grain structure by Dr.Ram

Grain structure

Grain is a small region of a metal, having a given and continuous crystal lattice orientation. Each grain represents small single crystal.

Grains form as a result of solidification or other phase transformation processes. Grains shape and size change in course of thermal treatment processes (for example recrystallization annealing). The normal grain size varies between 1µm to 1000 µm.

Grain structure of a solid is an arrangement of differently oriented grains, surrounded by grain boundaries.

Formation of a boundary between two grains may be imagined as a result of rotation of crystal lattice of one of them about a specific axis.

Depending on the rotation axis direction, two ideal types of a grain boundary are possible:

• Tilt boundary – rotation axis is parallel to the boundary plane;

• Twist boundary - rotation axis is perpendicular to the boundary plane;

• An actual boundary is a “mixture” of these two ideal types.

Grain boundaries are called large-angle boundaries if misorientation of two neighboring grains exceeds 10º-15º.

Grain boundaries are called small-angle boundaries if misorientation of two neighboring grains is 5º or less.

Grains, divided by small-angle boundaries are also called subgrains.

Grain boundaries accumulate crystal lattice defects (vacancies, dislocations) and other imperfections, therefore they effect on the metallurgical processes, occurring in alloys and their properties.

Since the mechanism of metal deformation is a motion of crystal dislocations through the lattice, grain boundaries, enriched with dislocations, play an important role in the deformation process.

Diffusion along grain boundaries is much faster, than throughout the grains.

Segregation of impurities in form of precipitating phases in the boundary regions causes a form of corrosion, associated with chemical attack of grain boundaries. This corrosion is called Intergranular corrosion.

Crystallization process by Dr.Ram

Crystallization

• Nucleation
• Crystal growth
• Constitutional undercooling
• Dendrites

Some metallurgical processes involve phase transition.

The typical example of phase transition is crystallization.

Crystallization is transformation of liquid phase to solid crystalline phase.

There are two general stages of phase transformation (crystallization) process – nucleation and growth:

Nucleation

Nucleation is a process of formation of stable crystallization centers of a new phase.

Nucleation may occur by either homogeneous or heterogeneous mechanism, depending on the value of undercooling of the liquid phase (cooling below the equilibrium freezing point).

Presence of foreign particles or other foreign substance in the liquid alloy (walls of the casting mold) allows to initiate crystallization at minor value of undercooling (few degrees below the freezing point). This is heterogeneous nucleation.

If there is no solid substance present, undercooling of a hundred degrees is required in order to form stable nuclei or “seeds” crystals, providing following crystal growth (homogeneous nucleation)

Undercooling value determines quantity of nuclei, forming in the crystallizing alloy. When a liquid comes into a contact with cold and massive mold wall (chill zone), it cools fast below the freezing point, resulting in formation of a large quantity of stable nuclei crystals.

In order to promote the nucleation process, surface-active additives are used. They decrease interfacial energy of the nuclei crystals, causing formation of many more new stable nuclei.

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Crystal growth

Number of stable nuclei per unit volume of crystallizing alloy determines the grain size.

When a large number of stable nuclei are present in chill zone of mold, fine equiaxed grains form. Latent crystallization heat, liberating from the crystallizing metal, decreases the undercooling of the melt and depresses the fast grains growth.

At this stage some of small grains, having favorable growth axis, start to grow in the direction opposite to the direction of heat flow. As a result columnar crystals (columnar grains) form.

Contrary to the pure metals, in alloys different type of undercooling takes place. It is called constitutional undercooling.

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Constitutional undercooling

Since solubility of an alloying element in solid is lower, than in liquid at the same temperature, this element (solute) is rejected by the solidifying metal to the liquid phase, enriching the region of liquid adjacent to the crystallization front.

For the most of the alloys: the higher the concentration of alloying element in the alloy, the lower its liquidus temperature (temperature at which crystallization of the alloy starts).

Thus crystallization temperature of the liquid, adjacent to the crystallization front, rises with increasing the distance from the front surface. Therefore there is a layer of the liquid, where its temperature is lower, than its crystallization temperature. This is the region of constitutional undercooling (see the figure below).

