Reaction sintering of an Fe-6 Wt Pct B-48 Wt Pct Mo alloy in the presence of liquid phases

The mechanism of liquid phase sintering of an Fe-6 wt pct B-48 wt pct Mo alloy was investigated by means of thermal dilatometry, differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). A fine composite microstructure of Mo₂FeB₂ and ferrite was produced from powders of Fe, Mo, and FeB by a reaction sintering process involving two liquid phases. The hard phase Mo₂FeB₂ is produced in the compact prior to liquid formation initially by the reaction 2Mo+2FeB=Mo₂FeB₂+Fe and later by the reaction 2Mo+2Fe₂B=Mo₂FeB₂+ 3Fe. Above 1365K, considerable densification results from the initial stage rearrangement of the solid phases (austenite and Mo₂FeB₂) coexisting with the first formed liquid phase (L₁). Another liquid (L₂) which forms above 1415K, where Mo₂FeB₂ is the only coexisting solid phase, is required for complete densification. L₂, having a high solubility for Mo₂FeB₂, provides for solution/reprecipitation processes which characterize the intermediate stage of liquid phase sintering. The effective separation of the initial and intermediate stages by L₁ and L₂ is considered essential for the control of the sintered microstructure of the ternary alloy. 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.