Formation Of Subboundaries Research Articles

Ferrite and Spheroidized Cementite Ultrafine Microstructure Formation in an Fe-0.67 Pct C Steel for Railway Wheels under Simulated Service Conditions

The microstructure development of a high-carbon steel (0.67 pct C) for railway wheels as they are affected by rolling contact with rail tracks and by cyclic frictional heat from braking is studied in the vicinity of the contact surface by scanning electron microscopy (SEM) and electron backscattered diffraction. An ultrafine microstructure consisting of ferrite grains with a size of less than 1 μm and spheroidized cementite particles is formed in the region up to 100 μm below the contact surface. The generation of low-angle sub-boundaries associated with the rearrangement of accumulated dislocations involved in continuous recrystallization of ferrite microstructure contributes to the microstructure refinement at temperatures lower than A1 temperature (1000 K). Fine spheroidized cementite particles with uniform distribution obstruct the migration of high-angle grain boundaries, by which the dislocation density is maintained sufficiently high for the formation of sub-boundaries. The formation of a texture, corresponding to the surface texture typically formed in low-carbon steels by hot rolling without lubrication at ferritic temperatures, is observed in the ultrarefined microstructure region. The results drawn from this study strongly indicate the occurrence of “in-situ microstructure control” under service conditions.

Dislocation analysis of die-cast Mg–Al–Ca alloy after creep deformation

Tensile creep tests were combined with detailed transmission electron microscopy in order to characterize the dislocation movements during creep and to explain the creep properties of the Mg–Al–Ca AX52 die-cast alloy at 473 K and stresses from 15 to 70 MPa. TEM observations indicate that dislocations are generated within the primary α-Mg grain in the die-casting process, which consist of both the basal and non-basal segments. The basal segments of dislocations are able to bow out and glide on the basal planes under the influence of a stress, and the jogs follow the basal segments with the help of climb during creep. The creep mechanism for the alloy is deduced as dislocation climb due to the formation of sub-boundaries during creep, while the easy glide of the basal segments of dislocations is controlling the creep rates immediately after the stress application of creep tests.

The formation of broken subboundaries in the elastic field of a planar mesodefect

Based on an energy analysis within the framework of the disclination model, it is shown that a planar mesodefect can (similarly to the dipole of partial disclinations) initiate the formation of a band of reorientation. The process of band formation has been studied by means of computer simulation. It is established that the reorientation band formed in the elastic field of the planar mesodefect represents a system of broken subboundaries, the appearance of which has a kinetic nature.

Dynamic Recrystallization of Austenite in Ni-30 Pct Fe Model Alloy: Microstructure and Texture Evolution

The microstructure and crystallographic texture development in an austenitic Ni-30 pct Fe model alloy was investigated within the dynamic recrystallization (DRX) regime using hot torsion testing. The prominent DRX nucleation mechanism was strain-induced grain boundary migration accompanied by the formation of large-angle sub-boundaries and annealing twins. The increase in DRX volume fraction occurred through the formation of multiple twinning chains. With increasing strain, the pre-existing Σ3 twin boundaries became gradually converted to general boundaries capable of acting as potent DRX nucleation sites. The texture characteristics of deformed grains resulted from the preferred consumption of high Taylor factor components by new recrystallized grains. Similarly, the texture of DRX grains was dominated by low Taylor factor components as a result of their lower consumption rate during the DRX process. The substructure of deformed grains was characterized by “organized,” banded subgrain arrangements, while that of the DRX grains displayed “random,” more equiaxed subgrain/cell configurations.

Computer simulation of terminated sub-boundary formation in the disclination elastic field

Computer simulation of the dislocation ensemble behavior in the elastic field of disclination dipole and the external stress during deformation is performed. It is shown that disclination dipole disturbs the laminar flow of dislocations. This leads to the appearance of sub-boundary dislocations of opposite signs, which effectively screens the disclination elastic field. Formation of junction disclinations and terminated dislocation boundaries during plastic deformation is investigated in the model bi-crystal and tri-crystal.

A model of the accommodation nucleation of broken subboundaries on grain boundaries

A model of the accommodation mechanism of the formation of broken subboundaries has been studied by means of computer simulation. It is established that an increase in the power Ω of a disclination on a grain boundary leads to the activation of a source of dislocations in the grain volume, which results in the formation of a broken subboundary. The growth of Ω also leads to an increase in the subboundary misorientation θst, which (at every current moment of time) is approximately equal, on the one hand, to half of the disclination power (θst ≃ Ω/2) and, on the other hand, to the grain deformation ɛ.

