Materials Science/Material Characterization

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Materials Characterization[edit]

An important aspect of materials science is the characterization of the materials that we use or study in order to learn more about them. Today, there is a vast array of scientific techniques available to the materials scientist that enables this characterization. These techniques will be introduced and explained in this section.

Microscopy[edit]

Microscopy is a technique that, combined with other scientific techniques and chemical processes, allows the determination of both the composition and the structure of a material.

Optical Microscopy[edit]

Optical microscopes are formed of lenses that magnify and focus light. This light may have been transmitted through a material or reflected from a material's surface and can be used to ascertain a great deal of information about that material under evaluation. This can include whether the material is dense or contains porosity, what color the material is, whether the material is composed of a single phase or contains multiple phases etc. A common practice performed in conjunction with optical microscopy is that of targeted and controlled chemical attack of the material using one of many chemical reagents available. For metallic materials, this technique combined with optical microscopy is know as optical metallography. The basis of this combined technique is that regions of different composition within a material as well as entirely different materials are affected differently when exposed to certain chemicals. These chemical effects are cataloged in various works and through an understanding of these effects and a systematic experimental process they can be used to determine material composition and structure.

Materials Characterisation[edit]

An important aspect of materials science is the characterisation of the materials that we use or study in order to learn more about them. Today, there is a vast array of scientific techniques available to the materials scientist that enables this characterisation. These techniques will be introduced and explained in this section.

Macroscopic Observation[edit]

The first step in any characterisation of a material or an object made of a material is often a macroscopic observation. This is simply looking at the material with the naked eye. This simple process can yield a large amount of information about the material such as the colour of the material, its lustre (does it display a metallic lustre), its shape (whether it displays a regular, crystalline form), its composition (is it made up of different phases), its structural features (does it contain porosity) etc. Often, this investigation yields clues as to what other tests could be performed to fully identify the material or to solve a problem that has been experienced in use.

Microscopic Observation[edit]

Microscopy is a technique that, combined with other scientific techniques and chemical processes, allows the determination of both the composition and the structure of a material. It is essentially the process of viewing the structure on a much finer scale than is possible with the naked eye and is necessary because many of the properties of materials are dependent on extremely fine features and defects that are only possible to observe using one of the following techniques in this field.

Optical Microscopy[edit]

Optical microscopes are formed of lenses that magnify and focus light. This light may have been transmitted through a material or reflected from a material's surface and can be used to ascertain a great deal of information about that material under evaluation. This can include whether the material is dense or contains porosity, what colour the material is.

A common practice performed in conjunction with optical microscopy is that of targeted and controlled chemical attack of the material using one of many chemical reagents available. For metallic materials, this technique combined with optical microscopy is know as optical metallography. The basis of this combined technique is that regions of different composition within a material as well as entirely different materials are affected differently when exposed to certain chemicals. These chemical effects are catalogued in various works (for example the ASM Metals Handbook or Metallographic Etching by G. Petzow) and through an understanding of these effects and a systematic experimental process they can be used to determine material composition and structure.

There are several limitations to the usefulness of optical microscopy. The first is that the maximum resolving power is limited by diffraction effects to approximately 0.2 micrometres at a magnification of around 1500X). Many of the defects and structural features important in determining material properties, and therefore of interest to materials scientists, are of atomic scale. (for , the diameter of a helium atom is approximately 100 picometers) The second major limitation in optical microscopy is limited depth of field. This limitation means that surfaces with features at different heights - such as the rough surfaces of a fractured specimen for example - cannot be seen in sharp focus at the same time. This means that flat or polished surfaces are preferred for this technique. Furthermore, the chemical techniques required for identifying different phases within a structure are destructive. Thus, if a only a small amount of a certain portion of the sample is present then this may be destroyed by the process by the etching technique.

Electron Microscopy[edit]

Scanning Electron Microscopy[edit]
Transmission Electron Microscopy[edit]
Chemical Analysis in Electron Microscopy[edit]

Diffraction Techniques[edit]

Principles of Diffraction[edit]

X-Ray Diffraction[edit]

Neutron Diffraction[edit]

Electron Diffraction[edit]

Spectroscopic Techniques[edit]

Energy Dispersive X-Ray Spectroscopy[edit]

Wavelength Dispersive X-Ray Spectroscopy[edit]

Electron Energy Loss Spectroscopy[edit]

X-Ray Photoelectron Spectroscopy[edit]

X-ray photoelectron spectroscopy (XPS) is surface analytical technique used to characterize materials.

Auger Electron Spectroscopy[edit]

Infra-red and Raman Spectroscopy[edit]

Ultra-violet and Visible Spectroscopy[edit]

Electrical and Magnetic Techniques[edit]

Electrical Resistance[edit]

Impedance Spectroscopy[edit]

Thermal Techniques[edit]

Thermogravimetric Analysis (TGA)[edit]

Differential Scanning Calorimetry (DSC)[edit]

Mechanical Testing[edit]

Strength[edit]

Hardness[edit]

Hardness is defined as the resistance of a material to penetration by an indentor. The Mohs scale of hardness has ten level and diamond is the material with the highest level of hardness ever known. There are several methods used to determine material's hardness, such as: Brinell, Rockwell, Vickers and Poldy.

Hardness Brinell (HB)[edit]

Is the method used for raw metallic materials. It uses a spherical ball indentor in order to stamp a print in the material. An external force transmitted through the indentor over the surface of the material determines the material's penetration.

Hardness Rockwell (HRB/HRC)[edit]

Is the method used for heat treated metallic materials. It has two variants regarding the indenter shape (ball or cone).

Hardness Vickers (HV)[edit]

Is a method used for the determination of hardness of special metallic materials, such as high alloyed materials, characterized by a very high degree of hardness.

Non destructive testing (NDT)[edit]

Some of the NDT methods available are: ultrasonic method, radiation penetration method.

Creep[edit]

Creep is defined as time-dependent strain under stress that is lower than the yield point.

Creep will be significant if T>=Tm

There are 3 creep regimes:

  1. primary -creep rate decreases towards a constant value
  2. secondary(steady state) -creep rate keeps constant; most important for design
  3. tertiary -creep rate increases until rupture, appears under tensile loading only

Steady state creep rate:

rate= Aσ^n exp(-Q/kT)

A=statistical entropy factor of the system,

Q=activation energy, obtained as slope of In(rate) vs 1/T plot,

k=Boltzmann constant,

T=temperature,

σ=stress,

n=creep exponent, obtained as slope of In(rate) vs Inσ plot,

if n≈1 at low stress, low stress regime is called linear creep regime; if 3<n<8 at high stress, high stress regime is called power creep law regime.

if T>0.5Tm, Q≈QSD(activation energy for self-diffusion)