Technical Programs under XRD System

Analytical support for structural analysis

We extend X-Ray diffraction analytical support for qualitative and quantitative estimations in the range of 0.1 to 100% phase identification in a phase pure/mixture samples up to about 25 phase complex phase mixture in a single sample.

Generally fine powder samples (grain size less than around 30 micron size) or solid samples with one of the faces that is to be exposed to X-Rays with solid size maximum of 35 mm diameter and up to about 50-60 mm high can be tolerated for XRD investigations.

The powder samples are generally dusted on a ‘Zero Background Plates (ZBP)’ to avoid background noise arising from the sample holder. Monolayer thick samples are generally used to avoid Bragg peak broadening due to scattering of X-rays from different layers across the sample thickness to sample absorption effects. Normally all the powder samples are continuously rotated at 60 rpm in every Bragg plane to minimize / average out the preferred orientation effects of the grains present in the sample.

Sample holder to accommodate large sized samples for XRD investigations

Our existing D8 Advance XRD system has been extremely fine tuned to produce distinct Bragg peaks of Si (111) K a 1, 2. The Full-Width-at-Half-Maximum (FWHM) of Si (111) K a 1 in the present condition is 0.016 ° 2 q (lowest recorded so far any where in the world using sealed XRD tubes under normal laboratory conditions).

Quantitative phase analysis of XRD
High resolution of XRD system

The phase analysis is generally carried out by superposing the Bragg Peak positions obtained from latest PDF-4 database of the desired phase on the experimental pattern and measuring the peak height(s) of individual most intense Bragg peaks of individual phases after subtracting their local background at the foot of the Bragg peak. The concentrations of individual phases are then evaluated on the basis of the Bragg peak heights of all the phases, taking into effect their I/I cor values (necessary software for ICDD data superposing on the expt. Patterns, Deconvolution of highly merged peaks and search /match programs for proper identification of correct phase(s) has been developed in-house). It is observed that this way of quantitative analysis results match close to those reported by other experimental techniques like ICP AES, Dilatometer, EDS etc.

Crystallite (Particle) size analysis by XRD Using Scherrer formula

Crystallite (particle) size estimation from XRD methods which produces information from the bulk of the sample is far better than those numbers produced by highly localized information obtained from TEM / HREM etc. Since in XRD the FWHM of the Bragg peak arising out of the crystallite Bragg peak is considered for size measurements it is appropriate to call as Crystallite size. Crystallite size estimation from XRD instruments depend on a few important factors which include (1) Instrumental broadening (2) Strain (uniform/non uniform) effects present in the samples (3) Preferred orientation effect of the sample (4) Amorphous nature of the sample material (5) Stacking faults etc. Crystallite size below approximately 300 nanometers can only be estimated by XRD methods since FWMH becomes normally positive because of absolute value of (FWHM - (Instrumental Broadening width + Strain factors)).

Since our XRD system has been extremely fine tuned, the instrumental broadening effect is negligible. The appropriate shape factor depending up on the shape of the material under consideration (some times after investigating the shapes of the same materials in EDS or TEM) is plugged into the equation.

Retained Austenite content

Retained austenite content is generally measured by XRD methods from the measurement of peak intensities of Austenite and Magnetite + Peralite Bragg peaks from the experimentally observed sample data.

Direct comparison method is assumed as the greatest metallurgical interest, as it can be applied directly to polycrystalline aggregates, as developed by Averbach and Cohen [1]. This method has been widely used for measurement of retained austenite in hardened steels.

The hardening of steel requires two operations: (1) heating to a high temperature to form a homogeneous, face-centered cubic solid solution called Austenite and (2) quenching the austenite to room temperature to transform it to a hard, metastable, body-centered-tetragonal solid solution called Martensite.

In practice, quenched steel may contain some un-dissolved carbides and because of incomplete transformation, some austenite is often retained at room temperature. The effect of this austenite on the service behavior of the steel is usually detrimental, but sometimes beneficial. At any rate there is a considerable interest in methods of determining the exact amount of austenite content present. Quantitative microscopic examination is fairly satisfactory as long as the austenite content is fairly high, but becomes unreliable below about 15 % of austenite in many steels.

The X-ray method, on the other hand, is quite accurate in low-austenite range, often the range of greatest practical interest.

Assume that a given hardened steel contains only two phases, martensite and austenite. The problem is to determine the composition of the mixture, when the two phases have the same composition but different crystal structure. The external standard methods cannot be used, because it is usually impossible to obtain a reference sample of pure austenite or of known austenite content, of the same chemical composition as the austenite in the unknown. Instead, we proceed as follows:

We can make an absolute measurement of austenite content of the steel by direct comparison of the integrated intensity of an austenite line with the integrated intensity of the martensite line. By comparing several pairs of austenite and martensite line, we can obtain several independent values of the austenite content. If steels contains a third phase, Fe3C (Cementite) we can determine the Cementite concentration either by quantitative microscopic examination or by diffraction. If we measure Ic, the integrated intensity of a particular Cementite line and calculate Rc, then we can set up an equation similar to the earlier one from which c/cc can be obtained. The value of c can be found from the relation

c + cx + cc = 1

In choosing diffraction lines to measure, we must be sure to avoid overlapping or closely adjacent lines from the different phase. The austenite and martensite 1.0% carbon steel, made with Cr K alpha radiation. Unfortunately strong (111) austenite line is too close to the (101)-(110) martensite line. Suitable austenite lines are the (200) and (220); these may be compared with (002)-(200) and (112)-(211) martensite doublets. These doublets due to tetragonality of the martensite unit cell are not usually resolved into separate lines because all lines are usually quite broad, both from martensite and austenite. The unresolved martensite lines are then indexed as a cubic line: for example the (002) - (200) doublets are called the (200) line. The line broadening is due to the non-uniform microstrain in both phases of the quenched steel and very often the fine grain size.

Bragg Peak Positions for relevant Diffraction line using Cu Ka Radiation

Phase (hkl) (~) 2
Austenite (200) 74.6
Martensite (002) 61.4
Martensite (200) 65.5

Austenite (222) 95.9
Martensite (112) 79.3
Martensite (211) 82.1

In the measurement of diffraction intensity, it is essential that the integrated intensity (area under the curve) and NOT the maximum intensity, be measured. Large variations in the shape can occur because of variations in the micro-strain and grain size. These variations in line shape will not affect the integrated intensity, but they can make the values of maximum intensity absolutely meaningless.

It is also essential to subject the sample to at least at the rate of 1 revolution per second in every plane of diffraction during the sample data collection to minimize the preferred orientation of grains. Using auto convergence/auto divergence slits at the time of X-ray diffraction experiments helps the X-ray beam foot print to be same irrespective of angle of data collection, there by making intensity information/final results more reliable.

• Quantitative phase estimation from multiple phases
• Theoretical modeling of XRD phases
• Rietveld refinement methods for identified phases

XRD Pattern of LAB 6 Powder