Transformation of Fe–C system to high pressured hexagonal structures by mechanical alloying of elemental powders
Introduction
The Fe–C system has two phase diagrams as the equilibrium Fe-graphite and the metastable Fe-cementite [1], and the Co–C and Ni–C systems have only the metal-graphite diagrams without carbides. The authors have studied the structure changes of these three systems by mechanical alloying of elemental powders and have reported their transformation to the hexagonal structures as [2], [3], [4], [5], [6],
Orthorhombic phases are formed on the way in the Fe–C and Co–C system, but they finally transforms to the hexagonal ones. These hexagonal structures also have been reported by mechanical alloying [7], [8] and ion implantation [9], and it is known that pure iron transforms with pressure from α-Fe (bcc) to ε-Fe (hcp) over 11.5 GPa at room temperature [10] and the inner core of the earth is the metallic solid Fe–Ni alloy with large amount of metalloid elements [11], [12]. Dachille and Ray [13] have reported that the high pressure phases of PbO, SiO2 and B2O3 could be prepared by milling. In this paper the transformations of the Fe–C system, from Fe75C25 to Fe50C50, and Ni75C25 by mechanical alloying of their elemental powders were studied by X-ray diffraction and 57Fe transmission Mössbauer spectroscopy.
Section snippets
Experimental procedure
Milling of powders was carried out using a fluctuating mixer (TKMAC-1200) and under an argon atmosphere. The vial and the balls were made of yttria toughened zirconia (YTZ) to avoid metallic impurities. The purities of iron and nickel were 99.9 at.% and their sizes were under 50 μm. The purity of carbon was 99.9 at.% and its size was about 5 μm. All amount of powders were recovered at each milling time. The crystal structure changes of powders were studied using a X-ray diffraction (CuKα, 40
Fe75C25
Fig. 1 shows the XRD pattern change by mechanical alloying of Fe75C25 powder. The diffraction was α-Fe up to 9 ks, but another peaks appeared after 18 ks. These new peaks grew with milling time and the orthorhombic cementite-like peaks were obtained after 36 ks, but were not clearly distinguished from α-Fe because they were broad and overlapped as 2θ(110)=44.7 of α-Fe and 2θ(031)=45.0 of cementite.
Fig. 2 shows the 57Fe Mössbauer spectrum change of Fe75C25 powder with milling time. The spectrum
Conclusion
Supersaturated hexagonal solid solutions were formed by mechanical alloying of elemental powders from Fe75C25 to Fe50C50 and Ni75C25 as bcc→orthorhombic structure (cementite)→hcp in the Fe–C system and fcc→hcp in Ni75C25. It is considered that the formation of these hexagonal solid solutions is related to the locally high compression generated by the forced solid dissolving of large amount of carbon atoms into the interstitially octahedral sites of the host metals. Their 57Fe Mössbauer spectra
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