Abstract
Sintering of powders is a common means of producing bulk materials when melt casting is impossible or does not achieve a desired microstructure, and has long been pursued for nanocrystalline materials in particular. Acceleration of sintering is desirable to lower processing temperatures and times, and thus to limit undesirable microstructure evolution. Here we show that markedly enhanced sintering is possible in some nanocrystalline alloys. In a nanostructured W–Cr alloy, sintering sets on at a very low temperature that is commensurate with phase separation to form a Cr-rich phase with a nanoscale arrangement that supports rapid diffusional transport. The method permits bulk full density specimens with nanoscale grains, produced during a sintering cycle involving no applied stress. We further show that such accelerated sintering can be evoked by design in other nanocrystalline alloys, opening the door to a variety of nanostructured bulk materials processed in arbitrary shapes from powder inputs.
Highlights
Sintering of powders is a common means of producing bulk materials when melt casting is impossible or does not achieve a desired microstructure, and has long been pursued for nanocrystalline materials in particular
The resulting powder particles are micron size in diameter as shown in Fig. 1a, each particle being much larger than the average grain size of about 13 nm, as shown in the transmission electron microscopy (TEM) micrograph in Fig. 1b; each powder particle is polycrystalline with nanoscale grains[14,15,16], which is an important distinction as compared with, for example, nanopowders, where every particle is of nanometre scale dimension and typically is a single crystal, and where some interesting sintering phenomena have been observed[17]
Chromium has almost no equilibrium solubility in tungsten at room temperature[18], high-energy ball milling is widely known to achieve supersaturation[19,20,21] and the Cr is fully dissolved in W here; this supersaturated solution is poised to phase separate on heating
Summary
Sintering of powders is a common means of producing bulk materials when melt casting is impossible or does not achieve a desired microstructure, and has long been pursued for nanocrystalline materials in particular. We further show that such accelerated sintering can be evoked by design in other nanocrystalline alloys, opening the door to a variety of nanostructured bulk materials processed in arbitrary shapes from powder inputs. Sintering is a common processing method for manufacturing bulk polycrystalline materials, it can often require long time-at-temperature cycles that pose problems for structural stability, for example, grain growth. Powder processing and sintering have long been studied as a route to achieve bulk nanocrystalline materials[1,2,3,4,5,6,7,8], it is a challenge to use enough of a thermal cycle to remove all the porosity without seeing large changes in grain size. We show that accelerated sintering is possible in some nanocrystalline alloys that are designed to exhibit nanoscale phase separation, which in turn leads to a dual-phase structure that accelerates sintering
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