The mechanical response of the Ni−Ni₃Nb eutectic composite: Part I. Monotonic behavior

Abstract

To understand the mechanical behavior of the Ni−Ni₃Nb eutectic composite, it was necessary to determine the operative deformation and fracture mechanisms in the Ni₃Nb intermetallic phase. It was found that Ni₃Nb deforms primarily by twinning along {112} planes and {011} planes when tension and compression, respectively, are applied parallel to the [100] growth direction. The {112} twins were observed to serve as crack nucleation sites with cracks forming along the twin boundaries. The monotonic response of the Ni−Ni₃Nb eutectic composite was investigated with tension and compression tests, metallography, and electron fractography. Room temperature tensile testing of the Ni−Ni₃Nb composite revealed this material to be capable of sustaining tensile strains in excess of 11 pct. This large composite ductility was associated with extensive {112} twinning of the Ni₃Nb lamellae and subsequent twin boundary cracking. When amassed in sufficient numbers in a given cross-section, these {112} twin boundary fissures initiated composite rupture. The room temperature ultimate tensile and compressive strengths of the alloy were found to be 109 and 235 ksi, respectively.