U.D. Kulkarni

Microstructure and properties of a Cu-Cr-Zr alloy

Abstract Copper-based dilute Cu–Cr–Zr alloys and their minor modifications, because of their excellent thermal conductivity, strength and fatigue resistance, are commonly used in an aged condition in heat transfer elements. However, in comparison to dilute Cu–Cr binary alloys in which the precipitation of chromium has been studied extensively in the last two decades, attempts at delineating the morphology, composition and crystallography of precipitates in Cu–Cr–Zr alloys have been few and only partially successful. The role of zirconium has also remained largely unresolved. In the present work, the precipitation in an alloy having a nominal composition of Cu–1 wt% Cr–0.1 wt% Zr has been shown to take place through the formation of a metastable ordered fcc phase. Also, the improvement in fatigue resistance due to the addition of zirconium has been ascribed to lowering of the stacking fault energy (SFE) of the alloy.

Precipitation in a Cu-Cr-Zr alloy

Transmission electron microscopy (TEM) investigations of the early stages of precipitation of a b.c.c. phase in f.c.c. matrix have been carried out in a dilute Cu-Cr-Zr alloy. The initial product of decomposition, characterized by a state of order, which mimics a cube-on-cube orientation relationship (OR) with the matrix appears prior to ordered b.c.c. phase precipitates showing Nishiyama-Wassermann (N-W) OR.

On the sequence of clustering and ordering in a meltspun Cu–Ti alloy

Abstract Although it is well established that dilute Cu–Ti alloys with titanium in the range of 2.5–5 wt.% decompose by a spinodal mechanism, the sequence of ordering and clustering processes in the early stages has been a matter of controversy. An attempt has been made in this work to resolve some of the issues by carrying out transmission electron microscopy (TEM) investigations on a dilute meltspun Cu–Ti alloy. Decomposition was studied as a function of ageing over a temperature range of 573–723 K. The results seem to suggest that clustering precedes the formation of long range ordered (LRO) phases in this alloy. Formation of a transitory special point N 3 M phase was observed for the first time in this work. This disappeared on prolonged ageing giving rise to the metastable Cu 4 Ti (D1 a ).

Laser surface alloying of nickel by molybdenum and aluminium— microstructural studies

Abstract It is now well established that a considerable improvement in the mechanical and chemical properties of the near surface regions of materials may be achieved by the use of high energy laser beams. By manipulating the laser power density and the time of interaction of the laser beam with an appropriately coated work piece, it is possible to achieve a surface chemistry that would have an improved resistance to wear, fatigue and corrosion failures. The change in chemistry at the surface is attained through the process of melting and mixing of the coating and a thin layer of the substrate. Solidification of this molten region at the surface results, due to an interplay of various forces, in the development of very complex microstructures. To analyse these in a piece of nickel that had been coated with a mixture of molybdenum and aluminium powders and then treated with a continuous wave CO 2 laser, extensive transmission electron microscopy was done on a thin foil obtained from near the bottom region of the recast pool. The foil was found to have a cellular microstructure comprising cells of the Ni 3 Al (γ′) ordered phase. In the intercellular regions, two dispersed phases, namely, martensitic Ll 0 phase and Ni 2 Al 3 , and a contiguous phase, identified as a close derivative of the equilibrium δ-NiMo phase were found. A gradient in the degree of order could be observed within the γ′ cells. In the central portion of the cells, rapid solidification resulted, by the process of sequential ordering, in the development of a partially ordered alloy with very fine domains. In the peripheral regions of the cells, the alloy solidified by direct ordering into larger domains. This paper discusses some of these results.

A near icosahedral quasiperiodic phase in rapidly solidified Mg32Al49

Abstract We report the occurrence of a low symmetry quasiperiodic derivative of the icosahedral phase in a rapidly solidified Mg32Al49 alloy. Selected area diffraction patterns obtained from this phase have been indexed using the projection formalism.

