Ductility of Bulk Nanocrystalline and Ultrafine Grain Iron and Steel


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This paper reviews the ductility of nanostructured and ultrafine iron obtained using a variety of methods. Mechanical milling of powder and subsequent hot consolidation, one of the most popular methods offer high mechanical strength but poor ductility. Improvements made in the consolidation processes and the introduction of final heat treatments, in addition to new approaches such as spark plasma sintering and high pressure torsion, have increased the total plastic strain of nanostructured iron. The development of bimodal structures enables the existence of strain hardening and more uniform deformation. The paper also includes a steel study, which finds that the hardness of milled powder and the role of carbon atoms inside ferrite grains make it more difficult to improve the ductility of nanostructured samples.



Materials Science Forum (Volumes 633-634)

Edited by:

Yonghao Zhao and Xiaozhou Liao






J. A. Benito et al., "Ductility of Bulk Nanocrystalline and Ultrafine Grain Iron and Steel", Materials Science Forum, Vols. 633-634, pp. 197-203, 2010

Online since:

November 2009




[11] shows pure iron processed by high pressure torsion (HPT) with an average grain size below 0. 5 µm that has an UTS of 1. 9 GPa and 30% plastic deformation in a tensile test. In this case, the engineering curves do not show much strain hardening and plastic deformation is mainly observed during necking. Bimodal grain size distributions appear to be more efficient in producing uniform deformation. Figure 3. Comparison of the mechanical properties for nanocrystalline and ultrafine iron samples. The average grain size is listed in Table 3. Steels Microstructures. The microstructures for a 0. 55%C steel with different grain sizes in the nanocrystalline and ultrafine ranges are shown in Fig. 4. In the case of nanocrystalline steels obtained by MM, the presence of higher amounts of carbon led to the development of a complex microstructure during subsequent heat treatments. Several studies have shown that in MM steel.

DOI: 10.1016/s1875-5372(10)60104-x

[12] , 13] the carbon atoms are distributed along ferrite grain boundaries and also dissolved inside ferrite grains. The atomic percentage of carbon atoms into ferrite is far beyond the limit established by the Fe-C phase diagram [12, 13]. When the temperature is increased during the consolidation process or heat treatments, the carbon solved in the ferrite grains moves out to the grain boundaries to form nanoprecipitates of cementite. The final size of these particles increases as the temperature and time of treatment rises. For nanocrystalline steels, to avoid grain growth, the consolidation temperature must be as low as possible, usually lower than 500 ºC. Consequently the diffusion of carbon out of the interior of the ferrite is not complete. This renders that nanocrystalline ferrite is not free of carbon atoms and that they are sited in the dislocation cores.

DOI: 10.1520/e0089

[12] This fact promotes a change in the deformation mechanism in comparison with pure iron [12-14] and obviously hinders the movement of dislocations. Therefore, the ductility of the steel is expected to decrease as the carbon content increases. When the heat treatment temperatures are increased, and especially when the A1 critical temperature is reached, carbon migrates from ferrite. The ferrite then becomes ductile and allows strain hardening and larger amounts of plastic deformation, but its grain size can be in the ultrafine 0 2 04 06 08 01 0 0 500 1000 1500 2000 2500 3000.

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[7] Ultimate Compressive Strength (MPa) Total Plastic Deformation (%) * Ultimate tensile strenght range (100-1000nm) and the mechanical strength is then lower. In this range, the microstructures are often similar to those obtained by SPD or thermomechanical methods such as caliber warm rolling.

[15] Mechanical behavior. As in pure iron, weak interparticle bonding appears to be the main cause of the fragility of the MM consolidated specimens. Moreover, as can be observed in Table 1, the MM steel powder has a higher hardness than pure iron and makes it less efficient at reducing processing flaws during consolidation processes. Since hardness increases as carbon content increases, a more fragile behavior is expected in high carbon steels. As shown in Table 2 and Fig. 2, 0. 3 wt. %C steel with an average grain size of 135 nm has an 8% plastic deformation in compression with no higher mechanical strength than pure iron. With heat treatments at temperatures over 725º C, its ductility is clearly enhanced, but the strain hardening ability is not. In the case of 0. 55 wt. %C steels, brittleness is even more pronounced since samples in the nanocrystalline range undergo failure before plastic deformation takes place (Figº 2). Only with larger grain sizes (200 nm) is some plastic deformation achieved depending on the heat treatment (Fig. 3 and Table 2), but always without strain hardening.

DOI: 10.1007/bf00700694

[7] If strain hardening is required, heat treatment temperatures should be increased to allow carbon atoms to completely migrate from the ferrite grains forming precipitates in grain boundaries. This implies larger ferrite grain sizes, i. e. above 0. 5 µm. Figure 4. TEM microstructures for a 0. 55wt. % C with different average grain sizes. (A) 30 nm, (B) 45 nm, (C) 115 nm, (D) 460 nm. The lack of good interparticle bonding in warm consolidated specimens from MM powders seems to be palliated when the SPS technique is used for consolidation.

[16] In this case, with a consolidation temperature of 600 ºC, a 0. 8wt. %C alloy displayed a 40% plastic deformation in compression, showing a large work hardening range with a compressive strength of 3500 MPa. In tensile tests, the brittleness of bulk nanocrystalline steels is more evident, and the failure occurs in the elastic range well below the yield strength values found in compression tests. Our own experiments have shown that no large plastic deformation in 0. 55%wt. C steel is obtained until grain sizes close to 1 µm are achieved. SPD or thermomechanical processes are free from the problems associated with powder consolidation, although the grain sizes achieved are larger than the ones obtained in MM processes. In the case of caliber warm rolling.

[15] a 10% plastic deformation is reported for the majority of steel grades tested from 0. 05 to 0. 6%wt. C, with a ferritic grain size of approximately 0. 5 µm. However, as the carbon content increases, the strain hardening capacity is enhanced simultaneously with the mechanical strength. This effect is related to the most homogeneous dispersion of cementite precipitates giving rise to a more uniform deformation. Summary The MM and HC procedures have enabled the production of bulk nanocrystalline iron and steels with high mechanical strength but low ductility. Recently the improvement of these techniques and the development of bimodal structures, together with new consolidation approaches such as SPS and HPT have shown that some ductility and strain hardening can be obtained with a lower loss in mechanical strength. Rferences.

DOI: 10.1016/b978-1-4832-8412-5.50133-8

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