Stress Induced Boundary Migration Research Articles

Fostering deep drawability through recrystallization texture strengthening in aluminum-rich interstitial free steel

The development of strong normal direction (ND)║〈111〉 fiber texture is crucial for deep drawability in interstitial free steels used for automotive applications. The present work focuses on comparative investigations on texture development, Lankford parameter measurement, microstructural evolution, recrystallization kinetics, and grain growth kinetics of titanium‑niobium stabilized interstitial free steels with different aluminum (Al) content. Industrially hot-rolled interstitial free steels with 0.06 wt% Al and 0.17 wt% Al were cold-rolled (∼90%), followed by isothermal annealing at 650 °C, 675 °C, and 700 °C for various time intervals. Texture studies using electron backscatter diffraction and X-ray diffraction showed the formation of a stronger ND fiber texture in 0.17 wt% Al steel compared to 0.06 wt% Al Steel after full recrystallization. A high r m value in 0.17 wt% Al steel than in 0.06 wt% Al Steel indicates the beneficial effects of Al addition in interstitial free steel. The study of recrystallization kinetics depicted faster recrystallization kinetics in interstitial free steel with 0.17 wt% Al. The grain growth kinetics of recrystallized grains illustrated similar behavior in both steels. At the onset of recrystallization in 0.06 wt% Al steel, a transient strengthening of the {001} 〈110〉 component (an effect of stress-induced boundary migration) was observed, which obstructed the nucleation of ND fiber grains. In contrast, the initial rapid nucleation of ND fiber grains in the 0.17 wt% Al steel was responsible for stronger ND fiber texture development. • Aluminum-interstitial-free steel developed stronger {111} <uvw> fiber texture. • Aluminum-interstitial-free steel showed high r m value. • Aluminum-interstitial-free steel possesses faster recrystallization kinetics. • Strain-induced boundary migration is dominant initially in normal interstitial free steel. • Rapid nucleation of {111} <uvw> grains occurs in Aluminum-interstitial free steel.

Correlation between the mechanical properties and the 〈110〉 texture in a hot-rolled near β titanium alloy

Correlation between mechanical properties and texture characteristics was investigated in a hot-rolled Ti-7333 alloy. After rolling deformation, two strong fiber-textures with 〈100〉 parallel to rolling direction (RD) and 〈110〉 parallel to RD were determined. The mechanism of stress-induced boundary migration mainly controls the nucleation of recrystallized grains holding the similar orientation with the adjacent deformed grains on the bulged grain boundaries. It was revealed that the 〈110〉 fiber-texture with higher Schmid factor (SF) possesses the high deformation ability along RD.

Grain Boundary Dynamics under an Applied Stress

The response of various structurally different planar grain boundaries in aluminum bicrystals to an applied stress was experimentally investigated. Stress induced boundary migration was observed to be coupled to a tangential translation of the grains for symmetrical and asymmetrical and tilt boundaries with both low and high misorientation angles. The activation enthalpy of high angle boundary migration was found to vary non-monotonously with misorientation angle, whereas for low angle boundaries the migration activation enthalpy was virtually the same. The Σ7 CSL boundaries in bicrystals of different geometry were observed to move under an applied stress, but their migration did not produce shear. These crystallographically equivalent boundaries, however, were found to behave different with respect to migration rate and its temperature dependence. The stress driven migration of the mixed tilt-twist boundary was observed to be accompanied by both the translation of adjacent grains parallel to the boundary plane and their rotation around the boundary plane normal. This behavior was interpreted in terms of the structure of the investigated tilt-twist boundary.

Stress Induced Grain Boundary Motion in Al Bicrystals

Recent results of experimental research into stress induced grain boundary migration in aluminum bicrystals are briefly reviewed. Boundary migration under a shear stress was observed to be coupled to a lateral translation of the grains for any &lt;100&gt; tilt boundary in the entire misorientation range (0-90°). Measurements of the temperature dependence of coupled boundary migration revealed that there is a specific misorientation dependence of migration activation parameters. Grain boundaries can act during their motion under the applied stress as sources of lattice dislocations that leads to the generation and growth of new grains in the boundary region. The rate of stress induced boundary migration decreases with increasing solute content in aluminum. Both the migration activation enthalpy and the pre-exponential mobility factor were found to increase with rising impurity concentration.

