Grain Refinement Of Magnesium Alloy Research Articles

Phase field simulating grain refinement of magnesium alloy by thin strip second phase particles

The study has investigated the grain growth of AZ31 magnesium alloy matrix with fine strip second phase particles, by the phase field model in the real space and time, through introducing free energy equation, and compared with the simulated results containing spherical particles. The results showed that both the thin strip ellipsoidal particles and the cuboid particles have grain refinement effect on the microstructure, moreover, when the content of the second phase particles is the same, the refining effect of thin strip particles could be better than the spherical particles on the matrix microstructure. This study provides the real scale phase field models to research the grain refinement by second phase particles.

Investigation on the Atomic Mechanism for Grain Refinement of Magnesium Alloys by Mg-Zr Master Alloy.

The valence electron structure, bond energy, and cohesive energy of Mg, Zr, and α-Mg containing Zr, and α-Zr containing Mg crystals were calculated using the empirical electron theory of solids and molecules (EET). The calculation results show that the bond and cohesive energies of Zr were much greater than those of Mg, so Zr particles could precipitate ahead of α-Mg in general magnesium alloy melts or insoluble Zr particles exist when the magnesium melt temperature is relatively low. The bond energy of α-Zr decreases with the increase in Mg content; therefore, at the end of the growth of Zr particles, the remaining Zr atoms in the melt exist in the form of Mg-Zr clusters. In order to reduce the surface energy of Zr particles, the outer surface of Zr particles tends to terminate with a Zr-Mg atomic layer, that is, a stable two-dimensional Zr-Mg atomic layer is formed first on the (0001) crystal surface of the outermost surface of Zr particles. Furthermore, on the basis of the calculated results, a complementary criterion to the edge-to-edge model of heterogeneous nucleation is also proposed. {ure and single Zr particles cannot become the heterogeneous nucleus of α-Mg, but when there is an atomic layer of two-dimensional Zr-Mg on its surface, the nucleation of particles can be activated. Mg atoms in the liquid phase preferentially attach to the Zr-Mg/Mg-Zr atomic layer on the surface of Zr particles to grow and form a stable ordered structure, which lastly transforms Zr particles into efficient heterogeneous cores.

Improvement of edge crack damage of magnesium alloy by optimizing the edge curve during cross variable thickness rolling

According to the MAS rolling and cross rolling theory, the hot rolling experiments were carried out on the AZ31 magnesium alloy specimens at the initial rolling temperature of 350 °C, and the grain distribution and edge damage of the strip after rolling by a controlled rolling reduction in different regions were analyzed. Meanwhile, the edge curves were designed to improve the edge crack and grain refinement of magnesium alloy during cross variable thickness rolling. The results indicated that the variable thickness rolling with the edge curve could effectively reduce the edge crack of magnesium alloy. The results also indicated that different edge curves could produce varying degrees of grain refinement at the strip edge and realize the ideal grain refinement in different regions during the rolling process; meanwhile, the optimization of the edge curve during cross variable thickness rolling could provide a theoretical basis for improving the magnesium alloy edge crack.

Microstructure Evolution of a Semisolid Magnesium Alloy Slurry Obtained via an Internal Rapid Cooling Stirring Process (IRCSP)

Grain refinement of magnesium alloys is an effective way to improve their tensile strength and ductility to meet industrial application requirements. Herein, a new rheocasting process, internal rapid cooling stirring process (IRCSP), was used to obtain a fully grain-refined spherical semi-solid slurry. Rapid quenching was used to study the microstructure evolution during IRCSP. The experimental results showed that the solid fraction was mainly determined by nucleation, and grain growth played only a minor role. The particle density was determined by a combination of the fragmentation and coalescence of solid particles. Changes in grain size over time followed a 1/3 power law with a ripening coefficient. In addition, the particle distribution was determined using shear stress flow and cooling rates. The microstructural analysis suggested that a fast cooling rate led to grain refinement, and rapid stirring enhanced the interfacial energy and stability of the solid-liquid interface. As a result, fine globular primary α-Mg particles were obtained during IRCSP.

