Coarse Dendrites Research Articles

The Influence of Severe Plastic Deformation on Microstructure and In Vitro Biocompatibility of the New Ti-Nb-Zr-Ta-Fe-O Alloy Composition.

In this work, severe plastic deformation (SPD) of the newly designed Ti-Nb-Zr-Ta-Fe-O GUM metal was successfully conducted at room temperature using high speed high pressure torsion (HSHPT) followed by cold rolling (CR) to exploit the suitability of the processed alloy for bone staples. The Ti-31.5Nb-3.1Zr-3.1Ta-0.9Fe-0.16O GUM alloy was fabricated in a levitation melting furnace using a cold crucible and argon protective atmosphere. The as-cast specimens were subjected to SPD, specifically HSHPT, and then processed by the CR method to take the advantages of both grain refinement and larger dimensions. This approach creates the opportunity to obtain temporary orthopedic implants nanostructured by SPD. The changes induced by HSHPT technology from the coarse dendrite directly into the ultrafine grained structure were examined by optical microscopy, scanning electron microscopy and X-ray diffraction. The structural investigations showed that by increasing the deformation, a high density of grain boundaries is accumulated, leading gradually to fine grain size. In addition, the in vitro biocompatibility studies were conducted in parallel on the GUM alloy specimens in the as-cast state, and after HSHPT- and HSHPT+CR- processing. For comparative purposes, in vitro behavior of the bone-derived MC3T3-E1 cells on the commercially pure titanium has also been investigated regarding the viability and proliferation, morphology and osteogenic differentiation. The results obtained support the appropriateness of the HSHPT technology for developing compression staples able to ensure a better fixation of bone fragments.

Effect of heat treatment on the microstructure and properties of in-situ Mg2Si reinforced hypereutectic Al-18%Si matrix composites

The in situ preparation of particle reinforced metal matrix composites (PRMMCs) can prevent the particle surface from pollution, improve the wettability of particles and matrix metals, and reduce the production cost of composites. In this paper, in situ Mg2Si reinforced hypereutectic Al-18%Si matrix composites were prepared by adding an appropriate amount of magnesium. However, the Mg2Si enhanced phase often has developed dendrites, which are easy to cause stress concentration and cut apart the matrix, resulting in the decrease of the mechanical properties of the material. Therefore, in this study, necessary solution treatment and aging treatment were carried out, and the effect of heat treatment on the microstructure and properties of the prepared composites was discussed. The results showed that the in situ Mg2Si reinforced Al-18%Si matrix composites with an ideal microstructure could be prepared by adding 4% Mg. After solution treatment at 545 °C for 8 h and aging treatment at 170 ± 2 °C for 8 h, the coarse dendrites of Mg2Si were broke and granulated, which improved the strength, hardness and wear resistance of the composites. Compared with the common as cast hypereutectic Al-18%Si alloy, the hardness and the strength of the prepared composites were increased by 30% and 46%, respectively. Besides, the granulation process of the in situ Mg2Si phase was explained with the Gibbs-Thomson theorem in this paper.

Influence of aging on microstructure and hardness of lead-free solder alloys

Purpose The purpose of this paper is to carry out a study of the evolution of the microstructure and the microhardness of Sn-Cu-Ag alloys from as-cast condition and under artificial isothermal aging at different temperatures (100ºC and 180ºC) for a treatment time up to 500 h. A comparison with Sn-37% Pb eutectic solder samples was also made. Design/methodology/approach Sn-3.5%Ag, Sn-0.7%Cu and Sn-3.5%Ag-0.9%Cu were poured in two different cooling rate conditions and then aged at 100ºC (373ºK) and 180 °C (453ºK) during 500 h. Microstructural changes were observed by optical microscopy, scanning electron micrograph and energy dispersive X-ray microanalysis. Differential scanning calorimetry technique (DSC) was also used to confirm the obtained results. Findings A decrease up to 20% in microhardness respect to the value of the as-cast alloy was observed for both aging temperatures. These changes can be explained considering the coarsening and recrystallization of Sn dendrites present in the microstructures of all the systems studied. Originality/value There is no evidence of dissolution or precipitation of new phases in the range of studied temperatures that could be detected by DSC calorimetry technique. The acting mechanisms must be the result of coarsening of Sn dendrites and the residual stresses relaxation during the first stages of the isothermal aging.

