EFFECT OF FROZEN-IN VACANCIES ON HARDNESS AND TENSILE PROPERTIES OF POLYCRYSTALLINE B2 FEAL

Polycrystalline B2 FeAl sheets containing 40 and 46 mol% aluminum were prepared by arc-melting and hot-rolling. After annealing at 1123 K, they were cooled to room temperature at three cooling rates; i.e. i) air-cooling (about 25 K.s-1), ii) 0.065 K.s-1 and iii) 0.01 K.s-1. Hardness values are localized around the average value except Fe-46 mol%Al cooled at 0.065 K.s-1. Hardness of the air-cooled samples is much higher than that of slowly cooled samples at 0.01 K.s-1 because of the introduction of frozen-in vacancies. The hardness values of Fe-46 mol%Al cooled at 0.065 K.s-1 are widely distributed from grain to grain, suggesting that the elimination rate of the vacancies is different on each grain. The temperature dependence of yield stress and elongation is obtained for both alloys air-cooled and annealed at 713 K for 150 h after air-cooling to eliminate frozen-in vacancies. Yield stress of the air-cooled Fe-40 mol%Al at 473 K is much higher than that of the vacancy-eliminated Fe-40 mol%Al and shows a strong negative temperature dependence. The vacancy-eliminated Fe-40 mol%Al shows a weak positive temperature dependence at 673 K to 873 K. The difference in the yield stress between the air-cooled and the vacancy-eliminated samples disappears above 873 K. The introduction of vacancies by air-cooling raises the ductile-brittle transition temperature (DBTT), and reduces elongation below the DBTT. Fe-46 mol%Al exhibits similar tensile properties to Fe-40 mol%Al, while the positive temperature dependence of yield stress does not occur even though the vacancy elimination treatment is done. Furthermore the DBTT of Fe-46 mol%Al vacancy-eliminated is higher than that of Fe-40 mol%Al, which is considered to be intrinsic for the aluminum composition dependence of the DBTT in B2 FeAl.