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Crack growth threshold analysis in TiAl alloys

Periodic Reporting for period 3 - TiAlCracks (Crack growth threshold analysis in TiAl alloys)

Reporting period: 2019-03-01 to 2019-10-31

Titanium Aluminide alloys form a relatively new class of materials that provides high temperature resistance at a very low specific weight and thus make an ideal choice for future turbine blades. However the material class is very brittle, i.e. it has a low toughness with respect to the much heavier high-temperature materials such as Ni-base alloys. To establish design criteria and ensure product integrity it is critical to understand the fracture behaviour of the new material class. In modern gas turbines the blades are subject to high temperatures (750°C and higher) and high cyclic loading, therefore it is of the utmost importance that the fracture behaviour is assessed and understood for these harsh conditions.

The measurement of crack growth of several μm wide cracks at temperatures above 700°C is experimentally challenging and far from being standard. The objective of the project is to investigate the fatigue crack growth (FCG) threshold and rate for long as well as short cracks of second generation TiAl alloys suitable for use in the Intermediate Pressure Turbine (IPT) of the UltraFanTM engine. To this end, theproject work plan defined a test matrix with second generation TiAl specimens.

The specimens were produced by the Topic Manager Rolls Royce plc (R-R) and delivered to the Coordinator Materials Center Leoben Forschung GmbH (MCL).

The test matrix included crack growth and fatigue experiments with:
• 3 different specimen geometries (CC, SE, TE/HCF)
• 7 different testing temperatures (RT, 400°C, 650°C, 700°C, 750°C, 775°C and 800°C)
The objective of the TiAlCracks project was to investigate the fatigue crack growth (FCG) threshold and rate for long as well as short cracks of second generation TiAl alloys suitable for use in the Intermediate Pressure Turbine (IPT) of the UltraFanTM engine.

Main Aim:
The experiments performed by Materials Center Leoben Forschung GmbH (MCL) had to generate and deliver data that the topic manager, Rolls Royce plc (R-R), could implement it in its database for the design of components for the UltraFanTM engine.

The main challenges were:-
(1) the joint definition of sample geometries and test procedures without violation of IPR from other test houses collaborating with R-R,
(2) the adaption of MCL test facilities to the defined specimen geometries and test procedures,
(3) the parallel R-R TiAl alloy development programme and corresponding specimen manufacturing by R-R and delivery to MCL to the project timescales, and
(4) the performance of experiments at temperatures up to 800°C.

In order to master the stated challenges,
(1) a close contact was established between MCL and R-R (monthly WebEx meetings and steering face to face meetings),
(2) an approval package was defined utilising 1st gen. TiAl,
(3) an external company located close to MCL was engaged for the production of special starter notches (EDM cuts < 80µm),
(4) a new test plan for the 2nd gen TiAl alloy was developed and processed (fewer material variations, more temperature variations, overall less specimens but longer test times per specimen) and
(5) corresponding shifts in the personnel and test facility work loads between the work packages.
The corresponding changes to the work pan resulted in an amendment to the original work plan submitted and accepted in 12/2017.

The main results were
(1) All pre-defined deliverable reports had been cross-checked by the topic manager prior to the submission to the joint undertaking and all reports were submitted on time.
(2) MCL has been accredited as a test house for specific materials characterisation experiments by R-R.
(3) The approval package and the resulting evaluation of starter notch and specimen geometry effects on the FCG threshold resulted in a presentation at the international conference MS&T 2019 in 10/2019 in Portland, Oregon and the submission of a scientific paper to the renowned Elsevier journal “Intermetallics” (submitted 09/2019, reviewing in progress).
(4) The FCG threshold data generated on the 2nd gen. TiAl alloy is now available for R-R for use in the design process of jet engine components.
Fatigue crack growth (FCG) measurement is not a standard procedure and on the same material the results may depend on the chosen starter notch geometry and testing procedure. A comparison of FCG curves and thresholds achieved with corner crack (CC) and single edge notched tensile (SENT) specimens is therefore scientifically and technically interesting and the dissemination of the comparison in scientific journals has started (submitted to the journal Intermetallics).
Testing innovative high-temperature materials for aircraft engine