The brittle-ductile transition in gamma-TiAl single crystals

Fracture tests in the temperature range 20-800 degrees C were performed on single crystal gamma-TiAl (Ti-54.7% Al) specimens. Ail tests were by four-point bending; the specimens were precracked by spark discharge so that K-lc could be determined. Two orientations of specimens were used, chosen so that the influence on the fracture behaviour of the 'ordinary' dislocations (b = a/2 < 110]) and 'superdislocations' (b = a < 101]) could be distinguished. K-lc rises steadily from similar to 1.5 MPa m(1/2) at 20 degrees C to similar to 3.5 MPa m(1/2) at similar to 600 degrees C for both specimen types. The brittle-ductile transition (BDT) temperature, above which specimens yielded rather than fracturing, is in the range 600-700 degrees C, and varies with strain rate. The strain rate variation allows activation energies for the BDT process to be determined: 1.4 eV for the 'ordinary' dislocation specimens and 5.9 eV for the 'superdislocation' specimens. Below similar to 300 degrees C, fracture is by cleavage from the precrack; above similar to 300 degrees C, stable crack growth occurs before final fast cleavage fracture.