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.

Tailoring bimodal grain structure of Mg-9Al-1Zn alloy for strength-ductility synergy:Co-regulating effect from coarse Al_(2)Y and submicron Mg_(17)Al_(12) particles

Grain boundary strengthening is an effective strategy for increasing mechanical properties of Mg alloys.However,this method offers limited strengthening in bimodal grain-structured Mg alloys due to the difficultly in increasing the volume fraction of fine grains while keeping a small grain size.Herein,we show that the volume fraction of fine grains(FGs,~2.5μm)in the bimodal grain structure can be tailored from~30 vol.%in Mg-9 Al-1 Zn(AZ91)to~52 vol.%in AZ91-1Y(wt.%)processed by hard plate rolling(HPR).Moreover,a superior combination of a high ultimate tensile strength(~405 MPa)and decent uniform elongation(~9%)is achieved in present AZ91-1Y alloy.It reveals that a desired bimodal grain structure can be tailored by the co-regulating effect from coarse Al_(2)Y particles resulting in inhomogeneous recrystallization,and dispersed submicron Mg_(17)Al_(12)particles depressing the growth of recrystallized grains.The findings offer a valuable insight in tailoring bimodal grain-structured Mg alloys for optimized strength and ductility.

A CELLULAR AUTOMATON-APPROACH TO SIMULATION OFGRAIN STRUCTURE DEVELOPMENT INELECTROSLAG CASTING

A 3-D cellular automaton model of thermal transfer and solidification has been developed, aiming at a simulational study of the grain structure development in electroslag casting. The program we developed for simulation of the model allows the effects of both metallurgical factors, including solidification point, supercooling required for nucleation and its scattering, and liquid/solid interface energy, and thermophysical factors, including heat conduction coeffcients, heat transfer coefficients and latent heat, to be investigated. The effect of process control can be indirectly inspected with the simulation by varying the melting rate. A box counting algorithm was employed to estimate the local curvature of liquid/solid interface. A series of simulated experiments of electroslag casting processes have been carried out. The simulation started from the beginning of the electroslag casting and proceeds by iteration of certain rules, during which a uniform constant slag temperature and a constant melting rate were assumed. It has been observed that a pool of molten metal forms and deepens gradually under constant melting rate. The deepening of the pool slows down with the simulated electroslag casting process, and the depth and shape of the pool tends to be steady after certain height of cast is formed. A finger-like grain structure with the fingers approximately normal to the bottom of the molten metal pool was generally observed. Higher latent heat was found to enhance dendritic growth. The results agree well with general observation of the grain structures in electroslag castings and demonstrate the applicability of cellular automaton modeling to structural development in casting.

Effect of grain boundary sliding on the toughness of ultrafine grain structure steel: A molecular dynamics simulation study

Molecular dynamics simulations are carried out to investigate the mechanisms of low-temperature impact toughness of the ultrafine grain structure steel. The simulation results suggest that the sliding of the {001 }/{ 110} type and { 110}/{ 111 } type grain boundary can improve the impact toughness. Then, the mechanism of grain boundary sliding is studied and it is found that the motion of dislocations along the grain boundary is the underlying cause of the grain boundary sliding. Finally, the sliding of the grain boundary is analyzed from the standpoint of the energy. We conclude that the measures which can increase the quantity of the {001}/{110} type and {110}/{ 111} type grain boundary and elongate the free gliding distance of dislocations along these grain boundaries will improve the low-temperature impact toughness of the ultrafine grain structure steel.

Role of Ordering Energy in Formation of Grain Structure and Special Boundaries Spectrum in Ordered Alloys with L12 Superstructure

It was revealed that an average energy of special boundaries is proportional to APB energy in the alloys with the L12 superstructure. This fact proves the appearance of the GAPBs in the planes of location of special boundaries in coincidence sites of ordered alloys. It was determined that the more energy of special boundaries in ordered alloys, the more energy of complex stacking fault. There is a correlation between the distribution of special boundaries as a function its relative energy and ordering energy: the more ordering energy, the more degree of washed away of distribution. The correlation between average relative energy of special boundaries and ordering energy was detected: the more ordering energy, the more average energy of special boundaries. The reverse dependence between ordering energy and average number of special boundaries in grains limited by boundaries of general type was discovered.

