Microstructure and solidification analysis of melt-spun AlTi and AlTiB alloys

Abstract

Two alloy compositions (Al5wt.% Ti and Al4.6wt.%Ti–0.2wt.%B) were cast using the melt spinning process to provide a ribbon approximately 100 μm thick which could be used to investigate the effect of rapid solidification on an as-cast structure. The alloys were commercial-grade grain-refining master alloys used to control the grain structure in as-cast Al alloys. Equilibrium Al 3Ti phase particles, with sizes ranging from 5 to 100 μm, were major components in these master alloys before remelting and rapid solidification. The microstructure of the rapidly solidified alloys was examined using scanning electron microscopy and transmission electron microscopy. Four different morphologies of L1 2-type metastable intermetallic particles were observed in a region within 30 μm of the chill surface of the ribbon. These included (1) single-phase cuboidal particles, (2) nanocellular cuboidal particles, (3) duplex structure (single phase + nanocellular) cuboidal particles and (4) nanocellular angular particles. The average size of the cubodial particles was 0.3 μm, while the average size of the angular particles was about 0.6 μm. The growth morphologies observed in (2) and (3) are reported here for the first time. The different structures found within the particles are the result of different growth rates during solidification. The growth rate of L1 2-type metastable intermetallic particles is estimated to be up to 0.7 m s −1. The formation of a single phase or a nanocellular structure inside metastable intermetallic particles is believed to be related to the absolute stability of the smooth front—a mechanism which is generally used to account for the transition of plane front growth to a cellular structure for α-Al phase during rapid growth. This paper presents the details of the as-cast microstructure of the alloys and describes factors that affect the internal structure, the size and the shape of the L1 2-type metastable intermetallic particles over the ribbon thickness.