Bulk Nanocrystalline Al 7050 Prepared via Cryomilling

The operation with a combination of three processing routes: cryomilling, hot isostatic pressing (HIPping) and hot extrusion was adopted in the present study for preparation of the bulk nanocrystalline Al 7050. The microstructure and fractography of the bulk material were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. Furthermore, the chemical composition, density and tensile properties of the material were also measured. Microstructural investigation showed that the grain size of the bulk nanocrystalline Al 7050 ranged from 100nm to 200nm. Numerous dispersoids with a diameter/length of ~50nm were observed on grain boundaries and inside the grains. Besides, one phase of these dispersoids existed in the bulk nanocrystalline Al 7050 was identified as Al6(FeMn). These dispersoids dispersed within the bulk nanocrystalline Al 7050, to some extent, increased the mechanical properties and thermal stability of the material. The resulted sample exhibited ultimate strength of 412MPa with an elongation of 5.2% when tested under tensile load, which was a bit lower than that of the traditionally wrought Al 7050-T6. The present results suggested that improper selected starting powder and milling parameters resulted in the flake-like morphology of the cryomilled powder. The flake-like morphology made it difficult for the cryomilled powder to fill the can entirely and achieve a high density material, which led to the weak interface within the bulk material and in turn degraded the mechanical properties of the bulk nanocrystalline Al 7050 prepared in the present work.

Strain Rate Sensitivity Studies of Cryomilled Al Alloy Performed by Nanoindentation

Al 5083 alloy powder was mechanically milled in liquid nitrogen to achieve a nanocrystalline (NC) structure having an average grain size of 50 nm with high thermal stability, and then consolidated by quasi-isostatic (QI) forging. The consolidation resulted in ultrafine grains (UFG) of about 250 nm, and the bulk material exhibited enhanced strength compared to conventionally processed Al 5083. The hardness of as-cryomilled powder and the UFG material was measured by nanoindentation using loading rates in the range of 50−50,000 /N/s, and results were compared with the conventional grain size alloy. Negative strain rate sensitivity was observed in the cryomilled NC powder and the forged UFG plate, while the conventional alloy was relatively strain rate insensitive.

Thermal stability of nanocrystalline WC–Co powder synthesized by using mechanical milling at low temperature

Nanostructured WC–18% Co powder was synthesized by using cryogenic mechanical milling, and the thermal stability of the nanostructured powder was investigated in detail. The results indicated that the as-synthesized WC–18% Co powder had an average WC particle size of 25 nm. Growth of WC particles occurred above 873 K; however, the average WC particle size remained smaller than 100 nm in the powder isothermally heated for 4 h at 1273 K. Thermal exposure in air at T < 623 K did not result in significant oxidation of the cryomilled powder. The thermal exposure did promote the formation of WO2 and WO3 oxides. The Co6W6C phase was detected by x-ray diffraction in the powder heated in nitrogen at 1273 K, and the phases associated with decarburization of WC, such as W2C, W3C phases, were not observed. With increasing temperature, the dissolution of W and C elements in the Co matrix led to a gradual increase in {111} crystallographic plane spacing, eventually leading to the formation of an amorphous phase.