Amber Genau

Morphological characterization of the Al–Ag–Cu ternary eutectic

Abstract Ternary eutectic microstructures have a level of complexity far exceeding that of binary eutectics. As there is currently no detailed description of ternary eutectic structures in the literature, we have undertaken a systematic investigation of the structures formed by directional solidification of the ternary eutectic Al–Ag–Cu. Using a constant temperature gradient of 3 K mm−1 and solidification velocities between 0.2 and 5 μm s−1, we are able to observe different forms of the eutectic structure with varying degrees of alignment. We describe these structures both qualitatively and quantitatively, using measures including the volume fraction and composition of all phases, the distribution of interphase separations, and the number of nearest neighbor phases. The effects of solid-state transformations on the microstructure are also discussed.

Crystal orientation and morphology in Al-Ag-Cu ternary eutectic

Ternary eutectics provide a unique opportunity for studying the effects of complex microstructure formation, as three distinct phases must be formed simultaneously from the melt. In order to produce fully coupled three-phase growth, Al-Ag-Cu at the ternary eutectic composition was directionally solidified in a constant temperature gradient of 3 K/mm at velocities between 0.2 and 5.0 μm/sec. Under these conditions, the two intermetallic phases appear to grow as closely coupled rods in an α(Al) matrix, with the solidification velocity affecting the specific morphologies chosen by the rods and the general degree of alignment of the structure. Crystal orientations were examined by EBSD to determine if variations in morphology within a single sample are due to specific orientation relationships. Although no conclusive connection to morphology has yet been found, two different sets of orientation relationships between the three phases have thus far been identified.

A model for eutectic growth in multicomponent alloys

A model for eutectic growth of multicomponent alloys is proposed. Among several other modifications brought to the classic theory of eutectic growth, the model introduces the phase fraction change as the main mechanism for maintaining an isothermal solid/liquid interface during solidification of eutectic alloys. A comparison between the theoretical results and experimental measurements for the Fe-C based alloys showed a good agreement for alloys of composition not too far removed from the eutectic point. The analysis also showed that an earlier proposed criterion for spacing selection of binary irregular eutectics, based on volume fraction of the faceted phase, can be applied to multicomponent alloys as well. The investigation performed on the effect of Si on the growth kinetics of Fe-C-Si eutectic alloys showed that the growth coefficient μ decreases with the increase of Si content.

The Morphological Evolution of Low Volume Fraction Tin Dendrites During Coarsening

Tin dendrites tend to grow in closely spaced, parallel sheets; thus, within a certain range of solid volume fraction, it is possible to obtain samples with both dense dendritic regions and dendrite-free regions of liquid. Such samples were produced by directionally solidifying Pb-69.1 wt pct Sn, allowing us to compare tin dendrite structure and coarsening in a traditional dense mushy zone with the same dendrites in much lower volume fraction solid regions. The morphology of the dendrites, both in the dense and less-dense regions is analyzed using three-dimensional reconstructions obtained by serial sectioning. Quantitative measurements of these complex structures were obtained by calculating interfacial curvature and interfacial normal distributions, and the spatial correlations of interfacial curvature. We find that the spatial correlation measurement can be used to determine average secondary or tertiary arm length. We find also that coarsening proceeds in this system by both welding of secondary arms and dissolution and growth via long-range diffusional interactions and that the microstructure becomes more morphologically anisotropic as coarsening proceeds.

Alloying effects on graphite spacing in gray iron

Even with decades of study, the complex development of solidification microstructures in cast iron is incompletely understood. In order to observe the effect of ternary and quaternary alloying additions on the solidification behaviour and graphite spacing of gray iron, directional solidification in a Bridgman furnace was used to carefully control solidification conditions. Specifically, alloys with industrially relevant amounts of silicon and manganese were studied. Average spacings for five compositions, containing varied amounts of Si and Mn, and velocities from 0.5 to 10 μm/s are reported. A critical velocity of around 1 μm/s was observed, above which the graphite structure loses directionality and austenite dendrites appear. Spacing measurements are compared to historical data and a new model, highlighting the importance of sulphur in determining spacing. Spacing measurements obtained in the traditional way were compared to results obtained using a recently developed automated MATLAB algorithm. The automated results compare favourably with traditional manual measurements and will allow for more robust measurement of eutectic spacing in a variety of systems where the spacing is highly irregular.

Characterization of Directionally Solidified Gray Iron

Even with decades of study, the complex development of solidification microstructures in cast iron is incompletely understood. Because Fe-C eutectic can produce different morphologies, and even different phases, depending on growth velocities and composition, understanding the conditions under which each forms is important. Directional solidification was used to investigate the effects of alloying additions and solidification velocity on graphite spacing in gray iron. Average and minimum spacing for five compositions, containing varied amounts of Si and Mn, and velocities from 0.5 to 5 µm/s are reported. A critical velocity of around 1 µm/s was observed, above which the graphite structure loses directionality and austenite dendrites appear. A semi-automated MATLAB code was developed for quickly and objectively measuring graphite spacing. The automated results compare favorably with traditional manual measurements, and will allow for more robust measurement of eutectic spacing in systems where the spacing is highly irregular.