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ADVANCE: Sophisticated experiments and optimisation to advance an existing CALPHAD database for next generation TiAl alloys

Periodic Reporting for period 1 - ADVANCE (ADVANCE: Sophisticated experiments and optimisation to advance an existing CALPHAD database for next generation TiAl alloys)

Reporting period: 2018-10-01 to 2020-03-31

Reduction of aircraft CO2 and NOx emissions is achievable using lightweight materials such as TiAl alloys that are used in the design and manufacture of aircraft engines. A reduction in aeroengine weight directly contributes to a reduction in the airline industry’s global fuel consumption and thus its overall environmental footprint.
TiAl is stable at high temperatures with low weight and high strength, i.e. high specific strength, making it a good substitute for Ni-based alloys that have a lower specific strength and are traditionally the material used to make the rotating blades in the last stages of the low-pressure aero engine turbines. The next generation of aero engine designs aims to use TiAl alloys in at least one more stage of the low-pressure turbine because of TiAl alloys’ enormous weight saving potential. To cope with the higher operating temperatures of the more forward stages of the turbine, the temperature capability of TiAl alloys must be increased by 50–100 °C. This temperature requirement is a technical challenge requiring precise scientific data to determine the influence from chemistry on microstructure and its stability for this class of alloy. To be both precise and effective requires predictive simulation capabilities that will enable the evaluation of a large number of potential TiAl alloys in a very short time.
The ADVANCE project consists of an extensive and ambitious experimental program to generate detailed and accurate phase equilibrium data for a series of homogeneous alloys of high purity in the ternary systems Ti-Al-X (X = Nb, Mo, W, O, B, Zr, C, Si) and quaternary systems Ti-Al-Nb-Z (Z = Mo, W). The aim is to resolve existing experimental controversies, determine missing data points, assess and re-assess these individual subsystems, and integrate the data into an advanced CALPHAD database for TiAl alloys. The extended CALPHAD database will be capable of rapid computational design and will facilitate the optimisation of new TiAl alloys and the respective processing conditions with a high degree of confidence.
A series of experimental ternary Ti-Al-X (X = Nb, Mo, W, O, B, Zr, C, Si) and quaternary Ti-Al-Nb-Z (Z = Mo, W) alloys have been produced from elements of the highest available purity. To ensure homogeneity, alloys were primarily produced by levitation melting as well as using an advanced arc melting device. After casting the composition of all alloys and the content of the most prominent impurities have been determined by wet chemical analysis, which was complemented by electron probe microanalysis (EPMA). To date, the majority of alloys produced by levitation or arc melting showed only minor deviations from the intended compositions and has little or no segregation, ensuring that those specimens can be considered as representing the intended overall chemical compositions to be investigated.
To generate samples for equilibrium heat treatment, a diamond wire was used to cut the cast alloys into slices of 2-10 mm thickness. These slice samples were encapsulated and annealed at a range of temperatures and at times sufficiently long to ensure equilibrium. Following equilibrium heat treatment all the samples have been, or will be, subject to metallographic inspection and phase analysis using a range of techniques. These techniques include light optical and scanning electron microscopy (LOM, SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), electron probe microanalyzer (EPMA), differential thermal analysis (DTA), high-energy X-Ray diffraction (HEXRD) and transition electron microscopy (TEM). This work that is still ongoing allows the project team to accurately determine equilibrium phases and the composition, as well as phase transition temperatures, which is all important data needed to better understand the microstructure and chemistry of the chosen alloys.
The experimental work to date is expected to contribute to the detailed understanding of phase equilibrium present in ternary systems Ti-Al-X (X = Nb, Mo, W, O, B, Zr, C, Si) and quaternary systems Ti-Al-Nb-Z (Z = Mo, W). This scientific data will successfully be used to obtain an accurate CALPHAD thermodynamic database that has the necessary predictive capabilities to computationally study the state and properties for a large number of potential alloy compositions in a reasonably short analysis time. This functionality will directly enable scientists and research and development teams in industry to become more efficient and innovative in their processes to develop and manufacture new and improved lightweight TiAl alloys. The short-term goal, to provide an accurate database that will aid in the design of a lightweight, high temperature tolerant TiAl alloy used to make aeroengines rotating blade parts, will likely be adapted in other transportation sectors where such materials are also beneficial.
Conceptual overview of ADVANCE project