Secondary Arm Research Articles

Multiple Spiral Arms in Protoplanetary Disks: Linear Theory

Abstract Recent observations of protoplanetary disks, as well as simulations of planet–disk interaction, have suggested that a single planet may excite multiple spiral arms in the disk, in contrast to the previous expectations based on linear theory (predicting a one-armed density wave). We reassess the origin of multiple arms in the framework of linear theory by solving for the global two-dimensional response of a nonbarotropic disk to an orbiting planet. We show that the formation of a secondary arm in the inner disk, at about half of the orbital radius of the planet, is a robust prediction of linear theory. This arm becomes stronger than the primary spiral at several tenths of the orbital radius of the planet. Several additional, weaker spiral arms may also form in the inner disk. On the contrary, a secondary spiral arm is unlikely to form in the outer disk. Our linear calculations, fully accounting for the global behavior of both the phases and amplitudes of perturbations, generally support the recently proposed WKB phase argument for the secondary arm origin (as caused by the intricacy of constructive interference of the azimuthal harmonics of the perturbation at different radii). We provide analytical arguments showing that the process of a single spiral wake splitting up into multiple arms is a generic linear outcome of wave propagation in differentially rotating disks. It is not unique to planet-driven waves and also occurs in linear calculations of spiral wakes freely propagating with no external torques. These results are relevant for understanding formation of multiple rings and gaps in protoplanetary disks.

Microstructural refinement of 355 aluminium alloys by ultrasonic melt treatment (UST)

Chemical grain refining of high-silicon-content aluminium alloys, such as 355 alloy, is impaired by the high silicon content. One solution is the use of ultrasonic melt treatment (UST). This study sought to determine the duration of UST and type of horn (steel or Ti) required to achieve optimal grain refining in these alloys. Samples produced by conventional casting underwent UST for different times with steel and Ti horns and were compared with as-cast samples. For all the conditions studied, analysis of the samples showed that UST is an effective grain refining technique and yields satisfactory values of average grain size and primary and secondary dendrite arm spacing (λ1, λ2) as well as low porosity. The chemical composition of the samples was analysed by SEM–EDS mapping and point analysis to identify the intermetallic phases before and after UST. Best results were achieved after only 20 s of UST with a steel horn. UST for this length of time with a steel horn produced a homogeneous microstructure and possible homogeneous mechanical properties. Grain size was 160 µm; primary dendrite arm spacing, or dendrite cell size, was 130 µm; secondary arm spacing was approximately 18 µm; and Vickers hardness was approximately 95 HV.

On the statistical distribution of primary and secondary dendrite arm spacing in cast metals

ABSTRACTStatistical distributions of dendrite arm spacing have not been addressed in the literature. Two histograms for primary dendrite arm spacing in a cast nickel-based superalloy, and four histograms from a chill cast 5182 aluminium alloy ingots for secondary dendrite arm spacing have been analysed. For all datasets, lognormal distribution provided excellent fits to the data. Moreover, the Lifshitz–Slyozov–Wagner distribution was found to have systematic error in the fits to the secondary arm spacing data.

Dendrite Coarsening and Secondary Arm Migration in the Mushy Zone During Directional Solidification

Dendrite Coarsening and Secondary Arm Migration in the Mushy Zone During Directional Solidification

Phase Field Simulations of Microstructure Evolution in IN718 Using a Surrogate Ni–Fe–Nb Alloy during Laser Powder Bed Fusion

The solidification microstructure in IN718 during additive manufacturing was modeled using phase field simulations. The novelty of the research includes the use of a surrogate Ni–Fe–Nb alloy that has the same equilibrium solidification range as IN718 as the model system for phase field simulations, the integration of the model alloy thermodynamics with the phase field simulations, and the use of high-performance computing tools to perform the simulations with a high enough spatial resolution for realistically capturing the dendrite morphology and the level of microsegregation seen under additive manufacturing conditions. Heat transfer and fluid flow models were used to compute the steady state temperature gradient and an average value of the solid-liquid (s-l) interface velocity that were used as input for the phase field simulations. The simulations show that the solidification morphology is sensitive to the spacing between the columnar structures. Spacing narrower than a critical value results in continued growth of a columnar microstructure, while above a critical value the columnar structure evolves into a columnar dendritic structure through the formation of secondary arms. These results are discussed in terms of the existing columnar to dendritic transition (CDT) theories. The measured interdendritic Nb concentration, the primary and secondary arm spacing is in reasonable agreement with experimental measurements performed on the nickel-base superalloy IN718.

