EFFECTS OF OXYGEN, WATER-VAPOR, ARGON, AIR AND EXPOSURE HISTORY ON FRACTURE STRENGTH OF TI48AL-2NB-2CR NOTCHED TENSILE SPECIMENS

Thin plate notched tensile specimens with 0.14 mm radius 45 degrees no tches were deformed in tension with step-wise load increments in 2-3 t orr gas environments in-situ in an environmental scanning electron mic roscope and in air. The load was applied in small Increments near the fracture load, resulting in average net area stressing rates near 10 k Pa s(-1) Fracture occurred at net area stresses near 200 MPa, for all conditions. Contrary to experiments in vacuum, 16 out of 17 specimens in gas environments fractured suddenly without stable crack growth bei ng observed. Specimens in which the grip slipped before fracture were unloaded, exposed to air and reloaded to failure. Specimens deformed w ith various loading and aging histories in air were evaluated to deter mine how air can attack TiAl that is slightly damaged. Specimens with prior net area stress history above 100 MPa and exposed to air had a h igher probability of failing at lower stress levels. The effects of lo ading history on damage nucleation and environmental degradation is di scussed with the potential to use this effect to help identify stress and/or strain criteria for damage nucleation. (C) 1997 Elsevier Scienc e S.A.