High-temperature tensile rupture property of (Nb,W) co-alloying TiAl-based alloys

Abstract

In order to explore whether (Nb,W) co-alloying TiAl-based alloys with relatively higher W addition have better high-temperature tensile rupture property, Ti-44Al-4Nb-1 W-0.1B alloy is designed and prepared. Ti-44Al-8Nb-0.1B alloy and Ti-44Al-7.2Nb-0.2 W-0.1B alloy are also prepared for comparative study. The rupture property testing is carried out at 800 °C and different tensile stresses. The property data, macro/microstructure evolution, fracture surface, W content, crack failure behaviors are studied. The results show that the (Nb,W) co-alloying alloys have better rupture property than the pure Nb alloying alloy. For the (Nb,W) co-alloying alloys, the higher W contained Ti-44Al-4Nb-1 W-0.1B alloy has the better property under lower tensile stress, and the lower W contained Ti-44Al-7.2Nb-0.2 W-0.1B alloy has the better property under higher tensile stress. The relationships of rupture life $t$ and stress $\sigma$ for the three alloys are given. All the three alloys have a coupling fracture mode of ductile fracture and brittle fracture. The ductile fracture exhibits the typical dimple characteristics. The brittle fracture exhibits the typical trans-granular cleavage, river-like pattern and trans-lamella fracture characteristics. The higher the stress, the more brittle fracture characteristics there are. After rupture property testing, the (α₂ + γ) lamella colony sizes of the three alloys all decrease, indicating that DRX and grain boundary slip occur not only along (α₂ + γ) lamella colony boundary, but also inside it. The colony boundary regions have the stress concentration, where the B2 phase produces the better buffering and coordination, and as well as the dislocation tangles, DRX and grain boundary slip can be found by EBSD and TEM. EPMA results show that the more W added in the alloy, the more W content is in the (α₂ + γ) lamella matrix, which is beneficial for the rupture property. However, more W addition will also lead to the formation of more B2 phase in the initial as-cast microstructure. So that, under higher tensile stress, when the stress intensity factor K is higher, the crack failure is more likely to occur.