Metallographic, Structural and Mechanical Characterization of REM-Containing Fe-30Mn-8Al-1.8C Low Density Steel in As-Cast Condition

ABSTRACTRecently, low-density steels have received increased attention as promising alternatives for automotive applications of the next generation of Advanced High-Strength Steels (AHSS), considering that vehicle´s weight decrease has been the subject of intense interest. It is well-known that the addition of rare earth metals (REM) has a remarkable effect on shape control and the modification of inclusions. Also, REM additions affect the grain size refinement as well as the tendency to form oxides and sulfides. The aim of this research work was to determine the effect of REM (Ce, La) addition on the microstructure and mechanical properties of the Fe-30Mn-8Al-1.8C low-density steel in as-cast condition. In order to clarify the REM effect on the Fe-Mn-Al-C system, non-microalloyed (LD-NM) and REM microalloyed (LD-REM) specimens were examined in detail by means of light optical and scanning electron microscopy for microstructural characterization. In the same way, the primary and secondary phases founded in the studied steels were identified by X-ray diffraction (XRD). Meanwhile, in order to evaluate the mechanical properties, ten microhardness measurements were carried out on the overall bulk by the Vickers hardness testing. In general, the results showed a dendritic refinement effect due to the addition of REM to low-density steel. REM acted as effective inoculants agents which reduced the primary and secondary arm spacing. Also, the strong segregation tendency at the grain boundaries in the liquid phase was limited. XRD profiles revealed the presence of austenite, ferrite, κ and DO3 phases. Low density steel microalloyed with REM showed a moderate increase in hardness compared to the non-microalloyed steel in the as-cast condition.

Effect of fluid flow on solidified equiaxed dendrite morphology evolution based on phase field - lattice Boltzmann method

Fluid flow can significantly change the evolution of microstructural morphology. However, relatively little is known how the fluid flow, concentration and microstructure affect each other quantitatively, which is essential to optimize processing parameters. A quantitative simulation study of Al-Cu solidified equiaxed dendrite evolution under forced flow based on phase field - lattice Boltzmann method (PF-LBM) is carried out. Results obtained are validated by Gibbs-Thomson relation at the dendrite tip. Compared with the equiaxed dendrite evolution without flow, the upstream dendrite arm is enhanced while the downstream arm is inhibited. Besides, as the inlet flow rate increases, the secondary arms attached onto the upstream primary arm and the upstream side of the primary arm normal to the inflow velocity has been well developed. Results show that sidewise instabilities of the primary dendrite arm and onset of secondary arm is caused by the local concentration perturbation and will be enhanced or inhabited by the flow. It is believed that the coupled PF-LBM method is able to handle dendrite evolution under forced flow quantitatively, which helps in investigating the solidified dendrite morphology evolution.

In situ observation and phase-field simulation on the influence of pressure rate on dendritic growth kinetics in the solidification of succinonitrile

The influence of pressure rate on dendritic growth kinetics in solidification of succinonitrile was studied via in situ observation based on a novel apparatus established in our previous work and using phase-field modeling where a pressure term associated with pressure rate was introduced. The experimental and simulation results revealed that dendrites grew much faster at higher pressure rate, resulting in dendrites characterized by more developed secondary arms and larger secondary dendrite arm spacing (SDAS), while dendrites growing at lower pressure rate was more cellular-like with small secondary arms. Higher pressure rate facilitated the competitive growth of dendrites, which led to fewer but larger dominate primary dendrites and larger primary dendrite arm spacing (PDAS) in the final microstructure. The cellular-to-dendrite transition (CDT) was more advanced at higher pressure rate, and it was demonstrated that it was the higher pressure rate not the high value of pressure that motivated CDT, via elevating effective undercooling and thus growth velocity at CDT moment. Furthermore, the growth kinetics was analyzed quantitatively, and the variation of tip velocity at different pressure rates was consistent with that of the corresponding undercooling induced by pressure and thermal condition. Moreover, the slope of growth and re-melting velocity—the tip acceleration—increased with pressure-rising rate and pressure-declining rate, respectively, even in a complicated periodic pattern, which was qualitatively consistent with the theoretical relationship of tip acceleration and pressure rate.

