M. Zehetbauer

Bulk nanostructured materials

The processing and mechanical behaviour of bulk nanostructured materials are one of the most interesting new fields of research on advanced materials systems. Many nanocrystalline materials possess very high strength with still good ductility, and exhibit high values of fatigue resistance and fracture toughness. There has been continuing interest in these nanomaterials for use in structural and biomedical applications, and this has led to a large number of research programs worldwide. This book focuses on the processing techniques, microstructures, mechanical and physical properties, and applications of bulk nanostructured materials, as well as related fundamental issues. Only since recently can such bulk nanostructured materials be produced in large bulk dimensions, which opens the door to their commercial applications.

Fundamentals of Superior Properties in Bulk NanoSPD Materials

Bulk nanoSPD materials are materials with nanostructural features, such as nanograins, nanoclusters, or nanotwins, produced by severe plastic deformation (SPD) techniques. Such nanostructured materials are fully dense and contamination free and in many cases they have superior mechanical and functional properties. Here, we provide a critical overview of such materials, with a focus on the fundamentals for the observed extraordinary properties. We discuss the unique nanostructures that lead to the superior properties, the underlying deformation mechanisms, the critical issues that remain to be investigated, future research directions, and the application potential of such materials.

Deformation Induced Vacancies with Severe Plastic Deformation: Measurements and Modelling

In discussing hardening characteristics in terms of crystalline lattice defects, in most cases the properties and kinetics of dislocations and their arrangement have been considered. However, during plastic deformation also vacancies and/or vacancy type defects are produced in very high densities which are typically close to those of vacancies in thermal equilibrium at the melting point. The effect of high vacancy concentrations on the hardening characteristics is twofold: (i) direct effects by impeding the movement of dislocations (ii) indirect one by inducing climbing and annihilation of edge dislocations leading to softening or even absolute decreases in strength. This paper presents first measurements of deformation induced vacancies in SPD materials which have been achieved by combined evaluation of resistometry, calorimetry and X-ray diffraction. The density of vacancies during and after SPD deformation is found to be markedly higher than in cases of conventional deformation and/or coarse grained material which may be partly attributed to the particular conditions of SPD namely the enhanced hydrostatic pressure as well as the changes in deformation path. It is suggested to make this high vacancy concentration responsible for both dynamic and static recovery and/or recrystallisation processes recently found during and after SPD, being potential reasons for enhanced ductility and superplasticity which only occur with nanomaterials originating from SPD. Recent publications show that in alloys, SPD induced vacancies can also enable the existence of phases which do not appear in the equilibrium diagram.

Measurement of screw and edge dislocation density by means of X-ray Bragg profile analysis

Abstract The type of dislocations produced during plastic deformation is necessary to know for the fundamental understanding anisotropy of hardening processes. By studying X-ray line broadening in several reflections, the fraction of certain dislocation types can be determined experimentally. The contribution of strain to line broadening is generally anisotropic. On the basis of the dislocation model, the modified Williamson–Hall and Warren–Averbach procedures have been suggested where g and g2 are replaced by g C and g2 C , with C as the average dislocation contrast factor. C can be evaluated theoretically for different crystal systems and different dislocation types, i.e. edges and/or screws, by numerical methods. On the other hand, by analyzing strain anisotropy C can be determined from experiment. Comparing experimental with theoretical C-values the specific fraction of dislocation types can be determined. In the present paper this procedure has been carried out for fine grained Cu 99.9% for deformation well into stage III. The ratio starts with a high fraction (90%) of screw dislocations. During deformation up to e=0.3 this picture changes in favor of edge dislocations (75%).

Thermal stability and phase transformations of martensitic Ti-Nb alloys.

Abstract Aiming at understanding the governing microstructural phenomena during heat treatments of Ni-free Ti-based shape memory materials for biomedical applications, a series of Ti–Nb alloys with Nb concentrations up to 29 wt% was produced by cold-crucible casting, followed by homogenization treatment and water quenching. Despite the large amount of literature available concerning the thermal stability and ageing behavior of Ti–Nb alloys, only few studies were performed dealing with the isochronal transformation behavior of initially martensitic Ti–Nb alloys. In this work, the formation of martensites (α′ and α″) and their stability under different thermal processing conditions were investigated by a combination of x-ray diffraction, differential scanning calorimetry, dilatometry and electron microscopy. The effect of Nb additions on the structural competition in correlation with stable and metastable phase diagrams was also studied. Alloys with 24 wt% Nb or less undergo a transformation sequence on heating from room temperature to 1155 K. In alloys containing >24 wt% Nb α″ martensitically reverts back to β0, which is highly unstable against chemical demixing by formation of isothermal ωiso. During slow cooling from the single phase β domain α precipitates and only very limited amounts of α″ martensite form.

