Material characterisation is of vital significance to identify and understand the component, microstructure, surface characteristic and other properties of a material. Common characterisation methods include SEM, TEM for surface morphology; XRD, XPS, FTIR for chemical composition and microstructure; DSC, DTA for crystal structure and chemical components and other major methods ^[1].

This page proposes three different characterisation methods including SEM, XPS and XRD in order to inspect the microstructure, element composition and content, chemical state, and crystal structure of the sample. The experiments are not conducted by the group due to objective conditions, however, example data are given as references for upcoming experiments.

1. Introduction to SEM

To research the microstructure of Nickel-chromium alloy, scanning electron microscope (SEM), a precise characterisation method to observe the surface ultrastructure of various solid substances, can be applied to observe the structure of Nickel chromium alloy. ^[1] In SEM, the electrons are applied to interact with atoms in the sample, producing different signals which contain information about composition and surface topography of the sample. The SEM can provide high-resolution microstructure images of micro-morphology of nickel-chromium system, which facilitates the investigation of the crystal morphology, metallography structure, and the atomic arrangement of Nickel-chromium alloy.

2. Principles for SEM

In SEM, the electrons are generated from the electron source at the top of the instrument and emitted with enough thermal energy. Then they are accelerated by the positively-charged anode to interact with the sample through the condenser lens and objective lens. In this process, secondary electrons and electromagnetic radiation are emitted, both of which would be collected by detector and converted into a video signal to screen. Finally, the signals are magnified by electronic amplifier and displayed as variations in brightness on computer monitor to represent surface of specimen. The whole process will be in a vacuum sealed environment to prevent pollution and interference. Moreover, the vacuum environment also improves the efficiency and resolution of the detector to collect electrons.

3. Identifying information and quality of SEM

SEM displays digital images of the intensity of signals emitted from scanned regions of specimen to qualitatively identify the microstructure of the material. Energy Dispersive X-Ray Spectroscopy (EDX) is usually used in combination with SEM to provide complementary information in terms of element composition. In this way, structure and composition of the specimen can be identified comprehensively with both qualitative and quantitative information.

SEM has extremely high accuracy in detection due to its strong magnification ability. Magnification in SEM can be controlled in the ranges of about 6 orders of magnitude, from 10 to 300000 times. The resolution of the latest SEM can reach 1nm, with large depth of field, high field of vision, and good stereo imaging effect.^[2]

4. SEM examples for Nickel chromium stainless steel

Figure 1. a, b) Scanning electron micrographs of Nickel chromium alloy; c, d) EDS of light and dark regions ^[2].

As is shown in the figure, the microstructure of nickel chromium alloy consists of light region, dark region and light grey region. Light regions are mainly spherical and separated by continuous dark regions, and there are some light grey regions located at the junction of the light region and dark region. Through EDS analysis in the SEM, it can be found that the light region contains 27 at% Cr and 73 at% Ni, indicating it to be the Ni-Cr alloy phase. While the dark region comprises 95 at% Cr and 5 at% Ni, indicating it to be the Cr₃C₂ phase. The light grey regions consist of the two phases, Cr₃C₂, Ni-Cr, indicating it to be a composite phase ^[2].

1. Introduction to XPS

For information on the element composition and content, chemical state, molecular structure, and chemical bonds of the compound, XPS technology can be utilized. Not only that, the inner electron binding energy and its chemical shift, molecular structure, atomic valence, and surface depth distribution can be detected. Also, the excitation source has very little damage to the sample.

2. Principles for XPS

The principle of XPS is to use X-rays to irradiate the sample to stimulate the emission of inner electrons or valence electrons of atoms or molecules. Electrons excited by photons are called photoelectrons whose energy can be measured. The kinetic energy or binding energy is taken as the abscissa, and relative intensity as the ordinate to make a photoelectron energy spectrum, which obtains information.

3. Identifying information and quality of XPS

XPS can perform qualitative and quantitative analysis on the element composition, surface depth and oxidation state. This is because the energy represented by the vertical axis is proportional to the number of electrons. Its detecting depth is 0.5-2nm. The spatial resolution is lower than 3μm and the detection sensitivity is 1-50nm.

4. XPS examples for nickel chromium stainless steel

Through searching the literature, it can be found that XPS mainly contains three functions in terms of chemical analysis which is suitable for the characterization of 4Cr14Ni14W2Mo.

