Search

• Microgravity experiments yield nucleation and kinetic parameters. • Neopentylglycol-Camphor shows a change in growth orientation. • Composition effects on interface properties are discussed. We investigate the effect of composition of hypoeutectic Neopentylglycol-(D)Camphor (NPG-DC) alloy on nucleation and microstructure kinetics during alloy solidification in low gravity conditions without convection and buoyancy effects. Two alloys with compositions of 0.2 and 0.3 wt-frac. DC have been solidified in a small thermal gradient to obtain equiaxed dendritic microstructures with different crystallographic growth orientation. In-situ optical and thermal characterization in the transparent material provided evolution of equiaxed dendrite density, nucleation undercooling, dendrite tip velocity and kinetic law. Significant differences in the results for the two compositions were found, which are discussed in relation to temperature and concentration dependent solid–liquid interfacial properties. For NPG-0.2 wt-frac. DC equiaxed dendrites with 〈 100 〉 crystallographic growth orientation exist which tend to grow faster and fill the liquid volume quicker compared to the NPG-0.3 wt-frac. DC alloy. This behavior hinders nucleation of further dendrites and decreases their final number. In contrast, for NPG-0.3 wt-frac. DC equiaxed dendrites with 〈 111 〉 crystallographic growth orientation nucleate at higher undercooling and the kinetic law indicates smaller dendrite tip velocities correlated to a larger number of dendrites. [ABSTRACT FROM AUTHOR]

[Display omitted] • Microgravity results for alloy solidification including grain refinement. • Validated model of grain texture for solidified metallic alloy. • Progressive and sharp columnar to equiaxed transitions modelled. • No requirement for a blocking criterion as used in other models. Columnar and equiaxed structures, which occur during solidification of metallic alloys, influence the texture and properties of castings, welded joints and additively manufactured components. During transient solidification, where grain refiner particles provide the predominant nucleation mechanism, a Columnar to Equiaxed Transition (CET) occurs when conditions that had originally favoured directional columnar growth change to those favouring equiaxed. Constitutional undercooling ahead of the columnar front can permit equiaxed nucleation and growth. By carrying out experiments in microgravity conditions, liquid flows due to thermal and solutal buoyancy effects are suppressed. In these diffusion-controlled conditions, we have observed examples of both sharp (clear) and progressive (gradual) CET. The experimental outcomes, especially the observation of a progressive CET, has highlighted the need for a continuum model that allows for competitive columnar and equiaxed structure development; hence, the Concurrent Columnar to Equiaxed Transition (C2ET) model is proposed. The C2ET is thermally transient and relies on the well-known concept of extended growth for impingement mechanics; thereby, greatly reducing numerical complexity. Importantly, the proposed approach removes the need for a specific equiaxed-blocking criterion, which is often proposed as an essential requirement in other CET models. The C2ET model is validated by four experimental solidification scenarios: two velocity jumps and two thermal-gradient decreases. The velocity jumps induced sharp CETs; whereas, thermal-gradient decreases gave progressive CETs. The C2ET model gave good agreement for the columnar and equiaxed transition zones for both sharp and progressive CET. Results are compared with the classic Hunt model. Unlike Hunt's model, the C2ET model predicted all macrostructure transitions faithfully using a single (or consistent) set of nucleation input parameters across all four scenarios. Since, the same level of grain refinement was used in each experiment, a consistent set of nucleation parameters was expected. The validated approach can enable effective simulation at lower computational cost for industrial processes that rely on a solidification processing step. [ABSTRACT FROM AUTHOR]

A Nucleation Progenitor Function (NPF) approach that accounts for the interdependence between nucleation and growth during equiaxed solidification is proposed. An athermal nucleation density distribution, based on undercooling, is identified as a progenitor function. A Kolmogorov statistical approach is applied assuming continuous nucleation and growth conditions. The derived progeny functions describe the (supressed) distribution of actual nucleation events. The approach offers the significant advantage of generating progeny functions for volumetric (3D) data and projected image (2D) data. The main difference between 3D and 2D data in transparent alloy experiments is due to a stereological correction for over-projection. Progeny functions can be analysed to obtain statistical output information, e.g., nucleation counts, average nucleation undercooling and standard deviation. The statistical output data may be calculated in a formative (running) or a summative (final) mode. The NPF kinetics have been incorporated into a transient thermal model of equiaxed solidification. The model has been applied to characterise a microgravity solidification experiment with the transparent alloy system Neopentylgycol-30 wt%(d)Camphor. The model predicted thermal and observed nucleation and growth data with a good level of agreement. [ABSTRACT FROM AUTHOR]