Deformation-induced grain coalescence in an electrodeposited pure iron sheet studied by in situ neutron diffraction and electron backscatter diffraction

The plastic deformation behavior of an ultrafine-grained electrodeposited pure iron sheet with a strong {1 1 1}〈h k l〉 texture was studied by in situ neutron diffraction during tensile deformation at room temperature and by electron backscatter diffraction (EBSD). The combination of volume-averaged crystallographic orientation changes determined by neutron diffraction and the local orientation relationship determined by EBSD reveals a texture change to {1 1 1}〈1 1 0〉 and corresponding microstructural changes with tension deformation. Related to such grain rotation, grain coalescence on deformation was found using semi in situ EBSD. The results obtained are explained using a characteristic slip model, which also gives a reason for the ultrahigh Lankford value of this material.     

Effects of temperature on structure and mobility of the 〈1 0 0〉 edge dislocation in body-centred cubic iron

Dislocation segments with Burgers vector b = 〈1 0 0〉 are formed during deformation of body-centred-cubic (bcc) metals by the interaction between dislocations with b = 1/2〈1 1 1〉. Such segments are also created by reactions between dislocations and dislocation loops in irradiated bcc metals. The obstacle resistance produced by these segments on gliding dislocations is controlled by their mobility, which is determined in turn by the atomic structure of their cores. The core structure of a straight 〈1 0 0〉 edge dislocation is investigated here by atomic-scale computer simulation for α-iron using three different interatomic potentials. At low temperature the dislocation has a non-planar core consisting of two 1/2〈1 1 1〉 fractional dislocations with atomic disregistry spread on planes inclined to the main glide plane. Increasing temperature modifies this core structure and so reduces the critical applied shear stress for glide of the 〈1 0 0〉 dislocation. It is concluded that the response of the 〈1 0 0〉 edge dislocation to temperature or applied stress determines specific reaction pathways occurring between a moving dislocation and 1/2〈1 1 1〉 dislocation loops. The implications of this for plastic flow in unirradiated and irradiated ferritic materials are discussed and demonstrated by examples.     

In situ TEM study of microplasticity and Bauschinger effect in nanocrystalline metals

In situ transmission electron microscopy straining experiments with concurrent macroscopic stress–strain measurements were performed to study the effect of microstructural heterogeneity on the deformation behavior of nanocrystalline metal films. In microstructurally heterogeneous gold films (mean grain size d_m = 70 nm) comprising randomly oriented grains, dislocation activity is confined to relatively larger grains, with smaller grains deforming elastically, even at applied strains approaching 1.2%. This extended microplasticity leads to build-up of internal stresses, inducing a large Bauschinger effect during unloading. Microstructurally heterogeneous aluminum films (d_m = 140 nm) also show similar behavior. In contrast, microstructurally homogeneous aluminum films comprising mainly two grain families, both favorably oriented for dislocation glide, show limited microplastic deformation and minimal Bauschinger effect despite having a comparable mean grain size (d_m = 120 nm). A simple model is proposed to describe these observations. Overall, our results emphasize the need to consider both microstructural size and heterogeneity in modeling the mechanical behavior of nanocrystalline metals.     

Structure and free volume of 〈1 1 0〉 symmetric tilt grain boundaries with the E structural unit

Atomistic simulations are used to investigate the structure and interfacial free volume of 〈1 1 0〉 symmetric tilt grain boundaries in copper containing the E structural unit from the Σ9(2 2 1)θ = 141.1° grain boundary. In this work, a stereologically-based methodology is used to calculate the grain boundary free volume along with the spacing and connectivity of free volume. After generating the minimum energy equilibrium grain boundary, we examine (i) the grain boundary structure, (ii) a measure of free volume associated with the grain boundary, (iii) spatial correlation functions of the distribution of free volume, and (iv) images of grain boundary free volume distribution. Using the results from these calculations, the influence of free volume spatial distribution and grain boundary structure on dislocation dissociation and nucleation is briefly discussed for boundaries with the E structural unit subjected to tensile loading normal to the interface along with the potential implications of free volume connectivity.     

Texture memory effect in heavily cold-rolled Ni3Al single crystals

In Ni₃Al the cold-rolled Goss texture changed to a complicated one after primary recrystallization and returned to the original Goss during the subsequent grain growth, which can be referred to as the texture memory effect. In this study, we examined the evolution of grain orientations during the grain growth using the electron backscatter diffraction (EBSD) method. It was found that just after the primary recrystallization most of the grains had a 40°〈1 1 1〉 rotation relationship to the Goss texture, the remaining grains being Goss and other textures. The formation of the 40°〈1 1 1〉 rotated grains can be explained by a multiple twinning mechanism. In the grain growth, the Goss grains, which were surrounded by the 40°〈1 1 1〉 rotated grains, grew preferentially due to the high mobility of the 40°〈1 1 1〉 grain boundaries, leading to the texture memory effect.     

