Solidification Behaviour of Al Particles Embedded in an Ni Aluminide Matrix
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ABSTRACT A hypereutectic A1-40wt%Ni alloy has been manufactured by melt spinning, and the resulting microstructure examined by transmission electron microscopy. As-melt spun hypereutectic Al-40wt%Ni consists of an Ni aluminide matrix and an Al-rich phase distributed in the form of particles with sizes - 50-100 nm, and as an irregular layer at the cell and grain boundaries. Diffraction analysis of the Ni aluminide matrix is consistent with the ASTM x-ray diffraction standard 20 values for the orthorhombic NiAl 3 phase, a=6.6114 A, b =7.3662 A and c=4.8112 A. The solidification nucleation kinetics of Al-rich particles have been examined by heating and cooling experiments in a differential scanning calorimeter over a range of heating and cooling rates. Solidification of the Al-rich phase at the cell and grain boundaries nucleates catalytically on the surrounding Ni aluminide matrix at an undercooling of - 3 K. Analysis of the solidification nucleation kinetics of the Al-rich phase in AI-40wt%Ni supports the hypothesis [1-4] that the classical spherical cap model of heterogeneous nucleation breaks down at low undercoolings and small contact angles. INTRODUCTION The nucleation of solidification in ingots and castings is important in determining microstructural features such as phase composition, grain size and structure, and second phase particle distribution, which influence final material properties. Nucleation usually takes place by a heterogeneous process, with solid nuclei forming on an external catalytic surface. Unfortunately there are serious experimental difficulties in studying the heterogeneous nucleation of solidification in ingots and castings [1,5], because of the difficulty of excluding extraneous impurities which can otherwise act as heterogeneous nucleation catalysts. An experimental embedded particle technique designed to give reproducible measurements of the liquid undercooling during nucleation by a known heterogeneously nucleating substrate was first devised by Wang and Smith [6] and subsequently used by Chadwick and co-workers [7-11]. A binary alloy is thermally manipulated to produce a microstructure of low melting point particles embedded in a higher melting point matrix, and is then heat treated to melt the particles and monitor subsequent particle solidification. Embedded particle experiments of this type are particularly suitable for controlled studies of heterogeneous nucleation, with the solidification of the particles nucleated catalytically by the surrounding high melting point matrix. More recently Cantor and co-workers [1-4,12-19] have used the embedded particle technique to investigate heterogeneous nucleation in a number of binary monotectic and immiscible eutectic alloys prepared by rapid solidification. For Al alloys, grain refinement is of particular interest, to provide reproducible properties for subsequent working, to disperse brittle constituents which may otherwise lead to deleterious internal stresses, and to maximise casting speed during continuous or semicontinuous casting w
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