Effect of grain structure on fatigue crack propagation behavior of Al-Cu-Li alloys

Recrystallization behavior during optimized heat treatments provides a potential to obtain desirable grain structure,which significantly improves the mechanical properties of aluminum alloys.The influence of grain structures on fatigue crack propagation(FCP)behaviors of Al-Cu-Li alloy with hot-rolled(HR)and cold-rolled(CR)was investigated.Subgrain boundaries have a significant impact on small crack growth rates,which is reflected in the pronounced fluctuation of fatigue crack growth of HR specimens after solution treatment.Moreover,the specific cellular structure within grains can improve the deformation capacity of alloys due to their accommodation of plastic deformation,which contributes to the lower fatigue crack growth rates and higher threshold values in HR specimens.The intragranular deflection also decelerates the FCP rate and occurs in these regions of large grain without subgrain boundaries.Recrystallization occurs in the CR specimens,resulting in small anisotropy on the fatigue resistance for the different orientations in the Paris stage due to the recrystallization texture.Fatigue cracks can be deflected and tend to propagate along the grain boundaries when it goes into the grain with a relatively low Schmidt factor value.

Relationship between the fatigue behavior and grain structures of an as-extruded Mg-6.2%Zn-0.6%Zr(in wt.%)alloy

Through investigating the tension-tension fatigue behavior of an as-extruded Mg-6.2 wt.%Zn-0.6 wt.%Zr(ZK60)alloy,it revealed that the determined fatigue strength at 107 cycles was quite sensitive to the grain structure.Among them,the fine grain structure had the highest fatigue strength of 130 MPa,whereas the typical“bi-modal”grain structure had the lowest fatigue strength of 110 MPa.Failure analysis demon-strated that for the fine grain structure,fatigue cracks preferentially nucleated at grain boundaries.For the“bi-modal”and coarse grain structures,the fatigue crack initiation was dominated by the cracking along slip bands.

Effect of grain structure on the mechanical properties of a Monel alloy fabricated by laser-based directed energy deposition

Monel K-500 is a Ni–Cu alloy widely used in the marine and offshore industry due to their superior resistance to corrosion in seawater and hence easily degraded.To address this problem,laser-based directed energy deposition(LDED)is used to repair or refabricate these high-value worn parts.To optimize the mechanical properties of repaired parts,the commonly applied solution and aging is not ideal because it also changes the properties of the base materials.Consequently,in situ control of the grain structures during the LDED process becomes an effective approach for high-performance repair.In this study,we fabricated a duplex grain structure with small grain size and low texture intensity using low laser power and scanning velocity.The duplex microstructure consists of short columnar grains and zigzag-distributed fine equiaxed grains.The formation of this grain structure is dependent on both the solidification and recrystallization mechanisms.The strength of this grain structure is improved to 523.5 MPa without the sacrifice of ductility,which is instead 20%higher than that of the counterpart consisting of typical columnar grains due to the grain refinement and crack toughening.The mechanical properties of the alloy with the duplex grain structure are even comparable to heat-treated Monel K-500 fabricated by wire arc additive manufacturing.This work provides valuable insights into the in situ optimization of the microstructure and mechanical properties of LDED-fabricated parts.

Effect of scanning strategy on grain structure and crystallographic texture of Inconel 718 processed by selective laser melting

Two types of scanning strategies were adopted to study the effect of scanning strategy on grain structure and crystallographic texture of selective laser melted （SLM） Inconel 718. The results show that bidirec-tional scanning without and with a 90°-rotation for every layer produced the bimodal grain structure and the directional columnar grain structure, respectively. Controlling the heat flux direction between the successive layers via scanning strategy enabled the formation of such different grain structures. Fur-thermore, when the 90°-rotation was applied, the competitive grain growth mechanism became more pronounced and the strong cube texture developed.

