Strain Rate Contribution due to Dynamic Recovery of Ultrafine-Grained Cu−Zr as Evidenced by Load Reductions during Quasi-Stationary Deformation at 0.5 Tm

During quasi-stationary tensile deformation of ultrafine-grained Cu-0.2 mass%Zr at 673 K and a deformation rate of about 10−4 s−1 load changes were performed. Reductions of relative load by more than about 25% initiate anelastic back flow. Subsequently, the creep rate turns positive again and goes through a relative maximum. This is interpreted by a strain rate component ϵ˙− associated with dynamic recovery of dislocations. Back extrapolation indicates that ϵ˙− contributes the same fraction of (20±10)% to the quasi-stationary strain rate that has been reported for coarse-grained materials with high fraction of low-angle boundaries; this suggests that dynamic recovery of dislocations is generally mediated by boundaries. The influence of anelastic back flow on ϵ˙− is discussed. Comparison of ϵ˙− to the quasi-stationary rate points to enhancement of dynamic recovery by internal stresses. Subtraction of ϵ˙− from the total rate yields the rate component ϵ˙+ related with generation and storage of dislocations; its activation volume is in the order expected from the classical theory of thermal glide

Characterization of grain boundary deformation processes in commercial purity titanium using nanoindentation and crystal plasticity modeling

The nature of plastic deformation across grain boundaries in α-titanium has been studied using instrumented sphero-conical nanoindentation in complement with crystal plasticity modeling. Following crystal orientation mapping using electron backscattered diffraction, indention was preformed (i) in the center of grains to mimic single crystal indentation and (ii) near grain boundaries to carry out bicrystal experiments. The pile-up topography that forms as a function of nanoindentation axis was characterized using atomic force microscopy (AFM). Crystal plasticity finite element simulations were carried out iteratively by varying the slip parameters until a best fit was found between the experimentally measured single crystal pile-up topographies and the simulated single crystal pile-up topographies of several orientations. AFM characterization of bicrystal nanoindents revealed changes in the pile-up topographies compared to those for single crystal nanoindents carried out in the same grain. In general, the pile-up topographies were not strongly influenced by low angle grain boundaries, indicating significant strain transfer across the boundaries. In contrast, many high angle boundaries strongly influenced the pile-up morphology, with changes in the pile-ups noted in both the grain containing the pile-up and in the neighboring grain. The ability to accurately simulate the influence of grain boundaries pile-up development was examined by carrying out CPFE simulations of bicrystal nanoindentation. Direct comparison of the AFM-measured topographies with simulated topographies reveals that the general nature of bicrystal pile-up topography is well simulated, but that in general, the measured pile-up transfer is somewhat smaller than the simulated pile-up transfer. The results of the study will be discussed in terms of a number of generalized slip transfer criteria. This research was supported the National Science Foundation through a Materials World Network Grant DMR-1108211 and the complementary Deutsche Forchungsgemeinschaft grant ZA523/3-1

Nondestructive determination of subsurface grain morphology

Recent progress in experimental and numerical methods enables to scrutinize simulated polycrystal surface micromechanics at high spatial resolution. For the correct interpretation of similarities and deviations between experiment and simulation, the consideration of subsurface grain morphology is imperative because of its significant impact on the surface layer boundary condition. A novel method is presented that coarsely scans a relatively large area for subsurface crystallite orientation up to depths of ~0.2 mm by means of differential aperture X-ray microscopy. The resulting point set is categorized into grains according to proximity in physical and orientation space. Reconstruction of the subsurface grain structure starts with a Voronoi tessellation using the categorized set as seed points. Progressive smoothing of the resulting ragged grain boundary surfaces is achieved through mean curvature flow. As it turns out that the reconstruction quality of the bulk and on the surface are related, the latter can serve as guidance for optimum subsurface reconstruction

Quasi-Stationary Strength of ECAP-Processed Cu-Zr at 0.5 Tm

The influence of the grain structure on the tensile deformation strength is studied for precipitation-strengthened Cu-0.2%Zr at 673K. Subgrains and grains are formed by equal channel angular pressing (ECAP) and annealing. The fraction of high-angle boundaries increases with prestrain. Subgrains and grains coarsen during deformation. This leads to softening in the quasi-stationary state. The initial quasi-stationary state of severely predeformed, ultrafine-grained material exhibits relatively high rate-sensitivity at relatively high stresses. This is interpreted as a result of the stress dependences of the quasi-stationary subgrain size and the volume fraction of subgrain-free grains

A study on nanosized certium oxides systems for environmental catalysis

NR 2014080

In-situ characterization of twin nucleation and correlated crystal plasticity modeling of deformation behavior in alpha titanium using 3D X-ray diffraction data

The goal of this study is to gain a clearer understanding of the conditions that favor nucleation of T1 twinning in commercial purity titanium. A tensile specimen with a strong c-texture was deformed to about 2% global strain. Concurrently, far field 3D X-ray diffraction from the synchrotron source at the APS was used to identify type 1 (T1) deformation twinning events. Identification of the twin-parent grain pairs was done using criteria for c and a axes misorientation and spatial proximity. Two approaches are used to assess the geometrical plausibility of a twin resulting from slip transfer (S+T); or twin induced shear transfer (T+T) across a grain boundary. The first step involved calculation of the slip transfer parameter. Second, the relative elevation (along the tensile direction) of the parent grain with respect to the adjacent grain(s) is examined. In addition, Voronoi tessellation is used to generate a representative volume element of the microstructure. Local stress–strain data from a spectral crystal plasticity based model is compared to experimental values, to identify local stress states associated with nucleation of a twin