Dendritic Trunks Research Articles

Thermal-solutal convection-induced low-angle grain boundaries in single-crystal nickel-based superalloy solidification

Low-angle grain boundaries (LAGBs) are one of the solidification defects in single-crystal nickel-based superalloys and are detrimental to the mechanical properties. The formation of LAGBs is related to dendrite deformation, while the mechanism has not been fully understood at the mesoscale. In this work, a model coupling dendrite growth, thermal-solutal-fluid flow, thermal stress and flow-induced dendrite deformation via cellular automaton-finite volume method and finite element method is developed to study the formation of LAGBs in single crystal superalloys. Results reveal that the bending of dendrites is primarily attributed to the thermal-solutal convection-induced dendrite deformation. The mechanical stress of dendrite deformation develops and stabilises as solidification proceeds. As the width of the mushy zone gets stable, stresses are built up and then dendritic elastoplastic bending occurs at some thin primary dendrites with the wider inter-dendritic space. There are three characteristic zones of stress distribution along the solidification direction: (i) no stress concentration in the fully solidified regions; (ii) stress developing in the primary dendrite bridging region, and (iii) stress decrease in the inter-dendritic uncontacted zone. The stresses reach maximum near the initial dendrite bridging position. The lower temperature gradients, the finer primary dendritic trunks and sudden reductions in local dendritic trunk radius jointly promote the elastoplastic deformation of the dendrites. Corresponding measures are suggested to reduce LAGBs.

Human Purkinje cells outperform mouse Purkinje cells in dendritic complexity and computational capacity

Purkinje cells in the cerebellum are among the largest neurons in the brain and have been extensively investigated in rodents. However, their morphological and physiological properties remain poorly understood in humans. In this study, we utilized high-resolution morphological reconstructions and unique electrophysiological recordings of human Purkinje cells ex vivo to generate computational models and estimate computational capacity. An inter-species comparison showed that human Purkinje cell had similar fractal structures but were larger than those of mouse Purkinje cells. Consequently, given a similar spine density (2/μm), human Purkinje cell hosted approximately 7.5 times more dendritic spines than those of mice. Moreover, human Purkinje cells had a higher dendritic complexity than mouse Purkinje cells and usually emitted 2–3 main dendritic trunks instead of one. Intrinsic electro-responsiveness was similar between the two species, but model simulations revealed that the dendrites could process ~6.5 times (n = 51 vs. n = 8) more input patterns in human Purkinje cells than in mouse Purkinje cells. Thus, while human Purkinje cells maintained spike discharge properties similar to those of rodents during evolution, they developed more complex dendrites, enhancing computational capacity.

Amplifying Mn Segregation and Enhancing Damping Capacity via Adding Sn in Mn–44Cu–1.5Al as‐Cast Alloys

As a metal functional material, the Mn–Cu‐based alloys are regarded to be able to cope with the vibration and noise from machines. In particular, the as‐cast alloys often exhibit an excellent damping capacity, which is attributed to the Mn segregation in the alloys. Herein, to amplify the kind of segregation, a trace of Sn is added in as‐cast Mn–Cu–Al alloys. The phase structures of the as‐cast Mn–44Cu–1.5Al alloys are analyzed in detail through X‐Ray diffraction (XRD). The distribution characteristics of face‐centered cubic (FCC) phases and face‐centered tetragonal (FCT) phases are observed using electron backscatter diffraction (BSE). The alloys’ dynamic mechanical spectra and internal friction (IF) are measured using an inverted torsion pendulum device. It is found that the addition of Sn promotes the Mn segregation in the as‐cast Mn–44Cu–1.5Al alloys. A large amount of FCT phases appears in the dendritic trunks, enhancing the damping capacity of the alloys. In the meantime, the Sn atoms enhance the twin boundary mobility in Mn‐rich dendritic trunks. As a result, the alloy containing 1 wt% Sn has the highest damping capacity in the alloys.

