Polycrystalline Texture Stress (PTS) X-Ray Diffractometer (XRD)
Scintag's Polycrystalline Texture Stress (PTS) / X-ray Diffractometer (XRD) is designed to do the work of three instruments, they are as follows: 1) Polycrystalline X-ray Diffraction; 2) Crystallographic Texture Determination (Preferred Orientation Measurement); and 3) Residual Stress Determination. These techniques are used for solving problems in metallurgy, materials science, ceramics, mineralogy and chemistry.
X-ray diffraction analysis is based on the well-known Bragg equation which relates wavelength to angle of incidence and spacing between atomic planes: nλ=2d sinθ , where λ is the of the incident x-ray beam wavelength (typically measured in angstroms), d is the atomic spacing for a particular set of planes (also in angstroms), and θ is the angle to the surface normal. When these conditions are satisfied, a peak in the x-ray intensity will occur. A typical powder or polycrystalline diffraction pattern spectrum consists of a plot of diffracted intensities (peaks) that are caused by layers of atoms in the material versus the diffraction (detector) angle 2θ. The diffraction angle, 2θ , is the angle between the incoming x-ray beam and the diffracted intensity. The 2θ values for the peaks also depend on the wavelength of the anode in the x-ray tube. Furthermore, the areas under the peaks are relative to the amount of each phase present in the spectrum.
The combination of x-ray optics and the intrinsically low background of the high-purity germanium crystal (80 mm2 active area) solid state detector, which operates near liquid nitrogen temperatures, defines the ultimate resolution, peak-to-background ratio, sensitivity, and speed of data acquisition. The resolution of the germanium x-ray detector at the energy of the Cu Kα x-ray tube (radiation source) is approximately 200eV Full Width Half Maximum (FWHM) at 5.9 keV guaranteed. The thin window material is beryllium.
XRD techniques are used to identify and quantify the following:
- Foreign material in various manufactured products
- Metallurgical phases (Ferrite, Austenite, etc.)
- Metallurgical corrosion products
- Filler materials in paper, paint, plastic, polymers, rubber, textiles, etc.
- Unknown solids
- Solids filtered from solution
- Relative crystallinity of plastics
- Quantitative measurements of crystalline phases
- Crystalline size measurements
- Lattice dimension measurements
PTS/XRD Data Collection Modes
Polycrystalline X-ray Diffraction — In powder or polycrystalline diffraction it is important to have a sample with a smooth plane surface. If possible, the sample should be ground down to particles of about 0.002mm to 0.005mm cross sections. The ideal sample is homogeneous and the crystallites are randomly distributed. The powder is pressed into a sample holder creating a smooth flat surface. Ideally a random distribution of all possible h,k,l planes is now present, and only crystallites having reflective planes (h,k,l) parallel to the sample surface will contribute to the reflected intensities. In a truly random sample, each possible reflection from a given set of planes will have an equal number of crystallites contributing to it. The sample is moved through the glancing angle θ in order to produce all possible reflections.
About 95% of all solid materials can be described as crystalline. When x-rays interact with a crystalline substance (phase) the result is a diffraction pattern. Furthermore, the same crystalline substance always gives the same pattern, and in a mixture of substances each produces its pattern independently of the others. Therefore, the diffraction pattern is a summation of each crystalline phase present in the sample. The x-ray diffraction pattern of a pure substance is, therefore, like a fingerprint of the substance. The powder diffraction method is thus ideally suited for characterization and identification of polycrystalline phases. About 30,000 inorganic and 12,000 organic diffraction patterns are available as standards for use in a computer search to match for qualitative phase identification of unknown components in a sample. Any single component of a specimen generally needs to be present at 2% to be detectable. For quantitative XRD, relative precision that can be achieved under favorable conditions is generally 2-10% of the amount present.
Crystallographic Texture (Preferred Orientation) — The determination of preferred orientation of the crystallites in polycrystalline material is referred to as texture analysis. The term texture is used as a broad synonym for preferred crystallographic orientation in a polycrystalline material, normally a single phase. Furthermore, preferred orientation is usually described in terms of pole figures. A pole figure is scanned by measuring the diffracted intensity of a given reflection (2θ is constant) at a larger number of different angular orientations of the sample. A contour map of the intensity is then plotted as a function of the angular orientation of the sample. The intensity of a given reflection (h, k, l) is proportional to the number of h, k, l planes in reflecting condition (Bragg's Law). Hence, the pole figure gives the probability of finding a given crystal-plane-normal as a function of the sample orientation. If the crystallites in the sample have a random orientation the recorded intensity will be uniform. By collecting data for several reflections and combining several pole figures we can arrive at the complete Orientation Distribution Function (ODF) of the crystallites within the single polycrystalline phase that makes up the material. A sample oscillator helps average the effects of coarse crystallite (grain) material.
Residual Stress Measurement — Residual stress can be introduced by any mechanical, chemical or thermal process. e.g. machining, plating and welding. Residual Stress is the stress that remains in a material after the external forces that caused the stress have been removed. Stress is defined as force per unit area. Positive values indicate tensile (expansion) stress, negative values indicate a compressive state. The stress causes microscopic deformations in the sample. The deformation per unit length is called strain.
The principle of stress analysis by x-ray diffraction is based on measuring angular lattice strain distributions. That is, we choose a reflection at high 2θ and measure the change in the d-spacing with different orientations of the sample. Using Hooke's law the stress can be calculated from the strain distribution. The PTS calculates residual stress by measuring the change in peak position for a given reflection as a function of sample tilt.
