Grain refinement of magnesium alloys processed by severe plastic deformation

Grain refinement of AZ31 Mg alloy during cyclic extrusion compression （CEC） at 225-400 ℃ was investigated quantitatively by electron backscattering diffraction （EBSD）. Results show that an ultrafine grained microstructure of AZ31 alloy is obtained only after 3 passes of CEC at 225 ℃. The mean misorientation and the fraction of high angle grain boundaries （HAGBs） increase gradually by lowering extrusion temperature. Only a small fraction of {101^-2} twinning is observed by EBSD in AZ31 Mg alloys after 3 passes of CEC. Schmid factors calculation shows that the most active slip system is pyramidal slip {101^-1}〈1120〉and basal slip {0001}〈1120〉 at 225-350 ℃ and 400 ℃, respectively. Direct evidences at subgrain boundaries support the occurrence of continuous dynamic recrystallization （CDRX） mechanism in grain refinement of AZ31 Mg alloy processed by CEC.

Research progress on semi-continuous casting of magnesium alloys under external field

High-performance magnesium alloys are moving towards a trend of being produced on a large scale and in an integrated manner.The foundational key to their successful production is the high-quality cast ingots.Magnesium alloys produced through the conventional semi-continuous casting process inevitably contain casting defects,which makes it challenging to manufacture high-quality ingots.The integration of external field assisted controlled solidification technology,which combines physical fields such as electromagnetic and ultrasonic fields with traditional semi-continuous casting processes,enables the production of high-quality magnesium alloy ingots characterized by a homogeneous microstructure and absence of cracks.This article mainly summarizes the technical principles of those external field assisted casting process.The focus is on elaborating the refinement mechanism of different types of electromagnetic fields,ultrasonic fields,and combined physical fields during the solidification of magnesium alloys.Finally,the development prospects of producing highquality magnesium alloy ingots through semi-continuous casting under the external field were discussed.

Grain refinement mechanism of Al-5C master alloy in AZ31 magnesium alloy

Al-5C master alloy was prepared by powder in situ synthesis process, and its effects on grain refinement of AZ31 alloy and refining mechanism were investigated. The results indicate that the AI 5C master alloy consists of a（Al） and A14C3 phases, and the size distribution of Al4C3 particles is controlled by sintering time. The AI 5C master alloy can remarkably reduce the grain size of AZ31 alloy, which decreases with the increasing addition amount of AI-SC master alloy when the addition amount is below 2%. The refining mechanism is attributed to the formation of new compounds of Al-C-Mnparticles by Al4C3 and Mn, which might act as nucleating substrates for a-Mg grain.

Correlation of recalescence with grain refinement of magnesium alloys

The grain refinement of Mg-Al based alloys with carbon inoculation was investigated by a computer-aided cooling curve analysis(CA-CCA) system.The results show that carbon inoculation decreases the main parameters of the recalescence regime during the initial stage of solidification.These parameters include the recalescence undercooling(Δθ_(rec)),duration of recalescence (t_(rec)),and liquid peak parameter(LPP) which is firstly introduced into magnesium alloys.The resultant grain size decreases with increasing nucleation temperature(θ_n) and decreasing values ofΔθ_(rec),t_(rec) and LPP.

Grain refinement of Mg-Al magnesium alloys by carbon inoculation

C2Cl6 was used as grain refiner for AM60 magnesium alloys. The effects of grain refinement process on chemical composition, microstructure, impact energy, hardness and mechanical properties of magnesium alloys were investigated with XRF spectrometer, optical and electronic microscopes, pendulum impact tester, hardness tester and MTS material testing machine. The results show that C2Cl6 has good effects on microstructure and mechanical properties of AM60 magnesium alloys. The optimum usage of C2Cl6 in AM60 for getting the best properties is 1.0%. The results of electronic microscopic examination and theoretical analyses show that Al4C3 should be the potent heterogeneous nucleant for Mg-Al magnesium alloys.

Grain refinement mechanism of as-cast aluminum by hafnium

The effect of Hf on the grain refinement of as-cast aluminum was investigated using optical microscopy, electron microscopy and X-ray diffraction. The result shows that the grain size of studied alloy decreases effectively with the addition of Hf,Hf can react with Al to form Al3Hf particles during the solidification, the primary Al3Hf particles are highly potent nucleants for Al and the nanoscale coherent Al3Hf particles can inhibit the grain growth by pinning effect. The grain refinement mechanism of studied alloys was verified by the solute theory and the crystallography study, and it can be divided into two distinct types: At low Hf contents, there are no primary Al3Hf phases to form, the acquired grain refinement is primarily attributed to the constitutional undercooling induced by the Hf solute. At medium and high Hf contents, both Hf solute and Al3Hf particles contribute to the refinement.

