In the present study, the effect of lamellar spacing, volume fraction of equiaxed gamma grains and lamellar orientation on fatigue crack propagation threshold have been assessed for three as-cast γ-TiAl alloys, Ti45Al2Mn2Nb1B (4522XD), Ti45Al2Mn2Nb (4522) and Ti46Al8Nb (at %). The influence of alloying elements, Nb and Ta on fatigue threshold was also studied with five forged alloys, Ti45Al2Mn2Nb1B, Ti45Al2Nb2Ta1B, Ti45Al2Mn2Ta1B, Ti45Al2Mn4Ta1B and Ti45Al2Mn4Nb1B. The fatigue crack propagation threshold tests were carried out at 650°C in air at a stress ratio R=0.1 and frequency of 10HZ. In addition, the microstructural characterisation of fatigue crack growth fracture surface was also examined by SEM. The samples assessed for fatigue crack propagation threshold failed mainly by translamellar fracture and interlamellar fracture. Interlamellar cracking occurs in lamellar colonies at any lamellar orientation at high stress intensity factor, ΔK, while at low ΔK cracking occurs in lamellar colonies only when the lamellar interface is parallel to the fracture plane. Therefore, the fracture behaviour is determined by both lamellar orientation and ΔK. The fatigue crack propagation threshold shows little sensitivity to lamellar spacing and volume fraction of equiaxed gamma grains with 4522XD variants in the studied range.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:633428 |
| Date | January 2015 |
| Creators | Yang, Jing |
| Publisher | University of Birmingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://etheses.bham.ac.uk//id/eprint/5620/ |
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