Investigation On Graded Tungsten Heavy Alloys

The tungsten heavy alloys (WHAs) are expected to replace the depleted uranium penetrator materials due to their excellent mechanical properties and free of radioactive contamination. However, the mushrooming occurs due to the inability of WHAs to self-sharpen by shear localization, which would detrimentally affect penetration efficiency. Contradiction of hardness and toughness is an obvious issue when WC-Co cermet is applied as cutting and mining tools. A potential solution to this problem is to employ functionally graded cemented tungsten carbides. At the same time, WHAs with gradient structures might be the desired materials which can improve penetration performance.The effects of Mn, Cr, Mo, Co, Sn, Ti to the formation of gradient were discussed. It is found that the high oxidation potential of Mn and Cr makes it difficult to get high performance tungsten alloys in conventional sintering with Mn or Cr addition. Titanium would react with hydrogen and pores would form in hydrogen atmosphere. It is impossible to obtain ideal WHAs with titanium addition in hydrogen atmosphere. The microstructures of WHAs with Co and Sn additions would be homogenous after2hours sintering as the diffusional potential for Co and Sn in tungsten heavy alloys is strong. As the commonly used addition element, molybdenum can infinitely dissolve in tungsten, and the diffusion rate of molybdenum in Ni-Fe-W liquid phase is rather slow in WHAs. All these make it to be the proper additive for gradient formation in WHAs.The effects of Mo addition on creating gradient microstructure of a W-Ni-Fe bi-layer sample (one with Mo addition and the other without Mo) by liquid phase sintering were investigated. Mo distribution, W grain size, volume fraction of binder phase as well as micro-hardness between the two layers in the sintered sample was examined. The grain size decreases from23μm in initial molybdenum-free zone to16μm in the section of molybdenum rich. The volume fraction of binder phase increases from12.5%in initial molybdenum-free zone to18.5%in the section of molybdenum rich. The variation of micro hardness is similar to that of the grain size and volume fraction of binder phase. The micro hardness increases from252HV to374HV. The grain size, volume fraction of binding phase and micro hardness vary gradually due to the graded distribution of molybdenum.The volume fraction of binder phase of W-Ni-Fe bi-layer sample with different grain size and different composition were determined which shows grain size gradient and solid/liquid phase tension gradient were the reason for the gradient distribution of binder volume fraction.It has been observed from the fracture surfaces of graded WHAs, the fracture mode in Mo-rich section was W cleavage and in Mo-free section the fracture paths were W-matrix interfacial separation and W cleavage. It would promote the self-sharping ability of WHAs when served as penetrator core materials.