Dynamic Fatigues Testing of Titanium Diboride in Molten Aluminum

Open Access
Author:
Sworts, Lance M
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
October 22, 2012
Committee Members:
  • John Richard Hellmann Jr., Thesis Advisor
Keywords:
  • Dynamic fatigue
  • titanium diboride
Abstract:
A test methodology has been utilized to identify stress corrosion susceptibility of a commercially available titanium diboride in a molten aluminum environment. Specimens were fractured utilizing a dynamic fatigue approach (constant stressing rate) to calculate a stress corrosion resistance parameter. Analysis of the stress corrosion parameter, variations in loading trends and fracture strengths were correlated to microstructural features of the fracture surfaces and starting material characterization. Evidence for slow crack growth mechanisms has been identified and strategies for future investigations and microstructure modifications have been suggested. The titanium diboride specimens were tested in molten aluminum at 970°C, argon atmosphere at 970°C, air atmosphere at 970°C and air atmosphere at 25°C. Stressing rates ranged from 0.2 to 900 MPa/s. The fracture stresses of the specimens tested in molten aluminum at 970°C ranged from 55 to 490 MPa and have low N-values indicating high susceptibility to stress corrosion. The fracture stresses of the specimens tested in argon atmosphere at 970°C, air atmosphere at 970°C and air atmosphere at 25°C ranged from 270 to 550 MPa and have high N-values indicating a resistance to stress corrosion. The fracture stresses of specimens which had been pre-exposed to kerosene (for density analysis) prior to mechanical testing in molten aluminum at 970°C ranged from 180 to 360 MPa and had a negative N-value suggesting a mechanism other than stress corrosion may be operative. Intergranular fracture mode was observed in the aluminum penetrated samples, while transgranular fractured mode was observed in samples fractures in non aluminum environments. Increased exposure time in molten aluminum resulted in increased migration of aluminum along the grain boundaries and significantly decreased the fracture strength of the material. The fracture strength further decreased when specimens were tested perpendicular to hot pressing direction as compared to tested normal to the hot pressing direction. The occurrence of TiC precipitates was observed to potentially influence crack propagation at low stressing rates. This could have significant implications in developing future materials for resistance to stress corrosion. A need for molten aluminum wetting and fracture analysis on single crystal titanium diboride as a function of crystal orientation was recognized.