Strain-induced Boundary Migration Research Articles

Goss Texture Formation by Strain Induced Boundary Migration in Semi-Processed Nonoriented Electrical Steels

Semi-processed nonoriented electrical steels are very attractive products whose magnetic properties are significantly improved through annealing treatment in customers plant. The improvement is due to strong Goss texture formation by strain induced boundary migration (SIBM). In order to the effect of temper rolling reduction on the strengthening of Goss texture, temper rolling reduction was changed in the range of 2% to 8%. The annealing times was changed from 10 minutes to 180 minutes. A mechanism of grain growth during SIBM is suggested from our experimental data. In the specimen temper-rolled by 2%, relatively strong {111}<112> texture develops, whereas in the specimens temper-rolled by 4% through 8%, strong Goss texture develops as a result of SIBM during final annealing. It can be found from observed EBSD data that the Goss grains have the lowest stored energy in all temper-rolled specimens, which is confirmed by average image quality value in EBSD measurements. However, for the Goss grains to grow preferentially, stored energy difference between Goss grains and their neighboring grains may have to be higher than a certain critical value.

Effect of Initial Grain Sizes on Strain Induced Boundary Migration

Examination of the SIBM mechanism based on the dislocation substructure at the interior of the Goss oriented grain was carried out by changing the grain size prior to the temper rolling. The following results were obtained. 1) SIBM significantly increased Goss orientation during the eminent grain growth with the initial grain sizes from 18 to 55μm. 2) When the initial grain sizes were large, i.e. 37μm and 55μm, the rolling with the reduction beneath the critical value could not promote SIBM, even the normal grain growth could also be hindered. Consequently a proposal was made that the nucleation of the recovery appeared among substructure domains containing sluggish strain. There exists an adequate size of the domain which varies with the change both of the rolling reduction and the initial grain size.

Strain Induced Inward Grain Growth during Recrystallisation in Steel Sheets with BCC Crystal Structure

The present paper investigates the potential application of Strain Induced Boundary Migration mechanism on the two different types of surface textures developed after α-γ-α phase transformation annealing, one with preferred cube and Goss orientation at the surface and the other with random surface texture without preferred orientations. It has been demonstrated that these surface texture components grow in across the thickness of the sheet after an appropriate combinations of a critical amount of rolling reductions and an annealing treatment at the recrystallisation temperature.

Microstructure and Texture Evolution of Fe-Si Steels after Hot Dipping and Diffusion Annealing

A homogenous intensity distribution along the cube texture fibre is important to achieve an easy magnetization in non-oriented electrical steels. Several alternatives have been discussed in literature to achieve this goal namely, tertiary recrystallization (surface energy controlled), decarburization annealing, two step cold rolling (strain induced boundary migration), twin-roll thin strip casting (directional solidification), phase transformation (surface energy anisotropy) and columnar grains formation (selective grain growth). In the present study, a hypoeutectic Al-Si alloy was deposited on the surface of cold rolled Fe-Si steels with a hot dipping simulator and subsequently annealed at 1000°C for different times. This procedure was developed previously in order to enrich the substrate with Al and/or Si and consequently improve their resistivity. Of specific interest was the formation of columnar grains in the low Fe-Si steel after annealing. These columnar grains were found to grow from the surface towards the centre of the substrate. The microstructure and texture in the columnar grains were significantly different than those in the middle of the material. Therefore, the evolution of these features during processing was studied in detail in this work.

Reinforcement of the Cube texture during recrystallization of a 1050 aluminum alloy partially recrystallized and 10% cold-rolled