Modeling of dendrite arm fragmentation and dendrite arm coarsening⋆.

A two-dimensional quantitative cellular automaton (CA) model is employed to simulate dendrite arm fragmentation and dendrite arm coarsening in mushy zones. The phenomenon of dendrite arm fragmentation of an Al-Cu alloy during heating is well represented by the CA simulation, and it is analyzed in detail by comparing the local actual concentration and local equilibrium concentration. The CA simulations for the dendritic microstructures of SCN-ACE alloys during isothermal holding in a mushy zone reproduce the typical dendrite coarsening features as observed in experiments. The effects of holding temperature and alloy composition on the microstructures and dendrite coarsening kinetics are investigated. It is found that the melting of small dendrite arms and interdendritic groove advancement are the two main mechanisms in dendrite coarsening. The mechanism of coalescence by joining arm tips is more likely to take place at a lower temperature or for a lower alloy composition, while the dendrite arm fragmentation mechanism tends to occur at a higher temperature. The coarsening rate constant is found to decrease with increasing holding temperature and alloy composition.

Wire and arc additive manufacturing of 4043 Al alloy using a cold metal transfer method

Cold metal transfer plus pulse (C + P) arc was applied in the additive manufacturing of 4043 Al alloy parts. Parameters in the manufacturing of the parts were investigated. The properties and microstructure of the parts were also characterized. Experimental results showed that welding at a speed of 8 mm/s and a wire feeding speed of 4.0 m/min was suitable to manufacture thin-walled parts, and the reciprocating scanning method could be adopted to manufacture thick-walled parts. The thin-walled parts of the C + P mode had fewer pores than those of the cold metal transfer (CMT) mode. The thin- and thick-walled parts of the C + P mode showed maximum tensile strengths of 172 and 178 MPa, respectively. Hardness decreased at the interface and in the coarse dendrite and increased in the refined grain area.

Modification of Mg2Si Phase Morphology in Mg-4Si Alloy by Sb and Nd Additions

We reported the effects of compound modification with Nd and Sb on the microstructure evolution and mechanical properties of Mg-4Si alloys. The characterization results showed that adding 1.0 wt.% Sb and 1.0 wt.% Nd to the alloy can effectively change the morphology of Mg2Si particles. The primary Mg2Si particles changed from coarse dendrites to regular polygons, and the average particle size decreased from 78.3 to 6.5 μm. Meanwhile, the Chinese eutectic Mg2Si became small short fiber. The experimental results showed that the Nd4Sb3 phase could be formed after adding 1.0 wt.% Sb and 1.0 wt.% Nd to the alloy. The Nd4Sb3 phase could act as the heterogeneous nucleation core of Mg2Si phase, which increased the nucleation rate of Mg2Si and improved the morphology of Mg2Si particles. The mechanical properties test found that the tensile properties and Brinell hardness of the alloy were improved with Sb and Nd alloyed. After adding 1.0 wt.% Sb and 1.0 wt.% Nd to the alloy, the ultimate tensile strength increased from 113 to 184 MPa, the elongation increased from 2.23 to 4.61%, and the Brinell hardness increased from 65.45 to 87.32 HB.

Microstructure and tribological properties of laser in-situ synthesized Ti3Al composite coating on Ti-6Al-4V

Titanium intermetallics are highly significant to protect surfaces of lightweight structures against corrosion and wear. In this study, Ti and Al powders were uniformly mixed with an atomic ratio 65:35 and in-situ synthesized on Ti-6Al-4V substrate by laser cladding with Ar cooling. The microstructure and high temperature tribological behavior were studied. The results show that Ti3Al (α2) phase was first precipitated in the form of coarse primary dendrites during cladding solidification, and subsequently α-Ti (α) phase of needle-shaped martensite was formed by fast cooling. The 6:4 ratio of phase α2 to α of the composite coating provides optimal toughness and high hardness (680 HV). Under the testing up to 500 °C, the average friction coefficient of the composite coating is 0.22 ± 0.011 and a 35% lower wear rate than that of Ti-6Al-4V substrate.

