Techniques/facilities

What is the lab capable of?

Arc melting alloys

The atomic percent of each pure metal in the alloy are converted to mass percent, weighed out, and placed into a hemispherical hole in the arc melter container, in an order determined by their relative metling points – for example, a lower melting point material should be covered by the others, to avoid vaporisation and loss of material, changing the composition of the melted alloy. Alongside the weighed out metal, a piece of material with high oxygen affinity (a ‘getter’), for example titanium, is placed into the chamber. The air is then evacuated from the arc melter, and a current is passed between two electrodes. This forms a high temperature plasma arc of temperature variable with current, which is used to melt the metals. First, the getter is melted, to absorb any residual oxygen remaining in the chamber after evacuation, in order to further minimise oxidation of the melted alloy. The getter is melted on one side, before being flipped and re-melted on the opposite side. This process is repeated four times to ensure a thorough melt. This process is then repeated with the weighed out metal, until an irregularly-shaped blob of fully melted alloy is obtained. A variable polariser should be used at the window to the chamber, in order to both allow the user to assess the quality of the melt, and also protect their eyes from the bright plasma arc when the current is active.

 

Encapsulation

https://www.youtube.com/watch?v=HSmLw_VGz3g

In order to conduct heat treatments to obtain the desired microstructure,  samples may be placed into a non-vacuum furnace. This can only be done successfully, and without oxidation, if the samples are first sealed into an ampoule which is evacuated of oxygen, and filled with an inert gas, such as argon. Encapsulation is the process of wrapping the sample in molybdenum foil before placing it into a quarz glass tube (sometimes along with a getter if the sample is suspected to have a high oxidation susceptibilty), evacuating the oxygen from it before filling it with argon, and heating and twisting the tube to seal the sample inside it, over a gas flame. The seal can be tested by submerging into water, to ensure no oxidation of the sample will occur in the furnace.

Grinding/polishing for metallography

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Before analysing samples via microscopy or X-ray diffraction, it must be ensured that the sample surface is planar and completely cleaned. To ensure this, samples are ground and polished before characterisation.

For ease of handling, the samples are first mounted in bakelite under elevated temperature and pressure.  OPAL460 mounting press apparatus is used, at a temperature and pressure of 180C and 180atm, respectively.

To grind the samples – therefore ensuring a smooth, planar surface – the exposed edges are ground against silicon carbide paper on a grinding wheel. Papers of decreasing grit are used to smooth the surface further, with higher grits requiring a shorter time, and lower grits requiring more grinding time as their ability to remove surface material is reduced.

After ensuring a smooth surface, the samples are then polished with a suspension containing hard abrasive particles, made of diamond or silicon carbide, against a neoprene polishing cloth. The purpose of this is to reveal the microstructure, by ensuring a smooth surface down to 0.25um, such that electon beam microscopy may be employed.

After polishing, the sample should be cleaned in an ultrasound bath, which removes small particulates left over from preparation with ultrasonic vibrations to dislodge them.To reveal the microstructure further, an etchant may be applied to the surface of the sample. Etchants preferentially erode grain boundaries, making them more prominent under a microscope.

Unpolished sample – original cut surface, mounted in bakelite.

 

Mirror finish sample – camera lens visible in surface. This sample was ground against SiC grinding papers of decreasing grit, before polishing with a 0.25um silicon carbide suspension.

Scanning electron microscopy (SEM)

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Scanning electron microscopy can be used to achieve high resolution imaging of very small surfaces. An electron beam scans over the surface of the sample, and the interactions that occur between the electrons and the sample produce information about the topography and composition.

In the M4X group, SEM is used to produce detailed images of the topography, phase and composition of the superalloys produced.  This allows us to examine the grain structure, composition changes across grain boundaries, and secondary phase particles in the microstructure; all of which have effects on the mechanical properties of the alloy.

To obtain information about the topography of the sample, Secondary Electron (SE) imaging is used: Secondary electrons are a result of near-surface inelastic interaction between the atoms and the incident electron, and therefore provide information about the sample surface. The secondary electron information is recorded by placing a detector into the electron chamber at an angle, so as to optimise the intensity of the beam detected. Before detetction, the electron beam is converted to a light beam by attacting them to a positively-charged Faraday case, in which sits a scintillator. The scintillator accelerates such that they present at a photomultiplier as light, allowing their signal to be amplified before detetction.

