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Characterisation

Characterisation in ASRC

1. Confocal Scanning Laser Microscope

The High Temperature Confocal Scanning Laser Microscope (HT-CSLM) is used to image metallurgical reactions and transformations at temperatures up to 1700oC. Using a scanning laser combined with confocal optics, the CSLM is able to image the sample's surface, in spite of the glow, and detect clear changes in contrast. The confocal nature detects only signals from the focal plane while decreasing the intensity of signals from other planes. The laser further increases the signal / noise ratio. This makes the CSLM ideal for imaging surfaces at high temperature.

The instrument has recently been upgraded to include an in-situ compression/tension stage allowing for the observation of phenomena such as recrystallization, deformation due to inclusion and measurement of material expansion due to temperature changes. With the previous high temperature stage being used for a range of investigations such as exploring peritectic solidification, slag-metal emulsification, oxide fluxing and isothermal solidification due to chemistry changes.

The equipment works on the premise of a gold-plated elliptical furnace which serves to reflect the IR-radiation from a halogen bulb (in 1 focal point) to the sample (in the second focal point). The sample is held in a crucible of suitable material on a platinum ring, to which a control thermocouple is attached to. This temperature reading is sent to an external temperature controller, which uses the reading to control the amperage (and therefore the intensity) of the halogen lamp.

The video output from the HT-CSLM in sent to a computer by way of analog-digital converter and is recorded by a software package. The temperature controller is connected to a computer by serial port, and so can have two-way communication with the program. Examples of HT-CSLM images and movies are shown here. Due to the surface imaging and high temperatures, the equipment is highly sensitive to atmospheric purity, as such pre-getters and filters are used to achieve up to parts per-trillion levels of oxygen when required.

The instrument has also been developed to offer unique capabilities such as: in-situ powder addition, thermal imaging of temperature gradients, high resolution cross imaging for sessile drop testing and gas humidification for moisture experimentation.

2. Differential Scanning Calorimetry (DSC)

The DSC is a thermo-analytical technique measuring the heat required to change a samples temperature, which allows phase changes (solidification, glass transition points, solid state transformation) or lattice restructuring (recovery, recrystallisation) to be characterised from the precise thermal and energy measurement.

The DSC in the ASRC (figure 1) has a high temperature furnace, capable of heating up to 1600°C and holding an atmosphere (usually inert high purity argon). Heating rates of up to 20°C/min and cooling rates of 20°C/min allow for investigations into the effect of heating rates on phenomena such as undercooling or latent heat emission. The equipment takes sample sizes from 10 mg to 2 g. Samples sit on a Pt stage (figure 2) with space for the sample crucible (alumina or Pt) and an empty reference crucible.

Multiple heating and cooling runs of a sample and reference are conducted to ensure material preparation effects are excluded from the phenomena characterised. As energy is lost or gained during a phase-change, peaks are formed along the energy/temperature profile indicating the occurrence of a given phenomenon (figure 3).

3. High Temperature Rheometer System FRS1600

The High Temperature Rheometer System in the Advanced Steel Research Centre is used to carry out viscosity measurements especially suitable for materials with high melting point. The FRS1600 consists of a rotational rheometer using a Double Concentric Cylinder, the first includes the molten material and the second creates the shear stress. The attached furnace goes up to 1520 °𝐶 after calibration. Viscosity is a physical property that characterises the fluidity of a liquid.

4. Nanoindenter

The nanoindenter at the AMMC is an innovative nanomechanical testing platform (Micro Materials Ltd: NanoTest Xtreme) with a nanoscale positioning stage sitting on the anti-vibration table inside a vacuum chamber. It not only allows for quasi-static indentation testing at room temperature but also enables the analysis to be performed at low (- 40⁰C) and high (up to 800⁰C) temperatures using the elaborately designed sample stages.

The instrument has the capacity of operating at the maximum 500 mN load at a thermal drift rate below 0.01 nm/s. The nano-impact and fatigue module to be delivered will enable the analysis of impact resistance, dynamic hardness and low cycle fatigue, providing complementary information to nanoindentation. The analysis can be carried out at the maximum static load of 100 mN with the maximum acceleration distance of 18 µm and a strain rate up to 103 s-1.

The graph to the upper right shows two indentation curves from a steel sample at room temperature, where a discontinuity (also called the ‘pop-in’ event) in the measured depth can be seen in the blue curve. The observation of a sudden burst of displacement signifies the occurrence of plastic deformation such as dislocation nucleation or phase transformation. The figure to the right below is a scanning electron microscope (SEM) image of four typical indentation imprints that are 20 µm apart. (Courtesy of M. Ghasri Khouzani, C. Slater and C. Davis)