Microstructure evolution and nano-hardness modulation of rapidly solidified Ti–Al–Nb alloy

Microstructure evolution, the formation of B2 (ordered BCC) phase and its relation to the nano-mechanical properties of the rapidly solidified Ti 68 Al 32 , Ti 63 Al 32 Nb 5 and Ti 58 Al 32 Nb 10 alloys were investigated by the drop tube technique. For the Ti 68 Al 32 alloy droplets, the solidified microstructures are composed of α 2 -Ti 3 Al phase, when the droplet diameters (D) are ranging from 226 to 1100 μm. With the decrease of droplet diameters, the microstructural characteristics of α 2 phase transform from coarse dendrites to the fully equiaxed grains. Notably, at D < 226 μm, the formation of α 2 phase is suppressed, resulting in the retention of metastable α-Ti phase. For the Ti 68-x Al 32 Nb x (x = 5, 10) alloys, the increasing of Nb contents promotes the formation of B2 phase and diminishes the size of the α 2 grains. Upon further decreasing the droplet diameter, the microstructure evolves from dendrite dendrites to equiaxed dendrites, and the B2 phase precipitates from the core of α 2 dendrites to α 2 grain boundaries, which ascribes to Nb-segregation from the dendrite core area to the grain boundary via L→β and β→α, respectively. Nanoindentation test reveals that the hardness of α 2 phase first increases and then decreases with the decrease of droplet diameters, which corresponds to the microstructure morphology, phase constitution, grain refinement of the alloys. Meanwhile, the additions of Nb exhibit enhanced properties of α 2 phase compared with those with low or without Nb additions, which can effectively modulate the hardness of these alloys. • TEM images confirmed that the formation of α 2 phase is suppressed, resulting in the retention of metastable α-Ti phase. • The increasing of Nb contents promotes the formation of B2 phase and diminishes the size of the α 2 grains. • B2 phase precipitates from the core of α 2 dendrites to α 2 grain boundaries with the decreases of droplet diameter • The addition of Nb can effectively modulate the hardness of Ti 3 Al–Nb alloys.

Effects of Y2O3 on the microstructure and wear resistance of WC/Ni composite coatings fabricated by plasma transferred arc

The present study envisaged the WC-reinforced nickel composite coatings prepared on the surface of 4145H steel using the plasma transferred arc (PTA) method. The effects of different content of Y2O3 on the microscopic morphology, phase composition, micro-hardness and wear behavior were investigated by scanning electron microscopy (SEM) with an energy-dispersive spectrometer (EDS), X-ray diffraction (XRD), micro-hardness test and wear experiments. The deposited coatings consisted of WC-reinforced Ni59 composite coatings with the addition of 0 wt%, 0.5 wt% and 1 wt% Y2O3. Microstructural observation showed that the addition of 0.5 wt% Y2O3 had a better improvement on the coarse dendrites and the grain refinement. The main phase composition of the composite coatings was γ-Ni, FeNi3 solid solution, γ(Fe, Ni) eutectic phase, and the hard phase of WC, W2C. The cladding layer added with 0.5 wt% Y2 O3 was preferable over the Y2O3-free coating, as the microhardness was improved from 500 HV to 800 HV with a slight fluctuation of values. The coating with 0.5 wt% Y2O3 offered an excellent wear resistance to the extrusion and cracks for the uniformity of organizational structure compared with the substrate and the other two coatings. Furthermore, the results of weightlessness and coefficient of friction proved the positive effect of the rare earth Y2O3 on the wear resistance.

