Residual Liquid Phase Research Articles

Microstructure and mechanical properties of high-strength Al-Zn-Mg-Ni alloys with excellent twin-roll castability

This paper proposes a new high-strength Al-Zn-Mg-Ni alloy with excellent twin-roll castability compared with conventional Al-Zn-Mg-Cu alloys. Thermodynamic calculations were carried out to identify the solidification range and amount of residual liquid phase in the Al-Zn-Mg-(Cu, Ni) alloys, which are factors exerting significant influence on the formation of defects in the final strips. The designed Al-Zn-Mg-Ni alloys exhibited a narrower solidification range and smaller amount of residual liquid at the final solidification temperature, resulting in excellent surface quality after twin-roll casting compared with Al-Zn-Mg-Cu alloys. A series of rolling and heat treatment processes was performed on the Al-Zn-Mg-Ni alloy strips, and the mechanical properties of the final sheets were evaluated. The results showed that the yield and tensile strength were increased without affecting the elongation up to approximately 1.0wt.% Ni addition. Additionally, compared with conventional Al-Zn-Mg-Cu alloys, equivalent or higher tensile properties were achieved after T6 tempering. In conclusion, the Al-Zn-Mg-Ni alloy exhibits high-strength with excellent twin-roll castability.

Nutrient recovery from source-separated urine via sorption and a comparative investigation on the improvement of the residual liquid quality

Human urine, a highly saline solution rich in plant-available nutrients, leaves behind significant organic matter after nutrient recovery, necessitating additional treatment for environmental protection. While nutrient recovery from human urine is well-documented in the literature, research on the safe handling of the residual liquid phase is notably lacking. This study investigates nutrient recovery from source-separated human urine using clinoptilolite for the ion exchange/adsorption process and evaluates the safe management of the residual liquid through anaerobic granular sludge and a second-stage of sorption. The results indicated that the ion exchange/adsorption process, using an ammonium loading of 15 mg NH4+/g clinoptilolite, removed the majority of nutrients, achieving 82% ammonium removal and 100% phosphorus removal, along with 30% removal of organic matter. The residual liquid phase from the nutrient removal stage was treated separately with anaerobic digestion and a second-stage of sorption for further processing. Results showed that anaerobic processing removed 68%–84% of organic matter, with no additional nitrogen removal observed as expected, and produced 0.20–0.46 L CH4/L urine. The second-stage of sorption removed 59%–62% of organic matter and nearly all nitrogen. Both processes effectively removed organic matter, with sorption also eliminating nitrogen and anaerobic processing potentially generating biogas, making them recommended for improving the quality of the residual liquid phase before final disposal.

In Situ Observation of Microstructure and Precipitate Phase Transformation during the Solidification of Mg‐Containing GH3625 Alloy at Different Cooling Rates

In practical applications, intermetallic compounds like Laves phase and metal carbides adversely affect the performance of nickel‐based superalloys. Using a high‐temperature confocal laser scanning microscope, the solidification process of as‐cast GH3625 alloy containing Mg at different cooling rates (−20, −35, and −50 °C min−1) is studied. Fitting curves of the volume fraction of the solid phase with solidification temperature before and after Mg treatment are obtained. Trends of solid phase transformation rates with solidification temperature are determined. Differential scanning calorimetry is employed to analyze and statistically evaluate the melting temperature range and enthalpy of each phase during the melting process. Experimental results demonstrate that Mg treatment significantly accelerates the alloy solidification at the cooling rates of −20 and −35 °C min−1, while reducing the area of residual liquid phase at the same solidification temperature, disrupting the Laves/NbC eutectic relationship, and regularizing NbC morphology, transitioning its distribution from aggregation to dispersion. After Mg treatment, the precipitation of the Laves phase is significantly reduced. As a result, the influence mechanism of Mg treatment on the phase transformation and microstructure of GH3625 is clarified based on homogeneous nucleation theory.

