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INTRODUCTION

COMPOSITES comprising Mo2FeB2 [1] as a hard phase and a ferrous binder phase alloyed with Mo, Cr, Ni, etc. have been reported to show high flexural strength (2.5 GPa) and wear resistance, t2'3[ The hard alloys, also having good corrosion resistance and high temperature properties, have been successfully applied to can-making tools, plastic and ceramic injection molding machine parts, etc. where a combination of the latter properties is essential.I3'4t Essential to obtaining optimum mechanical properties is the refinement of microstructure in terms of both size and uniformity. Direct liquid phase sintering of a mixture of synthesized Mo2FeB2 and elemental powders is not favored because of coarsening of the hard phase during sintering, which lowers the flexural strength significantly. Also the use of synthesized Mo2FeB2 gives rise to a high production cost. When MoB and an iron powder are used to form in situ Mo2FeB2, some MoB remains unreacted in the sintered structure, causing deleterious effects on the mechanical properties. The process which gives the finest and most uniform microstructure, and hence the best mechanical properties, as well as reduced production costs, is a liquid phase sintering process and involves a reaction during sintering which forms the hard phase, Mo2FeB 2, from powders of Fe, Mo, Ni, Cr, and FeB. The liquid phase sintered microstructure is composed of fine grains (1 to 2 ~m) of Mo2FeB2 and a ferrous binder phase. However, the exact evolution of the fine composite microstructure during the reaction sintering has not been fully understood, owing partly to the complexity of the multicomponent alloy system. The purpose of the present investigation is to deepen the understanding of the basic sintering mechanisms underlying the microstructural development by studying a more researchable ternary Fe-6 wt pet B-48 wt pct Mo alloy by TSUNEYUKI IDE is Senior Researcher. Technical Research Laboratory, Toyo Kohan Co., Ltd., 1296 Higashitoyoi. Kudamatsu-Shi. Yamaguchi-Ken, 744 Japan. TEIICHI ANDO, formerly with the Technical Research Laboratory, Toyo Kohan Co.. Ltd.. is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted August 4, 1987. METALLURGICAL TRANSACTIONS A

means of DTA, thermal dilatometry, XRD, and SEM, and provide a metallurgical basis for controlling the microstructure of the liquid phase sintered composites. II.

EXPERIMENTAL PROCEDURE

The composition of the test alloy was adjusted by blending a 99.9 pct pure Mo powder, a carbonyl iron powder, and a gas a t o m i z e d FeB powder. A small amount of graphite was also added up to 0.35 wt pct to the powder mixture to facilitate reduction of oxides during sintering or thermal analyses above approximately 1175 K. Table I shows the chemical analysis of the FeB powder. The carbon in the FeB powder is also consumed in the oxide reduction. Careful control of carbon content in the powder mixture limited the C content after sintering to less than 0.