Ultramlcroscopy 40 (1992) 265-270 llhti~llll~l North-Holland Morphology of silicon grain boundaries in Sr-modified AI-Si eutectic alloys by HREM M Shamsuzzoha, P A Deymler Department of Matertals Sctence and Engmeermg, Unwerstty of Artzona, Tucson, AZ 85721, USA and David J Smith Center for Sohd State Sctence and Department of Phystcs, Artzona State Unwerstty, Tempe, AZ 85287, USA Received 6 January 1991, at Edltorml Office 28 August 1991 Crystallographic changes m the ~'3(110)/{111}, ~f3(110)/{112} and .~9(110)/{112} boundaries reduced by the presence of strontmm m the slhcon phase of St-modified A1-SI alloys have been studied by high-resolution electron microscopy The 2'3 boundaries possessing {111} and {112} planes m the pure silicon phase often exhibit asymmetric morphologies m the Sr-modlfled phase Interaction of various 23(110)/{111} boundaries results m both 23(110)/{112} and 2'9(110)/{112} boundaries The resulting ~9 boundaries also show asymmetrical mchnatlon 1. Introduction Impurmes can have a very strong effect on the morphology of grain boundaries For example, it ts well estabhshed that segregatton at gram boundartes m a wide range of matermls promotes the formation of new gram boundary configura- tions such as faeetmg [1-3] Unfortunately, very httle is known about the crystallographic struc- ture of chemtcally modified boundaries Stgmficant advances have been made towards understanding the atomic structure of gram boundaries in pure metals, ceramics and semicon- ductors For instance, determmattons of the atomtc structure of pure symmetrical-tilt high-an- gle colnodent boundaries m slhcon and germa- nmm such as ~3(110)/{111} [4], 23(110)/{112} [5] and 29(110)/{112} [6] by high-resolution electron microscopy (HREM) have firmly estab- hshed that such boundaries assume a symmetric morphology in their equlhbrmm conflgurat|on Moreover, it is always possible to descrtbe the boundartes m terms of s~mple structural units, and tetrahedral coordination is maintained every- where so that there are no danghng bonds associ- ated with the (periodic) structural umts [6] Htgh-resolutlon electron microscopy can also be apphed to obtam mformatton about the struc- ture and morphology of similar grain boundartes m doped SI Hence, we have recently directed our attention to Sr-~mpurtty-lnduced morphological changes m Z3(l10)/{lll}, Z3(110)/{112} and ~9(110)/{112} grain boundaries m sthcon using HREM Th~s paper describes some h~gh-resolu- tlon imaging of the morphological changes in gram boundaries present m the sthcon phase of Sr-modlfied AI-St eutechc alloys 2. Experiment AI-12 7wt%S1 eutectlc alloys with 0 05% Sr were vacuum cast from A1 and S1, each of 99 999% 0304-3991/92/$05 00 © 1992 - Elsevier Science Pubhshers B V All rights reserved