The Research On The Microstructure And Mechanical Properties Of A Highly-alloyed TiAl-base Alloy

A highly-alloyed TiAl ingot alloy Ti-44Al-4Nb-4Hf-0.2Si-1B was grain refined by adding 1% B,and then isothermally forged in a singleα-phase region(at 1310℃).The microstructure and mechanical properties produced by the forging have been investigated in detail in this project.The corresponding changes in the microstructure and mechanical properties of the ingot alloy have been characterized andquantified,using electron microscopy as well as tensile and S-N fatigue testing techniques.It has been found that,different from the normal outcomes ofα+γtwo-phase forging,thesingeα-phase forging has caused a significant increase in the lamellar colony size and a noticeable increase in theα2+γlamellar thickness for the fully lamellar ingot alloy.Also found is that a more thermodynamically stable condition has been reached through the forging process,which is characterized by an increased volume fraction ofγlame]lae inside the iamellar colonies,compared with the ingot alloy.Another important change is that the large boride ribbons in the ingot alloy have been broken down by the forging and the distribution of the borides therefore becomes more uniform.As a result,the isothermally forged alloy shows a significant increase in room temperature ductility,while tensile strengths remain essentially unchanged.This mechanical behaviour tends to indicate that the forging in the singleα-phase region is still beneficial to the grain refined TiAl alloy in a way,in which the beneficial effects of breaking/refining borides outweigh the detrimental effects of increasing colony size and lamellar thickness.The positive impact from the forging is also observed in the S-N high-cycle fatigue performance;a significantly high fatigue limit has been obtained after isothermal forging,although the S-N curve is quite flat.