Heinz Werner Höppel

Microstructural study of the parameters governing coarsening and cyclic softening in fatigued ultrafine-grained copper

Abstract The cyclic deformation behaviour of ultrafine-grained (UFG) copper produced by equal-channel angular pressing was investigated. Special attention was paid to the parameters governing cyclic softening and cyclic grain coarsening. UFG copper shows significant cyclic softening for the tests performed at intermediate plastic strain amplitudes Δϵp1/2, that is in the range 2 × 10−4 ≤ Δϵpl/2 ≤ 1.0 × 10−3. Within this range, the cyclic softening as well as the intensity of grain coarsening increase with decreasing plastic strain amplitude. By contrast, under stress control, corresponding to a plastic strain amplitude range 2.4 × 10−5 ≤ Δϵpl/2 ≤ 1.2 × 10−4, cyclic softening as well as the intensity of grain coarsening decrease with decreasing plastic strain amplitude. Furthermore, cyclic softening and grain coarsening were also found to be enhanced by decreasing the deformation rate (and thus increasing the test time) and/or by increasing the temperature. These findings indicate that the responsible microstructural processes are thermally activated. Based on the experimental results, a dynamic ‘recrystallization’ mechanism is proposed and suggested to be responsible for the cyclic grain coarsening and to some extent also for the cyclic softening.

Annealing treatments to enhance thermal and mechanical stability of ultrafine-grained metals produced by severe plastic deformation

Ultrafine-grained (UFG) metals produced by techniques of severe plastic deformation, such as equal channel angular pressing (ECAP), exhibit extraordinary strength properties. However, in the as-ECAP-processed state, the heavily deformed microstructure of such UFG metals is rather unstable and is prone to undergo grain coarsening (recrystallization) at moderate temperatures. This microstructural instability is enhanced in the presence of modest mechanical stressing as, for example, in cyclic deformation. Thus, all measures to enhance the thermal stability are also considered as beneficial for the improvement of the mechanical stability. One main objective of the present work is to analyse the thermal and mechanical stability of ECAP-processed metals during specific annealing and cyclic deformation tests. As a by-product, some conclusions relating to the separate effects of dislocation density, grain size (in the UFG regime) and internal stresses on the (micro)yielding behaviour will be drawn. Another goal is to explore the potential of different annealing treatments with respect to the stabilization of the microstructure and the optimization of the mechanical properties of ECAP-processed UFG metals in terms of an optimal combination of strength and ductility. In order to demonstrate the potential and the limitations of this approach, experimental work performed on UFG copper, aluminium and α-brass produced by ECAP will be reported and discussed. The results presented indicate strongly that a heat treatment leading to a bimodal grain size distribution provides the best compromise between strength and ductility.

Strain-rate sensitivity of ultrafine-grained materials

Abstract The strain-rate sensitivity of commercial purity Aluminium (Al 99.5) and of α-iron, with both conventional (CG) and ultrafine (UFG) grain sizes, are investigated by compression and tension tests at different temperatures. Microstructural investigations were performed before and after compression testing in order to investigate the microstructural stability. Pronounced strain rate sensitivity was found for UFG Al at room temperature as well as at elevated temperatures, while for UFG α-iron an enhanced strain-rate sensitivity was found only at elevated temperatures.

Cyclic Deformation and Fatigue Properties of Ultrafine Grain Size Materials: Current Status and Some Criteria for Improvement of the Fatigue Resistance

In this review, some general conclusions based on studies performed to date on fatigued materials of ultrafine grain (UFG) size produced by equal channel angular pressing (ECAP) are drawn, and open issues of current interest are defined. Important aspects addressed include the apparent discrepancy between improved fatigue strengths in Wohler (S-N) plots as opposed to inferior fatigue strengths in Manson-Coffin plots, the clarification of the microstructural mechanisms of severe cyclic softening in conjunction with dynamic (local) grain/subgrain coarsening and damaging large-scale catastrophic shear banding. The important roles of the cyclic slip mode, the friction stress, the crystal structure and the temperature of cyclic deformation with respect to stable cyclic deformation behaviour are emphasized. Based on such considerations, criteria are formulated that must be observed, when designing ECAP-processed UFG-materials of superior fatigue strength.

Deformation behaviour, microstructure and processing of accumulative roll bonded aluminium alloy AA6016

Abstract The technologically relevant aluminium alloy AA6016 was successfully processed using accumulative roll bonding to 8 cycles (∊von Mises = 6.4) in order to obtain an ultrafine-grained microstructure with an average grain size of approximately 200 nm. With the aim of optimising the accumulative roll bonding process detailed investigations on the robustness, the type of rolling mill, the influence of temperature, the microstructural evolution and the mechanical properties have been carried out. Processing at 230 °C provided a good compromise between thermal stability and interlamellar bonding. Samples strained up to 6 cycles (∊von Mises = 4.8) showed an increase in the yield strength by a factor of 3 in comparison to the as-received material. The ductility of the roll bonded samples was slightly sacrificed, although an increase in ductility can be achieved by increasing the number of accumulative roll bonding cycles. The mechanical properties depend on the strain rate, as has also been found for many other ultrafine-grained materials. Annealing of ultrafine-grained samples revealed a stability limit of approximately 200 °C.