Study on the Deformation Control and Microstructures of Thin-Walled Parts Repaired by Laser Cladding

To reduce the deformation and improve the quality of thin-walled parts repaired by laser cladding, a three-factor, three-level orthogonal experimental scheme was employed to clad Ni60 powder on thin-walled parts with a thickness of 3.5 mm. To measure the deformation of the thin-walled parts, a method of combining the meshing of the backs of the thin-walled parts and fixing one end of the parts during cladding was used. The effects of the powder feed rate, laser power, and scanning speed on the deformation of the thin-walled parts were studied via visual analysis and analysis of variance, and the process parameters that resulted in the minimum deformation were determined. The deformation process of the thin-walled parts and the causes of cladding stress were also studied, and the microstructure of the cladding layer with the minimum deformation was analyzed via scanning electron microscopy (SEM). The results reveal that the deformation of the thin-walled parts increased with the increase of laser power. The increases of the scanning speed and powder feed rate were found to reduce the deformation of thin-walled parts; the laser power was found to have a significant effect, and the powder feed rate was found to have no significant effect, on the deformation of thin-walled parts. The order of the influence of factors on the deformation of thin-walled parts from greatest to least was determined to be as follows: laser power &gt; scanning speed &gt; powder feed rate. The optimal parameters to obtain the minimum deformation and good metallurgical bonding of thin-walled parts were found to be a powder feed rate of 1.4 r/min, a laser power of 1100 W, and a scanning speed of 8 mm/s. From the bottom to the top, the crystal structure of the coating with the minimum deformation was found to be coarse dendrite, dendritic crystal, and equiaxed crystal.

Sub-grain formation in Al–Li–Mg–Zn–Cu lightweight entropic alloy by ultrasonic hammering

In this paper, the ultrasonic hammering treatment is applied to the lightweight entropic alloy Al80Li5Mg5Zn5Cu5, contributing to the modification of coarse microstructures and the improvement of mechanical properties. The effects of ultrasonic hammering on the microstructures and mechanical properties of Al80Li5Mg5Zn5Cu5 are elaborately studied, and the formation of sub-grains is thoroughly analyzed. Results show that the coarse dendrite structure is crushed into diffusely distributed micron-sized particles, and the grain size is refined significantly from larger than 200 μm to about5 μm, along with the formation of sub-grains during instant dynamic recovery. Microhardness and modulus of the grain-refined alloy increase by 50% and 22%, respectively, compared to the coarse-grained counterpart without ultrasonic hammering.

The effect of Sc addition on the novel nano-AlCu phase in as-cast Al-5%Cu alloy

The effects of Sc addition on the primary α-Al phases and novel nano-AlCu phases were investigated in Al-5%Cu alloys without and with 0.8%Sc. The microstructural evolution was characterized by scanning electron microscopy (SEM) and transmission electron microscope (TEM) with energy dispersive X-ray spectroscopy (EDS). In the study, the novel ribbon nano-AlCu phases were found in as-cast Al-5%Cu alloy and the average length is about 200–300 nm. When 0.8%Sc was added into the Al-5%Cu alloy, the α-Al phases were obviously refined from coarse dendrites to fine equiaxed grains and the average grains size decreased to 29.8 μm. Moreover, the number of nano-AlCu phases significantly increases, but the length and width obviously decrease. In addition, the nano-AlCu phases have a good lattice matching relationship with α-Al phases, which can act as a precursor refining α-Al grains.

Effects of a high-voltage pulsed magnetic field on the solidification structures of biodegradable Zn-Ag alloys

The physical and chemical properties of biodegradable zinc alloys are closely related to their microstructures. Therefore, various techniques are being attempted to change their microstructures. However, little work has been done to control the initial solidification structures of the biodegradable zinc alloys, which play an important role in determining their final properties. In view of this, Zn-Ag alloys with different compositions were solidified under a high-voltage pulsed magnetic field (HVPMF) in this work. The results show that the primary η-Zn grains in the isomorphous Zn-2.5 wt% Ag alloy are transformed from coarse dendrites to fine globular grains with increasing the HVPMFs. Also, its macrostructural homogeneity in the specimens is enhanced gradually. Moreover, the primary ε-AgZn3 dendrites in the hypoperitectic Zn-4.0 wt% Ag and hyperperitectic Zn-9.0 wt% Ag alloys are refined and “levitated” by the HVPMFs. Correspondingly, the peritectic η-Zn grains that nucleate on the fine primary ε-AgZn3 particles are also significantly refined. In addition, when applying the HVPMFs in separate solidification stages of these two alloys, it is found that the HVPMFs mainly have a crucial effect on the structures in the early stage. The modification of the solidification structures in the Zn-Ag alloys is attributed to the forced convection and the Joule heat induced in the liquid-solid two-phase zone by the HVPMFs.

