Materials Science/Material Characterization

Materials Characterization

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

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

Optical microscopes are formed with 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 Characterization

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.

Macroscopic Observation

The first step in any characterization 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

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

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.

Spectroscopic Techniques

Electron Energy Loss Spectroscopy

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X-Ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) is surface analytical technique used to characterize materials.In this we use x rays to eject photoelectron when they strike on a material.

Electrical and Magnetic Techniques

Electrical Resistance

This tells about the transport of electrons within material.

Thermal Techniques

Thermal analysis is a very essential method to study the thermal behavior of materials and finds widespread applications in diverse industrial and research fields. It is a general term, which covers a group of related techniques in which the temperature dependence of the parameters of any physical property of a substance is measured. Thermoanalytical methods involve the measurement of various properties of materials subjected to dynamically changing environments under predetermined conditions of heating rate, temperature range and gaseous atmosphere or vacuum.

The thermoanalytical analysis includes the following three interrelated techniques: • Thermogravimetric analysis (TGA), which involves monitoring of change in weight while varying temperature. • Differential thermal analysis (DTA), which involves a measure of energy changes, comparing the precise temperature difference between a sample and an inert reference material, while heating both. • Differential scanning calorimetry (DSC), similar to that of DTA except that electrical energy is used to restore the cooler of the two materials to the same temperature as the other.

Thermogravimetric Analysis (TGA)

The thermogravimetric analysis (TGA) is a type of thermoanalytical testing performed on materials to determine changes in weight in relation to changes in temperature. The TGA relies on a high degree of precision in three measurements: weight, temperature and temperature change. A derivative weight loss curve can be used to tell the point at which weight loss is most apparent. The TGA is commonly employed in research and testing to determine characteristics of materials, to determine degradation temperatures, absorbed moisture content of materials, the level of inorganic and organic components in materials, decomposition points of explosives and solvent residues.

A simultaneous TGA-DTA measures both heat flow and weight changes in a material as a function of temperature or time in a controlled atmosphere. The simultaneous measurement of these two material properties not only improves productivity but also simplifies interpretation of the results. The complementary information obtained allows differentiation between endothermic and exothermic events which have no associated weight loss (e.g., melting and crystallization) and those which involve a weight loss (e.g., degradation).

The TGA analyzer usually consists of a high-precision balance with a pan (generally platinum) loaded with the sample. The pan is placed in a small electrically heated oven with a thermocouple to accurately measure the temperature. The atmosphere may be purged with an inert gas to prevent oxidation or other undesired reactions. A computer is used to control the instrument. An analysis is carried out by raising the temperature gradually and plotting weight against temperature. After the data is obtained, curve smoothing and other operations may be done such as to find the exact points of inflection.

Differential Thermal Analysis (DTA)

The differential thermal analysis (DTA), often considered an adjunct to TG, is far more versatile and yields data of a considerable more fundamental nature. The technique is simple as it involves the measurements of the temperature difference between the sample and inert reference materials, as both are subjected to identical thermal regimes, in an environment heated or cooled at a constant rate. The origin of the temperature difference in the sample lies in the energy difference between the products and the reactants or between the two phases of a substance. This difference is manifested as enthalpy changes-either exothermic or endothermic. The differential thermal curve would be parallel to the temperature axis till the sample undergoes any physical or chemical change of state. However, as soon as the sample has reached the temperature of this change of state, the additional heat flux reaching the sample will not increase the sample temperature at the same rate as that of reference and the differential signal appears as a peak. The differential signal would return to the baseline only after the change of state of the sample is completed and the temperature becomes equal to that of the reference material.

Differential Scanning Calorimetry (DSC)

The differential scanning calorimetry is a thermodynamical technique in which the difference in the amount of heat required to increase the temperature of a sample reference is measured as a function of temperature. Both the sample and reference are maintained at very nearly the same temperature throughout the experiment. Generally, the temperature program for a DSC analysis is designed such that the sample holder temperature increases linearly as a function of time. The reference sample should have a well-defined heat capacity over the range of temperatures to be scanned.

The basic principle underlying this technique is that, when the sample undergoes a physical transformation such as phase transitions, more or less heat will need to flow to it than the reference to maintain both at the same temperature. Whether more or less heat must flow to the sample depends on whether the process is exothermic or endothermic. For example, as solid sample melts to a liquid it will require more heat flowing to the sample to increase its temperature at the same rate as the reference. This is due to the absorption of heat by the sample as it undergoes the endothermic phase transition from solid to liquid. Likewise, as the sample undergoes exothermic processes such as crystallization less heat is required to raise the sample temperature. By observing the difference in heat flow between the sample and reference, differential scanning calorimeters are able to measure the amount of heat absorbed or released during such transitions. The DSC may also be used to observe more subtle phase changes, such as glass transitions.

Mechanical Testing

Hardness

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)

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)

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

Hardness Vickers (HV)

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)

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

Creep

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)