Preparation And Characterization Of Superfine W(Mo)-based Composite Powders

Powder metallurgical W(Mo)-based alloys are widely used in electricalcontacts, aerospace devices and many other components due to their highdensity, good electrical and thermal conductivity. They have attractedconsiderable attention in military and civilian. The development of modernscience and technology puts forward higher requirements on W(Mo)-basedalloys, and preparation of nanocomposite powder and fine-grain alloys haveattracted much attentions.Fine-grain W(Mo)-base alloys with homogeneous microstructure haveexcellent comprehensive properties. Therefore, development of W(Mo)-basedalloys has become one of the main topics in W(Mo)-based alloys. Nanosizedpowders can improve ability, reduce sintering temperature and refinemicrostructure. It is desirable to prepare superfine or nanosized W(Mo) basedalloys. Furthermore, the sintering process also has significant influence on theproperties of W(Mo)-based alloys.Because of the initial powders and the sintering process are two intrinsicfactors that affect the mechanical properties of W(Mo)-based alloys. In thisdissertation, the two factors were investigated respectively. Glycine-nitrateprocess(GNP) method has been used to prepare oxide ceramic powders and thepowders show good compositional uniformity, low residual carbon levels andsmall particle sizes. In this work, this method was employed to prepareprecursor powders of Mo-Cu and95W-3.5Ni-1.5Fe composite powders. Aftersubsequent calcination and reduction, superfine composite powders werefinally obtained. Sinterability of the composite powders were further studiedin this work.The main achievements and innovation points of the work are listed asfollowing:(1) Glycine-nitrate process (GNP)-reduction method was innovativelyused to synthetize Mo-Cu composite powders. Ultrafine Mo-20Cu、Mo-40Cu、Mo-45Cu(wt.%) composite powders were fabricated with ammoniumheptamolybdate and copper nitrate(Cu(NO3)2·3H2O) as raw materials andgylcine as a complexant and incendiary agent. Superfine resultant particle sizeof the Mo-Cu powders are about100nm.(2) The Mo-Cu composite powders prepared by the GNP method show good sinterability. After sintered at1050°C, relative density over98%isobtained for the Mo-40Cu samples, and the Mo-20Cu samples have relativedensity of99%after sintering at1150°C, which is close to the full density.(3) Mo-Cu alloys containing different Cu contents exhibit distinctdensification tendencies. For Mo-40Cu and Mo-45Cu, further increase of thetemperature to1100℃results in lower sintering density. This is due toexcessive Cu fluid seepage Mo skeleton during liquid phase sintering.(4) Nanocrystalline95W-3.5Ni-1.5Fe powder was successfully producedby the GNP method using ammonium paratungstate(APT), Fe(NO3)39H2O,Ni(NO3)36H2O as raw materials and glycine as a complexant and incendiaryagent. Grain particle size of the composite powders is about30nm and thecomposition distributes uniformly in the powders.(5) The solubility between W and γ-(Ni,Fe) was measured using EDXanalysis. The results show that the solution of W in binder phase is muchhigher than those of γ-(Ni, Fe) in W phase, and the solution of W in binderphase increases gradually as the temperature increasing when sinteredbelow1480℃. When the sintering temperature was elevated to above1500℃,the compacts were subjected to slight solution degradation.(6) The superfine95W-3.5Ni-1.5Fe powders show good sinterability.More than92%relative density was obtained at sintering temperature of1400℃。After sintering at1480℃, the tungsten heavy alloys have homogeneousmicrostructure with ultrafine tungsten particle size about4μm, and relativedensity above96%. The bending strength and tensile strength reach1359MPaand650MPa，respectively. A maximum relative density of98.59%is achievedat sintering temperature1500℃. However, tungsten grains grow rapidly, andthe bending strength and tensile strength also decrease due to the overburntphenomenon.（7）The higher strength of the heavy alloys are associated with the finertungsten grain size and cleavage fracture of tungsten particles. The tensilefracture characteristics was investigated by scanning electron microscope.SEM images of fractured surfaces clearly indicated that failure in case ofW-Ni-Fe heavy alloys sintered at1350℃due to matrix and/or interfacefailure, where as, W-Ni-Fe heavy alloy sintered at1450℃were failedpredominantly by cleavage fracture of tungsten particles and avulsion ofmatrix. In contrast to1500℃，the tensile strength decreases with tungstenparticles grow rapidly.