Melt flow-induced mechanical deformation of dendrites in alloy solidification: A coupled thermal fluid - solid mechanics approach

Melt flow during alloy solidification promotes more dendrite fragments or equiaxed grains due to dendrite fragmentation, while the complex interactions between melt flow and dendritic growth causing flow-induced dendrite deformation and dendrite fragmentations remain obscure. In this work, cellular automaton-finite volume approach and the displacement-based finite element method have been combined to simulate dendrite growth, fluid flow and flow-induced mechanical deformation in Al-4.5 wt%Cu alloy, and to reveal the stress evolution during dendritic growth under melt convection. It is found that dendrites can undergo visible mechanical bending under fluid flow. The stress increases with the enhancement of fluid flow. The primary dendritic trunk is the location of the dominant deformation under a parallel fluid flow. Though the secondary dendrite does not suffer the large stress, it significantly affects the stress concentration of the primary dendritic trunk. As flow velocity increases, the secondary dendrite arm spacing gradually decreases and bridging occurs due to the contact of tertiary dendrites. The bridging of secondary dendrites impedes the development of stress concentrations. Especially, as inflow velocity exceeds 0.05 m/s, the stress does not get larger than that under the velocity of 0.01 m/s under the complication interactions between the dendritic growth and flow patterns. Unlike previous understanding, the maximum stresses induced by melt flow are mainly located at the position where primary dendritic trunk intersects with unconnected secondary dendrites or the thin dendrite trunk.

Regulating the microstructure and hydrogen storage properties of Ti23V40Mn37 + 10 wt% ZrNi alloys via ultrasonic treatment

To improve the activation performance and kinetic property of the Ti23V40Mn37 + 10 wt% ZrNi alloy, ultrasonic treatment was applied during its solidification. Besides, the effect of microstructure evolution on the hydrogen storage properties was investigated. Both as-cast and ultrasonically-treated (UST) Ti23V40Mn37 + 10 wt% ZrNi alloys were composed of BCC, C14 Laves and β-Zr phases. BCC phases appeared to be dendritic-shaped, of which the lattice parameter increased and the dendritic trunk was refined from 13.5 μm to 5.0 μm by ultrasonic processing. Hydrogen ab-/desorption rate accelerated and the maximum hydrogen desorption capacity was improved to 1.52 wt% at 303 K, owing to the increasing number of grain boundaries formed. The apparent dehydriding activation energy of the UST alloy decreased to 32.530 kJ/mol, while its dehydriding enthalpy (ΔH) enlarged to 40.22 kJ/mol H2 due to the lowered stability of the hydride.

Electrochemical anisotropy of Hastelloy X fabricated by selective laser melting in a NaNO3 solution

The microstructure and electrochemical behavior of Hastelloy X fabricated by selective laser melting were investigated. The microstructure of selective laser melted Hastelloy X mainly comprises columnar crystals grown along the deposition direction, consisting of fine columnar dendrites. The results of the electrochemical tests conducted using a 10 wt% NaNO3 solution show that the passive film that formed on the horizontal section (HS) and vertical section (VS) presented p-type semiconductor properties, but the stability and protectiveness of the passive film that formed on the VS was better than those of that on the HS. The surface morphology after dissolution indicates that the dissolution rate of the interdendritic region was faster than that of the dendritic trunk, which resulted in a faster dissolution rate for the center of the molten pool than that of the edge of the molten pool, and the dissolution rate of the HS was faster than that of the VS.

S-Afadin binds to MAGUIN/Cnksr2 and regulates the localization of the AMPA receptor and glutamatergic synaptic response in hippocampal neurons

A hippocampal mossy fiber synapse implicated in learning and memory is a complex structure in which a presynaptic bouton attaches to the dendritic trunk by puncta adherentia junctions (PAJs) and wraps multiply branched spines. The postsynaptic densities (PSDs) are localized at the heads of each of these spines and faces to the presynaptic active zones. We previously showed that the scaffolding protein afadin regulates the formation of the PAJs, PSDs, and active zones in the mossy fiber synapse. Afadin has two splice variants: l-afadin and s-afadin. l-Afadin, but not s-afadin, regulates the formation of the PAJs but the roles of s-afadin in synaptogenesis remain unknown. We found here that s-afadin more preferentially bound to MAGUIN (a product of the Cnksr2 gene) than l-afadin invivo and invitro. MAGUIN/CNKSR2 is one of the causative genes for nonsyndromic X-linked intellectual disability accompanied by epilepsy and aphasia. Genetic ablation of MAGUIN impaired PSD-95 localization and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (AMPA) receptor surface accumulation in cultured hippocampal neurons. Our electrophysiological analysis revealed that the postsynaptic response to glutamate, but not its release from the presynapse, was impaired in the MAGUIN-deficient cultured hippocampal neurons. Furthermore, disruption of MAGUIN did not increase the seizure susceptibility to flurothyl, a GABAA receptor antagonist. These results indicate that s-afadin binds to MAGUIN and regulates the PSD-95-dependent cell surface localization of the AMPA receptor and glutamatergic synaptic responses in the hippocampal neurons and that MAGUIN is not involved in the induction of epileptic seizure by flurothyl in our mouse model.

