Formation Of Dislocation Cells Research Articles

Influence of low-frequency vibrations on the compression behavior and microstructure of T2 copper

In this study, the influence of low-frequency vibrations on the room-temperature compression behavior of T2 copper was experimentally investigated. Upon superposing a low-frequency vibration, the forming load noticeably reduced. Among the three vibration parameters investigated, viz. frequency, amplitude, and feeding rate, vibration amplitude plays the most important role in load reduction. The superposition of stress waves can explain the mechanism of forming load reduction at low-frequency vibrations. Due to an increase in the internal stress caused by the superposition of stress waves, the required forming load reduced. The influence of the vibrations on the microstructures of compressed specimens was also discussed. Under low-frequency vibration conditions, cyclic impact loading leads to an increase in the density of dislocations and the formation of dislocation cells, which gradually evolve into new grain boundaries, causing grain refinement.

Formation of nanocrystalline structure in pearlitic steels by dry sliding wear

Nanocrystalline grains, whose average size was approximately 35±5nm, were generated in the outermost layer of a pearlitic wheel steel by dry sliding wear. To explore the evolution of nanocrytalline grains, successive observations from the outermost layer of sliding wear to the undeformed matrix were done by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The investigations showed that the evolution process of nanometer-sized grains in pearlitic wheel steels included four steps: initially, pearlitic plastic deformation took place, lamellar spacing diminished, great amounts of dislocations occurred in ferrite, the thickness of ferrite lamellae decreased and some of ferrite lamellae fractured and dissolved; with increasing strains, considerable cementite started to dissolve into grains, and inside ferrite dislocations accumulated, tangled and formed dislocation walls, which cut ferrite into many short-rod shaped—and even equiaxed shaped dislocation cells; as strains continued to increase, dislocation cells gradually transformed into low-angle sub-boundaries. Accumulation and annihilation of dislocations and rotation of grains induced low-angle boundaries (LABs) to turn into high-angle boundaries (HABs), with the grains having random orientation. At this moment, cementite dissolution was relatively saturated and undissolved cementite particles scattered around the refined ferrite boundaries. Under higher strains and strain rates, refined ferrite grains repeatedly underwent plastic deformation, the formation of dislocation cells and the transformation from LABs to HABs, until finally the size of ferrite grains reached the stable minimum value and the refinement of grains ceased. Meanwhile, cementite continued to dissolve slightly due to dislocation accumulation. When dislocation accumulation rate and annihilation rate reached balanced, the dissolution of cementite ceased.

Influence of hydrogen on dislocation self-organization in Ni

The influence of hydrogen on the evolution of the deformation microstructure in Ni at the same specific macroscopic strain has been determined for the first time. The evolution of the microstructure was accelerated by hydrogen as evidenced by the formation of smaller dislocation cells and dense dislocation walls compared to only dislocation cells in the absence of hydrogen. This enhanced evolution is described in terms of the hydrogen-enhanced localized plasticity mechanism.

Effect of deformation induced precipitation on grain refinement and improvement of mechanical properties AA 7055 aluminum alloy

An improved TMT (thermomechanical treatment) double step hot rolling (DR) including low-temperature pre-deformation, intermediate short-term annealing and final hot rolling, was proposed to manufacture fine grained AA7055 sheets. Microstructural analysis results show that the low temperature pre-deformation can accelerate the formation and spheroidization of fine precipitates which can exert great drag force to the migration of grain boundaries and dislocations so as to promote the formation of high density dislocation cells and accommodate sufficient deformation storage energy subsequently. Then, these dislocation cells may turn into polygon sub-grains by short-period intermediate annealing. With the final hot rolling, low angle grain boundaries (LAGBs like sub-grain boundaries) were gradually transferred into high angle grain boundaries (HAGBs) and fine-grained structures were obtained. Tensile testing results reveal that samples processed by the optimal DR processing (300°C/60%+430°C/5min+430°C/50%) display superior tensile plasticity than the conventional hot rolling (CR) samples without strength loss, which could be correlated with the fine grained structure of the DR sample. It indicates that the present DR processing may be a good alternative to efficiently produce fine grain structured heat-treatable aluminum alloys.