Ultrasonic cavitation and acoustic streaming effects during liquid phase separation and dynamic solidification of ternary Al–Sn–Si immiscible alloy

Ultrasonic melt treatment is an environmentally friendly and no-polluting external field technology and has great application potential in materials fabrication. Power ultrasound with different intensities was introduced into the liquid phase separation and solidification process of ternary Al-45%Sn-5%Si immiscible alloy to explore the effects of ultrasound on the formation of macro- and microstructure. It was found that the previously apparent macrosegregation caused by the large density difference between immiscible liquid (Al) and (Sn) phases was gradually reduced and even suppressed with the increase of ultrasonic amplitude. Meanwhile, the ultrasonic waves also significantly modified monotectic structures, within which (Al) phase transformed from coarse dendrites to refined equiaxed grains, and (Si) phase evolved from long strips to small blocks. The introduction of ultrasound weakened the texture of solidified structure, whereas the growth orientation relationship between monotectic (Al) and (Si) phases remained unchanged. The theoretical analysis and calculation results indicated that ultrasonic cavitation and acoustic streaming were the dominant effects to modulate the macro- and microstructural evolution. This would improve the exploiting and utilization of immiscible alloys by ultrasonic processing technology.

The Role of the Primary Structure in the Formation of the Mechanical Properties of Steel Castings

The paper considers the total effect of primary structure parameters on steel mechanical properties depending on the wall thickness of castings. It is shown that during the formation of mechanical properties with an increase in the thickness of the work-piece, the role of the sizes of primary grains decreases and the role of reducing the dispersion of dendritic crystals increases, due to the process of dendritic branches coarsening.

Optimizing convolutional neural networks to perform semantic segmentation on large materials imaging datasets: X-ray tomography and serial sectioning

Machine learning was used to segment large materials science datasets resulting from synchrotron-based x-ray computed tomography (XCT) images of dendrite growth, and serial sectioning (SS) images of dendrite coarsening. Both neural networks (NNs) yielded quantitatively more accurate outputs than conventional segmentation techniques using only 30 XCT or 6 SS training images. We show that performance can be improved if NNs are trained using a large number of small images that are sampled from the fixed amount of training data. The optimal image size and number of training images was identified for the XCT and SS datasets. NN transferability was also tested by applying the highest performing XCT and SS NNs to related datasets. While the initial segmentations were successful, applying simple transformations to the raw images further improved NN performance. These results show the great predictive ability and promising future of using machine learning for segmentation of large materials science datasets.

Effect of iron content on microstructure and properties of FexNi2Co2CrTiNb high-entropy alloy coating

FexNi2Co2CrTiNb (x = 0, 0.5, 1, 2) high-entropy alloy coatings were fabricated on 45 steel by laser cladding. The phase, microstructure, hardness and wear resistance of FexNi2Co2CrTiNb coatings are studied systematically. The results behave that FexNi2Co2CrTiNb (x = 0, 0.5, 1, 2) coatings are composed of FCC and BCC solid solutions. When the Fe content increases from 0 to 2, the BCC phase in the coating increases significantly while the FCC phase decreases notably, the microstructure of FexNi2Co2CrTiNb coating evolves from coarse dendrites to the fine dendritic clusters in the general form of columnar or equiaxed crystals. The hardness of each FexNi2Co2CrTiNb (x = 0, 0.5, 1, 2) coating is much higher than that of the substrate. The highest hardness measured reaches up to 873.40 HV, which is about 3 times the hardness of the substrate. The average hardness of the FexNi2Co2CrTiNb coating presents an increasing trend with the increase of Fe content, the average hardness of Fe2Ni2Co2CrTiNb coating is 40.74 % higher than that of the Fe0Ni2Co2CrTiNb coating. Correspondingly, Fe2Ni2Co2CrTiNb coating with the highest hardness has the best wear resistance. The Fe2Ni2Co2CrTiNb coating with excellent wear resistance can be fully applied to various tools, molds and mechanical parts that require high hardness and superior wear resistance.