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Dendrites

If a protruding finger forms on the solidifying surface, its tip may reach the region of constitutional undercooling . In this case the protuberance starts accelerated growth, forming the main dendrite arms. Under certain conditions the same process may occur on the surface of the main dendrite arms, causing branching off the secondary arms and then arms of higher orders.

Process of solidification is considered in the article Solidification.

imperfections in crystal structures - Dr.Ram

mperfections of crystal structure

There are three conventional types of crystal imperfections:

• Point defects
• Line defects
• Planar defects

Point defects

The simplest point defects are as follows:

• Vacancy – missing atom at a certain crystal lattice position;

• Interstitial impurity atom – extra impurity atom in an interstitial position;

• Self-interstitial atom – extra atom in an interstitial position;

• Substitution impurity atom – impurity atom, substituting an atom in crystal lattice;

• Frenkel defect – extra self-interstitial atom, responsible for the vacancy nearby.

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Line defects

Linear crystal defects are edge and screw dislocations.

• Edge dislocation is an extra half plane of atoms “inserted” into the crystal lattice. Due to the edge dislocations metals possess high plasticity characteristics: ductility and malleability.

• Screw dislocation forms when one part of crystal lattice is shifted (through shear) relative to the other crystal part. It is called screw as atomic planes form a spiral surface around the dislocation line.

For quantitative characterization of a difference between a crystal distorted by a dislocation and the perfect crystal the Burgers vector is used.

The dislocation density is a total length of dislocations in a unit crystal volume. The dislocation density of annealed metals is about 1010 - 1012 m−². After work hardening the dislocation density increases up to 1015 - 1016 m-². Further increase of dislocation density causes crackes formation and fracture.

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Planar defects

Planar defect is an imperfection in form of a plane between uniform parts of the material. The most important planar defect is a grain boundary. Formation of a boundary between two grains may be imagined as a result of rotation of crystal lattice of one of them about a specific axis. Depending on the rotation axis direction, two ideal types of a grain boundary are possible:

• Tilt boundary – rotation axis is parallel to the boundary plane;

• Twist boundary - rotation axis is perpendicular to the boundary plane:

• An actual boundary is a “mixture” of these two ideal types.

Grain boundaries are called large-angle boundaries if misorientation of two neighboring grains exceeds 10º-15º.

Grain boundaries are called small-angle boundaries if misorientation of two neighboring grains is 5º or less.

Grains, divided by small-angle boundaries are also called subgrains.

Grain boundaries accumulate crystal lattice defects (vacancies, dislocations) and other imperfections, therefore they effect on the metallurgical processes, occurring in alloys and their properties.

Since the mechanism of metal deformation is a motion of crystal dislocations through the lattice, grain boundaries, enriched with dislocations, play an important role in the deformation process.

Diffusion along grain boundaries is much faster, than throughout the grains.

Segregation of impurities in form of precipitating phases in the boundary regions causes a form of corrosion, associated with chemical attack of grain boundaries. This corrosion is called Intergranular corrosion.

metallic crystal structure by Dr.Ram

Metals crystal structure

There are two main forms of solid substance, characterizing different atoms arrangement in their microstructures:

• Amorphous solid
• Crystalline solid

Amorphous solid

Amorphous solid substance does not possess long-range order of atoms positions. Some liquids when cooled become more and more viscous and then rigid, retaining random atom characteristic distribution.

This state is called undercooled liquid or amorphous solid. Common glass, most of Polymers, glues and some of Ceramics are amorphous solids. Some of the Metals may be prepared in amorphous solid form by rapid cooling from molten state.

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Crystalline solid

Crystalline solid substance is characterized by atoms arranged in a regular pattern, extending in all three dimensions. The crystalline structure is described in terms of crystal lattice, which is a lattice with atoms or ions attached to the lattice points. The smallest possible part of crystal lattice, determining the structure, is called primitive unit cell.

Examples of typical crystal lattice are presented in the picture:

Metal crystal structure and specific metal properties are determined by metallic bonding – force, holding together the atoms of a metal. Each of the atoms of the metal contributes its valence electrons to the crystal lattice, forming an electron cloud or electron “gas”, surrounding positive metal ions. These free electrons belong to the whole metal crystal.