Elastic anisotropy of γ′-Fe4N and elastic grain interaction in γ′-Fe4N1−y layers on α-Fe: First-principles calculations and diffraction stress measurements

The three independent single-crystal elastic-stiffness constants C_ij of cubic γ′-Fe₄N (face-centred cubic (fcc)-type iron substructure) have been calculated by first-principles methods using the density functional theory: C₁₁ = 307.2 GPa, C₁₂ = 134.1 GPa and C₄₄ = 46.0 GPa. The Zener elastic-anisotropy ratio, A = 2C₄₄/(C₁₁ − C₁₂) = 0.53, is strikingly less than 1, implying 〈1 0 0〉 as stiffest directions, whereas all fcc metals show A > 1. This elastic anisotropy is ascribed to the ordered distribution of N on the octahedral interstitial sites. X-ray diffraction lattice-strain measurements for a set of different h k l reflections recorded from γ′-Fe₄N_1−y layers on top of α-Fe confirmed the “abnormal” elastic anisotropy of γ′-Fe₄N_1−y. Stress evaluation, yielding a compressive stress of about −670 MPa parallel to the surface, was performed on the basis of effective X-ray elastic constants determined from the calculated single-crystal elastic constants C_ij and allowing a grain interaction intermediate between the Reuss and the Voigt models.     

Effects of solute and vacancy segregation on migration of a/4〈1 1 1〉 and a/2〈1 0 0〉 antiphase boundaries in Fe3Al

The effects of segregation of solute atoms and vacancies on migration of a/4〈1 1 1〉 and a/2〈1 0 0〉 antiphase domain boundaries (APDBs) in stoichiometric Fe₃Al at various temperatures are studied using a phase-field model [Koizumi Y, Allen SM, Minamino Y, Acta Mater 2008;56:5861] based on the Bragg–Williams approximation and kinetic parameters determined from experimental data. Boundary mobilities (M) were measured from the boundary velocity of shrinking circular antiphase domains (APDs). In the case of a/4〈1 1 1〉 APDBs, solute atmospheres follow the APDB until the APD vanishes by shrinking to zero radius, and therefore the Ms are always smaller than the intrinsic boundary mobilities because of the solute-drag effect. The M of a/4〈1 1 1〉 APDBs can be enhanced by up to 60% by vacancy segregation. On the other hand, the M of a/2〈1 0 0〉 APDBs is enhanced by only a few per cent. The a/2〈1 0 0〉 APDBs are observed to break away from the solute atmosphere during as the circular ABDBs shrink. The M increases by up to 40% associated with the breakaway, and becomes equal to the intrinsic boundary mobilities even though slight depletion and segregation of Al atoms remain ahead and behind the migrating boundary, respectively.     

Observations of a〈0 1 0〉 dislocations during the high-temperature creep of Ni-based superalloy single crystals deformed along the [0 0 1] orientation

A NASAIR-100 superalloy single crystal was tested in tension creep at 1000 °C at a stress of 148 MPa, for a time period of 20 h and to a strain of 1.1%. Analysis of the resulting dislocation structures after rafting was completed reveals the frequent presence of all three types of a〈0 1 0〉 dislocations in the γ′ particles. Two of these families experience no resolved forces due to the applied stress. It is proposed that these a〈0 1 0〉 dislocations form as a result of the combination of two dissimilar a/2〈0 1 1〉 dislocations entering from γ channels. The possible driving forces for the movement of these a〈0 1 0〉 dislocations are discussed, and a novel recovery mechanism during creep of rafted microstructures is introduced on the basis of these observations.     