Microstructural refinement and mechanical properties of direct extruded ZM21 magnesium alloys

Cast ZM21 magnesium alloys were subjected to symmetric extrusion at four different temperatures（200,250,300 and 350 ℃） with three extrusion ratios of 4：1,9：1 and 16：1,respectively.The effects of extrusion parameters such as temperature and extrusion ratio were studied by optical microscopy,X-ray diffraction（XRD） and tensile test.The optical micrographs exhibited various stages of recrystallization,i.e.,partial to full recrystallization influencing mechanical properties to good extent.Higher extrusion temperature resulted in coarse grains,whereas finer grains were obtained at higher extrusion ratios.Ultimate tensile strength of this alloy was increased from 160 MPa to 316 MPa after extrusion at 250 ℃ with an extrusion ratio of 9：1.

REVIEW ON RESEARCH AND DEVELOPMENT OF MAGNESIUM ALLOYS

The current research and development of magnesium alloys is summarized. Several aspects of magnesium alloys are described： cast Mg alloy, wrought Mg alloy, and novel processing. The subjects are discussed individually and recommendations for further study are listed in the final section.

Friction stir welding of AZ31 magnesium alloys processed by equal channel angular pressing

Equal channel angular pressing （ECAP） is an effective thermo-mechanical process to make ultrafine grains. An investigation was carried out on the friction stir welding （FSW） of ECAPed AZ31 magnesium alloys with a thickness of 15 mm. For different process parameters, the optimum FSW conditions of ECAPed AZ31 magnesium alloys were examined. The basic characterization of weld formation and the mechanical properties of the joints were discussed. The results show that the effect of welding parameters on welding quality was evident and welding quality was sensitive to welding speed. Sound joints could be obtained when the welding speed was 37.5 mm/min and the rotation speed of the stir tool was 750 r/min. The maximum tensile strength （270 MPa） of FSW was 91% that of the base materials. The value of microhardness varied between advancing side and retreating side because of the speed field near the pin of the stir tool, which weakened the deformed stress field. The value of microhardness of the welding zone was lower than that of the base materials. The maximum value was located near the heat-affected zone （HAZ）. Remarkable ductile character was observed from the fracture morphologies of welded joints.

Improved Mechanical Properties in AZ31 Magnesium Alloys Induced by Impurity Reduction

Mechanical properties and microstructures of AZ31 magnesium alloys containing different impurity levels but having the same alloying element content, were investigated at ambient temperature. These AZ31 alloys were produced by semi-continuous casting, wherein the content of impurity was varied systematically. Microstructure observation shows that finer grains are existent in the alloy with lower impurity level. Tensile testing reveals that a reduction of impurity content results in a noticeable increase of the strength and elongation in the alloys in the cast, homogenized and extruded states. As the impurity content decreases from 0.0462wt% to 0.0163wt%, the ultimate tensile strength is evidently enhanced by 62 MPa and the elongation is nearly doubled in the homogenized specimen. The observed property improvement was discussed in terms of the microstructure variation with impurity reduction.

Recent advances on grain refinement of magnesium rare-earth alloys during the whole casting processes:A review

High-performance cast magnesium rare-earth(Mg-RE) alloys are one of the most important materials among all developed Mg alloy families, and have shown great potential in military and weapons, aerospace and aviation, orthopedic implants, etc. Controlling grain size and distribution of it is key to the promising mechanical performance of Mg-RE alloy casting components. During the casting of a real component, nearly every procedure in the fabrication process will influence the grain refinement effect. The procedure may include and may not be limited to the chemical inoculations, possibly applied physical fields, the interfere between grain refiner and purifications, and the casting techniques with different processing parameters. This paper reviews the recent advances and proposed future developments in these categories on grain refinement of cast Mg-RE alloys. The review will provide insights for the future design of grain refinement techniques,the choosing of processing parameters, and coping strategies for the failure of coarsening for cast Mg-RE components with high quality and good performance.

Enhanced Grain Refining Effect of Mg–Zr Master Alloy on Magnesium Alloys via a Synergistic Strategy Involving Heterogeneous Nucleation and Solute‑Driven Growth Restriction

