The High-Temperature Creep Behavior Of Ti-44Al-5Nb-0.85W-0.85B

The creep behavior of a fine-grained cast TiAl alloy with nominal chemical composition of Ti-44Al-5Nb-0.85W-0.85B(at.%) was studied.The material was HIPped(hot isostatic pressed) at the temperatures of 1260℃and 1340℃respectively to produce different microstructures:the B2+ωphase was segregated at lamellar colony boundaries after 1260℃HIPping,and it was completely removed after 1340℃HIPping.Tensile creep tests were performed at the temperature of 700℃under a constant stress ranging from 150MPa to 300MPa.The influence of B2+ωprecipitates on creep under different stresses was studied and discussed.The results showed that the two conditions of the cast alloy subjected to creep for up to 1000 hours,even at the temperature of 700℃and at the stress of 300MPa,were still in the secondary creep regime,no fracture occurred.This reveals a good creep resistance for the W-containing fully lamellar TiAl alloy.Also found is that the fully lamellar microstructure with B2+ωsegregation shows a better creep resistance than that without B2+ωsegregation.Creep mechanism has been found to change between 200MPa and 300MPa.The microstructure evaluation was performed by scanning electron microscopy (SEM) and transmission electron microscopy(TEM).The results showed that coarseγlaths contained a relatively high dislocation density.Dislocation pile-ups were observed among lamellar tips,antiphase domain boundaries andα₂/γinterfaces.Theα₂+γlamellae exhibit relatively high stability.Parallel decomposition of coarseα₂ laths was rarely found,the deformation twins were found only inγlaths,and no any lamellae were observed to break-up and globularise.The research has found that that blocky B2+ωordered phase can be removed completely after 1340℃HIPping.However,the ordered phase was precipitated again around the colony boundaries after creep.It is deduced that the refractory elements W,Nb still distribute nearby after the HIPping due to sluggish diffusion.They would contribute to the re-precipitation of B2+ωduring long-time creep under the high temperature and high stress.It is therefore concluded that the removing of B2+ωby HIPping is not an effective way to strengthen theα₂ andγlamellae in TiAl alloys.