Characterization of Stress Relaxation, Dislocations and Crystallographic Tilt Via X-ray Microdiffraction in GaN (0001) Layers Grown by Maskless Pendeo-Epitaxy

AbstractIntrinsic stresses due to lattice mismatch and high densities of threading dislocations and extrinsic stresses resulting from the mismatch in the coefficients of thermal expansion are present in almost all III-Nitride heterostructures. Stress relaxation in the GaN layers occurs in conventional and in pendeo-epitaxial films via the formation of additional misfit dislocations, domain boundaries, elastic strain and wing tilt. Polychromatic X-ray microdiffraction, high resolution monochromatic X-ray diffraction and finite element simulations have been used to determine the distribution of strain, dislocations, sub-boundaries and crystallographic wing tilt in uncoalesced and coalesced GaN layers grown by maskless pendeo-epitaxy. An important parameter was the width-to-height ratio of the etched columns of GaN from which the lateral growth of the wings occurred. The strain and tilt across the stripes increased with the width-to-height ratio. Tilt boundaries formed in the uncoalesced GaN layers at the column/wing interfaces for samples with a large ratio. Sharper tilt boundaries were observed at the interfaces formed by the coalescence of two laterally growing wings. The wings tilted upward during cooling to room temperature for both the uncoalesced and the coalesced GaN layers. It was determined that finite element simulations that account for extrinsic stress relaxation can explain the experimental results for uncoalesced GaN layers. Relaxation of both extrinsic and intrinsic stress components in the coalesced GaN layers contribute to the observed wing tilt and the formation of sub-boundaries.

Superplasticity in a large-grained TiAl alloy

The superplastic behaviour was systematically investigated in a large-grained Ti–47Al–2Mn–2Nb–B alloy having nearly equiaxed γ-phase with grain size of 95 μm, in which a small amount of fine particles of α 2 distribute uniformly. Superplastic deformation was examined at a temperature range of 1025–1100 °C and strain rates range of 4×10 −5–1.28×10 −3 s −1. The large-grained TiAl alloy exhibits all deformation characteristics of conventionally fine-grained superplastic alloys without the prerequisites, fine grain size and grain boundary sliding. All the values of strain rate sensitivity, m are larger than 0.3. In most cases, an elongation over 200% was gained. A maximum elongation of 287.5% with an m value of 0.39 was obtained at 1100 °C and an initial strain rate of 4×10 −5 s −1. Microstructure evolution during superplastic deformation was characterized by optical microscopy, orientation imaging microscopy and transmission electron microscopy (TEM). Metallographic examination has shown that the average grain size of large-grained TiAl alloy decreased during superplastic deformation, after that a much finer grain size of 10 to 3–5 μm could be obtained. Electron back-scattered diffraction analysis revealed that significant grain refinement was obtained at different levels with an increase in the density of low and high angle grain boundaries. A direct evidence for dynamic formation of grain boundaries with misorientation of 15–30° was found, which was evolved from subboundaries. The evidence of subboundary formation and dislocation glide in the interior of grains was revealed by TEM observation. A continuous recovery and recrystallization process similar to that in FeAl and Fe 3Al alloys was proposed as the superplastic deformation mechanism in the large-grained TiAl alloy.

Superplasticity and deformation mechanisms in NiAl(- Mo) alloy

The mechanical behaviour and microstructural evolution in tension of NiAl - 9Mo eutectic alloy at 1100 ° C and strain rates from 10-5 to 10-3 s-1 have been investigated. High values of strain rate sensitivity index, but relatively small elongations between 150 and 200%, have been observed. Tensile specimens with various strains were quenched in water to preserve the dislocation structures for TEM examination. The TEM results show that the dislocation configuration and density change significantly with an increase in strain rate, and therefore correspond to different deformation mechanisms. At a low strain rate (5.5 × 10-5 s-1), the dislocation density is relatively low and dislocation activity is regarded as an accommodation mechanism for grain boundary sliding. In contrast, the high density of dislocations as well as clear subboundaries observed in grains at a high strain rate (5.5 × 10-4 s-1) suggest that the dislocations are active directly in response to the applied stress as well as participating in the relaxation process of grain boundary sliding by subboundary formation. Thus, grain boundary sliding is mainly responsible for superplastic deformation at a low strain rate, while superlastic deformation at a high strain rate is controlled by the combined operation of both grain boundary sliding and dynamic recovery.

Dynamical polygonization below the cellular-bifurcation threshold in thin-sample directional solidification

We show that a polygonization of the crystal (formation of grain subboundaries) takes place during the growth in thin-sample directional solidification of a transparent nonfaceted alloy at solidification rates lower than the cellular-bifurcation threshold. We give experimental evidence that this phenomenon is due to a dynamical coupling between the lattice defects intersecting the growth front and the diffusive modes of instability of the front. We present a theoretical model of this process in two-dimensional systems.