For information about the composition of the sample, Back-Scatter Electron (BSE) imaging may be used – BSE electrons originate from an elastic interaction between the incident beam and a large volume of the sample. Compositional data is obtained by consideration of the scattering angle of the BSE electrons, as this angle is proportional to the atomic number of the atoms: a larger atom will cause a greater scatter, giving a greater signal. Therefore, the intensity of the signal provides information about the constitutional elements in different phases. The BSE information is detected by a solid state detector, which contains p-n junctions connected to two electrodes. Electron-hole pairs generated by the BSEs which escape from the surface of the sample are attracted to the positivie and negative electrodes respectively, constituting an electrical current across them – the nature of which is dependent of the amount of electron-hole pairs generated, and therefore the number of BSEs escaped. This way, the current contains information about the atomic numbers of the atoms present.

SE micrograph of an aged FeNiAl-Mn sample, from JEOL 6060 SEM
Hitachi 3030+ Benchtop SEM (this apparatus is commonly used for a quick overview of the SE and BSE data for characterisation.  More sophisticated SEMs may be used for greater detail.

Transmission electron microscopy (TEM)

(Heymer, 2020)

Transmission electron microscopy can be used to image very thin slices (less than 100nm thick) of a sample with even better resolution than scanning electron microscopy (SEM). Whereas in SEM the electron beam scans across the surface and surface level information is gathered, in TEM the full electron beam is transmitted through the sample. An image is then formed from the transmitted beam, which can be used to analyse the microstructure in much greater detail than other microscopy methods.

 

 

Electron backscatter diffraction (EBSD)

Example of an EBSD mapping showing grain orientations variations across the sample surface (On a W-Mn sample)

Electron back-scatter diffraction can be used to identify the phase, crystal structure, orientation and texture of a material. A sample is placed at a 70° angle, and an electron beam scans across its surface. The electrons undergo Bragg diffraction, and back-scatter to the detector. Kikuchi bands (stripes of detected electrons across the detector) are observed, which are characteristic of specific crystal structures at specific angles. EBSD therefore analyses the crystal structure and orientation at each point on its surface. This describes the texture of the material, which has significant effects on its mechanical properties.

 

X-ray diffraction

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X-ray diffraction is a technique used to investigate the crystal structure of a sample. X-rays are incident on the sample at a range of angles, giving a diffraction pattern since the crystal lattice of the sample constitutues a diffraction grating.  At certain angles, the diffracted waves add constructively, so the XRD detector detects a “peak” at these angles. The angle that these peaks are found at depends on the crystallographic parameters of the material ie. the lattice spacing, so these can be extracted from the XRD results via Bragg’s law. This law describes the relationship between the diffraction angle, the wavelength of the incident beam, and the planar spacing in the lattice.

Information about the crystal structure of the sample, and the phases present may also be found, given that each crystal structure contains its own set of characteristic planes which appear in XRD analysis (these are not necessarily the only planes present – intermedicate planes may not be visible via XRD as the nature of the diffraction means that some signals may interfere destructively). For example, a body-centred cubic structure shows a trio of peaks corresponding to the 110, 200, and 211 planes, which will allow a phase of this structure to be identified.

Powder diffraction may also be used in order to provide a textured isotropic sample, therefore representing each plane in the signal due to the random orientation of each particle.

(Sub-scale) Three- and four- point flexural testing

Three-point flexural testing (or three-point bending) is a mechanical testing technique in which a cuboidal sample of a material is suspended on two support pins, and bent by lowering a third pin through its centre. The force required to lower the pin and bend the sample is recorded, and this data can be used to calculate a flexural stress-strain response for the material. This can then be used to analyse the deformation and fracture properties of the material, and compare it to other materials.

Three point bending (Heymer, 2020)

In four-point bending, two central pins are lowered to bend the sample. This creates a larger region of the sample that is maximally stressed, providing more reliable results.

Usually, mechanical tests require relatively large samples. This is not always a feasible option for expensive or exotic alloys that we may only produce in small quantities. Sub-scale three-point bending is suited towards samples as small as 12mm long and 1mm thick, and is ideal for analysing materials with limited volume.

Microhardness Testing

Vicker’s microhardness apparatus in action: the diamond indentor can be seen to be under loading, indenting the sample. This will leave a pyramidal indent whose dimensions may be determined under a microscope.

The samples are hardness tested using Mitutoyo HM-124 Vickers microhardness testing apparatus. This involves indenting the surface of the sample with a diamond pyramidal indentor of interfacial angle 136degrees under a 20kgf load, before measuring the diagonal diameter of the indent under an optical microscope.

The hardness of the sample is proportional to by the ratio between the diameter of the indentor and that of the indentation, and represents the ability of the material to resist plastic deformation. As the measurement is made by eye, and therefore prone to human error, a calibration is first performed for a material of known hardness, to provide a conversion for the operator.