Segregated AIP phase in a near-rapidly solidified Al-5Si-1Ti alloy

Abstract

During transition electron microscopy (TEM) studies of microstructures of a near-rapidly solidified (strip casting) AI-5Si-ITi (wt %) alloy, AlP phase was observed near the crotch end of the crown form (tree-like) Si phase, with superlattice diffraction patterns in electron diffraction and fine domains on the TEM image. The formation of the AlP phase was caused by the chemical segregation of the purity P. A series of studies of aluminium phosphide have been reported on its atomic arrangement, electron structure and metamorphism in aluminium alloys [1-6]. Wentore [2] suggested a hexagonal structures for AlP while Wang and Spinar [1] and Addamiano [5] found AlP to be of the zinc-blende type (B3). TEM analyses of AlP phase in aluminium alloys have not been demonstrated before. In the present work, AI-5Si-ITi strips with a thickness of 250/xm were prepared by DC casting at a cooling rate of 103 Ks -1. The strips were mechanically worn and jet-electropolished, and then ionpolished for examination in a JEM 2000-FX transmission electron microscope. Fig. 1 indicates that there were two phases distributed on the dentritic structures of the A1 matrix: the larger crown form Si phase (marked A) located on the dentritic boundaries; the smaller AlP phase (marked B) near the crotch end of the Si phase. These were defined by electron diffraction patterns, as shown in Fig. 2 and Fig. 3a. A sequence of diffraction patterns of Si and AlP phases with the unit stereographic triangle were obtained on tilting from [1 i 1], [001], [101] to [1 1 1]. The diffraction patterns of the Si phase could be indexed to be of diamond type with a cell parameter of 0.5404 nm; those of the AlP phase could be indexed as facecentred cubic (B3 type) with a cell parameter of 0.5428 nm, in agreement with Wang and Spinar (a = 0.5467 nm) [1] and Addamiano (a = 0.5451) [5]. The unit cell of B3-ordered superlattices is shown in Fig. 3b. Fig. 4a and b shows the (1 1 1) bright-field (BF) and dark-field (DF) electron micrographs while Fig. 5a and b shows those of (1 f 1). These all reveal the presence of extremely fine B3 domains. The presence of the extremely fine domains indicates that the ordering reaction occurred during quenching or near-rapid solidification. However, no Ti-enriched phases were found in the alloy, which indicates that Ti has been trapped in the A1 matrix at the cooling rate of 103 Ks -1. In this instance, Ti was more easily dissolved in the AI matrix than Si and P. This probably resulted from the strong interaction between Ti and vacancies in the A1 saturation [7]; therefore, the dissolution of other elements in the A1 matrix was restrained. In addition, since its crystal structure including the cell parameter was very near that of Si, the AlP is usually used as a nucleus of Si in conventional A1-Si alloy castings [6]. In this work, no evidence was found showing that the AlP helped Si to nucleate, for example, no constant orientation relationship was found between AlP and Si phases. One reason was that the P content in the alloy was too low, the other was that the rapid solidification has a strong ~ effect on the relative kinetics of the formation of the competing solid phases.