N. K. Mukhopadhyay

On the short-range order in Al-Mn quasicrystals during low-temperature ageing

We report the evolution of diffuse intensity during the low-temperature ageing of Al-Mn quasicrystals. This is taken as evidence of short-range order in the icosahedral phase prior to its decomposition. The implication of these diffuse intensities is discussed.

An analysis of microhardness of single-quasicrystals in the Al–Cu–Co–Si system

Abstract Microhardness tests were conducted on single grain quasicrystals of the decagonal phase in a Al–Cu–Co–Si system by varying the load from 15 to 500 g on the ten-fold plane and two-fold planes. Ultrasonic tests were also performed on ten-fold plane and its perpendicular planes. No significant difference was found in the microhardness and elastic properties measured on these two planes. The microhardness was found to be load dependent, varying from 9.01 to 7.86 GPa. The variation of hardness i.e. the indentation size effect (ISE) was attributed to both elastic recovery (important at small loads) and to cracking. The cracks arising during indentation up to a 200 g load were observed to be the Palmqvist type. Lateral cracks other than Palmqvist were also observed at 300 and 500 g loads. At 15 and 25 g loads, cracks were not observed, indicating a limited amount of plasticity. At a 100 g load the four corners of the indentation yielded well-developed and symmetrical radial cracks without any lateral cracks. The fracture toughness ( K IC ) was calculated from the fracture mechanics relation developed for Palmqvist type of cracking. The measured value was 1.40±0.1 MPa √ m , comparable to other data reported in the literature for quasicrystals. A simple model of cracking at indentations equating the release of stored elastic energy to the work of fracture, was used to explain the critical indentation size observed for crack nucleation.

An investigation of the failure of low pressure steam turbine blades

Abstract An analysis of the failure of LP turbine blades of a 210 MW thermal power plant has been presented in this paper. The blade material is of 12% Cr steel with tempered martensitic microstructure. Microstructural analysis as well as hardness and tensile tests did not indicate any degradation in terms of microstructure and mechanical properties. Physical discontinuities were observed in the braze joint which might have been formed due to improper brazing operation. Failure of the brazed joints between the blade and lacing rod was found to be due to improper brazing operations and corrosion effects during service. Fractographic evidence showed that the cracks were initiated from various points on the blade surface, which were at the interface with the lacing rod. Striations and beach marks were also observed which indicated the occurrence of high cyclic loading on the blades. Frequency data obtained from plant indicated the possibility of excessive vibration generated due to fluctuation in grid frequency during operation. Thus, the situation was aggravated due to a resonant condition of vibration, facilitating the propagation of cracks which were initiated earlier.

An electron microscopic study of quasicrystals in a quaternary alloy : Mg32(Al, Zn, Cu)49

The discovery of a solid exhibiting m 3 5 point group symmetry by Shechtman et. al. (l) in a rapidly solidified Al-14at%Mn alloy has activated intensive studies of a new class of solids, termed as quasicrystals (2). While the original discovery reported the existence of quasicrystals in AI-Mn. AI-Fe and AI-Cr alloys, subsequent work has revealed their existence in Mg-Zn-Al(3,4), Mg-A]-Cu(5), AI-Mn-Si(6) and Ti-Ni-V(7) alloys (Table l).

A comparative electron microscopic study of Al-based and Mg-based quasicrystals

It is shown that both icosahedral Al-Mn-Si and Mg-Al-Zn alloys give rise to the same variety of electron diffraction patterns as documented for icosahedral Al-Mn alloys. Subtle variations in intensity are ascribed to a different decorational motif in terms of the Mackay icosahedron and the Pauling triacontahedron. Icosahedral Al-Mn-Si alloys do not appear to be ordered on a superlattice basis.

Evolution of superlattice order in Al–Mn quasicrystals and its relation to face-centred icosahedral quasicrystals

We have established a simple icosahedral to face-centred icosahedral ordering transformation in Al-Mn quasicrystals. This result strongly supports the view that the recently discovered face-centred icosahedral quasicrystal in ternary Al-Cu-Fe and related alloys represents a long-range superstructure of simple icosahedral quasicrystals

Rational approximant structures and phason strain in icosahedral quasicrystalline phases

The degeneracy of the reciprocal space with respect to the icosahedral symmetry has been demonstrated by experimental diffraction patterns obtained from rapidly solidified A1-Mn-Si, Mg-A1-Ag, Mg-AI-Zn alloys. The distortion in diffraction patterns has been attributed to rational approximant (RA)structures. The distorted pattern obtained from Mg-A1-Zn system has been indexed based on 3/2 RA structure. The shift parameter has been quantified and shown to vary linearly with the pseudospace counterpart of the corresponding quasicrystalline reciprocal vector, which is the characteristic feature of phason strain.

Transformation of the decagonal quasicrystalline phase to a B2 crystalline phase in the Al±Cu±Co system by high-energy ball milling

Samples of a decagonal quasicrystalline phase, located in the Al-Cu-Co system and synthesized by a slow cooling technique, have been mechanically milled in a high-energy planetary ball mill for 10, 20 and 30h. The milled powders, as well as powders that had been annealed (after milling) for times ranging from 30 to 150min at 600C, were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. A phase transformation from the decagonal phase to a B2 crystalline phase during highenergy ball milling is reported here for the first time. Powders milled for more than 10h contained predominantly the B2-type crystalline phase with a lattice parameter of 0.29nm. This crystalline phase was found to be quite stable after milling for 30h and also on subsequent annealing at 600C. These experimental results lend support to an earlier suggestion that the decagonal phase in Al-Cu-Co is actually less stable than the B2 phase at low temperatures.

Synthesis and microhardness measurement of Ti–Zr–Ni nanoquasicrystalline phase

Abstract The Ti–Zr–Ni forms one of the interesting systems of quasicrystals and related structures. It is the only system in Ti-based quasicrystalline phases which belongs to Bergman class and at the same time gives rise to a stable quasicrystalline structure. We report the formation of nanoquasicrystalline phase directly from melt spinning of molten Ti 53 Zr 27 Ni 20 alloy. Such a phase has been obtained at an optimum copper wheel speed (40 m/s) and jet pressure of 90 atm. We have also measured the mechanical response of this material by the microhardness technique. It shows much better ductility, strength and fracture toughness than the usual alloy having micron-sized quasicrystalline phase. Even at higher load (200 g) no crack seems to appear. We do observe shear bands suggesting the better toughness of bulk material containing nanoquasicrystalline/nanocrystalline phase.

Nanomechanical characterization of Al–Co–Ni decagonal quasicrystals

This paper addresses some issues related to the nanomechanical responses of decagonal quasicrystals using nanoindentation techniques. A Hysitron Triboscope with a Berkovich indenter was used to carry out the nanoindentation tests on single crystals of Al–Co–Ni decagonal phase. The anisotropy in terms of nanohardness at higher load was observed, whereas it was not possible to establish the same at lower loads. However, the Youngs modulus was found to be indistinguishable in all these cases. Discontinuity in load displacement, which is known as a ‘pop-in’ effect, was observed frequently at various load regime. It was found that this discontinuity was not due to cracking or a phase transformation effect, but to a plastic yielding phenomena and nanodeformation of the material. Significant differences between the nanohardness and microhardness were noticed and are discussed based on various experimental parameters and the consequent mechanical response of materials.