Strain Rate Sensitivity of Ultrafine Grained FCC- and BCC-Type Metals

The dependence of the strain rate sensitivity (SRS) of α-Fe and Al 99.5, as typical representatives of fcc- and bcc-type metals, on the testing temperature and with respect to the microstructure is investigated. In particular, the differences between conventional grain size (CG) and ultrafine grain size (UFG) are pointed out. UFG Al 99.5 generally shows an elevated SRS compared to CG Al 99.5. In case of α-Fe the SRS of the UFG state is decreased at room temperature, but increased at 200 °C, compared to the CG state. It is shown that the SRS also influences the ductility of UFG-metals in tensile tests.

Influence of grain boundaries on the deformation resistance: insights from an investigation of deformation kinetics and microstructure of copper after predeformation by ECAP

Abstract The grain structure of coarse-grained Cu of different degrees of purity was modified by plastic deformation by , 4 and 8 passes of equal channel angular pressing (ECAP) at an ambient temperature with and without back pressure. The effect of this modification on deformation resistance was tested in rate change tests in compression at elevated temperatures up to 473. Electron microscopy and compressive strength evolution with strain confirmed that static recrystallization leads to grain coarsening during heating to test temperatures of 423 for and 473 for and that discontinuous grain coarsening by dynamic recry- stallization sets in already at . The influence of the grain structure on the deformation resistance is discussed in terms of a control loop considering generation and loss of free dislocations. The magnitude and rate dependence of the maximal deformation resistance of unrecrystallized ECAPed Cu were explained in terms of dynamic recovery at low- and high-angle boundaries as proposed previously. The interpretation sheds doubt on the common interpretation of rate change experiments in terms of thermally activated glide.

Strain Rate Sensitivity of Ultrafine Grained Aluminium Alloy AA6061

The strain rate sensitivity of the aluminium alloy AA6061 has been investigated in a conventional grain sized (CG) state and in two different ultrafine grained (UFG) conditions processed by Equal Channel Angular Pressing (ECAP) for 2 and 6 passes at 100o C. Strain rate jump tests in compression were performed at different temperatures and the strain-rate sensitivity exponent m was determined. The tests were accomplished by microstructural investigations before and after compression testing in CG and UFG conditions. It is shown that all UFG microstructures exhibit strongly increased strain-rate sensitivity (SRS) compared to the CG state. The SRS increases with increasing temperature and is more pronounced for the UFG material processed using 6 ECAP passes. The microstructural investigations show a rather high stability of the grain structure for the UFG conditions up to 250o C. The results are discussed with respect to the relevant deformation mechanisms.

Microstructure and Mechanical Properties of Accumulative Roll Bonded AA6014/AA5754 Aluminium Laminates

Among the well-known methods of severe plastic deformation the accumulative roll bonding (ARB) process is most promising for producing ultrafine-grained (UFG) materials with extraordinary mechanical properties at an industrial scale. Besides, it has also been shown that the ARB process can be successfully used to produce multi-component materials with tailored properties by reinforcement or grading, respectively. In this work, laminates with alternating layers of the high strength aluminium alloy AA5754 and the AA6014 alloy, well-known for good formability and high surface quality, were produced by ARB at 230 °C. Microstructural and mechanical investigations were performed after 2, 4 and 6 ARB cycles by means of light and electron microscopy, nanoindentation experiments and tensile testing. After ARB processing an ultrafine-grained microstructure is obtained. The UFG microstructure as well as the local mechanical properties alter with the layer composition. With increasing number of ARB cycles the interfaces between the layers become more and more wavy by shear band formation. Compared to the pure accumulative roll bonded AA6014 the yield and ultimate tensile strength of the multi-component laminates are considerably higher and are only slightly reduced in comparison to the high strength AA5754. In terms of elongation to failure no reduction in ductility is found. The serrated yielding effect, clearly visible in AA5754, is shifted to higher strains or fully disappears, respectively, whereas in AA5754 the magnitude of serrations increases with increasing number of ARB cycles. Combining AA5754 and AA6014 sheets by ARB results in well bonded ultrafine-grained laminates which exhibit a combination of the beneficial properties of the single-component materials: high strength of AA5754 and good surface quality of AA6014.

Bimodal grain size distributions in UFG materials produced by SPD: Their evolution and effect on mechanical properties

The mechanical properties of bulk ultrafine-grained materials produced by severe plastic deformation can be modified (sometimes enhanced) by a mild annealing treatment which leads, in some cases, to a bimodal grain size distribution, characterized by a good combination of strength and ductility. Bimodal grain size distributions can also evolve during cyclic deformation at rather low homologous temperature. Here, the conditions under which bimodal grain size distributions evolve and how they affect the mechanical properties, as studied by the authors and as reported so far in the literature, will be reviewed and discussed.