Solidification of Single-Phase Alloys_2007

Solidification of Single-Phase Alloys

Guided by Prof. B.J. Chauhan Sir

Prepared by Purvesh K. Nanavaty ME-I (Materials Technology)

Introduction The solidification process by which a liquid metal freezes in a mold plays a critical role in determining the properties of the as-cast alloy · The initial uniform composition in liquid becomes non uniform as the liquid transforms to solid · Different solidification conditions give rise to different microstructures of the solid

· Many casting defects, such as porosity and shrinkage, depend on the manner in which the alloy is solidified in a mold

Two important factors that control solidification microstructures 1. Alloy Composition A pure metal has a specific melting point Tm, while an alloy freezes over a range of temperatures

This freezing range is generally represented by a phase diagram, as shown in Fig. 1. The liquids line represents the temperature at which the liquid alloy begins to freeze, and the freezing process is complete when the solidus temperature is reached,

The ratio of the solid to liquid composition at a given temperature is called the solute distribution coefficient k. The first solid that forms at temperature TL will have a composition kCo, which is lower than the liquid composition Co. Thus, the excess solute rejected by the solid will give rise to a solute-rich liquid layer at the interface. This increase in liquid composition, along with the lowering of temperature, gives rise to solute segregation patterns in the solid 

A single-phase region of a phase diagram showing the liquidus and the solidus lines

The buildup of solute in liquid requires diffusion of solute in liquid for further growth. For efficient distribution of the solute in liquid. the interface may change its shape. In addition to the solute transfer, the interface shape is governed by the effective removal of the latent heat of fusion.

Heat Flow Conditions  Two distinctly heat flow conditions may exist in a mold. different  In the first case, the temperature gradients in the liquid and the solid are positive such that the latent heat generated at the interface is dissipated through the solid. Such a temperature field gives rise to directional solidification and results in the columnar zone in a casting.

In the second case, an equiaxed zone exists if the liquid surrounding the solid is under cooled so that a negative temperature gradient is present in the liquid at the solid/liquid interface. In this case, the latent heat of fusion is dissipated through the liquid. Such a thermal condition is generally present at the center of the mold.

A

B

C

D

Fig. 2 Effect of increasing growth rate on the shape of the solid/liquid interface in a transparent organic

system, pivalic acid-0.076 wt% ethanol, solidified directionally atG = 2.98 K/mm (75.7 K/in.). (a) v = 0.2 μm/s (8 μin./s). (b) v = 1.0 μm/s (40 μin./s). (c) v = 3.0 μm/s (120 μin./s). (d) v = 7.0 μm/s (280 μin./s)

Planar interface growth occurs only under directional solidification conditions and, for alloys, only under low growth rate or high-temperature gradient conditions. •consider an interface that is moving at a constant velocity v, with heat flowing from the liquid to the solid under temperature gradients GL and GS in liquid and solid, respectively •Constitutional supercooling diagram. The solute concentration profile in the liquid gives rise to the variation in the equilibrium freezing temperature Tf of liquid near the interface. The actual temperature in liquid is given by line 1, and the slope of Tf at the interface is given by line 2. A supercooled liquid exists in the shaded region.

Cellular and Cellular Dendritic Structures. Under directional solidification conditions.

Cellular /cellular dendritic interface is observed Which have two important characteristics. First, the length of the cell is small, and it is of the same order of magnitude as the cell spacing (Fig. b). Second, the tip region of the cell is broader and the cell has a larger tip radius. At higher velocities, a cellular dendritic structure forms (Fig. c) in which the length of

the cell is much larger than the cell spacing. Also, the cell tip assumes a sharper, nearly parabolic shape, which is similar to the dendrite tip shape so that the term cellular dendritic is used to characterize this structure

Dendritic Structures. . Dendritic structures are characterized by the formation of side branches (Fig. 2d). These side branches, as well as the primary dendrite, grow in a preferred crystallographic direction,

Directionally solidified peritectic cobaltsamarium-copper alloy showing primary cobalt dendrites when the Co17Sm2 matrix is etched away. The cut surfaces in the foreground indicate the structure that would be observed on the plane of polish if the matrix were not etched away.

Formation of cells with intercellular eutectic in the directionally solidified Sn-20Pb alloy. G = 31 K/mm (79 K/in.) and v = 1.2 μm/s (48 μin./s). The nearly flat eutectic interface is at the eutectic temperature

Dendritic structure in a directionally solidified transparent organic system, succinonitrile-4.0 wt% acetone. G = 6.7 K/mm (170 K/in.) and v = 6.4 μm/s (260 μin./s). The secondary dendrite arm spacing increases with the distance behind the tip.

Formation of equiaxed crystals at the center of the mold during the solidification of transparent ammonium chloride-water mixture

Schematic of microstructure zone formation in castings. Directional solidification conditions give rise to a columnar zone, while an equiaxed zone is formed at the center where the liquid is under cooled

References:ASM Metals Handbook Vol.15 Casting (R. Trivedi, Iowa State University; W. Kurz, Professor, Swiss Federal Institute of Technology, Switzerland) ASM Metals Handbook Vol.09 Metallography & Microstructures