Fabrication of Nano Grain Tungsten Compact by Mechanical Milling Process and Its High Temperature Properties

Abstract

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.

Nano Grain Formation in Tungsten by Severe Plastic Deformation-Mechanical Milling Process

“…This result clearly indicates that the nano grains are formed by increasing the misorientation of nano subgrains. The fact that the nano grain is formed by segmentations of nano layer grain 7,8,10,11) strongly supports the belief.…”

“…However, after milling for 360 ks the size of the powder became an average diameter of about 2 mm with agglomerate morphology and relatively smooth surface. Our previous paper 11) reported that particle morphology of W powders became finer by shredding at early stage of MM process, but the average particle size was kept to about 1 mm. These results indicate that the particle dimension of W powder is kept to about 1 micro meter during MM.…”

“…Various methods of severe plastic deformation (SPD), for example, High Pressure Torsion (HPT), [1][2][3] Equal Channel Angular Pressing (ECAP), 1,2,4) Accumulative Roll-Bonding (ARB), 5,6) Shot Peening, [7][8][9] and Mechanical Milling (MM), 7,[10][11][12][13] are used to fabricate ultrafine grain materials. It is well known that the minimal grain size is depending on the deformation conditions, i.e., amount of strain, strain rate, processing temperature.…”

Superplasticity and high temperature deformation behaviour in nano grain Tungsten compacts

“…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.…”

Initial-stage Sintering Kinetics of Nanocrystalline Tungsten

“…[3] Recently, Wang et al [4] reported as low a sintering temperature as 1373 K (1100°C) for nanocrystalline tungsten powders in hydrogen atmosphere without pressure, which was further reduced to 1273 K (1000°C) using spark plasma sintering (SPS). [5] Nanocrystalline tungsten powder of submicron sizes was also used to produce near fully dense (above 99.7 pct theoretical density) tungsten heavy alloy. [6] Tungsten is the most used high-temperature refractory metal in various applications, retention of the nanocrystalline/ultrafine grain (UFG) structure after sintering is a concern, and several processing strategies using newly developed consolidation techniques (e.g., SPS) have been reported earlier.…”