Entropy-Driven Grain Boundary Segregation: Prediction of the Phenomenon

The question is formulated as to whether entropy-driven grain boundary segregation can exist. Such a phenomenon would be based on the assumption that a solute can segregate at the grain boundary sites that exhibit positive segregation energy (enthalpy) if the product of segregation entropy and temperature is larger than this energy (enthalpy). The possibility of entropy-driven grain boundary segregation is discussed for several model examples in iron-based systems, which can serve as indirect evidence of the phenomenon. It is shown that entropy-driven grain boundary segregation would be a further step beyond the recently proposed entropy-dominated grain boundary segregation as it represents solute segregation at “anti-segregation” sites.

Preparation of 2A14 Aluminium Alloy Large-Sized Hollow Ingots by Electromagnetic Stirring DC Casting

The Φ730 / Φ340 mm hollow ingots of 2A14 aluminium alloy were produced by conventional and electromagnetic stirring (EMS) DC casting with extremely fine grain morphology. The results indicate that the metallographic microstructure of the alloy was more uniform and homogeneous in the EMS hollow ingot and the finer grain size was obtained. The average grain size dramatically decreased from 115 μm to 70 μm with applying EMS. The macrosegregation patterns of Cu element in EMS and conventional hollow ingots along the radial direction were both following the similar trend that positive segregation occurred in inner subsurface and middle section. Meanwhile negative segregation occurred in section offset to inside of centerline and outer surface. The extent of macrosegregation in EMS hollow ingot was severer than that in the conventional one. The mechanism of EMS was discussed to reveal its effect on the microstructure and macrosegregation.

Effect of Multi-Source Ultrasonic on Segregation of Cu Elements in Large Al–Cu Alloy Cast Ingot

The structure and composition of large-scale Al–Cu alloy ingots are inhomogeneous, and the segregation of (especially) elemental Cu negatively affects the uniformity and stability of the subsequent components. In this work, four ultrasonic generators were used to manipulate solidification/microstructures of cylindrical Al–Cu ingots (1250 mm in diameter; 3500 mm in length). The influence of ultrasonic configuration on both solidification microstructures and solute macrosegregation was investigated by changing the position parameters of generators for a fixed power. The results revealed that when the ultrasound is applied close to the center (I) from the 1/2 radius (II), the grain structure of the center undergoes significant refinement, degree of positive segregation in the center can be reduced, segregation index decreased from 0.2 to 0.15, and range of positive segregation in the center decreased from 200 to 150 mm. The segregation of elemental Cu was weakened by the combined effects of the ultrasound on the flow, heat transfer, and grain movement.

3D FEM Simulation of the Effect of Cooling Rate on SDAS and Macrosegregation of a High Strength Steel

Secondary dendrite arm spacing (SDAS) is a macrosegregation parameter directly linked to content of macrosegregation through cooling rates. The aim of this paper is to highlight the effect of cooling rate on the SDAS and macrosegregation patterns in a high strength steel. For this purpose, directionnal solidification in a cylinder was modeled with a plane-front solidification. Two cylinders were modeled with different boundary conditions (Tsurface = 1000°C and 1200°C). Using the FEM software Thercast, 3D macrosegregation maps were generated with thermomechanic algorithm taking into account metal shrinkage. Using Won’s equation, the influence of cooling rates in the mushy zone on SDAS was determined. The results indicated that a 72% lower difference in the area of negative macrosegregation zone (macrosegregation ratio (rseg) < -0.016%) for lower cooling rate (Ts = 1200°C). The difference of the area for positive segregation was 85% lower for higher cooling rates.