Effect of cold metal transfer and gas tungsten arc welding processes on the metallurgical and mechanical properties of Inconel® 625 weldings

Inconel® 625 weld metal has lower mechanical properties than its corresponding annealed base metal due to the precipitation of brittle phases during solidification. To control the precipitation of these phases, a reduction in heat input and amount of iron and silicon in the weld metal and an increase in the cooling rate are proposed. Cold metal transfer (CMT)–gas metal arc welding (GMAW) is a process that digitally controls filler metal transfer to the weld, which reduces the heat input and dilution ratio. In this work, weldments of 2-mm-thick Alloy 625 sheets were prepared using CMT and gas tungsten arc welding (GTAW) process to compare the mechanical properties (tensile and stress-controlled fatigue life tests) of the welded joints. An increment in the tensile properties and fatigue life of CMT welds was found when compared with the GTAW welds. Microstructure characterization, grain size in the heat-affected zone, phase content in the weld metal, secondary arm spacing, and Vickers hardness measurements were performed to establish the causes of the observed differences. This result was a consequence of the lower dilution of the base metal and amount of particles in the weld metal produced by a lower heat input supplied by CMT.

Influence of solute elements (Sn and Al) on microstructure evolution of Mg alloys: An experimental and simulation study

Wedge casting technique was employed for binary Mg alloys to obtain the solidified microstructure with the cooling rates ranging from 5 to 150 K/sec. Microstructural features such as secondary dendrite arm spacing (SDAS) and secondary phase fractions were experimentally determined. The experimental results of Mg-3.0, 6.0 and 9.0 wt% Sn alloys were compared with Mg-Al alloys to understand the solute effect upon the evolution of SDAS and second phase fraction in Mg binary alloys. Solidification calculations that incorporate solute back diffusion, secondary arm coarsening, dendrite tip undercooling and is dynamically linked with accurate thermodynamic databases were performed for accurate analysis of the experimental results.

Insight of the dendrite deformation in Ni-based superalloys for increased misorientation along convergent boundaries

The dendrite growth and the associated misorientation evolution on the platform base of a single crystal investment casting were investigated. It was found that the long secondary dendrite arm (LSDA) grew through the whole platform, and a convergent boundary of dendrite arms (CBDA) formed simultaneously between the corresponding branched arms and the extended secondary arms from the blade zone. The corresponding orientation measurement confirmed that the misorientation of dendrite arms between the two sides of the CBDA monotonically increased from 5.9° at one platform edge to 33.5° at the other platform edge. Additionally, the LSDA dendrite deformation was also identified by the accumulated misorientation within the LSDA dendrite core. Furthermore, it was deduced that the increased misorientation along the CBDA was caused by the LSDA dendrite deformation, and the reason of LSDA dendrite deformation was further discussed based on the associated strain distribution around the LSDA. Because of a 33.5° misorientation along the CBDA, it was indicated that the dendrite deformation might be the formation mechanism of high angle boundary.

Effect on microstructures, hardness and SDAS of primary Mg2Si/Al-Si eutectic phases of centrifugally cast functionally graded Al-(Mg2Si)p in-situ composites

In the present study, the centrifugal casting route was employed to synthesize the Al-(Mg2Si)p functionally graded in-situ composites at 1200 rpm rotating speed of mold. The formation of primary Mg2Si particles/Al-Si eutectic phases by the in-situ chemical reaction has been investigated. In the Al-Mg-Si alloys system, in-situ formed Mg2Si particles are lately introduced a new aspect, because of due to lighter density (1.99 gm/cm3) than Al-matrix (2.27 gm/cm3) the Mg2Si particles are more capable of migrating into inner zone of cast tubes due to centrifugal force. The X-Ray diffraction (XRD) technique was carried out to confirm the presence of in-situ formed Mg2Si/Si particles in the cast composites. Optical microscopy analysis was carried out to reveals distribution, size and volume fraction of Mg2Si particles of cast cylindrical tubes. Scanning electron microscopy (SEM) analysis was also carried out to reveal the morphological characteristics and some intermetallics (π & β phases) present into the cast FGM tubes. The Vickers hardness test was carried out along the radial direction of tubes. Resultant, the hardness value is remarkably improved by 20% to 30% at the inner zone due to high volume percent of reinforced primary Mg2Si hard particles. The objective of this investigation was to observe the effect on microstructures, hardness and secondary dendrites arm spacing (SDAS) of Mg2Si particles/Al-Si eutectic phases of fabricated FGMs tube and analyzed their gradient properties.

