Unusual Overgrowth Research Articles

A two-dimensional phase-field study on dendritic growth competition under convective conditions

In the present work, the growth competition of columnar dendrites is investigated in the presence of melt convection. By using a multiphase-field model, the microstructural evolution of unidirectionally solidified dendrites with different crystal orientations is addressed in detail. Generally, for converging bicrystal grains, the dendritic growth competition follows a conventional overgrowth model, where a favorably oriented dendrite eliminates an unfavorably oriented dendrite at the grain boundary. However, in the presence of lateral melt flow in the liquid phase, we discover that the conventional overgrowth mechanism transforms into an unusual overgrowth mechanism as the melt velocity gradually increases. Recently observed in bi-crystal studies, the unfavorably oriented dendrites overgrow at the expense of the favorably oriented dendrites during an unusual overgrowth behavior. From our simulations we elucidate that the presence of an additional convective transport in the liquid phase modifies the solute distribution at the grain boundary, which in turn affects the overgrowth mechanism of converging dendrites. We show that there exists a critical melt velocity to determine the growth competition at the grain boundary. In addition, the role of interfacial anisotropy on growth competition is analyzed in detail, where we discern that the conventional overgrowth behavior is dominant at large anisotropic strengths. Lastly, a morphological selection map is depicted to predict the crossover region between the two overgrowth behaviors.

Overgrowth behavior at converging grain boundaries during competitive grain growth: A two-dimensional phase-field study

Herein, three types of converging grain growth were investigated during directional solidification, through two-dimensional phase-field simulations. The three types were distinguished by the orientation of the favorably oriented (FO) dendrites. The growth orientation of the FO dendrites was parallel to the thermal gradient direction in type I, whereas the orientation of FO dendrites was inclined in the same and opposite lateral directions as those of the unfavorably oriented (UO) dendrites in types II and III, respectively. The largest difference between FO and UO grain inclination angles—that is, the threshold angle, for unusual overgrowth in different alloy systems were clearly different, in type I and III cases. In all three types of converging grain growth, overgrowth of UO dendrites by FO dendrites (usual overgrowth) was observed to occur more frequently than that of FO dendrites being overgrown by UO dendrites (unusual overgrowth). The asymmetry of the inclined dendrite diffusion layer caused different solute interactions in the three types of converging grain growths. Based on the different solute interactions, unusual overgrowth behavior occurred most readily in type III cases—and with most difficulty in type II cases. We also saw that the local GB orientation may alternate between FO and UO grain orientations in type III cases.

Effect of secondary arm orientation on unusual overgrowth at converging grain boundary during directional solidification in 3D

The competitive growth of two converging grains with multiple dendrite rows during directional solidification in three dimensions (3D) was investigated through phase-field simulations. We studied the effect of secondary arm orientation on the overgrowth of favorably oriented (FO) dendrite by unfavorably oriented (UO) dendrite, i.e. the so-called unusual overgrowth. It is observed that, when the initial spatial arrangement of FO and UO dendrites varies, the effect of secondary arm orientation on the unusual overgrowth will also be different. In addition to the unusual overgrowth similar to that previously illustrated in two dimensions (2D) and thin samples, we have observed a more complicated unusual overgrowth manner when the initial spatial arrangement between the FO dendrites and UO dendrites was the stagger configuration and there is a rotation of the secondary arm in either the FO grain or the UO grain. In these cases, the spatial arrangements of some UO dendrites with respect to FO dendrites can be automatically adjusted from the initial stagger configuration to the face-to-face configuration. Such an adjustment results in the interaction of FO dendrite not only with the UO dendrite penetrating into the liquid gap but also with the face-to-face UO dendrite. Hence, the unusual overgrowth of FO dendrites occurs easier than in the cases with only face-to-face interaction, i.e., the cases in 2D and thin samples. These findings illustrate the inconsistency between previous simulations and experiments regarding the effect of secondary arm orientation on the unusual overgrowth.

GPU-accelerated three-dimensional phase-field simulation of dendrite growth in a nickel-based superalloy

The microstructure formation of a nickel-based superalloy during solidification in three dimensions was investigated using the phase-field method. To accelerate the large-scale phase-field simulation, a parallel computing approach was developed using the graphic processing unit (GPU), and the limitation of insufficient GPU memory was circumvented by employing an asynchronous concurrent algorithm. The performance of the GPU-based parallel computing method was tested and the results demonstrate that a maximum performance of 774.292 GFLOPS (giga floating-point operations per second) can be obtained using a single NVIDIA GTX1080 GPU. In simulations of isothermal solidification, the microstructure evolution of a single and multiple dendrites under different undercooling levels was shown in detail. During the solidification, the dendrite tip growth velocity and fraction solid were recorded and then analyzed. In simulations of directional solidification, the formation of primary dendrite arms under different temperature gradients was investigated, and the simulated microstructure was in good agreement with experimental observations. Additionally, the distribution of primary dendrite arm spacing was quantitatively analyzed by Voronoi tessellation. Finally, simulation of polycrystalline growth in directional solidification was conducted to study the dendrite competitive growth. The unusual overgrowth phenomenon was observed in the initial growth stage, while as the solidification process proceeded, the dendrites with small inclination angles were more likely to overgrow the dendrites with large inclination angles.