Surface wave phenomenon and microstructure evolution mechanism of electromagnetically levitated ternary Ti-Al-Fe alloy

Electromagnetic levitation technique was performed to investigate the surface wave phenomenon and microstructure evolution mechanism of undercooled ternary Ti50Al30Fe20 alloy. Under the condition of high undercooling, the formation process of surface waves on the levitated alloy surface were directly detected by high-speed camera. The cooling gas provided the external perturbation that promoted surface nucleation, providing the suitable site for surface wave generation. On the rotating semisolid alloy surface, the competition between centrifugal force and surface tension caused the surface wrinkling, leading to the formation of surface waves. The residual liquid phase was solidified in the vortex circulation flow, and the vortex-shape pattern was preserved during rapid solidification. With the increases of undercooling, the microstructure evolved from dendrites and interdendritic lamellar eutectics into uniform anomalous eutectics because of the independent nucleation and competitive growth of (βTi) and TiFe phase. The surface wave was formed only at high undercoolings, indicating this phenomenon was related to the fluidity in eutectic growth. In this case, the uniform microstructure was closer to satisfying the continuum medium hypothesis, and the homogeneous medium reduced the energy damage during wave propagation. In addition, the micromechanical properties were remarkably increased as undercooling rose owing to the microstructural morphology and solute trapping effect.

Numerical simulation of jet boiling characteristics of high-pressure water injected into high-temperature liquid lead–bismuth in a confined space

The steam generator heat transfer tube rupture (SGTR) accident is potentially devastating and is one of the most serious design basis accidents for lead–bismuth cooled fast reactor (LFR). It is important to study the mechanism of water jet injecting into liquid lead–bismuth eutectic (LBE) for the safety analysis of SGTR accidents. In this paper, based on the volume of fluid (VOF) model, the large eddy simulation (LES) turbulence model and the Lee phase transition model, a three-dimensional numerical model is developed to investigate the typical characteristics of a high-pressure sub-cooled water jet injecting into high-temperature LBE in a confined space. The study focuses on examining the impact of water temperature, inlet pressure, and nozzle diameter on jet development and its surroundings, including the cover gas layer and LBE phase. The results indicate that the central region of the jet is primarily subject to flash boiling, particularly at the nozzle position, whereas the jet interface undergoes heat-transfer boiling. The residual liquid phase migrates upward along the interface and gradually boils. Based on the phase distribution characteristics, a typical jet can be categorized into four zones: the water-steam transition zone, the multiphase flow zone, the end water phase zone, and the steam block zone. Under the conditions analyzed in this paper (water temperature 140–260 °C, injection pressure 2–10 MPa, nozzle diameter 2–10 mm, LBE temperature 400 °C), the jet volume expands approximately exponentially, correlating positively with the three factors studied. It is also found that at high injection pressures and large nozzle diameters, the steam at the top of the jet is prone to destabilization and fragmentation, with steam volume and pressure oscillations being the dominant factors. Steam migration triggers pressure oscillations in the LBE region, and the intensity of these oscillations is closely linked to the migration rate. Under the conditions studied, the maximum pressure oscillation reached 5.5 times the initial pressure. The expansion of the jet steam drives the pressurization of the cover gas layer, which correlates positively with all three factors studied. The nozzle diameter has the greatest effect, increasing the maximum pressure by 168 % from the initial pressure.

Rapid dendritic growth kinetics of primary phase within supercooled Zr-V alloy at electrostatic levitation state

The liquid Zr100−xVx (x = 8.6, 16.5, 30) alloys were undercooled to the maximum undercooling of 364 K (0.18 TL), 405 K (0.21 TL), and 375 K (0.21 TL), respectively, by using electrostatic levitation technique. The Zr91.4V8.6 and Zr83.5V16.5 alloys present only one recalescence during liquid/solid phase transition, while the Zr70V30 alloy presents a transformation from two recalescence to one recalescence phenomenon with a critical undercooling of approximately 300 K. According to the LKT/BCT model, the calculated results of the primary β-Zr dendrite growth velocity in undercooled liquid Zr91.4V8.6 and Zr83.5V16.5 alloys agree well with the experiments. The velocity inflection points at 119 K of Zr91.4V8.6 alloy and 201 K of Zr83.5V16.5 alloy could be explained by the competition between solutal undercooling control and thermal undercooling control modes. For Zr70V30 alloy solidified in the P1 with twice recalescence, a critical second undercooling of 253 K and corresponding undercooling of 65 and 244 K are obtained. When the undercooling is in the range of 65–244 K, the second undercooling would be greater than 253 K, and the residual liquid phase would solidify into anomalous eutectic microstructure for Zr70V30 alloy. The Vickers hardness of Zr100−xVx (x = 8.6, 16.5, 30) alloys all show a quadratic relationship with undercooling. Under electrostatic levitation condition, the mechanical property of Zr-V alloys could be significantly regulated through solidifying the alloys at different undercoolings.