Microstructures, Mechanical Properties, and Corrosion Behaviors of Refractory High-Entropy ReTaWNbMo Alloys

A refractory ReTaWNbMo high-entropy alloy (HEA) with an equal atomic ratio prepared by rapid solidification displays a BCC crystal structure. After annealing the as-quenched samples at 673, 873, 1073, and 1273 K for 12 h, respectively, the crystalline phase still keeps its BCC structure, but the crystalline morphology transforms from coarse dendrites to fine equiaxed crystals together with gradual grain refinement. The intragranular regions are enriched in W and Ta, while the crystals in the intergranular regions are enriched in Nb, Mo, and Re. With the increase in annealing temperature, the yield strength increases, and the compressive plasticity and fracture strength are obviously enhanced. The failure mode gradually changes from transgranular to intergranular fracture. Furthermore, the corrosion behaviors of the as-cast ReTaWNbMo HEA and the annealed states in a 3.5 wt.% NaCl solution were also studied. The samples annealed at 1273 K exhibit the best corrosion resistance due to the elemental re-distributions within grains.

Microstructural evolution and mechanical properties of ternary Al–Fe–Nb alloy under free fall condition

Ternary Al55Fe40Nb5 alloy was rapidly solidified in 3 m drop tube to investigate its solidification evolution and mechanical properties at high cooling rates and undercoolings. The microstructure of this alloy was composed of the primary FeAl dendrite, the (FeAl + Nb(Fe, Al)2) eutectic and the composite of FeAl, FeAl2 and Nb(Fe, Al)2 phases. As the alloy droplet diameter varied from 1200 to 80 μm, the growth morphology of primary FeAl phase evolved from coarse dendrite to refined dendrite, and finally into equiaxed grain. Meanwhile, the eutectic and the composite exhibited a regular to anomalous microstructure transition, and the Nb(Fe, Al)2 phase changed from aggregation growth to uniform distribution. Consequently, the homogenous reticulate-shaped particulate structure formed. The microscopic structures of the three intermetallic compound phases were explored by HRTEM technique. Under the influences of large undercooling and cooling rate, the grain refinement of primary phase and the homogenous distribution of three phases led to the increase of Vickers microhardness for this rapidly solidified alloy.

Rapid solidification of a FeSi intermetallic compound in undercooled melts: dendrite growth and microstructure transitions

Rapid solidification of a FeSi stoichiometric intermetallic compound was studied using the glass fluxing method. The recalescence process was in situ observed for the first time by the infrared high-speed high-resolution cameras. The dendrite envelope was found to be non-isothermal during the recalescence process. The growth velocity increased first, then decreased and finally held nearly constant. The average dendrite growth velocity for the recalescence process increased monotonically with undercooling and was described well by the dendrite growth model for a stoichiometric intermetallic compound. At low undercooling, the microstructure transition from coarse dendrites to refined grains was consistent with the dendrite fragmentation model and the chemical superheating model. At high undercooling, dendrite deformation triggers stress accumulation upon rapid solidification, thus providing the driving force for recrystallization. However, there were no evidences for annealing twins accompanied by recrystallization as well as random textures due to recrystallization nucleation. From the local misorientation map, the grain refinement mechanism was suggested to be stress-induced dendrite fragmentation. This study is helpful for not only understanding the intrinsic mechanisms of microstructure transitions in theory but also controlling microstructures and performance of intermetallic compounds in practical applications.