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Forschungsarbeit

Nanoindentation as a tool for strain measurements in TiAl alloys experiments and Finite Element simulation

research project by Seema Rani Sen in cooperation with Prof. Dr. rer. nat. Mathias Göken and Dr.-Ing. Karsten Durst, FAU, Germany (04.09.2009)

The TiAl-based intermetallic alloys are attractive for high temperature structural applications due to their good high-temperature mechanical properties, low density, and good creep and oxidation resistance. These compounds are promising materials for turbine blades.  The application of these alloys is limited due to their poor fracture resistance. But the mechanical properties of these alloys can be improved by prestraining.  Here the lamellar TiAl alloys were deformed to introduce prestrains in the range 0%-15.3% by uni-axial compression testing. Then Nanoindentation experiments and Finite Element simulation were done to determine indentation hardness and elastic modulus. Our goal is to investigate the enhancement of the hardness and elastic modulus by prestraining.

[Bildunterschrift / Subline]: Figure 1: Schematic representation of Lamellar TiAl alloys ( α-2 and γ phases) (a), surface indents by Nanoindentation experiments (b), Load-displacement curve for complete cycle of loading and unloading(c).

Nanoindentation techniques are newly developed probing methods for measuring the mechanical properties of materials in nano scale. Here diamond tip is pressed into the surface of the materials and the load is measured the function as the indentation depth. The lowest indentation depth is as 20 nm and load is in the range μN to mN. The load (P) –displacement (h) curve for one complete cycle of loading and unloading, obtained by Nanoindention testing, shown in the figure 1(c). The elastic modulus and continuous indentation hardness (H= Pmax / A) can be determined from the load-displacement curve, using a method found by Oliver/Pharr [1]. Contact area A can be calculated from the indentation depth by the indenter shape function with proper tip shape calibration.

The Finite Element Method (FEM) is a continuum simulation technique in structural mechanics. The models have the ability to simulate the loading-unloading curves and the development of plastic deformation during indentation and to extract material property like hardness and elastic modulus from the tests. Here the indentation processes were simulated with the ABAQUS finite element software programme taking input data from compression testing of prestrained lamellar TiAl alloys.  The true hardness and elastic modulus were measured for simulation. The Von Misses stress profiles for 0.4% and 15.3% prestrained  lamellar TiAl alloys are shown in the figure 2.

[Bildunterschrift / Subline]: Figure 2: Schematic representation of simulation in Nanoindentation of 0.4 %( a) and 15.3%(b) prestrained lamellar TiAl alloys
[Bildunterschrift / Subline]: Figure 3: Elastic Modulus vs. Plastic strain curve (a) and Hardness vs. Plastic strain curve (b) by Nanoindentation experiments and FEM simulation of lamellar TiAl alloys.

The results from the FEM simulation were compared with the Nanoindentation experiments. Here the results show a good agreement with the Tabor correlation which are regulated by the flow stress increments in the plastic region in the stress-strain curve. The experimental result has higher hardness values due to materials pile up around the indents and residual stresses from sample preparation. But both results show that the indentation hardness increases linearly with prestrains and the elastic modulus stays nearly constant, shown in the figure 3. There is a good agreement between simulation and experimental results. Finally it is found that Prestraining enhances the mechanical properties of lamellar TiAl alloys.

References

1. Oliver, W. C. and Pharr, G. M. 1992. An improved technique for determining

hardness and elastic modulus using load and displacement sensing indentation

experiments. J. Mater. Res. 7 : 1564-1583.

2. Fisher-Cripps, A. C., Nanoindentation, Second Edition, Springer-Verlag, NY, 2004


Seema Rani Sen
Seema Rani Sen

Education
  • 2002-2007
  • Bangladesh University of Engineering and Technology (BUET), BSc.Engineering
  • 2007-2009
  • Member of Graduate Program Advanced Materials and Processes, Friedrich Alexander University (FAU), Erlangen-Nuremberg

Awards
  • 2007-2009
  • Siemens Masters Program Scholarship
  • 2002-2007
  • Awarded University merit scholarship for outstanding result in Engineering in BUET, Bangladesh