Failure during hot working of spray-formed Rene’ 88

Isothermal, hot compression tests were conducted on spray-formed Rene’ 88 to establish the effects of strain, strain rate, and temperature on the closure of porosity and the initiation of ductile fracture during hot working. In the as-sprayed condition, porosity was evenly distributed throughout the material in both the transverse and longitudinal directions. Hot compression tests were conducted at temperatures between 760 and 1090 °C at strain rates of 0.01, 0.1, and 1.0 s−1 to axial height strains of 0.4, 0.7, or 1.1. The deformation gave rise to near full-density (0.06% porosity) at the center and to porous regions (∼0.5% porosity) ∼1–2 mm from the barreled free surface on the equatorial plane. From these observations and parallel finite-element-method (FEM) simulations, it was concluded that pore closure was enhanced by a state of triaxial compressive stress. Observations of pore coalescence and fracture via splat boundary separation at the free surface were also interpreted via FEM simulations. The simulation results for the near-surface region revealed that a maximum-tensile-work, rather than a tensile-stress, criterion controlled the process of splat-boundary separation, pore linkage, and final ductile fracture.