Non-spherical voids and lattice reorientation patterning in a shock-loaded Al single crystal

An Al single crystal shock loaded in the <1 2 3> direction and captured at incipient spallation was examined by combining X-ray tomography, electron backscatter diffraction on a scanning electron microscope, and transmission electron microscopy (TEM). Octahedral voids with {1 1 1} faces were a characteristic feature in the spall region. Regular patterns of lattice reorientation were found around individual voids, with lattice rotation being up to 25–30°. Each reorientation pattern consists of a number of reoriented zones. The direction of lattice rotation varies systematically from one zone to another. Four groups of reorientation patterns were identified morphologically in the same metallographic section, which result from different sectioning positions relative to the voids and thus provide equivalently a “serial sectioning” investigation of the deformed volume around the voids. An analysis of the observed reorientation patterns based on active slip systems rationalizes the key features observed and suggests that the systematic reorientation patterns result from the dominance of a single slip system in each individual zone. Microstructures revealed by TEM in the spall region show formation of dislocation cells and extended dislocation boundaries, illustrating the importance of plastic deformation during void growth.

Laser Shock Peening for Suppression of Hydrogen-Induced Martensitic Transformation in Stress Corrosion Cracking

The combination of a susceptible material, tensile stress, and corrosive environment results in stress corrosion cracking (SCC). Laser shock peening (LSP) has previously been shown to prevent the occurrence of SCC on stainless steel. Compressive residual stresses from LSP are often attributed to the improvement, but this simple explanation does not explain the electrochemical nature of SCC by capturing the effects of microstructural changes from LSP processing and its interaction with the hydrogen atoms on the microscale. As the hydrogen concentration of the material increases, a phase transformation from austenite to martensite occurs. This transformation is a precursor to SCC failure, and its prevention would thus help explain the mitigation capabilities of LSP. In this paper, the role of LSP-induced dislocations counteracting the driving force of the martensitic transformation is explored. Stainless steel samples are LSP processed with a range of incident laser intensities and overlapping. Cathodic charging is then applied to accelerate the rate of hydrogen absorption. Using XRD, martensitic peaks are found after 24 h in samples that have not been LSP treated. But martensite formation does not occur after 24 h in LSP-treated samples. Transmission electron microscopy (TEM) analysis is also used for providing a description of how LSP provides mitigation against hydrogen enhanced localized plasticity (HELP), by causing tangling and prevention of dislocation movement. The formation of dislocation cells is attributed with further mitigation benefits. A finite element model predicting the dislocation density and cell formation is also developed to aid in the description.

Influence of ferrite matrix and precipitation status on the mechanical properties of low carbon low alloy steel during high temperature tension

Utilizing the inhomogeneity of temperature and stress status during heavy plate rolling, with controlled cooling, polygonal ferrite and quasi-polygonal ferrite with two grain sizes for each were obtained, and their precipitation status varied with different microstructures. GLEEBLE 3500 was used to investigate the influence of microstructure and precipitation status during high temperature tensile deformation. The results revealed that ferrite matrix microstructure and nucleation of precipitates were determined by the overall cooling process, while the final precipitation status was mainly determined by the cooling rate within the temperature range of (Ti,Mo)C precipitation during the subsequent isothermal process. Also, dislocations and subboundaries acting as containers facilitated the growth of precipitates along some particular directions. Pre-existing substructures, such as low-angle boundaries and statistically-stored dislocations, along with small precipitates, influenced the formation of subgrains and dislocation cells during high temperature deformation. These newly-generated substructures involved rotation or rearrangement of dislocations, and finally translated into recrystallized grains. Additionally, the grain size was closely related to the probability of interactional of subboundaries. Although the polygonal ferrite matrix showed more stable stress status, the quasi-polygonal ferrite matrix with fine precipitates and small grains exhibited better mechanical properties.

