Influence of Ir Additions and Icosahedral Short Range Order (ISRO) on Nucleation and Growth Kinetics in Au-20.5Wt Pct Cu
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URTULDU et al. have presented recently a new mechanism of nucleation in fcc alloys based on icosahedral quasicrystals (iQC) or clusters formation in the liquid phase and acting as pre-nuclei for the fcc phase. This so-called ‘‘iQC-mediated nucleation’’ mechanism was evidenced first in Al-20wt pctZn:Cr,[2] and then in Au-12.5wt pctAg-12.5wt pctCu:Ir.[1] It is still a conjecture but based on several strong experimental facts, mainly based on EBSD observations: (i) The addition of minute amounts of Cr in aluminum alloys or Ir in gold alloys drastically reduces the grain size with an abnormal fraction of twinned grain boundaries; (ii) Several assembly of nearest-neighbor (nn) grains exhibit multiple-twins (MT) relationship, with a common h110i direction; (iii) Up to 5 nn fcc grains have MT relationship compatible with the symmetry of a decahedron and up to 9 nn grains exhibit a MT symmetry compatible with an icosahedron, (iv) Some nn grains exhibit a MT [1,2]
J. ZOLLINGER, B. ROUAT, and S.K. PILLAI are with the Institut Jean Lamour, Universite´ de Lorraine, Alle´e Andre´ Guinier ARTEM Campus, 54000 Nancy, France. Contact e-mail: [email protected] J. GUYON is with the Laboratoire d’Etude des Microstructures et de Me´canique des Mate´riaux, Universite´ de Lorraine, 7 rue Fe´lix Savart, 57070 Metz, France. M. RAPPAZ is with the Institute of Materials, E´cole Polytechnique Fe´de´rale de Lausanne, Station 12, 1015 Lausanne, Switzerland. Manuscript submitted December 4, 2018. Article published online February 25, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
relationship compatible with the facets of a so-called interlocked icosahedron. The formation of twins and MT relationships between nn grains become clear when heteroepitaxy between icosahedral and fcc phases are considered, with f111gfcc -planes and h110ifcc -directions perpendicular to threefold and twofold symmetry axes of an icosahedron, respectively. Approximant phases exhibiting icosahedral building blocks as well as iQC’s are known in Al-Cr and formation of (Al,Zn)7 Cr-type iQC are thermodynamically compatible in Al-Zn alloys even at low Cr composition.[2–4] Although Tsai-type QC have been identified in Au49 Al34 Tm17 ,[5] Au51 Al34 Yb15 and Au44 Ga41 Yb15 ,[6] and Au44:2 Ca14:1 In41:7 ,[7] there is no known intermediate phases between Ir and Au, Ag or Cu, and thus no fivefold symmetry clusters or QC.[8–10] Yet, the potency of Ir- and Ru-additions for the grain refinement of gold alloys is known in dental and jewellery industries for more than a century, mainly for yellow gold alloys,[11–13] and this mechanism has been attributed recently to iQC-mediation.[1] A first question then arises: is the grain refinement with Ir additions to gold alloys sensitive to composition variations, such as Al-Zn-Cr alloys are? No literature has been found to determine whether such mechanism would be efficient for other Au-Cu-Ag gold alloys. Minute additions decrease atoms mobility in the liquid phase by promoting Icosahedral short range order (ISRO), as shown for Cr in Al
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