On the relationship between cubic crystalline coincidence site lattices and quasiperiodic superlattices

Quasiperiodic superlattice (QPSL) can be understood as a structure consisting of quasiperiodic arrangements of tiles of rational dimension and can be viewed as a quasiperiodic atomic decoration of a disordered lattice. Varieties of QPSLs have been generated by varying the orientation of the projection plane in the projection formalism. The coincidence site lattice (CSL) has been found to be related to the interfaces of these QPSLs. The structural relationship between the CSLs and the QPSLs related to the octagonal quasilattice has been pointed out for the first time. The physical significance of CSLs have been discussed in the context of the projection formalism.

Microstructural studies on rapidly solidified Mg32 (AlxZn1−x)49 potentially quasicrystal forming alloys

Abstract Rapidly solidified microstructures of four different alloys in the Mg 32 (Al x Zn 1− x ) 49 series, Mg 32 Al 49 , Mg 32 Al 24.5 Zn 24.5 , Mg 32 Al 17 Zn 32 and Mg 32 Al 7 Zn 42 were examined using transmission electron microscopy. A single-phase microstructure comprising grains of a low symmetry quasiperiodic derivative of the icosahedral phase was found to be present in the first alloy, while icosahedral phases showing considerable distortion of the icosahedral symmetry were found to coexist with different crystalline phases in the second and the third alloys. Certain features, notably sectorial division and particulate contrast, were found to be common to the icosahedral phases in the second and the third alloys and the derivative of the icosahedral phase in the first alloy. The extent of departure from ideal icosahedral symmetry appears to be a function of the [Al]/[Zn] ratio. The crystalline phase in the third alloy was identified as MgZn 2 (C14), which is a well known tetrahedrally close-packed phase. This phase comprises the bulk of the fourth zinc-rich alloy. A propensity for faulting was observed in the MgZn 2 phase in both these alloys. These and several other finer microstructural features are discussed in the light of the experimental evidence and relevant analyses of earlier reports on this system.

Formation of metastable phases in rapidly solidified Mg32(AlxZn1-x)49 alloys

In the pentagonal Frank-Kasper phases, a large proportion of the atoms are present in an icosahedral coordination. A near five fold diffraction symmetry observed in some of these phases indicates structural similarity of these with quasicrystals. Ramchandrarao et al. and Zhang et al. were the first to predict that these alloys are likely to yield quasicrystals on rapid solidification. One of the most important phases falling in this category is the T-phase Mg[sub 32](Al,Zn)[sub 49]. The T-phase is an equilibrium phase and is known to form in slowly cooled specimens. This study has indicated that the T-phase can also from on rapid solidification. It has also been observed in this study that the amount of quasicrystalline phase in these ternary alloys is a strong function of the Al/Zn ratio in the alloys and contrary to the observations made in some of the earlier studies, the alloys containing higher Al contain the highest amount of quasicrystalline phase. It should be noted that though Yamane et al. have carried out studies on some alloys of this system, they have limited their studies only to X-ray Diffraction (XRD) experiments. Investigations using XRD sometimes fail to reveal the presence of phases present in smallmorexa0» quantities. This study uses XRD as well as TEM for characterization of the phases in the different alloys. It has been possible to observe the formation of the quasicrystalline phase in many of the compositions examined in this study.«xa0less

Precipitation in a Cu–4Ti–2Be alloy

It is known that the binary Cu–2u2009wt.% Be alloy can be strengthened by ageing its supersaturated solution at 523u2009K. The increase in the strength arising from ageing is because of the formation of GP zones. The same level of strength is achieved by ageing the supersaturated solid solution of a dilute binary Cu–4u2009wt.% Ti alloy at 723u2009K. In contrast to Cu–2u2009wt.% Be alloy, the increase in strength in this alloy occurs on account of spinodal decomposition of the as-quenched microstructure through the mechanism of clustering and ordering. In the present study, decomposition of a supersaturated solid solution of the ternary Cu–4u2009wt.% Ti–2u2009wt.% Be alloy was studied. Whereas ageing of this alloy at 523u2009K resulted in the formation of GP zones like in a Cu–2u2009wt.% Be alloy, ageing at 723u2009K resulted in a spinodally decomposed microstructure as in a Cu–4u2009wt.% Ti alloy. The decomposition of the ternary alloy at the two temperatures by two different mechanisms indicates that Be and Ti in solution in Cu act, more or less, independently of each other during ageing.