Synthesis and structural aspects of quasicrystals in Mg-Al-Ag system: Mg 4 Al 6 Ag
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I.
INTRODUCTION
A F T E R the discovery of binary quasicrystals Eq in aluminum transition metal systems, attempts have been made to understand the effect of ternary additions. [2,3m Special mention may be made of the role of silicon which enhances the stability of quasicrystals, fS~All these A1 based quasicrystals can be classified as belonging to the A1Mn-Si class.t6] Quasicrystals have also been synthesized in Mg-A1-Zn, tTj Ti-V-Ni, [81 and U-Pd-Si t91 alloy systems. It now appears that there are two major classes of quasicrystals: A1-Mn-Si and Mg-A1-Zn/6~ The structure of these two classes can be described in terms of quasiperiodic arrangements of the Mackay icosahedral clusters in A1Mn-Si and Pauling triacontahedral clusters in Mg-A1-Zn quasicrystals, r1~ These special types of clusters can be identified in the respective crystalline c~-(A1-Mn-Si)[13~ and T-Mg32(A1-Zn)48 structures, t14~ After surveying all the experimental results, it is reasonable to explore the underlying rationale governing the successful synthesis of the icosahedral phases. At present, there are three approaches for the basis for synthesis: (1) icosahedral crystal structure criterion (ICS) which has a crystallographic basis; tTm (2) quantum structural diagram (QSD), which has a quantum basis; tt51 and (3) the phase diagram features. According to ICS, crystal phases which possess a predominant short-range icosahedral order can give rise to the icosahedral phase on rapid solidification. Alternately based on the quantum properties of the individual elements, the formability of the icosahedral phases can be predicted by QSD. In fact, QSD criteria have found applications in isolating complex ternary phases ~16~ and high temperature superconducting phases. [171A third criterion stresses the occurrence of quasicrystals in systems where phase diagram features peritectic reactions t~8,19~ and is an echo of the criterion for glass formation in systems displaying deep eutectic reaction. It is worthwhile to study the complex crystalline phases to understand the synthesis, structure, and stability of quasicrystals. By QSD, it is possible to synthesize quasiN.K. MUKHOPADHYAY, Graduate Student, K. CHATTOPADHYAY, Associate Professor, and S. RANGANATHAN, Professor, are with the Department of Metallurgy, Indian Institute of Science, Bangalore-560012, India. Manuscript submitted July 6, 1988. METALLURGICAL TRANSACTIONS A
crystal, but structural modelling is difficult. In fact, the issue as to whether the transformation between quasicrystal and crystal phases is of group-subgroup type is an interesting one and is treated only by the crystallographic basis. Earlier, we had established the occurrence of quasicrystals in a quaternary alloy T-Mg32(m1Znfu)48, [2~ based on the crystallographic basis. In this paper, we have followed the ICS basis to synthesize the quasicrystal in the Mg-A1-Ag system where the existence o f a complex crystalline phase has been reported by Wheeler et al., [2n and the structure has been confirmed to be identical to that of T-Mg32(AI
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