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  • Fig presents the EBSD micrographs

    2018-11-07

    Fig. 3 presents the EBSD micrographs obtained for the nanometric powders. Analyzing these data revealed the lack of any preferential orientation. Relative densities were measured at 95.8% for the direct-SPS sample (a), and 93.9 for the CIP-SPS sample (b), with a relative density of the green sample of 87% after CIP at 1.9GPa. Grains sizes were measured at 0.25µm and 1.75µm, respectively. Σ3 grain boundaries, including large twins, i.e. grain boundaries with a misorientation of 60±2.5° represent a fraction number of 20% in the direct-SPS sample, and 30% in the CIP-SPS sample. A pre-compaction appears to be a mean to increasing Σ3 grain boundaries in the processed samples. Low angle grain boundaries, with a misorientation lower than 15° represent 2% and 3%, respectively. Fig. 4 presents the EBSD micrographs obtained for the mixture of nanometric powders, representing 40% of the total amount, with micrometric powders, 60%. Relative densities were measured at 99.4% for the direct-SPS sample (a) and 96.3 for the CIP-SPS sample (b), with a relative density of the green sample of 87% after CIP at 1.1GPa. After densification, no significant variation of the nanometric–micrometric volume fractions is observed, with a calculated value of 44% nanometric – 56% micrometric grains for the direct-SPS sample. Average grains sizes were measured separately for micrometric and nanometric grains. For the direct-SPS sample, the grain sizes were measured at 3.5µm and 580nm, respectively, and for the CIP-SPS sample, at 2.8µm and 520nm, respectively. Σ3 grain boundaries, including twin boundaries, are depicted in red on this figure. For the direct-SPS they order Fulvestrant represent 40% of the grain boundaries in the micrometric grains, and 17% in the nanometric ones. For the CIP-SPS sample, these proportions are 52% and 23%, respectively. Low angle grain boundaries (<15°) are represented in green. In the direct-SPS sample, they represent 1.4% in the boundaries in the case of the micrometric grains component of the microstructure, and 2.5% for the order Fulvestrant nanometric ones. For the CIP-SPS sample, these proportions are 1.4% and 3%, respectively. It can be noticed that large isolated nanometer-sized grain domains are observed for the direct-SPS processed sample, contrariwise to the case of the pre-compacted sample. In the latter case a more or less connected nanometer-size grain shell enclosing coarser grains is observed. Fig. 5 presents the X-Ray diffraction patterns of the two bimodal samples, with and without CIP pre-compaction. In all our samples, pure Ni is by far the dominant phase, with a residual NiO phase being detected only when nanometric powders were used, as their specific surface is much higher. Rietveld analysis was performed using the MAUD program [5] and revealed a maximal NiO volume fraction of 0.0513 for the direct-SPS sample. As can be seen on this figure, peaks stay slightly larger in direct-SPS sample, due to smaller grain sizes. However, NiO contamination is 25% smaller in the CIP-SPS sample with a volume fraction of 0.0378, probably due to the fact that the powders spend less time in contact with air. These volume fractions correspond to a percentage of oxidized nickel of 0.4% and 0.3%, respectively. Table 2 summarizes the room temperature compression data at the strain rate of 2 10-3s-1 for the different samples presented here above. For the samples sintered from micrometric powders, Yield stresses of 400MPa for the direct-SPS sample, and 200 for the CIP-SPS sample were measured. Both samples depicted a similar strain hardening behavior (not shown here), up to 50% true plastic strain, at which point the capacity of the Deben machine was reached (5000N) and the test was interrupted. For the samples sintered from nanometric powders, an elastic limit of 900MPa was measured for the CIP-SPS sample. The direct-SPS sample possesses a typical brittle behavior without measurable plastic deformation before failure (therefore the data presented in Table 1 for that sample has no real meaning), while the CIP-SPS sample presents a sharp strain hardening with a measured true stress to failure of 1.57GPa for a plastic strain of about 4%.