Microstructure and growth mechanism of tin whiskers on RESn3 compounds

An exclusive method was developed to prepare intact tin whiskers as transmission electron microscope specimens, and with this technique in situ observation of tin whisker growth from RESn₃ (RE = Nd, La, Ce) film specimen was first achieved. Electron irradiation was discovered to have an effect on the growth of a tin whisker through its root. Large quantities of tin whiskers with diameters from 20 nm to 10 μm and lengths ranging from 50 nm to 500 μm were formed at a growth rate of 0.1–1.8 nm s^?1 on the surface of RESn₃ compounds. Most (>85%) of these tin whiskers have preferred growth directions of 〈1 0 0〉, 〈0 0 1〉, 〈1 0 1〉 and 〈1 0 3〉, as determined by statistics. This kind of tin whisker is single-crystal β-Sn even if it has growth striations, steps and kinks, and no dislocations or twin or grain boundaries were observed within the whisker body. RESn₃ compounds undergo selective oxidation during whisker growth, and the oxidation provides continuous tin atoms for tin whisker growth until they are exhausted. The driving force for whisker growth is the compressive stress resulting from the restriction of the massive volume expansion (38–43%) during the oxidation by the surface RE(OH)₃ layer. Tin atoms diffuse and flow to feed the continuous growth of tin whiskers under a compressive stress gradient formed from the extrusion of tin atoms/clusters at weak points on the surface RE(OH)₃ layers. A growth model was proposed to discuss the characteristics and growth mechanism of tin whiskers from RESn₃ compounds.     

High-temperature mechanical spectroscopy of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) with different amounts of intergranular phase

Y-TZP ceramics of different grain sizes and with various content of amorphous intergranular phase have been studied by mechanical spectroscopy and transmission electron microscopy. In all grades both clean grain boundaries and grain boundaries wetted by an amorphous film are present. In the materials with higher amount of intergranular phase, large amorphous pockets are present. A thermally activated mechanical-loss peak is observed in all grades at 1600 K (0.1 Hz; ΔH_act=560±20 kJ/mol). This peak evolves into a background at low frequencies and high temperature. The peak position and height depend on the grain size; the background is grain-size independent. The peak is interpreted as the limited sliding of those grains that are separated by an amorphous film. The background is attributed to the beginning of microcreep. In the grades with amorphous pockets an additional peak, interpreted as the α-relaxation of these pockets, is observed at 1425 K (0.1 Hz; ΔH_act=570±20 kJ/mol).     

Microstructure and strengthening mechanisms in Cu/Fe multilayers

Nanostructured Cu/Fe multilayers on Si (1 1 0) and Si (1 0 0) substrates were prepared by magnetron sputtering, with individual layer thicknesses h varying from 0.75 to 200 nm. The growth orientation relationships between Cu and Fe at the interfaces were determined to be of the Kurdjumov–Sachs and Nishiyama–Wasserman type. Nanoscale columnar grains in Fe, with an average grain size of 11–23 nm, played a dominant role in the strengthening mechanism when h ? 50 nm. At smaller h the hardness of Cu/Fe multilayers with (1 0 0) texture approached a peak value, followed by softening due to the formation of fully coherent interfaces. However, abundant twins were observed in Cu/Fe films with (1 1 1) texture when h = 0.75 nm, which led to the retention of high hardness in the multilayers.     

Generalized type III internal stress from interfaces,triple junctions and other microstructural components in nanocrystalline materials

The microstructure in polycrystalline materials consists of four types of geometric objects: grain cells, grain boundaries, triple junctions and vertex points. Each of them contributes to internal stress differently. Due to experimental limitations, the internal stresses associated with the microstructural components are difficult to acquire directly, particularly for polycrystalline materials with nanometer-scale grain sizes. Using newly developed computational methods, we obtained the type III internal stress associated with each of these microstructural objects in a stress-free nanocrystalline Cu. We found significant variation of the internal stresses from grain to grain, and their magnitudes descended in the order of vertex point, triple junction, grain boundary and grain cell. We also examined the effect of grain size and temperature. The change in the internal stresses inside the grains is found to follow a scaling relation of Ad^?x, using the mean grain diameter d from our results. For pressure, we found x = 1 and the effective interface stress A ～ 1 N m^?1, and for shear stress x = 0.75 and A ～ 14.12 N m^?1. On the other hand, the directly calculated interface stress is about 0.32–0.35 GPa for hydrostatic pressure and 12.45–12.60 GPa for von Mises shear stress. We discuss issues in treating the two-dimensional interface stress and one-dimensional triple junction line tension in nanocrystalline materials, as well as the potential impact of the type III internal stress on mechanical behavior of poly- and nano-crytalline materials.     

