Metals

Nanocrystalline metallic powder can be densified using conventional sintering. The largest departure from nonnanocrystalline powder sintering behavior is seen in the initial sintering stages. Here, lower activation energies and sintering onset temperatures are observed. For instance, an activation energy of 134 kJ/mol was reported for 40 nm tungsten (Ref 52), which is lower than the value for volume diffusion (580 kJ/mol) and surface diffusion (—300 kJ/mol) (Ref 53). Two apparent activation energy values were calculated in nanocrystalline iron: a low value for low-temperature sintering (125 kJ/mol) and a high value (248 kJ/mol) corresponding to high-temperature densification (Ref 54). The latter value is close to that of volume diffusion in iron and is similar to the value for coarse-grained iron. The low-temperature value is less than the grain boundary diffusion value.

The sintering onset temperature is particle-size dependent (Eq 5). In pure tungsten, the onset is 1100 K (0.3 Tm) for 31 nm particles and 900 K (0.24 Tm) for 9 nm particles (Ref 55). The onset temperature for densification of 40 nm iron powder is as low as 370 K (0.21 7m). as compared to —900 K (0.5 Tm) for 2 /'m iron powder (Ref 10, 56), as shown in Fig. 12. Alloying iron with aluminum improves fine structure after sintering (Fig. 13) due to formation of AlN dispersoids (Ref 56).

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Fig. 12 Effect of sintering temperature on density for 15 nm iron powder hot pressed at 10 MPa for 1 h. Dashed lines show the powder density before (11%) and after (23%) compaction. Source: Ref 10

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Fig. 13 Effect of fine-particle AlN as a grain-growth retardant for nanocrystalline iron cryomilled in nitrogen. Source: Ref 56

Sintering of iron-nickel nanocrystalline powder is initiated when oxide layers are reduced (Ref 57). This occurs above 500 K in hydrogen. When vacuum is used, no sintering takes place up to 725 K. To avoid further contamination, as-produced nanocrystalline powder can be either consolidated in situ or handled in a controlled environment prior to sintering. The oxygen role in nanocrystalline, evaporation-condensation, iron sintering is important (Ref 10). Although sintering is carried out in a reducing hydrogen atmosphere, oxygen is present in the final product as fine, grain-refining oxides. This is due to adsorbed oxygen entrapment at pore closure.

Iron, nickel, copper, and aluminum have been consolidated using high-pressure densification. The data conform to the compaction law given as Eq 8 and illustrated for iron and copper in Fig. 9. The transition point at which the slope changes (Fig. 9) is associated with agglomerate failure. When agglomerates are strong, they do not break and consequently, no transition point is seen. Rate controlled sintering of pure nickel resulted in 99% density with 70 to 80 nm grain size (Ref 58). Densities up to 97% at room temperature and 99% at 575 K in nickel (65 nm), iron (30 to 50 nm), and copper (58 nm) have been obtained using 3 GPa in a high pressure cell, as seen in Fig. 14 (Ref 41, 59). Historically, high pressure consolidation has been the preferred in situ consolidation method for classical nanocrystalline powders synthesized using inert gas condensation (Ref 60, 61, 62, 63, 64). A dense product with 22 nm final grain size has been achieved in Al-Fe (Mo, Si, B) alloys by high pressure consolidation (3 GPa at 1073 K) of alternating amorphous layers of constituent metals (Ref 65). Fully dense iron, nickel and Cu-50Ag specimens with less than 20 nm grain size were produced by severe plastic deformation consolidation in which torsional true strains of the order of 7 are imposed (Ref 66, 67, 68).

Fig. 14 Bright-field transmission electron microscopy image of 58 nm copper powder cold compacted at 750 MPa. Arrows indicate deformed particles presenting flat surfaces. Source: Ref 41

Sinter Forging. In numerous cases, sinter forging is shown to be very effective in achieving full or near full densities and grain sizes less than 100 nm in metals, as shown in Fig. 15 (Ref 49, 69, 70, 71). The composite Fe-10Cu has been densified by sinter forging (Ref 49). The benefits of sinter forging in comparison to HIP are evident, as sinter forging produced material with 45 nm grain size at 800 K and 525 MPa compared to HIP at 975 K and 170 MPa with a final grain size of 130 nm (Ref 49, 72). Sinter forging of ceramics is equally effective (see the section "Zirconia" in this article).

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Fig. 15 Effect of sinter-forging temperature on the density and grain size of mechanically milled Fe-29Al-2Cr. The initial particle size was 30 nm and forging was affected with 1.25 GPa for 5 h. Source: Ref 71

Shock wave consolidation has been applied to consolidation of metallic nanoparticles. For example, fully dense specimens were obtained from ball-milled Fe-C and Fe-N solid-solution, powders by shock wave consolidation (Ref 73, 74). The ball-milled iron powder (20 nm grain size) was consolidated by dynamic compaction (12-19 GPa) followed by 1 h anneal at 500 °C (930 °F).

Hot extrusion usually involves high stresses that can be applied at relatively lower temperatures than in other pressure-assisted techniques, such as HIP. A compromise is usually sought between the strength, which requires high temperatures, and final grain size, for which low temperatures are desired. Hot extrusion has been primarily used to consolidate metallic nanopowders (Ref 75, 76, 77, 78). Grain sizes less than 100 nm have been achieved at 1120 K using a 0.5 GPa stress in nickel and iron (Ref 75), Al-Ni-Zr with mischmetal additions under an extrusion ratio of 10 to 1 (Ref 70), and in Mg-10Y-5Cu at 323 to 773 K with extrusion ratios of 5 to 1 to 10 to 1 (Ref 70).

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