Research on the thermal stability of a near α titanium alloy before and after laser shock peening

The room-temperature tensile properties and thermal stability of Ti834 alloy before and after laser shock peening (LSP) were investigated. Micro-hardness measurement of the untreated and LSP treated specimens were carried out by using a Vickers indenter. The characteristic of microstructures were analyzed by TEM. The results show that both of the strength and ductility decrease for the 600°C/100h exposed Ti834 alloy, especially the decrease of ductility is more pronounced than that of strength. The strength and micro-hardness of Ti834 alloy can be improved by LSP, which is attributed to the high-density dislocation and dislocation-cell formation in the near-surface of the specimen. LSP treatment has a detrimental effect on the thermal stability of Ti834 alloy because of the formation of high density defects in the surface of the alloy.

Microstructural Evolution of an Al-Alloyed Duplex Stainless Steel During Tensile Deformation Between 77 K and 473 K (−196 °C and 200 °C)

Tensile deformation behavior of an Al-alloyed Fe-17Cr-6Mn-4Al-3Ni-0.45C (mass pct) duplex stainless steel containing approximately 20 vol pct ferrite was studied in the temperature range from 77 K to 473 K (−196 °C to 200 °C). While the elongation exhibited a maximum near room temperature, the yield strength continuously increased at lower tensile test temperatures. According to the microstructural examinations, the twinning-induced plasticity and the dislocation cell formation were the dominant deformation mechanisms in the austenite and ferrite, respectively. Reduction of the tensile ductility at T < 273 K (0 °C) was attributed to the ready material decohesion at the ferrite/austenite boundaries. Tensile testing at 473 K (200 °C) was associated with the serrated flow which was ascribed to the Portevin–Le Chatelier effect. Due to a rise in the stacking fault energy of austenite, the occurrence of mechanical twinning was impeded at higher tensile test temperatures. Furthermore, the evolution of microstructural constituents at room temperature was studied by interrupted tensile tests. The deformation in the austenite phase started with the formation of Taylor lattices followed by mechanical twinning at higher strains/stresses. In the ferrite phase, on the other hand, the formation of dislocation cells, cell refinement, and microbands formation occurred in sequence during deformation. Microhardness evolution of ferrite and austenite in the interrupted tensile test specimens implied a higher strain-hardening rate for the austenite as it clearly became the harder phase at higher tensile strain levels.

Analysis of partially pulsating fatigue process on carbon steel with microstructural observation

Pulsating low-cycle fatigue processes, up to the present, have been divided into three states: the transient state, steady state, and accelerating state of ratcheting. In our previous work, we suggested that fatigue behavior of pulsating fatigue process should be classified into five stages in which the plastic strain amplitude and the ratcheting strain rate are plotted on the X and Y axis, respectively. In this study, at the condition of R=−0.3 (partially pulsating fatigue), the change in the plastic strain amplitude and ratcheting strain rate for each cycle to failure was examined on AISI 1025 carbon steel. The dislocation substructure was examined using transmission electron microscopy (TEM) for each stage, except for stage I. It was also demonstrated that the fatigue process can be divided into five stages: stage I corresponds to the un-pinning of dislocations from the Cottrell atmosphere and propagation of the Luders band. Stage II corresponds to the restriction of dislocation movement by dislocation tangles. Stage III corresponds to the formation of dislocation cells. Stage IV corresponds to the promotion of the to-and-fro (back-and-forth) motion of dislocations by a re-arrangement of the dislocations in the cells. Stage V corresponds to the release of dislocation movement by the collapse of dislocation cells.

