In-Situ SEM/EBSD Analysis of Al 0.1% Mg Alloy During Strain Test and Heating

In-situ experiments in the SEM combined with crystallographic data gathered using EBSD can provide insight into the mechanisms operating during deformation and recrystallisation of materials.

In order to conduct such experiments suitable EBSD systems are required that can handle infrared radiation emitted from hot samples and in-situ stages that can be inserted into
standard scanning electron microscope chambers.

The Oxford Instruments Nordlys Max2 EBSD detector incorporates a infrared filter. This design filters the infrared emissions which can intefere with EBSD patterns. This solution gives superior signal to noise ratio compared to the use of phosphor screens coated with a thicker 300 nm aluminium coating used to absorb the infrared signal.

In-situ stages are commercially available - for this work a GATAN Microtest EH2000 tensile stage with heated grips was used.

Experimental

An experimental Al 0.1% Mg alloy was cold rolled to 1.2 mm. Tensile samples were cut using spark erosion with the gauge length set parallel to the rolling direction. The specimens were then annealed at 350ºC for an hour and electro-polished for EBSD.

The samples were carefully mounted into the GATAN in-situ tensile stage. The stage was mounted in a TESCAN Mira XM FEGSEM in which imaging and EBSD was conducted. An Oxford Instruments Nordlys Max2 detector coupled with AZtec® was used to acquire the forescatter diffraction (FSD) images and EBSD data.

The sample was tested at a strain rate of 1.8 x 10-4s-1 to a strain level of 1, as shown in the stress strain curve in Figure 1. The test was interrupted at regular intervals at which point the loading was stopped and EBSD data acquired. The load was then removed and the sample heated up to 320ºC, as shown in the heating temperature profile curve in Figure 2.

Stress strain curve for the in-situ experiment

Figure 1. Stress strain curve for the in-situ experiment

Heating temperature profile after tensile straining

Figure 2. Heating temperature profile after tensile straining

Results - Deformation Cycle

A series of forescatter detector and SEM images after strain are shown in Figure 3. With increasing strain increasing topography is generated on the surface, mainly due to slip processes.

Figures 3a and 3b are FSD images: as strain increases topography
develops and the contrast increases in the image. Figures 3c -3h are secondary electron images which show the topography.

FSD forescatter images at 0 and 0.04 d-h) SEM secondary electron images at 0.2, 0.4, 0.6, 0.7, 0.9 and 1.0 strain respectively. (Note: tensile axis is parallel to the X axis)

Figure 3. a, b) FSD forescatter images at 0 and 0.04 d-h) SEM secondary electron images at 0.2, 0.4, 0.6, 0.7, 0.9 and 1.0 strain respectively. (Note: tensile axis is parallel to the X axis)

EBSD IPF coloured maps at 0.0, 0.04, 0.2, 0.4, 0.6, 0.7, 0.9 and 1.0 strain respectively. (Note, tensile axis is parallel to the X axis)

Figure 4. a-h) EBSD IPF coloured maps at 0.0, 0.04, 0.2, 0.4, 0.6, 0.7, 0.9 and 1.0 strain respectively. (Note, tensile axis is parallel to the X axis)

Accumulation of deformation during tensile strain is shown in Figures 5 a-h), where {100} pole figures are plotted for IPF maps in Figure 4 a-h) respectively. Figure 5a) indicates that the sampled area consists of grains that have the (100), (110) and (111) planes approximately in the plane of the sample surface and that strain was applied in the [001] direction. As strain is applied, a spread of orientations is observed, increasing with applied strain, as shown in Figures 5 b-h).

It is also evident that, while the orientation spread during straining accumulates uniformly about the mean orientation, the rotated Goss grain oriented grains (green) tend to rotate about the Z axis. Further evidence of the accumulation of deformation is illustrated in a series of grain boundary maps in Figure 6 a-h).

EBSD {100} scattered pole figures at 0.0, 0.04, 0.2, 0.4, 0.6, 0.7, 0.9 and 1.0 strain respectively. (Note, tensile axis is parallel to the X axis)

Figure 5. a-h) EBSD {100} scattered pole figures at 0.0, 0.04, 0.2, 0.4, 0.6, 0.7, 0.9 and 1.0 strain respectively. (Note, tensile axis is parallel to the X axis)

In Figure 6a, it is observed that the starting micro-structure comprises high angle boundaries, shown in black >10 degrees misorientation and that some low angle boundaries shown in blue >1.5 degrees misorientation also exist. A total number of 46 grains (borders not included) was determined in the sampled area.

From Figures 6 b-h), it is clear that under strain high angle boundaries get stretched along the tensile axis and accumulation of deformation within grain takes place by the generation of low angle boundaries or substructure.

EBSD grain boundary maps at 0.0, 0.04, 0.2, 0.4, 0.6, 0.7, 0.9 and 1.0 strain respectively. (Note, tensile axis is parallel to the X axis. Black boundaries >10º and blue boundaries>1.5º misorientation)

Figure 6. a-h) EBSD grain boundary maps at 0.0, 0.04, 0.2, 0.4, 0.6, 0.7, 0.9 and 1.0 strain respectively. (Note, tensile axis is parallel to the X axis. Black boundaries >10º and blue boundaries>1.5º misorientation)

Figure 7 plots the grain orientation spread (GOS) versus strain. It is clear from this plot that, up to about 0.7 strain, there is a linear variation in GOS. The loss of linearity beyond this strain is probably significantly influenced by the lower hit rates achieved at these strains.