Structure of Milled Galena (PbS) Particles as a Result of Grinding: Observations by Electron Microscopy

We have examined galena powders with the aim of providing information about the preparation mode of such powders from ancient Egyptian burial objects. Two extreme conditions of milling have been used to prepare galena powders in the laboratory, and the resulting products have been examined using scanning electron microscopy and transmission electron microscopy (TEM). The microstructure of hand-crushed coarse particles consists mainly of dislocation tangles. Annealing at 300 °C promotes a substantial recovery of the dislocation structure with the formation of subboundaries. Energetic ball milling produces a large variety of particle sizes, from 10 nm to several micrometers, with grains containing very high dislocation densities. Although PbS is a soft plastic compound, its fragmentation occurs down to very small sizes along various fracture regimes like in many brittle materials. Comparisons are made between TEM observations and the data obtained from x-ray diffraction peak profile analysis.

Microstructure and Creep of Mo–ZrC <I>In-situ</I> Composite

Microstructure and creep of arc-melted and annealed Mo-40 mol%ZrC in-situ composite were investigated in this study. Microstructure after annealing is found to consist of primary ZrC particles and fine Mo/ZrC eutectic structure, i.e. hyper-eutectic structure. Compressive creep tests in the temperature range from 1673 to 1873 K indicate that the stress exponent, n, is approximately 5 in the stress range from 100 to 500 MPa, and the apparent activation energy for the creep deformation is estimated to be about 500 kJ/mol at 200 MPa and 300 MPa. TEM observation for crept specimens shows that the creep deformation is accompanied with sub-boundary formation in Mo solid solution phase. It is suggested that the creep deformation of the composite in the temperature range examined is interpreted in terms of the dislocation creep of Mo solid solution phase controlled by the lattice diffusion of either Mo or Zr.

Softening processes in single-crystal tungsten ribbons

Recovery processes in rolling-deformed (001)[110] tungsten single crystals having various degrees of purity are investigated. It is shown that the dislocation structure formed in the plastic deformation of tungsten single crystals is transformed in subsequent high-temperature anneals to a system of dislocation subboundaries; only polygonization, which preserves the single-crystal structure, takes place in samples heated to a temperature close to the melting point. The formation of subboundaries proceeds in two stages with subsequent transformation of the unstable structure to a configuration having an energy minimum. The decisive factor affecting the polygonization rate is the stacking fault energy; the presence of impurities also has a significant influence.

SiC nanoparticle-reinforced Al 2O 3 matrix composites: Role of intra- and intergranular particles

Submicron (or ‘nanocrystalline’) SiC-reinforced Al 2O 3 matrix composites have been prepared by co-milling followed by hot-pressing at 1650 °C. Both intra- and intergranular SiC particles were present. The volume ratio for intra- to intergranular particles was approximately 30:70 for 6 vol% SiC composites and 20:80 for 12 vol% SiC composites. The intragranular SiC (50–200 nm) formed mainly by growth of the Al 2O 3 matrix grains, while Zener pinning, mainly by the larger (200–300 nm) particles, was thought to effectively limit the Al 2O 3 grain size. Approximately 15% toughening was achieved by incorporating 12 vol% SiC nanoparticles into the matrix. The intergranular particles provided most of the toughening, mainly through a SiC particle-attracted crack deflection mechanism and crack impediment. No evidence was found of toughening by the intragranular particles, although they did cause some grain refinement by dislocation sub-boundary formation. A mixed fracture mode was observed, changing to a mostly intergranular mode with increasing volume of SiC particles.

Creep deformation in near-γ TiAl: Part 1. the influence of microstructure on creep deformation in Ti-49Al-1V

The influence of microstructure on creep deformation was examined in the near-y TiAl alloy Ti-49A1-1V. Specifically, microstructures with varying volume fractions of lamellar constituent were produced through thermomechanical processing. Creep studies were conducted on these various microstructures under constant load in air at temperatures between 760 °C and 870 °C and at stresses ranging from 50 to 200 MPa. Microstructure significantly influences the creep behavior of this alloy, with a fully lamellar microstructure yielding the highest creep resistance of the microstructures examined. Creep resistance is dependent on the volume fraction of lamellar constituent, with the lowest creep resistance observed at intermediate lamellar volume fractions. Examination of the creep deformation structure revealed planar slip of dislocations in the equiaxed y microstructure, while subboundary formation was observed in the duplex microstructure. The decrease in creep resistance of the duplex microstructure, compared with the equiaxed y microstructure, is attributed to an increase in dislocation mobility within the equiaxedy constituent, that results from partitioning of oxygen from the γ phase to the α2 phase. Dislocation motion in the fully lamellar microstructure was confined to the individual lamellae, with no evidence of shearing of γ/γ or γ/α2 interfaces. This suggests that the high creep resistance of the fully lamellar microstructure is a result of the fine spacing of the lamellar structure, which results in a decreased effective slip length for dislocation motion over that found in the duplex and equiaxed y microstructures.

