ELEVATED-TEMPERATURE FRACTURE OF RS PM ALLOY 8009 .1. FRACTURE-MECHANICS BEHAVIOR

Increasing temperature and decreasing loading rate degrade the plane s train initiation (K(ICi) from the J integral) and growth (tearing modu lus, T(R)) fracture toughnesses of RS/PM 8009 (Al-8.5Fe-1.3V-1.7Si, wt pct). K(ICi) decreases with increasing temperature from 25-degrees-C to 175-degrees-C (33 to 15 MPa square-root m for an extrusion and 28 t o 11 MPa square-root m for hot cross-rolled plate) and further decline s to 10 MPa square-root m at 316-degrees-C without a minimum. T(R) is greater than zero at all temperatures and is minimized at 200-degrees- C. A four order-of-magnitude decrease in loading rate, at 175-degrees- C, results in a 2.5-fold decrease in K(ICi) and a 5-fold reduction in T(R). K(ICi) and T(R) are anisotropic for extruded 8009 but are isotro pic for cross-rolled plate. Cross rolling does not improve the magnitu de or adverse temperature dependence of toughness. Delamination occurs along oxide-decorated particle boundaries for extruded but not cross- rolled 8009. Delamination toughening plays no role in the temperature dependence of K(ICi), however, T(R) is increased by this mechanism. Ma croscopic work softening and flow localization do not occur for notch- root deformation; such uniaxial tensile phenomena may not be directly relevant to crack-tip fracture. Micromechanical modeling, employing te mperature-dependent flow strength, modulus, and constrained fracture s train, reasonably predicts the temperature dependencies of K(ICi) and T(R) for 8009. While E and sigma(ys) decrease with increasing temperat ure for all aluminum alloys, the strain to nucleate crack-tip damage d ominates the fracture toughness of 8009 and decreases with increasing temperature for a range of constraint. Damage mechanisms for this nove l behavior are evaluated in Part II.