Experimental determination of mushy zone permeability in aluminum-copper alloys with equiaxed microstructures

An experimental apparatus for measuring the mushy zone permeability of aluminum-copper alloys with equiaxed microstructures has been constructed. Permeabilities have been measured for high solid fractions (0.68 to 0.91) and different dendrite morphologies. Microstructure characterizations on both the interdendritic and extradendritic length scales have been performed on the samples. The results are in fairly good agreement with the predictions of the Kozeny-Carman relation and with more recent theory that takes flow partitioning between interdendritic and extradendritic regions into account.     

A process model for the heat-affected zone microstructure evolution in duplex stainless steel weldments: Part I. the model

The present investigation is concerned with modeling of the microstructure evolution in duplex stainless steels under thermal conditions applicable to welding. The important reactions that have been modeled are the dissolution of austenite during heating, subsequent grain growth in the delta ferrite regime, and finally, the decomposition of the delta ferrite to austenite during cooling. As a starting point, a differential formulation of the underlying diffusion problem is presented, based on the internal-state variable approach. These solutions are later manipulated and expressed in terms of the Scheil integral in the cases where the evolution equation is separable or can be made separable by a simple change of variables. The models have then been applied to describe the heat-affected zone microstructure evolution during both thick-plate and thin-plate welding of three commercial duplex stainless steel grades: 2205, 2304, and 2507. The results may conveniently be presented in the form of novel process diagrams, which display contours of constant delta ferrite grain size along with information about dissolution and reprecipitation of austenite for different combinations of weld input energy and peak temperature. These diagrams are well suited for quantitative readings and illustrate, in a condensed manner, the competition between the different variables that lead to structural changes during welding of duplex stainless steels.     

A process model for the microstructure evolution in ductile cast iron: Part I. the model

In the present investigation, the multiple phase changes occurring during solidification and subsequent cooling of near-eutectic ductile cast iron have been modeled using the internal state variable approach. According to this formalism, the microstructure evolution is captured mathematically in terms of differential variation of the primary state variables with time for each of the relevant mechanisms. Separate response equations have then been developed to convert the current values of the state variables into equivalent volume fractions of constituent phases utilizing the constraints provided by the phase diagram. The results may conveniently be represented in the form of C curves and process diagrams to illuminate how changes in alloy composition, graphite nucleation potential, and thermal program affect the microstructure evolution at various stages of the process. The model can readily be implemented in a dedicated numerical code for the thermal field in real castings and used as a guiding tool in design of new treatment alloys for ductile cast irons. An illustration of this is given in an accompanying article (Part II).     

Grain boundary engineering: an overview after 25 years

Abstract

In 1984, 'grain boundary design', later known as 'grain boundary engineering (GBE)', was proposed. The central premise of GBE is that specific thermomechanical treatments, mainly on face centred cubic materials which readily form annealing twins, can be used to improve resistance to various forms of intergranular degradation such as corrosion, cracking or embrittlement. Engagement with the concept has accelerated in recent years. This overview charts the progress of GBE from its inception 25 years ago to the present day, including suggestions of key topics for ongoing or future research. These topics comprise confirmation of which boundaries are 'special' in terms of crystallography and properties, optimisation of processing regimes, new approaches to GBE in systems without annealing twinning and incorporation of connectivity metrics, especially in three dimensions.     

The origin of transformation textures in steel weld metals containing acicular ferrite

The present investigation is concerned with basic studies of the development of transformation textures in steel weld metals, using the electron backscattering pattern (EBSP) technique. It is shown that the acicular ferrite (AF) plates exhibit an orientation relationship with both the austenite and the prior delta ferrite columnar grains in which they grow. The observed orientation relationship lies within the Bain orientation region and can be described by three texture components,i.e., a 〈100〉 component and two complementary 〈111〉 components. Each of these texture components is orientated approximately parallel with the original cell/dendrite growth direction. Measurements of the spatial misorientation between neighboring plates confirm that the morphology of AF in low-alloy steel weld metals bears a close resemblance to upper bainite.     