Investigations of the processing–structure–performance relationships of an additively manufactured AlSi10Mg alloy via directed energy deposition

In this paper, the microstructure formation mechanism of an AlSi10Mg alloy prepared by laser directed energy deposition (DED-LB) and its influence on the mechanical properties was fully studied. The relationships between the microstructure characteristic scale, the molten pool solidification/thermal cycle conditions, and the tensile mechanical properties were established by combining the numerical simulation, theoretical calculation, and experimental validation. The as-deposited microstructure consists of the columnar α-Al dendritic array growing epitaxially along the building direction with the primary dendritic arm spacing of 18 ± 4.5 µm, the dendritic arm cell of 3 ± 1 µm surrounded by the refined eutectic network, and the Si precipitation of 17.7 ± 0.8 nm dispersed in the α-Al dendritic trunk. The feature size of the primary dendritic arm spacing and the dendritic arm cell size can be well predicted using the Kurz-Fisher and KGT dendritic growth models. The formation and size evolution of the Si precipitation was well described by the non-isothermal aging KWN model for the first time. The as-DED-LB-processed AlSi10Mg alloy exhibits a good comprehensive mechanical property with a yield strength of 187 ± 1.5 MPa and elongation to fracture of 7.4 ± 0.5%. The boundary strengthening from the eutectic phase network, the load-bearing capacity for dislocations caused by refined dendritic arms, and the precipitation strengthening of nano-Si particles play a major role in the improvement of the tensile strength and hardening ability. As a result, a quantitative relationship of the processing–microstructure–performance has been systematically investigated and established, which explores a method for the precision control and large-scale application of DED-AlSi10Mg alloys.

Effect of Mo on the morphology, microstructure and mechanical properties of NbTa0.5TiMox refractory high entropy alloy fabricated by laser powder bed fusion using elemental mixed powders

The production of refractory metal pre-alloy powder is difficult and the related production cost is high. In this paper, NbTa0.5TiMox series refractory high entropy alloy was prepared by laser powder bed fusion (L-PBF) with elemental mixed powders. The effects of Mo content on the morphology, microstructure, hardness, density, and compression properties of the NbTa0.5TiMox alloy were studied. All NbTa0.5TiMox alloys fabricated by L-PBF present a single-phase BCC. Typical dendrites are formed within the grains and the elements are enriched in the dendrite trunk and inter-dendrite region from inside to outside depends on the melting point of the different materials. It was observed that the Mo content improves significantly the hardness and compressive strength of the alloy by solution strengthening. NbTa0.5TiMo1.0 alloy has the highest yield strength (1609 MPa), but with reduced ductility, while NbTa0.5TiMox (x ≤ 0.5) alloy has excellent ductility (e > 80%) at room temperature. The increase in Mo content leads to the increase of cracks, pores and non-penetration defects, resulting in the loss of sustained strength & ductility and decrease in density. The results provide a basis for the design and preparation of ductile refractory high entropy alloys by L-PBF of elemental mixed powders.

The non-dendritic microstructure arising from grain boundary formation and wetting: A phase-field simulation and experimental investigation of semi-solid deformation

The semi-solid metal forming process can prevent the formation of unpopular dendritic microstructures but the mechanism remains unclear. To explore the underlying causes, a challenging phase-field simulation was performed, supported by a semi-solid experiment. The simulation managed to couple dendritic growth with melt flow and solid deformation, a process that few existing models or in situ experimental techniques could analyze. This study found that under deformation, grain boundary formation and wetting play a pivotal role in the formation of non-dendritic structures: it is the grain boundary formation and wetting that split the bending side-branch from dendrite trunk; the detached side-branch could then evolve into a non-dendritic morphology. The simulation showed that wetting could finish almost immediately after a moderate bending. This process appears similar to but is essentially distinct from the conventional explanation that describes dendrite fragmentation as mechanical breakage.

