Microstructure characterisation in metal powders by EBSD

Introduction

Many of the physical properties of metal powders, such as powder densification and strain hardening, are strongly dependent on the materials properties of the individual particles. Therefore, the microstructural characterisation of metal powders, providing information about particle grain size, texture and deformation, is of paramount importance, yet is significantly lacking in the field of additive manufacturing.

Scanning electron microscope (SEM) based electron backscattered diffraction (EBSD) is a fast, automated technique that provides information about the crystallographic orientation and phase on the surface of a sample with sub-micrometre resolution. Large area EBSD orientation maps can now be generated in a matter of a few minutes using the latest generation of fast CMOS-based EBSD detectors, such as the Oxford Instruments Symmetry S2 detector. A single orientation map can provide all the relevant information about a sample's microstructure, including phase fraction, texture, grain size, plastic strain and boundary types. Unlike other characterisation techniques, EBSD provides statistically relevant data from large areas on the sample surface but also enables detailed spatially-resolved variations to be observed.

In this brief application note, we demonstrate how gas atomised copper powders can be effectively characterised using EBSD data collected with the latest high-speed Symmetry EBSD detector.

Materials and Analytical Details

Gas atomized copper powders were evenly dispersed and mounted in a conductive mount so that they could be mechanically polished for EBSD analysis. EBSD characterisation took place in a field emission gun (FEG) SEM, using the Symmetry S2 EBSD detector and the AZtecHKL acquisition software. The electron beam was set to an energy of 20 kV using a probe current of approximately 12 nA. Orientation map data were acquired automatically covering a 0.6 mm × 0.4 mm field of view, with a measurement step size of 0.5 µm; the data were collected within 10 minutes.

Results

An orientation and boundary map was immediately plotted, as shown in Fig. 1, and grain size was calculated automatically, enabling a rapid assessment of the grain characteristics.

Fig 1. An orientation and boundary map of copper powder. Orientations are coloured according to the crystal direction parallel to the surface normal direction (IPF-Z colouring), with grain boundaries ≥15º shown in black, sigma 3, 5, 7 & 9 boundaries shown in red, green, purple and yellow.

An advantage of the EBSD technique is being able to measure the grain size excluding Coincident Site Lattice (CSL) boundaries. The mean grain size (equivalent circle diameter) in the powder excluding CSL boundaries is 6.75 µm: note that this is not a measurement of the particle size, but a measurement of the all-important internal crystallographic grain size. In this particular powder the grain size is clearly heterogeneous, and this may be a result of variations in the cooling history of individual particles during the cooling process.

The cooling rates of the powders are affected by various processing parameters in the Gas Atomization (GA) technique. Some of the particles will undergo rapid cooling, resulting in numerous nucleation events and a relatively homogeneous grain structure, as shown by Particle 1 in Figure 1. For particles that have undergone a slower cooling rate, abnormal grain growth can occur resulting in a much coarser microstructure (e.g. Particle 2).

The spatial variation of plastic strain within the particles is given by a Kernel Average Misorientation (KAM) map, as shown in Figure 2. The KAM value effectively quantifies the degree of local distortion of the crystal lattice and can be linked to the density of geometrically necessary dislocations.

The KAM values clearly vary considerably within and between individual particles: higher KAM values (e.g. green and yellow colours) are associated with intracrystalline distortion and the presence of low angle boundaries, whereas lower KAM values (blue) may indicate recovery or recrystallisation.

Fig 2. Kernel Average Misorientation map (KAM) of the same area shown in Fig. 1 highlighting the heterogeneous distribution of strain in the particles.

Conclusions

The results demonstrate that EBSD can be used to effectively characterise the microstructure of metal powders in a matter of minutes, providing detailed information on a multitude of microstructural parameters, including grain size and plastic strain. Accurate grain characterisation helps researchers to understand better the microstructural processes active during GA, whereas the localisation of strain may help to further refine the grain size in powders. All the measurements provided by EBSD will improve modelling of the GA process and enable better understanding of the use of GA during powder development, in turn helping to optimise any subsequent additive manufacturing process.

References

1. Genaro Pérez-de León, Vincent E. Lamberti, Roland D. Seals, Taher M. Abu-Lebdeh and Sameer A. Hamoush. Gas Atomization of Molten Metal: Part I. Numerical Modeling Conception.