Thermal expansion of skutterudites

The current paper gives an overview of the newly obtained thermal expansion coefficients of skutterudites as well as those so far available in literature. Thermal expansion was determined for CoSb3, Pt4Sn4.4Sb7.6, for As- and Ge-based skutterudites as well as for various high-ZT skutterudites (micro- and nanostructured) with didymium (DD) and mischmetal (Mm) as filler atoms in frameworks of (Fe1−xCox)4Sb12 and (Fe1−xNix)4Sb12, and for double and triple-filled skutterudites such as Ca0.07Ba0.23Co3.95Ni0.05Sb12 and Sr0.025Ba0.075Yb0.1Co4Sb12. For low temperatures, a capacitance dilatometer was used (4–300 K), whereas for temperatures 300<T<750 K, a dynamic mechanical analyzer was employed. For a set of Ge-, P-, and Sb-based skutterudites, lattice parameters of single crystals, measured at three different temperatures, were used to derive the thermal expansion coefficient. The semiclassical model of Mukherjee [Phys. Rev. Lett. 76, 1876 (1996)] has been successfully used to quantitatively describe the thermal...

Work hardening and microstructure of AlMg5 after severe plastic deformation by cyclic extrusion and compression

Deformation by cyclic extrusion/compression in AlMg5 leads to the same stages of work hardening as unidirectional deformation. The analogy is confirmed by studies of the microstructure, by analysis of long range internal stresses and by evaluation of dislocation densities. The strains leading to the various stages of work hardening are much higher than those in conventional deformation modes while the dislocation densities in the stages are about the same. The strain shift in cyclic extrusion/compression is attributed to the reversal of strain path. The resulting subgrain size is smaller than that resulting from conventional deformation modes which seems to be a consequence of the higher hydrostatic pressure of cyclic extrusion/compression.

Vacancy concentrations determined from the diffuse background scattering of X-rays in plastically deformed copper

The background intensity of X-ray diffraction patterns is analysed in terms of point defects, especially vacancies, in poly- and single-crystalline copper specimens. The samples have been deformed by compression in-situ in a synchrotron peak profile experiment. Systematic comparative analysis of X-ray, electrical resistivity, and calorimetric measurements indicate that (i) point defects, especially vacancies, are produced during plastic deformation and (ii) that the point defect concentration is increasing concomittantly with the actual dislocation density. The point defect production rate in poly- and single-crystalline specimens is observed to be drastically different. This difference is interpreted as different vacancy production rates in the grain interior and in the grain-boundary regions. With increasing deformation, the vacancy concentration in the grain-boundary regions is found to approach the equilibrium values corresponding to the melting temperature of copper. This result would support the assumption that in severely deformed metals the grain boundary region is a highly distorted, almost amorphous phase of the material.

Dislocation densities and internal stresses in large strain cold worked pure iron

Abstract Polycrystalline samples of Fe-0.005%-C were deformed by torsion at 300 K far into stage IV of deformation and investigated by X-ray peak profile analysis (XPA) for the microstructural evolution. The long-range internal stresses Δτ w −δτ c (τ w , τ c are the shear stresses in the cell wall and cell interior regions) pass through a maximum at the onset of stage IV, but reincrease within stage IV at higher deformation. A similar maximum is observed in the formal dislocation density derived directly from XPA which, however, is not seen in residual electrical resistivity. These results can be consistently explained by the assumption that in stages II and III the cell walls are formed as polarized dipole walls (PDW) which in stage IV change into polarized tilt walls (PTW), similarly to recent findings in cold rolled Cu. The significant constancy of total volume fraction of cell walls as well as of specific internal stresses in stage IV confirm the importance of the PTW s for stage IV strengthening.

TEM investigation of multidirectionally deformed copper

Abstract Polycrystalline samples of Cu 99.95% were deformed by equal channel angular (ECA) pressing by rotating the sample after each pass and by subsequent compression or combined compression torsion. The resulting microstructure was analysed by TEM methods. In case of pure ECA deformation the grain sizes remain unchanged with the number of passes; the misorientation of the grains increases and the distribution of grain sizes gets narrower. When applying additional deformations subsequent to ECA pressing both grain sizes and the number of adjacent grains with large misorientations increase. The changes in distribution of grain sizes depend on the mode of deformation following the ECA deformation.