Neurons in the Caudate Nucleus of the Camel and Human: A Comparative Study

The caudate nucleus is described as a huge, subcortical grey mass in the forebrain known for its role in motor function and involved in a number of clinical syndromes. During ontogenesis, the caudate nucleus follows the curvature of the telencephalic vesicle, hence its curved shape. Its rostral portion referred to as the head, is far more voluminous than the body.AimsThis study aimed to describe the different types and characteristics of neurons in the caudate nucleus of the brain of camel and human and compare these neurons with their counterparts in other species.Method4mm thick blocks containing caudate nucleus was cut, dry and processed according to the rapid Golgi and or the Golgi‐Kopsch methods (double impregnation). Well‐impregnated neurons studied under light microscope.ResultsAccording to the present study, neurons were classified according to the density and distribution of the spines along their axons and dendrites. Neurons in this study classified into three main types in the caudate nucleus of camel and human. According to the somal size each group includes one or more of the following cell size: large, medium and small. Rich‐spiny neurons (Type I): include medium and small subtypes. Hairy‐like, mushroom‐like and stubby spines were present in the secondary dendrite and their branching. Sparsely‐spiny neurons (Type II): they have relatively larger cell bodies and thicker primary dendrites than (Type I). Characterized by their very low density of spines and poorer branching of their dendrites. Aspiny‐neurons (Type III): are large, medium and small size. May have varicose, appendages with various types and short axon which gives collaterals mostly within the dendritic domain.ConclusionThe neuronal structure of the caudate nucleus in the camel is differ from that of the human. The human neurons appear slightly larger than those of the camel and they have more dendritic trunks that break up more often than the neurons of the camel. Although the types of spines are similar in both species, the density of spines appears to be greater in the camel than in the human. The structural differences of the caudate neurons between two species may result from the fact that the homo sapeneis belongs to a phylogenetically younger order (Anthropoidea) than the camel. (Artiodactyla) which support the fact that the number and diversity of the types of neurons go to increase throughout phylogenesis.

Numerical Analysis of Nucleation and Growth of Stray Grain Formation during Laser Welding Nickel-Based Single-Crystal Superalloy Part III: Kinetic and Thermodynamic Obviation of Solidification Cracking

To pertinently balance growth kinetics, solidification thermodynamics and dendrite expitaxy of multicomponent nickel-based single-crystal supralloy during laser processing, effect of thermometallurgy determinant factors, including laser power, welding speed and welding configuration, on solidification behavior, such as nonequilibrium solidification temperature range, and dendrite growth, such as dendrite trunk spacing, are progressively advanced to forestall solidification cracking phenomena. Symmetric developments of dendrite trunk spacing and solidification temperature range alongside solid/liquid interface are crystallographically driven by useful (001)/[100] welding configuration to auspiciously bring about crack-insusceptible and well-oriented dendrite growth. Dissimilarly, unsymmetrical developments of dendrite trunk spacing and solidification temperature range alongside solid/liquid interface are crystallographically driven by (001)/[110] welding configuration to insidiously favor crack-unresistant and disoriented dendrite growth. Higher heat input thermodynamically and kinetically boosts wide solidification temperature range, appalling stray grain growth with excess of solute ahead of dendrite tip and large size of crack-unresistant region to thermometallurgically disintegrate epitaxial growth for untoward solidification cracking, and therefore should be strictly withstood. Although geometry of symmetrical weld pool both sides is the same in infelicitous (001)/[110] welding configuration, [100] region of dendrite growth is more liable to ruinous stray grain growth and extensive solidification temperature range than [010] region of dendrite growth to complicate dendrite growth and exacerbate weld integrity. The determinant mechanism of crystallography-aided amelioration of solidification cracking resistance as result of kinetics-and thermodynamics-driven dendrite growth is propitiously proposed. Furthermore, the credible and understandable theoretical predictions are in conformity with the experiment results.

