Zusammenhang zwischen Gefüge und Schädigungsmechanismen bei mehrphasigen Stählen
Final Report Abstract
Efforts have been made to add to the understanding of the damage behaviour of lowalloyed TRIP steels. To this end mechanical testing and detailed microstructural investigations were carried out on an Al-alloyed and a Si-alloyed TRIP steel submitted to different heat treatments. In a first step the observations were interpreted on a phenomenological basis using a newly developed damage tolerance parameter. It could be shown that Al-TRIP steels show a certain variability of this parameter which could be related to the heat treatment of this steel. Sialloyed TRIP steels, however, are less sensitive to the heat treatment which was interpreted to be due to the higher strength of the bainitic-ferritic matrix. In a second step detailed microscopical observations were carried out to better understand the mechanisms of damage, in particular of damage nucleation. A first study in this context concerned the statistical relevance of local microscopical observations for the macroscopical characterization of a material. This study is particularly important for heterogeneous materials as TRIP steels are. The method can be applied to parameters like texture, phase volume fraction, grain size etc. A new measurement scheme for EBSD (electron backscatter diffraction)- based orientation microscopy was developed that improves large-scale measurements with statistical relevance. A second study focussed on the microstructure evolution of an Al-TRIP steel sample during deformation. These observations were performed on in-situ and ex-situ bending and tensile tests in an SEM using EBSD. These investigations showed a clear difference between stable und unstable austenite grains in terms of their crystallographic orientations, Taylor and Schmidt factors, grain size, grain shape and grain orientation with respect to principal loading axis. Particularly unstable are large grains with elongated shape and a high Schmid factor. The damage evolution in the TRIP steel was followed by tension tests together with digital image correlation (DIC) for accurate strain measurements. Damage nucleation always occurred in the vicinity of martensite either during or shortly after the γ−α transformation. The first visible voids were either in between different martensite variants or in a single martensite variant located at shear bands of a neighboring ferrite. The formation rate of voids was high in the beginning and slowed down after 20% of strain. With further strain the already formed decohesions run through the martensite variants and widened into voids. In the neighbourhood of the voids the ferritic matrix deforms drastically and fails due to void coalescence in the necking zone.
Publications
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“Crack development in dependence of the local microstructure in Al-TRIP-steels”. Proceedings “New Developments on Metallurgy and Applications of High Strength Steels”, Buenos Aires, Argentinien
Imlau, J.; Bleck, W.; Zaefferer, S.
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(2009). Comparison of damage development in dependence of the local microstructure in low alloyed Al-TRIP-steels, IF steel and a DP steel. Int. J. Materials Research 100, 584-593
J. Imlau, W. Bleck, S. Zaefferer
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“Effect of step size and scanned area on phase fraction and texture quantification from EBSD data”. DGM-DVM, AK-Treffen 2009, „Mikrostrukturuntersuchungen im REM", EBSD-Workshop 2009, 18-19 May 2009, Chemnitz-Germany
Kemal Davut and S. Zaefferer
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“Phase fraction and texture quantification of Al- TRIP steel from EBSD data”. 3rd Int. Conf. On Texture and Anisotropy of Polycrystals (ITAP-3), 23-25 September 2009, Göttingen-Germany
Kemal Davut and S. Zaefferer
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“Phase fraction and texture quantification of Al-TRIP steel from EBSD data”. 14th UGent-KUL-RWTH-MPIE-TUDelft Workshop (GLADD meeting), 2 October 2009, Aachen-Germany
Kemal Davut and S. Zaefferer
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"Statistical Reliability of EBSD Data Sets for the Characterization of Al-alloyed TRIP Steels". CD Proc of 15th Int Metallurgy-Materials Congress, 11-13 Nov 2010 İstanbul, Turkey 2010
K. Davut, S. Zaefferer
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"The effect of texture on the stability of retained austenite in Al-alloyed TRIP steels of Al-alloyed TRIP Steels". MRS 2010 Fall Meeting 29 Nov - 3 Dec 2010 Boston, MA, USA 2010
K. Davut, S. Zaefferer
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“Relation between damage nucleation and microstructure in TRIP steels”. Mat. Sci. and Eng. MSE 2010 (DFG Congress), 24-26 August 2010, Darmstadt Germany
Kemal Davut and S. Zaefferer
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“Statistical Reliability of Phase Fraction Determination based on EBSD investigations on the example of a TRIP steel”. Royal Microscopy Society - EBSD 2010 Meeting, 29-30 March 2010, Derby-UK
Kemal Davut and S. Zaefferer
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“Statistical reliability of phase fraction determination based on electron backscatter diffraction (EBSD) investigations on the example of an Al-TRIP steel”. Metallurgical and Materials Transactions A, Vol. 41A, (2010), pp. 2187-2196
Kemal Davut and Stefan Zaefferer
