77 research outputs found
Non-equilibrium solidification of concentrated Fe-Ge alloys
The solidification of concentrated alloys containing ordered compounds is less well understood. These alloys often exhibit complex phase change like peritectic reaction during liquid to solid transformation. The Fe-rich part of Fe-Ge binary alloy system consists of several critical points and ordered-disorder transitions and can be used as a model system to study the effect of departure from equilibrium on the solidification microstructure. In order to understand the phase selection and morphological transitions, undercooling and recalescence behaviour; growth rate of the solidifying phases and microstructure need to be explored. In the present paper, we summarise the results obtained in several iron rich alloy compositions (Fe-(14-25) at.% Ge) using techniques of melt quenching, levitation and laser resolidification. These results provide insight to the current theories of dendritic growth and reveals possibility of a new pathway for phase evolution in peritectic alloys at high undercooling involving a massive transformation
Parameter-Free Test of Alloy Dendrite Growth Theory
In rapid alloy solidification the dendrite-growth velocity depends sensitively on the deviations from local interfacial equilibrium manifested by kinetic effects such as solute trapping. The dendrite tip velocity undercooling function was measured in dilute Ni~Zr! over the range 1–25 m/s and 50–255 K using electromagnetic
levitation techniques and compared to theoretical predictions of the model of Trivedi and colleagues for dendritic growth with deviations from local interfacial equilibrium. The input parameter to which the model predictions are most sensitive, the diffusive speed VD characterizing solute trapping, was not used as a free parameter but was measured independently by pulsed laser melting techniques, as was another input parameter, the liquid diffusivity DL. Best-fit values from the pulsed laser melting experiment are VD526 m/s and DL
52.731029 m2/s. Inserting these values into the dendrite growth model results in excellent agreement with experiment with no adjustable parameters.Engineering and Applied Science
Solidification of undercooled peritectic Fe-Ge alloy
Bulk samples of Fe-25 at.% Ge peritectic alloy are undercooled up to 260 K using electromagnetic levitation technique. The growth rate of the primary phase is measured using a capacitance proximity sensor technique. Solidification microstructure is studied as a function of undercooling. The microstructure of samples at low undercoolings consists of a residual primary phase α2, peritectic phase ε and inter-dendritic ε-β eutectic. Microstructure at higher undercoolings is nearly phase-pure ε . Time resolved diffraction analysis of the levitated droplets using synchrotron radiation indicates the nucleation of primary α2 in all cases. The growth rate is analysed using current theories to explain the experimental observations. Interfacial undercooling is found to play an important role in the growth kinetics. Our results also suggest suppression of peritectic reaction
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