Effects of deviations from stoichiometry on the strength anomaly and fracture behavior of B-doped FeAl

Abstract

Abstract Tensile tests were conducted at temperatures ranging from 145–1123 K on four different FeAl alloys, containing 40, 43, 45, and 48 at.% Al, each doped with 0.12 at.% B. The alloys were initially heat treated to obtain a relatively large grain size (~200 μm), after which they were given a long, low-temperature anneal (673 K for 5 d), to minimize, respectively, the effects of grain boundary strengthening and thermal vacancies on the measured yield strengths. Each alloy displayed bcc-type behavior at low temperatures (yield strength decreasing with increasing temperature), followed by a strength anomaly at intermediate temperatures (yield strength increasing with increasing temperature), and a sharp drop in yield strength at elevated temperatures (beyond the anomalous strength peak). Thermal vacancies that are generated during the hold time at the test temperature may contribute to the production of the strength anomaly. In specimens not given the vacancy-minimizing anneal, quenched-in vacancies were found to substantially increase low-temperature strength, thereby masking the yield strength anomaly. As the Al concentration of FeAl increased, the prominence of the yield strength anomaly decreased. Ductility also exhibited a peak at elevated temperatures, first increasing with temperature until it reached a maximum value and then decreasing with further increases in temperature. The peak in ductility occured at lower temperatures as the Al content increased. The fracture mode in all four alloys was mixed (intergranular + transgranular) at cryogenic temperatures, predominantly intergranular at around room temperature, dimpled rupture at peak ductility, and intergranular cavitation at elevated temperatures where the ductility dropped.