Characterization Techniques Flashcards
X-Ray Diffraction
X-ray diffraction is a technique used to study the atomic structure of materials. When X-rays are directed at a solid, they interact with the electrons of the atoms in the material, scattering in various directions. For diffraction to occur, the arrangement of atoms in the material must cause the scattered X-rays to constructively interfere, creating distinct patterns. This is described by Bragg’s law, which states that diffraction occurs when the path difference between X-rays scattered from parallel planes of atoms equals a whole number of wavelengths.
Bragg’s Law
Bragg’s law helps determine the relationship between the X-ray wavelength, the angle of incidence, and the distance between atomic planes in the crystal. By analyzing the angles and intensities of these diffracted beams, scientists can deduce the atomic structure of the material. Different crystal structures, like BCC or FCC, have specific diffraction conditions based on how atoms are arranged, and reflection rules help predict which planes will produce diffracted beams. X-ray diffraction is widely used to identify unknown materials and understand their crystallographic structure.
Light Microscopes
Light Microscopes use visible light (400-700 nm) to illuminate the object of viewing. Light passes through the specimen, and an optical lens system magnifies the image. The image is viewed directly through an ocular lens.
Light Microscope Pros
No risk of radiation leakage.
Inexpensive with low maintenance costs.
Light Microscope Cons
Lower magnification (500x to 1500x).
Low resolution compared to electron microscopes.
Limited for detailed structural studies.
Electron Microscope
Electron microscopes use a beam of electrons (approx. 1 nm) to scan or pass through the specimen. Electrons interact with the studied object, and the image is formed based on electron scattering. The image is projected onto a zinc sulfate fluorescent screen for viewing.
Electron Microscope Pros
Higher magnification (up to 16000x directly, and up to 1000000x photographically).
High-resolution images provide detailed structural information.
Widely used in scientific research for in-depth analysis of materials.
Electron Microscope Cons
Risk of radiation leakage.
Expensive with high maintenance costs.
Parts of a TTT Diagram 1
X-axis: Time (often on a logarithmic scale), which shows how long the material is held at a certain temperature.
Y-axis: Temperature, which indicates the temperature at which the material is processed. Transformation Lines: These lines represent different phase transformations (e.g., austenite to martensite for steels, crystallization for ceramics). Nose Curve: The curve where transformation starts and completes, indicating the optimal time and temperature for achieving specific microstructures.
Parts of a TTT Diagram 2
Start Line: Marks the beginning of a phase transformation (e.g., the start of crystallization).
Finish Line: Marks the end of the transformation, indicating when the phase change is complete. Isothermal Transformation Region: Area within the diagram where a material is held at a constant temperature to allow specific transformations to occur. No-Transformation Region: Areas where no significant phase changes occur during the given time and temperature conditions.
Interpreting TTT Diagrams
Phase Changes: The TTT diagram provides insight into what happens to the material’s structure when subjected to different heating and cooling schedules.
Timing and Temperature: The diagram shows the optimal time and temperature needed to achieve desired transformations.
Phase Keeping Diagram
Phase change diagrams are graphical representations that illustrate the relationships between temperature, pressure, and the phases of a substance (solid, liquid, gas) at equilibrium. These diagrams help visualize how a material transitions between different states under varying conditions, which is crucial in materials science and engineering for predicting phase behavior during processes like melting, solidification, and chemical reactions.
unary phase change diagram
A unary phase change diagram illustrates the phase behavior of a single substance as temperature and pressure change. This type of diagram usually features a simple phase boundary that divides solid, liquid, and gas regions, along with critical points like the triple point (where all three phases coexist) and the boiling point. Unary phase diagrams are useful for understanding the basic phase transitions of pure substances without the complexity of multiple components.
binary phase change diagram
A binary phase change diagram depicts the phase behavior of two components (A and B) as temperature and composition vary. The diagram typically features regions indicating different phases (e.g., solid, liquid, solid solution) and critical points such as eutectic and eutectoid points, where specific compositions exhibit unique phase transitions. The diagram helps determine melting points, solidification pathways, and the composition of phases present at various temperatures.
Binary Phase Diagram: Axes
X-Axis: Typically represents the composition of the two components, ranging from pure A (0% B) to pure B (100% B). This is often expressed as weight percent or mole percent.
Y-Axis: Represents temperature, indicating how temperature affects the phase behavior of the mixture.
Binary Phase Diagram: Phase Regions
Single-Phase Regions: Areas where only one phase exists (either solid or liquid).
Two-Phase Regions: Areas where two phases coexist, such as a solid and liquid mixture. These regions are typically represented by horizontal lines or curves on the diagram.
Binary Phase Diagram: Phase Boundaries
Liquidus Line: The upper boundary of the two-phase region, indicating the temperature above which the mixture is entirely liquid. Below this line, solid phases may start to form.
Solidus Line: The lower boundary of the two-phase region, indicating the temperature below which the mixture is entirely solid. Above this line, both solid and liquid phases coexist.
Binary Phase Diagram: Critical Points
Eutectic Point: A special composition point where a liquid phase can transform into two solid phases simultaneously at a specific temperature. This point is crucial for understanding alloy solidification.
Eutectoid Point: A point where a solid phase transforms into two different solid phases at a specific composition and temperature.
Peritectic Point: A point where a solid and a liquid phase transform into a second solid phase upon cooling.
How to read a binary phase diagram
Identify the Composition: Determine the composition of the mixture you are interested in, locating it on the x-axis.
Find the Temperature: Locate the temperature of interest on the y-axis.
Determine the Phase Present: Use the composition and temperature coordinates to find which region of the diagram you are in:
If you fall within a single-phase region, only one phase is present (either solid or liquid).
If you are in a two-phase region, identify the phases present using the liquidus and solidus lines.
Analyze Phase Changes: As temperature changes, observe how the composition moves through different regions of the diagram, indicating potential phase transformations.
Ceramic Phase Change Diagrams: Al2O3–Cr2O3 System
This system resembles the Cu-Ni isomorphous diagram, with single liquid-phase and solid-phase regions and a two-phase solid-liquid region shaped like a blade. The solid solution is substitutional, as Al³⁺ and Cr³⁺ ions have similar radii and charges, with the same crystal structure, allowing solubility below the Al₂O₃ melting point.
Ceramic Phase Change Diagrams: MgO–Al2O3 System
The system features an intermediate phase, spinel (MgAl₂O₄), which has a nonstoichiometric range of compositions. Limited solubility exists between Al₂O₃ and MgO due to the ionic size and charge differences. Two eutectics are present on either side of the spinel field, and spinel melts congruently at about 2100°C.
Ceramic Phase Change Diagrams: ZrO2–CaO System
The system includes eutectic and eutectoid reactions and three ZrO₂ phases—tetragonal, monoclinic, and cubic. Adding 3-7 wt% CaO stabilizes zirconia by preventing crack formation caused by the tetragonal-to-monoclinic transformation. Partially stabilized zirconia (PSZ) retains cubic and tetragonal phases at room temperature. Y₂O₃ and MgO can also stabilize ZrO₂.
Ceramic Phase Change Diagrams: SiO2–Al2O3 System
This system is significant for ceramic refractories. Silica and alumina do not form solid solutions, with an intermediate compound, mullite (3Al₂O₃–2SiO₂), which melts incongruently at 1890°C. A eutectic occurs at 1587°C with 7.7 wt% Al₂O₃.
