Fractographic Interpretation of Subcritical Cracking in a High Strength Titanium Alloy (IMI 550)

Abstract

Abstract A detailed fractographic study of subcritical crack growth in 3.5% aqueous NaCl and liquid mercury was made for IMI 550 (Ti-4AI-4Mo-2Sn-0.5Si) in an (α + β) heat treated condition. The work of Cox and Ajtchison, which showed that river markings are at high angles to genuine cleavage planes, is confirmed. Mechanisms for formation of the river markings are postulated, and a distinction is made between the cleavage topographies in salt water and mercury. Support is also found for the mechanism of liquid metal embrittlement postulated by Hancock and Ives. The overall appearance of the subcritical fracture surfaces provides an explanation for the low KISCC (salt water) customarily found for IMI 550.