3.3. γ′ size versus distance from the weld interface
TEM analysis of the thin foils provided selected area diffraction patterns and dark field images showing the γ′ precipitates at the three positions. Although it is difficult to get an accurate size measurement from the images, the γ′ appears to consist of unimodal spherical precipitates at the 6 and 50 mm distances, and both non-spherical and spherical precipitates at the 1 mm distance from the weld interface. The TEM images for the 6 mm position are shown in Fig. 5. Ordered reflections clearly indicate the presence of γ′ precipitates. Because of poor image contrast, thedark field images for the 6 and 50 mm positions were used to provide only rough measurements of the γ′ volume fraction and size to help validate the APT values. Instead, the APT reconstructions were used to provide the size and volume fraction information for these 2 positions. Since the APT data from the 1 mm position did not contain complete γ′ precipitates, the software package ImageJ was employed to analyze the TEM images for the 1 mm position. 10 separate images were used in the analysis. Fine spherical tertiary and larger non-spherical γ′ precipitates are both present at the location (Fig. 6). The results show spherical precipitate diameters in the range of 50–100 nm, and larger precipitates with diameters of between 300 and 500 nm. The APT reconstructions and TEM images show the spherical γ′ has an approximate diameter of 10–15 nm and volume fraction of 33–36% at the 6 mm, and a smaller 10 nm size with slightly higher volume fraction of 38–40% at the 50 mm distance. The TEM images indicate the presence of very fine spherical γ′ within the first 50 mm of the interface in the LSHR material, indicating rapid nucleation and growth of γ′ during the welding process. The large grain boundary γ′ evident at 1 mm indicates the process did not completely dissolve the original phases present in the sample past this length. SE images of the 1 mm position (Fig. 7) show large primary γ′ at grain boundaries and in the interior of the γ, in agreement with earlier results reported by Gabbet al. [4]. Because these images provided a large population set of multi-sized precipitates, they were used to more accurately determine area fractions and sizes for primary and secondary γ′ at the 1 mm position instead of the TEM results. The slightly etched SE images were analyzed at high magnifications using Photoshop, and manually segregated to separate γ′ from γ. The segregated images were then thresheld to separate specific precipitates and determine individual precipitate pixel counts. The pixel counts were converted to areas by calibrating the image magnification, and then converted to average spherical diameters for size estimates. The area values were obtained by dividing the relevant precipitate pixel counts by the total pixel count for each image.The main error associated with this technique involves overestimating the diameter of the precipitate because of the etching process. For the larger secondary and primary γ′, this is not a significant issue. However, for the smaller tertiary γ′, it is important to carefully avoid over etching the sample prior to imaging. The resulting grain boundary primary γ′ area fraction was approximately 15.6%, with an equivalent diameter of 1.04 mm. The total primary γ′ was measured at 34.6%, providing an interior grain primary γ′ area fraction of 19%. The measured tertiary γ′ was 9.16% area fraction, and diameter of 74.6 mm. When combined with the interior grain gamma prime values, the values climbed to 28.16% area fraction with an equivalent diameter of 76.88 mm. Please note the differences between the γ′ morphologies.
3.3. γ′ขนาดเมื่อเทียบกับระยะทางจากอินเทอร์เฟซการเชื่อมTEM analysis of the thin foils provided selected area diffraction patterns and dark field images showing the γ′ precipitates at the three positions. Although it is difficult to get an accurate size measurement from the images, the γ′ appears to consist of unimodal spherical precipitates at the 6 and 50 mm distances, and both non-spherical and spherical precipitates at the 1 mm distance from the weld interface. The TEM images for the 6 mm position are shown in Fig. 5. Ordered reflections clearly indicate the presence of γ′ precipitates. Because of poor image contrast, thedark field images for the 6 and 50 mm positions were used to provide only rough measurements of the γ′ volume fraction and size to help validate the APT values. Instead, the APT reconstructions were used to provide the size and volume fraction information for these 2 positions. Since the APT data from the 1 mm position did not contain complete γ′ precipitates, the software package ImageJ was employed to analyze the TEM images for the 1 mm position. 10 separate images were used in the analysis. Fine spherical tertiary and larger non-spherical γ′ precipitates are both present at the location (Fig. 6). The results show spherical precipitate diameters in the range of 50–100 nm, and larger precipitates with diameters of between 300 and 500 nm. The APT reconstructions and TEM images show the spherical γ′ has an approximate diameter of 10–15 nm and volume fraction of 33–36% at the 6 mm, and a smaller 10 nm size with slightly higher volume fraction of 38–40% at the 50 mm distance. The TEM images indicate the presence of very fine spherical γ′ within the first 50 mm of the interface in the LSHR material, indicating rapid nucleation and growth of γ′ during the welding process. The large grain boundary γ′ evident at 1 mm indicates the process did not completely dissolve the original phases present in the sample past this length. SE images of the 1 mm position (Fig. 7) show large primary γ′ at grain boundaries and in the interior of the γ, in agreement with earlier results reported by Gabbet al. [4]. Because these images provided a large population set of multi-sized precipitates, they were used to more accurately determine area fractions and sizes for primary and secondary γ′ at the 1 mm position instead of the TEM results. The slightly etched SE images were analyzed at high magnifications using Photoshop, and manually segregated to separate γ′ from γ. The segregated images were then thresheld to separate specific precipitates and determine individual precipitate pixel counts. The pixel counts were converted to areas by calibrating the image magnification, and then converted to average spherical diameters for size estimates. The area values were obtained by dividing the relevant precipitate pixel counts by the total pixel count for each image.The main error associated with this technique involves overestimating the diameter of the precipitate because of the etching process. For the larger secondary and primary γ′, this is not a significant issue. However, for the smaller tertiary γ′, it is important to carefully avoid over etching the sample prior to imaging. The resulting grain boundary primary γ′ area fraction was approximately 15.6%, with an equivalent diameter of 1.04 mm. The total primary γ′ was measured at 34.6%, providing an interior grain primary γ′ area fraction of 19%. The measured tertiary γ′ was 9.16% area fraction, and diameter of 74.6 mm. When combined with the interior grain gamma prime values, the values climbed to 28.16% area fraction with an equivalent diameter of 76.88 mm. Please note the differences between the γ′ morphologies.
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