Elevated temperature fracture of RS/PM alloy 8009: part i. fracture mechanics behavior

Increasing temperature and decreasing loading rate degrade the plane strain initiation (K _ICifrom theJ integral) and growth (tearing modulus,T _R) fracture toughnesses of RS/PM 8009 (Al-8.5Fe-1.3V-1.7Si, wt pct).K _ICidecreases with increasing temperature from 25 ‡C to 175 ‡C (33 to 15 MPa√m for an extrusion and 28 to 11 MPa√m for hot cross-rolled plate) and further declines to 10 MPaVm at 316 ‡C without a minimum.T _Ris greater than zero at all temperatures and is minimized at 200 ‡C. A four order-of-magnitude decrease in loading rate, at 175 ‡C, results in a 2.5-fold decrease inK _ICiand a 5-fold reduction inT _R.K _ICiandT _Rare anisotropic for extruded 8009 but are isotropic for cross-rolled plate. Cross rolling does not improve the magnitude 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 ofK _ICi, however,T _Ris increased by this mechanism. Macroscopic 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 temperature-dependent flow strength, modulus, and constrained fracture strain, reasonably predicts the temperature dependencies ofK _ICiandT _Rfor 8009. While E and σ_ys decrease with increasing temperature for all aluminum alloys, the strain to nucleate crack-tip damage dominates the fracture toughness of 8009 and decreases with increasing temperature for a range of constraint. Damage mechanisms for this novel behavior are evaluated in Part II. Formerly Graduate Student Department of Materials Science and Engineering, University of Virginia