Mechanism of improving solidification structure of super austenitic stainless steel by feeding strip into molten steel

Feeding steel strip into molten steel is an effective technique for refining the solidification structure of steel, especially for high-alloy steel. This study experimentally and numerically investigated the effects of feeding strip on the solidification structure of super austenitic stainless steel S32654. The results revealed that feeding strip noticeably increased the proportion of equiaxed grains, refined the dendritic structure, and improved Mo segregation and the precipitation of σ phase. The relevant mechanism was clarified as follows. Firstly, feeding strip promoted the formation and survival of a large number of floating dendrites by cooling the molten steel. As the temperature decreased, these floating dendrites sharply increased their quantity by repeated splitting and then acted as nucleation particles. Numerous equiaxed grains grew around these floating dendrites, significantly refining the central dendritic structure. Meanwhile, the doubling of the equiaxed grains has two effects on Mo segregation. First, it prevented the growth of columnar grains and the movement of Mo segregation interface towards the center, thus reducing the Mo macrosegregation. On the other hand, it dispersed and refined the residual liquid phase (Mo segregation regions), thereby reducing the Mo microsegregation. Additionally, the reduced Mo segregation lowered the σ phase precipitation temperature, allowing it to form in solid austenite with a shortened growth duration. The lighter Mo segregation and slower Mo diffusion in solid austenite led to insufficient Mo supply for the σ phase growth. Moreover, dendritic refinement limited the growth space of the σ phase. Finally, less and finer σ phase particles precipitated in the S32654 steel fed with strip. The improvement of feeding strip on the solidification structure will reduce the temperature and time required for high-temperature homogenization, which is beneficial for the subsequent hot working.

Effect of additive content on texture evolution and mechanical properties of Si3N4 ceramics prepared by hot pressing

The purpose of this research is to investigate influence of additive content on sintering behavior, texture evolution and mechanical properties of Si3N4 ceramics systematically. Si3N4 ceramics with different additive contents are prepared by hot pressing at 1750 °C for 1h. XRD analysis confirms that the densification process is dominated by particle rearrangement instead of phase transformation. Grain morphology characterization demonstrates that additive content affects relative amount between textured β-Si3N4 grains and β-Si3N4 nuclei during initial sintering. During subsequent sintering, β-Si3N4 nuclei surrounded by residual liquid phase elongated randomly, and then weaken texture degree of Si3N4 ceramics. The texture degree of Si3N4 ceramics decreases first and then increases as additives increase steadily. Based on the CTE misfit model and the stochastic model of crack propagation, texture degree, residual stress and grain size have a synergistic effect on crack propagation behavior of β-Si3N4 ceramics. The outstanding flexural strength (1096 MPa) and fracture toughness (10.61 MPa ∙ m1/2) is achieved in hot pressed SN21 with a relatively high texture degree (fL = 0.213).

Research on gas pore formation and inhibition mechanism of high nitrogen steel during laser direct metal deposition

Nitrogen pore formation is an inherently tricky problem that limits the application of high nitrogen steel (HNS) in various fields. In the present study, a CA-LBM coupling model was proposed to understand the nitrogen pore formation mechanism during laser direct metal deposition (DMD) of HNS. Based on the proposed model and theoretical analysis, the effects of pulse frequency and shielding gas nitrogen content on the porosity were clarified systematically. Results showed that after the onset of solidification, the ferrite phase with extremely low nitrogen solubility precipitated first, inducing the continuous enrichment of nitrogen in the residual liquid phase, which in turn provided the prerequisite for nitrogen pore nucleation. The application of quasi-continuous-wave (QCW) laser mode accelerated the molten pool solidification, and shortened the nitrogen supersaturated region, allowing more nitrogen to be solid dissolved in the matrix. On this basis, the porosity was inhibited to a certain extent. Compared with the high-frequency pulse, the low-frequency pulse showed better porosity inhibition. In addition, following the classic nucleation theory, nitrogen segregation and nitrogen solubility synergistically dominated nitrogen pore formation. The nitrogen partial pressure, determined by the shielding gas nitrogen content, played a dual role in porosity defects. Elevated nitrogen partial pressure served to increase nitrogen solubility while promoting the nitrogen absorption process to intensify nitrogen segregation. However, due to the short duration of the molten pool, the nitrogen absorption process could not be fully processed. Nitrogen solubility enhancement was the key factor in inhibiting porosity during the DMD process.

