Abstract: A fine microstructure stabilised by dispersed particles is an interesting option for
increasing the resistance of Al-Sn tribo-alloys without sacrificing ductility. Particle stimulated and continuous recrystallisation allow achieving this goal, but require careful preconditioning of the microstructure to obtain the desired particle distribution. As-cast slabs of Al 12%Sn2%Pb3%Si contain large plates of b-AlFeSi and modified silicon in an aluminium matrix. Sn forms a reticular network which is liquid at conventional annealing temperatures. Experiments show that metastable
b-AlFeSi initially coarsens due to Ostwald ripening, but later dissolves and the remainder transforms into FeSiAl12. Silicon partitions to the liquid, allowing the nucleation of numerous small Si-grains.
Abstract: Impurity segregation at grain boundaries in polycrystalline alloys is known to have a
tremendous impact on the material properties such as grain boundary mobility, cohesion... But direct measurement of grain boundary chemistry is quite complex and there are few results concerning polycrystals. In this paper we present an indirect method to measure segregationmisorientation dependence on polycrystalline Ni-S alloys using 3D reconstruction of etch grooves. Samples of Ni-S alloy (1 ppm at) have been cold rolled at respectively 0.3 and 0.9 Von Mises
equivalent strain and then annealed at 455°C to promote recrystallization. Then they have been etched near the transpassive potential to form etch grooves, whose geometry depends on the sulfur segregation level. It is found that the sulfur concentration at grain boundary decreases significantly when the driving force for grain boundary migration (i.e. the initial strain) increases, as predicted by solute drag theory.
Abstract: This paper presents some preliminarily results on microstructure modifications associated with static recrystallization in an ODS alloy. The morphology of the grains issued from static recrystallization is influenced significantly by the alignment of the oxide particles in the as-extruded starting material. Grain growth modeling confirms the effect of particle alignment on the grain morphology and shows significant control of the particle distribution nature and the initial grain size on the grain anisotropy.
Abstract: This paper deals with the study of particle shape effect in grain growth inhibition. As it is difficult to quantify analytically such effect during growth kinetic because of particle distribution variation, computer simulation was implemented. Monte Carlo simulation was chosen to follow grain growth in presence of incoherent particles varying their size, fraction and morphology. Results suggested a correlated effect between particle eccentricity and fraction to control growth inhibition. Moreover, particle size controlled better growth kinetic, as it was possible to inhibit particle shape effect by giving a size advantage to particles having a high symmetry.
Abstract: The current literature reports the quantitative analysis of the kinetics of grain growth
influenced by second-phase particle mechanisms for a powder metallurgy nickel-base superalloy: APK-6. Annealing treatments in the superalloy are shown to involve coarsening/dissolution of γ’ particles, and these particles mechanisms are shown to influence the kinetics of grain growth. The grain-growth exponent, n, is computed, and the γ’-solvus temperature of the superalloy is determined to lie between 220 and 270 oC. The kinetic data is interpreted to establish dependence of
γ’ particles coarsening/dissolution mechanisms, grain size, γ’-solvus temperature, and annealing time and temperature on the rates of grain growth in the superalloy.
Abstract: The usual expression to calculate the limiting grain radius due to particle pinning is
RL=ar/f, where r is the particle radius, f is the volume fraction of the particles, RL is the limiting grain radius and a is a constant equal to 1/6 according to Rios[Acta Metall. Vol. 35 (1987) p. 2805]. This form is not convenient for comparison with experimental measurements. In this work two alternative expressions are proposed: 1)SVL=3SVP where SV is the grain boundary area per unit of volume and SVP is the interface area per unit of volume and 2)H=SVP where H is the total grain boundary curvature. These expressions are compared with experimental measurements carried out in a high purity Al-1mass%Mn alloy containing Al6Mn precipitates annealed for 3600 s at temperatures ranging from 500 to 620 oC. Good agreement is obtained between theory and experiment. The relative merits and shortcomings of each expression are discussed in detail from a fundamental and experimental point of view. It is concluded that the first expression is probably the
best choice for practical purposes.
Abstract: Aluminum alloys exhibit recrystallization kinetics that vary strongly with composition. The conventional understanding is that certain alloying elements, e.g. chromium, retard grain boundary motion due to the formation of fine dispersions of second phase particles, giving rise to particle drag of boundaries. There is countervailing evidence, however, that suggests that solute drag provides a stronger influence on grain boundary mobility. This paper presents new evidence for a pronounced effect of solute based on experiments in which individual boundaries migrate under the driving pressure of stored energy from prior plastic strain. As supported by the literature, boundaries exhibit a maximum mobility for a 38-39 degree <111> misorientation in initial annealing experiments. Specifically, this mobility maximum is asymmetric with a sharp cutoff below 38-39 degrees but a more gradual decrease at misorientations beyond 40 degrees. The occurrence of other, smaller mobility peaks is discussed within the context of the sharpening of evolving maxima with discussed within the context of the sharpening of evolving maxima with increased recrystallization. The presence of a minimum at 38-39 degrees is found at both higher temperatures and higher solute concentrations. This transition from a local mobility maximum to a minimum is discussed within the context of recent theories solute drag activity.
Abstract: The distribution of sizes for grain growth in presence of pinning centers (Zener pinned growth) is communicated at different times. The experimental approach uses the well-known similitude between growth in polycrystalline aggregates and
cellular soap froths. Two-dimensional results are communicated with grain growth
limited by a set of randomly distributed rounded pins.
Abstract: An original model, based on a variational formulation for boundary motion by viscous
drag, is developed to simulate single grain boundary motion and its interaction with particles. The equations are solved by a 3D finite element method to obtain the instantaneous velocity at each triangular element on the boundary surface, before, during and after contact with one or more particles. After validation by comparison with some simple, analytical and numerical cases, it is adapted to model curvature driven grain growth. For single phase material, the single grain boundary model closely matches the grain coarsening kinetics of a 3D multi boundary vertex model.
In the presence of spherical incoherent particles the growth rate slows down to give a growth exponent of 2.5. When the boundary is anchored there is a significantly higher density, by a factor of 4, of particles on the boundary than the density predicted by the classic Zener analysis, and many particles exert less than this Zener drag force. As a result the Zener drag is increased by a factor of about 2.2. The limiting grain radius is compared with some experimental results.
Abstract: We describe a general approach to obtaining 3D microstructures as input to computer simulations of materials properties. We introduce a program called MicroConstructor, that takes 2D micrographs and generates 3D discrete computer microstructures which are statistically equivalent in terms of the microstructural variables of interest. The basis of the code is a genetic algorithm that evolves the 3D microstructure so that its stereological parameters match the 2D data. Since this approach is not limited by scale it can be used to generate 3D initial multiscale microstructures. This algorithm will enable microstructural modellers to use as their starting point, experimentally based microstructures without having to acquire 3D
information experimentally, a very time consuming and expensive process.