Assessment of the Fitness for Service of the Flamanville EPR Reactor Pressure Vessel Closure Head and Bottom Head Domes Containing a Segregation Zone Characterized by a High Carbon Content

To complete the manufacturing process qualification of the closure head and bottom head domes of the Flamanville EPR RPV in 16MND5 ferritic steel (similar to A508 cl3), AREVA NP performed in 2014 material testing on a sacrificial dome so-called UA Sup. These tests revealed that the Charpy impact energy at 0°C — in the positive segregation zone at ¼ thickness — was lower than 60 J, the minimum value required by the French regulation. Thereafter, this result was explained by the high carbon content close to 0.3 % — against 0.18 % in the ladle — due to the forging process. The positive segregation was insufficiently eliminated to avoid high carbon content in certain location. As the level of Charpy impact energy is an indicator of the level of fracture toughness, a thorough material characterization of the segregation zone, as well as a brittle fracture analysis have been proposed and carried out by AREVA NP to demonstrate that the Flamanville EPR RPV domes are tough enough for a safe operation. The principles of the demonstration approach were reviewed by ASN and IRSN in 2015 and the final safety case was submitted to the ASN and IRSN review in December 2016. The latter led to the conclusion that, even if the segregation zone fracture toughness is lowered in comparison to that of the acceptance test zone, this does not put into question the Flamanville EPR RPV fitness for service as long as adequate in-service inspection of the domes is periodically performed. This paper aims at presenting the key elements of the IRSN assessment from part of which this conclusion was drawn, namely the material testing program, the description of the loadings in normal and accidental conditions, and the brittle fracture analysis.

Effect of Electromagnetic Field on the Macrosegregation Behaviour of Al-Cu Alloy Ingot Prepared with Direct-Chill Casting Process

A low frequency electromagnetic field was introduced into the direct chill (DC) casting process and the ingots of Al-Cu alloy were prepared to study the macrosegregation behaviour of the ingots under the influence of the electromagnetic field. The experimental results showed that there is an obvious positive segregation near to the surface and a negative segregation in the centre area of the ingot. Cu shows the highest segregation tendency among the main elements of Cu, Mg and Mn. Grain refiner element Ti shows a segregation trend opposite to that of Cu. With the application of electromagnetic field, the negative centreline segregation in the centre area of the ingot was evidently reduced although it didn’t show significant effect on the segregation near to the ingot surface. A significant grain refinement was also achieved with the application of electromagnetic field. The mechanism of the reduction of macrosegregation with electromagnetic field was also analyzed in the present work.

Microstructure and Hot Tearing Susceptibility of K418 and K419 Superalloy for Auto Turbocharger Turbine Wheel

K419 superalloy turbine wheel was more susceptible to hot tearing than K418 one when they were used for auto turbocharger turbine wheel. The fracture and microstructure characteristics in the K418 and K419 turbine wheel blades were analyzed. The segregation of alloying elements was analyzed by EDS. The probable equilibrium phases in the two kinds of superalloys, the effects of aluminum, titanium and niobium contents on the precipitation of γ and γ/γ eutectic phase and the segregation of alloying elements were studied by Thermo-Calc software. The results show that the hot tearing in the K418 and K419 superalloy turbine wheel blades is caused by the fracture of dendrites structures, while the amount of γ/γ eutectic in K419 is more than that in K418, resulting in K419 being more susceptible to hot tearing. Titanium and niobium, the strong positive segregation elements promote the formation of γ/γ eutectic, which lead to severe hot tearing susceptibility of the superalloy.

The Mathematical Model and Simulation of Macrosegregation during Solidification Process of Super Heavy Steel Ingot

Through establishing solidification-heat transfer mathematical model of the super heavy steel ingot, the temperature distribution at different moments in the solidification process of steel ingot has been simulated. Based on the simulated temperature field and the law of solute redistribution, the carbon macrosegregation in the solidification process has been calculated. The results show that, the complete solidification time of the 600T steel ingot is 68.34 hours, and positive segregation of carbon forms in the ingot head, and the concentration of carbon near the upper axis which is higher than 81 percent of the ingot’s height is over 2 times than the average value.

Study of Centerline Segregation in Alloy Tool Steel on CSP Technology

After studying the slab of the CSP, the composition segregation and the finished products, we drew the conclusion that the low magnification of CSP thin slab is denser, columnar grain more developed; Loose and segregation exist while the segregation is smaller than traditional one. From the surface to the center, obvious carbon segregation exists: nearby the center is negative segregation and in the center is positive segregation. With the increase of carbon content, carbon segregation also increases. The carbon content of tundish is different from that of rolled steel coil --namely, the carbon content of the rolled coil is significantly lower.