯2 twinning are more easily formed in the Ca-containing alloy compared with those in the ZK10 alloy. The weakened basal texture involving Ca addition leads to promoted activation of 101¯2 twinning, without significant impact on the Schmid factor distribution of pyramidal <c+a> slip. Simulation results of the visco-plasticity self-consistent model further indicate that Ca reduces the ratio of critical resolved shear stress for pyramidal <c+a> slip and 101¯2 twinning versus basal <a> slip, leading to a significant improvement in ductility. This work may provide a new strategy to develop wrought Mg alloys with enhanced ductility.

A good balance between strength and ductility in Mg–Zn–Mn-Gd alloy

Microstructure evolution, dynamic recrystallization (DRX) behavior and mechanical properties of Mg–6Zn–1Mn-xGd alloys (x = 0, 0.2, 1.0 and 3.0 wt%) were investigated. The results showed that the phase components of as-cast Mg–6Zn–1Mn alloy were comprised of Mg7Zn3, α-Mn and α-Mg, while Mg3Zn6Gd (I phase) and Mg3Zn3Gd2 (W phase) were detected with the addition of Gd. Gd addition led to finer recrystallized grains, and it retarded DRX process due to the obstruction of nano-scale particles to grain boundaries. All as-extruded alloys exhibited typical basal texture, but Gd addition resulted in a low pole density of basal texture. Mg–6Zn–1Mn-3.0Gd alloy (ZMG613) exhibited a good balance between the strength and ductility with an ultimate tensile strength (UTS) of 354 MPa, yield strength (YS) of 306 MPa and elongation (EL) of 15.7%. Fine grain strengthening, second phase strengthening and dislocation strengthening were mostly contributed to the achievement of high strength. And the excellent ductility was mainly ascribed to the finer grains, weak basal texture, nano-scale I phase and non-basal slip activation.