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EBSD ANALYSIS ON DEFORMATION OF ULTRAFINE AND NANO-GRAINS

EBSD ANALYSIS ON DEFORMATION OF ULTRAFINE AND NANO-GRAINS
EBSD ANALYSIS ON DEFORMATION OF ULTRAFINE AND NANO-GRAINS

EBSD ANALYSIS ON DEFORMATION OF ULTRAFINE AND NANO-GRAINS

IN ECAP-PROCESSED COPPER

H. Kimura1, Y. Akiniwa2, K. Tanaka2 and T. Ishida3

1. EcoTopia Science Institute, Nagoya University, Nagoya, Japan

2. Department of Mechanical Science and Engineering, Nagoya University, Nagoya, Japan

3. Graduate School of Mechanical Science and Engineering, Nagoya University, Nagoya, Japan

ABSTRACT

Neutron diffraction method as well as electron backscattering diffraction, EBSD, technique was employed to investigate deformation mechanism of ultrafine and nano-grains in copper processed by equal channel angular pressing, ECAP. Special attention was paid to the grain boundary sliding in the nano-sized grains in comparison with coarse grains. The results of the neutron diffraction measurement show that the grain boundary sliding occurs in the ECAP-processed specimen based on the recovery of the full width at half maximum of the diffraction profiles upon unloading. The grain boundary sliding is considered to occur before transgranular slip. Nano-scale EBSD analysis based on orientation change and change of misorientation spread in each grain shows that the grain boundary sliding was dominant in nano-sized grains. Grain boundary sliding can be observed in relatively large nano-sized grains in ECAP-processed material because of larger grain boundary energy.

Introduction

Nanocrystalline materials produced by severe plastic deformation techniques have captured great attention because of the excellent mechanical properties. Equal channel angular pressing (ECAP) [1-3] is especially attractive because the simple technique can produce large bulk samples with little residual porosity, elevating the strength without adding expensive alloying elements. Though ECAP hold many advantages in the practical application over other severe plastic deformation processes that can produce nanocrystals only on the specimen surface, the average grain size is larger in ECAP-processed materials with nano-sized and ultrafine grains. The deformation and damage mechanism in the characteristic microstructure needs to be clarified for the practical application based on microscopic as well as macroscopic analysis. Electron backscattering diffraction (EBSD) technique is a useful tool for microscopic investigation based on crystallographic orientation [4]. In this paper, EBSD analysis was conducted on pure copper processed by ECAP to investigate the microscopic deformation mechanism. Neutron diffraction measurement was employed for the macroscopic investigation on the material with nano-sized and ultrafine grains.

Material and Experiment

Annealed pure Cu with purity of more than 99.95% was processed by ECAP with 4 and 12 Rout B C passes. Tensile specimens were prepared with the dimension as shown in Fig. 1 for neutron diffraction measurement and Fig. 2 for EBSD measurement.

Fig. 1. Tensile specimen for neutron diffraction measurement. Fig. 2. Tensile specimen for EBSD measurement.

The measurement of crystallographic orientation was conducted by orientation imaging microscopy (TexSEM laboratory: Orientation imaging microscope) based on EBSD. The specimen surface was polished by colloidal silica before EBSD

measurement with the scanning resolution of 30 nm. For the analysis of plastic deformation by EBSD, grain orientation spread, GOS , was defined as the average of misorientation angles between all measurement points in a grain, whether a pair of two points for the calculation is adjacent or distant. The value is a measure for the orientation consistency throughout the grain. Grain boundary was defined as the boundary between two measurement points with the orientation difference more than 15 degrees.

(),1(-1)n ij i j i j GOS n n α≠==

∑ (1)

Fig. 3. Definition of grain orientation spread (GOS ).

Neutron diffraction measurements were conducted at Sirius of High Energy Accelerator Research Organization. Specimens were loaded during the measurement. The lattice spacing, d , and the full width at half maximum, FWHM , of the diffraction profile from 0.5 to 3 angstrom were measured by TOF method from the diffraction volume of 6.8 mm in diameter.

Results and Discussion

Figure 4 shows the inverse pole figure maps before ECAP, (a), after 4 ECAP passes, (b) and 12 passes, (c), where the orientation of each measurement point in the direction of sample normal is depicted by the color of the unit triangle in (a).

(a) Before ECAP (b) After 4 ECAP passes (c) After 12 ECAP passes

Fig. 4. Inverse pole figure map (orientation in sample normal direction depicted by color of unit triangle) based on EBSD.

Twins are evident in the annealed material. However, no twin boundaries are observed after ECAP of 4 or 12 passes. The misorientation within a grain is larger after 4 ECAP passes than that after 12 passes. High angle grain boundaries with more

than 15 degree orientation difference, black lines in Fig. 4, are dominant after 12 passes. The relation between the average

grain diameter, D, and the number of ECAP passes, M, is presented in Fig. 5. The grain diameter decreases from 350 micrometer of annealed specimen to about 500 nm after 4 passes. The tensile strength was doubled from 205 MPa to 400 MPa. After 12 ECAP passes, the grain diameter is about 200 nm. Figure 6 shows the change of GOS with the number of ECAP passes. With increase in M, GOS decreases indicating that the transgranular strain becomes small and that the recrystallization is more readily completed after 12 passes than 4 passes. The ratio of high angle boundaries is also larger after 12 passes. Therefore, the following investigation was conducted on specimens with 12 ECAP passes.

