Grain refinement of Mg-Ca alloys by native MgO particles

In Mg-Ca alloys the grain refining mechanism,in particular regarding the role of nucleant substrates,remains the object of debates.Although native MgO is being recognised as a nucleating substrate accounting for grain refinement of Mg alloys,the possible interactions of MgO with alloying elements that may alter the nucleation potency have not been elucidated yet.Herein,we design casting experiments of Mg-xCa alloys varied qualitatively in number density of native MgO,which are then comprehensively studied by advanced electron microscopy.The results show that grain refinement is enhanced as the particle number density of MgO increases.The native MgO particles are modified by interfacial layers due to the co-segregation of Ca and N solute atoms at the MgO/Mg interface.Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy,we reveal the nature of these Ca/N interfacial layers at the atomic scale.Irrespective of the crystallographic termination of MgO,Ca and N co-segregate at the MgO/Mg interface and occupy Mg and O sites,respectively,forming an interfacial structure of a few atomic layers.The interfacial structure is slightly expanded,less ordered and defective compared to the MgO matrix due to compositional deviations,whereby the MgO substrate is altered as a poorer template to nucleate Mg solid.Upon solidification in a TP-1 mould,the impotent MgO particles account for the grain refining mechanism,where they are suggested to participate into nucleation and grain initiation processes in an explosive manner.This work not only reveals the atomic engineering of a substrate through interfacial segregation but also demonstrates the effectiveness of a strategy whereby native MgO particles can be harnessed for grain refinement in Mg-Ca alloys.

Strength and plasticity improvement induced by strong grain refinement after Zr alloying in selective laser-melted AlSiMg1.4 alloy

In order to enhance the mechanical properties of the selective laser-melted(SLM) high-Mg content AlSiMg1.4 alloy,the Zr element was introduced.The influence of Zr alloying on the processability,microstructure,and mechanical properties of the alloy was systematically investigated through performing microstructure analysis and tensile testing.It was demonstrated that the SLM-fabricated AlSiMg1.4-Zr alloy exhibited high process stability with a relative density of over 99.5% at various process parameters.Besides,the strong grain refinement induced by the primary Al3Zr particle during the melt solidification process simultaneously enhanced both the strength and plasticity of the alloy.The values for the yield strength,ultimate tensile strength,and elongation of the SLM-fabricated AlSiMg1.4-Zr were(343±3) MPa,(485±4) MPa,and(10.2±0.2)%,respectively,demonstrating good strengthplasticity synergy in comparison to the AlSiMg1.4 and other Al-Si-based alloys fabricated by SLM.

Effect of addition temperature on dispersion behavior and grain refinement efficiency of MgO introduced into Mg alloy

The effect of addition temperature of MgO particles(MgOp)on their dispersion behavior and the efficiency of grain refinement in AZ31 Mg alloy was investigated.In addition,the grain refinement mechanism was systematically studied by microstructure characterization,thermodynamic calculation,and analysis of solidification curves.The results show that the grain size of AZ31 Mg alloy initially decreases and then increases as the MgOp addition temperature is increased from 720 to 810℃,exhibiting a minimum value of 136μm at 780℃.The improved grain refinement efficiency with increasing MgOp addition temperature can be attributed to the reduced Mg melt viscosity and enhanced wettability between MgOp and Mg melt.Furthermore,a corresponding physical model describing the solidification behavior and grain refinement mechanism was proposed.

