Refining of coarse lamellar microstructure of TiAl alloys by rapid heat treatment

It is now well known that the room temperature tensile yield strength (sigm a(y)), fracture strength (sigma(f)), and ductility (delta) of TiAl alloys i ncrease with decreasing the colony size (d) of the fully lamellar (FL) stru cture. Therefore searching a cost-effective approach to refine the coarse F L structure becomes critical to the engineering application of this materia l. In this study, we report our recent observation that, merely by rapid he at treatment, a FL structure with d = 500 mu m in a TiAl alloy of Ti-48Al-2 Cr (at%) could be refined to a FL structure with d being as fine as 10 mu m . No thermomechanical processing such as canned forging and hot extrusion, as done in previous studies, was required. Mechanical testing in tension sh ows that with the refinement from 500 to 10 mu m, the room temperature yiel d strength increased from 330 to 610 MPa, fracture strength from 415 to 825 MPa and ductility from 0.7 to 3.3%, although the fracture toughness decrea sed only slightly from 24 to 19 MPa m(1/2). (C) 2000 Elsevier Science Ltd. All rights reserved.