THE RELATIONSHIP BETWEEN COOLING RATE, GRAIN-SIZE AND THE MECHANICAL-BEHAVIOR OF B2 FE-AL ALLOYS

The effects of cooling rate on the room temperature mechanical behavior of B2 Fe-Al alloys has been examined. Increases in cooling rate result in large increases in the yield strength for both stoichiometric Fe-50Al and iron-rich Fe-40Al. This increase in strength is accompanied by a decrease in plastic elongation for Fe-40Al alloys that display up to 5% plastic strain when slowly cooled but were essentially brittle when oil quenched. Hall-Petch analysis of mechanical tests performed over a wide range of grain sizes indicates that these cooling-rate effects are primarily related to increases in the basic lattice resistance to flow and not strongly associated with grain boundary strengthening. These changes in mechanical behavior can be attributed to the large number of thermal vacancies that can be retained when these materials are quenched from high temperatures. The addition of boron to these alloys does not have a systematic effect on the relationship between cooling rate and mechanical behavior.