In high purity Aluminum, very strong {100}<001> recrystallization texture is developed after 98% cold rolling and annealing at 500 °C. On the contrary, in Aluminum alloys of commercial purity, the Cube component hardly exceeds 30% after complete recrystallization. Parameters controlling Cube orientation development are mainly the solute dragging due to impurities in solid solution and the stored deformation energy. In the present study, besides the 85% cold rolling, two extra annealings and a slight cold rolling are introduced in the processing route to increase the Cube volume fraction. The Cube development was analyzed by X-ray diffraction and Electron BackScattered Diffraction (EBSD). The nucleation and growth mechanisms responsible for the large Cube growth were investigated using FEG/EBSD in-situ heating experiments. Continuous recrystallization was observed in Cube oriented grains and competed with SIBM (Strain Induced Boundary Migration) mechanism. This latter was favored by the stored energy gap introduced during the additional cold-rolling between the Cube grains and their neighbors. Finally, a Cube volume fraction of 65% was reached after final recrystallization. ► EBSD in-situ heating experiments of aluminum alloy of commercial purity. ► A 10% cold-rolling after a partial recrystallization improved Cube nucleation and growth. ► Annealing before cold-rolling limited the solute drag effect and permitted a large Cube growth. ► Cube development is enhanced by continuous recrystallization of Cube sub-grains. ► The preferential Cube growth occurs by SIBM of small Cube grains.

Texture and Grain Boundary Character Distribution in a Thermomechanically Processed OFHC Copper

The effect of texture on grain boundary character distribution (GBCD) in thermomechanically processed oxygen-free high-conductivity copper has been investigated. Copper samples were cold rolled to a reduction in thickness of 50% and then annealed for 60 min in the range of 400–600°C. GBCD and texture were measured using electron backscatter diffraction. The fraction of special boundaries (Σ3, Σ9, and Σ27) varied from 59% to 71%, with the maximum in the sample annealed at 500°C. The results indicate that cold rolling provided a strong texture of brass type. It was found that the sample annealed at 500°C have texture components of cube, Goss, rotated-Goss, and Y orientations. These texture components were in relation with the formation of annealing twins and Σ3 boundaries. It was also shown that twin-induced GBCD evolution occurred by strain-induced boundary migration, multiple twinning, and conventional recrystallization. Annealing at 600°C caused full recrystallization and grain growth, showing a strong cube recrystallization texture. The grain growth was found to hinder the formation of special boundaries.

Microstructural Evolution of NiTi47.7Cu6.3 Alloy during Hot Deformation

In the present work, the hot workability and microstructural evolution of NiTi47.7Cu6.3 (at.%) shape memory alloy were investigated by using wedge-rolling test. The wedge specimens were subjected to hot rolling at the temperatures of 700–1000°C. The microstructural evolutions at the strains of 0.05, 0.15, 0.2, 0.25 and 0.3 were then characterized by optical microscopy and scanning electron microscopy (SEM) as well as energy dispersive spectrometry (EDS) technique. Depending on the deformation temperature and strain, the dynamic recrystallization (DRX) could occur, leading to the refining of the microstructure. At low temperatures of 700 and 800°C, there was no sign of DRX at all studied strains. At these temperatures, the formed fine needle-like precipitates pinned the grain boundaries and prevented them from bulging/migration. By contrast, at higher temperatures of 900 and 1000°C, the DRX took place at the critical strains lower than 0.25 and 0.15, respectively. However, at higher temperatures, strain-induced-boundary-migration (SIBM) mechanism resulted in the formation of DRX grains.

Alternating-current induced thermal fatigue of gold interconnects with nanometer-scale thickness and width

With dramatic reduction in sizes of microelectronic devices, the characteristic width and thickness of interconnects in large-scale integrated circuits have reached nanometer scale. Thermal fatigue damage of so small interconnects has attracted more and more attentions. In this work, thermal fatigue of Au interconnects, 35 nm thick and 0.1-5 μm wide, is investigated by applying various alternating current densities to generate cycling temperature and strain in them. A multi-probe measuring system is installed in a scanning electron microscope and a probe-type temperature sensor is for the first time introduced into the system for real-time measuring the temperatures on the pads of the tested interconnects. A one-dimensional heat conduction equation, which uses measured temperatures on the pads as boundary conditions and includes a term of heat dissipation through the interface between the interconnect and the oxidized silicon substrate, is proposed to calculate the time-resolved temperature distribution along the Au interconnects. The measured fatigue lifetimes are presented versus current density and thermal cyclic strain, and the results show that narrower Au lines are more reliable. The failure mechanism of those Au interconnects differs from what is observed in thick interconnects with relatively larger grain size. Topography change caused by localized plasticity on the less-constrained surfaces of the interconnects have not been observed. Instead, grain growing and reorienting due to local temperature varying appear, and grain boundary migration and mergence take place during high temperature fatigue in such thin and narrow interconnects. These results seem to reflect a strain-induced boundary migration mechanism, and the damage morphology also suggests that fatigue of the interconnects with decreased grain size and film thickness is controlled by diffusive mechanisms and interface properties rather than by dislocation glide. Open circuit eventually took place by melting at a region of severely damage cross-sectional area with the grain growing and reorienting.