Al-CeO_(2)-Mg refiner on grain refinement,cast fluidity and mechanical properties of AZ91 alloy

Grain refinement is a satisfying method to enhance the comprehensive performance of Mg alloys,and using grain refiners to improve the quality of Mg castings has become a common practice in the current casting industry.In this study,the effects of Al-CeO_(2)-Mg grain refiner addition on microstructure,cast fluidity,and mechanical properties of AZ91 alloy were systemically investigated by OM,SEM,DSC,TEM,sand Archimedean spiral mold fluidity test,and tensile test.The results show that the Al-CeO_(2)-Mg grain refiner has a remarkable effect on as-cast AZ91 alloy including notable grain refinement,enhancement of cast fluidity,and improvement of tensile properties.The average grain size of AZ91 alloy is refined by 63%with 1.0wt.%Al-CeO_(2)-Mg grain refiner.Meanwhile,the casting flow length of the as-cast AZ91 alloy is increased from 332 mm to 440 mm.The improvement of cast fluidity is attributed to the decreased liquidus temperature,shortening of solid-liquid two-phase temperature range,and refinement of microstructure.

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.

Grain refinement of as-cast superalloy IN718 under action of low voltage pulsed magnetic field

The grain refinement of superalloy IN718 under the action of low voltage pulsed magnetic field was investigated. The experimental results show that fine equiaxed grains are acquired under the action of low voltage pulsed magnetic field. The refinement effect of the pulsed magnetic field is affected by the melt cooling rate and superheating. The decrease of cooling rate and superheating enhance the refinement effect of the low voltage pulsed magnetic field. The magnetic force and the melt flow during solidification are modeled and simulated to reveal the grain refinement mechanism. It is considered that the melt convection caused by the pulsed magnetic field, as well as cooling rate and superheating contributes to the refinement of solidified grains.

Structure uniformity and limits of grain refinement of high purity aluminum during multi-directional forging process at room temperature

The effect of forging passes on the refinement of high purity aluminum during multi-forging was investigated. The attention was focused on the structure uniformity due to deformation uniformity and the grain refinement limitation with very high strains. The results show that the fine grain zone in the center of sample expands gradually with the increase of forging passes. When the forging passes reach 6, an X-shape fine grain zone is initially formed. With a further increase of the passes, this X-shape zone tends to spread the whole sample. Limitation in the structural refinement is observed with increasing strains during multi-forging process at the room temperature. The grains size in the center is refined to a certain size （110 μm as forging passes reach 12, and there is no further grain refinement in the center with increasing the forging passes to 24. However, the size of the coarse grains near the surface is continuously decreased with increasing the forging passes to 24.

Numerical simulation of acoustic pressure field for ultrasonic grain refinement of AZ80 magnesium alloy

Ultrasound with different intensities was applied to treating AZ80 alloy melt to improve its solidification structure.The average grain size of the alloy could be decreased from 303 to 148 μm after the ultrasound with intensity of 30.48 W/cm2 was applied.To gain insight into the mechanism of ultrasonic treatment which affected the microstructure of the alloy,numerical simulations were carried out and the effects of different ultrasonic pressures on the behaviors of cavitation bubble in the melt were studied.The ultrasonic field propagation in the melt was also characterized.The results show that samples from different positions are subjected to different acoustic pressures and the effect of grain refinement by ultrasonic treatment for these samples is different.With the increase of ultrasonic intensity,the acoustic pressure is increased and the grain size is decreased generally.

Grain refinement of pure aluminum by direct current pulsed magnetic field and inoculation

The combined effects of direct current pulsed magnetic field （DC-PMF） and inoculation on pure aluminum were investigated, the grain refinement behavior of DC-PMF and inoculation was discussed. The experimental results indicate that the solidification micro structure of pure aluminum can be greatly refined under DC-PMF. Refinement of pure aluminum is attributed to electromagnetic undercooling and forced convection caused by DC-PMF. With single DC-PMF, the grain size in the equiaxed zone is uneven. However, under DC-PMF, by adding 0.05% （mass fraction） Al5Ti-B, the grain size of the sample is smaller, and the size distribution is more uniform than that of single DC-PMF. Furthermore, under the combination of DC-PMF and inoculation, with the increase of output current, the grain size is further reduced. When the output current increases to 100 A, the average grain size can decrease to 113 μn.

