Environmental sensitivity and mechanical behavior of boron-doped Fe-45at.%Al intermetallic in the temperature range from 77 to 1000 K

The tensile properties and fracture behavior of a coarse-grained (grain size similar to 420 mu m) Fe-45at.%Al intermetallic doped with 0.05 at.% boron were examined at ambient temperature in air, argon and vacuum as well as in the 77-1000 K temperature range in liquid nitrogen, dry ice and air. Before testing the alloy was low temperature annealed (vacancy annealed) in order to remove all the retained vacancies. At ambient temperature ductility increases accordingly to decreasing water vapor (moisture) content in each environment. The mixed transgranular cleavage (TGC) + intergranular failure (IGF) mode in vacuum, which is associated with the highest elongation (similar to 6%), exhibits around 40% of IGF and the mixed fracture mode in argon, which is associated with the second highest elongation (similar to 3.2%), exhibits similar to 15% of IGF. The TGC fracture mode in air is associated with the lowest elongation (similar to 1%). Elongation in the cryogenic temperature range from 77 to 213 K is very low being in the range from 0.6 to 2.8%, and is associated with a mixed transgranular + intergranular fracture mode. Elongation increases gradually from 300 to 800 K attaining a ductility peak at 800 K and then decreases rapidly with increasing temperature. At 600-800 K, the yield strength of Fe-45Al-0.05B exhibits anomalous temperature dependence with the yield strength peak at 800 K. The mode of fracture from 300 to 700 K is predominantly TGC and that at the ductility peak is ductile rupture with very deep dimples. At temperatures above 800 K the mode of fracture changes to a typical intergranular creep (fibrous) failure with numerous flat dimples (voids/cavities) at the grain boundary facets, which is associated with a tensile ductility drop. Fine particles (borides) are observed at the grain boundary facets, which assist the development of intergranular creep fracture. (C) 1999 Elsevier Science S.A. All rights reserved.