Numerical Analysis of Nucleation and Growth of Stray Grain Formation during Laser Welding Nickel-Based Single-Crystal Superalloy Part I: Morphology and Size of Dendrite Growth

The dependency of morphology development and dendrite growth on welding conditions (laser power, welding speed and welding configuration) is numerically analyzed to decrease nucleation and growth of stray grain formation during laser processing aerospace component surface of ternary Ni-Cr-Al single-crystal superalloy. Proper (001)/[100] welding configuration crystallographically initiates three axisymmetrical distributions of microstructure development, i.e. stray grain formation, morphology development and dendrite trunk spacing, alongside the advancing solid/liquid interface of molten pool, whereby metallurgical properties are increased. Unpromising (001)/[110] welding configuration tends to crystallographically possesses unaxisymmetrical microstructure development to favor substantial crack-vulnerable dendrite size and morphology. Epitaxial [001] columnar dendrite growth region is favored for single-crystal dendrite growth, while vulnerable [100] equiaxed dendrite growth region is more susceptible to solidification cracking. The lower heat input is used, the smaller stray grain formation, negligible columnar/equiaxed transition (CET) and finer dendrite trunk spacing are consistently promoted by narrower constitutional undercooling ahead of solid/liquid interface to improve crack-resistant microstructure development and weld integrity. When comparing between [100] dendrite growth region on the right side and [010] dendrite growth region on the left side, (001)/[110] welding configuration spontaneously engenders severer stray grain formation, insidious columnar/equiaxed transition and coarser dendrite trunk spacing on the right side to deteriorate microstructure development with restriction of the same heat input on both sides of weld pool. The mechanism of asymmetrical solidification cracking as result of crystallography-induced microstructure degradation is therefore proposed. The theoretical predictions of asymmetrical solidification cracking susceptibility are comparable with experiments.

Modeling of microstructure formation with gas porosity growth during columnar dendritic solidification of aluminum alloys

In this paper, microstructure formation with gas bubble growth during columnar dendritic solidification has been numerically investigated by using the coupled lattice Boltzmann-cellular automaton-finite difference (LB-CA-FD) model, which combines the physical mechanisms of the redistribution of hydrogen components at the solid–liquid interface and the transportation of hydrogen atoms in liquid. Hydrogen transportation, bubble nucleation and growth are computed by the LB method while the growth of columnar dendrite and the transport of solutes are described by the CA-FD model. Predicted results show that the bubbles begin to nucleate at the roots of primary dendritic trunk and secondary arms, and elongated irregular bubbles are sandwiched between neighboring dendrites after merging and moving during the growth stage. Columnar dendritic change from coarse and ordered to fine and disordered as the cooling rate is increased. Meanwhile, the cooling rate has a significant effect on the incubation time for gas bubbles and the maximum growth rate of porosity. It was found that the misorientation angle has limited effects on the solid fraction and porosity percentage but strongly influences on the morphologies of the columnar dendritic arrays, resulting in the different distributions of bubbles.

Interaction of MnS inclusion behaviors and macrosegregation during solidification by multi-phase modelling

Manganese sulfide (MnS) is an important kind of non-metallic inclusion in steel which precipitates at the final stage of solidification and greatly affects the properties of steels. Plenty models have been developed to simulate the dynamics of MnS during solidification, while few considered its effects on macrosegregation formation in return. Here a four-phase model has been developed to couple the formation and growth mechanisms of MnS with the evolution of macrosegregation during the solidification. Classical nucleation theory and a diffusion-governed growth model are applied to illustrate the behaviors of MnS. The nucleation and growth of inclusions are fully coupled with a columnar-equiaxed ternary alloy (Fe-C-Mn) solidification model. The model simulates the nucleation and growth of equiaxed crystal, the sedimentation of equiaxed crystal, the growth of columnar dendritic trunks, thermo-solutal convection of the melt and solute transport in a benchmark case. The prediction of MnS is verified reasonably by comparison with many experimental results in perspectives i.e., volume fraction, diameter, criteria of precipitation and so forth. The effects of inclusion on macrosegregation and preferential formation location of MnS are further discussed in detail. The critical condition of such inclusion precipitate is proposed based on various S concentration.

