Correlating Atom Probe Crystallographic Measurements 285


reconstruction parameters and same pole selection. The third measurement is for the calibrated reconstruction using a different selection of poles. The average minimum misorientation angle/axis pair calculated from the calibrated reconstruction with different pole selections, is 40.7°/[0.045 0.198 0.979]. The misorientation angle and axis deviate 0.7° and 2.6°, respectively, from the TKD results.


Reflectron Fitted Atom Probe Analyses Versus TKD Misorientation Measurements


A similar crystallographic analysis procedure was then undertaken for the technically pure Mo specimen. Figure 4a shows the calibrated atom probe reconstruction collected for the sample (ICF = 1.28, kf = 2.56, ε = 0.37, L = 40mm). Strong segregation of theNand P impurity species can be seen at the grain boundary running down the centre of the recon- struction. Once again, pole and zone line structure could be observed throughout the reconstruction, although this was weaker than in the previous example (Figs. 4b, 4f). Density maps from approximately one million atom slices of two regions within the reconstruction have been used. It was still possible to index individual poles and clear periodicity of lat- tice planes was observed within these regions as indicated by the POE and SDMs (Figs. 4c–4e, 4g–4i). Using this informa- tion, the orientation of each grain, as well as the misorienta- tion between them, could be calculated using the method described in the previous section. These measurements could then be compared directly to those fromthe TKDmaps taken of the tip before the atom probe experiment. Figure 5 shows a comparison between the TKDand APT


analyses of the same grain boundary region. Once again, the grains have been colored according to the crystallographic direction in the vertical direction, as measured from both techniques. Figure 5a shows four TKD maps of the specimen at projections spaced ~90° apart, the relative measured orientation of each grain is also provided. Figure 5b shows the corresponding 3D orientation maps of the APT reconstruc- tion, also at four projections spaced ~90° apart.Amovie of the 3D orientationmap of the reconstruction can be found in the Supplementary Material (movie file 2). The coloring from each technique is very similar, indicating close agreement in the crystallographic measurements and facilitates easy com- parison between the two results.A1Dconcentration profile of the impurity elements along the direction normal to the grain boundary is also provided, which again highlights the ability to combine very precise atomic chemical and crystallographic information of individual grain boundaries with this analysis approach. A summary of the minimum misorientation measure-


ments is given in Table 2. As done earlier, the average orientation of each grain was calculated and a minimum misorientation was measured using these values. Three separate measurements of the same boundary in the atom probe data were conducted from the calibrated reconstruc- tion, non-calibrated reconstruction, and the calibrated


reconstruction with a different selection of poles. The aver- age minimum misorientation angle, calculated from the measurements taken on the calibrated reconstruction with different pole selections, is 33.4°/[0.099 0.640 0.761]. The misorientation angle and axis deviate 1.4° and 5.7°, respec- tively, from the TKD result.


DISCUSSION


Angular Resolution From the presented results, it was possible to gain insight into the angular resolution of orientation and misorientation mea- surements made directly from APT reconstructions through the use of complementary TKD results on the same tip speci- men. Such information is useful, because APT crystallographic studies are starting to become popular as ameans of combining atomic scale chemical and crystallographic measurements across individual grain boundaries, yet there has been limited investigation into the accuracy of such measurements. It is importanttopoint outthat the angularresolutionofTKD, while significantly high, still has amargin of error. A quick way to estimate this error is to perform a transect across a defor- mation free grain and view the pixel to pixel misorientation data. For the data presented, this was found to be on average approximately ±0.3°. When measuring the misorientation between two separate grains, the margin of error is double this value or ±0.6°. Higher angular resolutions would be possible with higher diffraction pattern resolutions but this would increase the time required to generate the TKD maps. An accurate estimate of the true angular resolution using


APT is difficult due to the time each individual measurement takes, the influence of so many variables, including the reconstruction calibration, and the protocols used to make the measurement itself and falls outside the scope of this manuscript. However, to gain some insight into the effect of reconstruction calibration and pole selection, three separate measurements were taken changing these parameters on each sample. It was presumed that the carefully calibrated recon- structions would result in significantly more accurate mis- orientation measurements, but based on the measurements in Tables 1 and 2, this could not be substantiated and is perhaps an indication of the robustness in making misorientation measurements from APT reconstructions. Nevertheless, some variance in the results, depending on parameter selec- tion, was observed. For the calibrated atom probe recon- structions, the maximumdeviation of misorientation angle is 2°. However, when the measurements using different pole selection are averaged, this falls to 1.4° and suggests that the accuracy can be improved by averaging multiple measure- ments using a different selection of poles. The deviation in the misorientation axis is larger, i.e., on average ~4°. This was to be expected as the nature of the misorientation axis calcula- tion is even more sensitive to variation in the orientation of each grain. The same compounding error is also seen in calculations from EBSD and TKD data. It is also worth pointing out that this is particularly the case when low angle


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188  |  Page 189  |  Page 190  |  Page 191  |  Page 192  |  Page 193  |  Page 194  |  Page 195  |  Page 196  |  Page 197  |  Page 198  |  Page 199  |  Page 200  |  Page 201  |  Page 202  |  Page 203  |  Page 204  |  Page 205  |  Page 206  |  Page 207  |  Page 208  |  Page 209  |  Page 210  |  Page 211  |  Page 212  |  Page 213  |  Page 214  |  Page 215  |  Page 216  |  Page 217  |  Page 218  |  Page 219  |  Page 220  |  Page 221  |  Page 222  |  Page 223  |  Page 224  |  Page 225  |  Page 226  |  Page 227  |  Page 228  |  Page 229  |  Page 230  |  Page 231  |  Page 232  |  Page 233  |  Page 234  |  Page 235  |  Page 236  |  Page 237  |  Page 238  |  Page 239  |  Page 240  |  Page 241  |  Page 242  |  Page 243  |  Page 244  |  Page 245  |  Page 246  |  Page 247  |  Page 248  |  Page 249  |  Page 250  |  Page 251  |  Page 252  |  Page 253  |  Page 254  |  Page 255  |  Page 256  |  Page 257  |  Page 258  |  Page 259  |  Page 260  |  Page 261  |  Page 262  |  Page 263  |  Page 264  |  Page 265  |  Page 266  |  Page 267  |  Page 268  |  Page 269  |  Page 270  |  Page 271  |  Page 272  |  Page 273  |  Page 274  |  Page 275  |  Page 276  |  Page 277  |  Page 278  |  Page 279  |  Page 280  |  Page 281  |  Page 282  |  Page 283  |  Page 284