Hot Tearing Susceptibility and Solidification Behavior of Mg-xGd-(4-x)Y-2Ni-0.5Zr Alloys

The hot tearing susceptibility (HTS) of Mg-xGd-(4-x)Y-2Ni-0.5Zr (x = 0, 1, 2, 3, 4, wt%) alloys was evaluated using a T-shape mold method. When the Gd element increased and the Y element decreased, the HTS of the alloys decreases gradually. The Mg-4Y-2Ni-0.5Zr has the highest while Mg-4Gd-2Ni-0.5Zr has the lowest HTS value. Microstructure observation reveals that the grains of the magnesium alloy were refined to a certain extent when the Gd element increased and the Y element decreased. The ‘pinning’ effect of the LPSO phase precipitated along the grain boundary effectively increases the solid skeleton strength of the alloy and reduces the HTS of the magnesium alloy to a certain extent. The thin and uniform liquid film at the fracture can disperse the solidification shrinkage stress. In addition, the binary phase precipitated in the alloy forms a fast channel of residual liquid phase feeding on the liquid film, which is beneficial to feeding hot cracks. Solidification behavior of the alloys by thermal analysis showed that with the increase of the Gd element and the decrease of the Y element, the dendrite coherency temperature of the dendrites of the alloy decreases, and the residual liquid phase feeding is easier. The solid fraction of dendrite coherency increases, the skeleton strength of the alloy increases, and the HTS of the alloy decrease. In Mg-xGd-(4-x)Y-2Ni-0.5Zr (x = 0, 1, 2, 3, 4, wt%) alloys, when x is 1, 2, 3, 4, the HTS of the alloy is reduced by 25%, 49%, 63%, 81%, respectively compared with that when x is 0.

In Situ Observation the Effect of Y on the Solidification Process of 7Mo-SASS under a Low Cooling Rate.

The effects of Y on the solidification process of 7Mo super austenitic stainless steel (7MoSASS) under low cooling rate conditions (10 °C/min) were investigated using high-temperature confocal laser scanning microscopy (HT-CLSM). The in situ observation results indicate that Y samples promote an increase in austenite nucleation density. After 10 s of nucleation, the nucleation density increased by 149.53/mm2 for the Y sample. Furthermore, variance analysis indicated that Y addition improved the uniformity of the 7MoSASS solidification microstructure under low cooling rate conditions. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory results showed that when the solid phase ratio was 0.5, the nucleation mode of the Y sample transitioned from saturation site nucleation to saturation site nucleation + Avrami nucleation. YAlO3 has a low lattice disregistry value with austenite, making it a suitable heterogeneous nucleation core for promoting the early nucleation of austenite. During the late stages of solidification, Y accumulates in the residual liquid phase, providing a greater degree of compositional undercooling. SEM-EDS analysis showed that Y contributed to the refinement of the 7MoSASS solidification microstructure, with the proportion of precipitated phases decreasing by approximately 7.5%. Cr and Mo were the main elements exhibiting positive segregation in 7MoSASS, and the Cr segregation ratio increased in the Y sample, while the Mo segregation ratio decreased.