Impact of He and Cr on defect accumulation in ion-irradiated ultrahigh-purity Fe(Cr) alloys

The effect of He on the primary damage induced by irradiation in ultrahigh-purity (UHP) Fe and Fe(Cr) alloys was investigated by transmission electron microscopy (TEM). Materials were irradiated at room temperature in situ by TEM in a microscope coupled to two ion accelerators, simultaneously providing 500 keV Fe⁺ and 10 keV He⁺ ions. Single Fe ion and dual Fe and He ion beam experiments were performed up to a dose of 1 dpa and to a He content of up to 1000 appm. Defects appear in the form of nanometric black dots with sizes between 1 and 5 nm. Defocused images reveal a dense population of sub-nanometric cavities after both single-beam and dual-beam irradiation. In Fe(Cr) alloys, the number densities of visible black dot defects still resolved in TEM are significantly higher after single than after dual-beam irradiation. In UHP Fe, conversely, the presence of He strongly increases the defect number density. The presence of He changes a a₀〈1 0 0〉 dominated defect population to a 1/2a₀〈1 1 1〉 dominated one in all materials, and the more so in UHP Fe. It appears that Cr increases the number of visible defects relative to UHP Fe. The dependence with increasing Cr content is weak, however, showing only a slight decrease in the number densities. The decrease in the density of visible a₀〈1 0 0〉 loops and increase in the visible 1/2a₀〈1 1 1〉 loops in all materials when He is present supports the idea that visible a₀〈1 0 0〉 loops are formed by the interaction between mobile 1/2a₀〈1 1 1〉 loops, as the latter would be immobilized by He already at sub-microscopic sizes. It is concluded that the primary loop population is dominated by 1/2a₀〈1 1 1〉 loops.     

Microstructure,texture and magnetic properties of strip casting Fe–6.2 wt%Si steel sheet

An Fe–6.2 wt%Si strip with equiaxed grains and mild {0 0 1}〈0 v w〉 fiber texture was produced by twin-roll strip casting process. Then the as-cast strip was treated with or without the hot rolling prior to the warm rolling and annealing. When the hot rolling was not introduced, a fine and heterogeneous warm-rolled microstructure was produced and led to a fine recrystallization microstructure and very weak {0 0 1}〈0 v w〉 fiber texture in the annealed sheets. When the hot rolling was introduced, a coarse and homogeneous warm-rolled microstructure was produced and led to a very coarse recrystallization microstructure and much stronger {0 0 1}〈0 v w〉 fiber texture in the annealed sheets. The annealed sheets with hot rolling showed a higher magnetic induction and a higher core loss than those without hot rolling.     

Heterogeneous nucleation on potent spherical substrates during solidification

For a spherical-cap nucleus to become a “transformation nucleus”, the linear dimension (d) of the flat substrate must exceed the critical nucleus size (2r¹). This Turnbull criterion (d ⩾ 2r¹) defines a minimum undercooling for grain formation on, and effective inoculation with, flat nucleating substrates. However, for nucleation on potent substrates the spherical-cap model is no longer tenable. The free growth model has in general considered the growth of a two-dimensional nucleus on a potent flat substrate. Inspired by the particle-core structures observed in magnesium alloys after inoculation with nearly spherical zirconium particles, a model has been proposed, on the basis of an adsorption and surface diffusion mechanism, for heterogeneous nucleation and grain formation on potent spherical substrates of d ⩾ 2r¹. The critical undercooling required is found to be approximately the same as that defined by Turnbull’s patch nucleation theory. The model shows excellent agreement with experiments compared from different perspectives.     

Microstructure and texture evolution in a twinning-induced-plasticity steel during uniaxial tension

A combination of electron back-scattering diffraction and X-ray diffraction was used to track the evolution of the microstructure and texture of a fully recrystallized Fe–24 Mn–3 Al–2 Si–1 Ni–0.06 C twinning-induced-plasticity steel during interrupted uniaxial tensile testing. Texture measurements returned the characteristic double fibre texture for face-centred cubic materials, with a relatively stronger 〈1 1 1〉 and a weaker 〈1 0 0〉 partial fibre parallel to the tensile axis. The interaction with the stable 〈1 1 1〉 oriented grains results in preferential plastic flow in the unstable 〈1 1 0〉 oriented grains. Consequently, the grains oriented along the 〈1 1 0〉 and 〈1 0 0〉 fibres record the highest and lowest values of intragranular local misorientation, respectively. The viscoplastic self-consistent model was used to simulate the macroscopic stress–strain response as well as track the evolution of bulk crystallographic texture by detailing the contributions of perfect and/or partial slip, twinning and latent hardening. The simulations revealed the dominant role of perfect slip and the limited volume effect of twinning on the texture development. The effects of initial orientation and grain interaction on the overall orientation stability during uniaxial tension showed that while the 〈1 0 0〉 fibre remains stable and does not affect the unstable orientations along the 〈1 1 0〉 fibre, the orientations along the stable 〈1 1 1〉 fibre strongly affect the unstable 〈1 1 0〉 orientations.     

