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4.0 Solidification of metals and alloys
4.1 Introduction
Most metals are melted and then cast into semifinished or finished shape. Solidification of a metal can be divided into the following steps: Formation of a stable nucleus Growth of a stable nucleus. Figure 4.1 (a)Formation of stable nuclei (b) Growth of crystals (c) Grain structure 4.1.1 Polycrystalline Metals In most cases, solidification begins from multiple sites, each of which can produce a different orientation. The result is a “polycrystalline”material consisting of many small crystals of “grains” Each grain has the same crystal lattice, but the lattices are misoriented from grain to grain
4.1.2 Cooling curves
1. Undercooling ‐The temperature to which the liquid metal must cool below the equilibrium freezing temperature before nucleation occurs. 2. Recalescence ‐The increase in temperature of an undercooled liquid metal as a result of the liberation of heat during nucleation. 3. Thermal arrest ‐A plateau on the cooling curve during the solidification of a material caused by the evolution of the latent heat of fusion during solidification.
4. Total solidification time ‐The time required for the casting to solidify completely after the casting has been poured. 5. Local solidification time ‐The time required for a particular location in a casting to solidify once nucleation has begun.
Figure 4.2 Cooling curve
(a) (b) Figure 4. 3 (a) Cooling curve for a pure metal that has not been well inoculated. Liquid cools as specific heat is removed (betweens points A and B ). Undercooling is thus necessary (between
Figure 4. 5 Various Stages during Solidification of a Crystalline Material (a)Small crystallite nuclei; (b) growth of crystallites; (c) solidification complete; (d) grain structure as it appears under a microscope Growth (a) Nuclei grow and then aggregate (b) Occurs as heat of fusion is extracted from liquid (c) Direction and rate of growth can be controlled by the manner of heat removal Directional solidification: Movement of S/L interface opposite to the direction of
heat flow and modify heat flow. direction
4.2 Cooling curves and equilibrium diagrams
The relationship between the composition, temperature and microstructure of alloys are illustrated by equilibrium diagrams. Equilibrium diagrams are constructed from cooling curves. Cooling curves are determined by experiment of applying thermocouples
4.2.1 Equilibrium Diagrams When a pure metal solidifies it changes from a liquid to a solid state. An intermediate state of liquid and solid exists (sometimes known as the pasty state). These states are known as phases , a phase being defined as regions that differ from one another, either in composition or in structure or in both. In a metal the liquid state consists of atoms randomly arranged whereas in the solid state the atoms are arranged regularly in crystal lattices. Therefore the structure of the two states is different and is referred to as phases. When a pure metal is cooled from the liquid state is produces a cooling curve as in Figure 4.6. The change from the liquid to the solid state occurs at a definite temperature. Where solidification begins and finishes at the same temperature but the time increases. Examples of metals that have this are lead, copper, aluminum. A pure metal solidifies at one fixed temperature, a fact which can be checked by plotting a cooling curve. A cooling curve may be obtained by melting a small amount of a metal and recording the temperature drop at suitable time intervals as this metal solidifies (the metal must be allowed to cool very slowly i.e. under equilibrium conditions). We can then plot a graph of temperature against time to give us the cooling curve for that particular metal. At temperatures above and below the curve falls smoothly without "kinks". When the solidification temperature is reached, the temperature remains constant for some time thus giving rise to the step in the curve. Down to the temperature of the liquid drops in a regular manner as heat is being lost to the surroundings at a nearly constant rate. The step is due to latent heat. This leads to zero change in temperature until the last drop of liquid has solidified. After no more latent heat is available the solid continues to cool in a regular manner giving the smooth curve.
Figure 4.7 Cooling curve for an alloy Some alloys complete their solidification at a constant temperature and a curved temp/time as is shown in Figure 4.8. Examples are cadmium and bismuth or lead and tin alloys. Figure 4.8 Cooling curves for some alloys
4.2.3 Alloying metals Most pure metals are soft and not very useful in their pure state. There are of course exceptions i.e copper is an excellent electrical conductor in its pure state. Therefore in order to increase properties like strength, hardness and corrosion resistance, we mix two or more pure metals together to give us an alloy. Everyday examples of alloys include Bronze which is an alloy of Copper and Tin where the Tin content is usually less than 20%.
4.3 Equilibrium
Equilibrium may be defined as a state of balance of stability. When a metal solidifies, equilibrium will occur under conditions of slow cooling where the fall in temperature is small in relation to the time elapsed. To achieve equilibrium it would be necessary, at every stage of cooling, to give the alloy elements time to diffuse (mix through on another) which would lead to a state that each grain of metal would have the same composition throughout. Complete diffusion seldom takes place in casting because solidification usually takes place before diffusion is complete. 4.3.1 Thermal Equilibrium diagrams. Instead of dealing with several different cooling curves for any alloy a quicker graph has been created using the various arrest points of all the alloys. When these points are marked on a graph and joined up we get a thermal equilibrium diagram which looks like this in Figure 4.9. Figure 4.9 Creating a thermal equilibrium diagram.
- An alloy system in which the two metals are soluble in each other in all proportions in both liquid and solid state.
- An alloy system in which the two metals are soluble in each other in all proportions in the liquid but not in the solid state.
- An alloy system in which the two metals are soluble in each other in all proportions in the liquid but only partially in the solid state.
