Elevated-temperature environmental embrittlement and alloy design of L1 2 ordered intermetallics

Abstract

Ordered intermetallic alloys with L1 2, crystal structure such as Ni 3Si exhibit environmental embrittlement with much lower ductilities in air than in vacuum when being tested at elevated temperatures. The environmental embrittlement is sensitive to test temperature, alloy composition and grain geometry. The embrittling behavior at elavated temperatures is a dynamic phenomenon, i.e. gaseous oxygen in test environments is chemically absorbed at the tip of cracks induced on alloy surfaces by deformation and then drives into the metal along grain boundaries, leading to a reduction of the grain boundary cohesion. The embrittlement can be alleviated by formation of protective oxide films on alloy surfaces, control of grain shape or addition of beneficial elements such as chromium. In contrast, ordered intermetallics based on b.c.c. crystal structures such as FeAl(B2) and Fe 3Al(DO 3) do not show environmental embrittlement at elevated temperatures. The embrittling behavior as well as alloy design to alleviate the embrittlement is discussed in this paper.