Deformation Of Mg Research Articles

Severe Plastic Deformation of Mg–Zn–Zr–Ce Alloys: Advancing Corrosion Resistance and Mechanical Strength for Medical Applications

Magnesium-based alloys hold potential for medical applications, but face challenges like rapid bioresorption and limited mechanical strength during early bone healing. In our study, we present a novel Mg–Zn–Zr–Ce alloy with low cerium content (up to 0.1 wt.% Ce) processed using two severe plastic deformation (SPD) techniques. Through an innovative combination of multiaxial forging and multipass rolling, we have achieved a fine-grained structure with an average grain size of the primary α-Mg phase of 1.0 μm. This refined microstructure exhibits improved mechanical properties, including a substantial increase in yield strength (σYS) from 130 to 240 MPa, while preserving ductility. The alloy’s composition includes α-Mg grains, cerium and zinc hydrides, and intermetallic phases with cerium and zinc elements. Tensile testing of the fine-grained alloy demonstrates an enhancement in yield strength (σYS) to 250 MPa, marking a 2.8-fold improvement over the conventional state (σYS = 90 MPa), with a modest 2-fold reduction in ductility. Crucially, electrochemical tests conducted in physiological solutions highlight substantial advancements in corrosion resistance. The corrosion current was reduced from 14 to 2 μA/cm2, while polarization resistance decreased from 3.1 to 8.1 kΩ∙cm2, underlining the alloy’s enhanced resistance to biodegradation. Our results show that the novel Mg–Zn–Zr–Ce alloy, after combined SPD, demonstrates mitigated bioresorption and enhanced mechanical properties. Our findings highlight the fact that the introduction of this innovative alloy and the application of SPD represent significant steps towards addressing the limitations of magnesium-based alloys for medical implants, offering potential improvements in safety and effectiveness.

Achieving low activation energy and double-peak texture after hot deformation of Mg–Al–Zn–Ca–Mn–Zr alloy and enabling the strength and ductility

In this study, the hot-deformation behavior of a die-casted Mg–Al–Zn–Ca–Mn–Zr alloy was investigated in the temperature range of 548–673 K and the strain rate range of 0.0001–1 s−1. The constitutive equation and processing maps were also developed to describe the flow stress behavior. The numerical simulation result revealed an average activation energy of ∼139.4 kJ mol−1, which is lower than the previously reported AZ31 Mg alloys. Subsequently, the low activation energy imparted a high degree of dynamic recrystallization and was mainly attributed to alloying elements Ca, Mn, and Zr. For optimum hot working parameters, processing maps displayed two different domains, the temperature range of 573–673 K for a strain rate range of 0.001–0.01 s−1 with a peak efficiency of 33% and a temperature of 673 K at a high strain rate of 0.01 s−1 with a peak efficiency of 31%. The microstructure evolution confirmed the validity of parameters and revealed high dynamic recrystallization. The average grain size ∼21.1 μm, ∼15.4 μm, and ∼16.1 μm were achieved at temperatures of 573 K, 623 K, and 673 K under a strain rate of 0.001 s−1. The profuse grain refinement is attributed to the pronounced accumulation of dislocation on grain boundaries and on the eutectic phases. In addition, the morphology of the eutectic phases was broken with the increase in the strain rate and temperature. Thus, the α-Mg and eutectic phases bear the compressive stresses and promote the dynamic recrystallization activity. Last but not least, the double peak texture after hot deformation was developed, which is one of the engineers' main requirements for enhancing the Mg alloy's ductility. Based on the above discussion, the hot deformed specimens exhibited an ultimate compressive strength of ∼340 MPa and elongated to fracture >25%. Consequently, this study enables the understanding of the broad use of Mg–Zn–Al–Ca–Mn–Zr alloys for automobile applications.

Investigation of superplastic behaviour in double-pass friction stir processed Mg–Al–Zn alloy

Double-pass friction stir processing (FSP) engendered intense plastic deformation in Mg–3Al–1Zn alloy with variation in the most influential process parameters, namely tool rotation speed and tool traverse speed. The effect of change in the FSP processing route on the resulting grain size was also analysed. Uniformly distributed fine equiaxed grains (average grain size ∼ 6.15 μm) were acquired in the stir-zone (SZ) due to extensive dynamic recrystallization (DRX). Attainment of large grain refinement encouraged for further investigation of the alloy's superplastic behaviour at elevated temperatures. Microstructural examination was followed by uniaxial tensile tests that were carried out at three distinct temperatures of 350, 400 and 450 °C at the constant strain rate of 1.3 × 10−3 s−1. With the rising deformation temperature, reduction in the flow stress led to significant increase in the tensile elongations of all the processed specimens. Micro-grain superplasticity (elongation >200% under tension) was observed at the highest deformation temperature for the friction stirred specimen with the finest grains.

