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, t hey were cooled to room temperature at three cooling rates; i.e. i) ai r-cooling (about 25 K.s-1), ii) 0.065 K.s-1 and iii) 0.01 K.s-1. Hardn ess 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 high er than that of slowly cooled samples at 0.01 K.s-1 because of the int roduction 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, sugg esting that the elimination rate of the vacancies is different on each grain. The temperature dependence of yield stress and elongation is o btained for both alloys air-cooled and annealed at 713 K for 150 h aft er air-cooling to eliminate frozen-in vacancies. Yield stress of the a ir-cooled Fe-40 mol%Al at 473 K is much higher than that of the vacanc y-eliminated Fe-40 mol%Al and shows a strong negative temperature depe ndence. The vacancy-eliminated Fe-40 mol%Al shows a weak positive temp erature dependence at 673 K to 873 K. The difference in the yield stre ss between the air-cooled and the vacancy-eliminated samples disappear s 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 F e-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 tha n that of Fe-40 mol%Al, which is considered to be intrinsic for the al uminum composition dependence of the DBTT in B2 FeAl.