Joseph Vallone

Cryomilling of Nano-Phase Dispersion Strengthened Aluminum

In recent years considerable effort has been expended on the development of dispersion strengthened alloys by mechanical alloying. Our research has shown that considerable improvement in microstructure control and properties can be gained by carrying out milling at cryogenic temperatures. We have found that aluminum and dilute aluminum alloys can be dispersion strengthened with aluminum oxy-nitride particles by the use of a slurry milling technique where the fluid medium is liquid nitrogen. The alloyed powders produced by this technique are strengthened by aluminum oxy-nitride particles which are typically 2–10 nm in diameter and with a mean spacing of 50–100 nm. The dispersoids are generated during the milling process by adsorption and reaction with components of the liquid nitrogen bath. On thermal treatment prior to consolidation, the alloyed powders recrystallize to a grain size which is typically in the range 0.05 to 0.3 μm. The alloys exhibit a yield stress in excess of 325 MPa at room temperature and a virtually temperature independent yield stress of about 130 MPa at temperatures greater than 375° C. The paper describes the preparation of dispersion strengthened aluminum by cryomilling, the characteristics of the microstructure and discusses some aspects of the mechanical properties.

The effect of nitrogen and oxygen on the synthesis of B2 NiAl by cryomilling

Abstract Cryomilling of elemental Ni-50 at.%Al powders in liquid nitrogen resulted in phase transformation through two opposing mechanisms: the formation of B2 NiAl and the elimination of NiAl by displacement reactions between NiAl and the liquid nitrogen and oxygen to form AlN and γ-Al 2 O 3 . During annealing, the NiAl reacts to form AlN and γ-Al 2 O 3 , thereby moving the composition of the balance of the mixture toward the Ni-rich end of the phase diagram. The resultant nanoscale AlN and γ-Al 2 O 3 particles were found to impede grain growth of the nanocrystalline structure at high temperatures.

Formation of nanocrystalline B2 NiAl through cryomilling of Ni-50 at.%Al at 87 K

Abstract A wear-like mechanism occurs between balls and the entrapped nanocrystalline mixture of elemental Ni and Al powders resulting in the formation of B2 NiAl at the process temperature at 87 K during cryomilling. Nanocrystalline structure of B2 NiAl with average size of 6 nm was synthesized after 45 hours of milling. This B2 NiAl was found to be at nonstoichiometric compositions. The kinetic study of the transformation exhibited approximately 90 wt.% of completion after 50 hours of milling. The transformation curve is sigmoidal in form which suggests that the process can be represented by the Johnson-Mehl-Avrami relation.