Precipitation of β-phase and twinning during deformation of Mg–Al alloy AZ91 at 150 °C after solution treatment

Abstract Precipitation of β-phase and deformation twinning have been studied in a solid solution-treated AZ91 alloy, which was subjected to compressive creep loading at 150 °C and 100 MPa for 163 h. It was found that nucleation of β-phase in solution-treated AZ91 is enhanced by concurrent deformation under creep conditions at 150 °C. The relatively fine precipitate structure leads to a reduction in minimum creep rate and, therefore, to an increase in creep life by a factor of 10 compared to the as-cast condition at the investigated conditions. In addition, twinning occurred in coarse grains of hard orientation. The twin boundaries lead to formation of incoherent precipitates and discontinuous precipitation within the former grains.

Influence of homogenization on the processing map for hot working of as-cast Mg–2Zn–1Mn alloy

Abstract Processing maps have been developed for hot deformation of Mg–2Zn– 1Mn alloy in as-cast condition and after homogenization with a view to evaluate the influence of homogenization. Hot compression data in the temperature range 300 – 500 °C and strain rate range 0.001 – 100 s– 1 were used for generating the processing map. In the map for the as-cast alloy the domain of dynamic recrystallization occurring at 450 °C and 0.1 s–1 has merged with another domain occurring at 500 °C and 0.001 s– 1 representing grain boundary cracking. The latter domain is eliminated by homogenization and the dynamic recrystallization domain expanded with a higher peak efficiency occurring at 500 °C and 0.05 s–1. The flow localization occurring at strain rates higher than 5 s–1 is unaffected by homogenization.

Local deformation of Mg–7Gd–5Y–1.2Nd–0.5Zr magnesium alloy under high speed impact along extrusion direction

The high strain rate deformation behavior of Mg–7Gd–5Y–1.2Nd–0.5Zr alloy was carried out at several temperatures. Through the truncated test of cap specimen, it was found that the local deformation occurred in the design shear zone, but no adiabatic shear band was observed. The deformation mechanism was mainly pyramidal slip, supplemented by prismatic slip, basal slip, and dynamic recrystallization. Among them, pyramidal 〈c + a〉 slip could coordinate deformation through the slip component along the 〈c〉 direction. The Johnson–Cook constitutive model and failure model were modified, and then the local deformation of the cap specimen along the extrusion direction was simulated by LS-DANY software. The results showed that the local severe deformation occurred in the area of the cap specimen with sharp change in shape, which was at an angle of 45 ° with the impact compression loading along extrusion direction. The temperature increased continuously with the amount of deformation until fracture occurred. The results showed that the alloy had high resistance to local high strain deformation and fracture.

Insights from the MEDE program: An overview of microstructure–property linkages in the dynamic behaviors of magnesium alloys

Magnesium (Mg) and its alloys have been the subject of intensive scientific research and development in the communities of materials science and engineering, mechanical engineering and manufacturing. Considering their light weight and high specific strength, current and potential applications include the aerospace industry, automobiles, and vehicle and personnel armors. This range of applications demands a good understanding of the behavior under extreme conditions such as impact or high strain rate loading. The past two decades have witnessed a surge of studies of the mechanical responses of Mg and its alloys under impact loading, both experimentally and using simulations and modeling at different spatial and temporal scales. Experimental examinations at strain rates up to 107 s−1 (shock wave loading) have been published. In terms of simulations and modeling efforts, multi-physics, multi-scale investigations, from first principles calculations (density functional theory, DFT), molecular dynamics (MD), discrete dislocation dynamics (DDD), crystal plasticity (CP) and continuum mechanics have all been explored. To address the challenges in design, manufacturing and application of Mg alloys, the US Army Research Laboratory (US-ARL) created the Materials in Extreme Dynamic Environments (MEDE) Collaborative Research Alliance (CRA) in 2012. The goal of the Metals Program within the MEDE CRA has been to observe, understand, and design the mechanisms active within Mg and Mg alloys in these extreme conditions.In this paper, fundamental aspects of plastic deformation of Mg and Mg-alloys and the history of the research efforts in experiments, modeling, and simulations available in the literature are critically reviewed. Key findings and contributions from the Materials in Extreme Dynamic Environments (MEDE) Metals Collaborative Materials Research Group (CMRG) are presented, followed by summary and future perspectives.