Semi-macrosegregation and carbide banding in high-carbon chromium bearing steels: Characteristics, evolution, and control

Semi-macrosegregation in continuously cast blooms and its associated carbide banding defects in hot-rolled bars were investigated to explore the characteristics, evolution, and control methods in high-carbon chromium bearing steel. It was found that the semi-macrosegregation exists only in the central equiaxed crystal zone of the blooms, and C, Cr solute elements inside show typical spot-like positive segregation. Carbide banding appears as white bright bands along the rolling direction, composed of high number density M3C carbide particles, with a significant effect on the mechanical properties of the rolled products. The hereditary evolution from semi-macrosegregation to carbide banding was established by means of banded segregation. Morphologically, normal semi-macrosegregation and carbide-typed semi-macrosegregation are formed based on whether Cr-rich primary carbides are generated. These two types of semi-macrosegregation evolved into two kinds of carbide banding respectively, that is, dark bands and bright bands defined according to the ability to reflect light under OM. To alleviate the carbide banding defect in rolling products of high-carbon chromium bearing steel, the morphology and area ratio of equiaxed crystals in continuous cast blooms should be modified. It is observed that the equiaxed crystal ratio in the bloom can be decreased from 62.5 % to 28 % by increasing liquid steel superheat and inhibiting superheat dissipation in the mold through casting practices. Additionally, the morphology of equiaxed crystals is related to the as-cast solidification structure, and when the equiaxed crystal ratio is large, the equiaxed crystals turn out to be coarser ones with only cellular primary dendrite; when the columnar crystals are longer, however, the equiaxed crystals appear as a dendrite structure with fine secondary dendrites. As a result, semi-macrosegregation was improved obviously with the fine dendrite structure splitting the local residual liquid phase, leading to the decrease of the maximum width of carbide banding from 268.41 μm to 73.72 μm, and the increase of mechanical properties in the rolled bar of high-carbon chromium bearing steel.

Evaluation of solidification cracking susceptibility factors in carbon steel and availability of Ti addition based on theoretical analysis

This study conducted a basic investigation to improve the solidification cracking susceptibility of carbon steel. BTR is one of the susceptibilities to solidification cracking, and it is said that solidification cracking susceptibility is reduced when BTR is smaller. BTR of three commercial filler metals was evaluated by using Trans-Varestraint test. EPMA and EDS analysis of the weld metal microstructure revealed two compounds. The white structure was composed of silicon, manganese, and sulphur, and the black structure was Ti-based compound. Theoretical analysis of BTR was conducted by applying a two-phase peritectic solidification model in FA mode and considering MnS crystallization during solidification segregation. The analysis results indicated that BTR of carbon steel is attributed to the difference in solidification completion temperatures, and carbon is the most deleterious element. Also, MX compound (Ti, C, N) crystallized in addition to MnS at the end of solidification. Theoretical analysis was conducted for Ti virtual addition materials because MX compound (Ti, C, N) crystallization by Ti addition may lead to BTR reduction by trapping C in the liquid phase. The results showed that Ti addition reduced BTR. The amount of C in the residual liquid phase was reduced by an increase in MX compound (Ti, C, N) crystallization due to Ti addition. Based on theoretical considerations, it can be concluded that Ti addition to carbon steel is effective in reducing BTR.

Precipitation mechanism of σ phase in S32654 super austenitic stainless steel

By utilizing High-Temperature Confocal Laser Scanning Microscopy (HT-CLSM), the microstructure evolution of S32654 super austenitic stainless steel was observed in situ during both solidification and solid-state phase transition. Moreover, the predominant focus was on investigating the precipitation mechanism of the σ phase. The findings revealed that the initial stages of the solidification process involved the nucleation and growth of austenite, followed by a gradual reduction in the residual liquid phase between dendrites until complete solidification was achieved. Furthermore, as the temperature decreased, the composition of the austenite formed from the residual liquid phase differed from the initially formed austenite, ultimately leading to the formation of granular austenite through extrusion. Below the critical temperature, the excessive amounts of Cr and Mo could not be dissolved in the supersaturated austenite between dendrites, thereby resulting in the instantaneous eutectoid decomposition of austenite, namely γ → γ2 + σ.