In Situ EBSD Investigation of Recrystallization in a Partially Annealed and Cold‐Rolled Aluminum Alloy of Commercial Purity

AbstractThe microstructure of partially annealed and additionally rolled 1050 aluminum alloy of commercial purity was monitored using FEG/EBSD in situ heating experiments in order to understand the preferential growth of Cube‐oriented grains. A fast reorganization of the deformation sub‐structure of Cube and S1{214} &lt;‐1‐21&gt; grains has been observed by sub‐grain growth that consists in a continuous recrystallization mechanism. This occurs as the main recrystallization mechanism at the beginning of the annealing but soon competes with a preferential Cube growth by Strain Induced Boundary Migration (SIBM). An EBSD pattern Image Quality (IQ) analysis attested that Cube grains store less energy than non‐Cube oriented grains. TEM observations allowed studying the effect of the additional rolling on the deformation sub‐structures depending on the crystallographic orientation. It appears that Cube grains close to the exact orientation contain isolated dislocations and that rotated Cube grains (with 15° spread to the exact orientation) are composed of dislocation cells with thin walls. At the opposite, non‐Cube grains are organized in sub‐grains with thicker walls and rich in dislocations. Finally, this difference of sub‐structure associated to an energy difference between Cube and non‐Cube grains explains the preferential Cube growth by SIBM evidenced by the sequence of in situ scans during heating.

Recrystallization in the particles interfacial region of the cold-sprayed aluminum coating: Strain-induced boundary migration

A cold-sprayed aluminum coating was fabricated using a mixture of atomized and cryomilled powder as the feed stocks. The microstructure in the particles' interfacial region of the atomized/cryomilled particles was investigated by TEM. The tongue-shaped grains with very low dislocation density were observed. The formation of the tongue-shaped grains is attributed to recrystallization dominated by strain induced boundary migration (SIBM). The driving force of SIBM in this work is an obvious difference in substructure on opposite sides of the grain boundary. A large boundary misorientation makes SIBM more viable and facilitates the grain boundary bulging into adjacent region of high stored energy. This partial reaction is also attributed to adiabatic shear instability due to severe impact during cold spraying, and it might be helpful to form ductile bonding in coatings and composite bulk materials.

Tension and strain annealing for abnormal grain growth in magnetostrictive Galfenol rolled sheet

This paper investigates the effect of tension annealing and of strain annealing on abnormal grain growth (AGG) in polycrystalline rolled sheet of NbC-added Fe81Ga19. Electron backscattering diffraction patterns captured by orientation imaging microscopy were used for analyzing texture and grain configuration of tension annealed samples. Tension annealing, with simultaneous application of tension and high temperature annealing, with strain rate control, was conducted to study “dynamic recrystallization” for AGG. Strain annealing, application of tension to produce uniform strain and subsequent high temperature annealing, was carried out to study the effect of “strain induced boundary migration” on AGG. A small amount of texture variation resulted, but no significant AGG was observed during tension annealing. During strain annealing, no texture or grain size variation was observed in the gauge region of uniform strain in the sample; however, abnormally grown Goss-texture was observed in regions of high stress concentration outside of the gauge region of the sample.

Development of strong {001}〈210〉 texture and magnetic properties in Fe–6.5wt.%Si thin sheet produced by rolling method

Fe–6.5wt.%Si thin sheets were successfully produced by conventional rolling and annealing methods. The deformation and recrystallization textures were investigated by means of x-ray diffraction and electron backscattered diffraction analysis. It was found that a strong {001}〈210〉 recrystallization texture was obtained and the magnetic property was significantly improved under the appropriate rolling temperature. The formation of the {001}〈210〉 texture was explained in terms of preferred nucleation by the strain-induced boundary migration mechanism and selective growth by grain size advantage established during primary recrystallization.

