Orientation Of Surface Grains Research Articles

Dependence of the nitriding rate of ferritic and austenitic substrates on the crystallographic orientation of surface grains; gaseous nitriding of Fe-Cr and Ni-Ti alloys

Gaseous nitriding of ferritic Fe–Cr and austenitic Ni–Ti solid solutions reveals that the extent of the uptake of dissolved nitrogen depends on the crystallographic orientation of the surface grains of the substrate. In both ferritic and austenitic substrates, the surface nitrogen concentration and the nitriding depth decrease upon increasing the smallest angle between the surface normal and the normal of a {1 0 0} plane of the surface grain considered. This phenomenon could be ascribed to the residual compressive macrostress developed during nitriding which varies as a function of crystallographic orientation of the (surface) grains due to the elastically anisotropic nature of ferrite and austenite solid solutions investigated in this study.

Influence of the crystalline orientations on microcrack initiation in low-cycle fatigue

Present study aims at analyzing the crack initiation in an austenitic stainless steel in low-cycle fatigue. A fatigue test was carried out using a polished specimen. The surface of the specimen was observed in situ during the fatigue test, in order to establish the time of slip activity or crack initiation. After a number of cycles sufficient to initiate small cracks, the test was stopped and the surface observed by scanning electron microscopy. The electron backscattered diffraction technique (EBSD) was used to identify the orientations of surface grains in the central zone of the fatigue specimen. Crack-initiation sites and slip systems associated to the initiated microcracks were identified. The criterion of the maximum Schmid factor explains two-thirds of the cracks initiated in slip systems; however if the favorably oriented slip band with respect to this criterion makes an angle of around 45° to the loading direction, a crack may initiate in another slip system.

Mechanism of size effects in microcylindrical compression of pure copper considering grain orientation distribution

In microscale deformation, the magnitudes of specimen and grain sizes are usually identical, and size-dependent phenomena of deformation behavior occur, namely, size effects. In this study, size effects in microcylindrical compression were investigated experimentally. It was found that, with the increase of grain size and decrease of specimen size, flow stress decreases and inhomogeneous material flow increases. These size effects tend to be more distinct with miniaturization. Thereafter, a modified model considering orientation distribution of surface grains and continuity between surface grains and inner grains is developed to model size effects in microforming. Through finite element simulation, the effects of specimen size, grain size, and orientation of surface grains on the flow stress and inhomogeneous deformation were analyzed. There is a good agreement between experimental and simulation results. The deformation behavior in micro compression of pure copper has been investigated by a modified surface model considering orientation distribution of surface grains and continuity between surface grains and inner grains.

Evolution of surface topography in dependence on the grain orientation during surface thermal fatigue of polycrystalline copper

Surface degradation due to cyclic thermal loading plays a major role in the Accelerating Structures (AS) of the future Compact Linear Collider (CLIC). In this article results on surface degradation of thermally cycled polycrystalline copper as a function of the orientation of surface grains are presented. Samples with different grain sizes were subjected to thermal fatigue using two different methods and were then characterized using roughness measurements and Orientation Imaging Scanning-Electron-Microscopy (OIM-SEM). Samples fatigued by a pulsed laser show the same trend in the orientation-fatigue damage accumulation as the sample fatigued by pulsed Radio-Frequency-heating (RF). It is clearly shown that [1 1 1] surface grains develop significantly more damage than the surface grains oriented in [1 0 0] and three reasons for this behaviour are pointed out. Based on observations performed near grain boundaries, their role in the crack initiation process is discussed. The results are in good agreement with previous findings and can be interpreted on the basis of existing theories. Finally the fatigue effects induced by the two different applied types of thermal cycling are compared.

Microcrack initiation in low-cycle fatigue of an austenitic stainless steel

The present study aims at analyzing the crack initiation in an austenitic stainless steel in low-cycle fatigue. A fatigue test was carried out using a polished specimen. The surface of the specimen was observed in-situ during the fatigue test, in order to establish the time of slip activity or crack initiation. After a number of cycles sufficient to initiate small crack, the test was stopped and the surface observed in Scanning Electron Microscope. Electron backscattered diffraction (EBSD) was used to identify the orientations of surface grains in the central zone of the fatigue specimen. Crack-initiation sites and slip systems associated to the initiated microcracks have been identified.