Zirconium(Zr)emerges as the most effective grain refiner for magnesium(Mg)alloys incorporating Zr.Typically,Zr is introduced in the form of an Mg–Zr master alloy.However,within Mg–Zr master alloys,Zr predominantly exists in a particle form,which tends to aggregate due to attractive van der Waals forces.The clustered Zr is prone to settling,thereby reducing its refining impact on Mg alloys.In this work,a combined pretreatment process for Mg–Zr master alloys was proposed,encompassing the introduction of a physical field to intervene the agglomeration of particle Zr and the employ of high-temperature dissolution and peritectic reactions to promote the solid solution of Zr.The results demonstrate that the particle Zr within the pretreated Mg–Zr master alloy is effectively dispersed and refined,and greater solute Zr levels can be achieved.The subsequent grain refinement ability was studied on a typical Mg–6Zn–0.6Zr(wt%)alloy.The outcome highlights that an improvement in the grain refinement efficacy(32.4%)of Mg–Zr master alloys was obtained with a holding time of 60 min.The pretreated Mg–Zr master alloy significantly augments the efficiency of grain refinement for Mg alloys through a synergistic strategy involving heterogeneous nucleation and solute-driven growth restriction.The crucial factor in achieving effective grain refinement of Zr in Mg alloys lies in regulating the presence and morphology of Zr in the Mg–Zr master alloy,distinguishing between particle Zr and solute Zr.This study introduces a novel method for developing more efficient Mg–Zr refiners.

Insight of magnesium alloys and composites for orthopedic implant applications-a review

Magnesium(Mg)and its alloys have been widely researched for orthopedic applications recently.Mg alloys have stupendous advantages over the commercially available stainless steel,Co-Cr-Ni alloy and titanium implants.Till date,extensive mechanical,in-vitro and in-vivo studies have been done to improve the biomedical performance of Mg alloys through alloying,processing conditions,surface modification etc.This review comprehensively describes the strategies for improving the mechanical and degradation performance of Mg alloys through properly tailoring the composition of alloying elements,reinforcements and processing techniques.It also highlights the status and progress of research in to(i)the selection of nutrient elements for alloying,reinforcement and its effects(ii)type of Mg alloy system(binary,ternary and quaternary)and composites(iii)grain refinement for strengthening through severe plastic deformation techniques.Furthermore it also emphasizes on the importance of Mg composites with regard to hard tissue applications.

Effect of equal channel angular pressing on AZ31 wrought magnesium alloys

AZ31 wrought magnesium alloys are light weight materials which play an important role in order to reduces the environmental burdens in modern society because of its high strength to weight ratio,corrosion resistance,and stiffness and machinability.Applications of this material are mainly in structural component i.e.,in constructions,automobile,aerospace,electronics and marine industries.In the present work,the microstructure characterization of the AZ31 alloys up to four ECAP passes at temperature of 573 K was observed for route Bc.Average grain size of the material was reduced from 31.8μm to 8μm after four ECAP passes.Mechanical properties of the alloy improved with increase in number of ECAP passes.Moreover,X-ray diffraction analysis was carried out for as received and ECAP processed material.

Bimodal grain structure formation and strengthening mechanisms in Mg-Mn-Al-Ca extrusion alloys

The effects of small additions of calcium (0.1%and 0.5%~1) on the dynamic recrystallization behavior and mechanical properties of asextruded Mg-1Mn-0.5Al alloys were investigated.Calcium microalloying led to the formation of Al_(2)Ca in as-cast Mg-1Mn-0.5Al-0.1Ca alloy and both Mg_(2)Ca and Al_(2)Ca phases in Mg-1Mn-0.5Al-0.5Ca alloy.The formed Al_(2)Ca particles were fractured during extrusion process and distributed at grain boundary along extrusion direction (ED).The Mg_(2)Ca phase was dynamically precipitated during extrusion process,hindering dislocation movement and reducing dislocation accumulation in low angle grain boundaries (LAGBs) and hindering the transformation of high density of LAGBs into high angle grain boundaries (HAGBs).Therefore,a bimodal structure composed of fine dynamically recrystallized (DRXed) grains and coarse un DRXed regions was formed in Ca-microalloyed Mg-1Mn-0.5Al alloys.The bimodal structure resulted in effective hetero-deformation-induced (HDI) strengthening.Additionally,the fine grains in DRXed regions and the coarse grains in un DRXed regions and the dynamically precipitated Mg_(2)Ca phase significantly enhanced the tensile yield strength from 224 MPa in Mg-1Mn-0.5Al to335 MPa and 352 MPa in Mg-1Mn-0.5Al-0.1Ca and Mg-1Mn-0.5Al-0.5Ca,respectively.Finally,a yield point phenomenon was observed in as-extruded Mg-1Mn-0.5Al-x Ca alloys,more profound with 0.5%Ca addition,which was due to the formation of (■) extension twins in un DRXed regions.

Effect of grain refinement induced by wire and arc additive manufacture (WAAM) on the corrosion behaviors of AZ31 magnesium alloy in NaCl solution

Additive manufacturing(AM)of Mg alloys has become a promising strategy for producing complex structures,but the corrosion performance of AM Mg components remains unexploited.In this study,wire and arc additive manufacturing(WAAM)was employed to produce single AZ31 layer.The results revealed that the WAAM AZ31 was characterized by significant grain refinement with non-textured crystallographic orientation,similar phase composition and stabilized corrosion performance comparing to the cast AZ31.These varied corrosion behaviors were principally ascribed to the size of grain,where cast AZ31 and WAAM AZ31 were featured by micro galvanic corrosion and intergranular corrosion,respectively.

