K. Muraleedharan

Bio: K. Muraleedharan is an academic researcher from Defence Metallurgical Research Laboratory. The author has contributed to research in topics: Microstructure & Ohmic contact. The author has an hindex of 8, co-authored 18 publications receiving 230 citations.

The quasi-crystalline phase in the Mg—Al—Zn system

Abstract: Shechtman et al.1 have reported a phase in rapidly solidified Al86Mn14 alloy with long-range orientational order, but with icosahedral point-group symmetry, which is inconsistent with lattice translations. However, Pauling2 has argued that the apparent icosahedral symmetry in A186Mn14 alloy is due to directed multiple twinning of cubic crystals with a cube edge of 26.7 A. We report here the powder X-ray diffraction study of a rapidly solidified Mg32(Al, Zn)49 alloy which shows 5–3–2 symmetry diffraction in transmission electron microscopy (TEM). Also reported is a study of rapidly solidified Mg2A13 alloy. The advantage of these alloys is that β-Mg2Al3, which has nearly the same structure as proposed by Pauling for the rapidly quenched Al86Mn14, is an equilibrium phase. This is unlike the Al–Mn system, where, according to Pauling, the cubic phase occurs in a metastable and already twinned form. The present choice of alloy systems makes it possible to compare the X-ray diffraction patterns from the quasi-crystalline phase and a cubic phase with nearly the same structure as proposed by Pauling. We observe that the X-ray diffraction pattern of rapidly solidified Mg32(Al, Zn)49 alloy is distinct from those of the equilibrium phases and that the pattern can be completely indexed to an icosahedral phase. We argue that the above observation is inconsistent with the proposal that the material consists of an aggregate of twinned cubic crystals. Therefore, the alternative is to invoke a quasi-crystalline lattice to explain the observed 5–3–2 symmetry diffraction in TEM.

Studies on structural transformation and magnetic properties in Sm2Co17 type alloys

Abstract: Structural transformations and microstructural characterisation of Sm2Co17 alloys containing Fe, Cu and Zr at different stages of thermal processing have been investigated by X-ray diffraction, optical, scanning electron and transmission electron microscopes. Solution treated samples consist of a mixture of hexagonal TbCu7 (1:7 H) and rhombohedral Th2Zn17 (2:17 R) structure types of 2:17 phase. After isothermal aging, TbCu7 + Th2Zn17 structures transform into Th2Zn17 type structure with precipitation of Cu-rich hexagonal SmCo5 (1:5 H) and Zr-rich platelet phases. In addition to the main phases, a soft magnetic phase of composition Zr6(FeCo)23 is formed in alloys containing higher Zr composition. Isothermal aging studies reveal that magnetic properties show a peak value when aged at 1108–1123 K for 10 h. TEM studies show cellular precipitate structure with cell interiors having 2:17 R structure, while the fully coherent cell boundaries have the 1:5 H structure. Zr-rich platelets which run across many cells and cell boundaries were found to have 1:7 H structure.

Effect of titanium substitution on the structure and properties of Fe3Al-based intermetallic alloys

Abstract: Substitutions (0–16 at%) of titanium for iron in Fe3Al-based alloys rapidly solidified by chill-block melt-spinning stabilize the D03 and B2 ordered structures. Rapid solidification results in total suppression of D03 order in binary alloys, whereas alloys containing titanium have D03 structure. D03 and B2 antiphase domain size, lattice parameter and hardness increase with increasing titanium content of the alloy. The deformation mode changes from single (unit) bcc dislocation in binary alloys to two-fold superdislocation configuration in D03Fe3Al-Ti alloys. Mechanical antiphase boundaries are generated by the movement of these imperfect configurations. All the alloys exhibited cleavage tensile failure. The mechanical properties are correlated with the observed structural changes.

Precipitation and Hardening in Magnesium Alloys

Abstract: Magnesium alloys have received an increasing interest in the past 12 years for potential applications in the automotive, aircraft, aerospace, and electronic industries. Many of these alloys are strong because of solid-state precipitates that are produced by an age-hardening process. Although some strength improvements of existing magnesium alloys have been made and some novel alloys with improved strength have been developed, the strength level that has been achieved so far is still substantially lower than that obtained in counterpart aluminum alloys. Further improvements in the alloy strength require a better understanding of the structure, morphology, orientation of precipitates, effects of precipitate morphology, and orientation on the strengthening and microstructural factors that are important in controlling the nucleation and growth of these precipitates. In this review, precipitation in most precipitation-hardenable magnesium alloys is reviewed, and its relationship with strengthening is examined. It is demonstrated that the precipitation phenomena in these alloys, especially in the very early stage of the precipitation process, are still far from being well understood, and many fundamental issues remain unsolved even after some extensive and concerted efforts made in the past 12 years. The challenges associated with precipitation hardening and age hardening are identified and discussed, and guidelines are outlined for the rational design and development of higher strength, and ultimately ultrahigh strength, magnesium alloys via precipitation hardening.

Quasicrystals in as-cast Mg-Zn-RE alloys

Abstract: Since the epoch-making discovery of a quasicrystal (QC) in rapidly solidified Al-Mn alloys by Shechtman et al., numerous quasicrystalline alloys have been found. Most of these works were concerned with Al alloys. Rajasekharan et al. reported the quasicrystal in a rapidly solidified Mg-Al-Zn alloy in 1985, which was followed by work on the quasicrystal in rapidly solidified Ga-Mg-Zn alloy reported by Chen and Inoute. In this paper, the authors discuss the presence of quasicrystals in the Mg-Zn-RE system alloys.