Microstructure and sinterability of nano-crystal tungsten powders

Tungsten (W) is a useful material because of its high performances such as high melting temperature (Tm: 3653 K) and high strength (Young's modulus: 403 GPa). However, its high melting temperature requires high sintering temperature for producing a bulk W material, and it has a high Ductile-Brittle Transition Temperature which leads to brittleness at room temperature. To improve such negative properties, a nano grain structure formation is very effective. In the present study, a Mechanical Milling (MM) process is applied to W powder and W-Re powder mixtures. The MM process enables to produce a nano grain microstructure very easily and has been applied to many powder materials. Pure W powder or W-3 mass% or 10 mass% Re powder mixture is charged into steel vials with steel balls and rotated using a planetary ball milling at 200 rpm for 360 ks under Ar atmosphere. The MM powders are then sintered by spark plasma sintering (SPS) equipment at the temperature range of 1273 K to 1873 K under the compaction pressure of 50 MPa. The compacts and powders are examined by means of XRD, SEM and TEM/ EDS. As a result, in the MM powder a nano grain structure, whose grain size is approximately 10 nm, is obtained after MM for 360 ks. The MM process enables sintering of the powders at 1273 K, while approximately 1673 K is necessary for non-MM powder to be sintered. Re addition prevents grain growth during sintering and thus increases hardness of the compacts.