T. Ungár

Dislocations, grain size and planar faults in nanostructured copper determined by high resolution X-ray diffraction and a new procedure of peak profile analysis

Abstract The particle size and the dislocation structure in inert gas condensed nanocrystalline copper were determined by high-resolution X-ray diffraction profile analysis. Well-behaved smooth curves were obtained in the modified Williamson–Hall plot and the modified Warren–Averbach plot through knowledge of the variation in dislocation contrast with Bragg reflection and the effect of twinning on particle size. The particle size was between 14 and 30xa0nm, in agreement with TEM results. The root-mean-squared strains were explained by the presence of dislocations, with a dislocation density of about 5×1015xa0m−2. The dislocations were found to have screw character probably related to the particle growth mechanism.

An X-ray method for the determination of stored energies in texture components of deformed metals: Application to cold worked ultra high purity iron

Abstract An X-ray method has been developed to evaluate the stored energy of cold work in different texture components of plastically deformed metals. The dislocation density and the outer cut-off radius of dislocations are obtained from Bragg peaks recorded from single texture components. The stored energy is approximated by the energy of dislocations, which is calculated according to the anisotropic theory of elasticity. As an example the method is applied to the case of two major texture components developed in cold rolled ultra high purity (UHP) iron. The stored energy of the {111}〈112〉-γ fibre component of the 88% cold rolled UHP iron is about 3.6 times larger than that of the {001}〈110〉-α fibre component. The present results, of significantly higher accuracy than those of previous methods, are in good agreement with data obtained from microhardness and recent calorimetric measurements.

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%).

Long-range internal stresses in cell and subgrain structures of copper during deformation at constant stress

Long-range internal stresses in dislocation cell and subgrain structures were investigated experimentally. The transition of the dislocation structure from cells to subgrains was achieved by deforming copper polycrystals in compression creep tests at constant stress normalized by the shear modulus in the temperature range from 298 K to 633 K. The long-range internal stresses were investigated by two methods. The first one was the evaluation of characteristically asymmetric X-ray line profiles. The internal stresses are the result of the analysis of the X-ray line profiles. The second one was the measurement of local lattice parameters by convergent beam electron diffraction. The internal stresses can be determined from the changes in the local lattice parameters. The results obtained from both methods show that long-range internal stresses of the same type exist in the cell as well as in the subgrain structures.

Correlation between resistivity increment and volume fraction of G.P. zones in an Al-3·2 wt % Zn-2·2 wt % Mg alloy

Abstract The correlation between resistivity increment caused by G.P. zones and their volume fraction has been investigated in an Al-3·2 wt % Zn-2·2 wt % Mg alloy. It is found that the resistivity increment is a linear function of the 2/3 power of the volume fraction. The results show that the resistivity increase is mainly determined by the total volume of zones and is not sensitive to their size distribution

Scanning X-ray diffraction peak profile analysis in deformed Cu-polycrystals by synchrotron radiation

Abstract Densities and arrangements of dislocations and mesoscopic long-range internal stresses in cold-rolled polycrystalline copper were determined in scanning mode by X-ray peak profile analysis with synchrotron radiation. The high brilliance enabled the use of a focal spot of less than 50xa0μm. The diffraction profiles of the 200 reflections were measured by scanning the specimen step by step in front of the beam. In this way, the heterogeneities of the deformation-induced microstructure within single grains were obtained. At small deformations including stage III, the dislocation densities and the internal stresses were uniform within the grain interior, but larger near the grain boundaries where the dislocations showed a tendency to form stress-intensive arrangements. At higher deformations towards and in stage IV, the dislocation density and the internal stresses increasingly fluctuated within the whole grain. The fluctuations were interpreted as the transformations of polarized dipolar dislocation walls (PDW) into polarized tilt walls (PTW) observed recently by macroscopic diffraction experiments on highly deformed specimens. These results are important for the current research of strengthening processes in stage IV.

On the decomposition behaviour of Al-4.5 at% Zn-2 to 3 at% Mg alloys during continuous heating

The decomposition processes taking place in the Al-4.5 at % Zn-2 to 3 at % Mg alloys were studied during continuous heating by means of electrical resistivity, XSAS and DSC measurements and by TEM investigations. It was found that the room temperature pre-ageing has no significant influence on the processes taking place above 230° C. Several temperature ranges were determined in which the decomposition of the solid solution and/or the transformation of the different particles of the second phases take place by different mechanisms.

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.

Hardening mechanisms in Al-Sc alloys

The hardening mechanism in Al-Sc alloys with scandium content of 0.11 and 0.19 at% is studied. Applying theoretical results due to the yield stress and work hardening of two phase alloys as a function of volume fraction and precipitate particle size, it is shown that after ageing at above 300° C the Orowan mechanism operates in these alloys. Using the experimental results, the volume fraction and average radius of the precipitate particles are determined.

Dislocation arrangement and residual long-range internal stresses in copper single crystals at large deformations

Copper single-crystals were wire drawn far into stage III and into the onset of stage IV. TEM investigations revealed the well known dislocation cell structure. High resolution X-ray diffraction line profile analysis was performed on the deformed specimens. The diffraction experiments have provided characteristically asymmetric line profiles which were evaluated for the dislocation densities and arrangements and for the residual long-range internal stresses prevailing in a dislocation cell structure. The formal dislocation densities were found to be spatially anisotropic and were interpreted in terms of anisotropic Burgers vector populations. The residual long-range internal stresses were found to form plane-stress states in conformity with the applied stress conditions. A microscopic model has been suggested accounting for the density and arrangement of dislocations and for the residual long-range internal stresses, at the same time, in a self consistent manner.