Nanoparticle-induced growth behavior of primary α-Mg in AZ91 alloys

Significant grain refinement of AZ91 alloys was achieved by ceramic nanoparticle(NP)-induced growth control. Advanced characterization techniques including 3D tomography and in-situ observation were carried out to examine the microstructure evolution. The results show that the NPs are distributed in matrix in three ways, i.e. along the grain boundaries, inside the α-Mg grains and within the interior of β phases. The intergranular NPs that are distributed along the grain boundaries can form a multilayer of NPs covering the surface of α-Mg, greatly restricting the grain growth. A numerical model is proposed to predict the grain size, in which the solute concentration gradient at dendrite tip and the impingement of grains are recognized as two pivotal factors in modelling. It is demonstrated from the model prediction that a reduced growing rate of α-Mg can be obtained upon NP addition due to the lowered solute concentration gradient at dendrite tip, which could increase the nucleation events, accelerate the grain impingement and thus induce significant grain refinement. Given the chemically and thermally stability of ceramic NPs, which can avoid the intrinsic drawbacks of conventional inoculation, e.g. element poisoning, NP-induced growth control (NIGC) may provide a new avenue for grain refinement in magnesium alloys.

Numerical investigation of grain refinement of magnesium alloys: Effects of cooling rate

Solidification experiment of Mg-4Y-3Nd-1.2Al (wt%) alloy cast in a step-like sand-mold casting shows that a slow cooling rather than a fast cooling leads to grain refinement during solidification with in-situ formed nucleant particles (Al2RE) in molten melt. A two-dimensional cellular automaton (CA) model is adopted to fundamentally understand the mechanism of grain refinement. Conservation equation of energy is solved at macro-scale. The generated thermal history serves as input for the CA simulation. The model predictions are verified against the experiment. A high cooling rate leads to fast dendrite growth and solute enrichment in the interdendritic region. The nucleant particles, in-situ formed under the fast cooling condition, have small sizes. A larger number of small-sized particles are suppressed because they do not experience enough constitutional undercooling to nucleate. A critical particle size exists, above which the grain refinement becomes less sensitive to the cooling rate measured in the experiment. The simulation of one particle transported to the interdendritic area shows it is more difficult for this particle to develop under the faster cooling condition. We expect this fundamental understanding of the physics of cooling rate for the grain refinement would have implications on future complex magnesium alloy designs as well as solidification process control.

Tailoring Microstructure and Properties of Fine Grained Magnesium Alloys by Severe Plastic Deformation

Modern wrought magnesium alloys have poor formability at room temperature, which impedes the wider uptake of these alloys by the industry. Over the last decades, research activities in the area of magnesium alloy development have grown enormously and have produced a pallet of exciting findings, which the authors summarize in this concise review focused on the effect of the microstructure, primarily of grain size, on room temperature ductility of wrought Mg‐based alloys. Well‐established paths and modern strategies to control over the grain size distribution are discussed. It is demonstrated that the use of severe plastic deformation techniques for ultimate grain refinement in magnesium alloys opens new windows for improving their mechanical properties profile by managing both strength and ductility in a wide range. However, it is shown that grain size alone cannot be regarded as a key parameter controlling the mechanical behavior of Mg alloys. The effect of texture is of paramount importance for the overall mechanical response of Mg alloys and this may supersede the influence of grain size.

On the role of twinning and stacking faults on the crystal plasticity and grain refinement in magnesium alloys

We have investigated the detailed microstructural mechanisms associated with the excellent crystal plasticity and ultra-fine grain refinement observed under high strain-rate deformation of Mg alloys, focusing on ZK60. Firstly, we have identified the clear formation of stacking faults in deformation-induced twinned crystal segments. Specifically, we have found that intrinsic I1 and I2 stacking faults bounded by 16<2¯ 023> and 13<101¯ 0> partial dislocations, respectively, were found to occur in very high number densities within the twins. This was due to the high Schmid factor for stacking fault shearing in twins and the critical role that twin boundaries played in emitting partial dislocations. Secondly, we have clarified the interplay between twinning and stacking faults on the enhanced crystal plasticity. Apart from the strain accommodated by the extensive twinning itself, we propose that the improved plasticity during high strain-rate deformation is mainly due to the nucleation of 13<11¯ 23>{112¯ 2} dislocation within twins, which provides enough independent slip systems to achieve a homogeneous deformation in the material. Finally, we have demonstrated the interplay between twinning and stacking fault formation on the nucleation of new grains via dynamic recrystallisation. The twin boundaries and stacking faults, especially those of the I1 type, facilitate the formation of low-angle grain boundaries that can subsequently transition into high-angle grain boundaries, and form ultra-fine dynamically recrystallised grains.

Processing, Superplastic Properties and Friction Stir Welding of Fine-Grained AZ31, AZ91, AE42 and QE22 Magnesium Alloys

The grain refinement after thermo-mechanical treatment (TMT) was investigated in AZ91, AE42, und QE22 magnesium alloys. The optimal over-aging temperature was determined to be 300 °C in the case of AZ91 and AE42 alloys and 350 °C for QE22 alloy. After optimized TMT, the average grain sizes were 13.5 µm (AE42), 11.1 µm (AZ91) and 1.9 µm (QE22). The QE22 alloy exhibited the superior superplastic properties, with maximum elongation to failure 750 % and strain rate sensitivity parameter m=0.73. The Friction Stir Welding showed that the original base material grain structure of the alloys AZ31 and AZ91 replaced by ultrafine grains in the stir zone. The purpose of the present paper is to present the results of the grain refinement in magnesium alloys by thermo mechanical treatment and stir welding.

