MEEN 360 Chapter 10

What is the size factor of the hume-rothery rules?

Size factor: The atoms or ions must be of similar size, with no more than a 15% difference in atomic radius, in order to minimize the lattice strain (i.e., to minimize, at an atomic level, the deviations caused in interatomic spacing).

What are the different factors of the Hume-Rothery rule?

1. Size Factor 2. Crystal Structure 3. Valence 4. Electronegativity

What are the three characteristics of a phase?

1. the same structure or atomic arrangement throughout 2. roughly the same composition and properties throughout 3. a definite interface between the phase and any surrounding or adjoining phases.

What are the two conditions that are required for growth of the solid in a phase diagram?

1.) growth requires that the latent heat of fusion (delta)Hf, which evolves as the liquid solidifies, be removed from the solid-liquid interface. 2.) unlike the case of pure metals, diffusion must occur so that the compositions of the solid and liquid phases follow the solidus and liquidus curves during cooling.

How does atomic size between the original (host or solvent) atom and the added (guest or solute atom) affect the strengthening effect?

A large difference in atomic size between the original (host or solvent) atom and the added (guest or solute) atom increases the strengthening effect. A larger size difference produces a greater disruption of the initial crystal structure, making slip more difficult.

What is a phase?

A phase can be defined as any portion, including the whole, of a system that is physically homogeneous within itself and bounded by a surface that separates it from any other portions.

What is a phase diagram?

A phase diagram shows the phases and their compositions at any combination of temperature and alloy composition.

What is a plain carbon steel?

A plain carbon steel is an alloy of iron and carbon.

What is a triple point?

A point on the phase diagram at which the solid, liquid, and gaseous phases coexist under equilibrium conditions.

What is a solid solution?

A solid solution contains two or more types of atoms or ions that are dispersed uniformly throughout the material.

True or false: a solid solution is a mixture

A solid solution is not a mixture. A mixture contains more than one type of phase, and the characteristics of each phase are retained when the mixture is formed. In contrast to this, the components of a solid solution completely dissolve in one another and do not retain their individual characteristics.

What is the effect of solid solution strengthening on ductility of an alloy?

Almost always, the ductility of the alloy is less than that of the pure metal. Only rarely, as in copper-zinc alloys, does solid-solution strengthening increase both strength and ductility.

What is sublimation?

At very low pressures, a solid such as magnesium can sublime, or go directly to a vapor form without melting, when it is heated.

How can the mechanical and other properties of a solid solution be manipulated?

By controlling the amount of the point defects (solute atoms) via the composition, the mechanical and other properties of solid solutions can be manipulated.

How can gibb's phase rule be expressed?

C is the number of chemically independent components, usually elements or compounds, in the system; F is the number of degrees of freedom, or the number of variables (such as temperature, pressure, or composition), that are allowed to change independently without changing the number of phases in equilibrium; and P is the number of phases present (please do not confuse P with "pressure"). The constant "2" in Equation 10-1 implies that both the temperature and pressure are allowed to change.

What is the freezing range?

Copper-nickel alloys melt and freeze over a range of temperatures between the liquidus and the solidus. The temperature difference between the liquidus and the solidus is the freezing range of the alloy. Within the freezing range, two phases coexist: a liquid and a solid.

What are stainless steels?

Corrosion-resistant stainless steels are alloys that usually contain iron, carbon, chromium, nickel, and some other elements.

What is the crystal structure factor of the hume-rothery rules?

Crystal structure: The materials must have the same crystal structure; otherwise, there is some point at which a transition occurs from one phase to a second phase with a different structure.

Depending on the temperature and pressure, how many phases may be present at one time?

Depending on the temperature and pressure, however, there may be one, two, or even three phases present at any one time: solid magnesium, liquid magnesium, and magnesium vapor.

What is the effect of solid solution strengthening on conductivity of an alloy?

Electrical conductivity of the alloy is much lower than that of the pure metal. This is because electrons are scattered by the atoms of the alloying elements more so than the host atoms. Solid-solution strengthening of copper or aluminum wires used for transmission of electrical power is not recommended because of this pronounced effect. Electrical conductivity of many alloys, although lower than pure metals, is often more stable as a function of temperature.

What is a unary phase diagram?

Figure 10-2 shows a unary (C 5 1) phase diagram in which the lines divide the liquid, solid, and vapor phases.

What is a phase?

For now, we consider a "phase" as a unique form in which a material exists.

What is unlimited solubility?

Furthermore, they display unlimited solubility. Regardless of the ratio of water and alcohol, only one phase is produced when they are mixed together. Similarly, if we were to mix any amounts of liquid copper and liquid nickel, only one liquid phase would be produced. This liquid alloy has the same composition and properties everywhere [Figure 10-3(a)] because nickel and copper have unlimited liquid solubility.

