Thermomechanical Fatigue Crack Propagation in an Anisotropic (Directionally Solidified) Nickel-Base Superalloy

Abstract

Strain-controlled thermomechanical fatigue tests were performed on two nickel-base superalloys, directionally solidified (DS) Mar-M200 plus hafnium and conventionally cast B-1900 plus hafnium. The DS alloy was tested at various angles (θ) to the direction of grain growth. It was found that crack growth rates were a minimum for the DS alloy tested parallel to the direction of grain growth (crack propagating normal to the direction of grain growth). The crack growth rate increased for other orientations in the sequence θ = 15, 30 or 90, and 45 deg until equivalence was reached with rates for the conventionally cast alloy, B-1900 plus hafnium. A linear elastic fracture mechanics approach was extended to strain-controlled crack growth as a function of θ for the DS alloy. Prediction of crack growth rates as a function of ϑ was achieved by normalizing with the elastic modulus. The sequencing of crack growth rates with θ and a smooth-to-rough fracture surface transition are explained on the basis of the orientation dependence of crack opening displacement for a given strain intensity range.