Grain-size effect on the deformation of Mg–3Al–3Sn alloy: Experiments and elastic-viscoplastic self-consistent modeling

Grain refinement can significantly change the tensile behavior of Mg alloys. The underlying mechanisms are unclear yet. Here we report that grain refinement of a Mg–3Al–3Sn (wt.%, AT33) alloy increases the yield strength (σ0.2) following the Hall-Petch law, while decreases the strain hardening exponent (h). Tensile stress-strain curves and lattice strain evolution of four rolled AT33 samples with different grains sizes (10–97 μm) can be simulated by an elastic-viscoplastic self-consistent (EVPSC) model that takes grain size into account. The key to the model's success is that it allows both the initial critical resolved shear stress (CRSS) and the dislocation nucleation coefficient of different slip modes to be functions of the grain size. The coarse-grained samples have higher dislocation nucleation coefficient due to concurrently formed deformation twins. The model predicts that prismatic slip is the dominant deformation mode in this AT33 alloy; pyramidal <a> and <c+a> slips are more active in coarse-grained samples than in fine-grained samples.

Superplastic deformation of Mg\u20139Li\u20132Al\u20130.5Sc alloy after grain refinement by KoBo extrusion and cyclic forging

The quaternary Mg–9Li–2Al–0.5Sc alloy (in wt%) was prepared from pure components. After homogenization, the alloy was subjected to severe plastic deformation by KoBo extrusion and cyclic forging leading to grain refinement in the range of 0.5–2 µm of hexagonal close-packed (HCP) α phase. Deformed alloys showed high ultimate tensile strength near 200 MPa and good elongation in the range 30–40% at room temperature (RT). Large elongations close to 200% were obtained during the tensile test at a temperature of 200 °C. Deformed samples showed the presence of multiple voids confirming grain boundary sliding mechanism of deformation. Twins on {10overline{1}2} planes were identified using electron backscatter diffraction analysis, being in a good agreement with the earlier observation of Mg–Li and Mg–Sc alloys. Intermetallic phases such as cubic MgSc were identified in deformed alloys mostly within HCP α phase, whereas HCP MgSc2 particles were observed within body-centered cubic (BCC) β phase. Intermetallic phases were responsible for RT strengthening of alloys and slightly lower tensile elongation during superplastic deformation. Formation of the HCP α phase was observed within the BCC β phase in tensile deformed alloys. Atomic-level nucleation of HCP phase within the β phase was identified by the use of high-resolution transmission electron microscopy technique.

Effect of temperature on the suppression of twinning in textured magnesium

In this work, the effect of temperature, in the range of 25 to 250 °C, on deformation twinning in textured polycrystalline pure magnesium (Mg) was investigated. Compression loading was applied perpendicular to the c-axis texture direction. The yield strength and strain hardening rate are shown to drastically decrease with increasing temperature with total suppression of twinning at 200 °C. This behavior is attributed to the decrease in the critical resolved shear stress for prismatic slip and temperature insensitivity of tensile twinning. These results provide a first step in fundamentally understanding the deformation of Mg at elevated temperatures and quantify the mechanisms that lead to their improved formability at elevated temperatures.

A micromechanical constitutive model for unusual temperature-dependent deformation of Mg–NiTi composites

Based on the mean-field approach, a micromechanical constitutive model is established to describe the unusual temperature-dependent deformation of Mg–NiTi composites. Firstly, the constitutive equations of two phases are proposed: for NiTi shape memory alloy (SMA), a simplified Hartl–Lagoudas model (Hartl and Lagoudas, 2009) which considering the martensite transformation and plastic deformation is adopted; while, for magnesium (Mg), an elastic-plastic model including a new nonlinear plastic hardening law is employed. The dependence of elastic moduli and yield surfaces of two phases at ambient temperature is addressed. Then, a non-isothermal incremental Mori–Tanaka homogenization method is employed and further extended to describe the interaction between two phases and calculate the macroscopic overall stress-strain response of Mg–NiTi composites. Finally, comparisons between the simulated results and the corresponding experimental ones show that the temperature-dependent deformation of the composites with different volume fractions of NiTi SMA phase can be well captured by the proposed model. Predicted results demonstrate that the unusual temperature-dependent deformation of Mg–NiTi composites originates from the change in the inelastic deformation mechanism of NiTi SMA with the variation of temperature.

