AZtec Grain Analysis

Introduction

The grain size is an important parameter of a material; it will strongly affect mechanical and physical properties. Understanding how grain size is influenced through the processing of materials can assist in engineering materials with optimised properties.

The current development is towards materials with increasingly smaller grains, often on the nanoscale. Traditional methods for grain size measurements are not all suited for nanoscale materials; however the combination of SEM combined with EBSD is an ideal solution. This application of EBSD is not new, however as this technique becomes more widespread, the requirement is to develop a robust solution which delivers accurate results every time. Oxford Instruments AZtec EBSD system has a dedicated Grain Analysis mode which is both reliable and flexible for this application.

The following applications demonstrate the AZtec Grain Analysis.

Example 1. Grain characterisation through recovery and recrystallization of a folded Al alloy

An investigation into the microstructural changes during recovery and recrystallisation was undertaken using a folded Al 0.1% Mg alloy. Changes in the microstructure were monitored during an in-situ heating experiment.

The sample was heated slowly to a holding temperature of 295°C which was held for several hours. During this period recovery and recrystallisation was observed, resulting in a large change to the microstructure. Grain size was measured before and after the heating cycle using the AZtec Grain Analysis mode.

A grain map, showing the grains randomly coloured is an effective way to illustrate the grain distribution. Fig. 1 shows the grain maps before and after heating; the fold of the sample is clearly visible. The data used in this analysis is as collected.

The initial microstructure has a mean grain size (equivalent circle diameter) of 85.9 µm. In the starting sample the grain size is clearly inhomogeneous with larger grains towards the sample edges and smaller grains in the centre. In the heated sample this microstructure has been replaced with much larger grains, with a mean grain size of 170.9 µm. The larger grains are in the centre of the sample. The system is fully interactive, shown in Fig. 1B, so that any grain selected from the map is highlighted in the grain details table.

Fig. 1(A). Grain map before heating cycling, mean grain size is 85.9 mm. (B) Grain map and grain details table after heating cycle, mean grain size is 170.9 mm. A large grain selected from the centre of the map is highlighted in the grain details table, it has a grain size of 504.7mm.

Example 2. Grain size variation through a spot welded steel

Resistance spot welding (RSW) is the predominant technique used for joining steel in automotive applications, and as such it is important to understand the impact of the welding on microstructure. This is studied using AZtec EBSD and the Grain Analysis tool.

Characterising the whole welded sample with a non-uniform microstructure requires analysis over multiple fields. AZtec large area mapping was used to collect EBSD data over an area 6 mm by 2 mm. 132 continuous fields were collected with a step size of 0.7 µm. The data was then montaged and analysed as a single dataset; 113,376 grains were detected.

Three distinct regions of the sample were investigated: the nugget (or joined region), a heat-affected zone (HAZ), and the initial base metal (BM). These are annotated in the image below.

Fig. 2. Grain map of cross-section in a RSW steel.

The grain size in the three regions is distinctly different, as is clear in the grain map Fig. 2. Grains with less than 10 pixels were filtered out and over 5,000 grains from each region were evaluated.

• The grains in the BM zone are elongated parallel to the surface of the original sheet, with mean grain size of 16.3 µm.
• The HAZ is characterised by a relatively homogeneous microstructure with small, recrystallised, equiaxed grains with mean grain size of 7.7 µm.
• The nugget has a more complex columnar martensite microstructure with grain size between 3.5 µm and 142.2 µm.

Table 1. Grain statistics from the three identified regions.

Accurate grain characterisation aids in understanding the mechanisms underway during RSW. This in turn enables better modelling and use of RSW in materials development.

Conclusion

Characterising and controlling grain size is important in tailoring material properties and understanding material performance. EBSD offers a direct, automatic and accurate quantitative method for grain analysis. It can measure grain sizes from tens of nm to hundreds of µm, and as the trend moves to smaller grain size in materials, EBSD analysis is a powerful solution to this application.

The Grain Analysis mode in AZtec is an ideal tool for accurately identifying grains and measuring grain size. It is optimised for easy to use, fast and reliable grain analysis directly after data acquisition.