Abstract:
Benefiting from good specific mechanical properties, exceptional oxidation resistance,
and high temperature capability, Titanium Alloys are used in Gas Turbine Engines,
especially in the early stages of the compressor. However they are subject to stresscorrosion
cracking in the laboratory when subjected to stresses and contaminated with
salts at elevated temperatures. The lack of in-service failures of titanium components
due to Hot-Salt Stress-Corrosion Cracking (HSSCC) is not yet understood.
The parameters influencing the HSSCC of titanium alloys (temperature, load, stress
and temperature cycling, quantity and kind of salt, air velocity, water vapour or
oxygen content of the atmosphere, composition, texture, and microstructure of the
alloy, surtace conditions), cannot account for the lack of in-service failure. After an
examination of the service conditions within a typical gas turbine engine compressor,
it was considered that the high pressures prevailing may extend the life of titanium
alloys subjected to HSSCC.
This work used a unique high temperature, high pressure, servo-hydraulic facility in
order to carry out hot-salt stress-corrosion testing on titanium alloy 1M! 834 at high
pressure. The results obtained show that high oxygen partial pressures extend
significantly the life of 1M! 834 subjected to HSSCC.
Continuous thermogravimetric measurements both in oxidising and salt-corroding
environments were carried out to study the kinetics of the hot-salt attack of IMI 834.
Basic metallography revealed the formation of channels which extend deep into the
metal during the initial stages of hot-salt-corrosion.
Theoretical thermodynamic studies highlighted the role of alloying elements and
vapour phase metallic chlorides in the mechanisms of the HSSCC of titanium alloys.
A new model for the hot-salt stress-corrosion of titanium alloys is proposed. It is
based on the establishment of a self sustaining cycle where vapour phase metallic
chlorides act as hydrogen carriers and can diffuse quickly into the material through
channels.
