Lutz Krüger

High strength Fe-Mn-(Al, Si) TRIP/TWIP steels development - properties - application

Abstract Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe-(15–30) wt.%Mn steels with additions of aluminium and silicon have been investigated. It is known that additions of aluminium increase the stacking fault energy γfcc and therefore strongly suppress the γ→e transformation while silicon decrease γfcc and sustains the γ→e transformation. The γ→e phase transformation takes place in steels with γ fcc ⩽20 mJ m 2 . For steels with higher stacking fault energy twinning is the main deformation mechanism. Tensile tests were carried out at different strain rates and temperatures. The formation of twins, α- and e- martensite during plastic deformation was analysed by optical microscopy, X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The developed light weight high manganese TRIP (“transformation induced plasticity”) and TWIP (“twinning induced plasticity”) steels exhibit high flow stress (600–1100 MPa) and extremely large elongation (60–95%) even at extremely high strain rates of about 103 s−1. Recent trends in the automotive industry towards improved safety standards and a reduced weight as well as a more rational and cost effective manufacturing have led to great interest in these high strength and “super tough” steels.

Mechanical properties and microstructural changes of ultrafine-grained AA6063T6 during high-cycle fatigue

Abstract Fatigue behaviour and mechanical properties of peak-aged AA6063T6 with ultra-fine grain size, produced by equal channel angular extrusion, were evaluated with special emphasis on the microstructure before and after cyclic loading. The strength significantly increased with grain size reduction and is described by an exponential power-law constitutive relationship. A remarkable enhancement of fatigue life compared to commercial AA6063T6 with coarse grains was found in the high-cycle regime after the first two extrusions. Further extrusions eliminated this improvement. It is shown that the optimum fatigue performance correlates very well with the minimum tensile ductility. Electron backscatter diffraction revealed that the material behaviour can basically be attributed to the grain boundary characteristics. Low grain boundary misorientation angles yield the best fatigue performance in the ultrafine-grained microstructure.

Corrosion behaviour of aluminium alloys in ethanol fuels

In this study, the corrosion behaviour of several aluminium alloys in ethanol fuels was investigated by immersion and polarization tests. The corrosion properties of cast aluminium alloys (Al–17wt%Si–4wt%Cu–Mg, Al–8wt%Si–3wt%Cu, Al–7wt%Si–Mg and Al–17wt%Si–4wt%Cu–Mg with a chemically deposited nickel layer) in ethanol blended gasoline fuels were examined at various ethanol and water contents and various temperatures. Electrochemical and gravimetric measurements revealed a pronounced acceleration of the corrosion process above the boiling point. Additions of water restrain the corrosion. Increasing the ethanol content and the temperature leads to a higher corrosion sensitivity of the aluminium alloys. Furthermore, the nickel layer is very protective in all tested fuels. For aluminium alloys, a theory of the corrosion process in ethanol blended gasoline fuels is proposed.

Combining equal-channel angular extrusion (ECAE) and heat treatment for achieving high strength and moderate ductility in an Al-Cu alloy

The effect of equal-channel angular extrusion (ECAE) on mechanical properties of an AA2017 produced by powder metallurgy is investigated. Special attention is given to the influence of heat treatment, processing temperature and backpressure on the workability for achieving high strength and moderate ductility. This is of special interest, since it is often reported that Al-Cu alloys have low ductility and therefore are prone to cracking during severe plastic deformation. It is shown that ECAE at high temperatures (>220°C) does not necessitate backpressure to ensure homogeneous deformation but leads to a significant sacrifice in strength due to in-situ precipitation. Thus, most of the extrusions are done at considerably low temperatures. Performing room temperature-extrusion is most effective in achieving high strengths but also requires high backpressures. Due to severe strain hardening during processing, the strength increase is combined with a reduction in ductility. Recently it was reported that a post-ECAE aging of pre-ECAE solution treated material is effective in enhancing the ductility of aluminium alloys. This approach was successfully transferred to the current alloy. A high-temperature, short-time aging after only one extrusion, for example, doubles the failure strain to a value of ~13%. Compared to the naturally aged condition with coarse grains that serves as reference (T4), an increase of 15 % in yield stress (YS) was obtained while retaining the ultimate tensile stress (UTS). Another effective approach is the combination of a pre-ECAE solution treatment with subsequent under-aging prior to ECAE. It is shown that performing ECAE at medium temperatures (160-180°C) enables a better workability and additionally gives higher strengths and better ductility compared to the processing in the water quenched condition. A remarkable YS of 530 MPa and an UTS of 580 MPa combined with a moderate failure strain of 11.6 % were achieved.

Compressive behaviour of ultrafine-grained AA6063T6 over a wide range of strains and strain rates

Abstract The flow stress behaviour of commercially available AA6063-AlMg0.5Si0.4 with ultrafine-grain size is investigated over a wide range of strain and strain rates under uniaxial compression. The ultrafine-grained microstructure is achieved by equal channel angular extrusion and characterised by grain sizes well below 1 μm. Results of quasi-static loading show that the ultrafine-grained states behave in an elastic – nearly perfect plastic manner with significantly reduced strain hardening capacity. When compared to the coarse grained counterpart, no change in strain rate sensitivity was measured for the material after two extrusions. In contrast to that, after eight extrusions the material shows significantly increased strain rate sensitivity especially in the range of highest rates above 102 s−1.

The Preparation of Magnesium Specimens for EBSD Using Ion Polishing

Abstract EDSD measurements were made for various thermally treated or deformed states of the magnesium alloy AZ31, which were produced from twin-roll cast strips. The article shows that a preparation route via the ion polishing method produces the best condition of the sample surface for the examinations and, in particular, yields very good results even in case of samples which are deformed or those the chemical composition of which is inhomogeneous.

Effect of zirconia and aluminium titanate on the mechanical properties of transformation-induced plasticity-matrix composite materials

Metal-matrix composite materials composed of an austenitic stainless steel with different ceramic particle reinforcements were investigated in this study. The test specimens were prepared via a powder metallurgical processing route with extrusion at room temperature. As reinforcement phase, either magnesia partially stabilized zirconia or aluminium titanate with a volume content of 5% or 10% was used. The mechanical properties were determined by quasi-static compressive and tensile loading tests at ambient temperature. The microstructure characteristics and failure mechanisms during deformation contributing to significant changes in strength and ductility were characterized by scanning electron microscopy including energy dispersive X-ray spectroscopy and electron back-scatter diffraction, and by X-ray diffraction. The composite materials showed higher stress over a wide range of strain. Essentially, the deformation-induced formation of α′-martensite in the steel matrices is responsible for the pronounced strain hardening. At higher degrees of deformation, the material behavior of the composites was controlled by arising damage evolution initiated by particle/matrix interface debonding and particle fracture. The particle reinforcement effects of zirconia and aluminium titanate were mainly controlled by their influences on martensitic phase transformations and the metal/ceramic interfacial reactions, respectively. Thereby, the intergranular bonding strength and the toughness of the steel/ceramic interfaces were apparently higher in composite variants with aluminium titanate than in composites with magnesia partially stabilized zirconia particles.

Crack initiation at high loading rates applying the four-point bending split Hopkinson pressure bar technique

Dynamic crack initiation with crack-tip loading rates of

Hydrogen-induced embrittlement of fine-grained twin-roll cast AZ31 in distilled water and NaCl solutions

\dot{K} \approx 2 \cdot 10^6\,\mathrm {MPa\,m^{0.5}\,s^{-1}}