Formation Mechanism and Annealing Behavior of Nanocrystalline Ferrite in Pure Fe Fabricated by Ball Milling.

Abstract

Nanocrystalline ferrite formation by ball milling in pure Fe (0.004 mass% C) has been studied through morphology observation and microhardness measurements. It was found that nano-ferrite first formed at the outer surface area in the powders at the early stage of ball milling, which was attributed to the deformation localization. The boundaries between the nano-ferrite and work-hardened regions are quite clear under SEM and the hardness of nano-ferrite region is more than 3 GPa higher than that of the work-hardened region. Further milling led to the particle refinement and formation of nano-ferrite of the whole particles. It has been suggested that the nanocrystalline ferrite formed through a transition from dislocation cell wall created by work hardening during ball milling to grain boundary, which is regarded to contribute to the hardness gap between the work hardened structure and the nanocrystalline ferrite. A relatively high stability against temperature was observed in the nano-ferrite formed by ball milling. The irregular grain boundaries of nano-ferrite annealed at high temperature was attributed to that the grain growth of nanocrystalline ferrite takes place by coalescence of neighboring grains.KEY WORDS: nanocrystalline ferrite; pure Fe; ball milling; heavy deformation; structure.and a length of several mm were cut from iron block and subjected to ball milling. Ball milling was performed in a conventional horizontal ball mill under the protection of pure Ar atmosphere up to 1 800 ks. A vial with inner diameter of 128 mm, a rotation speed of 95Ϯ1 rpm and a ball to powder weight ratio of 100 : 1 were applied. Structural characterization was performed in scanning electron microscopic (SEM) and transition electron microscopic (TEM) observations. SEM observations were carried out on a JEOL JSM-6 300 microscope after polishing and etching the samples in 3% Nital. TEM was performed on a Hitachi H-800 working at 200 kV. Microhardness measurement was carried out using a MVK-G1 Vicker's hardness tester with an applied load of 1.96 N for 10 s. ResultsThe microstructure evolution of pure Fe with ball milling was observed by SEM. It was found that heavy deformation taking place during ball milling led to the formation of two types of structure in all the specimens with intermediate milling times (see Fig. 1). One is a uniformly layered structure near the outer surface area of the particles, which is difficult to be etched by Nital (the bright area near or on the surface). The other is a conventionally deformed structure in the interior of the particles (the interior darker area). Generally, the boundaries between these two types of structures are quite clear under SEM observation. It was also observed that the amount of the layered structure increased with increasing milling time. Among the powders with certain milling time, the finer the particles are, the larger the fraction of layered structure. Figure 2 shows the microstructure change with ball milling time. The starting microstructure of pure Fe is ferrite wit...

Nanocrystallization in Fe-C Alloys by Ball Milling and Ball Drop Test.

“…Such microstructural evolution was observed in the milled powders irrespective of the carbon content (0.004-0.89 mass% C) or starting microstructure (ferrite, martensite, pearlite or spheroidite). [11][12][13][14] It should be noted that there is a drastic difference in hardness between these two types of structures as is shown in Fig. 2(a).…”

“…[11][12][13][14] However, nanocrystallization by ball drop test is limited in high carbon steels (Fe-0.89C with pearlite or spheroidite structure). This reason is considered as follows.…”

“…In our previous papers, [11][12][13][14] we have reported the microstructural evolution and nanocrystallization in the carbon steels by ball milling. However, the deformation mode in a ball milling process is substantially complicated, and thus the deformation conditions to induce nanocrystallization have been unknown.…”

Comparison of the Characteristics of Nanocrystalline Ferrite in Fe-0.89C Steels with Pearlite and Spheroidite Structure Produced by Ball Milling

“…It has been generally understood that ball milling gradually refine the grain size to the final nanocrystalline structure. However, our previous works 14,[17][18][19] revealed that nanocrystallization in steels by ball milling is not a gradual process, but there is a sharp transition from work-hardening state to nanocrystalline state.…”

“…Recently, the formation of nanocrystalline ferrite in Fe-C alloys (including pure iron) by ball milling has been reported. 7,8,[14][15][16][17][18][19] Various mechanisms of nanocrystallization have been proposed by many researchers according to their experimental results. It has been generally understood that ball milling gradually refine the grain size to the final nanocrystalline structure.…”