ABSTRACT

The previous design of engines used for commercial aircrafts fan and turbine were driven with same shaft; new geared engines decouple fan and the low pressure turbine increasing angular velocity, and also centrifugal forces, so a change from conventional superalloy blades to lighter ones is needed. Titanium aluminides are interesting alloys for new turbine blades.

In this regard,during some years several compositions of γ TiAl alloys have been studied. This work is focused on the 3rd generation TNM alloy with chemical composition Ti44Al4Nb1MoXB in order to understand the micromechanisms governing deformation and

fracture. TNM alloys solidify through the β phase, present at high temperature allowing hot workability in near-conventional conditions and grain refinement through the β → α phase transformation. Nevertheless, uncontrolled thermal history might induce the formation of brittle β0 from the residual β phase, which should be reduced by means of heat treatments in order to maintain mechanical properties. Moreover, heat treatments are also crucial to control the final microstructure in order to obtain fully lamellar, near lamellar, duplex or near gamma microstructures. Previous studies have shown that microstructures with

narrow lamellar spacings and small volume fractions of globular γ and β0 present the best balance in mechanical properties.

The main objective of this doctoral thesis is to understand microstructural effects on the fatigue resistance of Ti44Al4Nb1MoXB alloys. This first year assessment summarises the preliminary studies carried out with this final objetive in mind that can be sum- marised in

two sets of studies. In the first one, the microstructure of industrially cast turbine blades with 0.7%atB was analysed in different areas of the blade to assess the effect of solidification rate on the cast microstructure. The microstructure evolution in different areas of the blade was afterwards studied upon hot isostatic pressing (HIP) and different thermal treatments to evaluate the efficiency of this post thermal treatments on erasing the cast microstructure. The second part of the work focuses on the setting up of the required experimental methodology to analyse fatigue crack growth behavior for specimens with a range of microstructures.

Starting from cast ingots with 0.2%atB, an experimental procedure has been established to thermally treat flat specimens of this alloy with a great accuracy in temperature and heating

and cooling rates using a Gleeble thermomechanical simulator. Special emphasis has been put on optimising the control parameters in order to obtain sufficiently large homogeneously heat treated areas with- out loss of elements and preventing the formation of thick surface oxide layers. Future work will focus on the characterization of the monotonic and fatigue behavior as a func- tion of microstructure. For this, preliminary tests to determine mechanisms governing deformation and fracture have been performed by means of micropillar compression. Fi- nally, a crack growth measurement set up based on the potential drop method has been established for the future fatigue studies.