What is gibb's phase rule?

Gibbs developed the phase rule in 1875-1876. It describes the relationship between the number of components and the number of phases for a given system and the conditions that may be allowed to change (e.g., temperature, pressure, etc.).

What is immiscibility?

Immiscibility, or lack of solubility, is seen in many molten and solid ceramic and metallic materials.

What is spray atomization?

In applications in which porosity, microsegregation, and macrosegregation must be minimized, powders of complex alloys are prepared using spray atomization [Figure 10-17(b)]. In spray atomization, homogeneous melts of complex compositions are prepared and sprayed through a ceramic nozzle.

What is solid solution strengthening?

In metallic materials, one of the important effects of solid-solution formation is the resultant solid-solution strengthening (Figure 10-6). This strengthening is caused by increased resistance to dislocation motion.

How do point defects created by impurity or solute atoms affect metallic materials?

In metallic materials, the point defects created by the impurity or solute atoms disturb the atomic arrangement in the crystalline material and interfere with the movement of dislocations. The point defects cause the material to be solid-solution strengthened.

What is segregation in the solidification of a solid-solution alloy?

In order to achieve this equilibrium final structure, the cooling rate must be extremely slow. Sufficient time must be permitted for the copper and nickel atoms to diffuse and produce the compositions given by the phase diagram. In many practical casting situations, the cooling rate is too rapid to permit equilibrium. Therefore, in most castings made from alloys, we expect composition differences to exist, a phenomenon known as segregation.

How many components are there in a unary phase diagram?

In the unary phase diagram, there is only one component; in this case, magnesium.

True or false: Chapter 10 is about single and multi-phase alloys

In this chapter, we will examine the behavior of single-phase alloys.

What is an isomorphous phase diagram?

Isomorphous phase diagrams are found in a number of metallic and ceramic systems. In the isomorphous systems, which include the copper-nickel and NiO-MgO systems [Figure 10-8(a) and (b)], only one solid phase forms; the two components in the system display complete solid solubility.

What is the assumption needed for Gibb's phase rule?

It is important to note that the Gibbs phase rule assumes thermodynamic equilibrium and, more often than not in materials processing, we encounter conditions in which equilibrium is not maintained.

What is hot shortness?

Microsegregation can cause hot shortness, or melting of the lower melting point interdendritic material at temperatures below the equilibrium solidus. When we heat the Cu-40% Ni alloy to 1225 °C, below the equilibrium solidus but above the nonequilibrium solidus, the low nickel regions between the dendrites melt.

In a pressure/temperature diagram unary phase diagram, what does the atmospheric pressure indicate?

Note that at atmospheric pressure (one atmosphere, given by the dashed line), the intersection of the lines in the phase diagram give the usual melting and boiling temperatures for magnesium.

How many degrees of freedom are there at the triple point?

Now we have no degrees of freedom; all three phases coexist only if both the temperature and the pressure are fixed. A point on the phase diagram at which the solid, liquid, and gaseous phases coexist under equilibrium conditions is the triple point (Figure 10-2).

What is dispersion strengthening?

Of course, if too much of a large or small atom is added, the solubility limit may be exceeded and a different strengthening mechanism, dispersion strengthening, is produced. In dispersion strengthening, the interface between the host phase and guest phase resists dislocation motion and contributes to strengthening.

What is the engineering application for pure metallic elements (not a solid-solution)?

Pure metallic elements have engineering applications; for example, ultra-high purity copper or aluminum is used to make microelectronic circuitry.

What is the effect of solid solution for ceramics?

Recall from Chapter 7 that the strength of ceramics is mainly dictated by the distribution of flaws and not the propagation and interaction of dislocations; thus, solidsolution formation does not have a strong effect on the mechanical properties of ceramics.

How does the amount of alloying element affect solid solution strengthening?

Second, the greater the amount of alloying element added, the greater the strengthening effect (Figure 10-6). A Cu-20% Ni alloy is stronger than a Cu-10% Ni alloy.

What are the set of rules that act as conditions for unlimited solid solubility called?

The Hume-Rothery rules

What is the electronegativity factor of the hume-rothery rules?

The atoms must have approximately the same electronegativity. Electronegativity is the affinity for electrons (Chapter 2). If the electronegativities differ significantly, compounds form—as when sodium and chloride ions combine to form sodium chloride.

What are the two mechanisms for which solid solutioncs may form?

The guest atoms or ions may enter the host crystal structure at regular crystallographic positions or the interstices.

What is a solute?

The impurity or solute atoms

Where can the solute atoms occupy in another solid?

The impurity or solute atoms may occupy regular lattice sites in the crystal or interstitial sites.

What does introducing alloying elements during process changes affect the material?

The introduction of alloying elements or impurities during processing changes the composition of the material and influences its solidification behavior.

What is the valence factor of the hume-rothery rules?

