THE EFFECT OF MO ADDITION ON THE LIQUID-PHASE SINTERING OF W-HEAVY ALLOY

The morphological and compositional changes of grains have been invest igated in the initial stage of liquid-phase sintering of W-Mo-Ni-Fe po wder compacts. Both large (5.4-mu m) and small (1.3-mu m) W powders ha ve been used to vary their time of dissolution in the liquid matrix. W hen 80W-10Mo-7Ni-3Fe (wt pct) compacts of fine (about 1- to 2-mu m) Mo , Ni, and Fe and coarse (5.4-mu m) W powders are liquid-phase sintered at 1500 degrees C, the Mo powder and a fraction of the W powder rapid ly dissolve in the Ni-Fe liquid matrix. The W-Mo grains (containing sm all amounts of Ni and Fe) nucleate in the matrix and grow while the W particles slowly dissolve. In this transient initial stage of the liqu id-phase sintering, duplex structures of coarse W-Mo grains and fine W particles are obtained. As the W particles dissolve in the liquid mat rix during the sintering, the W content in the precipitated solid phas e also increases. The dissolution of the small W particles is assessed to be driven partially by the coherency strain produced by Mo diffusi on at the surface. During sintering, the W particles continuously diss olve while the W-Mo grains grow. When the compacts are prepared from a fine (1.3-mu m) W powder, the W grains dissolve more rapidly, in abou t 1 hour, and only W-Mo grains remain. These observations show that th e morphological evolution of grains during liquid-phase sintering can be strongly influenced by the chemical equilibrium process.