S. N. Ojha

Microstructure and mechanical properties of Al–Si alloys produced by spray forming process

Abstract The microstructural characteristics and mechanical properties of Al–6.5Si and Al–18Si alloys have been investigated. The alloys were spray-deposited and hot extruded at 480°C with an area reduction ratio of 6:1. The microstructure of the spray-deposit of Al–6.5Si alloy showed spherical morphology of the primary α-phase with a globular shape of the eutectic Si phase in the inter-particle boundaries. On the other hand, a fine particulate morphology of primary Si phase uniformly dispersed in the matrix of the α-phase was observed in spray-deposit of Al–18Si alloy. Microstructural refinement was further increased in the hot extruded alloys. The room temperature tensile tests of spray formed and extruded alloys showed considerable increase in their strength and ductility over that of the as cast alloys. The improvement in mechanical properties of spray formed alloys is discussed in light of the microstructural modifications induced during spray deposition process.

Metastable phase formation during solidification of undercooled melt

Abstract Undercooling behaviour of micron-sized droplets entrained in the matrix of the primary phase of Ag–Ge, Al–Ge and Cu–Ag alloys was investigated. Thermal cycle consisted of equilibration of hypoeutectic alloys above the eutectic temperature in two phase liquid–solid region followed by their slow cooling or quenching from the mushy state. Repeated thermal cycling generated a wide size range of droplets. During slow cooling droplets exhibited a large undercooling below the eutectic temperature of alloys. Solidification structure of the undercooled droplets revealed presence of metastable phases in Ag–Ge and Al–Ge alloys and a non-equilibrium solid solution phase in Cu–Ag alloy. Solidification behaviour of undercooled melt and formation of metastable microstructure are discussed.

Microstructural control by spray forming and wear characteristics of a Babbit alloy

The effect of process variables during spray forming of a commercial Babbit alloy containing Pb74–Sn12–Sb11.5–Cu1.25–NiO.75–Cd0.3–As0.2 on its microstructure and wear characteristics were investigated. Variation in atomization gas pressure from 0.6 to 1.2 MPa and nozzle to substrate distance from 0.2 to 0.4 m revealed considerable change in the nature of porosity and microstructural features of the spray deposits. The process variables during spray deposition were optimized to achieve microstructural homogeneity and refinement in second phase particles of this alloy. The wear study of both the spray formed and as-cast alloy under an applied load of 10 to 70 N and sliding velocity of 0.2 to 1.5 ms-1 indicated two distinct regimes of mild and severe wear. In both the regimes, the spray-formed alloy consistently indicated a low wear rate compared to that of the as-cast alloy. In addition, the mild wear regime of the spray-formed alloy was extended to higher load and sliding velocity. Wear characteristics of the spray formed alloy is discussed in light of its microstructural features induced during spray deposition processing.

Solidification of undercooled cadmium-zinc eutectic melts

Molten specimens of Cd-Zn eutectic alloy (26.50 at.% Zn) were undercooled by the glass-slag technique using glass-forming ZnCl2 as the slag. The microstructure of the solidified samples, weighing about 20 g each, was found to be a function of the undercooling produced. At lower undercoolings the microstructure consisted of supersaturated primary cadmium dendrites co-existing with the eutectic, while at higher coolings duplex structures were observed. Possible causes for the differences in microstructure are discussed with due importance given to the recorded recalescence effects.

Studies on spray casting of Al-alloys and their composites☆

Abstract A spray nozzle of convergent–divergent configuration was used for spray castings (SC) of A16061/LM25 alloys and their composites. Both monolithic alloys and composites showed equiaxed fine grained microstructure with a variation in grain size from 20 to 30xa0μm. Some of the regions of the preform also showed cellular solidification structure with globularization of most of the eutectic Si suggesting the effect of rapid solidification. High defect concentration in both SC matrix and their composites was observed with bulk SC samples showing uniformly distributed porosity. TEM studies showed decoration of grain boundaries/presolidified droplets by some precipitates and also vacancy loops. Their aging characteristics indicated lowering of solutionising temperature and aging time to get the same peak hardness as obtained in the alloy produced by ingot metallurgy methods. An ordered phase with unit cell four times larger than Al seems to form during the in situ heating of LM25–SiCp samples while dislocations seem to emerge from the interface between the particle and the matrix. The results are discussed in the light of reported literature on spray casting/melt spinning of Al-alloys and composites.

Microstructural evolution during spray forming of an Al-18Si alloy

Spray forming of Al-18Si alloy and its behaviour in semi-solid state holding has been studies.It is observed that a smaller deposition distance leads to large scale compositional inhomogeneity in the preform. nThis effect is attributed to increased incoming fraction of liquid at the deposition surface with growing thickness of the deposit. A large liquid pool thickness leads to development of microstructure with a typical characteristics of conventional casting process. A uniform nmicrostructure is evolved when the thickness of liquid pool is comparable to the thickness of the interaction ndomain formed during spray deposition.