Grain refinement and mechanism of CAl_(0.5)W_(0.5) compound in Mg-Al alloys

The effect of CAl0.5W0.5（CAW） compound on the grain refinement of Mg-Al based alloys was investigated.The results show that CAW compound is an effective and active grain refiner.The grain size of binary Mg-Al alloys is more than 500 μm,and it is changed to about 110 μm with a 1 wt.% CAW addition.The hardness increased with the decease of grain size monotonously.The mechanical properties are improved by the addition.The fine grain size is mainly ascribed to the dispersed Al2CO particles,which are very potent nucleating substrates for Mg-Al alloys.The nucleation cores formed by chemical reaction directly are well-distributed in the matrix.

Grain refinement and strength enhancement in Mg wrought alloys: A review

Low absolute strength becomes one major obstacle for the wider applications of low/no rare-earth(RE) containing Mg alloys. This review firstly demonstrates the importance of grain refinement in improving strength of Mg alloys by comprehensively comparing with other strategy, e.g., precipitation strengthening. Dynamic recrystallization(DRX) plays a crucial role in refining grain size of Mg wrought alloys.Therefore, secondly, the DRX models, grain nucleation mechanisms and the related grain refinement abilities in Mg alloys are summarized,including phase boundary, twin boundary and general boundary induced recrystallization. Thirdly, the newly developed low-RE containing Mg alloy, e.g., Mg-Ce, Mg-Nd and Mg-Sm based alloys, and the RE-free Mg alloys, e.g., Mg-Al, Mg-Zn, Mg-Sn and Mg-Ca based alloy,are reviewed, with the focus on enhancing the mechanical properties mainly via the grain refinement strategy. At the last section, the perspectives and outstanding issues concerning high-performance Mg wrought alloys are also proposed. This review is meant to promote the deep understanding on the critical role of grain refinement in Mg alloys and provide reference for the development of other high strength and low-cost Mg alloys which are fabricated by the conventional extrusion/rolling processing.

Grain refinement in AZ31 magnesium alloy rod fabricated by extrusion-shearing severe plastic deformation process

A new severe plastic deformation （SPD） method that is extrusion-shearing （ES）, which includes initial forward extrusion and shearing process subsequently, was developed to fabricate the fine grained AZ31 Mg alloys. The components of ES die were manufactured and installed to gleeble1500D thermo-mechanical simulator. Microstructure observations were carried out in different positions of ES formed rods. The results show that homogeneous microstructures with mean grain size of 2 μm are obtained at lower temperature as the accumulated true strain is 2.44. Occurring of continuous dynamic recrystallization （DRX） is the main reason for grain refinement during ES process. The experimental results show that the ES process effectively refines the grains of AZ31 magnesium. The production results of ES extrusion with industrial extruder under different extrusion conditions show that the ES extrusion can be applied in large-scale industry.

Effect of refinement of grains and icosahedral phase on hot compressive deformation and processing maps of Mg-Zn-Y magnesium alloys with different volume fractions of icosahedral phase

The effect of the volume fraction of I-phase on the hot compressive behavior and processing maps of the extruded Mg-Zn-Y alloys was examined, and the obtained results were compared with those of the cast alloys in a previous work. The average grain sizes, fractions of dynamically recrystallized(DRXed) grains,and sizes of DRXed grains of the extruded alloys after compressive deformation were significantly smaller,higher and smaller, respectively, than those of the cast alloys after compressive deformation under the same experimental conditions. This was because the microstructures of the extruded alloys, having much more grain boundaries and more refined I-phase particles than the cast alloys, provided a larger number of nucleation sites for dynamic recrystallization than those of the cast alloys. The constitutive equations for high-temperature deformation of the extruded and cast alloys could be derived using the same activation energy for plastic flow, which was close to the activation energy for lattice diffusion in magnesium.Compared with the cast alloys, the onset of the power law breakdown(PLB) occurred at larger ZenerHolloman(Z) parameter values in the extruded alloys. This was because the extruded alloys had finer initial grain sizes and higher fractions of finer DRXed grains compared to the cast alloys, such that the onset of PLB caused by creation of excessive concentrations of deformation-induced vacancies was delayed to a higher strain rate and a lower temperature. The flow-stress difference between the extruded alloys and the cast alloys could be attributed to the difference in the fraction of DRXed grains. According to the processing maps, the extruded alloys exhibited higher power dissipation efficiency and flow stability than the cast alloys. This agreed with the microstructural observations.