Ep. George et al., ENVIRONMENTAL EMBRITTLEMENT AND OTHER CAUSES OF BRITTLE GRAIN-BOUNDARY FRACTURE IN NI3AL, Materials science & engineering. A, Structural materials: properties, microstructure and processing, 193, 1995, pp. 277-288
In this paper we review intrinsic and extrinsic sources of grain bound
ary brittleness in Ni3Al. Special attention is given to the recently d
iscovered phenomenon of environmental embrittlement in Ni3Al, and how
variables such as test environment, strain rate, temperature, grain bo
undary character, and microalloying additions affect ductility and fra
cture behavior. It is shown that environmental embrittlement is a majo
r reason for the low ductility of Ni3Al in air; consistent with this,
ductility is found to increase at high strain rates, cryogenic tempera
tures, and in environments containing low levels of water vapor. When
environmental effects are carefully suppressed, the tensile ductility
of Ni3Al increases dramatically, indicating that Ni3Al is not as intri
nsically brittle as once thought. It is shown that the character of gr
ain boundaries in Ni3Al depends strongly on the processing conditions.
Some of these processing-related changes appear to be associated also
with changes in ductility; however, additional research is needed to
understand the detailed connection between the two. The ductilizing ef
fect of boron in Ni3Al is shown to be related principally to its role
in suppressing environmental embrittlement (partly by slowing down hyd
rogen diffusion). In addition, B enhances grain boundary strength and
suppresses intergranular fracture. Much less is known about the way in
which other alloying elements improve ductility. One possibility, whi
ch needs additional research, is that they alter the grain boundary ch
aracter distributions in Ni3Al.