Grain refinement of magnesium alloys processed by severe plastic deformation

Grain refinement of AZ31 Mg alloy during cyclic extrusion compression (CEC) at 225–400 °C 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 °C. The mean misorientation and the fraction of high angle grain boundaries (HAGBs) increase gradually by lowering extrusion temperature. Only a small fraction of 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 and basal slip {0001} at 225–350 °C and 400 °C, 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.

Development of extrusion-torsion simultaneous processing for grain refinement in Magnesium alloys

We have proposed a new extrusion process combined with torsion. Extrusion-torsion simultaneous processing is a very attractive technique for fabricating a rod-shape material with high strength and excellent workability. The grain size of AZ91D magnesium alloy was gradually decreased with increasing the torsion speed in as extrusion-twisted conditions. Grain refinement under 2um was achieved by the optimized extrusion-torsion condition. Hardness was increased by the addition of torsion speed comparing with as solution-treated and as extruded samples. Hardness change was dependent on the extrusion-torsion temperature.

The study of microstructure and mechanical properties of twin-roll cast AZ31 magnesium alloy after constrained groove pressing

Microstructure investigation and microhardness mapping were done on the material with ultra-fine grained structure prepared by constrained groove pressing of twin-roll cast AZ31 magnesium strips. The microstructure observations showed significant drop of the grain size from 200 gm to 20 gm after constrained groove pressing. Moreover, the heterogeneities in the microhardness along the cross-section observed in the as-cast strip were replaced by the bands of different microhardness in the constrained groove pressed material. It is shown that the constrained groove pressing technique is a good tool for the grain refinement of magnesium alloys.

Multi-temperature equal channel angular pressing of Mg-3 wt%Al-1 wt%Zn alloy

Grain refinement of magnesium alloys at temperatures below 200 °C via deformation processes is a challenging task. Equal channel angular pressing is one of the severe plastic deformation techniques which can significantly refine grain size. Refined grains consequently improve mechanical properties of magnesium alloys. In this paper, Mg-3 wt%Al-1 wt%Zn magnesium alloy was processed by different multi-temperature equal channel angular pressing cycles without backpressure. The lowest finishing temperature of equal channel angular pressing was 150 °C leading to fine-grained microstructure with sporadically large elongated grains. Shear bands were observed after equal channel angular pressing finishing at temperatures of 175 and 150 °C. Electron backscattered diffraction revealed that grain boundary distribution is qualitatively the same for all equal channel angular pressing cycles. Crystallographic analyses of large elongated grains showed that {0 0 0 1} and {} planes in these grains tend to be aligned parallel to the shear plane. Microtexture measurements showed split of (0 0 0 2) poles. The shape of stress–strain curves and consequently yields tensile strength of Mg-3 wt%Al-1 wt%Zn alloy after multi-temperature equal channel angular pressing was influenced by final texture and twinning.

Grain Refinement of Magnesium Alloys by Mg–Zr Master Alloys: The Role of Alloy Chemistry and Zr Particle Number Density

AbstractAlthough refinement of the as‐cast grain size of magnesium alloys by Zr is well established commercially, little research has been undertaken to optimize this refinement technology and to quantify the relative performance of different Mg–Zr master alloys. The performance of Mg–Zr master alloys was found to be related to (1) the number density of Zr particles between 1 and 5 µm in size where the master alloy with the largest number density of these particles exhibits the best refinement, and (2) the alloy's growth restriction factor (Q) where a linear relationship between grain size and 1/Q, was found to exist for each master alloy. An equation for predicting grain size based on the Interdependence model was developed. Further, the Interdependence Model was improved to be able to cater for an increasing Zr particle number density with increasing values of Q.