The effect of Sc on plastic deformation of Mg–Sc binary alloys under tension

An investigation into the effect of scandium on the mechanical properties of dilute binary Mg–Sc alloys deformed under tension has been conducted at 298 K, 78 K and 4.2 K. The relatively small improvement in strength and ductility is discussed in terms of the effect of Sc on texture, deformation modes and solute strengthening. Constitutive modelling results predict an increased relative activity of prismatic 〈a〉 and 〈c+a〉 slip with Sc solute addition. A minor decrease in stacking fault energy (SFE) of Mg due to Sc addition and its small atomic size are considered as determining factors in overall mechanical properties of alloys. No significant effect of alloy composition on strain rate sensitivity (SRS) is observed. An activation distance, d≈0.2b, is measured at 298 K and 78 K and is interpreted as being determined by dislocation–dislocation interactions, which use sequential activation of component partial dislocations during dislocation cutting.

Clustering on Magnesium Surfaces \u2013 Formation and Diffusion Energies

The formation and diffusion energies of atomic clusters on Mg surfaces determine the surface roughness and formation of faulted structure, which in turn affect the mechanical deformation of Mg. This paper reports first principles density function theory (DFT) based quantum mechanics calculation results of atomic clustering on the low energy surfaces {0001} and {bar{1}011}. In parallel, molecular statics calculations serve to test the validity of two interatomic potentials and to extend the scope of the DFT studies. On a {0001} surface, a compact cluster consisting of few than three atoms energetically prefers a face-centered-cubic stacking, to serve as a nucleus of stacking fault. On a {bar{1}011}, clusters of any size always prefer hexagonal-close-packed stacking. Adatom diffusion on surface {bar{1}011} is high anisotropic while isotropic on surface (0001). Three-dimensional Ehrlich–Schwoebel barriers converge as the step height is three atomic layers or thicker. Adatom diffusion along steps is via hopping mechanism, and that down steps is via exchange mechanism.

Constitutive modeling and understanding of the hot compressive deformation of Mg–9.5Zn–2.0Y magnesium alloy with reduced number of strain-dependent constitutive parameters

The hot compressive flow behavior of the cast Mg–9.5Zn–2.0Y alloy as a function of strain was analyzed, and the degree of dependence of the parameters (A: material constant, n 2: stress exponent, Q c: activation energy for plastic flow and α: stress multiplier) of the constitutive equation ( $$\dot \varepsilon = A{\left[ {\sinh \left( {\alpha \sigma } \right)} \right]^{{n_2}}}\exp \left( {\frac{{ - {Q_c}}}{{RT}}} \right)$$ ) upon the strain was examined in a systematic manner. This is to explore the possibility of representing the hot compressive deformation behavior of metallic alloys in a simple way by using a reduced number of strain-dependent constitutive parameters. The analysis results for several different cases can be interpreted as follows: (1) Q c can be treated as being strain-independent, which is physically sensible; (2) while only the microstructure changes as a function of strain at low flow stresses, as the flow stress increases, the power-law creep deformation and power-law breakdown mechanisms change; (3) the regime where only A is strain dependent expanded to higher strain rates and lower temperatures as the strain increased, suggesting that the number of the strain-dependent parameters decreases as the initial microstructure is refined by dynamic recrystallization, and the microstructure approaches a steady state.

Plastic Work to Heat Conversion During High-Strain Rate Deformation of Mg and Mg Alloy

Magnesium and magnesium alloy were investigated for plastic work to heat conversion (β). Thermomechanical response was measured employing the shear-compression specimen geometry, a split-Hopkinson pressure bar, and an infra-red detector. β of both materials measured to be less than the common assumption of 0.9; however, heat conversion was observed to be greater for magnesium alloy. Thus, results suggest that alloying and grain size refinement not only improved yield strength but also affected the thermomechanical response.

Modeling dislocation glide and lattice friction in Mg2SiO4wadsleyite in conditions of the Earth’s transition zone

Thermally activated dislocation glide in Mg 2 SiO 4 wadsleyite at 15 GPa has been modeled to investigate its potential contribution to plastic deformation of wadsleyite in the Earth’s transition zone. Modeling is based on a multiphysics approach that allows calculating the constitutive equations associated with single slip for a wide range of temperatures and strain rates typical for the laboratory and the Earth’s mantle. The model is based on the core structures of the rate limiting ½ {101} and [100](010) dissociated screw dislocations. After quantifying their lattice friction, glide is modeled through an elastic interaction model that allows calculating the critical configurations that trigger elementary displacements of dissociated dislocations. The constitutive relations corresponding to glide are then deduced with Orowan’s equation to describe the average intracrystalline plasticity. The high stresses predicted by the model are found to be in good agreement with experimental data on plastic deformation of wadsleyite at high-pressure conditions. Moreover, it is found that even at appropriate mantle strain rates, glide of dislocations remain difficult with critical resolved shear stress ( CRSS ) values typically larger than 100 MPa. This implies the inefficiency of dislocation glide to the overall plastic deformation of Mg 2 SiO 4 wadsleyite under transition zone conditions.