Ability of the valence free electrons to travel throughout the solid explains both the high electrical conductivity and thermal conductivity of metals.
Other specific metal features are: luster or shine of their surface (when polished), their malleability (ability to be hammered) and ductility (ability to be drawn).
These properties are also associated with the metallic bonding and presence of free electrons in the crystal lattice.

The following elements are common metals:

aluminum(Al), barium(Ba), beryllium(Be), bismuth(Bi), cadmium(Cd), calcium(Ca), cerium(Ce), cesium(Cs), chromium(Cr), cobalt(Co), copper(Cu), gold(Au), indium(In), iridium(Ir), iron(Fe), lead(Pb), lithium(Li), magnesium(Mg), manganese(Mn), mercury(Hg), molybdenum(Mo), nickel(Ni), osmium(Os), palladium(Pd), platinum(Pt), potassium(K), radium(Ra), rhodium(Rh), silver(Ag), sodium(Na), tantalum(Ta), thallium(Tl), thorium(Th), tin(Sn), titanium(Ti), tungsten(W), uranium(U), vanadium(V), zinc(Zn).

solid solutions by Dr.Ram

Solid solutions

• Alloy
• Phase
• Substitution solid solution
• Interstitial solid solution

Alloy

Alloy is a metal, composing of a mixture of elements. Most of alloys are composed of a base metal with small amounts of additives or alloying elements. The typical examples of alloys are steel/cast iron (iron base alloys), bronze/brass (copper base alloys), aluminum alloys, nickel base alloys, magnesium base alloys, titanium alloys.

Alloys may be prepared by different technological methods: melting, sintering of a powders mixture, high temperature diffusion of an alloying element into the base metal, plasma and vapor deposition of different elements, electroplating etc.

Alloy structure may be a single phase or a multi phase.

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Phase

Phase is a uniform part of an alloy, having a certain chemical composition and structure, and which is separated from other alloy constituents by a phase boundary.

An alloy phase may be in form of valence compound (substance formed from two or more elements), with a fixed ratio determining the composition) or in form of solid solution.

Solid solution is a phase, where two or more elements are completely soluble in each other.

Depending on the ratio of the solvent (matrix) metal atom size and solute element atom size, two types of solid solutions may be formed: substitution or interstitial.

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Substitution solid solution

If the atoms of the solvent metal and solute element are of similar sizes (not more, than 15% difference), they form substitution solid solution, where part of the solvent atoms are substituted by atoms of the alloying element (see the picture below).

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Interstitial solid solution

If the atoms of the alloying elements are considerably smaller, than the atoms of the matrix metal, interstitial solid solution forms, where the matrix solute atoms are located in the spaces between large solvent atoms (see the picture below).

When the solubility of a solute element in interstitial solution is exceeded, a phase of intermediate compound forms. These compounds (TiN, WC, Fe₃C etc.) play important role in strengthening steels and other alloys.

Some substitution solid solutions may form ordered phase where ratio between concentration of matrix atoms and concentration of alloying atoms is close to simple numbers like AuCu₃ and AuCu.

Solid solution formation usually causes increase of electrical resistance and mechanical strength and decrease of plasticity of the alloy.

Hume Rotherys Rules for solid solutions by Dr.Ram

The Hume-Rothery Rules

How can you tell if two solids will form a solid solution? A great place to start is the Hume-Rothery Rules. William Hume-Rothery researched metal mixtures and created these rules to predict whether two elements can form solid solutions. The Hume-Rothery rules state that two elements must be very similar to each other in order to form a solid solution, because any dissimilarities can cause separation like in the case of oil and water. The two elements must therefore meet all of the following conditions in order to mix and form a solid solution.

• Atomic Size: The atomic radii of the elements must be within about 15% of each other.
• Crystal Structure: The two elements must have the same crystal structure. This means that the atoms in one element must arrange in the same way as the atoms in the other element.
• Electronegativity: The two elements must have similar electronegativity values. Electronegativity is a measure of an atom's ability to attract electrons. Electronegativity values can be found online or in chemistry books.
• Valency: The two elements must have the same valency, which is a measure of an atom's ability to combine with other atoms. Valency values can be found online or in chemistry book.