Synthesis Mechanism And Mechanical Properties Of Laminated TiAl Composite

As a new type of structural materials with excellent properties, Ti Al-based alloys can satisfy the dual requirements of structure weight reduction and high temperature strength of the aerospace supersonic aircrafts and automotive engines. However,owing to intrinsic brittleness and poor forming ability of Ti Al-based alloys, it is rather difficult to fabricate Ti Al-based alloy sheets by means of traditional hot rolling. In this paper, the mechanical properties of Ti Al sheet are improved by the design of laminated structure and the intruduction of the Ti B whisker enhanced phase.The pure Ti foils and the in situ formation Ti Bw/Ti composite foils with the pure Al foils were alternately stacked, then Ti-Al laminate and(Ti Bw/Ti)-Al laminate were prepared by vacuum hot pressing. The laminated Ti Al sheet and the laminated Ti Bw-Ti Al composite sheet were synthesized by reaction heat treatment. The phase composition and phase transition rule of the stacked layer during the heat treatment were analyzed. The forming mechanism of the laminated structure studied.The mechanical performance of the laminated Ti Al sheet and the Ti Bw-Ti Al composite sheet were tested and revealed the strengthening and toughening mechanisms and the fracture mechanism.For Ti-Al laminate with good interface bonding fabricated by hot pressing, after the initial reaction annealing during 660 oC, Al and Ti atoms interdiffused constantly and Ti Al3 phase produced with linear growth kinetics model. After 2h, Al element in the material was consumed completely, and obtained the Ti-Ti Al3 laminate.Because of the different diffusion rates of Ti and Al in the reaction of annealing, Kirkendall effect existed and the Ti Al3 was porous structure. When the laminate was subsequently annealed at 1200oC/35MPa/2h, the holes in the material were removed. Al and Ti interdiffused constantly and then produced Ti Al and Ti3 Al. As the material composition was not uniform, the poor Al area and the rich Al area existed. With the heat treatment, the poor Al area produced fully lamellar(α2-Ti3Al/γ-Ti Al) layer and the rich Al area produced the equiaxed γ-Ti Al layer, finally the double layered structure Ti Al sheet was prepared.Due to the diffusion components of Ti-Al laminate and(Ti Bw/Ti)-Al laminate are identical, and the thickness is basically not changed.Therefore also use the same heat treatment process for(Ti Bw/Ti)-Al laminate and then the laminated Ti Bw-Ti Al composite was prepared. The laminated Ti Bw-Ti Al composite is composed by Ti Al layer and Ti Bw-rich layer, and the Ti Al layer is composed by alternately arranged fully lamellar(α2-Ti3Al/γ-Ti Al) layer and the equiaxed γ-Ti Al layer, and the Ti Bw-rich layer was a high volume fraction of Ti B whisker aggregation layer.The results showed that the tensile strength and room temperature elongation of the laminated Ti Al sheet were high, which was because the(α2-Ti3Al/γ-Ti Al) layer could refine the organization. Ti Bw-rich layer of the laminated Ti Bw-Ti Al composite sheet increased the strength. The tensile fracture surface analysis showed that fracture in the layer tissue was mainly interlamellar cracks and translamellar cracks, and the fracture in the equiaxed microstructure was mainly intergranular fracture and transgranular fracture.The fracture toughness results showed that the fracture toughness in transverse direction was higher than in normal direction of laminated Ti Al sheet and the fracture toughness was increased with the introduction of Ti B whisker. The layered structure and the presence of Ti B whisker increased the resistance of crack initiation and propagation, and extended the path of crack growth.The three-point bending results showed that the room temperature bending strength of the laminated Ti Bw-Ti Al composite sheet was increased which was enhanced by the introduction of Ti Bw-rich layer.