Faceting behaviour of primary phase in Ag-Bi alloys

In the past the faceting behaviour of crystals, during their growth from the melt, has been the subject of extensive research. It is generally considered that factors which influence the structure of the solidliquid interface predominantly control the growth morphology of crystals. One of the first and simplest analyses of the interface structure of pure component was provided by Jackson [1]. The faceting behaviour of the interface was correlated using the concept of an a-factor or roughness parameter, which is a function of the entropy of fusion (AS f) of the component. It was shown that phases with a 16.8 Jmo1-1 K -1 exhibit faceted morphology during crystallization from their melt. According to this analysis most of the semiconductors or metalloids grow with faceted morphology, whereas common metals have non-faceted or dendritic growth features. The Jackson criteria for pure materials has been further extended by Kerr and Winegard [2] for the solid solution phase. These investigators used the entropy of solution (AS ~L) of the phase in the expression of the a-factor. Later, more general analysis by Jackson [3] suggested that the entropy of solution of the phase is indeed an important parameter, not only to control the faceting behaviour of crystals but also to determine the rate of crystal growth and its anisotropy. Although the a-factor criterion is adequate to explain most of the experimental observations [4], a few notable exceptions to this rule was observed in the cases of CuA12 [5] and phosphorus [6]. Even though these materials have a-factor <2, they still exhibit faceted growth morphology. In a recent publication Saroch et al. [7] proposed a thermodynamic model to emphasize the importance of composition and temperature dependence of entropy of solution (dASaL/dT) of the primary phase in the A1-Sn system to explain the faceting behaviour of the primary Al-rich phase. The origin of large values of this parameter was traced to the occurrence of retrograde solid solubility and nearly fiat liquidus lines in this system. Similar features are also observed in the phase diagram of the Ag-Bi system [8].

Microstructural features associated with spray atomization and deposition of Al-Mn-Cr-Si alloy

An inert gas atomization process was employed in production of rapidly solidified powders as well as disc-shape preform by spray deposition of an Al75Mn10Cr5Si10 alloy. Microstructural features of atomized powders and spray deposited preforms were evaluated by scanning and transmission electron microscopy and X-ray diffractometry techniques. Solidification structure of powders revealed cellular and dendritic morphology, depending on their size. The interdendritic regions consisted of second phase particles. In contrast the spray formed alloy exhibited microstructural homogeneity with distribution of ultra-fine second phase particles of intermetallic compound. The structure of second phase intermetallics was identified as a cubic α-Al(Mn,Cr)Si, in both the atomized powders and the spray-deposits. The formation of cubic phase is discussed as rational approximant structure of an icosahedral quasicrystal.

Undercooling and solidification of droplets of Cu-Ag alloy entrained in the primary phase

The undercooling behaviour and formation of metastable microstructures have been studied in the melt of Cu-Ag alloy entrained in its primary phase. A maximum undercooling of 180 °C below the liquidus temperature was observed in isolated liquid droplets. The highly undercooled droplets underwent a massive transformation which resulted in the formation of a metastable solid-solution phase containing Ag-28 at% Cu. The metastable phase decomposed on ageing to form the equilibrium phases. The undercooling behaviour and evolution of metastable microstructures in droplets are discussed.

On spinodal boundaries of the AgCu system

Introduction The Ag-Cu system satisfies the Hume-Rothery criteria for the formation of a solid solution phase over the entire range of compositions. However, the phase diagram of this system exhibits a eutectic reaction between the two terminal solid solution phases, ~ and I~. Duwez et. al. [1], in their gun quenching experiments, have shown the complete solubility of Ag and Cu in the solid state. Following Duwezs work, several investigations have reported the formation and characterization of metastable solid solution phases in this system. Nagakura et al. [2], in their experiments, have shown that the metastable a and 13 phases form from the y phase on aging at room temperature. Systematic studies of Stoering and Conrad [3], and Boswell and Chadwick [4] confirmed the transformation of metastable y to a and 13 phases on aging. Stoering and Conrad [3] sometimes observed only the a phase in the thicker region of the foil, without evidence for the existance of the y phase. The route of formation of a metastable ~ phase in such a case is not very clear. It could form either directly from the liquid or as a decomposition product of the y phase. However, in their mushy-quenching experiments, Prasad et al. [5] reported the formation of an a phase from the highly undercooled liquid in metastable equilibrium with the primary 13 phase. In contrast, our recent investigation on a mushy-quenched Cu-Ag alloy showed the metastable a phase as a decomposition product of a solid solution y phase, having an Ag-28at.% Cu composition. This ~, phase was shown to form by the diffusionless solidification of liquid, undercooled below a T o temperature. On further cooling, the y phase transforms through the spinodal route [6]. In order to understand the mechanism of formation and decomposition of metastable phases in this system, the location of the spinodal boundary in the phase diagram is essential. The present work aims to present the chemical and coherent spinodals for this system. Thermodynamic Formulation Various authors have calculated the spinodal boundaries, using different thermodynamic approaches [4,7-12]. Boswell and Chadwick [4] calculated the spinodal boundary, using the expression of Cook and Hilliard [8]. Ishihara and Shingu [9], Murray [10] and Hayes et al. [11] performed a thermodynamic analysis of the phase diagram. However, there are some limitations in their procedures. As a result, conclusions drawn from these are valid only for the conditions arising for low underoooling of the melt. Jonsson and Agren [12] provided a new model, which is applicable, even at high undercooling of the melt. These investigators calculated the metastable solidus and liquidus curves in the Ag-Cu system. For calculating the metastable boundaries of the Ag-Cu system, Jonsson and Agren [12] followed the procedure of Jansson [13], while utilizing the thermodynamic data given by Murray [10], and Choudhary and Ghosh [14].