Microhardness and Fracture Toughness Studies of Decagonal Quasicrystal in Al-Cu-Co System

Abstract

After the discovery of icosahedral (I) quasicrystals(QC) by Shechtman et al. [1], many more alloysystems exhibiting various classes of quasicrystalhave been reported [2]. Among the various types ofQCs, the icosahedral and the decagonal (D) QCshave been studied most extensively. I-phase isquasiperiodic in three dimensions with point group(m 3 5), while the D-phase is quasiperiodic in twoand periodic in the other dimension, with pointgroup 10=m mm. Although many metallic alloys areknown to form QC phases, only a few of them canreally be grown as large stable QCs, in order tounderstand the mechanical and other physicalproperties of these phases. Now it is understoodthat these quasiperiodic phases can be regarded as anew class of complex intermetallics.Bhaduri and Sekhar [3] reported hardness andtoughness properties on the stable icosahedral Al-Cu-Li, using Knoop’s indentation techniques. In asimilar approach, Wittmann et al. [4] performedVickers’ indentation and scratch testing on D-phasein the Al-Cu-Co-Si alloy. The hardness of quasi-crystals in the Al-Co-Ni system was also reported byTakeuchi et al. [5]. Plastic deformation studies werecarried out at various temperatures by compressiontesting on bulk Al-Ru-Cu samples [6], Al-Fe-Cu [7],and on other Al-base QCs [8]. Koster et al. [9]investigated the deformation and fractographicbehavior on Al-Cu-Fe quasicrystal in detail. Fromthese studies, it appears that QCs possess very highhardness, low toughness, and a high capability forelastic recovery resembling the properties of crystal-line intermetallics. However, the deformation me-chanism in QC at room temperature and hightemperature differed [6–8]. At room temperature itexhibits brittleness, whereas at high temperature itshows ductility due to the mobility of thermallyactivated distributions. When these phases arepresent along with other ductile crystalline phasesas a composite, limited ductility was observed inQCs. Further research in this direction is required formaking use of the properties of QC for sometechnological applications. The aim of the presentstudy is to investigate the mechanical properties ofthe D-phase, synthesized by a slow-cooling methodin Al-Cu-Co alloys by means of a hardness-indentation technique, and point out some interestingfeatures in relation to the mechanical response ofthis new type of intermetallics.Alloys with nominal composition Al