High resolution electron microscopy studies of rure earth-rich phase particles in a melt-quenched Ti-5Al-4Sn-2Zr-1MO-0.25Si-1Nd alloy indicate that the α-Ti matrix crystallizes prior to the rare earth-rich phase particles. According to a new nucleation model, the heteropeneous nucleation of the rare earth-rich phase particles at the matrix interface is easier than the homogeneous nucleation off the interface matrix, which is consistent with the results of the HREM obseroations.
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