Experimental Techniques for Identification of Microstructure and Defects

Various Instruments for the Study of Microstructure It is important to study the behavior of materials based on their microstructures, and existing defects. Various Instruments are designed to reveal information about the internal structure of material at length scales from micro to nano range. In this range, the structure of grains, gain boundaries, line defects, surface defects may be studied. Instruments for the microstructure study includes: optical metallurgical microscope, scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM, and scanning probe microscopy.

Optical Metallography Study the features and internal makeup of materials at micrometer level. Provide qualitative and quantitative information on grain size, grain boundary, internal damage, etc. Quality of specimen is very important in the outcome of the process using optical metallography. Require procedure to prepare the material sample: 1) surface grinding, 2) polishing, 3)etching.

Optical Metallography Surface grinding stages remove large scratches and thin plastically deformed layers from the surface of the specimen. Polishing stages remove fine-scratches formed during grinding stages. Polishing aims to produce smooth, mirror-like surface without scratches---minimize topographic contrast. Etching process involving chemical etchant, and etching time.

Etching Atoms at the grain boundary will be attacked by etchant at a much rapid rate as compared to those inside the grains. Reasons: atoms at grain boundary possess higher state of energy due to less efficient packing. As a result, etchant produces tiny groves along the boundaries of the grains. The sample is then examined using the optical metallurgical microscope.

optical metallurgical microscope The microscope rely on visible incident light. When exposed to incident light, the groves at the grain boundary do not reflect the light as intensely as the remainder of the grain. The reduced light reflection causes the tiny grooves to appear as dark lines to the observer. Impurities and internal defects react differently to the etchant and reveal themselves in photomicrograph taken from the sample surface. Grain size and average gain diameter may be determined using the photomicrograph.

Optical metallurgical microscope

Effect of Etching Etched Unetched Etched Brass Unetched Brass Steel 200 X Unetched Brass 200 X Etched Steel 200 X Unetched Steel 200 X 4-32

Grain Size Important for polycrystalline material as the amount of grain boundary has significant effect on material strength. Affects the mechanical properties of the material More grain boundaries means higher resistance to slip (plastic deformation occurs due to slip). At high temperature, grain boundary are weaker, grain sliding may occur. 4-30

Measuring Grain Size ASTM grain size number ‘n’ is a measure of grain size. N = 2 n-1 N = Number of grains per square inch of a polished and etched specimen at 100 x. n = ASTM grain size number. n< 3 – Coarse grained 4 < n < 6 – Medium grained 7 < n < 9 – Fine grained n > 10 – ultrafine grained 200 X 200 X 1018 cold rolled steel, n=10 1045 cold rolled steel, n=8 4-31

Average Grain Diameter Average grain diameter , d, more directly represents grain size. Random line of known length is drawn on photomicrograph. Number of grains intersected is counted. Ratio of number of grains intersected to length of line, nL is determined. d = C/nLM C=1.5, and M is magnification 3 inches 5 grains.

An ASTM grain size determination is being made from a photomicrograph of a metal at a magnification of 100X. What is the ASTM grain-size number of the metal if there are 64 grains per square inch?

If there are 60 grains per square inch on a photomicrograph of a metal at 200X, what is the ASTM grain-size number of the metal?

Scanning Electron Microscopy (SEM) Used for fracture characterization, microstructure studies, surface contamination examination, and failure analysis. Impinges a beam of electron in a pinpointed spot on the surface of the specimen. Consists of an electron gun that produce electron beam in an evacuated column. Low-angle backscattered electrons interact with the protuberances of the surface and generate secondary backscattered electrons to produce an electronic signal. The signal in turn produces image with depth of field 300X that of optical microscope.

Scanning Electron Microscopy (SEM) Scanning electron fractograph of intergranular corrosion fracture

Scanning Electron Microscopy (SEM) SEM fractographs are used to determine whether fractured surface is intergranular (along the grain boundary) and transgranular (across the grain) or a mixture of both. Samples to be analyzed are coated with gold or other heavy metals to achieve better resolution and signal quality.

Transmission Electron Microscopy (TEM) Resolves features in the nanometer range. Defects such as dislocation can be observed on the image screen of a TEM. Sample preparation is complex. (e.g. electric-discharge machining, rotating wire saw, electropolishing, and ion-beam thinning. Specimen must have a thickness of several hundred nanometers or less depending on the operating voltage of the instrument. Specimen must have flat parallel surface.

Transmission Electron Microscopy (TEM) Electron beam is produced by heated tungsten filament and accelerated by high voltage. Electromagnetic coils are then used to condense the electro beam which is then passed through the specimen. A thick sample will not allow the passage of electrons due to excessive absorption and diffraction. Differences in crystal atomic arrangements will cause electron scattering that will form images on dislocations.

Transmission Electron Microscopy (TEM) Dislocation structure in iron deformed at 14% at -195 degree Celsius. The dislocations appear as dark lines.

High-resolution Transmission Electron Microscopy (HRTEM) Has resolution of 0.1nm Allow viewing of crystal structure at the atomic level. Basic concept similar to TEM, but the sample is significantly thinner—on the order of 10 to 15nm.

Scanning Probe Microscopy (SPM) Able to magnify the features of surface to the subnanometer scale, producing an atomic-scale topographic map. Important to analyze the arrangement of atoms and their bonding. Important to metrology on surface roughness Important to nanotechnology where position of atoms and molecules may be manipulated to investigate new nanoscale phenomena. E.g. Scanning Tunneling Microscope and Atomic Force Microscope.

Scanning Tunneling Microscope (STM) Consists of an extremely sharp tip made of metals such as tungsten, nickel, gold, and carbon nanotubes. The tip if first positioned a distance in the order of an atom diameter from the surface of the sample. At such proximity, the electron clouds of the atoms in the tip interact with the electron clouds of the surface atoms. If small voltage is applied across the tip and the surface, the electrons will “tunnel” the gap, producing small current that can be detect and monitored. The produce current is extremely sensitive to the gap size between the tip and the surface. Only materials that can conduct electricity may be mapped, include metal and semiconductor.

Scanning Tunneling Microscope STM tip sharpened using chemical etching techniques

Scanning Tunneling Microscope

Scanning Tunneling Microscope The quality of the surface topography achieved by STM is striking

Atomic Force Microscope (AFM) Similar to STM except that the tip is attached to a small cantilever beam. As the tip interacts with the surface of the sample, the forces (Van de Waals) acting on the tip deflect the cantilever beam. The deflection is monitored using a laser and a photodetector. The deflection is used to calculate the force acting on the tip. AFM can be applied to all materials even nonconductors. Application areas include DNA research, polymer technology, nanotechnology.