Formation of metastable phases of Ni-C
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I.
INTRODUCTION
S I M I L A R to the system Fe-C, the phase diagrams of the systems Ni-C and Co-C are known to exhibit socalled "double diagram" 1~-5]in which the stable M (M = Fe, Co, Ni)-graphite and the metastable M-M3C equilibrium phase boundaries may be drawn on the phase diagrams. Experimental investigation of these metastable phase boundaries by the use of rapid quenching of liquid alloys has been reported previously, t6'v] For the purpose of investigation of metastable phases, the mechanical alloying (MA) technique has been proven to be quite effective, ts~ In addition, since the MA is a solid state reaction, it allows the study of alloy phases in composition ranges wider than the melting and casting method. For the Fe-C system, we have previously reported the formation of metastable phases Fe3C and fe7C 3 by the MA method starting from the elemental powder mixture of iron and graphite. ~9'1~ The purpose of the present work is to investigate the metallographic structure change and phase formation in MA process of the high-carbon Ni-C and Co-C alloys by ball-milling. The formation of two kinds of metastable phases depending on carbon concentration of starting mixture and on processing time will be reported. The formation ability of these metastable phases will also be discussed from the viewpoint of the formation enthalpy of alloy while comparing with the case of Fe-C system.
II.
EXPERIMENTAL PROCEDURES
Mechanical alloying of Ni~_xC~ was performed by the use of a conventional ball mill. Carbon contents, X = 0.10, 0.15, 0.20, 0.25, 0.40, 0.50, 0.70, and 0.90, were examined. Elemental powders of nickel (99.9 pct; average size: 80/xm) and graphite (99.9 pct; average size: 5 /xm) were used as starting materials. Both the vial T. TANAKA, Lecturer, is with the Department of Mechanical Engineering, Osaka Sangyo University, Daito, Osaka 574, Japan. K.N. ISHIHARA, Assistant Professor, and P.H. SHINGU, Professor, are with the Department of Metal Science and Technology, Kyoto University, Kyoto 606-01, Japan. Manuscript submitted January 2, 1992. METALLURGICAL TRANSACTIONS A
(1600 cc) and the balls (9.5 mm in diameter) were made of stainless steel. The weight of sample was 40 g, and the weight ratio of sample to balls was 1/100. Ball-milling was carried out up to 2000 hours in argon atmosphere, and 1.5 cc of methyl alcohol was added to the sample for the purpose of lubrication. After several fixed time intervals, a small amount of sample was taken out in the glove box filled with argon gas for the structural examination. The metallographic structure was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The sample for TEM observation was sliced mechanically into a thickness of several tens nanometers by using a diamond blade. Identification of phases and the measurement of lattice constant were performed by X-ray diffractometry using Cu-K~ radiation with a graphite monochromator. The thermal analysis was performed by differential scanning calorimetry (DSC) at the heating rat
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