Deformation and Recrystallization of Interstitial Free (IF) Steel

The rolling texture of Body Centered Cubic (BCC) low carbon and interstitial free (IF) steel consists of two crystallographic fibers, the α fiber (⟨110⟩ //RD) and the γ fiber (⟨111⟩ //ND). Cells, subgrains, and microbands, where these are not associated with significant lattice curvature, provide the driving forces for recrystallization. However, when these microstructures are associated with significant short range lattice curvature, as they are in deformation or shear bands, they can provide the nucleation sites. The process of recrystallization in IF steel is shown to occur in two stages: the first where the nuclei are contained in the original rolled-out grains belonging to the γ fiber, and where the essential lattice curvature is derived from shear and deformation bands. The second stage involves the impingement/coalescence of several recrystallized grains in the as-rolled envelope of an original hot band grain which provides a super-nucleation event, in which the as rolled boundaries bow out to give an equiaxed microstructure. An elegant proof is provided for this process, along with a simple model based on Strain Induced Boundary Migration (SIBM). The well-known optimal cold rolling, beyond which the drawability is known to degrade, is explained by deformation banding of a particular subset of the grains belonging to the α fiber. Finally, a novel experiment is described, based on the observed deformation banding behavior of crystals belonging to the γ fiber, which leads to very strong γ recrystallization textures, which are desirable in deep drawing.

Numerical simulation of recrystallization in BCC metals

A Monte Carlo simulation technique was utilized to model the recrystallization process of elongated grains in deformed metals with body-centered cubic crystalline structure. The initial microstructure was determined using Voronoi polygons. Two mechanisms, subgrain growth and strain-induced boundary migration, were considered for the nucleation of new grains during modeling of recrystallization. Each subgrain was considered to have specific orientation and stored energy. The simulation results indicated that the strain-induced boundary migration mechanism was the main contributor to the recrystallization process. The recrystallization kinetics can be described by an Avrami-type equation with a time exponent n of about 3.52, which is in agreement with time exponents reported in interstitial-free ferritic steels. A normal grain growth process was also observed in the simulated microstructure.

Characteristics of Strain-Induced Boundary Migration as Evaluated by the Crystal Rotation Axis Method in (001) [100] and (112) [11\\bar1] Aluminum Bicrystal Deformed by Plane-Strain Compression

An aluminum bicrystal with (001) [100] and (112) [11 1] orientation was deformed by plain-strain compression to a strain of 0.34. The types of slip systems responsible for producing crystal rotations relative to the initial orientations were clarified by the crystal rotation axis method. In the (001) [100] crystal, an affected zone developed along the grain boundary, which revealed a variety of slip systems in operation; the operation of some of these systems could be predicted from the Schmid factor while that of the remaining systems could not. The unpredictable slip systems were activated by the formation of cross slips. After annealing at 400 C for 30 s, recrystallization that occurred was characterized by strain-induced boundary migration. The recrystallized area had the same crystal orientation as that of the affected zone of the (001) [100] crystal. By using the crystal rotation axis method, it was determined that the affected zone was the origin of the strain-induced boundary migration. [doi:10.2320/matertrans.L-MG200952]

Recrystallized microstructure of Al–0.7%Mg–0.4%Si–0.2%Cr alloy with cold-rolling at 30%

Recrystallization behaviors in an Al–Mg–Si alloy have been intermittently observed by means of SEM-EBSD in detail. A specimen of Al–0.73 Mg–0.38 Si–0.26 Cr (mass%) alloy was cold-rolled with reduction rate of 30%, then annealed at 673 K. Annealing, ion-polishing and EBSD measurement were repeated and changes in microstructures were observed on the same area in the specimen during annealing up to 4.8 ks. At the early stage of annealing, crystallites (subgrains) formed near grain boundaries, and they grew into recrystallized grains without changing their orientations. The orientation of the crystallite is in the range of lattice rotation due to cold-rolling. Prior grains in which {100} planes were parallel to the specimen surface tend to form crystallites with {100} and {110} orientations. While prior grains with {110} orientation tend to form {100} oriented crystallites. Crystallite invaded neighboring prior grains with the strain induced boundary migration mechanism. The invaded grain was separated from the mother grain by other invading grains. Grain boundary migration and mutual intersection resulted in formation of recrystallized grains surrounded by large angle grain boundary in the neighboring prior grain. Orientations of the prior grains are maintained in the recrystallized grains with changing in the range of lattice rotation.