Effectively mitigated macro-segregation and improved tensile properties of twin-roll cast Al–Mg–Si (6022) alloy strips via Zr addition

It is a great challenge to avoid the central macro-segregation during the solidification process of twin-roll casting (TRC), which seriously limits the mechanical properties of 6xxx series Al alloys. In this study, we find that the trace addition of 0.08 wt% Zr to Al-1.0Mg-1.2Si (6022) alloy can effectively promote the columnar-to-equiaxed transition and refine the solidification microstructure (particularly α-Al dendrites and π-AlFeMgSi phase). More importantly, the macro-segregation of the as-cast TRC strip is well mitigated with Zr addition. The refinement of α-Al dendrites and π-AlFeMgSi phase may be attributed to the formation of (Al, Si)3Zr phases when introducing Zr element, which can act as heterogeneous nucleation sites for both of them. The secondary dendrite arm spacing (SDAS) in the central region of strips is sharply decreased from ∼33 to ∼16 μm, which facilitates the incorporation of more solute elements into the Al matrix, thus increasing the partition coefficients of Mg (kMg) and Si (kSi) elements. As a result, there are fewer solute atoms in the residual liquid phase during the final stage of solidification, leading to the alleviation of central macrosegregation. The tensile properties of the as-cast, solid solution treated (T4) and paint baking treated (T6) alloys are synchronously optimized with Zr addition. In particular, the fracture elongation of the T4 alloy is increased from ∼30.9% to ∼33.4%, indicating excellent formability. The yield strength of the T6 alloys is increased from ∼227 MPa to ∼239 MPa, satisfying the more demanding service requirements. This work can provide a strategy for the effective control of macro-segregation and produce alloy strips with high performance by the TRC process.

Influence of gadolinium on solidification behavior of Ni–Mo-Cr-Gd alloy

The influence of Gd content on the solidification behaviors and as-cast microstructures of Ni–Mo-Cr-Gd alloy have been investigated systematically. It was found that the γ/Ni5Gd eutectic phases were formed in the interdendritic region, which exhibits the divorced character. With the Gd adding, the γ/M6C eutectic phases were replaced by the γ/M2C eutectic ones. Most M2C carbides locate at the interfaces of γ/Ni5Gd eutectic phases or are enclosed by Ni5Gd phases. The segregation of alloying elements was mitigated by Gd adding. Based on the thermal analysis, composition analysis and thermodynamic calculation, the solidification behaviors, and the affecting mechanism of Gd adding were briefly discussed. During the solidification process, the γ/Ni5Gd eutectic reaction occurs first, which interrupts the solute redistribution process and thus mitigates the segregation of some alloying elements. The Ni5Gd phases accommodate a mass of Si and keep the Si content in the residual liquid phase low, which decreases the nucleation rate of M6C carbides. Then, the γ/M2C eutectic phases become the dominant ones at the final stage of the solidification process. Our results are useful in future optimization and processing research of Ni-Mo-Cr-Gd alloy for applications in molten salt reactors.

Stable Water Isotopologue Fractionation During Soil‐Water Evaporation: Analysis Using a Coupled Soil‐Atmosphere Model

AbstractThe atmosphere‐soil system forms a highly coupled system, which makes key processes such as evaporation complex to analyze as the mass, energy, and momentum transfer is influenced by both domains. To enhance the understanding of evaporation processes from soils, stable water isotopologues are suitable tools to trace water movement within these systems as heavier isotopologues enrich in the residual liquid phase. Due to the complex coupled processes involved in simulating soil‐water evaporation accurately, quantifying fractionation during flow and transport processes at the soil‐atmosphere interface remains an open research area. In this work, we present a multi‐phase multi‐component transport model that resolves flow through the near‐surface atmosphere and the soil, and models transport and fractionation of the stable water isotopologues using the numerical simulation environment DuMux. Using this coupled model, we simulate transport and fractionation processes of stable water isotopologues in soils and the atmosphere by solving compositional flow equations and by using suitable coupling conditions at the soil‐atmosphere interface instead of commonly used parameterization. In a series of examples of evaporation from bare soil, the transport and distribution of stable water isotopologues are evaluated numerically with varied conditions and assumptions, including different atmospheric conditions (turbulent/laminar flow, wind speed) and their impact on the spatial and temporal distribution of the isotopic composition. Building on these results, we observed how the enrichment of the isotopologues in soil is linked with the different stages of the evaporation process. A qualitative study is conducted to verify single fractionation processes in our approach.

