Microtest tensile stage used for grain rotation studies in the SEM

Acknowledgement: Dr. R. Mishra and Mr. R. Kubic, General Motors R&D Center, Warren, MI, USA.

Introduction

In most alloys the material properties, such as strength and formability, are strongly affected by the development of texture. The scanning electron microscope (SEM) fitted with electron backscatter diffraction (EBSD) instrumentation is widely used to study texture. The current example shows the power of adding a tensile stage in situ with the SEM / EBSD to study microstructural and texture development during forming. In this configuration, the Microtest 5000N tensile stage allows tensile testing dynamically in the SEM, enabling investigations of the relation of crystallographic rotations of grains with traditional stress-strain analysis.

Tensile stage

The Microtest range of modules from Gatan offer dynamic tensile, compression and bending tests of specimens in the SEM. Microstructural changes at the region of interest can be examined with the large depth of field offered by the SEM, while software simultaneously controls the experiment and records stress-strain information.

These modules are optimized for working in the SEM environment, e.g., allowing short working distance, and are manufactured using non-magnetic metals. They can also be used on the bench top or under a light microscope. When configured for the vacuum chamber of the SEM, Gatan also offers the option to heat or cool the specimen in situ over a wide temperature range.

In this study, a room temperature module with a maximum load of 5000N was fitted on a replacement SEM door and X, Y, Z translation stage specially configured to fit the stringent criteria of specimen tilt, working distance and diffraction pattern illumination for in-situ EBSD studies.

The EBSD camera is inserted once the stage is positioned and an IR chamber scope is helpful in this regard. A single connector using a vacuum feedthrough provides stress and strain control and information to the computer.

The photograph shows the 5000N tensile module and the replacement SEM stage and door. In this example the microscope pole piece and tensile module geometry precluded 70° tilting of the stage, so 70° tilt of the specimen using specially designed grips. This design allows reverse tilted grips to mount specimens normal to the electron beam for non EBSD dynamic testing studies, or standard microcharacterization techniques.

Experimental

An allow specimen (Aluminum A5754-0) was polished to obtain a surface suitable for EBSD analysis.

The initial EBSD grain map prior to deformation is shown in Figure 1(a). The sample was then subjected to in-situ tensile loading to Point A in Figure 2. At this point, the tensile experiment was paused.

The EBSD map collected at 1500N load is shown in Figure 1(b) from the same area as in Figure 1(a). The straining was resumed to points B and C in Figure 2, stopping at each point to collect matching EBSD data.

Note, for other types of specimens where strain rate is important, the Microtest software provides a choice of strain speeds and allows relaxation to be studied by providing options of continuing data acquisition while extension is topped, or by controlling extension to keep a constant load. In addition thresholds of load, extension or time can be set in software, providing useful dynamic control of the experiment.

The changing microstructure under plastic deformation as measured using EBSD can be displayed in many ways to quantify texture, grain size and boundary orientation.

Grain Rotation (º) Spread before (º) Spread after (º)
1 3.5 0.6 7.2
2 7.4 0.6 5.4
3 11.5 0.7 5.7

Table 1 lists the measured values of grains marked 1, 2, and 3 in Figure 1. These values can be quantitatively correlated with strain distribution inside the material.

Figure 4 shows grain rotation and orientation maps of the specimen after loading to 2225N which corresponds to 22% strain. The density of low angle grain boundaries inside grains is noticeably higher in Figure 4(a) and the corresponding grain orientation figure shows color variations corresponding to orientation spreads. The dark region in Figure 4(a) corresponds to gross topographic variation on the surface.

As the material deforms, grains undergo lattice rotation. The amount of rotation depends on the amount of slip in individual grains and their relative orientation and location with respect to the tensile axis. The dislocation networks inside the grains increase the spread of local orientations.

Summary

This work has documented the ability of the Microtest tensile stage configured for EBSD analysis, to show microstructural changes in an aluminum alloy sample subjected to in-situ tensile deformation. Increase in lattice misorientation in the grains and lattice rotations can be directly measured in samples deformed to large strains using this combination of equipment. Resulting measurements attest to the positional and mechanical stability of the stage in this application.