Andrzej Baczmanski

Changes in the misfit stresses in an Al/SiCp metal matrix composite under plastic strain

Results are presented from neutron diffraction measurement of the strains in each phase, matrix and reinforcement, of a metal matrix composite bar before and after deformation beyond the elastic limit by four-point bending. The strains in each phase have been converted to stress. A stress separation technique was then applied, and the contributing mechanisms separated and identified. In this way the changes in the different contributions owing to plastic deformation have been determined. It is found that, initially, the average phase stresses can be explained in terms of a combination of essentially hydrostatic phase average thermal misfit stresses in the matrix (tension) and particles (compression) combined with a parabolic macrostress from quenching. After plastic bending the change in axial macrostress is as expected for that for a monolithic bar, but unexpectedly the misfit stresses had relaxed to approximately zero in both the tensile and compressive plastically strained regions of the bar.

Examination of the residual stress field in plastically deformed polycrystalline material

Abstract A new method for the determination of residual stresses in plastically deformed polycrystalline materials is presented. This method is based on standard X-ray and neutron diffraction measurements of the 〈d hkl〉 interplanar spacings in different directions. Until now the estimation of the residual stresses was limited to those of the first-order but the method presented takes into account both the first-and second-order stresses.

Microstresses in textured polycrystals studied by the multireflection diffraction method and self-consistent model

A new version of the X-ray diffraction method for determining macrostresses and microstresses in textured polycrystalline material is presented. In this method the lattice strains for various orientation of the scattering vector as well as for various crystallographic planes {hkl} are measured. The interpretation of the experimental data is based on the least-squares fitting procedure, in which the diffraction elastic constants and theoretical values of microstresses are used. The diffraction elastic constants and the microstresses are calculated by the self-consistent model. The new method was successfully applied for stress determination in one- and two-phase steels subjected to elastoplastic deformation, and the significant anisotropy of the incompatibility stresses was observed in textured samples.

Relaxation of thermal mismatch stress due to plastic deformation in an Al/SiCp metal matrix composite

Abstract The neutron diffraction method was applied to determine the evolution of internal strains (and stresses) within an Al/SiCp metal matrix composite. The specimen was subjected to thermal treatment followed by different types of elastoplastic deformation. An Eshelby type model was used to study the contribution of different phenomena to the evolution of stress. The results of the experiment were compared with model predictions obtained using a self-consistent model of elastoplastic deformation. The validity of the use of this model for stress analysis was verified by comparison with a neutron diffraction measurement performed for an external load applied ‘in situ’. Hydrostatic phase stresses were experimentally found in the samples after thermal treatment. These stresses were modified due to plastic deformation, and significant relaxation of the microstresses was observed.

Quantitative estimation of incompatibility stresses and elastic energy stored in ferritic steel

Plastic incompatibility second-order stresses were determined for different orientations of a polycrystalline grain, using X-ray diffraction data and results of the self-consistent elasto-plastic model. The stresses in cold rolled ferritic steel were determined both in as-received and under tensile loaded conditions. It has been shown that the Reuss model and the self-consistent model applied to near surface volume provide the best approaches to determine diffraction elastic constants. For the first time, the elastic energy in an anisotropic material (arising from plastic incompatibilities between grains having various lattice orientations) has been determined. The second-order incompatibility stresses and stored elastic energy are presented in Euler space.

New Type of Diffraction Elastic Constants for Stress Determination

A new method for calculation of the diffraction elastic constants, based on the selfconsistent model, is proposed and tested. This method is especially useful in the interpretation of the results of X-ray measurements since the ellipsoidal inclusion near the sample surface is considered. In X-ray diffraction the information volume of the sample is defined by absorption, causing unequal contribution of different crystallites to the intensity of the measured peak. Consequently, the surface grains participate more effectively in diffraction than the grains which are deeper in the sample.

Neutron time-of-flight diffraction used to study aged duplex stainless steel at small and large deformation until sample fracture

Owing to its selectivity, diffraction is a powerful tool for analysing the mechanical behaviour of polycrystalline materials at the mesoscale (phase and/or grain scale). In situ neutron diffraction during tensile tests and elastoplastic self-consistent modelling were used to study slip phenomena occurring on crystallographic planes at small and large deformation. The critical resolved shear stresses in both phases of duplex stainless steel were found for samples subjected to different thermal treatments. The evolution of grain loading was also determined by showing the large differences between stress concentration for grains in ferritic and austenitic phases. It was found that, for small loads applied to the sample, linear elastic deformation occurs in both phases. When the load increases, austenite starts to deform plastically, while ferrite remains in the elastic range. Finally, both phases undergo plastic deformation until sample fracture. By using an original calibration of diffraction data, the range of the study was extended to large sample deformation. As a result, mechanical effects that can be attributed to damage processes initiated in ferrite were observed.

X-ray grazing incidence technique—corrections in residual stress measurement—a review

Using X-ray grazing incidence diffraction (GID) it is possible to perform a non-destructive analysis of the heterogeneous stress field for different volumes below the surface of the sample. The stress can be measured at very small depths, of the order of a few m. The penetration depth of radiation is almost constant in a wide 2  range for a given incidence angle . It can be easily changed by an appropriate selection of  angle (or also by using a different type of radiation). There are, however, some factors which have to be corrected in this technique. The most important is the refraction of X-ray wave: it changes the wave length and direction of the beam. The both effects modify a pick position. A corresponding correction was calculated and tested on ferrite powder and on 316L austenite stainless steel sample.

Residual stress field in steel samples during plastic deformation and recovery processes

An X-ray diffraction method was applied to measure residual stresses and stored elastic energy in deformed and annealed polycrystalline ferritic and austenitic steel samples. The orientation distribution of plastic incompatibility second-order stresses created during elastoplastic deformation was determined and presented in Euler space. Using deformation models, these stresses were correlated with different types of intergranular interactions occurring in the studied materials. An important decrease of the first- and the second-order residual stresses was observed during recovery and recrystallisation processes. Diffraction peak widths, related to dislocation density, were studied and correlated with stress variation during annealing process. Differences in stress relaxation between ferritic and austenitic samples were explained by different values of the stacking fault energy, which influences dislocation climb and cross-slip.

Corrections for residual stress in X-ray grazing incidence technique

Grazing incidence technique can be used to study samples with important stress gradients. The stress can be measured at very small depths, of the order of a few μm. The penetration depth of radiation is almost constant in a wide 2θ range for a given incidence angle α. It can be changed by an appropriate selection of α angle. This enables the investigation of stress variation with depth below the sample surface. There are, however, some factors which have to be corrected in this technique. The most important one is the X-ray wave refraction: it changes the wave length and direction of the beam inside a sample. These two effects cause some shift of a peak position and they have to be taken into account. For small incidence angles (α≤100) the corrections are significant and can modify the measured stress even of 70 MPa. The refraction correction decreases with increasing of the incidence angle. The corrections were tested on ferrite powder and on the ground AISI316L steel samples.