Dominic Phelan

High-temperature laser-scanning confocal microscopy as a tool to study the interface instability during unsteady-state solidification of low-carbon steel.

Solidification microstructure is a defining link between production techniques and the mechanical properties of metals and in particular steel. Due to the difficulty of conducting solidification studies at high temperature, knowledge of the development of solidification microstructure in steel is scarce. In this study, a laser‐scanning confocal microscopy (LSCM) has been used to observe in situ and in real‐time the planar to cellular to dendritic transition of the progressing solid/liquid interface in low carbon steel. Because the in situ observations in the laser‐scanning confocal microscopy are restricted to the surface, the effect of sample thickness on surface observations was determined. Moreover, the effect of cooling rate and alloy composition on the planar to cellular interface transition was investigated. In the low‐alloyed, low‐carbon steel studied, the cooling rate does not seem to have an effect on the spacing of the cellular microstructure. However, in the presence of copper and manganese, the cell spacing decreased at higher cooling rates. Higher concentrations of copper in steel resulted on an increased cell spacing at the same cooling rates.

Atomic Scale Simulation of Deformation in Magnesium Single Crystals

The deformation behaviour of magnesium single crystals under plane strain conditions has been examined using molecular dynamics modelling. The simulations were based on an existing atomic potential for magnesium taken from the literature. A strain of 10% was applied at rates of 3x109s-1 and 3x107s-1. The simulations predicted the formation of mechanical twins that accommodated extension in the c-axis direction of the hexagonal unit cell. However, the predicted twin is not of the same kind found in magnesium, but is that commonly observed in titanium. It is believed that further analysis of the physical properties predicted by this interatomic potential will shed more light on the atomic processes controlling twinning in Magnesium alloys. It also highlights the need for improvements to the interatomic potential such that more accurate deformation behaviour can be attained.

Experimental Studies into Strip Casting of Steel

The strip casting of steel, whereby liquid steel is solidified between twin water cooled copper rolls directly into its final shape, is a radical, energy efficient, cost effective route for the production of steel products that also provides exciting opportunities for the development of new products. An experimental program is currently underway to study phenomena associated with rapid solidification of steel using levitating droplet techniques and Gleeble®3500 thermo-mechanical processing. For example, studies have been conducted to investigate the heat transfer, nucleation behaviour and microstructure development during solidification of a low carbon steel and a peritectic steel on copper substrates hard coated electrolytically or using Filtered Arc Deposition (FAD). It was found that peak and average heat fluxes were significantly higher for steels solidified on the first substrate than for the FAD coated substrates. Maximum heat flux on the respective substrates was 36.5 to 39.0 MW/m2 and 8.3 to 9.4 MW/m2. The average heat flux on the respective coated substrates ranged between 9.6 to 12.5 and 5.5 to 6.6 MW/m2.

Influence of Undercooling on the Kinetics of the Peritectic Transition in an Fe-4.2wt%Ni Alloy

In-situ Laser Scanning Confocal Microscopy observations are presented that assess the influence of undercooling before the initiation of the peritectic transition in a Fe-4.2wt%Ni alloy on the resulting kinetics of the peritectic reaction and transformation. In a series of experiments varying the cooling rate, increasing the cooling rate led to a lower temperature at the L/ interface. The resulting peritectic reaction changed from slow 840m/s - 1500m/s, with limited growth into the  to rapid ~11mm/s with significant growth into . In continuous cooling experiments when the nucleation temperature was low, growth into  was high and the reacting species was observed to propagate along the liquid/delta-ferrite interface at a rate of ~11mm/s. The peritectic reaction rate did not appear to be a function of temperature over a measured nucleation temperature range of 5 K. Conversely, the growth rate of austenite into the delta-ferrite in the first 0.03 seconds was observed to increase from 1.5mm/s to 8mm/s as the measured temperature at nucleation decreased.

Synthesis of Intermetallics Based on the Fe-Al-Si-Zn Alloy System by Magneto-Mechanical Milling of Ductile Elemental Powders

The Zincalume hot-dip coating process is a well-established technique for excellent corrosion protection of steel products. This paper describes the study of two intermetallics based on the Fe- Al-Si-Zn alloy system, α-AlFeSi and α-AlFeSi(+Zn), pertinent to the Zincalume process. These intermetallics are difficult to characterize in-situ due to their formation at high temperature and because they form as very thin intermetallic layers on a steel substrate, which inhibits accurate quantitative analysis. Controlled magneto-ball milling and hot-pressing have been employed in an attempt to synthesise these intermetallic compounds. Magneto-ball milling, under a He atmosphere, was conducted in shearing mode to enable controlled milling of elemental powders, namely Zn and Al, without the excessive cold-welding often associated with milling of ductile powders with high coefficients of surface friction. XRD analysis indicated that uniform mixtures of highly reactive fine-structured powders were produced, as indicated by diffuse elemental peaks of low intensity. Hot-pressing was utilized to sinter the powders into compact intermetallic compounds without sintering aids. The intermetallics were characterized by DTA, XRD and elemental contrast mapping performed on an SEM with EDS.