Refinement of Microstructures and Improvement of Mechanical Properties in Intermetallic TiAl Alloys by Rheocasting

Abstract

An investigation was made on the refinement of microstructures and improvement of mechanical properties in intermetallic TiAl binary alloys containing 44, 49 and 54 at% aluminum produced by the rheocasting in which the solidifying alloy was vigorously agitated at high rotation speeds of 15 to 70 rev/s s of a stirrer immersed in the alloys in argon gas atmosphere. In the microstructure of the rheocast Ti-44at%Al alloy such a lamellar structure as seen in the alloy solidified without stirring was destroyed completely, and an extremely refined microstructure composed of dispersed agglomeration aggregates was formed. The mean sizes of agglomeration aggregates and individual crystal grains of the Ti-44%Al alloy rheocast at a rotation speed of 70 s -1 were 6.4 ± 3.7 μm and 2.2 ± 1.3 μm in major axis and 2.0 ± 1.1 μm and 1.0 ± 0.6 μm in minor axis, respectively. However, oval lamellar grains were seen in the microstructure of the rheocast Ti-49at%Al alloy and lamellar substructure was observed in the oval primary crystal grains in the microstructure of the rheocast Ti-54at%Al alloy. In the microanalysis of the Ti-44%Al alloy rheocast at 70 s -1 the uniform distribution of Al and Ti elements could be achieved by destruction of the lamellar structure with the mechanical stirring, although lamellar segregation was observed in the alloy solidified without stirring. The room temperature elongations of the Ti-44%Al alloy rheocast at 15 and 70 s -1 were 2.7% and 3.3%, respectively, being higher than the elongation of the alloy solidified without stirring, i.e. 2.0%. The room temperature tensile strengths of Ti-44%Al alloy rheocast at 15 and 70 s -1 were 359 and 468 MPa, respectively. They were also higher than the tensile strength of the alloy solidified without stirring, i.e. 245 MPa. The elevated temperature elongation and tensile strength of the rheocast Ti-44%Al alloys were improved as compared with those of the alloy solidified without stirring, although a fluctuation existed. The tensile strengths of the alloys rheocast at 70 s -1 and solidified without stirring were 538 and 363 MPa at 1173 K, and 439 and 389 MPa at 1273 K, respectively.