The Effect of Grain Boundaries on the Surface Rearrangement during Wuestite Reduction within its Range of Existence

Dense polycrystalline wüstite samples equilibrated with 50%CO-CO2 gas mixture for 432 ks has been reduced at 1073 K using 60%CO-CO2 gas mixture to study the effect of grain boundaries on the surface rearrangement process. Orientations of surface grains on the wüstite specimens are measured by applying electron backscattering pattern technique. The development of the surface arrangement away from grain boundaries is significantly larger than that near grain boundaries. Quick establishment of the equilibrium with the reacting gas in the region near grain boundaries at the early stage of the reduction makes it difficult to continue the transportation of Fe2+ ions into deep interiors of wüstite by the volume diffusion process. As a result, the surface rearrangement is retarded at the surface near grain boundaries after the equilibration of the surface. The transportation of Fe2+ ions at the surface layer away from grain boundaries by volume diffusion is small but not negligible so that the surface rearrangement away from the grain boundary is possible to continue until almost the whole sample is equilibrated.

Correlation between Surface Structure and Iron Nucleation in the Wustite Reduction

The effects of the orientations of surface grains on the iron nucleation process during the wustite reduction at 1073 K were studied. The orientation of grains in polycrystalline wustite was measured by applying electron backscattering patterns technique. Comparing the numbers of the formed iron nuclei in the grains with different orientations, the reactivity of iron nuclei formation was established as a function of grain orientation. The highest number of iron nuclei was observed in the grain with the orientation of (649). The nucleation behaviour was discussed based on the BCF (Burton‐Cabrera‐Frank) model of crystal growth. The step density in the surface structure of the grains was found to be the dominant controlling factor for the iron nuclei formation during the wustite reduction process.

Formation of mesoscale roughening in 6022-T4 Al sheets deformed in plane-strain tension

Mesoscale surface-roughening evolution in 6022-T4 Al sheets was investigated using plane-strain tension. The formation of grain-scale hills and valleys and their relation to the morphologies and corresponding orientations of surface grains after deformation were examined experimentally. These observations were analyzed using various approaches based on the Schmid and Taylor crystal plasticity models. It was observed that surface grains with and without slip bands tend to form valleys and hills, respectively, wherever these two types of grains are adjacent to each other along the plane-strain tension direction. When the sample was pulled along the transverse direction, the formation of hills and valleys by unbanded and banded grains was more lineally organized in the plane-strain (rolling) direction than in the sample that was pulled along the rolling direction (RD). Slip banding and valley formation were principally observed in the surface grains with either very few (1 to 2) slip systems of high Schmid factors or with low Taylor factors, in contrast to nonslip-banded and hill-forming surface grains. Quantitative analysis using correlation coefficients showed that the Schmid factor provided slightly better agreement than the Taylor factor in predicting the slip-banding (and valley-forming) and nonslip-banding (and hill-forming) behaviors of surface grains. In addition, measures that quantify the image qualities of electron backscattered diffraction (EBSD) patterns for selected surface grains suggested that the slip-banded and valley-forming grains contain less lattice distortion than the nonslip-banded and hill-forming grains, despite the larger strains experienced by these grains. This indicates that dislocations in the slip-banded grains move out of the surface to create deformation without lattice distortion. Plastic interactions between specific neighboring grains are central to the formation of mesoscale surface roughening.

The Effect of Grain Boundaries on Iron Nucleation during Wuestite Reduction Process

Dense polycrystalline wustite samples equilibrated with 50%CO-CO 2 gas mixture for 432 ks were reduced at 1 073 K using 75%CO-CO 2 to study iron nucleation process. Especially the effects of grain boundaries and grain orientations on the iron nucleation were focused. The orientation of surface grains in the wustite specimens was measured by applying electron backscattering pattern technique. The number of formed iron nuclei near grain boundaries was found to be significantly larger than that away from grain boundaries. The prevalence of the iron nucleation process near grain boundaries was mainly due to the fast transport process of holes and Fe divalent cation by surface and grain boundary diffusion. The dominant effect of the grain boundary on the iron nucleation was found on the all grains with different grain orientations.

Determination of the Orientation of Metal Grains by a New Kossel Line Technique

The orientation of surface grains in a thick crystalline specimen can be determined by means of a Kossel line technique. In this work the Kossel lines are recorded on a cylindrical film coaxial with the electron beam of an electron probe microanalyzer. These Kossel lines are analyzed by a mathematical technique to reveal the orientation of a grain. The technique will also yield a rough crystal analysis of unknown materials; therefore, it may be useful in analyzing inclusions and precipitates in solid specimens.