| dc.description.abstract | The celebrated discovery of quasicrystals has broadened our outlook towards the various possibilities of atomic arrangements in the solid state. Many fascinating problems in the field of quasicrystals have emerged, which need attention to enhance the status of our understanding. The aim of the present study is to address some of the issues related to the synthesis, structure and stability of icosahedral (I) and decagonal quasicrystals.
A number of selected Al-base (Al??Mn??Ge?, Al??Mn??Si?, Al??Mg?Cr?, Al??Mn??) and Mg-base (Mg??(Al,Zn)??, Mg??(Al,Zn,Cu)??, Mg??(Al,Ag)??) alloys were prepared under controlled atmosphere. Rapid solidification was accomplished using the gun quenching, melt spinning and twin roller techniques. The characterization of the phases was performed by TEM and XRD techniques. For theoretical modelling and simulation, HP-1000, a 16-bit mini computer has been employed.
SYNTHESIS
Three new alloy systems have been shown to give rise to the formation of I-phase after rapid solidification. Two systems among these three, namely, Mg??(Al,Zn,Cu)?? and Mg??(Al,Ag)?? alloys, have been proved to be members of rational approximant (RA) structures of the I-phase. Another one, i.e., Al??Mg?Cr? phase, has been shown not to belong to either pentagonal Frank-Kasper (FK) or RA structures, even though it has quasicrystal formability. All the guidelines which have been advanced for quasicrystal formability have been critically analysed in the light of present experimental data. It has been shown that icosahedral quasicrystal structure (ICS) criterion is better applicable as the basis for synthesis rather than those based on structure maps and phase diagram features.
We have shown by e/a (electron/atom) calculation of all our alloy systems that the I-phase can be treated as a Hume-Rothery phase. The experimental data is close to the value calculated in literature (Friedel and Denoyer; Bancel and Heiney) based on Fermi surface-Brillouin zone interaction. The classification of Al-Mn-Si and Mg-Al-Zn class quasicrystals is best understood from the plot aR/a (quasicrystal constant / average atomic spacing) vs. e/a parameter, which has been proposed in the present investigation.
The effect of ternary additions (such as Si and Ge) in Al-20% Mn has been studied. It has been established by the systematic tilting based on the stereographic approach that the addition of Si does not lead to a new type of quasicrystal as claimed by Chen and Chen. The superlattice ordering due to Si addition has been ruled out experimentally and also from geometrical considerations. We have not found any decagonal quasicrystal in Al-Mn-Ge system. Based on structural argument Schaefer and Bendersky have established the existence of decagonal phase in Al-Mn-Ge. Our experiments do not confirm the above observation. However, we have noticed more diffraction spots in Al-Mn-Si compared to Al-Mn-Ge. This has been attributed to ordering among the atoms in the structure due to Si but not the superlattice ordering in the sense implied by Chen and Chen.
An unusual banded microstructure has been observed in the roller quenched Al-10% Mn alloy. All the theories based on interface instability have been evaluated. Qualitatively we have proposed a new model based on the nucleation and growth mechanisms of alternate I-phase and ?-Al during solidification using the metastable phase diagram reported in the literature.
STRUCTURE
The various sections of the reciprocal space of Al-Mn-Si and Mg-Al-Zn-Cu have been explored and compared. Some subtle differences between them have been recorded. The quasilattice constants in Al-Mn-Si and Mg-Al-Zn-Cu were shown to be 4.63 Å and 5.15 Å respectively. We were the first to draw attention to this feature. Based on our data the classification scheme of quasicrystals has been proposed by Henley and Elser. The intensity differences between these two systems have been attributed to decoration effect. The computation of the diffraction patterns of the quasilattice structure with decoration and without decoration has been performed. The decoration was made using Mackay icosahedral (MI) clusters and Pauling triacontahedral (PT) clusters in Al-Mn-Si and Mg-Al-Zn-Cu structures respectively. From the calculation many interesting features have been noted and partial success with the decoration model has been pointed out.
The decagonal phase structure has been modelled as the periodic arrangements of 2D PT sheets in real space structure. The diffraction along 10-fold axis was computed by taking a Fourier transform of the window function in the orthogonal space. From the experimental data available in the literature, attempts were made to find out the quasilattice parameter and to propose a new indexing scheme. The redundancy problem during indexing in the incommensurate plane has been tackled by the least path criterion developed in the present study.
The twinning of the icosahedral grain has been noticed in Al-10% Mn alloy. The twin axis is found to be one of the five-fold axes of I-phase. The symmetry of the composite patterns has been derived to be 10/mmm, which is similar to that of decagonal phase. The simulation of the reciprocal space on the basis of hyperdimensional projection has been successfully worked out. The bi-quasicrystallography concept has to be introduced to understand further the details of the twin boundary and also grain boundaries in quasicrystals. | |