Asymptotic Input-Output Relationship Predicts Electric Field Effect on Sublinear Dendritic Integration of AMPA Synapses.

An extracellular electric field (EF) induces transmembrane polarizations on extremely inhomogeneous spaces. Evidence shows that EF-induced somatic polarization in pyramidal cells can modulate the neuronal input-output (I/O) function. However, it remains unclear whether and how dendritic polarization participates in the dendritic integration and contributes to the neuronal I/O function. To this end, we built a computational model of a simplified pyramidal cell with multi-dendritic tufts, one dendritic trunk, and one soma to describe the interactions among EF, dendritic integration, and somatic output, in which the EFs were modeled by inserting inhomogeneous extracellular potentials. We aimed to establish the underlying relationship between dendritic polarization and dendritic integration by analyzing the dynamics of subthreshold membrane potentials in response to AMPA synapses in the presence of constant EFs. The model-based singular perturbation analysis showed that the equilibrium mapping of a fast subsystem can serve as the asymptotic subthreshold I/O relationship for sublinear dendritic integration. This allows us to predict the tendency of EF-mediated dendritic integration by showing how EF changes modify equilibrium mapping. EF-induced hyperpolarization of distal dendrites receiving synapses inputs was found to play a key role in facilitating the AMPA receptor-evoked excitatory postsynaptic potential (EPSP) by enhancing the driving force of synaptic inputs. A significantly higher efficacy of EF modulation effect on global AMPA-type dendritic integration was found compared with local AMPA-type dendritic integration. During the generation of an action potential (AP), the relative contribution of EF-modulated dendritic integration and EF-induced somatic polarization was determined to show their collaboration in promoting or inhibiting the somatic excitability, depending on the EF polarity. These findings are crucial for understanding the EF modulation effect on neuronal computation, which provides insight into the modulation mechanism of noninvasive brain modulation.

Multiphase Field Modeling of Dendritic Solidification of Low-Carbon Steel with Peritectic Phase Transition

In the present study, an improved multiphase field model with taking into consideration the S/L interface anisotropy is proposed to simulate the dendritic growth with peritectic transition of low-carbon steel, and a new random heterogeneous nucleation method is proposed to treat the γ phase nucleation during the peritectic solidification process. The γ phase growth around the dendrite (the single dendrite and polycrystalline ferrite) and the subsequent peritectic transformation during the peritectic solidification are simulated. The results show that when the temperature reaches the γ phase nucleation temperature, γ nuclei suddenly nucleate at the δ/L interface, and laterally grow with the movement of L/γ/δ triple point around the δ/L interface of δ dendrite. After the δ dendrite is fully wrapped by the continuous intervening γ phase, the γ phase continues to grow with direct phase transformation from both δ phase and liquid phase. With the increase of melt undercooling, more δ phase and liquid phase are consumed to form γ phase, and the intervening γ phase between the δ dendrite and liquid phase becomes more and more thick, but the δ dendrite arm becomes more and more thin. During the dendritic solidification process, the solute would be rejected from dendrite trunk with the dendritic growth and enriched at the dendrite boundary, especially at the dendrite root. The solute enrichment at the dendrite root would inhibit the γ phase growth at the dendrite root and thus the thickness of intervening γ phase at the dendrite root is thinnest around the dendrite. Moreover, the solute concentration in γ phase is among the δ phase and liquid phase, because the carbon solubility in γ phase is higher than that in δ phase, but lower than that in liquid phase.

Characterizations of micro-nano structure and tensile properties of a Sc modified Al Mn alloy fabricated by selective laser melting

In this work, bulk deposits of a Sc modified Al-Mn-Fe-Si alloy were prepared by selective laser melting (SLM), and the microstructural characteristics and tensile properties of the deposits were analyzed. The as-deposited sample presented a multiscale microstructure consisting of epitaxial columnar grains with width of 30–140 μm, in which there exist α-Al dendrite arrays with average primary dendrite arm spacing of 0.34 μm, and interdendritic nanoscale α-Al + Al6(Mn,Fe) divorced eutectic. After an aging treatment, Al6(Mn,Fe) transformed into Al12(Mn,Fe)3Si, and its morphology gradually evolved from skeleton into particle. Meanwhile, needle-like AlMnSi precipitated and grew up in the α-Al dendrite trunk. The ultimate tensile strength increased from 349 ± 9 MPa in the as-deposited state to 430 ± 3 MPa after aging at 300 °C. However, it dropped to 358 ± 1 MPa when the aging temperature increased to 350 °C. The addition of Sc caused precipitation strengthening of secondary Al3Sc after aging treatment, but the grain refinement effect of primary Al3Sc was inhibited since Sc tended to segregate at the frontier of the Mn-containing pre-precipitates.

