Lothar W. Meyer

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.

Adiabatic shear failure under biaxial dynamic compression/ shear loading

Abstract A new combined compression /shear testing method was developed in order to study the propensity of metallic materials to adiabatic shear failure under multiaxial stresses. The combined simultaneous loading is applied by a drop weight hammer to a cylindrical specimen which has its axis inclined to the loading axis. The results for titanium and tungsten alloys show a strong sensitivity of the onset of adiabatic shear failure which is promoted under small amounts of a shear component to the main compression stress. The results are characterized and compared with predictions using a finite element method analysis.

Reaction synthesis/dynamic compaction of titanium diboride

A new method for producing dense compacts of titanium diboride is described. This approach combines reaction synthesis with a high velocity forging step (5 to 15 m/s impact velocity) to achieve densification and near-net shape. By combining synthesis and densification, titanium diboride disks with over 96 pct of the theoretical density were produced. The highly exothermic (279.6 to 324.0 kJ/mol) synthesis reaction between titanium and boron produces temperatures of approximately 3000 K, at which the material is ductile and forgeable in a high-speed machine. The mechanical properties (quasi-static and dynamic) of reaction-synthesized/dynamically compacted titanium diboride are determined and compared with that of conventionally hot-pressed material.

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.

Metallurgical Effects on Impact Loaded Materials

The change of microstructure is a common process in impact-loaded materials. Around the path of projectile, for example, cracks and white streaks are visible. From the point of view of protection this deformation behavior of an armor plate must be considered an undesired process. In order to investigate these streaks a specimen was designed by which it is possible - using a Hopkinson bar - to produce white streaks in a laboratory with the advantage of good reproducibility. With different materials load versus time measurements using strain gauges were carried out. Comparing these results with those of the examination of the streaks by light and electron microscopy methods a relationship could be given between the load-time-diagram and the correspondent microstructure of the streaks. Furthermore the effects of material parameters on the intensity of deformation and the dimensions of the streaks are reported.

Processes of partial bulk metal-forming—aspects of technology and FEM simulation

Abstract This publication focuses on near-net-shape and net-shape forming of rotation-symmetrical workpieces by incremental rolling processes (cross rolling, spin extrusion). The combination of both processes is essential. The FEM simulation is compared with the model method of shape deformation (FMM), taking into account material flow and stress on the material. The flow behaviour is described in constitutive equations with respect to temperature and forming speed. The effect of the cyclic load on the material is examined. A description of the major development aims and application range in the process chain is intended.

Uniaxial and biaxial compressive response of a bulk metallic glass composite over a range of strain rates and temperatures

The uniaxial and biaxial compressive responses of Zr 57 Nb 5 Al 10 Cu 15.4 Ni 12.6 –W composite were investigated over a range of strain rates (∼10 −3 to 10 3 s −1 ) using an Instron universal testing machine (∼10 −3 to 10° s −1 ), drop-weight tower (∼200 s −1 ), and split Hopkinson pressure bar (10 3 s −1 ). The temperature dependence of the mechanical behavior was investigated at temperatures ranging from room temperature to 600 °C using the instrumented drop-weight testing apparatus, mounted with an inductive heating device. The deformed and fractured specimens were examined using optical and scanning electron microscopy. Stopped experiments were used to investigate deformation and failure mechanisms at specified strain intervals in both the drop weight and split Hopkinson bar tests. These stopped specimens were also subsequently examined using optical and scanning electron microscopy to observe shear band and crack formation and development after increasingly more strain. The overall results showed an increase in yield strength with strain rate and a decrease in failure strength, plasticity, and hardening with strain rate. Comparison of uniaxial and biaxial loading showed strong susceptibility to shear failure since the additional 10% shear stress caused failure at much lower strains in all cases. Results also showed a decrease in flow stress and plasticity with increased temperature. Also notable was the anomalous behavior at 450 °C, which lies between the T g and T x and is in a temperature regime where homogeneous flow, as opposed to heterogeneous deformation by shear banding, is the dominant mechanism in the bulk metallic glass.

Submicrosecond strength of ultrafine-grained materials

We present the results of measuring the strength properties of metals and alloys with face-centered cubic lattice (copper, aluminum), body-centered cubic structure (Armco iron, tantalum), hexagonal close-packed structure (titanium and titanium alloy BT6) in the original coarse-grained and submicrocrystalline state under shock-wave loading. The grain dimension of the materials under study was changed by intensive plastic deformation. The influence of the grain dimensions on the dynamic yield stress does not always agree with the data of low-rate test even in sign, which is interpreted in the framework of general laws of the strain rate influence on the metal and alloy flow stress. As the grain dimension decreases, there is an increase in the compression rate in the plastic shock wave, a small increase in the fracture strength (spall strength), and an increase in the spall fracture rate.

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.

Ultimate Strength of a Tungsten Heavy Alloy after Severe Plastic Deformation at Quasi-Static and Dynamic Loading

A tungsten heavy alloy (92%W, Ni-Co matrix) is subjected to severe plastic deformation (SPD) by high pressure torsion (HPT) at room temperature up to equivalent strains of 0.7, 5.3, 10.7 and 14.3. The microstructure and the mechanical properties are investigated by cylindrical compression samples at quasi-static and dynamic loading. The harder spherical W particles are homogeneously deformed within the softer matrix, becoming ellipsoidal at medium strains and banded at high strains without shear localization or fracture. Results of quasi-static loading show that the strength is approaching a limiting value at strains of ~10. At this strain for the matrix a grain size of ~80 nm and for W a cell size of ~250 nm was observed, suggesting strain concentration on the matrix. The initial yield stress of 945 MPa for the coarse-grained condition is increased thereby to an ultimate value of 3500 MPa, while a peak stress of ~3600 MPa is reached. Such remarkably strength has never been reported before for pure W or W-based composites. The strain hardening capacity as well as the strain rate sensitivity is reduced drastically, promoting the early formation of (adiabatic) shear bands.