Characterization of the Grain Structures in Vacuum-Assist HighPressure Die Casting AM60B Alloy

The structures in vacuum-assist high-pressure die casting （HPDC） AM60B alloy were studied by using an optical microscope and a scanning electron microscope with an energy dispersive spectrometer. It was found that the HPDC under the vacuum could significantly change the morphology and distribution of the microstructure. For both conventional and vacuum-assist HPDC processes, the externally solidified crystals （ESCs） tended to aggregate in the center along the thickness direction of the castings. Besides, the aggregation was more pronounced, and the number of ESCs decreased, and the ESCs tended to become smaller and more globular, as the distance between the specimen location and runner increased. Compared with the conventional castings, the vacuum-assist HPDC can significantly reduce the size and amount of ESCs, and the ESCs tended to be more globular. For the distribution of ESCs along the thickness of the specimens, the aggregation tendency was more pronounced in vacuum-assist die castings than that in conventional castings. Besides, the distribution of ESCs at different locations was more converged in the vacuum-assist HPDC than that in the conventional HPDC.

Stabilizing a severely deformed Al-7Mg alloy with a multimodal grain structure via Mg solute segregation

Single-phase Al-Mg alloys processed by severe plastic deformation(SPD)usually suffer from unsatisfactory thermal stability at moderate to high temperatures with recrystallization occurring and obvious grain coarsening.In the present work,an Al-7Mg alloy prepared by equal-channel angular pressing(ECAP)possessed markedly enhanced thermal stability upon annealing at moderate to high temperatures(200-275℃),compared with those ultrafine-grained dilute Al-Mg alloys with a uniform microstructure.The enhanced thermal stability is due primarily to the multimodal grain structure consisting of nano-,ultrafine-and micron-sized grains,strong segregation and/or clusters of Mg solute along grain boundaries(GBs),and Al_(3)Mg_(2)precipitates formed during annealing.First,extensive recovery predominates over recrystallization and consumes most of the stored energy in the ECAPed Al-7Mg alloy annealed at≤275℃,leading to the recrystallization and growth of nano/ultrafine grains being retarded or postponed.Moreover,Mg solute segregation and/or clusters along GBs of nano/ultrafine grains could further suppress grain growth via diminishing GB energy and dragging GBs efficiently.In addition,Al_(3)Mg_(2)precipitates formed with increasing annealing time could inhibit grain growth by pinning GBs.The present multimodal-grained Al-7Mg alloy with enhanced thermal stability is believed to be particularly attractive in potential engineering applications at moderate to high temperatures.

Achieving strength-ductility synergy in a non-equiatomic Cr_(10)Co_(30)Fe_(30)Ni_(30)high-entropy alloy with heterogeneous grain structures

Cold rolling and post-deformation annealing(PDA)heat treatments were used to produce heterogeneous grain structures(HGS)in a single-phase face-centered cubic(fcc)Cr_(10)Co_(30)Fe_(30)Ni_(30)high-entropy alloy(HEA).The microstructural evolution and microstructure-property relationship of the HEA were systematically studied under different states.HGS could be achieved in PDA-treated samples at 875℃for 20 s and at 900℃for 20 s(PDA-900-20 s).PDA-900-20 s sample exhibits the most excellent combination of strength and ductility,showing a tensile yield strength of~590 MPa,an ultimate strength of~706 MPa and a total elongation of~23.9%.Additionally,compared with the homogenized counterpart exhibiting homogenous grains,PDA-900-20 s sample displays a notable increment of~413%in yield strength and simultaneously maintains a good ductility.The dominated strengthening mechanisms in PDA-900-20 s sample are grain-boundary strengthening and heterogeneous deformation-induced(HDI)strengthening,whereas the good ductility is mainly resulted from the HDI ductility.Accordingly,the present study provides an effective and simple pathway to overcome the strength-ductility tradeoff of typical fcc HEAs through heterogeneous microstructure.