Grain refinement of Al-5%Cu aluminum alloy under mechanical vibration using meltable vibrating probe

A mechanical vibration technique to refine solidified microstructure was reported. Vibration energy was directly introduced into a molten alloy by a vibrating horn, and the vibrating horn was melted during vibration. Effects of vibration acceleration and mass ratio on the microstructure of Al-5% Cu alloy were investigated. Results show that the present mechanical vibration could provide localized cooling by extracting heat from the interior of molten alloy, and the cooling rate is strongly dependent on vibration acceleration. It is difficult to refine the solidified microstructure when the treated alloy keeps full liquid state within the entire vibrating duration. Significantly refined microstructure was obtained by applying mechanical vibration during the initial stage of solidification. Moreover, mechanisms of grain refinement were discussed.

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.

Effect of electrodes and thermal insulators on grain refinement by electric current pulse

The application of electric current pulse（ECP） to a solidification process refers to the immersion of electrodes into the liquid metal and the employment of thermal insulators on the upper surface of metal.In order to ascertain the effects of these two factors on the structure refinement by the ECP technique,three groups of experiments were performed with different types of electrodes or various thermal insulators.By the comparison between solidification structures under different conditions,it is followed that the electrode and the thermal insulator have an obvious influence on the grain refinement under an applied ECP,and further analysis demonstrates that the thermal conditions of the liquid surface play a vital role in the modification of solidification structure.Also,the results support the viewpoint that most of the equiaxed grains originate from the liquid surface subjected to an ECP.

Effect of Al_2Ca intermetallic compound addition on grain refinement of AZ31 magnesium alloy

The Al2Ca intermetallic compound was prepared by melting process in a vacuum induction furnace. And the A12Ca compound was added in as-cast AZ31 alloys for grain refinement. The effect of its additional levels on grain refinement of as-cast AZ31 alloy was investigated and the mechanism of the grain refinement was discussed. The results reveal that the addition of 1.1% Al2Ca （mass fraction） decreases the average grain size of as-cast AZ31 alloy from 354 to 198 μm. And the thermal stability of the grains refined by Al2Ca is superior. The grain refining mechanism is attributed to the combined effects of solute and heterogeneous nucleation from the Al2Ca.

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.

Effect of processing route on grain refinement in pure copper processed by equal channel angular extrusion

An experimental study of the microstructures in pure copper billets processed by 8 passes of equal channel angular extrusion （ECAE） via an extended range of processing routes with a 90° die is carried out. Each processing route is defined according to the inter-pass billet rotation angle （χ）, which varies from 0° to 180°. According to the generation of high-angle boundaries and reduction of grain size by electron backscatter diffraction （EBSD） measurements, the grain refinement is found to be most efficient for route with χ=90°and least efficient with χ=180°, among the seven routes studied. This trend is supported by supplementary transmission electron microscopy （TEM） measurements. Comparison of the EBSD and TEM data reveals the importance of considering the non-equiaxity of grain structures in quantitative assessment of microstructural differences in ECAE-processed materials.

Influence of grain refinement on the corrosion behavior of metallic materials:A review

Grain refinement can strengthen the mechanical properties of materials according to the classical Hall-Petch relationship but does not always result in better corrosion resistance.During the past few decades,various techniques have been dedicated to refining grain,along with relevant studies on corrosion behavior,including general corrosion,pitting corrosion,and stress corrosion cracking.However,the funda-mental consensus on how grain size influences corrosion behavior has not been reached.This paper reviews existing literature on the beneficial and detrimental effects of grain refinement on corrosion behavior.Moreover,the effects of microstructural changes(i.e.,grain boundary,dislo-cation,texture,residual stress,impurities,and second phase)resulting from grain refinement on corrosion behavior are discussed.The grain re-finement not only has an impact on the corrosion performance,but also results in microstructural changes that have a non-negligible effect on corrosion behavior or even outweigh that of grain refinement.Grain size is not the only factor affecting the corrosion behavior of metallic ma-terials;thus,the overall influence of microstructures on corrosion behavior should be understood.

