Röntgenweitwinkeldiffraktometer
Final Report Abstract
Shear softening of grain boundaries in nanocrystalline Pd: Nanocrystalline materials are polycrystals with grain sizes L of the order of 10 nm. Unlike conventional polycrystals, the volume fraction of matter in the core region of grain boundaries tends to approach the share of matter in the interior of the nanometer-sized grains since it scales as 1/L. The discovery of room-temperature grain growth in nanocrystalline Pd makes it possible to monitor the coarsening of the microstructure and to measure simultaneously, or even in situ, the concomitant evolution of properties. From the resulting scaling behavior of properties we deduced interface elastic moduli, using ultrasonic techniques. In particular, we find a 30% shear softening of grain boundaries and discuss its relation to the shear softening observed in bulk metallic glasses. Nanocrystalline metals go ductile under shear deformation: Nanocrystalline Pd90Au10 (grain size <10 nm) has been deformed under dominant shear and superim-posed compression to large plastic strains (ε> 20 %). Taking stress strain curves at different strain rates (10−4 < ˙ ε< 100) allowed us to extract the shear activation volume and strain rate sensitivity as a func-tion of plastic strain, which are of the order of 4b3 and 0.035, respectively. Particularly, the plastic strain-dependent shear activation volume evolves through a maximum value, which relates to yield-ing. Analyzing the strain hardening rate as a function of applied stress enables to determine strain rate dependent macroyield stresses. Scaling of the applied stress with the macroyield stress yields in addi-tion also the strain rate dependent onset stresses of microyield. Surprisingly, strain hardening associated with interfacial deformation modes dominates the stage of microplasticity, whereas, in the macroplastic regime the strain hardening rate saturates at strains >10% to approach a more or less generic value. Kinematics of polycrystal deformation by grain boundary sliding: We analyze the macroscopic deformation of a polycrystalline solid due to local deformation events in the core of grain boundaries. The central result is an equation that decomposes the effective macroscopic strain into contributions from three deformation modes, namely: (i) the elastic strain in the bulk of the crystallites; (ii) the results of dislocation glide and climb processes; and (iii) the deformation events in the grain boundary core. The latter process is represented by jumps in the displacement vector field that can be decomposed into tangential (“slip”) and normal (“stretch”) components. The relevant measure for the grain-boundary-mediated deformation is not the displacement jump vector but a grain-boundary discontinuity tensor that depends on the displacement jump and on the orientation of the grain boundary normal. Accommodation processes at triple junctions do not contribute significantly to the macroscopic strain. By means of example, the theory is applied to the effective elastic response of nanocrystalline materials with an excess slip compliance at grain boundaries. The predictions, specifically on the size dependence of the Poisson ratio, agree with recent experiments on nanocrystalline Pd. The value of the slip compliance for grain boundaries in Pd is obtained as 18 pm GPa_1. Neutron scattering study of the magnetic microstructure of nanocrystalline gadolinium: We report grain-size-dependent results on nanocrystalline bulk Gd obtained by magnetic small-angle neutron scattering (SANS) and magnetometry. This approach allows one to study systematically how the magnetic microstructure of this rare-earth metal is affected by defects in the atomic microstructure, which are largely present in nanocrystalline materials, predominantly in the form of grain boundaries. The neutron scattering data reveal two types of angular anisotropies in the magnetic-field-dependent scattering cross section that are typically not seen in the coarse-grained polycrystal. In particular, a cloverleaf-shaped anisotropy and an elongation of the scattering pattern in the direction of the applied magnetic field have been detected. While the first result, which is an exceptional finding even in the nanocrystalline state, can be attributed to pronounced spin disorder in the vicinity of the Gd grain boundaries, the second anisotropy is related to spin misalignment due to the random magnetocrystalline anisotropy within the individual crystallites. Furthermore, we have calculated the correlation function of the spin misalignment from the radially averaged data, which gives access to the characteristic length scales on which the magnetization is perturbed by crystal defects. The results of this real-space analysis independently support the findings from magnetometry and field-dependent SANS.Wide-angle x-ray diffraction data indicate that stacking faults may limit the range of spin-misalignment fluctuations due to random anisotropy in this material. Influence of crystallite size and temperature on the antiferromagnetic helices of terbium and holmium metal: We report on the results of grain-size and temperature-dependent magnetization, specific-heat, and neutronscattering experiments on the heavy rare-earth metals terbium and holmium, with particular emphasis on the temperature regions where the helical antiferromagnetic phases exist. In contrast to Ho, we find that the helical structure in Tb is relative strongly affected by microstructural disorder, specifically, it can no longer be detected for the smallest studied grain size of D = 18 nm. Moreover, in coarse-grained Tb a helical structure persists even in the ferromagnetic regime, down to about T = 215 K, in agreement with angle-resolved photoelectron spectroscopy (ARPES) data, which reveal a nesting feature of the bulk Fermi surface at the L point of the Brillouin zone at T = 210 K. As samples for the ARPES measurements, we used 10-nm-thick single-crystalline Tb films that show a bulk electronic valance-band structure. Thus our ARPES measurements are used to discuss temperature-induced effects observed in the coarse-grained samples.
Publications
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“Magnetic irreversibility, spin-wave excitation and magnetocaloric effect in nanocrystalline gadolinium,” J. Phys. Conf. Ser. 200, 072047 (2010)
Mathew, S., Kaul, S., Nigam, A., Probst, A.-C. and Birringer, R.
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“Mechanical testing via dominant shear deformation of small-sized specimen,” Mat. Sci. Eng. A 528, 526 (2010)
Ames, M., Markmann, J. and Birringer, R
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“Influence of crystallite size and temperature on the antiferromagnetic helices of terbium and holmium metal,” Phys. Rev. B 83, 224415 (2011)
Michels, A., Bick, J.-P., Birringer, R., Ferdinand, A., Baller, J., Sanctuary, R., Philippi, S., Lott, D., Balog, S., Rotenberg, E., Kaindl, G. and Döbrich, K.
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“Kinematics of polycrystal deformation by grain boundary sliding,” Acta Materialia 59(11), 4366 (2011)
Weissmüller, J., Markmann, J., Grewer, M. and Birringer, R.
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“Shear softening of grain boundaries in nanocrystalline Pd,” Acta Mater. 59(4), 1523 (2011)
Grewer, M., Markmann, J., Karos, R., Arnold, W. and Birringer, R.
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“Nanocrystalline metals go ductile under shear deformation,” Mater. Sci. Eng. A 546, 248 (2012)
Ames, M., Grewer, M., Braun, C. and Birringer, R.
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“Neutron scattering study of the magnetic microstructure of nanocrystalline gadolinium,” Phys. Rev. B 85, 094411 (2012)
Döbrich, F., Kohlbrecher, J., Sharp, M., Eckerlebe, H., Birringer, R. and Michels, A.
