Scanning Electron Microscopy of Fe 79.5 B 6.5 C 14 Network Alloys: Part I
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I.
INTRODUCTION
AMORPHOUS phase separation was first observed by Chen and Turnbull[1] in their studies of Pd-Si and PdAu-Si glassy foils by transmission electron microscopy (TEM). Chou and Turnbull[2] then employed the techniques of small-angle X-ray scattering (SAXS), confirming that the phase separation occurring in amorphous Pd-Au-Si is consistent with a spinodal mechanism. The amorphous phase separation that occurs in Pd-Si and Pd-Au-Si glassy alloys is unusual because they have a negative heat of mixing.[3,4] Since then, other research groups reported conflicting results on phase separation in metallic glasses or bulk metallic glasses (BMGs) of negative heat of mixing.[5–16] Tanner and Ray[5] were able to obtain TEM micrographs showing image contrast in glassy Zr-Ti-Be specimens. For a long while, they were regarded as direct evidence of amorphous phase separation. However, it was demonstrated later on that in the thinning process of a TEM specimen, artifacts may be introduced, casting doubt on the accuracy of the results obtained by Tanner and Ray.[15] As a result, amorphous phase separation in metallic glasses and BMGs of negative heat of mixing has become a controversial issue. Recently, Lan et al.[17] found direct evidence of amorphous phase separation in Pd41.25Ni41.25P17.5 BMGs (of negative heat of mixing). The nature was also examined. By studying Pd40 + 0.5xNi40.5 + 0.5xP20–x BMGs with 0 £ x £ 3.5, it was found that the characteristic size in the phase-separated specimens obeys the lever rule.[18] Together with Chou and Turnbull’s SAXS results,[2] it is suggested that the phase separation is due to a metastable liquid miscibility gap (MLMG). Finally, the spinodal mechanism that occurs in the MLMG of Pd-Ni-P BMGs is characterized.[19]
W.H. CHOW, Graduate Student, C.C. LEUNG, Y.L. YIP, S.W. MOK, Research Associates, and H.W. KUI, Professor, are with the Department of Physics, Chinese University of Hong Kong, Shatin, NT, People’s Republic of China. Contact e-mail: [email protected]. edu.hk Manuscript submitted August 14, 2012. Article published online April 25, 2013 3532—VOLUME 44A, AUGUST 2013
Inside an MLMG, a phase-separated melt becomes an amorphous nanostructure of network morphology[10] if crystallization is bypassed on cooling to room temperature. On the other hand, if crystallization is allowed to take place, the phase-separated melt becomes a crystalline nanostructure of network morphology on cooling to room temperature. In the case of Pd-Si[20,21] and Pd-NiP,[22] the as-formed crystalline nanostructures are brittle[23] because the constituent subnetworks are made up of brittle intermetallic compounds. Ho et al.[24,25] found that molten Fe100–xCx alloys, where x = 17 to 24, can be cast into solids of network morphology of Fe3C and aFe. For clarity, they are called network alloys. Because the aFe subnetwork is ductile, the network alloys have attractive mechanical properties. For instance, Fe83C17 network alloy has a yield strength (compressive) of ~2000 MPa and a plastic strain to failure of ~18 pct,
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