Grain Refinement of Magnesium Alloys: A Review of Recent Research, Theoretical Developments, and Their Application

This paper builds on the “Grain Refinement of Mg Alloys” published in 2005 and reviews the grain refinement research on Mg alloys that has been undertaken since then with an emphasis on the theoretical and analytical methods that have been developed. Consideration of recent research results and current theoretical knowledge has highlighted two important factors that affect an alloy’s as-cast grain size. The first factor applies to commercial Mg-Al alloys where it is concluded that impurity and minor elements such as Fe and Mn have a substantially negative impact on grain size because, in combination with Al, intermetallic phases can be formed that tend to poison the more potent native or deliberately added nucleant particles present in the melt. This factor appears to explain the contradictory experimental outcomes reported in the literature and suggests that the search for a more potent and reliable grain refining technology may need to take a different approach. The second factor applies to all alloys and is related to the role of constitutional supercooling which, on the one hand, promotes grain nucleation and, on the other hand, forms a nucleation-free zone preventing further nucleation within this zone, consequently limiting the grain refinement achievable, particularly in low solute-containing alloys. Strategies to reduce the negative impact of these two factors are discussed. Further, the Interdependence model has been shown to apply to a broad range of casting methods from slow cooling gravity die casting to fast cooling high pressure die casting and dynamic methods such as ultrasonic treatment.

Grain refinement due to complex twin formation in rapid hot forging of magnesium alloy

Twinning behavior and twin interactions were investigated to characterize grain refinement in magnesium alloys subjected to rapid hot forging. Formation of {101¯2} tensile twins with various twin variants and their subsequent mutual interaction resulted in new boundaries at low strain. With further straining, fragmentation of {101¯1}-{101¯2} double twin bands by the formation of more complex twins in the interior, accompanied by a drastic decrease in the stored energy, led to grain refinement.

Study of β-precipitates and their effect on the directional yield asymmetry of friction stir processed and aged AZ91C alloy

Friction stir processing (FSP) has emerged as a solid state deformation process for achieving significant grain refinement in magnesium alloys. Cast AZ91C alloy is solution treated and friction stir processed to achieve an average grain size of ∼6.5μm in the nugget zone. Ageing of the FSP alloy is conducted at different temperatures and times for continuous and discontinuous precipitation of the β-phase. In the as-FSP condition, significant yield asymmetry in the processing and transverse directions is observed due to strong basal texture produced in the processing direction. The yield asymmetry is found to get reduced upon ageing and minimized under optimized ageing conditions. Microscopic investigation of continuous and discontinuous β-precipitates is conducted using optical microscopy, SEM and TEM techniques and their effect on the directional yield asymmetry is discussed.

Grain Refinement of Magnesium Alloys by CONFORM: A Continuous Severe Plastic Deformation Route?

Effective grain refinement through equal channel angular pressing (ECAP) for magnesium (Mg) alloys has been demonstrated by many researchers. Although with the capability to achieve superplasticity, the batch mode nature of this method and the required repetitive processing to attain ultrafine grained structure have prohibited it from being widely used in large-scale industrial production. In this study, a well-established metal forming method – the continuous extrusion forming (CONFORM) process – was employed as a severe plastic deformation route to refine the microstructure of Mg alloys. Cast Mg-3%Al-1%Zn (AZ31) rods were used as the feedstock and the cast structure (grain size of ~150 microns) was refined to ~1 micron after one pass CONFORM extrusion. Uniaxial tensile tests of the as-extruded samples were conducted at a temperature of 473K and an elongation of ~200% was achieved under a strain rate of 1×10-4 s-1. The significant grain refinement effect was attributed to the severe shear deformation occurred during the CONFORM process, which is very similar to ECAP but with even higher effective strains. The most important advantage of CONFORM over ECAP is that the former is a continuous route, so it is able to produce long products. It was also shown that CONFORM could be an additional forming method for Mg alloys to conventional rolling, forging and extrusion.

Effect of Ultrasonic Treatment on Microstructures of Mg-Ca Binary Alloy

The effects of high intensity ultrasonic melt treatment on the microstructure of magnesium alloys were investigated in this paper. Magnesium melts were treated with power ultrasonic wave and then cooled to a predetermined temperature. With the increase in ultrasonic power, the structure exhibited refined and spheroidzed crystal grains. After further increasing the ultrasonic power, the grains tended to somewhat coarsened. Increasing the ultrasonic processing time led to a grain refinement of magnesium alloy.

Grain refinement and mechanical behavior of a magnesium alloy processed by ECAP

Equal-channel angular pressing (ECAP) is an effective tool for refining the grain structure of magnesium alloys and improving the ductility at moderate temperatures. However, grain refinement in these alloys differs from other metals because new grains are formed along the boundaries of the initial structure and these newly formed grains slowly spread to consume the interiors of the larger grains in subsequent passes. A model is presented for grain refinement in magnesium alloys processed by ECAP based on the principles of dynamic recrystallization where new fine grains are formed along the initial boundaries and along twin boundaries. This model provides an explanation for a wide range of experimental data and introduces the concept of grain size engineering for achieving selected material properties in magnesium alloys.