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FORMATION OF METASTABLE STRUCTURES IN DEEPLY UNDERCOOLED ALLOYS

Objective


Four alloy systems have been studied with respect to the microstructure development using the three processing techniques: Ni-Cu (complete), dilute solutions of C in Ni, Fe-rich Fe-Ni alloys, and Fe-Cr-Ni alloys on the 69 at% Fe isopleth. For Fe-Ni and Fe-Cr-Ni, the selection process of the primarily formed crystalline phases - stable or metastable - has been investigated. The microstructures obtained in all alloy systems studied were classified as either coarse-grained denditric or grain-refined equiaxed. Grain-refined microstructures are formed at low and high undercoolings. At intermediate undercoolings, coarse, dendritic microstructures prevailed. This clearly demonstrates the important role played by the melt undercooling in microstructure evolution. The sequence of microstructures has been successfully explained by a recently developed model for dendrite fragmentation. The characteristic differences between the refined droplets formed at low and high undercoolings were utilized to interpret the microstructures found in the spray methods (atomization and drop-tube processing), for which it has thus become possible to establish a link between the the droplet size and the undercooling reached prior to nucleation. The results obtained with atomization and drop-tube processing supplement each other with respect to the droplet size. They have been condensed in microstructure-selection maps with droplet size and alloy composition as variables. The results obtained with the levitation technique have been summarized in selection maps showing the microstructure and/or the primary formed crystal phase with the undercooling and the alloy composition as parameters. Computer modelling of these maps was fairly satisfactory.

The characteristic features of the texture of levitated droplets vary systematically with the undercooling. Similar variations have been found in droplets produced in the spray methods. Thus it has been demonstrated that investigations of the texture enable one to estimate the undercooling reached in atomization, something which was not possible up till now.

The combined results obtained with the main techniques used - levitation, drop-tube, atomization - have significantly deepened the understanding of droplet solidification processes. For the spray methods, fundamental links have been established between droplet diameter and undercooling by studying characteristic features of the microstructure and of the texture, showing that smaller droplets reach higher undercoolings. The identification of dendrite fragmentation as a key process in the microstructure evolution, and its theoretical description, have led to a predictive capability for similar alloy systems with respect to microstructure formation and phase-selection in droplet processing.
Rapid solidification is now well established as a technique for producing alloys in metastable structural states far from equilibrium with extraordinary properties. The novel microstructures exhibit refinement of scale, extended solid solubility, metastable crystalline, quasicrystalline and amorphous phases. In commercial practice rapid solidification is achieved by rapid external heat extraction, using melt spinning, planar flow casting, spray deposition and atomization. Direct observation and in-situ diagnostics of rapid solidification are very difficult in such processes. On the other hand, rapid solidification is also achieved by highly undercooling the melt, providing in this way a large driving force for crystallisation. This method will be subject of the present proposal giving access for direct diagnostics and even external stimulation of rapid solidification.

Large undercoolings will be achieved by containerless processing using electromagnetic levitation in the earth laboratory and in space as well as drop tubes and atomization. The scientific interest will focus on direct investigations of nucleation and growth of stable versus metastable microstructures in order to develop a comprehensive diagram capable to predict quantitatively the formation of microstructures of different morphology and state as a function of alloy concentration and undercooling. The selection of the alloys to be investigated will be done in agreement with the endorsing enterprises. Most of the work will be on Ni and Fe based alloys.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Deutsche Forschungs- und Versuchsanstalt für Luft- und Raumfahrt eV
Address
Linder Höhe
51147 Köln
Germany

Participants (1)

UNIVERSITA DEGLI STUDI DI PADOVA
Italy
Address
Via Venezia 1
35174 Padova