Formation and Growth of A15 Type Nb<SUB>3</SUB>Al in a Nb/Al Composite at 1873-2073 K: Part I. Planar Growth of the A15-Phase Layers

Abstract

Formation of A15 type Nb 3 Al phase at the interface between solid Nb and liquid Al in a bulky Nb/Al composite has been studied at as high temperatures as from 1873 to 2073 K. The A15-phase layer grows keeping the interfaces planar when heated at 2023 K or lower, whereas the interface adjacent to Nb solid solution, Nb S.S , becomes ''nonplanar'' when heated at 2073 K. It is suggested that the formation process of A15 phase consists of three stages; the rapid dissolution of Nb into liquid Al until nucleation of A15 phase at the Nb/Al interface, and the following two stages (1st and 2nd ones) for growth of A15 phase. The planar growth rate of A15-phase on the 1st stage is considerably faster than that on the 2nd one. A model for the planar growth rate of A15-lphase on the 2nd stage has been presented. The model is based on the unidirectional inter-diffusion fluxes of Nb and Al atoms in the three sequential regions of Nb S.S , A15 and '-phases. The ''nonplanar'' growth of A15-phase will be detailed in Part II of the present study.