Mechanical milling (MM), which is one of SPD process, is applied to W powder and W-Remixed powder. MM processed W and W-Re powders easily form nano grain structure even though they have high melting temperature. The nano grain formation mechanism in these powders is as follows: multi axial deformation of the powders by milling, at the temperature of 333K at most, produces pan-cake grain structure at first. Extremely dense dislocations result in grain sub-division, and finally nano grain structure with high angle boundary forms. Nano grains with approximately 10 nm in diameter are obtained. The MM powders are sintered using Spark Plasma Sintering process. Sintering and high temperature deformation behaviors of the MM powders are also investigated. The MM treatment enables W powder to be able to sinter at 1273 K while the powder without MM could never sintered at the same temperature. Re addition prevents grain growth during sintering and thus increases hardness of the compacts. A large deformation of W-10mass%Re sintered compact, whose grain size is approximately 450nm, is observed at elevated temperatures.

The mechanism of nano grain formation in tungsten by mechanical milling, which is one of the methods for severe plastic deformation, was investigated. The powder microstructure was divided in some layered grain structures, and finally consisted of equiaxed nano grains. This nano grain structure was formed by the subdivision of the layered grain. These nano grains have non-equilibrium grain boundary structure. It is considered that such a specific microstructure is formed by extremely heavy plastic deformation by due to the mechanical milling process.

Nano grain tungsten is fabricated by Mechanical Milling (MM) process, and its grain growth behavior and high temperature deformability is investigated. As a result, a nano grain structure, whose grain size is approximately 20 nm or less, is obtained after MM for 360ks. Those nano grains demonstrate an irregular grain boundary structure, i.e., "non-equilibrium grain boundary", and they change to a smooth grain boundary structure by annealing at 1023 K for 3.6 ks. Compacts with nano grain structure indicate superior sintering property even at 1273 K (0.35 T m ). Rhenium addition prevents grain growth during sintering and thus the compacts indicate a further improvement in deformability. The compact is composed of equiaxed grain, whose grain size is 420 nm, and has low dislocation density even after the large deformation. The strain rate sensitivity, i.e., m-value, of 0.41 is obtained in the W-Re compact at 1473 K. Those results strongly imply that the nano grain W-Re compacts show superplasticity at less than half of the melting temperature, i.e., 1473 K (0.42 of the solidus temperature).

Synopsis :Mechanical milling is one of the severe plastic deformation processes and can give a heavy deformation to a metallic powder. Using such a mechanical milling process nano crystalline material can be produced with relatively simple equipment, low cost and short time. The mechanical milling process leads to finer grain than other severe plastic deformation for bulk materials and the grain size produced by mechanical milling is less than 10 nm. Nano grain structure by mechanical milling is finally formed with nano layered structure though the grain refinement rate depends on the milling method and the milling condition. The minimum grain size produced by mechanical milling is related to many factors such as melting point and stacking fault energy. The nano grain tungsten powder demonstrates a good workability and its sintered compact demonstrates the superplastic deformation at 1473K. When the milling intensity is suitable, mechanically milled powder consisting of shell and core can be produced. The compacts produced from such mechanically milled powder have the nano / meso hybrid microstructure and demonstrate advanced mechanical properties.

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.