Characterising precipitate evolution in multi-component cast aluminium alloys using small-angle X-ray scattering

Aluminium alloys can be strengthened significantly by nano-scale precipitates that restrict dislocation movement. In this study, the evolution of inhomogenously distributed trialuminide precipitates in two multi component alloys was characterised by synchrotron small angle Xray scattering (SAXS). The appropriate selection of reference sample and data treatment required to successfully characterise a low volume fraction of precipitates in multi-component alloys via SAXS was investigated. The resulting SAXS study allowed the analysis of statistically significant numbers of precipitates (billions) as compared to electron microscopy (hundreds). Two cast aluminium alloys with different volume fractions of Al3ZrxV1-x precipitates were studied. Data analysis was conducted using direct evaluation methods on SAXS spectra and the results compared with those from transmission electron microscopy (TEM). Precipitates were found to attain a spherical structure with homogeneous chemical composition. Precipitate evolution was quantified, including size, size distribution, volume fraction and number density. The results provide evidence that these multi-component alloys have a short nucleation stage, with coarsening dominating precipitate size. The coarsening rate constant was calculated and compared to similar precipitate behaviour

Eutectic colony formation: A phase field study

Eutectic two-phase cells, also known as eutectic colonies, are commonly observed during the solidification of ternary alloys when the composition is close to a binary eutectic valley. In analogy with the solidification cells formed in dilute binary alloys, colony formation is triggered by a morphological instability of a macroscopically planar eutectic solidification front due to the rejection by both solid phases of a ternary impurity that diffuses in the liquid. Here we develop a phase-field model of a binary eutectic with a dilute ternary impurity and we investigate by dynamical simulations both the initial linear regime of this instability, and the subsequent highly nonlinear evolution of the interface that leads to fully developed two-phase cells with a spacing much larger than the lamellar spacing. We find a good overall agreement with our recent linear stability analysis [M. Plapp and A. Karma, Phys. Rev. E 60, 6865 (1999)], which predicts a destabilization of the front by long-wavelength modes that may be stationary or oscillatory. A fine comparison, however, reveals that the assumption commonly attributed to Cahn that lamella grow perpendicular to the envelope of the solidification front is weakly violated in the phase-field simulations. We show that, even though weak, this violation has an important quantitative effect on the stability properties of the eutectic front. We also investigate the dynamics of fully developed colonies and find that the large-scale envelope of the composite eutectic front does not converge to a steady state, but exhibits cell elimination and tip-splitting events up to the largest times simulated.Comment: 18 pages, 18 EPS figures, RevTeX twocolumn, submitted to Phys. Rev.

The Effect of Degassing on Grain Refinement in Commercial Purity Aluminum

Degassing of molten aluminum is used to remove dissolved hydrogen and impurity particles prior to casting. The most common method, rotary degassing, gives a small bubble size and distributes the bubbles throughout the melt by means of vigorous stirring. Although this is an efficient method for hydrogen removal, the purge gas may also inadvertently remove grain refining particles and thus reduce refinement efficiency. During degassing, particle removal occurs by physical attachment to the degassing bubbles and flotation, by turbulent transport due to the flow field generated by stirring, or by sedimentation. The experiments reported here were undertaken using a static graphite degassing lance (i.e., nonrotating) in order to prevent the formation of strong flow fields and to quantify the rate of grain refiner loss due to bubble attachment and floatation. By varying either (a) gas flow rate, (b) grain refiner addition level, or (c) type of grain refiner, it was found that, although grain size increased with time, the increase was predominantly due to particle sedimentation and not caused by attachment