Microstructure and Sinterability of Nano-Crystal Tungsten Powders

Abstract

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.

Superplasticity and high temperature deformation behaviour in nano grain Tungsten compacts

“…After compression test, the grain growth during compression test was restrained in the both compacts. As mentioned above Re segregation at the grain boundaries in the compact presumably resulted in the suppression of grain growth during sintering process [4]. In addition, as can be seen in Figs.5 (c) and (d), nano grain W-10Re compact has very few dislocations even after compression test, while coarse grain W-10Re compact has many dislocations.…”

“…It is therefore necessary to improve the thermal stability of the nano grain W for the improvement of deformability at elevated temperatures. Our previous papers have shown that Re solid solution is able to retard the grain growth by segregation of Re at the grain boundaries in the compact [4,13]. It is considered that nano grain structure materials with Re addition also enable to indicate superplasticity at low temperature or low flow stress.…”

“…To improve such negative properties, various methods like the grain refinement, solid solution of Re, addition of dispersions or grain refinement by severe plastic deformation, have been used [1 -8]. The grain refinement of W enables to obtain both of superior sinterability [3,4] and excellent deformability at high temperature [4,5,6,9].…”

Fabrication of Tungsten-Rhenium Cladding materials via Spark Plasma Sintering for Ultra High Temperature Reactor Applications

“…W-Cu [14] and W-Mo [15] alloys have been successfully produced using SPS. There were a few published papers by a single research group in Japan [16,17].…”

Synthesis and Characterization of Tungsten Micro-/Nanoparticles Using Tungstate-Based Inorganic-Organic Hybrid Nanobelts as Precursors

“…However, the high melting point of W 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 overcome those shortcomings, scientists have developed methods to synthesize nonstructural W materials [3], which have superplasticity and high temperature deformation behaviors [4], leading to mitigation of embrittlement by high ductile-to-brittle transition temperature (low temperature embrittlement) and by irradiation (radiation embrittlement) [5]. The synthesis of ultrafine W powders is an essentially important step to obtain the ultrafine cemented carbides.…”