Work hardening behavior involving the substructural evolution of an austenite–ferrite Fe–Mn–Al–C steel

The strain hardening behavior in relation with the evolving dislocation substructures during uniaxial tensile deformation for an austenite–ferrite Fe–Mn–Al–C steel was investigated in the article. After being solution treated at 1000°C for 0.5h and then water quenched, the steel possessed an excellent combination in mechanical properties, with the value of product of tensile strength and elongation over 35GPa%. The steel exhibited obvious three-stage strain hardening characteristics and austenite showed greater strain hardenability than ferrite. With high Al content included, the stacking fault energy (SFE) of austenite was calculated to be ∼66mJ/m2. The deformation mode of austenite is dominated by planar glide, relating to dislocation configurations, Taylor lattices and microbands. However, wavy glide in ferrite leads to the formation of dislocation nodes, dislocation cells (DCs) and cell blocks (CBs). The distinct substructural hardening in the duplex phases led to the three-stage strain hardening behavior and eventually considerable combination of mechanical properties. The crush and separation of banded-structure δ-ferrite grains during solution treatment enhanced the deformation coordination and improved the steady plastic deformation ability. Obvious ductile fracture accompanied with many deep dimples was observed in the as-solutionized steel.

Study of the formation of adiabatic shear bands in steel during impact

Both metals and other materials may exhibit the formation of narrow bands of extreme strains when impacted at high strain rates and large strains. These are known as Adiabatic Shear Bands (ASBs). They are observed during material processing such as forging and machining as well as wear and in armor plates during impact by projectiles. The prevailing theory for their formation is that they form in narrow bands because of two competing mechanisms occurring sequentially: strain hardening followed by thermal softening from the retained heat due to the impact. However, recent studies suggest that the formation of ASBs may be a simultaneous occurrence of different mechanisms which starts with the emergence of dislocations depending on the imposed local strain and strain rate. This study uses different methodologies to explore the microstructure of ASBs in a hardened low alloy steel. The study includes the effect of the initial microstructure on the formation of ASBs. The Focused Ion Beam technique was used to prepare transmission electron microscopy samples from regions within the shear bands to eliminate the induced further deformation which could be produced by conventional approaches of electropolishing. The present study reveals that each of these methodologies complements each other. Also more complex series of mechanisms including dislocation cell formation, texture development, dynamic recrystallization and carbide dissolution accommodate the excessive strains that occur during the evolution of the shear bands.

Evolution of microstructures and texture in the surface layer of copper during burnishing process

The evolution of microstructures and texture subjected to a burnishing process in pure copper was investigated by transmission electron microscopy and electron backscattered diffraction. The grains of the top surface underwent significant refinement with a minimum grain size of ∼100 nm. The grain refinement was dominated by dislocation activities, including formation of dislocation cells, transformation of cell walls into sub-grain boundaries, development of highly disoriented sub-boundaries and generation of equiaxed grains. Shear strain induced by friction affected the strain state at the first pass of burnishing, resulting in a {001}<110> shear texture. Brass orientation developed well at the early stage of burnishing. Then, with a further increase in plastic strain, Brass orientation decreased, while the Goss orientation was observed, which mainly owing to the transformation from Brass to Goss orientation by rotation along the α-fibre. Moreover, dynamic recrystallisation occurred as the strain increased.

Microstructure characterisation of an X70 grade pipeline steel and its dislocation development during Bauschinger testing

The microstructure of an X70 alloy and its dislocation development during one cycle of compression/tension strain has been analysed. The microstructure of the as received material and the dislocation arrangements before, during and after the Bauschinger test have been characterised. Results for this particular alloy show no evidence of grain boundary dislocation pile-ups or of precipitates pinning dislocations. A comparison between the dislocation arrangement in the as received, compressed and compressed-tensioned samples shows the formation of small dislocation cells, implying that the conception of the Bauschinger effect comes from dislocation-dislocation interactions.

The effect of size on dislocation cell formation and strain hardening in aluminium

The formation of dislocation cells has a significant impact on the strain hardening behaviour of metals. Dislocation cells can form in metals with a characteristic size defined by three-dimensional tangles of dislocations that serve as “walls” and less dense internal regions. It has been proposed that inhibiting the formation of dislocation cells could improve the strain hardening behaviour of metals such as Al. Here we employ in situ scanning electron microscope compression testing of pure Al single crystal pillars with physical dimensions larger, close to and smaller than the reported cell size in Al, respectively, to investigate the possible size effect on the formation of dislocation cell and the consequent change of mechanical properties. We observed that the formation of dislocation cells is inhibited as the pillar size decreases to a critical value and simultaneously both the strength and the strain hardening behaviour become strongly enhanced. This phenomenon is discussed in terms of the effect of dimensional restriction on the formation of dislocation cells. The reported mechanism could be applied in polycrystalline Al where the tunable physical dimension could be grain size instead of sample size, providing insight into Al alloy design.

