Study of the Plastic Deformation in Nanocrystalline and Ultrafine Iron and Carbon Steels

Abstract

Samples of nanostructured and ultrafine grained steels with carbon content ranging from 0.05 to 0.55%wt. have been obtained by a warm consolidation process from mechanically milled powders and subsequent heat treatments. In general, homogeneous grain size distributions were obtained except for the low-carbon steel in which a bimodal grain size distribution was observed when it was heat treated at high temperatures. The stress-strain response has been studied by means of compression tests. Nanostructured materials showed high strength but poor results in terms of ductility. In the low-ultrafine range (mean grain size between 100-500 nm) the three materials showed an increase in the ductility with strain softening. Finally, when the average grain size was close to 1 µm samples showed larger ductility and strain hardening.

The effect of oxide particles on the strength and ductility of bulk iron with a bimodal grain size distribution

“…Both samples are interesting, since for 675 ºC samples the strength was good (1244 MPa) with a 4% uniform strain and for 700 ºC the UTS decreased to 1028 MPa, but with a 15% total strain, being the percentage of uniform elongation closed to 9%. Unlike the as-consolidated samples, in the case of heat treatment at 700 ºC, there was similarity with previous compression tests [16]. The UTS and compressive stress were close, and the material showed large ductility with a slight strain hardening in both tests.…”

“…6A) in which the contour of the former powder particles are easy to identify. It is worth mentioning that the same samples but in compression tests were fairly good both in strength and ductility, since compressive stress was 1922 MPa and compressive strain arrived to 32% [16].…”

“…This is because these tests are not as sensitive to porosity and bonding defects as tensile test. Therefore, the yield strength calculated from the tensile test, the values of hardness tests and the yield strength obtained from compression tests [16] are displayed together in Table 2 The second consideration is related to the study of the misorientations between the ferrite grains in the consolidated samples. This study must be made since the exponent associated to the grain size in the calculation of the influence of grain boundaries could be affected [41].…”

“…Under compression, together with a high values of strength, a moderate ductility has been found even for average grain sizes close to the NC range [3,[16][17][18]. It must be noted that, in these cases, the strain hardening declined as the grain size was refined, being virtually zero below 300 nm.…”

“…Mechanical milling of pure iron and subsequent warm or hot consolidation has been one of the main techniques to obtain bulk samples of NC and UFG iron and steels with a homogeneous grain size distribution [2][3][4]16,18]. However, porosity and lack of bonding between powder particles in the consolidated specimens by these techniques has been pointed out as the main causes for the brittle behavior observed, especially in tensile tests.…”