The cyclic stress-strain behavior of nickel-base superalloys—II. Single crystals

Abstract

The cyclic stress-strain behavior of the single crystal nickel-base superalloy Mar-M200 has been investigated at ambient temperature and 843°C under constant strain rate cycling at 10 −2 and 10 −3/s. At ambient temperature, the cyclic stress-strain curve exhibits a clear strain rate independent plateau regime similar to that observed for f.c.c. metal single crystals and other coherent precipitate strengthened systems. At elevated temperature, the cyclic stress-strain curves are reduced to power law functions with the higher strain rate curve exhibiting a higher slope, or work hardening exponent than the lower strain rate curve. The forms of these curves, the cyclic hardening behavior and details of hysteresis behavior are rationalized with respect to substructural observations. At ambient temperature, the substructure consists of widely spaced, well defined slip bands which shear the γ′ precipitates and extend completely through the crystal on {111} planes. At 843°C and 10 −2/s, the slip bands initially shear the γ′ which then reform around the slip bands. In addition, a more homogeneous tangled structure which cages the γ′ and provides a work hardening component is found. The lower strain rate at elevated temperatures produces a homogeneous substructure throughout the crystal with evidence of less well defined bands. These results are compared and contrasted with the results of a companion study (Part I) on a representative polycrystalline superalloy.