In the present work, two powders of pure Al and of pure Mo, respectively, have been mixed (composition: Al with 15 at.% Mo) and processed by ball milling. The structural changes of the powder vs. milling time were analyzed by X ray diffraction and by scanning electron microscopy up to a milling time of 100 hours. The pitting potentials of the green and of the sintered alloy were determined in NaCl-solution.
Experimental Methods
MaterialsThe raw materials used in this work were elementary powders of aluminum and of molybdenum. The aluminum powder from Alcoa had a purity of 99.7% and the following particle size distribution: +100#: 0%, +200#: 0.1%, +325#: 11.5%, -325#: 88.4%, according to the manufacturer's analysis. The molybdenum powder from Aldrich had a purity of 99.9+% and an average particle size <10 µm (-100#). The two powders were mixed before milling and 1 wt.(%) of stearic acid was added as process controlling agent.
Milling processMilling was performed in a ball mill type Netzsch Molinex PE5. The mill had a stainless steel rotator and chromium steel balls of 6 mm diameter. The ball to powder weight ratio was 10:1. The mill was water cooled and the chamber was purged with argon during the milling process. Milling was carried out with a velocity of 400 rpm. The milling process was interrupted several times in order to remove a small quantity of powder for analysis.A maximum milling time of 100 hours was considered as convenient. Zdujic et al. 8 reported that after this milling time, with low energy, molybdenum was finely dispersed in the aluminum matrix.
IntroductionAluminum alloys are susceptible to pitting corrosion in chloride containing solutions. Massive pitting sets in when a critical potential, the so called pitting potential is reached. Alloying elements can influence this critical potential. However, among the alloying elements contained in technical alloys, only Cu shifts the pitting potential up to about 150 mV to the noble direction, but this is not enough to obtain a substantial improvement of the corrosion resistance. In the nineties researchers produced supersaturated Al alloys, mostly alloyed with transition elements such as Mo, W, Ta, Zr, Nb, Cr, Ni [1][2][3][4] . Also rare earth elements have been added more recently 5 . Since the solubility of these elements in Al is very low, they have to be produced by non-equilibrium methods like PVD, ion-implantation or meltspinning. Electrodeposition from ionic liquids has also been applied for supersaturated aluminum alloys 6 . Shifts of more than 1V were reported for the pitting potential of some alloys in chloride solution 1 . However, the drawback of these materials is that they can be produced only as deposited thin films or as thin ribbons, which does not permit their direct technical use. Although it is known that supersaturated alloys can also be produced by high energy ball milling 7 , until now the powder metallurgical way to corrosion resistant supersaturated aluminum alloys has not been examined. Therefore the aim of this paper is to st...
