σ-Phase Formation in Super Austenitic Stainless Steel During Directional Solidification and Subsequent Phase Transformations

The solidification path and the σ-phase precipitation mechanism in the S31254 (UNS designation) steel are investigated thanks to Quenching during Directional Solidification (QDS) experiments accompanied by scanning electron microscopy observations and electron backscattered diffraction (EBSD) analysis. Considering experimental conditions, the γ-austenite is found to be the primary solidifying phase (1430 °C), followed by δ-ferrite (1400 °C, ≈ 87 pct solid fraction). The σ-phase appears in the solid-state through the eutectoid decomposition of the δ-ferrite: δ → σ + γ2 (1210 °C), whereas the σ-phase is predicted to form from the austenite at 1096 °C in equilibrium conditions. The resulting temperatures of solidification path and phase transformation are compared with Gulliver–Scheil model and equilibrium calculations predicted using Thermo-Calc© software. It is shown that the thermodynamics calculations agree with experimental results of solidification path. The EBSD analysis show that the δ-ferrite has δNW2 ORs with the σ-phase

Solidification path and phase transformation in super-austenitic stainless steel UNS S31254

The solidification path and the σ-phase precipitation mechanism of UNS S31254 alloy were studied on the basis of directional solidified experiments accompanied by scanning electron microscopy observations and energy dispersive X-ray a nalysis. The resulting temperatures of solidification paths and phase transformation were compared with Gulliver-Scheil and equilibrium calculations predicted using ThermoCalc© software. It was confirmed that the experimental solidification path was in agreement with the thermodynamic calculations. The complementarity of the results have made it possible to propose a solidification path and a σ-phase precipitation mechanism for the UNS31254 steel

Influence of Minor Cr Additions on Crystal Growth in Rapidly Solidified Al-20Zn Alloys

It has been discovered quite recently that Icosahedral Short-Range Order (ISRO) of atoms in the liquid phase of metallic alloys surrounding some trace elements added to the melt can influence both the nucleation and growth of the primary phase. In this work, Al-20wt.%Zn alloys without and with 0.1 wt.% Cr additions have been processed using a free-falling droplet technique. This technique allows to undercool the liquid droplet during its fall and thus to have rapid directional solidification conditions when it collides a copper-cooled substrate. Under such rapid solidification conditions, microstructural and EBSD analyses have shown that, under such rapid solidification conditions, Cr addition is responsible for the nucleation and growth of feathery grains (or twinned dendrites). This morphology specific to aluminum alloys has been discovered more than seventy years ago without a clear identification of its origin. The angular analysis between twinned dendrites indicates a behavior similar to those of the propagation of topological defects, through an ISRO-induced stacking fault mechanism

Packing of sedimenting equiaxed dendrites

International audienceThe packing of free-floating crystal grains during solidification has a strong impact on the phase- change process as well as on the structure and the defects in the solidified material. The packing fraction is affected by the particular dendritic morphology of the grains and by their low inertia re- sulting from the small density difference between solid and liquid. Understanding the grain packing phenomenon during metal alloy solidification is not experimentally possible since packing is coupled to many other phenomena. We therefore investigate the packing of equiaxed dendrites on a model system, consisting of fixed-shape non-convex model particles sedimenting in conditions hydrodynam- ically similar to those encountered in solidifying metals. We perform numerical simulations by a discrete-element (DEM) model and experiments with transparent liquids in a sedimentation column. The combination of experiments and simulations enable us to determine the packing fraction as a function of: (i) the grain morphology, expressed by a shape parameter, and (ii) the hydrodynamic conditions, expressed by the particle Stokes number

Packing dynamics of spherical and nonconvex grains sedimenting at low Stokes number

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Effect of Inoculant Alloy Selection and Particle Size on Efficiency of Isomorphic Inoculation of Ti-Al

International audienceThe process of isomorphic inoculation relies on precise selection of inoculant alloys for a given system. Three alloys, Ti-10Al-25Nb, Ti-25Al-10Ta, and Ti-47Ta (at %) were selected as potential isomorphic inoculants for a Ti-46Al alloy. The binary Ti-Ta alloy selected was found to be ineffective as an inoculant due to its large density difference with the melt, causing the particles to settle. Both ternary alloys were successfully implemented as isomorphic inoculants that decreased the equiaxed grain size and increased the equiaxed fraction in their ingots. The degree of grain refinement obtained was found to be dependent on the number of particles introduced to the melt. Also, more new grains were formed than particles added to the melt. The grains/particle efficiency varied from greater than one to nearly twenty as the size of the particle increased. This is attributed to the breaking up of particles into smaller particles by dissolution in the melt. For a given particle size, Ti-Al-Ta and Ti-Al-Nb particles were found to have a roughly similar grain/particle efficiency

Three Dimensional Methodology to Characterize Large Dendritic Equiaxed Grains in Industrial Steel Ingots

International audienceThe primary phase grain size is a key parameter to understand the formation of the macrosegregation pattern in large steel ingots. Most of the characterization techniques use two-dimensional measurements. In this paper, a characterization method has been developed for equiaxed dendritic grains in industrial steel castings. A total of 383 contours were drawn two-dimensionally on twelve 6.6 cm 2 slices. A three-dimensional reconstruction method is performed to obtain 171 three-dimensional grains. Data regarding the size, shape and orientation of equiaxed grains is presented and thereby shows that equiaxed grains are centimeter-scale complex objects. They appear to be a poly-dispersed collection of non-isotropic objects possessing preferential orientations. In addition, the volumetric grain number density is 2.2 × 10 7 grains/m 3 , which compares to the 0.5 × 10 7 grains/m 3 that can be obtained with estimation from 2D measurements. The 2.2 × 10 7 grains/m 3 value is ten-times smaller than that previously used in the literature to simulate the macrosegregation profile in the same 6.2 ton ingot

Microsegregation Model Including Convection and Tip Undercooling: Application to Directional Solidification and Welding

International audienceThe microsegregation behavior of alloy filler metal 52 (FM 52) was studied using microprobe analysis on two different solidification processes. First, microsegregation was characterized in samples manufactured by directional solidification, and then by gas tungsten arc welding (GTAW). The experimental results were compared with Thermo-Calc calculations to verify their accuracy. It was confirmed that the thermodynamic database predicts most alloying elements well. Once this data had been determined, several tip undercooling calculations were carried out for different solidification conditions in terms of fluid flow and thermal gradient values. These calculations allowed the authors to develop a parametrization card for the constants of the microsegregation model, according to the process parameters (e.g., convection in melt pool, thermal gradient, and growth velocity). A new model of microsegregation, including convection and tip undercooling, is also proposed. The Tong-Beckermann microsegregation model was used individually and coupled with a modified Kurz-Giovanola-Trivedi (KGT) tip undercooling model, in order to take into account the convection in the fluid flow at the dendrite tip. Model predictions were compared to experimental results and showed the microsegregation evolution accurately