Formation of [formula omitted] twins from I1-type stacking faults in Mg: A molecular dynamics study

The nature of the reaction and transient behavior of I1-type stacking faults (SFs) under shear stress was investigated to understand the role of I1 SFs in non-basal plastic deformation of Mg. This was because it has been suggested that I1 SFs formed via solute (such as Y) additions to Mg may act as the nucleation source of non-basal dislocations, which serve as important deformation modes to improve the workability. Crystal models of pure Mg containing an I1 SF were deformed by simple shear along the basal plane using molecular dynamics simulations with many-body interatomic potentials. The characteristic dissociation reaction was observed at the boundaries of the I1 SF, where 〈c+a〉 dislocations on the pyramidal plane, Shockley partial dislocations on the basal plane, and stair-rod dislocations were nucleated. Interestingly, instead of activation of the non-basal dislocations, {112¯1} twins were nucleated in the early stage of the reaction and grew steadily as the applied stress was increased. It was suggested that the I1 SF was likely to act as a “reactive” defect to assist and accommodate the c-axis deformation. Further, the deformation-induced twin boundaries were found to act as moderate obstacles to dislocation movement on the basal planes in the Mg matrix, without significantly sacrificing its plasticity.

Microstructural development of high temperature deformed AZ31 magnesium alloys

Due to their significant role in automobile industries, high temperature deformation of Mg–Al–Zn alloys (AZ31) at constant stress (i.e. creep) were studied at a wide range of stresses and temperatures to characterize underlying deformation mechanism, dynamic recrystallization (DRX) and dislocation density evolution. Various microstructures (e.g. grain growth & DRX) are noted during steady-state creep mechanisms such as grain boundary sliding (GBS), dislocation glide creep (DGC) and dislocation climb creep (DCC). Although a combination of DRX and grain growth is characteristic of low stacking fault energy materials like Mg alloys at elevated temperatures, observation reveals grain growth at low strain-rates (GBS region) along with dynamic recovery (DRV) mechanism. X-Ray Diffraction (XRD) analysis revealed a decrease in dislocation density during GBS region while it increased under dislocation based creep mechanisms which could be related to the possible DRV and DRX respectively. Scanning Electron Microscopic (SEM) characterization of the fracture surface reveals more inter-granular fracture for large grains (i.e. GBS region with DRV process) and more dimple shape fracture for small grains (i.e. DGC & DCC region with DRX).

Rate controlling mechanisms during hot deformation of Mg–3Gd–1Zn magnesium alloy: Dislocation glide and climb, dynamic recrystallization, and mechanical twinning

The flow behavior of the Mg–3Gd–1Zn (GZ31) magnesium alloy during hot working was critically analyzed and dislocation glide in the form of a viscous drag process (viscous glide) was identified as the rate controlling mechanism due to interaction of rare earth Gd atoms with the moving dislocations. Mechanical twinning was shown to significantly affect the level of flow stress at high Zener–Hollomon parameters, i.e. low forming temperatures and high strain rates. Moreover, dynamic recrystallization (DRX) was found to be another responsible phenomenon for deviation of constitutive equations from the theoretical ones, namely the deformation activation energy based on diffusivity and the pre-defined Garofalo’s type hyperbolic sine power, during high-temperature thermomechanical processing of this creep resistant light alloy.

Modeling of the flow stress behavior and microstructural evolution during hot deformation of Mg–8Al–1.5Ca–0.2Sr magnesium alloy

Hot compression tests of Mg–8Al–1.5Ca–0.2Sr magnesium alloy were performed on the Gleeble-1500 machine at temperatures of 300, 350, 400, and 450 °C and strain rates of 0.01, 0.1 and 1 s−1. The flow stress behavior and microstructural evolution were followed. The work hardening was derived according to the Laasraoui–Jonas model. An improved approach, which considered the influence of yield stress on flow stress and the effect of grain boundary (GB) migration on the evolution of dislocation density during compression, was used to simulate the microstructural evolution, the flow stress and the volume fraction recrystallized of Mg–8Al–1.5Ca–0.2Sr magnesium alloy. The simulated results are in good agreement with experimental results.