Mechanism of Grain Growth During Annealing of Si-Al Electrical Steel Strips Deformed in Tension

AbstractThe mechanism of recrystallization as a result of annealing during 600–7200 seconds at 700 °C of a Si-Al, low C electrical steel strip is investigated in samples deformed in tension. The evolution of grain size during annealing is evaluated by optical microscopy and electron backscatter diffraction in the scanning electron microscope. It is found that grain growth starts after an incubation time of 600 s with no apparent evidence of primary recrystallization. After that, the grain size-time relationship exhibits two different stages. Initially, the grain size increases linearly with time up to about 3600 s. During this time, some selected grains grow until they consume the deformed microstructure. In the second stage, the rate of growth decreases significantly and a final grain size of about 150 m is reached after 7200 seconds of annealing. Grain orientation spread maps obtained from EBSD data of deformed and partially recrystallized samples during the stage of linear growth reveals that the growing grains exhibit lower misorientation and therefore smaller stored energy than the non-recrystallized matrix grains. Analysis of image quality maps reveal that the IQ values for {100}&lt;uvw&gt;orientations are higher than those observed for {111}&lt;uvw&gt;orientations thus suggesting that the {100}&lt;uvw&gt;orientations grow at the expense of {111}&lt;uvw&gt;orientations by a mechanism of strain-induced boundary migration.

Compressive Creep Behaviour of Cast Magnesium Under Stresses Above the Yield Strength and The Resultant Texture Evolution

Creep behaviour of commercial purity cast magnesium with preferred orientation was examined in compression at 100, 125 and 150°C and 35 and 50 MPa. The as-cast structure is composed of large columnar grains in which prismatic poles are parallel to the longitudinal direction. Cylindrical compression specimens were prepared from the cast material in the way that columnar grains were almost perpendicular to the compressive loading direction. Steady-state creep rate increased from -2 xl x 1 0-7/s to 5 x 10-7/s at 35 MPa and from 5.7 x 1O--7/s to 2 x 10- 6/s at 50 MPa with increasing temperature from 100 to 150 °C. The apparent activation energy, Qc, was determined to be 34 kJ/mol at 50 MPa and 29kJ/mol at 35 MPa. The stresses used were above the yield strength of the cast Mg and instantaneous plastic deformation occurred at creep loading. It was noticed that creep under the aforementioned conditions changed the initial grain orientation. Such evolution of orientation was stronger at higher temperatures. Some evidence of grain boundary serrations was also observed after creep and these were attributed to creep-deformation due to strain-induced grain boundary migration.

The evolution of cluster of grains with ∑3 n relationship in austenitic stainless steel

The evolution of cluster of grains with ∑3 n relationship (∑3 n CG) in austenitic stainless steel was investigated by in situ microstructural observations using electron backscatter diffraction (EBSD). The results showed that, after cold rolling with thickness reduction of 6%, ∑3 n CG developed from several existing grains during annealing at 1173 K. As annealing proceeded, ∑3 n CG was growing larger with the so-called strain induced boundary migration (SIBM), and in the meantime, the connectivity of random high angle grain boundaries (HABs) network was interrupted by low ∑-coincidence site lattice boundaries (∑CSLBs) effectively.

Strain-induced boundary migration of carbon steel at high temperatures

The rate of strain-induced boundary migration (SIBM), which appears during small strains in carbon steel under the critical strain for dynamic recrystallization, was measured through in situ observation at high temperatures. The rate of SIBM measured is much higher than the rate of grain growth without strain. The in situ observation method confirms that SIBM can be initiated by strains as small as 0.015.