Cryomilling of Nano-Phase Dispersion Strengthened Aluminum

Abstract

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.

Grain growth of nanocrystalline Ni powders prepared by cryomilling

“…EXPERIMENTAL PROCEDURE elemental and/or prealloyed powders are milled in cryogenic Commercially available pure Ni powders with a purity media, e.g., liquid nitrogen, liquid argon, etc., to form a Ն99.5 wt pct (Sulzer Metco Inc., Westbury, NY) and a slurry. [14] Reaction between powders and oxygen or nitrogen nominal particle size of 45 Ϯ 11 m were used in this study. from the environment or milling slurry under energetic mill-…”

Thermal Stability of Cryomilled Al-Mg-Er Powders

“…Due to the significant oxide content in the milled Al-Mg-Er powders, and their likely presence as nanoscale dispersoids, grain growth can be affected by second-phase pinning forces (i.e., Zener = 3 /2 ) [5,39]. Dispersoids formed in cryomilled ODS Al powders during milling were reported as being aluminum oxynitride particles 2-10 nm in size [40] and platelets of aluminum-nitrogen and aluminum-oxygen content a few atomic layers thick and 10-15 nm in two dimensions [41]. If the volume fraction of the oxide is estimated by assuming the oxygen is present as oxide dispersoids and taking the average oxide/oxynitride particle to be 5 nm in size, it is possible to estimate the Zener limit grain size due to second-phase pinning forces.…”

Cluster Assembly of Nanophase Materials

“…Nanometer size grains nucleate within the shear bands of heavily deformed materials converting a coarse-grained structure to nanophase. The heavy deformation is usually induced by means of high-energy ball milling (Hellstern et al, 1989;Luton et al, 1989;Jang and Koch, 1990;Trudeau et al, 1990Trudeau et al, , 1991Koch and Cho, 1991) but can result as well from surface wear phenomena (Ganapathi and Rigney, 1990). Thus, the individual grains are never isolated clusters and much synthesis and processing flexibility is lost.…”

“…This clearly shows that grain sizes down into the nanometer regime can be accessed, at least in relatively hard materials. Applying the method at low temperatures (so-called cryomilling) can, however, extend the range of applicability, as shown by Luton et al (1989) in their work on dispersion-strengthened Al. This technique is more fully treated in Chapter 5.…”