Microsample characterization of the tensile and compressive mechanical properties of single crystalline gamma-TiAl

Titanium-aluminide intermetallic compounds are promising as high temperature structural materials in both the automotive and gas turbine industries, because of their low density, relatively high Young's modulus and yield strength. The commercially used TiAl intermetallic alloys will most likely have a two-phase fully lamellar structure. However, due to the complex nature of these alloys this study focuses on the single-phase gamma, since it will be the major constituent in the two-phase materials and dislocation activity in two-phase alloys is believed to occur most readily in the gamma-TiAl phase.; Previously, the lack of sufficiently large high quality single crystals has precluded mechanical testing of gamma-TiAl in tension. In the present study, single crystals of gamma-Ti 55.5at%Al have been grown, oriented and cut into microsample tension/compression specimens with nominal dimensions of 3mm x 1mm. The small size of the microsample allows for both tension and compression testing of single crystals of gamma-Ti 55.5at%Al as a function of temperature and crystallographic orientation.; Single crystal microsample test specimens of gamma-Ti 55.5at%Al oriented along the ∼[001], ∼[010] and ∼[−110] crystal axes have been deformed in tension, compression, fully reversed compression-tension and fully reversed tension-compression tests at temperatures ranging from 723 K to 1273 K From these experiments measurements of the Young's modulus, coefficient of thermal expansion and 0.2% offset flow stress have been made as a function of temperature, crystal orientation and sense of applied load in the anomalous yielding regime.; A measurable flow strength anomaly is observed for all three orientations and a significant tension/compression asymmetry has been measured in the anomalous yielding temperature region. The asymmetric dissociation and motion of superdislocations, their ability to cross-slip under applied loads, the anisotropic interaction forces between dislocations and the constriction forces on dislocations have all been analyzed to determine their role in the cross-slip locking processes of superdislocations. The formation of locks is most favorable when the applied stress promotes cross-slip of the trailing dissociated superpartial onto the secondary octahedral plane, and is used to explain the flow stress anomaly, tension/compression asymmetry and the Schmid's law violations that have been measured.