Brief introduction
The microstructure of Titanium alloy can be detected by scanning electron microscope (SEM). Comparing with transmission electron microscope (TEM) and optical microscope, SEM will provide a stereoscopic 3D image and high-resolution observation. So it’s easier to obtain the image of the surface shape of the sample Ti-6Al-4V by SEM method.
Principle
Scanning electron microscope (SEM) is a large-scale precision instrument used for high-resolution micro-topography analysis. It is used to observe the morphology and composition of the surface ultrastructure of various solid substances. There are five main components of SEM which includes electron source, electron lenses, sample stage, detectors and display devices. The focused beam of electrons is shoot to the sample and generate a variety of signals which is captured by the detector and show in the display devices. These signals include secondary electrons backscattered electrons diffracted backscattered electrons photons visible light and heat. Among them, secondary electrons will produce SEM images, diffracted backscattered electrons are used to determine crystal structures and orientations of minerals.
SEM as a new technique of microscope imaging, it has a higher instrument resolution which can reach 3nm and the magnification range of the instrument is large and can be continuously adjusted.[1]
Image Analysis
There are 2 phases in titanium alloys which called α and β phase which α phase is HCP structure, β phase is BCC structure. In the graph, black part is β phase and the white acicular part is α phase. Obviously, the size of β grain is 550µm and the width of α platelets is about 2µm. Because of the existence of the two phases, Ti-6Al-4V alloy has high hardness and abrasion resistance. When detect a thick specimen, SEM can observe more real shape even the relationship between cross-sectional morphology and microstructure can also be obtained. After the machining process, the distribution of α and β phase will change and SEM image of Ti-6Al-4V will be different.
Reference
[1] Scanning Electron Microscopy (SEM), Susan Swapp, University of Wyoming, https://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html
[2] Joy D C. Introduction to the Scanning Electron Microscope[J]. Microscopy and Microanalysis, 2003: 1556-1557.
[3] Ma Y, Youssef S S, Feng X, et al. Fatigue crack tip plastic zone of α + β titanium alloy with Widmanstatten microstructure[J]. Journal of Materials Science & Technology, 2018, 34(11): 2107-2115.
Schematic Drawing of SEM[1]
Limitation
Because the display device of SEM can only show images of microstructure of the sample, it cannot give the quantitative information of the sample. Titanium alloy cannot be characterized by SEM without process and the sample Ti-6Al-4V need to be polished before characterizing.