The first application is to judge what types of elements exist in the material and calculate their compositions at the surface. Irradiating the samples with non-mono-chromatised Al Kα radiation, after subtracting the background, XPS peaks can be separated corresponding to the specific elements depending on the standard binding energies to tell the existence. In this case, Cr 2p, Mo 3d and Ni 2p are the most commonly used signals with relatively high intensity and obvious features. Using the deconvoluted spectra, the compositions of the elements are able to be calculated based on the corrected peak areas.

Figure 2. XPS spectra for Cr, Mo and Ni after smoothing and separating the peaks ^[3].

The second role is to determine the oxidation state of specific elements. As shown in figure 3, the changes of the valency of the particular element can be discovered when the corresponding peaks have shifted.

                           Figure 3. Normalised XPS spectra for Cr, Mo and Ni after 1 h oxidation under different electrode potentials ^[4].

Another advantage is to operate the depth profiling chemical composition analysis combining with the ion sputtering technique to study the impact from the environment.

1. Introduction to XRD

X-Ray diffraction analysis (XRD) is a technique that is able to determine the crystallographic structure as well as the composition of the material ^[5]. It could be used to measure the grain size, determine the crystal structure, or measure the stress in the material. In this experiment, XRD is used to identify the phase in 4Cr14Ni14Co2Mo. The principles for XRD are in traduced as followed.

2. Principles for XRD

There are three steps in XRD including electron emission, X-Ray generation, and secondary X-Ray generation. X-Ray is generated by the diode that is shown in Figure 4(a). The cathode is heated by current and electrons are ejected at a high temperature. After applying a voltage between the anode and the cathode, the electrons transmit to the anode. They eject the inner electrons of the anode atoms and form holes at the corresponding positions. The outer electrons fill the hole and there is an energy difference between their initial and final states that produces X-Ray, which is shown Figure 4(b). The X-Ray could hit the material for characterisation and produce Secondary X-Ray by the same mechanism, which is shown in Figure 5. Secondary X-Ray is used to identify the structure of the material.

3. Identifying information and quality of XRD

XRD pattern is an accurate and precise qualitative analysis in phase and crystal structure determination since the characteristic peaks are distinctive and nearly invariant for one individual phase, which ensures the accuracy of matching. In the case, comparing the experimental pattern with the standard patterns in the database, if diffraction peaks of the examined sample are matched with one pattern, the sample will correspondingly contain such a phase, and the accuracy of qualitative comparison is 100%.

4. XRD examples for nickel chromium stainless steel

                                                Figure 6. Standard XRD pattern of austenite for nickel chromium stainless steel ^[7].

  Figure 7. XRD pattern of 316L steel ^[8].

From Figure 6 and Figure 7, it is identified that, in 316L steel, the significant diffraction peaks respectively appear in 44°, 51°, and 75°, which are almost located at the same angles in standard pattern. In the case, it is proved that 4Cr14Ni14Co2Mo contains the austenite expressed as γ in Figure 5. Consequently, without any other impurity peaks, the dominated phase of 4Cr14Ni14Co2Mo can be determined as austenite after XRD.

1. Leng, Y. (2009). Materials characterization: introduction to microscopic and spectroscopic methods. John Wiley & Sons.
2. Cunha C, Lima N, Martinelli J, Bressiani A, Padial A, Ramanathan L. Microstructure and mechanical properties of thermal sprayed nanostructured Cr3C2-Ni20Cr coatings. Materials Research. 2008;11(2):137-143.
3. Risk assessment of XPS technique. CMM Labs: Hawken Building, Queensland Bioscience Precinct, Australian. Institute for Bioengineering and Nanotechnology, Chemistry Building. 1347.
4. Kocijan, A., Milošev, I. and Pihlar, B., 2004. Cobalt-based alloys for orthopaedic applications studied by electrochemical and XPS analysis. Journal of Materials Science: Materials in Medicine, 15(6), pp.643-650.
5. ASTM A276 / A276M-17, Standard Specification for Stainless Steel Bars and Shapes, ASTM International, West Conshohocken, PA, 2017, Available from: astm.org.
6. ASTM A240 316/316l Stainless Steel Plate, Anson steel, Access on: May 19, 2021, Available from: http://www.steels-supplier.com/steel-standard/astm-a240-316-316l-stainless-steel-plate.html.
7. 00-033-0397, Standard austenite diffraction pattern, JCPDF-ICDD, @2021 International Centre for Diffraction Data, Access on: May 19, 2021.
8. Mao Nan, Microstructure and mechanical properties of 316L stainless steel welded joint, Harbin Institute of Technology, 12.2012.