국민대학교

세미나&특강

제목 “Design of nanocrystalline bulk tungsten alloys via nano-phase separation sintering”

1. 연사: MIT 재료공학부 박만수 박사

2. 초록 (또는 요약문)

New accelerated sintering method called ‘nano-phase separation sintering’ is developed which is

specifically applicable to nanostructured tungsten alloys.  Nanocrystalline tungsten alloy with 15

atomic percent chromium is produced by high-energy ball milling and then consolidated.  With

an addition of 15 atomic percent chromium, nanocrystalline tungsten alloy exhibits the onset of

sintering at low temperature around 950 °C and the rapid rate of densification.  Sintering

mechanism is established through understanding the role of nano-scale second solid-state phase

precipitation analyzed by a thermomechanical analyzer, electron microscopy and x-ray

diffraction.  In addition, a series of control experiments reveals that the accelerated sintering

apparently stems from the combination of two features of the powders: (i) nanocrystallinity and

(ii) alloy supersaturation.  The addition of alloying elements and second phases are beneficial for

retaining grain size during thermal cycle and nano-phase separation sintering is thus naturally an

appropriate method to produce fine-grained materials.  Sintered products achieved through nano-

phase separation sintering display 10~30 times smaller grain sizes at comparable densities than

those produced by conventional accelerated sintering methods such as solid-state activated

sintering and liquid phase sintering.  The thermodynamic features and conditions for nano-phase

separation sintering are further explored based on the binary phase diagram in order to achieve

the generality of its mechanism to other alloy systems.  Based on the rules established, the

consolidation of chromium with the addition of nickel is accelerated.  Nanocrystalline bulk

tungsten alloys are finally accomplished employing titanium as a stabilizer suppressing grain

growth.