The ions must have the same valence; otherwise, the valence electron difference encourages the formation of compounds rather than solutions.

What is microsegregation?

The nonuniform composition produced by nonequilibrium solidification is known as segregation. Microsegregation, also known as interdendritic segregation and coring, occurs over short distances, often between small dendrite arms. The centers of the dendrites, which represent the first solid to freeze, are rich in the higher melting point element in the alloy. The regions between the dendrites are rich in the lower melting point element, since these regions represent the last liquid to freeze. The composition and properties of the a phase (in the case of Cu-Ni alloys) differ from one region to the next, and we expect the casting to have poorer properties as a result.

What is the effect of solid-solution strengthening on resistance to creep and strength at elevated temperatures?

The resistance to creep and strength at elevated temperatures is improved by solid-solution strengthening.

What is the solidus temperature?

The solidus temperature is the temperature below which the alloy is 100% solid.

How is solubility related to temperature?

The solubility increases with increasing temperature.

Are the solubility of interstitial atoms limited or unlimited?

The solubility of interstitial atoms is always limited. Interstitial atoms are much smaller than the atoms of the host element, thereby violating the first of Hume-Rothery's conditions (they are not similar size).

What does the term chemically independent mean?

The term "chemically independent" refers to the number of different elements or compounds needed to specify a system. For example, water (H2 O) is considered as a one component system, since the concentrations of H and O in H2 O cannot be independently varied.

What is the effect of solid solution strengthening on strength and hardness of an alloy?

The yield strength, tensile strength, and hardness of the alloy are greater than those of the pure metals. This is one reason why we most often use alloys rather than pure metals. For example, small concentrations of Mg are added to aluminum to provide higher strength to the aluminum alloys used in making aluminum beverage cans.

What are the two types of alloys?

There are two types of alloys: single-phase alloys and multiple-phase alloys.

What is macrosegregation?

There exists another type of segregation, known as macrosegregation, which occurs over a large distance, between the surface and the center of the casting, with the surface (which freezes first) containing slightly more than the average amount of the higher melting point metal.

How can the composition of each phase be determined in a phase diagram?

Through tie lines

How are copolymers similar to solid solutions?

We can form materials that are known as copolymers that consist of different monomers. For example, acrylonitrile (A), butadiene (B), and styrene (S) monomers can be made to react to form a copolymer known as ABS. This resultant copolymer is similar to a solid solution in that it has the functionalities of the three monomers from which it is derived, blending their properties.

What is polymeric material?

We can process polymeric materials to enhance their usefulness by employing a concept similar to the formation of solid solutions in metallic and ceramic systems.

What does homogenization heat treatment do?

We can reduce the interdendritic segregation and problems with hot shortness by means of a homogenization heat treatment. If we heat the casting to a temperature below the nonequilibrium solidus, the nickel atoms in the centers of the dendrites diffuse to the interdendritic regions; copper atoms diffuse in the opposite direction

How can macrosegregation be reduced?

We cannot eliminate macrosegregation by a homogenization treatment, because the diffusion distances are too great. Macrosegregation can be reduced by hot working, which was discussed in Chapter 8. This is because in hot working, we are basically breaking down the cast macrostructure

What is an alloy?

We define an "alloy" as a material that exhibits properties of a metallic material and is made from multiple elements.

What is the liquidus temperature?

We define the liquidus temperature as the temperature above which a material is completely liquid.

What does 2 degrees of freedom mean in terms of phase diagrams?

What does this mean? Within limits, as seen in Figure 10-2, we can change the pressure, the temperature, or both, and still be in an all-liquid portion of the diagram. Put another way, we must fix both the temperature and the pressure to know precisely where we are in the liquid portion of the diagram.

When does non-equilibrium solidification and segregation occur?

When cooling is too rapid for atoms to diffuse and produce equilibrium conditions, nonequilibrium structures are produced in the casting.

What is a binary phase diagram?

When only two elements or two compounds are present in a material, a binary phase diagram can be constructed.

When a small amount of elements are added, what may form?

When small amounts of elements are added, a solid material known as a solid solution may form.

What is limited solubility?

When we add a small quantity of salt (one phase) to a glass of water (a second phase) and stir, the salt dissolves completely in the water. Only one phase—salty water or brine—is found. If we add too much salt to the water, the excess salt sinks to the bottom of the glass [Figure 10-1(c)]. Now we have two phases— water that is saturated with salt plus excess solid salt. We find that salt has a limited solubility in water.

How can the amount of each phase be determined in a phase diagram?

the lever rule: To calculate the amounts of liquid and solid, we construct a lever on our tie line, with the fulcrum of our lever being the original composition of the alloy. The leg of the lever opposite to the composition of the phase, the amount of which we are calculating, is divided by the total length of the lever to give the amount of that phase.