Dynamics of grain boundaries under applied mechanical stress

Recent results of experimental research into stress-induced grain boundary migration in aluminum bicrystals are reported. Boundary migration under a shear stress was observed to be coupled to a lateral translation of the grains for planar symmetrical 〈100〉 tilt boundaries. This coupling proved to be the typical migration mode of any 〈100〉 tilt boundary, no matter whether low- or high-angle, low Σ CSL coincidence or non-coincidence boundary. The measured ratios of normal boundary motion to the tangential displacement of grains are in an excellent agreement with theoretical predictions. The migration-shear coupling is also observed for asymmetrical 〈100〉 boundaries. Measurements of the temperature dependence of coupled boundary migration revealed that there is a specific misorientation dependence of migration activation parameters. Grain boundaries can act during their motion under the applied stress as sources of lattice dislocations that leads to the generation and growth of new grains in the boundary region. The rate of stress-induced boundary migration decreases with increasing solute content in aluminum. Both the migration activation enthalpy and the pre-exponential mobility factor were found to increase with rising impurity concentration.

SEM-EBSD analysis on early stage of recrystallization in Al–0.7%Mg–0.4%Si–0.2%Cr alloy

Intermittent observations have been carried out on an early stage of recrystallization in Al–Mg–Si–Cr alloy with using an SEM-EBSD system. The specimen of Al–0.7%Mg–0.4%Si–0.2%Cr alloy was solution heat treated at 813 K for 3.6 ks, and then mechanically polished and finished by electropolishing. Vickers indentations were applied on the surface for landmarks. The specimen was cold-rolled at R. T. with about 30% of a reduction rate. After an EBSD observation, the specimen was cut into about 5×5×2 mm in size. Annealing the specimen at 673 K with using a molten salt bath, ion-polishing of the surface of the specimen for 180 s, and observing with the SEM-EBSD system were repeated. Intermittent observations on the same area were carried out during annealing up to 60 s. All the grains were rotated by cold-rolling. Deformation was inhomogeneity even in the same grain. Recovery was observed as formation of subgrains after annealing for 3 s. Subgrains having a similar orientation to the mother grain showed subgrain growth with annealing out of heavily deformed region. Such a subgrain growth was followed by formation of small grain without subgrains and surrounded by large angle grain boundary. Stress induced boundary migration was also observed.

Annealing of cold-worked aluminium (1st report)

These studies have been carried out paying the special attension to the heterogeneity of deformation and using the etching pit method together with other conventional methods. Cold-worked state: -Sub-grains are already formed in the cold-worked state due to heterogeneity of deformation. The sub-grains are found most prominently in the neighborhood of the main boundaries and the boundary of zone affected by adjacent grains. They have generally two types; (1) relatively large blocks, and (2) fine blocks smaller than 10-4cm.These sub-grains which are accompanied by large internal stress and some crystal rotation become prominent with an increase of the rate of working, and finally type (1) is replaced by type (2).In the metals heavily worked by rolling, a banded structure parallel to the rolling direction is found by macroscopic etching. These banded structure on the specimen surface is due to the different ratio of the frequency of crystal planes near (111) to crystal planes near (100).The ratio is small in light bands, but large in dark bands.Recovery: -When metals having the above-mentioned structures are annealed, the recovery without prominent structural change is recognized primarily. And the following characteristics are found; (1) the interal stress in fine blocks is relieved, and these low-stress blocks grow by the stress-induced boundary migration in the large block, (2) numerous polygonization walls are formed, (3) sub-boundaries in the cold-worked state become more distinct, (4) mechanical and physical properties are recovered markedly, (5) although they are very slight there are some structural changes and increases in micro-hardness, which become prominent with an increase of the rate of working. The results mentioned above will be theoretically discussed in the 3rd report.