A method based on the centroid of segregation points: A Voronoi polygon application to solidification of alloys

A new method for controlling segregation delicately based on the centroid of independent segregation points in an alloy macrostructure is established using Voronoi polygon method. Voronoi polygon area is found to nearly obey the log-normal distribution, which shows that a polygon may reflect a new solidifying unit. The relationship between Voronoi polygon characteristics and carbon segregation extent (segregation ratio) is investigated at different locations. In a pure columnar grain zone, the segregation extent worsens as the Voronoi polygon size decreases; in an equiaxed grain zone, this extent worsens as the size increases. A near-equilibrium solidification model for polygons under different conditions is studied, and the results are consistent with the aforementioned finding of segregation ratio. Compared with a secondary arm spacing, the fractal dimension of the solidification structure may be a better method to describe the characteristics of the solidification structure and the size change of Voronoi polygons as to relevant segregation. Thus, this study proposes a new experimental method that depicts the macrostructure from a different perspective, i.e., the segregation point instead of the solidification structure. The formation of Voronoi polygons created by segregation points during solidifying should be divided into two stages, i.e., appearance of the segregation point and determination of the polygon's outline. The segregation extent can be controlled by altering the size of the relevant Voronoi polygon, which is a unique alternative strategy for improving the uniformity of element distribution rather than simultaneously considering nucleus formation, solidification structure growth, and liquid flow as to the complex process of segregation formation.

Modeling of binary alloy solidification under conditions representative of Additive Manufacturing

Understanding the interplay among process parameters, microstructure, and presence of defects is critical to optimizing Additive Manufacturing (AM) routes for producing specialty alloy parts with unique geometry, properties, and compositional features. However, since it is not feasible to obtain experimental results for every possible set of AM process, process conditions, and alloy composition, computational modeling is becoming an increasingly important tool for understanding key physical processes, with the hope of developing a framework for AM materials design. One such physical process of interest is the dendritic solidification within grains; we implement a Cellular Automata (CA) model for coupled solute transport and growth of cellular and dendritic β-Ti alloy colonies under thermal conditions typically encountered in AM processes. Quantitative agreement with analytical models of the undercooling-solidification velocity relationship was achieved, as well as qualitative agreement with trends in primary dendrite arm spacing (PDAS), secondary arm development, and compositional profile with changes in solidification conditions. The roles of solute diffusivity, interfacial energy, and alloying addition are considered as well. Under rapid solidification conditions, extension to include local non-equilibrium for solute allowed for the modeling of solute trapping along dendrite stems as well as qualitative representation of the dendritic to banded morphology transition. To quantitatively reproduce non-equilibrium solidification at the dendritic colony scale and more accurately estimate rapid solidification microstructures, use of multiple grids or more complex CA rules would be in order.

Influences of different Zr additions on the microstructure, room and high temperature mechanical properties of an Al-7Si-0.4Mg alloy modified with 0.25%Er

In this work, three innovative age hardenable Al alloys based on the Al-7Si-0.4Mg-0.25Er system (A356 alloy + 0.25wt% Er), with nominal additions of Zr between 0 and 0.6wt%, are designed, cast, heat treated and characterized from a microstructural and mechanical point of view. Zr additions leads to a progressive reduction of the Secondary Dendrites Arm Spacing (SDAS) of the alloys, due to inoculant pro-peritectic intermetallics and growth rate restriction of dendrites during solidification, and a reduction in eutectic Si size. 0.6wt% Zr also causes a morphological modification of dendrites, which display a globular shape, thanks to the inoculation action of primary Zr-containing intermetallics. Tensile tests performed in the peak aged and overaged conditions reveal the beneficial effect of Zr on the mechanical properties of the alloys; in particular, the alloy with the highest Zr content shows the highest mechanical properties in all the tested conditions. This work shows the potentialities of Zr as an alloying element to increase the microstructural stability and mechanical properties of Al-Si-Mg-Er alloy.

Contest competition and men's facial hair: beards may not provide advantages in combat

Abstract In contemporary human societies, where direct male-male competition is reduced compared to ancestral societies, sporting competitions remain an avenue for status acquisition via intra-sexual competition. Beards are the most visually salient and sexually dimorphic of men's secondary sexual traits and play a strong role in communicating masculinity, dominance and aggressiveness intra-sexually. Hypotheses have been advanced that beards provide advantages in intra-sexual combat, as protective organs and honest signals of fighting ability. Here we provide the first test of these hypotheses using data from professional mixed martial arts fighters competing in the Ultimate Fighting Championship. We explored whether secondary sexual traits (height, weight, beardedness), fighting stance (southpaw, orthodox), arm reach and past contest experiences impact on contest outcomes. If beards function as protective organs, bearded fighters should succumb to fewer knock-outs, and hence protection to injuries to the jaw, fewer abrasions and lacerations to the face and brain damage than clean-shaven fighters. Alternatively, if beards signal fighting ability then bearded fighters should win more fights. We found no evidence that beardedness was associated with fewer losses by knock-out or greater fighter ability. While fighters with longer reaches won more fights, neither stance nor past experience influenced fight outcomes. We suggest that beards represent dishonest signals of formidability that may serve to curtail the escalation of intra-sexual conflict through intimidation rather than providing advantages in direct combat.