Achieving excellent energy storage reliability and endurance via mechanical performance optimization strategy in engineered ceramics with core-shell grain structure

Although dielectric ceramic capacitors possess attractive properties for high-power energy storage,their pronounced electrostriction effect and high brittleness are conducive to easy initiation and propagation of cracks that significantly deteriorate electrical reliability and lifetime of capacitors in practical applications.Herein,a new strategy for designing relaxor ferroelectric ceramics with K_(0.5)Na_(0.5)NbO_(3)-core/SiO_(2)-shell structured grains was proposed to simultaneously reduce the electric-field-induced strain and enhance the mechanical strength of the ceramics.The simulation and experiment declared that the bending strength and compression strength of the core-shell structured ceramic were shown to increase by more than 50% over those of the uncoated sample.Meanwhile,the electric-field-induced strain was reduced by almost half after adding the SiO_(2) coating.The suppressed electrical deformation and enhanced mechanical strength could alleviate the probability of generation of cracks and prevent their propagation,thus remarkably improving breakdown strength and fatigue endurance of the ceramics.As a result,an ultra-high breakdown strength of 425 kV cm^(-1) and excellent recoverable energy storage density(Wrec~4.64 J cm^(-3))were achieved in the core-shell structured sample.More importantly,the unique structure could enhance the cycling stability of the ceramic(Wrec variation<±2% after 105 cycles).Thus,mechanical performance optimization via grain structure engineering offers a new paradigm for improving electrical breakdown strength and fatigue endurance of dielectric ceramic capacitors.

Bimodal grain structures and tensile properties of a biomedical Co-20Cr-15W-10Ni alloy with different pre-strains

The influence of pre-strain on the formation of bimodal grain structures and tensile properties of a Co-20 Cr-15 W-10 Ni alloy was investigated.The bimodal grain structures consist of fine grains(FGs;2-3μm in diameter)and coarse grains(CGs;8-16μm in diameter),which can be manipulated by changing the pre-strain(ε=0.3-0.7)and annealing temperatures(1000-1100℃).High pre-strain applied in the samples can intensify the plasticity heterogeneity through increasing the total dislocation density and the local volumes of high-density dislocations.This can essentially result in finer FGs,a higher FG volume fraction,and overall grain refinement in the samples after annealing.High-temperature essentially increases both the size and volume fraction of CGs,leading to an increase in the average grain size.The tensile test suggests that the bimodal grain structured samples exhibited both high strength and ductility,yield strengths of621-877 MPa and ultimate tensile strengths of1187-1367 MPa with uniform elongations of 55.0%-71.4%.The superior strength-ductility combination of the samples arises from the specific deformation mechanisms of the bimodal grain structures.The tensile properties strongly depend on the size ratio and volume fraction of FGs/CGs in addition to the average grain size in the bimodal grain structures.The grain structures can be modified via changing the pre-strain and annealing temperature.

Thermal stability of bimodal grain structure in a cobalt-based superalloy subjected to high-temperature exposure

The present work investigates the thermal stability and mechanical properties of a Co-20 Cr-15 W-10 Ni(wt%) alloy with a bimodal grain(BG) structure.The BG structure consisting of fine grains(FGs) and coarse grains(CGs) is thermally stable under high-temperature exposure treatments of 760℃ for 100 h and 870℃ for 100-1000 h.The size of both FGs and CGs remains no significant changes after thermal exposure treatments.The microstructural stability is associated with the slow kinetics of grain growth and the pinning of carbides.The thermal stability enables to maintain the BG structures,leading to the same mechanical properties as the sample without thermal exposure treatment.In particular,the BG alloy samples after thermal exposure treatment exhibit superior mechanical properties of both high strength and high ductility compared to the unimodal grain(UG) structured ones.The BG structure of the alloy samples after thermal exposure is capable of avoiding severe loss of ductility and retaining high strength.More specifically,the ductility of the BG alloy samples after thermal exposure treatments of 870℃ for 500-1000 h is ten times higher(44.6% vs.3.5% and 52.6% vs.5.0%) than that of the UG ones.The finding in the present work may give new insights into high-temperature applications of the Co-20 Cr-15 W-10 Ni alloy and other metallic materials with a BG structure.