Grain growth competition during thin-sample directional solidification of dendritic microstructures: A phase-field study

We present the results of a comprehensive phase-field study of columnar grain growth competition in bi-crystalline samples in two dimensions (2D) and in three dimensions (3D) for small sample thicknesses allowing a single row of dendrites to form. We focus on the selection of grain boundary (GB) orientation during directional solidification in the steady-state dendritic regime, and study its dependence upon the orientation of two competing grains. In 2D, we map the entire orientation range for both grains, performing several simulations for each configuration to account for the stochasticity of GB orientation selection and to assess the average GB behavior. We find that GB orientation selection depends strongly on whether the primary dendrite growth directions have lateral components (i.e. components perpendicular to the axis of the temperature gradient) that point in the same or opposite directions in the two grains. We identify a range of grain orientations in which grain selection follows the classical description of Walton and Chalmers. We also identify conditions that favor unusual overgrowth of favorably-oriented dendrites at a converging GB. We propose a simple analytical description that reproduces the average GB orientation selection from 2D simulations within statistical fluctuations of a few degrees. In 3D, we find a similar GB orientation selection as in 2D when secondary branches grow in planes parallel and perpendicular to the sample walls. Remarkably, quasi-2D behavior is also observed even when those perpendicular sidebranching planes are rotated by a finite azimuthal angle about the primary dendrite growth axis as long as the absolute values of those azimuthal angles are equal in both grains. In contrast, when the absolute values of those azimuthal angles differ markedly, we find that unusual overgrowth events at a converging GB are promoted by a high azimuthal angle in the least-favorably-oriented grain. We also find that diverging GBs can be strongly affected by those azimuthal angles, while converging GBs exhibit a weak dependence on those angles. For diverging GBs, GB orientation is also strongly affected by the relative signs of the lateral components of the primary dendrite growth directions in both grains.

Two-dimensional phase-field simulations of dendrite competitive growth during the directional solidification of a binary alloy bicrystal

We investigated the competitive growth of dendrites at the converging grain boundaries (GBs) of bicrystals during the directional solidification of an Al–Cu alloy by means of two-dimensional phase-field simulations. In particular, the focus was on the recently observed phenomenon of unusual overgrowth during the directional solidification of a Ni-based superalloy, where the favorably oriented (FO) dendrites are overgrown by the unfavorably oriented (UO) ones. The phase-field simulations were accelerated by parallel computations on graphics processing units. The simulation results showed that unusual overgrowth occurs in Al–Cu alloys, indicating that this phenomenon is a common one in metallic materials. It was also concluded that the differences in the diffusion layers in front of the FO and UO dendrites had a dominant effect on the competitive growth of dendrites at the converging GB as well as on the unusual overgrowth. In addition, unusual overgrowth was observed in all the FO dendrites with a spacing that allowed the dendrite array to grow stably without necessitating a change in the number of dendrites. The FO dendrite at the GB is overgrown by the UO dendrite when the spacing between the FO dendrite at the GB and the next FO dendrite is approximately equal to the critical minimum spacing. However, the unusual overgrowth was not observed for UO dendrites with a large inclination angle. In this case, all the UO dendrites are blocked by the FO dendrite at the GB, and the FO dendrites migrate toward the UO dendrites.

<i>In Situ</i> and Real-Time Observation of the Solidification Process of Al–20 mass%Cu Alloy by Synchrotron X-ray Radiography

High brilliance synchrotron X-ray radiography has been used to in situ study the solidification process of Al­20mass%Cu alloy with nominal spatial and temporal resolutions of 7.4µm and 300ms, respectively. The block of favorably oriented dendrites by the unfavorably oriented ones was first observed in metal solidification, which related to solute interaction of neighbor dendrites. Solute interaction can also retard the growth of dendrites at the grain boundary, which makes the unusual overgrowth phenomenon possible. Fragmentation occurred when dendrite branches were detached from mother dendrite by local re-melting during the dendrite ripening. Fragmentations together with nucleated dendrites contributed to the columnar to equiaxed transition (CET) and the gravity played a significant role in the final microstructure due to the density difference between primary aluminum dendrites and the melt. In addition, the microporosity formation was observed in the inter-dendrite at the last stage of solidification. [doi:10.2320/matertrans.M2014034]