Sharp Increase of Hysteresis Area Due to Small Plastic Deformation Studied With Magnetic Barkhausen Noise

Three steels with similar microstructure but with very different mechanical properties were examined with nondestructive technique magnetic Barkhausen noise (MBN). All these steels were submitted to a very small plastic deformation (tensile test), varying between 0.5% and 4% of deformation. It was found that 180° domain wall movement becomes difficult, even after very small deformation of 0.5%. This is caused by grain fragmentation due to plastic deformation and by an increase of the dislocation density, with formation of dislocation tangles and dislocation cells. The change of the MBN envelope shape suggests that the magnetostrictive process becomes more important after plastic deformation, and this probably is related to the stress field around dislocations.

Dynamic Properties of Magnesium Alloys

With the rapidly expanding use of automobiles globally, the application of lighter structural materials in the form of magnesium alloys has become an urgent necessity to improve fuel efficiency and reduce the environmental impact. Within automobiles, magnesium alloys are used for components such as steering wheel frames and wheel rims, which are subjected to not only quasi-static but also dynamic strains. Indeed, catastrophic failure of a component by dynamic strain effects is a critical consideration, such as that of a wheel rim by braking at a high speed of 320 km/h. A recent consideration of magnesium for crash rail applications was rapidly brought to an end by the poor performance of initial crash test results. Dynamic strain rate testing is also important because of the emergence of magnesium alloys for armor with good ballistic properties. Just as with quasi-static strain, the mechanical behavior at dynamic strain rates depends on the microstructure. Deformation at dynamic strain rates produces more uniform distribution of dislocations and hinders the formation of discrete dislocation cells. Suppression of thermally activated dislocation processes occurs in this regime, which can lead to stresses high enough to activate deformation twins even in face-centered cubic (fcc) alloys with the high-stacking-fault energy. Among the fcc, body-centered cubic (bcc), and hexagonal closepacked (hcp) metals and alloys, high-purity fcc metals exhibit yielding that is nearly independent of the strain rate, while the strain hardening after yielding is strongly dependent on strain rate, which is due to the suppression of dynamic recovery process. In the case of bcc metals and alloys, yielding is strongly dependent on strain rate, while strain hardening is independent of the strain rate. For hcp metals, both the yielding and the strain hardening are dependent on strain rate. In hcp metals and alloys, twins play a very important role in deformation, especially for magnesium alloys in which nonbasal slips are not easily activated. The texture and c/a ratio greatly influence the deformation behavior of hcp metals. The effects of the polarity of twinning are similar to that at quasi-static tests. During in-plane testing of rolled plates with strong basal texture, in which the basal planes are parallel to the loading direction, it is observed that the twinning propensity increases with increasing strain rate. Through-thickness dynamic testing, in which the loading direction is perpendicular to the basal planes, reveals that the main deformation twinning mode, 1012 extension twinning, may not be activated. As a result, the work-hardening response in in-plane loading is more sensitive to strain rate than through-thickness loading. In this issue, we have four contributions on the dynamic behavior of magnesium alloys. The first is titled, ‘‘Dynamic Behavior of a Rare-Earth-Containing Mg Alloy WE43B-T5 Plate with Comparison to Conventional Alloy AM30-F’’ by S.R. Agnew, W. Wittington, A. Oppedal, H. El Kadiri, M. Shaeffer, K.T. Ramesh, J.J. Bhattacharyya, R. DeLorme, and B. Davis, which compares the dynamic behavior of two wrought alloys, a rolled WE43B alloy in peakaged condition, and an extruded AM30 alloy. The WE43B plate exhibited a relative isotropy of yield strength at quasi-static and dynamic strain rates, although it did show tension–compression strength asymmetry and anisotropy in strain hardening, which is consistent with the texture. The extruded AM30 alloy showed stronger anisotropy in strainhardening behavior related to the stronger texture in this material. At larger strain levels, the rate sensitivities along different directions converge in both the alloys. The authors also report that the yield strength is insensitive to strain rates along loading directions in which the rate-controlling process is 1012 type twinning, which is known to be relatively rate insensitive. The second contribution titled, ‘‘Dynamic Behavior of Some Magnesium Alloys’’ by E.S. Prasad, B. Li, N. Dixit, M. Shaefer, S.N. Mathaudhu, and K.T. Ramesh investigates the two important alloys, AZ31B and ZK60, at fine grain and ultrafine grain JOM, Vol. 66, No. 2, 2014