Grain refinement in the coarse-grained region of the heat-affected zone in low-carbon high-strength microalloyed steels

The microstructural features and grain refinement in the coarse-grained region of the heat-affected zone in low-carbon high-strength microalloyed steels were investigated using optical microscopy, scanning electron microscopy, and electron backscattering dif- fraction. The coarse-grained region of the heat-affected zone consists of predominantly bainite and a small proportion of acicular ferrite. Bainite packets are separated by high angle boundaries. Acicular ferrite laths or plates in the coarse-grained region of the heat-affected zone formed prior to bainite packets partition austenite grains into many smaller and separate areas, resulting in fine-grained mixed microstruc- tures. Electron backscattefing diffraction analysis indicates that the average crystallographic grain size of the coarse-grained region of the heat-affected zone reaches 6-9 μm, much smaller than that of anstanite grains.

Grain refinement of Mg-Al alloys inoculated by MgAl_(2)O_(4)powder

As a potent nucleating substrate forα-Mg grain,MgAl_(2)O_(4) powder was used to inoculate the Mg-Al melt in this study.The effects of MgAl_(2)O_(4)amount,holding time and Al content on the grain size and grain refining ratio of the inoculated Mg-Al alloys are systematically investigated.The results show that the minimum grain size of Mg-3Al alloy is achieved by adding 2wt.%MgAl_(2)O_(4)powder and this alloy exhibits higher grain refining ratio than Mg-5Al and Mg-8Al alloys.The crystallographic misfit calculation indicates the wellmatching and possible orientation relationships(ORs)betweenα-Mg and MgAl_(2)O_(4).Among these predicted ORs,[10–10]α−Mg//[110]MgAl2O4 in(0002)α−Mg//(1–13)MgAl2O4 possesses the smallest misfit,i.e.,2.34%(fr).Both results of the experiment and crystallographic calculation demonstrate that the grain refinement of Mg-Al alloys is attributed to the MgAl_(2)O_(4)particles acting as the heterogeneous nucleation substrates forα-Mg grains.

The role of Al_(2)Y in grain refinement in Mg-Al-Y alloy system

A series of Mge10(AlþY)alloys with various weight ratios of Al to Y were cast to investigate the role of Al_(2)Y in grain refinement in MgeAleY ternary system.Thermal analysis combined with microstructural and EDX analysis was used to determine the phase transformation temperatures during solidification process.Experimental results show that when the Al content is below 4 wt%,a peritectic reaction,LþAl_(2)Y/a-Mg,occurs after the intermetallic Al_(2)Y forms directly from the melt as a pro-peritectic phase.Once the Al content is above 4 wt%,an eutectic reaction occurs at a lower temperature.The presence of the pro-peritectic phase can lead to nucleation of a-Mg directly through a peritectic reaction although grain refining efficiency is also closely related to the active particle size.In the case where solidification does not involve a peritectic reaction,the growth restriction factor,quantitatively the Q-value,governs the grain refining efficiency.Higher Q-value corresponds to finer grains.

Kinetic behaviour of TiB particles in Al melt and their effect on grain refinement of aluminium alloys

Solidification experiments were carried out to investigate the kinetic behaviour of TiB2 particles in Al melt and their effect on the grain refinement of commercially-pure Al.A model was proposed to describe the kinetic behaviour of TiB2 particles during the whole process from the addition of TiB2 to the melt to the freezing of the melt.The results indicate that TiB2 particles are not stable in Al melt.They may dissolve and coarsen during the holding period and grow during the cooling period of the melt.The kinetic behaviour of TiB2 particles in the melt has a great influence on their number density and the grain refinement.Solute Ti addition can suppress the dissolution,Ostwald ripening and growth behaviours of TiB2 particles.