On Shocks Driven by High-mass Planets in Radiatively Inefficient Disks. III. Observational Signatures in Thermal Emission and Scattered Light

Abstract Recent observations of the protoplanetary disk around the Herbig Be star HD 100546 show two bright features in infrared (H and bands) at about 50 au,with one so far unexplained. We explore the observational signatures of a high-mass planet causing shock heating in order to determine if it could be the source of the unexplained infrared feature in HD 100546. More fundamentally, we identify and characterize planetary shocks as an extra, hitherto ignored, source of luminosity in transition disks. The RADMC-3D code is used to perform dust radiative transfer calculations on the hydrodynamical disk models, including volumetric heating. A stronger shock heating rate by a factor of 20 would be necessary to qualitatively reproduce the morphology of the second infrared source. Instead, we find that the outer edge of the gap carved by the planet heats up by about 50% relative to the initial reference temperature, which leads to an increase in the scale height. The bulge is illuminated by the central star, producing a lopsided feature in scattered light, as the outer gap edge shows an asymmetry in density and temperature attributable to a secondary spiral arm launched not from the Lindblad resonances but from the 2:1 resonance. We conclude that high-mass planets lead to shocks in disks that may be directly observed, particularly at wavelengths of 10 μm or longer, but that they are more likely to reveal their presence in scattered light by puffing up their outer gap edges and exciting multiple spiral arms.

Compact Wideband Dual Loop Coupler With High Power Handling Capability for Radar Applications

Couplers are widely used in microwave subsystems, especially for radar applications. They are used to extract a directive sample of the output signal of an RF source. Typically, a coupler consists of a main arm and a secondary arm connected via coupling section. For couplers used in radar applications, power handling is a limiting factor for designers. In addition, the coupling value must be kept a flat with a high directivity over the operating bandwidth of the coupler. In this letter, we provide designs for two compact loop couplers, operating in the frequency range of 1–2 GHz and 2–4 GHz. These designs have flat coupling of −60 dB and achieve matching below −22 dB.

Effect of Sn Addition on Corrosion Properties of As-Cast and Hot-Rolled AZ31 Magnesium Alloys

In this study, the effect of Sn addition on corrosion resistance of as-cast and hot rolled AZ31 magnesium alloy was investigated. Sn additions were made by 0.2 wt%, 0.5 wt% and 1 wt%. An electric resistance furnace was used to produce alloys. Hot rolling process was performed at 350 °C by 40% thickness reduction at one rolling pass. Microstructure characterizations were performed by optical (OM) and scanning electron microscope (SEM). Immersion tests and electrochemical analyses were performed to investigate the corrosion resistance of the alloys. A 3.5% NaCl working solution at room temperature was used in both corrosion tests. The results showed that Sn addition decreased the primary dentrite size and restricted the growth of secondary dentritic arm. The as-cast structures transformed to dynamically recrystallized grain structures after hot-rolling process in all the alloys. Corrosion resistance of AZ31 magnesium alloy tended to decrease with Sn addition. This decrease was more clear in homogenized and hot-rolled states while there were some flactuations in as-cast states.

Circumference-Based Criteria for Detection of Secondary Arm Lymphedema for Chinese Women.

Detection of upper limb lymphedema following treatment for breast cancer typically relies on interlimb circumference-based differences. Normative-determined criteria from an Australian population have high sensitivity and specificity for detection of mild lymphedema in Australian women. It is unknown whether these criteria are applicable to Chinese women whose body habitus is different from Australian women. The aims of this study, therefore, were to determine the normative-based interlimb circumference and volume differences in a Chinese population and whether specific population-based thresholds are required. Arm circumferences at the wrist and at 10 cm intervals proximally to 40 cm were measured on both the dominant and nondominant limb in 484 healthy women, aged ≥20 years. Absolute interlimb differences and interlimb ratios were determined for the circumferences and derived 10 cm volume segments. Analysis of variance determined if the differences varied, depending on location. The absolute interlimb circumference difference from 10 to 30 cm did not vary significantly, enabling determination of a single threshold applicable to all three circumferences. The interlimb ratios varied less, requiring only a threshold for the wrist and one for 10-40 cm and one ratio for interlimb volume segments. The interlimb thresholds determined in the Chinese population were within 2 mm to that reported in the Australian study. Even though the habitus of Chinese women living in China differed to the women living in Australia, similar normative-based thresholds for the detection of lymphedema were identified.