RELATIONSHIP BETWEEN COMPOSITION, MICROSTRUCTURE,AND STRESS CORROSION CRACKING IN TITANIUM ALLOYS,

摘要

Four alpha titanium alloys and 11alpha +beta titanium alloys were characterized to relate phase composition and associated microstructure to stress corrosion cracking (SCC). Of these alloys,only a low-interstitial,commercially pure alpha alloy (Ti-50A) was immune to SCC. Addition of oxygen,aluminum,or aluminum and tin restricted slip in the alpha phase and promoted stress corrosion susceptibility. Formation of ordered domains of Ti3(Al,Sn) further restricted slip and increased susceptibility. Stress corrosion resistance was improved by thermomechanical treatments that reduced alpha grain size or increased dislocation density. Alpha-phase susceptibility is qualitatively related to the intensity of the stress field surrounding a dislocation pileup. Alloying with molybdenum and/or vanadium increased strength and often improved stress corrosion resistance. This is attributed to stabilization of the ductile beta phase. However,precipitation of a fine dispersion of alpha or omega in beta caused embrittlement and reduced the stress corrosion threshold. Intermetallic compound formation in alloys containing copper or silicon similarly promoted susceptibility. Thermomechanical processing of Ti-4Al-4Mo-2Sn-0.5Si limited the embrittlement,probably by refining Ti5Si3particles in the beta phase. (Author)