Solute enrichment induced dendritic fragmentation in directional solidification of nickel-based superalloys

Dendritic fragmentation should be strictly prevented to avoid introducing new equiaxed grains of arbitrary orientations (known as freckles) in the single crystal blade, whose mechanism properties are extremely sensitive to the grain boundaries. However, the mechanism of the formation and distribution of freckles in superalloy castings remains debatable. Here the interactions among thermal-solutal convection, solute segregation and dendritic structure in the directional solidification is revealed via a cellular automaton-finite volume model developed and validated for nickel-based single-crystal superalloys. The simulated channel distribution, morphology of residual liquid region, dendritic tip velocity and freckle grains are in good agreement with the previous experimental observations. The present work directly predicts solute enrichment induced dendritic fragmentation, that is, the middle of the over-growing dendritic trunk is melted by the superheated liquid metal in the channel and thus the top of the dendrite becomes isolated fragments. These dendritic fragments rather than the newly nucleated grains are proved to be the root cause of freckle grains. The simulation results also provide new insight into the effect of the operating parameters on the solute distribution and dendritic structure. The undesired lateral heat flux in the directional solidification promotes the transfer of the solute-enriched solute and the migration of the channels, and thus results in the dendritic detachment. Increasing cooling rate makes significant effects on eliminating the freckle defect, because a higher cooling rate promotes the growth of the ternary dendritic arms on the side branches and inhibits the transfer of solute-enriched melt to the over-growing dendrite.

Numerical Analysis of Stray Grain Formation during Laser Welding Nickel-Based Single-Crystal Superalloy Part II: Multicomponent Dendrite Growth

Multicomponent dendrite growth is theoretically predicted to optimize solidification cracking susceptibility during ternary Ni-Cr-Al nickel-based single-crystal superalloy weld pool solidification. The distribution of dendrite trunk spacing along the weld pool solidification interface is clearly symmetrical about the weld pool centerline in beneficial (001)/[100] welding configuration. The distribution of dendrite trunk spacing along the weld pool solidification interface is crystallography-dependent asymmetrical from bottom to top surface of the weld pool in detrimental (001)/[110] welding configuration. The smaller heat input is used, the finer dendrite trunk spacing is kinetically promoted by less solute enrichment and narrower constitutional undercooling ahead of solid/liquid interface with mitigation of metallurgical contributing factors for solidification cracking and vice versa. Vulnerable [100] dendrite growth region is predominantly suppressed and epitaxial [001] dendrite growth region is favored to spontaneously facilitate single-crystal columnar dendrite growth and reduce microstructure anomalies with further reduction of heat input. Optimum low heat input (both lower laser power and higher welding speed) with (001)/[100] welding configuration is the most favorable one to avoid nucleation and growth of stray grain formation, minimize both dendrite trunk spacing and solidification cracking susceptibility potential, improve resistance to solidification cracking, and ameliorate weldability and weld integrity through microstructure modification instead of inappropriate high heat input (both higher laser power and slower welding speed) with (001)/[110] welding configuration. The dendrite trunk spacing in the [100] dendrite growth region on the right side of the weld pool is considerably coarser and grows faster than that within the [010] dendrite growth region of the left side in the (001)/[110] welding configuration to deteriorate weldability, although the welding conditions are the same on the either side. Furthermore, the alternative mechanism of crystallography-dependent solidification cracking as consequence of asymmetrical microstructure development and diffusion-controlled dendrite growth of γ phase is therefore proposed. The theoretical predictions are comparable with experiment results. The reliable model is also useful for welding conditions optimization for crack-free laser processing.