Creep deformation in near-γ TiAl: II. influence of carbon on creep deformation in Ti-48Al-1V-0.3C

The influence of interstitial strengthening and microstructure on creep deformation has been examined in the near-γ TiAl alloy Ti-48Al-lV-0.3C. Creep studies were conducted under constant load in air at 815 °C in the stress range of 50 to 200 MPa. Significant improvement in creep resistance was observed in this alloy compared with a similar alloy (Ti-49Al-lV) containing low levels of carbon (0.07 at. pct). The degree of strengthening resulting from the addition of carbon was found to be dependent on microstructure. At 815 °C and 150 MPa, the addition of carbon reduced the minimum creep rate by a factor of approximately 20 in the equiaxedy and duplex microstructures and by a factor of 3 in the fully lamellar microstructures. Carbide precipitation occurred in this alloy when aged in the temperature range of 700 °C to 950 °C. The addition of carbon leads to a decrease in the stress exponent from 4 to 3 in the duplex and equiaxedy microstructures and the inhibition of sub-boundary formation in the duplex microstructure. This suggests that solute/dislocation interaction mechanisms, rather than a direct effect of carbide precipitates, are responsible for the significant increase in creep resistance observed in this alloy.

Subboundary Characterization and Creep Deformation in Investment Cast Ti-48Al-2Nb-2Cr

AbstractSubgrain boundaries (subboundaries) in a creep deformed investment cast Ti-48Al-2Nb-2Cr specimen were characterized. A multi-stress drop test was performed at temperature 765 °C and stresses from 276 MPa to 103 MPa. The stress exponent n = 7. Subboundaries were observed both in equiaxed γ grains and within γ laths in lamellar grains. Dislocations within the subboundaries are characterized to be ordinary 1/2<110] dislocations. Subboundaries are found to occur on many crystallographic planes and no preferred crystallographic planes are found for subboundary formation in the crept TiAl specimen. Misorientations across the subboundaries are less than 1°. Analysis of subboundary formation mode indicates that the creep deformation in TiAl is pure metal type.

Recovery of highly strained Fe–28 at.-%Al alloy

Recovery of a highly strained Fe–28 at.-%Al alloy has been studied by means of electrical resistivity, Mössbauer spectroscopy, magnetic polarisation, and coercivity measurements. The amount and state of defect rich volumes (interfacial phase) were investigated. Three recovery steps were found in the temperature dependences of resistivity and magnetic polarisation. The first two steps are characterised by almost the same activation energies (ɛ1=120±5 and ɛ1=127±1 kJ mol−1) derived from resistivity measurements, and can be ascribed to the annihilation of defects and relaxation of stresses accompanied by cell wall sharpening and the onset of subboundary formation. The appearance of backward recovery stresses was manifested as changes of spin orientation. The third step is ascribed to the process of cell wall sharpening. The observation of the third and further steps was complicated by overlapping caused by magnetic transformations and ordering effects.MST/1663

Diffusion-deformational instability and subboundary formation in the recrystallization of semiconductor-on-insulator films

A novel (defect-deformational) mechanism for subboundary formation in laser beam recrystallization of semiconductor films is proposed. The theory is developed yielding explicit analytical expressions for the distance between subboundaries as a function of film thickness and the scanning velocity of the laser beam. The theoretical results obtained agree well with the experimental results.

Dislocation mechanisms, diffusional processes and creep behavior in NbC x

A brittle-ductile transition occurs around 1373 K in NbC x primarily as a result of the increase in the metal character of the bonding with increasing temperature. Steady-state creep in NbC x between 1373 and 1573 K occurs by dislocation glide to form equiaxed cell structures; it is controlled by the unlocking of dislocation interactions within the cell walls. Between 1573 and 1873 K, the kinetic data indicate a transition from dislocation glide to dislocation climb as the rate controlling creep process. Above 1873 K, subboundary formation, and polygonization confirms that dislocation glide and climb are occurring during creep. The activation energy values for creep in this temperature range are consistent with niobium lattice diffusion as the rate controlling mechanism.