Creep deformation of Ni3Al-Mo in situ composites

High temperature γ′(Ni₃A1)-α(Mo) in situ composites solidified with growth rates ranging from 0.12 cm/h to 2.4 cm/h, were creep tested at temperatures 710 °C, 830 °C, and 950 °C under a tensile stress of 300 MPa. Creep rupture lives of rapidly grown alloys were comparable to those of γ′/γ-δ and Nitac eutectics whereas strain to rupture was up to three times greater. In comparison, creep rupture lives of slowly grown alloys were more than ten times and strain to rupture about two times smaller than in rapidly grown alloys. In slowly grown alloys, failure occurred by formation of shear bands caused by cooperative shear of matrix and fibers whereas no shear bands were seen in rapidly grown samples. Shear band formation was due to pile-up stresses and parallel orientation of matrix and fiber slip systems, the latter resulting from a change in the crystallographic phase relationship as growth rate decreased. There was evidence that shear band formation depended on a threshold stress. The creep behavior of rapidly grown alloys was in qualitative agreement with predictions obtained from a linear visco-elastic model composite strengthened by the “mean matrix stress”.