Effect of Ca/Al ratio on hot tearing susceptibility of Mg–Al–Ca alloy

The hot tearing susceptibility (HTS) of Mg–xAl–yCa (x + y = 8) alloys with different Ca/Al ratios (namely 0.06, 0.34, 0.63, 1.04, 1.75, and 2.82) is experimentally investigated using a “T-shaped” hot tearing measuring system. Additionally, the HTS of the alloys is determined using an optimized version of the Clyne–Davies model in conjunction with the solidification parameters of the alloys. The results of the optimized model, i.e., the fact that the HTS decreases with the increase in the Ca/Al ratio, are in good agreement with both the numerical simulations and experimental results. Furthermore, the hot tearing curves, thermal analysis curves, and microstructure of the alloys show that with the increase in the Ca/Al ratio, the intergranular bonding before the hot tearing initiation changes from relying on a liquid film only to involving both intergranular bridging and the liquid film, which enhances the intergranular bonding ability. In addition, the increase in the eutectic phase content, number of dendrite skeleton voids, and dendrite gap width result in an increase in the number of feeding channels and a decrease in the flow resistance of the residual liquid phase, thus increasing its filling efficiency against tears. Therefore, in alloys with high Ca/Al ratios, the large grain size results in a reduction in both the number and total length of grain boundaries, thereby reducing the number of locations where hot tears may initiate. Therefore, the HTS of alloys with a high Ca/Al ratio is significantly reduced.

Significant refinement and spheroidization of harmful Fe-rich phases in Al–Cu–Mg alloys by small Sn addition

Solidification phases of conventional cast aerospace Al–Cu–Mg alloys and their evolution during homogenization were comprehensively studied. Significant refinement and spheroidization of solidification phases were found by the addition of 0.04, 0.15, and 1.00 mass % Sn to Al–Cu–Mg alloys. The soluble Sn component is dissolved in the Al matrix as solute atoms, while Sn in excess of the solid solubility segregates in the intradendrites in the form of Sn pools with a size (diameter) of one-half to several micrometers. These solute Sn atoms and Sn pools can capture Cu, Mg, and Fe atoms or hinder their diffusion into the residual liquid phase in the interdendritic region during solidification. Owing to the Sn addition, the contents of the Al2Cu, Al2CuMg, and Al7Cu2Fe phases in the interdendritic region are greatly reduced, thereby leading to significant refinement and spheroidization of these solidification phases. The formation of proeutectoid Mg2Sn phases is also found to be effective for the refinement of solidification phases.

Starchy and fibrous feedstuffs differ in their in vitro digestibility and fermentation characteristics and differently modulate gut microbiota of swine

BackgroundAlternative feedstuffs may contribute to reducing feed costs of pig production. But these feedstuffs are typically rich in fiber and resistant starch (RS). Dietary fibers and RS are fermented in the gastrointestinal tract (GIT) and modulate the microbial community. Certain microbes in the GIT can promote host health, depending on the type of fermentation substrates available. In this study, six alternative feedstuffs (three starchy: Okinawan sweet potato, OSP; yam, and taro, and three fibrous: wheat millrun, WMR; barley brewers grain, BBG; and macadamia nut cake, MNC) were evaluated for their in vitro digestibility and fermentation characteristics and their effects on pig’s hindgut microbial profile. After 2 steps of enzymatic digestion assay, residues were fermented using fresh pig feces as microbial inoculum, and gas production was recorded periodically for 72 h and modeled for fermentation kinetics. After fermentation, the residual liquid phase was analyzed for short-chain fatty acid (SCFA), and the solid phase was used to determine the nutrient’s digestibility and microbial community.ResultsIn vitro ileal digestibility of dry matter and gross energy was higher in starchy than fibrous feedstuffs. Total gas and SCFA production were significantly higher (P < 0.001) in starchy feedstuffs than fibrous feedstuffs. Both acetate and propionate production was significantly higher (P < 0.001) in all starchy feedstuffs than BBG and MNC; WMR was in between. Overall alpha diversity was not significantly different within and between starchy and fibrous feedstuffs. Beta diversity (measured using bray Curtis dissimilarity distance) of starchy feedstuffs was significantly different (P < 0.005) than fibrous feedstuffs.ConclusionStarchy feedstuffs acted as a substrate to similar types of microbes, whereas fibrous feedstuffs resulted in a more diverse microbial population. Such alternative feedstuffs may exert comparable beneficial effects, thus may be included in swine diets to improve gut health.