This study seeks to clarify the effects of gas contamination from milling atmospheres of mechanical alloying (MA) on mechanical properties. An iron-based dispersion alloy of Fe-13Cr-3W-0.5Ti-0.5Y2O3 (mass%) was selected as the experiment material. We prepared MA powders by milling mixed powders in atmospheres of argon, helium, hydrogen, nitrogen, and vacuum; then we made bulk alloys by groove rolling the MA powders. We then examined atmospheric elements trapped in the MA powders and their releasing processes with heat treatment in vacuum. For bulk alloys, we also examined the high-temperature behavior of residual atmospheric elements and their effects on impact strength as a function of heat-treatment time at 923 K.
Experiment results demonstrated that some of the atmospheric element trapped by MA powder was unexpectedly difficult to remove with heat treatment. The content of widely used argon in MA powder, for example, was 0.013 mass%, and the argon was difficult to remove even with treatment at 1473 K, which is considered the maximum allowable temperature. Therefore, most of the argon was introduced into bulk alloy as gas contamination. Nitrogen was effectively reduced with treatment at 1323 K; however, hydrogen could not be sufficiently removed with this treatment. Residual argon and helium in bulk alloys formed bubbles at elevated temperatures and caused density decrease (swelling). Impact strengths of the bulk alloys obtained through milling in argon, hydrogen, nitrogen, and vacuum decreased with increased treatment time; more remarkable decreases were observed in the alloys including argon and hydrogen. We concluded that nitrogen is the most suitable milling atmosphere to be applied as high-temperature materials.