Ultrahigh strength ductile nanocrystalline steel

Various unconventional properties such as ultrahigh strength, high hardness, superplasticity and unique magnetic properties were reported from nanocrystalline materials whose grain size is less than 100 nm. During the past two decades, many techniques have been developed to synthesize nanocrystalline materials, including inert gas condensation and consolidation (IGC) [1], severe plastic deformation (SPD) [2], crystallization of amorphous solid [3], electrodeposition [4], and surface mechanical attrition treatment (SMAT) [5-8]. However, producing industrially useful bulk materials with the microstructure in a nanometer scale is still a challenge for materials engineering. One of the widely-used promising methods to produce commercial bulk nanomaterials with a grain size of less than 100 nm is ball milling accompanied by a subsequent consolidation process. Due to the metastable nature of the nanocrystalline materials, the consolidation process must be able to eliminate extensive grain growth and retain the nanoscale grain size while obtaining a near theoretical density. Numerous traditional consolidation techniques, such as hot pressing [9], hot isostatic pressing [10] and hot-extrusion [11] can yield nearly fully dense compacts [12]; however, recrystallization and grain growth during high temperature consolidation processes retard the synthesis of nanocrystalline microstructure. An artifact-free bulk nanocrystalline Cu with a good combination of strength (770 MPa) and ductility ( 30%) was developed very recently by mechanical milling and in-situ consolidation [13]; however, the uncontrollable sample shape and size may be the restriction for engineering applications. A process that can be applied to produce bulk nanocrystalline materials at moderate temperature with low applied load and fast consolidation speed are necessary for potential engineering applications. Spark plasma sintering (SPS) can achieve rapid heating rate with pulsed electrical discharge that is required to densify powders rapidly without substantial grain growth. The SPS method has been used previously to consolidate a large variety of ceramic and metal powder materials to high densities [14]. The SPS method was used very recently to synthesize nanocrystalline Fe-Al with a near theoretical density with controlled grain growth [15]. The unique boundary cleaning effect of the SPS process makes the powder handling more convenient [16]. In the present investigation, we selected Fe-C powder as the precursor to fabricate dense bulk nanocrystalline materials by means of the SPS method, because a recent study showed that the Fe-C nanocrystalline microstructure that was produced by mechanical milling exhibits strong resistance to the grain growth by heating [17]. In addition, the Fe-C system has been used in wide industrial applications and is not susceptible to media pollution during mechanical milling with stainless steel balls.

Fe-0.8wt%C alloy powder was prepared by ball milling iron (99.99% purity) and graphite powder (99.999% purity) in a Fritsch P-6 planetary ball mill. Stainless steel media (10 mm in diameter) were used with a ball to a charge ratio of 10:1. Powder handling was conducted in an argon glove box to avoid contamination. Milling was conducted at 250 rpm for 100 h, with an intermediate stopping for 2 h after every 20 h milling to reduce temperature rise. The as-milled Fe-C powder was packed in a graphite die with an inner diameter of 10 mm and was densified by a SPS apparatus, Sumitomo Coal Mining Company Model 1050, in a vacuum of <10-3 Pa at a load of 5.5 kN (70 MPa) for 10 min. Temperatures of 400oC, 500oC, and 600oC were selected to compare the consolidation behavior, mechanical properties, and constituent phases. A thermocouple placed in the middle part of the graphite die was used to control the temperature during the SPS treatment.


Fig 1 Bright field image and selected area diffraction pattern of the nanocrystalline Fe-C alloy. The dark field image is excited with the cementite diffraction, so the brightly imaging particles are cementite

As shown in Fig. 1, bulk nanocrystalline Fe-C alloys with a grain size of 150 nm containing a few nanometer cementite dispersoids were fabricated from the mechanically milled Fe and graphite powder by SPS. Yield strength of 2000 MPa, ultimate strength of 3500 MPa, and a plastic strain over 40 % were obtained from this nanocomposite alloy as shown in Fig. 2. This deformation behavior is very unusual for the nanocrystalline material with the grain size of 150 nm, as it shows clear strain hardening. In addition, the plastic strain before fracture is about 40%, which is in contrast to brittle nature of the high strength nanocrystalline materials. Yield stregth of 1,800 MPa is equivalent to maraging steels, but the present alloy does not contain any precious alloying elements and exhibit superior elongation, so is promissing.


Fig. 2 .Compression stress-strain curve of the bulk nanocrystalline Fe-C alloy fabricated by mechanical milling and spark plasma sintering processes.

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Related Publications

Fabrication of bulk nanocrystalline Fe-C alloy by spark plasma sintering of mechanically milled powder
H. W. Zhang, R. Gopalan, T. Mukai, and K. Hono, Scripta Mater., 53, (2005), in press.

Characterization of nanocrystalline ferrite produced by mechanical millin of pearlitic steel
S. Ohsaki, K. Hono, H. Hidaka, S. Takaki, Scripta Materialia, 52, 217 - 276 (2005).

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