Equiaxed crystals

About: Equiaxed crystals is a research topic. Over the lifetime, 5898 publications have been published within this topic receiving 100065 citations.

Modification of alpha morphology in Ti-6Al-4V by thermomechanical processing

Abstract: The modification of lamellar alpha phase in Ti-6A1-4V by hot working was investigated with the aim of controlling morphology (aspect ratio) and final grain size. The effect of strain was studied using forging at 955 °C (1750 °F), followed by annealing at 925 °C (1700 °F) to allow the alpha morphology to adjust. Increasing the deformation from 6.5 pct to 80 pct reduction caused the lamellar alpha morphology to become progressively more equiaxed upon annealing. TEM observations showed that annealing of material deformed to 6.5 pct resulted in recovery of the alpha, without a noticeable change in the morphology, while higher deformation resulted in plate shearing and beta cusp formation. It was found that material with an initial thin alpha plate structure (thickness — 3.4 ώm) breaks up at a lower critical strain than a material with a thicker plate morphology (thickness ≃ 6 μm). The material with thin alpha plates more rapidly forms equiaxed alpha grains separated by beta phase, while the material with a thicker plate structure exhibits more alpha/alpha boundaries after deformation and annealing. The morphology change from alpha lamellae into lower aspect ratio grains was identified to be by a break-up of the alpha lamellae, essentially by a two-step process: a formation of low and high angle alpha/alpha boundaries or shear bands across the alpha plates followed by penetration of beta phase to complete the separation. This break-up takes place during hot deformation and subsequent annealing.

Modeling of equiaxed microstructure formation in casting

Abstract: A general micro/macroscopic model of solidification for 2-D or 3-D castings, valid for both dendritic and eutectic equiaxed alloys, is presented. At the macroscopic level, the heat diffusion equation is solved with an enthalpy formulation using a standard FEM implicit scheme. However, instead of using a unique relationship between temperature and enthalpy (i.e., a unique solidification path), the specific heat and latent heat contributions, whose sum equals the variation of enthalpy at a given node, are calculated using a microscopic model of solidification. This model takes into account nucleation of new grains within the undercooled melt, the kinetics of the dendrite tips or of the eutectic front, and a solute balance at the scale of the grain in the case of dendritic alloys. The coupling between macroscopic and microscopic aspects is carried out using two time-steps, one at the macroscopic level for the implicit calculation of heat flow, and the other, much finer, for the microscopic calculations of nucleation and growth. This micro/macroscopic approach has been applied to one-dimensional and axisymmetric castings of Al-7 pct Si alloys. The calculated recalescences and grain sizes are compared with values measured for one-dimensional ingots cast under well-controlled conditions. Furthermore, the influence of casting conditions on temperature field, undercooling, grain size, and microstructural spacings is shown to be predicted correctly from axisymmetric calculations with regard to the expected experimental behavior.

Separation of second phase particles in spheroidized 1045 steel, Cu-0.6pct Cr alloy, and maraging steel in plastic straining

Abstract: Experiments were performed on spheroidized 1045 steel, Cu-06 pct Cr alloy, and maraging steel containing respectively Fe3C, Cu-Cr, and TiC particles of nearly equiaxed shape The local interfacial stresses for separation of these particles during plastic deformation were evaluated by the methods described in the two preceding papers The results show that the interfacial strengths for these particles in their respective matrices are 242, 144, and 264 ksi In the spheroidized steel the average diam of the separated particles is distinctly larger than the average diam of the whole population This is quantitatively explained by the enhanced interfacial stresses developed in regions of above average volume fraction of second phase which frequently occur in very dense populations of particles No such effect was observed in the other two systems which is consistent with their much lower volume fraction of second phase Some tension experiments have also been performed with the spheroidized 1045 steel at elevated temperature, giving results qualitatively similar to those at room temperature

Flow visualization and residual microstructures associated with the friction-stir welding of 2024 aluminum to 6061 aluminum

Abstract: The friction-stir welding (FSW) of 0.6 cm plates of 2024 Al (140 HV) to 6061 Al (100 HV) is characterized by residual, equiaxed grains within the weld zone having average sizes ranging from 1 to 15 μm, exhibiting grain growth from dynamically recrystallized grains which provide a mechanism for superplastic flow; producing intercalated, lamellar-like flow patterns. These flow patterns are visualized by differential etching of the 2024 Al producing contrast relative to 6061 Al. The flow patterns are observed to be complex spirals and vortex-like, among others, and to change somewhat systematically with tool rotation (stirring) speed between 400 and 1200 rpm; depending on tool orientation. The equiaxed grain and sub-grain microstructures are observed to vary according to estimated temperature profiles (varying from 0.6 to 0.8 TM, where TM is the absolute melting temperature) referenced to the rotating tool axis. Dislocation spirals and loops are also observed in the 2024 Al intercalation regions within the weld zones at higher speeds (>800 rpm) corresponding to slightly elevated temperatures introducing dislocation climb, and residual microhardness profiles follow microstructural variations which result in a 40% reduction in the 6061 Al workpiece microhardness and a 50% reduction in the 2024 Al workpiece microhardness just outside the FSW zone.