Effect of Initial Microstructure Prior to Extrusion on the Microstructure and Mechanical Properties of Extruded AZ80 Alloy with a Low Temperature and a Low Ratio

Magnesium(Mg)alloys are the lightest metal structural material for engineering applications and therefore have a wide market of applications.However,compared to steel and aluminum alloys,Mg alloys have lower mechanical properties,which greatly limits their application.Extrusion is one of the most important processing methods for Mg and its alloys.However,the effect of such a heterogeneous microstructure achieved at low temperatures on the mechanical properties is lacking investigation.In this work,commercial AZ80 alloys with different initial microstructures(as-cast and as-homogenized)were selected and extruded at a low extrusion temperature of 220℃and a low extrusion ratio of 4.The microstructure and mechanical properties of the two extruded AZ80 alloys were investigated.The results show that homogenized-extruded(HE)sample exhibits higher strength than the cast-extruded(CE)sample,which is mainly attributed to the high number density of fine dynamic precipitates and the high fraction of recrystallized ultrafine grains.Compared to the coarse compounds existing in CE sample,the fine dynamical precipitates of Mg17(Al,Zn)12form in the HE sample can effectively promote the dynamical recrystallization during extrusion,while they exhibit a similar effect on the size and orientation of the recrystallized grains.These results can facilitate the designing of high-strength wrought magnesium alloys by rational microstructure construction.

COMPUTER SIMULATION OF GRAIN BOUNDARY STRUCTURES IN Ni_3Al

The embedded atom type potentials and static relaxation method combined with steepest gra- dient computational technique have been used to simulate the grain boundary cohesive ener- gies,the distribution of electron density and stress field in the grain boundary,region,and oth- er related problems of[100],[110]and[111]symmetric tilt grain boundaries in Ni_3Al with different grain boundary geometrical index and composition.Their relations with the segrega- tion or boron,behaviors or the grain boundary,and especially the stoichiometrical effect of B induced ductility have also been studied and discussed.

Using vibrational infrared biomolecular spectroscopy to detect heat-induced changes of molecular structure in relation to nutrient availability of prairie whole oat grains on a molecular basis

Background： To our knowledge, there is little study on the interaction between nutrient availability and molecular structure changes induced by different processing methods in dairy cattle. The objective of this study was to investigate the effect of heat processing methods on interaction between nutrient availability and molecular structure in terms of functional groups that are related to protein and starch inherent structure of oat grains with two continued years and three replication of each year.Method： The oat grains were kept as raw（control） or heated in an air-draft oven（dry roasting： DO） at 120 °C for 60 min and under microwave irradiation（MIO） for 6 min. The molecular structure features were revealed by vibrational infrared molecular spectroscopy.Results： The results showed that rumen degradability of dry matter, protein and starch was significantly lower（P 〈0.05） for MIO compared to control and DO treatments. A higher protein α-helix to β-sheet and a lower amide I to starch area ratio were observed for MIO compared to DO and/or raw treatment. A negative correlation（-0.99, P 〈 0.01）was observed between α-helix or amide I to starch area ratio and dry matter. A positive correlation（0.99, P 〈 0.01） was found between protein β-sheet and crude protein.Conclusion： The results reveal that oat grains are more sensitive to microwave irradiation than dry heating in terms of protein and starch molecular profile and nutrient availability in ruminants.

Study on grain refinement of interstitial-free steel during continuous frictional angular extrusion

To explore the application of severe plastic deformation for grain refinement in steel production, a new method called continuous frictional angular extrusion （CFAE） was applied to refine the grain of interstitial-free steel. The deformation was carried out at room temperature and individual sheet specimens were processed in different number of passes. An overall grain size of 200nm was achieved after 8 passes and the proportion of high-angle boundaries to the total boundaries was more than 60%. Through the characterization of high resolution EBSD, X-ray diffraction （XRD） and hardness testing,this paper discussed the evolution of microstructures and textures during deformation and explored the development direction of the method.

Gradient Ultra-fine Grained Surface Layer in 6063 Aluminum Alloy Obtained by Means of Rotational Accelerated Shot Peening

Gradient ultra-fine grained surface layer in 6063 aluminum alloy was obtained by means of a novel surface self-nanocrystallization technique,namely rotational accelerated shot peening(RASP)treatment.The average grain sizes along the vertical section vary from hundreds of nanometers in the top surface to micrometers in the matrix.By using orthogonal experimental design to compare roughness values and hardness values,we synthesized the processing parameters to obtain sample of smaller roughness values and higher hardness.