Plot of GOS versus strain

Figure 7. Plot of GOS versus strain

The accumulation of deformation in individual grains and its influence on neighbouring grain orientations, is illustrated in a series of EBSD maps and {100} pole figures in Figures 8 and 9 (overleaf).

Data is shown for a set of grains with a near (111) planes, surrounded by (100) grains in Figures 8 a-h) and in Figures 8 a-h) another (111) grain with neighbouring (110) and (100) oriented grains respectively.

It is evident from Figure 8, that the (111) grains with (100) neighbours accumulate mis-orientations uniformly about the existing starting orientations.

However, in the case of the (111) orientation surrounded by the (100) and (110) orientation initially and up to a strain of 0.4 rotation are taking place about the Z axis. 0.4 strain is about the ultimate tensile strength (UTS), and after this strain rotation take place about the Y axis up to 1.0 strain at which point load was removed. It is clear from this inference that neighbouring grains have a strong influence on the way deformation accumulates during tensile strain.

EBSD IPF coloured maps at increasing strain and corresponding {001} pole figures of highlighted grains.

Figure 8. EBSD IPF coloured maps at increasing strain and corresponding {001} pole figures of highlighted grains.

EBSD IPF coloured maps at increasing strain with corresponding {001} pole figures of highlighted grain

Figure 9. EBSD IPF coloured maps at increasing strain with corresponding {001} pole figures of highlighted grain

Results - Heating Cycle

In Figure 10, a series of local mis-orientaion maps are shown illustrating the sequence of replacement of strain free grains coloured blue during the heating cycle in Figure 2. The change in orientations from the deformed to the final heated state are shown in IPF maps and corresponding {100} pole figures in Figure 11.

Only the recrystallised grains are shown in pole figure 11d, which clearly shows that bulk of the new grains in this sample have attained the cube orientation. It is also clear from the set of maps in Figures 10 and 11 that the majority of the non indexed pixels are shadowed by the relief that was generated during tensile strain.

a) EBSD local mis-orientation maps after 1.0 strain and b-e) at 255ºC, 290ºC 1 hour, 290ºC 1.8 hours , 290ºC 1.9 hours, 300ºC 2.9 hours, 320ºC 3.9 hours, 320ºC 4.5 hours. respectively. Blue coloured regions are fully recrystallised.

Figure 10. a) EBSD local mis-orientation maps after 1.0 strain and b-e) at 255ºC, 290ºC 1 hour, 290ºC 1.8 hours , 290ºC 1.9 hours, 300ºC 2.9 hours, 320ºC 3.9 hours, 320ºC 4.5 hours. respectively. Blue coloured regions are fully recrystallised.

a and c) EBSD IPF maps of the sample stretched to 1.0 strain and after heating up to 320ºC, b and c) {100} pole figures from corresponding maps in a and c respectively.

Figure 11. a and c) EBSD IPF maps of the sample stretched to 1.0 strain and after heating up to 320ºC, b and c) {100} pole figures from corresponding maps in a and c respectively.

A comparison of the 1.0 strain IPF map with that after heating to 255ºC, Figure 12 shows that, while a lot of recovery has occurred, some new recrystallised grains have also appeared (labelled A, B, C, D, E and F). The crystal orientations of these are shown in the 3D views and their approximate point of nucleation/origin and orientation is shown in the map obtained from the corresponding deformed map.

Recrystallised grains A, B, D are near cube, but the origin of A and D cannot be ascertained as they appear from regions that were not indexed. Grain B appears to originate from a near S oriented grain.

Similarly, grain C’s origin which is near Brass orientation cannot be ascertained. Grain E is a near Goss grain which has originated from a similar oriented deformed grain, and grain F which is near rotated brass originated from a near S or cube grain F’.

An exhaustive work on the following new grains and their origins has not been carried out but preliminary interrogation of the data indicates that cube grains mainly occupy the areas that were initially of the rotated Goss orientation in the earlier stages, while cube grains maintain their original orientations.

a-b) EDSD IPF maps after heating to 290ºC and after 1.0 strain.

Figure 12. a-b) EDSD IPF maps after heating to 290ºC and after 1.0 strain.

A summary of the starting microstructure, the deformed structure to 1.0 strain and after the final heating stage are shown in Figure 13 a–c) with corresponding pole figures in Figures 14 a-c).

In Figure 15, a plot of how the grain orientation spread reduces with heating is shown. This plot indicates that recovery brings the orientation spread down very rapidly and further reductions are then due to recrystallisation and growth.

EBSD IPF maps of sample at 0, 1.0 strain and after heating to 320ºC respectively.

Figure 13. EBSD IPF maps of sample at 0, 1.0 strain and after heating to 320ºC respectively.

Grain orientation spread versus heating time.

Figure 14. Grain orientation spread versus heating time.

Conclusions

EBSD combined with in-situ tensile and heating experiments have been shown to be a powerful means to give insight into generation of deformation and subsequent recovered and recrystallised microstructure within the same set of grains.

This preliminary test has illustrated that accumulation of deformation occurs mainly by slip and is orientation dependent. Existing cube orientation give rise to new recrystallised cube grains at the expense of the elimination of rotated Goss oriented grains.

This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments NanoAnalysis.

For more information on this source, please visit Oxford Instruments NanoAnalysis.

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