Conditional Deletion of Cytochrome P450 Reductase in Osteoprogenitor Cells Affects Long Bone and Skull Development in Mice Recapitulating Antley-Bixler Syndrome: Role of a Redox Enzyme in Development

NADPH-cytochrome P450 oxidoreductase (POR) is the primary electron donor for cytochromes P450, dehydrocholesterol reductase, heme oxygenase, and squalene monooxygenase. Human patients with specific mutations in POR exhibit severe developmental malformations including disordered steroidogenesis, sexual ambiguities and various bone defects, similar to those seen in patients with Antley-Bixler syndrome (ABS). To probe the role of POR during bone development, we generated a conditional knockout mouse (CKO) by cross breeding Por lox/lox and Dermo1 Cre mice. CKO mice were smaller than their littermate controls and exhibited significant craniofacial and long bone abnormalities. Differential staining of the CKO mice skull bases shows premature fusion of the sphenooccipital and basioccipital-exoccipital synchondroses. Class III malocclusion was noted in adult knockout mice with an unusual overgrowth of the lower incisors. Shorter long bones were observed along with a reduction in the bone volume fraction, measured by microCT, in the Por-deleted mice compared to age- and sex-matched littermate controls. Concerted up- or down-regulation of proteins in the FGF signaling pathway observed by immunohistochemistry in the tibia samples of CKO mice compared to wild type controls shows a decrease in the FGF signaling pathway. To our knowledge, this is the first report of a mouse model that recapitulates both skull and long bone defects upon Por deletion, offering an approach to study the sequelae of POR mutations. This unique model demonstrates that P450 metabolism in bone itself is potentially important for proper bone development, and that an apparent link exists between the POR and FGF signaling pathways, begging the question of how an oxidation-reduction flavoprotein affects developmental and cellular signaling processes.

Competitive grain growth in directional solidification investigated by phase field simulation

During directional solidification, the competitive dendritic growth between various oriented grains is a key factor to obtain desirable texture. In order to understand the mechanism of competitive dendritic growth, the phase field method was adopted to simulate the microstructure evolution of bicrystal samples. The simulation has well reproduced the whole competitive growth process for both diverging and converging dendrites. In converging case, besides the block of the unfavorably oriented dendrite by the favorably oriented one, the unfavorably oriented dendrite is also able to overgrow the favorable one under the condition of relatively low pulling velocity. This unusual overgrowth is dictated by the solute interaction of the converging dendrite tips. In diverging case, it was found that the grain boundary can be either inclined or parallel to the favorably oriented grain depending on the disposition of two grains.

Phase-field study of competitive dendritic growth of converging grains during directional solidification

The microstructure evolution of grains with different orientations during directional solidification is investigated by the phase-field method. For converging dendrites, in addition to the usually accepted overgrowth pattern wherein the favorably oriented dendrites block the unfavorably oriented ones, the opposite pattern of overgrowth observed in some recent experiments is also found in our simulations. The factors which may induce this unusual overgrowth are analyzed. It is found that in addition to the difference in tip undercooling, the solute interaction of converging dendrites, which has been ignored in the classical theoretical model, also has a significant effect on the nature of the overgrowth at low pulling velocities. Solute interaction can retard the growth of dendrites at the grain boundary (GB) and induce a lag of these dendrites relative to their immediate neighbors, which gives the unfavorably oriented dendrite the possibility to overgrow the favorably oriented one. However, this unusual overgrowth only occurs when the spacing between the favorably oriented GB dendrite and its immediate favorably oriented neighbor decreases to a certain level through lateral motion. These findings can broaden our understanding of the overgrowth mechanism of converging dendrites.

An unusual case of hemifacial microsomia with overgrowth of the facial muscles

Aim To present a case of severe hemifacial microsomia with unusual muscle overgrowth at presentation.

A cluster of translocation breakpoints in 2q37 is associated with overexpression of NPPC in patients with a similar overgrowth phenotype

Overexpression of the C-type natriuretic peptide, encoded by the NPPC gene in 2q37.1, was recently reported in a patient presenting an overgrowth phenotype and a balanced t(2;7)(q37.1;q21.3) translocation. We present clinical, cytogenetic, and molecular data from two additional patients carrying balanced translocations involving the same 2q37.1 chromosome band and chromosomes 8 and 13, respectively. The clinical phenotype of these patients is very similar to the first patient described. In addition to the overgrowth syndrome, there is evidence of generalized cartilage dysplasia. In these two new cases, we found overexpression of NPPC, confirming that this unusual overgrowth phenotype in humans is due to the overexpression of this gene. The involvement of three different chromosomes and a cluster of breakpoints around the NPPC gene suggests that the overexpression of this gene in translocation patients could be due to its separation from a negative regulatory element located on chromosome 2, which would constitute a previously undescribed mutational mechanism.