Probing structure and microstructure of epitaxial Ni–Mn–Ga films by reciprocal space mapping and pole figure measurements

The crystal structure and complex twinning microstructure of epitaxial Ni–Mn–Ga films on (1 0 0) MgO substrates was studied by X-ray diffraction using 2θ scans, pole figure measurements and reciprocal space mapping (RSM). The orientation distribution of all variants is visualized by RSM, which forms the basis for a better understanding of the crystallographic relation between variants and substrate. Above the martensitic transformation temperature the film consists of single austenite phase with lattice constant a = 5.81 Å at 419 K. At room temperature some epitaxially grown residual austenite with a = 5.79 Å remains at the interface with the substrate, followed by an intermediate layer exhibiting orthorhombic distortion, a_trans = 6.05 Å, b_trans = 5.87 Å, c_trans = 5.73 Å and a major fraction of 14M (7M) martensite, a = 6.16 Å b = 5.79 Å c = 5.48 Å. The seven-layered modulation of this metastable martensite structure is directly observed by RSM. The intermediate phase observed close to interface indicates the existence of an instable, pre-adaptive martensite phase with a short stacking period.     

Observation and modeling of the through-thickness texture gradient in commercial-purity aluminum sheets processed by accumulative roll-bonding

The texture evolution in commercial-purity aluminum (AA1070) processed by accumulative roll-bonding (ARB) is investigated with the aid of X-ray diffraction and crystal plasticity modeling. The experimental results indicate strong texture gradients through the sheet thickness, from rolling-type textures with orthorhombic symmetry at the center to shear-type textures with monoclinic symmetry near the surface. The experimental textures are reproduced well by polycrystal plasticity modeling carried out with deformation histories from finite element simulations. The observations of a relatively strong {4 4 11}〈11 11 8〉 component at the center and a {0 0 1}〈1 1 0〉 component at the surface are attributed to their higher orientation stability than the other rolling- and shear-type orientations. Examination of the average through-thickness textures suggests that the ARB technique may not be an effective means to develop apparent {1 1 1}〈u v w〉 components and thus to enhance the normal anisotropy of plasticity of the bulk sheet materials.     

Growth behavior of superconducting DyBa2Cu3O7–x thin films deposited by inclined substrate deposition for coated conductors

Superconducting DyBa₂Cu₃O_7–x (DyBCO) films were grown on biaxially textured MgO buffer layers deposited by inclined substrate deposition (ISD) on Hastelloy substrates. Despite the large lattice mismatch (8.5%) between DyBCO and MgO, the DyBCO grew epitaxially on the MgO buffer layer and the biaxial texture of the MgO was well transferred to the DyBCO. Typical critical current densities, j_c, of the DyBCO film were 2.1 MA cm^?2 at 77 K in a self-field. Biaxial texturing is the key for reaching the high critical current densities and was investigated by transmission electron microscopy. DyBCO grains were found to be ～130–500 nm in size, with faceted grain boundaries. The c-axis of the DyBCO grains was tilted away from the substrate normal by 29° such that it was perpendicular to the MgO (0 0 2) facets. A high dislocation density of ～7.4 × 10¹¹ cm^?2 and stacking faults along the ab-planes were observed in the DyBCO film. Interface, grain boundary and volume energies of the DyBCO film were calculated and a growth model for the DyBCO film is discussed. ISD offers the potential for high-quality, biaxially textured MgO buffer layers suitable for long-length superconducting coated conductors.     

Evolution of orientation distributions of γ and γ′ phases during creep deformation of Ni-base single crystal superalloys

The evolution of orientation distributions of γ and γ′ phases in crept Ni-base single crystal superalloys have been investigated by theoretical calculations with elastic–plastic models and by experiments. As creep deformation proceeds, the crystallographic orientation distributions for both phases are broadened as a result of the waving of the raft structure, which occurs to reduce the total mechanical energy. The broadening of the orientation distribution occurs in such a way that the 0 0 1 pole broadens isotropically while the h k 0 poles broaden preferentially along the 〈0 0 1〉 directions. Since the extent of the broadening increases almost linearly with the number of creep deformation, the measurement of the broadening by X-ray diffraction can be utilized in non-destructive methods to predict the lifetime of Ni-base superalloys.