Ductile Fe 83 C 17 Alloys of Ultrafine Networklike Microstructure
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INTRODUCTION
THE Fe-C with more than 1.7 wt pct of carbon forms white cast iron readily on solidification.[1,2] It contains two phases, which are aFe and Fe3C. Together they form a eutectic morphology. aFe is ductile, but Fe3C is hard and brittle. Since the volume fraction of the latter greatly exceeds that of the former, white cast iron is brittle and abrasive. There are various methods to introduce ductility and toughness to white cast iron. In one practice,[2] heat treatments are employed, which transform white cast iron into malleable iron with appreciable ductility; for example, 32510 malleable iron has an ultimate tensile strength of 340 MPa and a strain to failure of 10 pct, and 90001 malleable iron has an ultimate tensile strength of 720 MPa and a strain to failure of 1 pct. In another practice,[3] as much as ~30 wt pct of Cr is added to a white cast iron, and the resulting alloy is called a high Cr white cast iron. Its toughness improves, but the alloy remains brittle.[4] Another type of cast iron of importance is the austempered ductile iron.[5–8] It has attractive mechanical properties; for example, 3.44C2.41Si-0.15Mn-94Fe (in wt pct) austempered ductile iron[8] has an ultimate tensile strength of 1510 MPa and a strain to failure of 2.8 pct, and 3.56C-2.8Si0.96Ni-92.68Fe (in wt pct) austempered ductile iron[6] has an ultimate tensile strength of 1170 MPa and a strain to failure of ~17 pct. Chen and Turnbull[9,10] demonstrated that Pd-Au-Si glassy foils undergo amorphous/liquid state spinodal decomposition when they are annealed at a temperature near the glass transition temperature, Tg. The microstructure can be described as networklike, and its wavelength k is in the nanometer range. Some of the mechanical properties of the phase-separated specimens were studied by Chou and Spaepen.[11] They found that
C.M. HO, Graduate Student, C.C. LEUNG, Y.L. YIP, and S.W. MOK, Research Associates, and H.W. KUI, Professor, are with the Department of Physics, Chinese University of Hong Kong, Shatin, N.T., Hong Kong, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted October 9, 2009. Article published online September 16, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
when completely crystallized, the phase-separated specimens are extremely brittle. Later, by a fluxing technique,[12,13,14] a Pd-Si melt can be quenched into a crystalline ingot (diameter ~1 cm) of networklike microstructure, which is due to the intermixing of Pd9Si2 and Pd3Si. Equivalently, the network morphology can be regarded as consisting of two subnetworks: a Pd9Si2 subnetwork and a Pd3Si subnetwork. The k of each subnetwork is
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