Micromechanisms of quasi-static and fatigue crack growth in titanium-aluminides.

摘要

The rapid development of advanced aerospace technology has put an increasing demand on the development of a new generation of structural materials for aircraft engines. Intermetallics materials with low densities and improved fracture toughness at elevated temperature show promise for replacing widely used Ni-base superalloys. TiAl and Ti{dollar}sb3{dollar}Al, ordered intermetallic compounds of Ti and Al, have been the subject of increasing research activity in the recent past because of their light weight and high strength. An understanding of the resistance to quasi-static and cyclic load fracture in these intermetallics at both ambient and elevated temperatures is of paramount importance for damage-tolerant, fatigue-critical structural applications for which the intermetallics are candidate materials.; This thesis explores the effects of microstructure, microstructural stability, mechanical variables (such as load ratio and fatigue frequency), and temperature on the quasi-static and fatigue crack growth characteristics of Ti-Al and Ti-Al-Nb alloys. With the aid of controlled heat treatments, duplex microstructures comprising either {dollar}alphasb2{dollar} and {dollar}beta{dollar} or {dollar}alphasb2{dollar} and {dollar}gamma{dollar} phases as well as predominantly {dollar}alphasb2{dollar} phase microstructures are produced. The fatigue crack growth characteristics are monitered over a range of {dollar}Delta{dollar}K values from threshold to final failure. The effects of controlled microstructures on the overall fatigue and fracture resistance are examined in light of TEM observations of crack tip damage, scanning auger spectroscopy of fracture surface oxidation and scanning electron fractography. The stability of these microstructures are examined in light of their response to both fatigue crack growth and fracture toughness. Microstructural features which lead to a betterment of fatigue crack growth and fracture toughness are discussed in context of the present experimental results.