- Iron/Carbon equilibrium diagram. 4.3.1.1 Solid Solution Alloys A solid solution occurs when we alloy two metals and they are completely soluble in each other. If a solid solution alloy is viewed under a microscope only one type of crystal can be seen just like a pure metal. Solid solution alloys have similar properties to pure metals but with greater strength but are not good electrical conductors. (a) Substitutional solid solution The name of this solid solution tells you exactly what happens as atoms of the parent metal ( or solvent metal) are replaced or substituted by atoms of the alloying metal (solute metal). In this case, the atoms of the two metals in the alloy, are of similar size. Here we see the white atoms have been replaced or substituted by the black atoms. Depicted in Figure 4.11. Figure 4.11 Substitutional Solid Solution
(b) Interstitial solid solution In interstitial solid solutions the atoms of the parent or solvent metal are bigger than the atoms of the alloying or solute metal. In this case, the smaller atoms fit into interstices i.e spaces between the larger atoms. The smaller atoms are small enough to fit into the spaces between the larger solvent atoms. Shown below in Figure 4.12. Figure 4.12 Interstitial solid solution In both substitutional and interstitial solid solutions the overall atomic structure is virtually unchanged. Examples of solid solution alloys include Copper-Nickel, Gold- Silver all whom has an F.C.C structure. Molybdenum-Tungsten is an example of an solid solution with a B.C.C structure. Thermal diagrams created using solid solution alloys are given the name binary alloys. One final thing we must deal with before we move on to the next type of alloy combination is the lever rule. The Lever Rule The equilibrium diagram for a solid solution alloy that we have just been dealing with contains two distinct phases, liquid and solid solutions. Between the liquidus and solidus lines these two phases exist together in equilibrium and hence the area between the curves is known as the two phase region. If a horizontal line is drawn through the two phase region, such a line is called a tie line. We see a tie line drawn in this equilibrium diagram. The lever rule may be introduced by considering the simple see - saw. For the see- saw to be balanced, i.e in equilibrium, without
Figure 4.13 Bismuth / Cadmium (Eutectic Alloy) As you would accept everything above the liquidus line is in the liquid state and in this state the two metals are totally soluble in each other. In the eutectic point region there is only the eutectic composition alloy. If you look at 100% Cadmium you will see that there is a large amount of solid Cadmium while this decreases in the alloys found nearer to the eutectic. The same applies for Bismuth. Therefore we can say that as the composition of the alloy moves away from the eutectic composition, grains of either Cadmium or Bismuth appear in the eutectic matrix.
Figure 4.14 Microstructures An examination at 80% Cadmium and 20% Bismuth. As the temperature falls crystal nuclei of pure cadmium begin to form. The temperature cuts the liquidus line at 80/20% and the other phase boundary is the 100% Cadmium ordinate. Dendrites of cadmium are deposited and the remaining liquid becomes increasingly richer in bismuth. Therefore the composition of the liquid moves to the right. As the temperature decreases more cadmium deposition takes place. The growth of cadmium dendrites and consequent enriching of the remaining liquid is bismuth. The changing properties depending on metal composition are shown in Figure 4.15.
Figure 4.16 Lead/Tin (solder) partial solubility Figure 4.17 Liquidus and Solidus Inter-metallic Compounds Whilst some metal alloy systems exhibit total or partial solubility and others are insoluble in the solid state, a number of metals combine together to form an intermediate phase or intermediate
compounds. There are two types of inter-metallic compounds which are often encountered in metallurgy. Electron compounds These compounds are of definite chemical crystal structure and arise if the two alloying metals are of different crystal structure, valency, and if one of these metals is electro- positive with the other being electro- negative an example of this type of electron compound would be an alloy of the elements Magnesium and Tin which combine to form an inter-metallic compound Mg 2 Sn. The composition of the compound is fixed and consists of two atoms of Magnesium combining with one atom of Tin. Metallic compounds form a crystal lattice with the atoms of the alloying metals taking up specific positions within the lattice. These compounds are usually hard and brittle. Interstitial compounds Interstitial compounds, as the name suggests form between metals, or metals and non- metallic elements, with atom sizes very similar to those that form interstitial solid solution. One set of atoms fit into the spaces, or interstices, between the larger atoms. Iron Carbide (Fe 3 C) or cementite which is important in the study of Iron- Carbon diagrams is an example of an interstitial compound. As the chemical symbol for Cementite is Fe 3 C we know that Cementite is an interstitial compound containing 3 iron atoms for every 1 atom of Carbon. The allotropy of iron Allotropy is the ability of some elements to exist in different physical forms (differing in color, hardness, melting point etc.). Iron is allotropic; at room temperature pure iron exists in the Body Centered Cubic crystal form but on heating transforms to a Face Centered Cubic crystal. The temperature that this first transformation takes place is known as a critical point and it occurs at 910 degrees Celsius. This change in crystal structure is accompanied by shrinkage in volume, since the atoms in the face centered crystal are more densely packed together than in the body centered cubic crystal. At the second critical point the F.C.C crystal changes back to a B.C.C crystal and this change occurs at 1390 degrees Celsius. Iron above 1390 degrees is known as delta iron ( δ ) BCC Iron between 1390 and 910 degrees is known as gamma iron (γ) FCC Iron below 910 degrees is known as alpha iron (α). BCC
The Steel Section of the Iron - Carbon Diagram Shown here is the steel part of the iron carbon diagram containing up to 2% Carbon. At the eutectoid point 0.83% Carbon, Austenite which is in a solid solution changes directly into a solid known as Pearlite which is a layered structure consisting of layers of Ferrite and Cementite. Notes added: Liquidus line - This is the line at which all the alloys begin to solidify. Solidus line - This is the line at which all the alloys complete their solidification. Anywhere below the solidus is solid. Solvus line - separates two different solid phases in the material