Comparative Study of Microstructure and Mechanical Properties of Al 6063 Alloy Processed by Multi Axial Forging at 77K and Cryorolling

In the present work, commercially available Al 6063 alloy was subjected to multiaxial forging (MAF) at 77K. The evolution of ultrafine grain structure during MAF at 77k at various strain levels (∑Δɛ=1.2 and ∑Δɛ =2.4) was investigated using Optical microscopy and Transmission electron microscopy. Cryorolling was performed up to true strain 2.4 and the mechanical properties and micro structural changes were compared with MAF at 77K sample of same strain. At lower strain level (∑Δɛ=1.2), the microstructure reveals formation of dislocation cells along with sub grain structure, predominantly low angle grain boundaries. Formation of elongated sub grain structure with high angle grain boundaries through dynamic recovery was observed with increasing strain up to (∑Δɛ=2.4) at shear bands. Microstructure observed in cryorolled sample is more homogeneous with low angle grain boundaries. The improvement in mechanical properties observed in both the samples processed through CR and MAF are nearly same.

Investigation of deformation and microstructural evolution in Grade 91 ferritic–martensitic steel by in situ high-energy X-rays

In situ wide- and small-angle X-ray scattering (WAXS/SAXS) were used to characterize microstructural evolution in Grade 91 ferritic–martensitic steel during tensile tests at room temperature (RT) and 650°C. Elastic lattice strains in the Fe matrix, M23C6 and MX precipitates were measured from peak shifts in WAXS patterns. Load transfer, as evidenced by precipitates developing much higher lattice strain than the Fe matrix in the plastic regime, was found for the RT test but was not obvious at 650°C. Detailed peak-broadening analysis as a function of strain, using modified Williamson–Hall plots, indicates strong dislocation activity in the Fe matrix during the RT test which can be divided into two stages: (i) dislocation multiplication (from 0 to 4.0% strain) and (ii) formation of dislocation walls and cells (after 4.0% strain). This framework is consistent with the observed precipitate strain response and can describe the macroscopic flow stress behavior. At 650°C, Fe peaks broadened much less with increasing strain, indicating a low dislocation density and providing an explanation for the low precipitate strains and macroscopic flow stress at this temperature. Changes in SAXS intensity were used to infer nanoscale void formation, which occurred immediately after necking at RT but long after necking at 650°C. Most voids were concentrated at the necking center, and the RT specimen developed more voids than the 650°C specimen.

Influences of magnetic annealing on the grain growth in a cryoECAPed 1050 aluminum alloy

The evolutions of hardness, grain microstructures and dislocations were investigated in cryoECAPed 1050 aluminum alloy after annealing at 423–673K for 1h without and with a magnetic field of 12T. Compared to those of samples annealed without the magnetic field, the hardness of samples annealed with the magnetic field is lower at 423–573K, and then higher at >573K. The high magnetic field accelerates the formation of dislocation cells or sub-grains. The interface dislocations frequently observed in materials deformed at high temperature were detected in samples annealed with the magnetic field at ≥423K. The fraction of low angle boundaries in sample annealed with the field is 49% and 34% in sample annealed without the field even at 673K. No abnormal grain growth occurs in samples annealed with the magnetic field due to the rapid consumption of the stored distortion energy during recovery.