Microscale investigations of the metal-dusting corrosion mechanism on mild steel

The internal and external products from metal-dusting corrosion of a mild steel specimen have been investigated, with the intention of further exploring the corrosion reaction mechanism. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been employed. A section of a steel tube, which had been subjected to heavily corrosive attack under controlled conditions, was studied. Electron-transparent TEM samples from this specimen were observed in the cross-sectional and plan-view orientations of the corrosion interface. Deposit on the corroded surface contained amorphous carbon, graphitic crystalline carbon, and decomposition products. Supersaturated cementite is an intermediate reaction product and was present at the surface of the exposed alloy. Surface cementite was seen to decompose locally into the graphite, where graphite basal planes were oriented perpendicular to the cementite surface. Iron was transported through the graphite, and the mild steel disintegrated by continuous formation and disintegration of the surface cementite. The observations are discussed with reference to the prevailing model mechanism for metal-dusting corrosion.     

A process model for active brazing of ceramics

In the present investigation process modelling techniques have been applied to describe reaction layer growth during active brazing of ceramics. As a starting point, the classical solution for parabolic growth of transformation products is considered. Specific computational features are then explicitly built into the model to allow for transient effects during heating and cooling as well as changes in the growth kinetics due to depletion of the active element during brazing. This approach gives considerable scope for optimization of both process and joint properties through adjustment of the filler metal composition and the temperature-time programme under which brazing takes place. The aptness of the process model is illustrated in an accompanying paper (Part II).Nomenclature A, C B reaction products in diffusion couple - C A ceramic component - C reactive element in braze alloy - C _B concentration of element B at a given position within the reaction layer, C B (mole m^–3) - C _B ^b bulk concentration of element B in ceramic, A B (mole m^–3) - C _B ⁱ (1), C _B ⁱ (2) concentration of element B in reaction layer at C B /A B and braze metal/C B interface, respectively (mole m^–3) - C _C concentration of element C at a given position within the reaction layer, C B (mole m^–3) - C _C ⁰ initial concentration of element C in braze metal (mole m^–3) - C _C ^b bulk concentration of element C in braze metal (mole m^–3) - C _C ⁱ (1), C _C ⁱ (2) concentration of element C in reaction layer at braze metal/C B and C B /A interface, respectively (mole m^–3) - D ₀, D ₀ ^* constants in expression for diffusion coefficient (m² s^–1) - D _B intrinsic diffusivity of B in C B (m² s^–1) - D _C intrinsic diffusivity of C in C B (m² s^–1) - J _B molar flux of element B (mole m^–2 s^–1) - J _C molar flux of element C (mole m^–2 s^–1) - k ₀ constant in expression for k_p (m² s^–1) - k ₀ ^* rate constant referring to infinite diffusion couple analogue (m² s^–1) - k _p parabolic growth rate constant (m² s^–1) - L half width of braze metal zone (m) - m proportionality constant (equal to the ratio between C _C ⁱ (1) and C _C ^b ) - Q _app apparent activation energy for diffusion of C in C B (J mole^–1) - Q _app ^* apparent activation energy for diffusion of B in C B (J mole^–1) - R universal gas constant (8.314 J mole^–1 K^–1) - t time (s) - t ₀ incubation time (s) - t ₁, t ₂ limits of integration (s) - t _i isothermal hold time (s) - t _i time increment used in the numerical integration procedure (s) - T absolute temperature (K or °C) - T _c chosen reference temperature (K or °C) - T _i isothermal hold temperature (K or °C) - X thickness of reaction layer (m) - X _c contribution of the cooling leg of the brazing cycle to the total reaction layer thickness (m) - X _h contribution of the heating leg of the brazing cycle to the total reaction layer thickness (m) - X _i contribution of the isothermal hold period to the total reaction layer thickness (m) - X _lim limiting thickness of reaction layer, C B (m) - X _i increase in reaction layer thickness due to a small time increment t _i (m) - y ₁, y ₂, y ₃ molar partitioning factors - , , , , , ) molar stoichiometric factors - molar volume of reaction product, C B (m³ mole^–1)     

Optimisation of the broiler production supply chain

In this paper, we propose a mixed integer programming (MIP) model for the Chicken Flock Sizing, Allocation and Scheduling Problem (CFSASP), which is an important planning problem in the broiler production supply chain. To solve the CFSASP efficiently, two variants of rolling horizon heuristics (RHHs) have been developed and applied on the case of a Norwegian broiler production company. Computational results show that the RHHs successfully obtain high-quality solutions within a reasonable time. The value of optimisation is verified through comparison with the case company's plans, where the solutions from optimisation outperforms the current solutions. Sensitivity analyses are also conducted to provide managerial insights regarding certain strategic decisions, such as how many and which days to use for hatching of chickens. Due to the promising results, the case company is now implementing an optimisation-based decision support system based on the MIP model and solution methods shown in this paper.