Reinhard Pippan

Technical parameters affecting grain refinement by high pressure torsion

Abstract High pressure torsion is a well known and widespread processing technique for severe plastic deformation. The aim of high pressure torsion and other comparable techniques is to obtain ultrafine-grained or even nanocrystalline materials with enhanced mechanical and physical properties compared with their coarse-grained counterparts. Generally this refinement process is strongly influenced by processing parameters such as temperature or accumulated strain, but can also simply be affected by the entire experimental setup. Therefore, the benefits and limitations of the process with regard to grain refinement, homogeneity and specimen size, underlined with experimental results using different tools, will be discussed.

Characterization of the fracture toughness of micro-sized tungsten single crystal notched specimens

Fracture experiments using micrometer-sized notched cantilevers were conducted to investigate the possibility of determining fracture mechanical parameters for the semi-brittle material tungsten. The experiments were also used to improve the understanding of semi-brittle fracture processes for which single crystalline tungsten serves as a model material. Due to the large plastic zone in relation to the micrometer sample size, linear elastic fracture mechanics is inapplicable and elastic-plastic fracture mechanics has to be applied. Conditional fracture toughness values J Q were calculated from corrected force vs. displacement diagrams. Crack growth was accessible by direct observation of in-situ experiments as well as with the help of unloading compliances. As a further tool, fracture toughness can be determined via crack tip opening displacement. The micro samples behave more ductile and exhibit higher fracture toughness values compared to macro-sized single crystals and fail by stable crack propagation.

Advantages and Limitations of HPT: A Review

The improvements in the design of the HPT tools lead to a well defined torsion deformation and permits, therefore, a comparison with other SPD-techniques. The design of the tools, the advantages and disadvantages of HPT, as well as the limitation in the sample size are discussed.

Influence of loading frequency on the high cycle fatigue properties of AlZnMgCu1.5 aluminium alloy

Abstract Fatigue investigations of AlZnMgCu1.5 aluminium alloy have been performed with conventional testing equipment (cyclic frequency 100 Hz) and with ultrasonic equipment (20 kHz). No statistically significant influence of cyclic frequency on lifetimes was found in the investigated regime, i.e. cycles to failure above 10 5 . Different heat treating influenced lifetimes and near threshold crack growth properties, and fatigue properties of AlZnMgCu1.5-T6 were superior to AlZnMgCu1.5-T66 and AlZnMgCu1.5-T64. Fatigue crack propagation in the range of approximately 10 −9 m/cycle and below measured at ultrasonic frequency is affected by air humidity, since growth rates are below the mean diffusion distance of hydrogen during one cycle. In ambient air, a minimum growth rate of a propagating crack of approximately 10 −10 m/cycle was found, whereas crack propagation rates may be as low as 10 −12 m/cycle in a vacuum. Threshold values of AlZnMgCu1.5-T6 in ambient air and in a vacuum are 1.5–1.55 MPa√m and 2.7–2.95 MPa√m, respectively.

An argument for a cycle-by-cycle propagation of fatigue cracks atsmall stress intensity ranges

Abstract A model is presented in which the plastic strains around a growing fatigue crack are discretized in the form of geometrically necessary dislocations. The model explains the existence of a threshold and the near threshold regime. It also explains the minimum value of striation spacing with decreasing amplitude of stress intensity. The model corresponds to a “cycle-by-cycle” crack growth at all amplitudes of stress.

Determination of the length dependence of the threshold for fatigue crack propagation

Abstract A rising load amplitude crack growth test on specimens pre-cracked in cyclic compression is presented as a procedure to determine the length dependence of the threshold of fatigue crack propagation described by the R (resistance)-curve for the threshold of stress intensity factor range. The experimental results show that the residual stress field in front of the pre-crack can significantly influence the R -curve. In order to measure the material specific R -curve which is not affected by the pre-cracking condition it is important to use the smallest possible load amplitude. To achieve this goal, a very small notch root radius is essential. It is shown that at notches machined by razor blade polishing technique the load amplitude for pre-cracking can be reduced to values where the load history does not influence the R -curve for the threshold of stress intensity range.

Generation of metallic nanocomposites by severe plastic deformation

Abstract In this article, the potential to fabricate composites with ultrafine grained structures, as well as composites with a nanostructure, by different severe plastic deformation methods, is reviewed. A broad spectrum of diverse composites produced by severe plastic deformation methods exist which include metal–metal composites, metal matrix composites and amorphous metal matrix composites. Furthermore, the influence of the strain path and initial structure on the final composite material is outlined.

Threshold and effective threshold of fatigue crack propagation in ARMCO iron I: The influence of grain size and cold working

The influence of grain size and cold working on the threshold of fatigue crack growth was studied in ARMCO iron (grain size, 3–3000 μm; degree of cold working, 0–90%). The role of crack tip shielding due to crack closure mechanisms and crack deflection was examined. The fracture surface roughness is influenced by the grain size and especially by the degree of cold working. However, the effective threshold ΔKeff th for all the microstructures is about 2.75 MPa m12. The increase in ΔKth with increasing grain size at R = 0.1 is attributed to the increase in crack tip shielding due to crack closure.

Homogeneous Cu–Fe supersaturated solid solutions prepared by severe plastic deformation

A Cu–Fe nanocomposite containing 50 nm thick iron filaments dispersed in a copper matrix was processed by torsion under high pressure at various strain rates and temperatures. The resulting nanostructures were characterized by transmission electron microscopy, atom probe tomography (APT) and Mössbauer spectrometry. It is shown that α-Fe filaments are dissolved during severe plastic deformation leading to the formation of a homogeneous supersaturated solid solution of about 12 at% Fe in fcc Cu. The dissolution rate is proportional to the total plastic strain but is not very sensitive to strain rate. Similar results were found for samples processed at liquid nitrogen temperature. APT data revealed asymmetric composition gradients resulting from deformation-induced intermixing. On the basis of these experimental data, the formation of the supersaturated solid solutions is discussed.

Cyclic high-pressure torsion of nickel and Armco iron

Cyclic high-pressure torsion, a modified version of high-pressure torsion, is applied to Armco iron and nickel. The results in terms of microstructure and flow stress are compared to samples deformed by conventional high-pressure torsion. For both processes and both materials, a saturation in the decrease of the structure size and the increase in the flow stress is observed. The minimum size of the structural elements which is obtainable is smallest for the conventionally high-pressure torsion deformed samples and increases with decreasing strain per cycle in cyclic high-pressure torsion.