Fig. 5. Relation between average grain diameter and ECAP passes. Fig. 6. Relation between GOS and ECAP passes.

The results of the neutron diffraction measurement are presented from Fig. 7 to 10. The relation between applied stress, σ, and the stroke of the loading head, x, is shown in Fig. 7 for annealed specimen. The dotted curve is σ - x relation and the solid square marks represent the neutron diffraction measurement points. Specimen was occasionally unloaded during the loading process. Figure 8 shows the relation between full width at half maximum, FWHM, of the 311 diffraction profile and x for annealed specimen. The value of FWHM increased with x due to the increase in inhomogeneous elastic and plastic strain. Upon unloading, FWHM decreased approximately by 1x10-4 angstrom. This is considered to result from the recovery of the elastic strain and the corresponding decrease in the elastic inhomogeneous strain. The value of FWHM in annealed specimen tends to increase with x because the change mostly depends on the irreversible plastic strain caused by the transgranular slips, which are not recovered upon unloading.

Fig. 7. σ vs. x for annealed specimen. Fig. 8. FWHM vs. x for annealed specimen.

The relation between σ and x for ECAP-processed specimen is shown in Fig. 9. The solid square marks represent the neutron diffraction measurement points. Figure 10 shows the relation between FWHM of the 311 diffraction profile and x for ECAP-processed specimen. In the first loading, FWHM increased with x. However, FWHM decreased upon unloading in contrast to

the result of the annealed specimen. The detailed observation on the specimen surface showed that the grain refinement doesn't occur during the loading. Therefore, the increase and decrease in FWHM are considered to be mostly due to the grain boundary sliding. In the process of plastic deformation caused by grain boundary sliding, FWHM recovers to the initial value after unloading [5] because irreversible transgranular strain by the tangle of dislocation is little and the elastic inhomogeneous strain decreases. Though the average grain size in ECAP-processed material is larger than nanocrystalline materials, grain boundary sliding occurs. This is considered because the grain boundaries are unstable with large boundary energy. In the range beyond 3.5 mm of x, FWHM after unloading becomes larger. The irreversible increase in FWHM is considered due to the increase in the transgranular strain by dislocation tangle within larger grains. The grain boundary sliding is considered to occur before transgranular slip.

Fig. 9. σ vs. x for ECAP-processed specimen. Fig. 10. FWHM vs. x for ECAP-processed specimen. EBSD observation was made on the surface of tensile specimen at plastic strain, ε, of 0, 4, 8 and 16 %. The change of orientation at the same measurement point, θ, was measured for all the grains at each ε. Figure 11 shows the results of the 7 representatives with 3 nano-sized grains in solid marks and 4 relatively coarse grains with the average grain size, d, more than 1 μm in open marks. With the increase in ε, the orientation change tends to increase regardless of the grain size. It is clear that θ is larger in nano-sized grains than in larger grains especially at higher ε. The values of θ of open marks are up to 3 %, while those of solid marks are up to 4 - 6 %. The result of all the measured grains is shown in Fig. 12 for θ between ε = 0 and 16 %

Fig. 11. Relation between orientation change and plastic strain. Fig. 12. Relation between orientation change and grain size.

in relation to d. The orientation change is obviously larger for smaller grains. Below the range of d < 500 nm, the orientation change tends to decrease linearly with d. Large θ in small d region is considered to result either from the increase in misorientation within the grains, observed as tangle of dislocations, or from grain rotation by grain boundary sliding without increase in the transgranular misorientation. In order to evaluate the change in transgranular strain, GOS was calculated for all the grains. Figure 13 shows the change of GOS between θ = 0 and 16 % in relation with d.

Fig. 13. Relation between change of GOS between 0 and 16% plastic strain and grain size.

The value of GOS slightly increases with d, indicating that the transgranular misorientation, i.e., plastic deformation by slip is more likely to be introduced in coarser grains. Judging from the results in Fig. 12 and 13, it is concluded that grain rotation occurs among nano-sized grains in ECAP-processed Cu. The rotation angle of nanoscopic grains is measured to be about 4 degrees under θ = 16 %. The rotation is small because of the larger grain size than that in nanocrystalline materials.

Conclusion

Deformation mechanism of ECAP-processed copper with ultrafine and nano-grains were investigated by EBSD analysis and neutron diffraction method. The ECAP-processed microstructure was finer after 12 passes with smaller transgranular strain and large amount of high angle grain boundary. The results of the neutron diffraction measurement show that the grain boundary sliding occurs in the ECAP-processed specimen based on the recovery of the full width at half maximum of the diffraction profiles upon unloading. The grain boundary sliding is considered to occur before transgranular slip. EBSD analysis was conducted under different applied stress to measure the orientation change of the same measurement point and change of misorientation spread in each grain. The results show that the grain boundary sliding was dominant in nano-sized grains. Grain boundary sliding can be observed in relatively large nano-sized grains in ECAP-processed material compared with other nanocrystalline materials because of the unstable grain boundary with larger grain boundary energy.

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