Amorphous phase formation in Al 70 Si 17 Fe 13 alloy
- PDF / 1,089,217 Bytes
- 4 Pages / 595.44 x 841.68 pts Page_size
- 82 Downloads / 182 Views
Recently, formation of amorphous aluminiumtransition metal alloys by vapor deposition was shown to be possible.1 It was found that the amorphous phase is formed in the limited composition range of about 75-90 at. % Al where this composition range is closely associated with the existence of complex intermetallic compounds. The substitution of Si for some of the Al makes possible the formation of amorphous phases by rapid liquid quenching. Thus amorphous phases have been reported in melt-quenched Al-based Al-Fe-Si, 2 " 5 AlMn-Si,4'6-7 and Al-F-Si (T= Cr, Co) 4 ' 8 alloys. Study of a structure of amorphous Al 70 Si j 7 Fe j 3 alloys by neutron diffraction shows a strong chemical order over distances as large as 15 A.3 This order was found to be related to the order in the quasicrystalline icosahedral phase. Similar conclusions about a structural relation between amorphous and icosahedral phases have been obtained for Al-Mn, where the observed structure with diffuse diffraction rings was called microquasicrystalline.9 This Rapid Communication reports the effect of the cooling rate on the microstructure of a rapidly quenched Al 70 Sii 7 Fe] 3 alloy. We have used electron-beam surface melting and melt spinning to study this effect. Phase separation, with amorphous phase forming as a primary phase, was found in this system and is discussed. Alloy bars were prepared by arc melting of 99.99% Al, electrolytic iron, and silicon. The samples were turned over and remelted several times to minimize segregation. Surface melting was carried out in a vacuum of 10 ~ 3 Pa using a 25 kV beam of electrons. The electron beam was scanned across the sample surface at velocities ranging from 10-500 cm/s. Thin slices were removed parallel to the melted surface, ground and electropolished from one side for transmission electron microscopy (TEM) examination. The melt-spun ribbons were produced from the same arc-melted alloy bars. Small pieces cut from these buttons were induction-heated in zirconia-coated J. Mater. Res. 2 (4), Jul/Aug 1987
quartz tubes. Immediately upon melting, the metal was squirted onto a copper wheel rotating at velocities ranging from 2000-7000 rpm. The process was carried out under helium at atmospheric pressure. The TEM specimens were prepared from the ribbons by standard electropolishing technique. The electron-beam surface melting was used to produce a wide range of solidification velocities10 or melt undercooling.'' The solidification velocity of primary dendrites (or eutectic) is proportional to the electronbeam scan velocity in the cases where growth kinetics are not a limitation (for example, Ag-Cu alloys10). However, in concentrated alloy systems where the primary solidification phase is a faceted intermetallic compound, considerable supercooling can be developed in the trailing part of the melt pool. In these cases the various phenomena associated with undercooled liquids can occur: formation of an amorphous solid or nucleation of quasicrystals or other metastable or stable phases (for example, Al-Mn alloys11)
Data Loading...
