The Effects Of Isothermal Forging Process On The Microstructures And Mechanical Properties Of TiAl Matrix Ingot Alloy

The microstructures and mechanical properties of both ingot-cast and isothermally foreged Ti-44Al-4Nb-4Zr-0.2Si-1B alloys have been studied and the mechanisms that govern the improvement in the microstructures and room temperature mechanical properties are summarised in this thesis.It has been found that the lamellae thickness of the forged Ti-44Al-4Nb-4Zr-0.2Si-1B alloy is decreased by 15% from 0.52μm to 0.44μm, and the volume fraction of B2+ωsegregation phases is reduced by 60%, compared with the cast-ingot alloy, and found that the coarse boride ribbons is refined and the distribution of borides becomes less heterogeneous after forging. Correspondingly, the room temperature tensile strengths and fatigue strength are increased, while the low value of the elongation to failure remains essentially unchanged.The improvement in room temperature tentile strengths and fagitue strength is attributed to the decreases inα₂+γlamellae thickness, volume fraction of B2+ωbrittle segregation phases and the size of borides. On the other hand, the unchanged low ductility at room temperature is suggested to be controlled by two factors: a) still segregated B2+ωbrittle phases after forging, although the volume fraction is reduced, b) a great number of borides remain in the alloy even though the size and distribution are improved. Both the B2+ωand borides are detrimental to the room temperature ductility of the alloy.