Atomistic simulations of grain boundary migration in copper

While the motion of twist boundaries can be readily studied by atomistic simulations with molecular dynamics (MD) under the action of an elastic driving force, the approach fails for tilt boundaries. This is due to the interaction of the elastic stress with the grain boundary (GB) structure, which causes plastic strain by GB sliding. A novel concept, the orientation correlated driving force, is introduced to circumvent this problem. It is shown that this concept can be successfully applied to the study of the migration of tilt boundaries. The migration behavior of several twist and tilt GBs was investigated. The transition from low-to high-angle boundaries can be captured, and a structural transition of tilt boundaries was found at high temperatures, which also affected the migration behavior. The results compare well with experimental results of the motion high-angle boundaries, but for low-angle boundaries, the agreement is poor. This article is based on a presentation made in the “Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,” December 2–3, 2004, organized by The Royal Institute of Technology in Stockholm, Sweden.     

Early Stages of Recrystallization in Equal-Channel Angular Pressing (ECAP)-Deformed AA3104 Alloy Investigated Using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) Orientation Mappings

Early stages of recrystallization in alloys containing complex structure of second phase particles are of considerable practical interest. They were observed for the AA3104 alloy in which large particles occur against the background of randomly distributed dispersoids. The samples were deformed by equal channel angular pressing and then slightly annealed to obtain the state of partial recrystallization. The highly deformed alloy contained a structure of flat grains with the spacing between high-angle grain boundaries ranging from 100 nm to 1 ??m. On annealing, the structure coarsened and got transformed into nearly equiaxed grains by both discontinuous and continuous recrystallization. The nucleation of new grains was observed in statically recrystallized bulk samples using scanning electron microscopy, and during in-situ recrystallization in a transmission electron microscope. Special attention was paid to the nucleation of new grains in areas close to large second phase particles, where a relatively high stored energy was expected to stimulate nucleation. A particular role in the rise of nuclei is attributed to migration of low angle boundaries. During recrystallization at 623 K (350?°C), in most of the observed cases, the growth of grains occurred by coalescence of neighbouring cells and by migration of high-angle grain boundaries. These processes led to nearly equiaxed grains of similar size. Orientation mappings showed that although orientations of new grains were widely scattered, they were not completely random.     

Evolution of texture and grain misorientation in an Al-Mg alloy exhibiting low-temperature superplasticity

Low-temperature superplasticity (LTSP) at 250 °C and 1 × 10⁻³ s⁻¹ was observed in a 5083 Al-Mg base alloy after thermomechanical treatments (TMTs). With a higher TMT rolling strain, the fraction of high-angle grain boundaries increased, which was favorable for the further operation of grain-boundary sliding (GBS) and LTSP. The near-brass {110}〈112〉, S {123}〈634〉, and Cu {112}〈111〉 texture components in the as-thermomechanically treated specimens gradually evolved into a random orientation distribution during LTSP straining from 30 to 100 pct. Static annealing at 250 °C itself could not alter the existing texture. The grain-misorientation distribution curves also showed that, after 100 pct LTSP elongation, the misorientation angles approached the random distribution. In the latter case, the low-, medium-, and high-angle boundaries each would partition around 10, 20, and 70 pct, respectively. When the LTSP elongation was greater than 150 pct, the macrodeformation anisotropy (R) ratio would reach a plateau value of ∼0.8. During the initial stage, a group of over 60 grains proceeded cooperative grain-boundary sliding (CGBS); most individual grain boundaries started to slide at the later stage. It seems that it is the high-angle boundaries, not the special coincidence-site lattice (CSL) boundaries, which could govern the LTSP performance.     

Damage Susceptibility of Grain Boundaries in HT9 Steel Subjected to High-Temperature Creep

HT9 steel is an attractive ferritic/martensitic steel that is used in components of nuclear and fossil power plants because of its high strength and good swelling resistance. Specific phenomena (such as segregation, voiding, cracking, etc.) are prevalent along grain boundaries since these interfaces act as efficient sources for vacancies. The accumulation of vacancies in grain boundaries may result in intergranular fracture. In this study, HT9 steel was subjected to creep tests at elevated temperature (about 0.5 T _m) and two different creep conditions (where creep lifetimes were about 100 and about 1000?hours, respectively). The grain boundaries in HT9 steel after creep tests were studied by the use of scanning electron microscopy in order to establish the relationship between the grain boundary structure and creep damage. Images and data obtained using electron backscatter diffraction reveal a high susceptibility of high-angle boundaries to creep cavitation, as expected. In addition, the ??3 boundaries are also susceptible to damage under these conditions at a similar or even higher rate as compared with random high-angle boundaries.     

晶界位向对硅钢立方/旋转立方取向双晶轧制取向变化的影响

晶界和{100}柱状晶在硅钢生产过程中对织构的遗传和演变有关键作用,因此本文利用晶体塑性有限元方法进行立方和旋转立方取向双晶在晶界不同位向时晶体取向演变的全场模拟。模拟显示,三种晶界位向下,晶界都具有诱发晶内产生S形状形变不均匀和缓解局部形变不均匀区取向转动的特点,立方和旋转立方取向双晶在带有剪切作用的轧制条件下都显示明显的取向稳定性。GB⊥RD(表示晶界垂直于轧向)晶界位向时,旋转立方取向晶粒优先在晶界中心位置发生取向转动,而立方取向则优先在远离晶界的端部发生取向转动。GB⊥TD(表示晶界垂直于横向)的晶界位向下,其晶界阻碍作用最小,双晶内产生的取向漫散度大,织构强度较低;除绕TD转动外,也具有复杂的绕RD、ND的取向转动。GB⊥ND(表示晶界垂直于法向)的晶界位向下,取向转动与GB⊥RD时相近,但有少量取向绕ND转动。     

The dislocation microstructure of aluminum

Aluminum of 99.999 pct purity was deformed in torsion at 644 K and an equivalent uniaxial strain rate of 5.04 × 10⁻⁴ s⁻¹ to various steady-state strains up to 16.33. The subgrain size and density of dislocations not associated with subgrain boundaries remained fixed throughout the wide steady-state strain range. The subgrain boundaries, however, underwent two important changes. At the onset of steady state (ε ~0.2) all of the subgrain boundaries had relatively small misorientation angles averaging about 0.5 deg. With increased strain, however, an increasing fraction of the subgrain facets were high-angle boundaries. At strains greater than about four nearly a third of the boundaries were high-angle. In specimens with both types of boundaries, the high-angle boundaries have misorientation angles (θ) greater than 10 deg, while θ for low-angle boundaries is nearly always less than 3 deg. Only rarely do subgrain boundaries have misorientation angles between 3 deg and 10 deg. In aluminum, the increased high-angle boundary area at larger strains originates from the extension of the initial boundaries through the mechanism, recently introduced by others, of “geometric dynamic recrystallization” in aluminum. The average misorientation across low-angle boundaries initially increases during steady state but eventually reaches a maximum value of about 1.2 deg at ε ≃ 1.2. Since the flow stress stays nearly constant, the dramatic changes in the character of the subgrain boundaries that are observed during steady state suggest that the details of the boundaries arenot an important consideration in the rate-controlling process for creep.     

A Study on Microstructural Evolution and Dynamic Recrystallization During Isothermal Deformation of a Ti-Modified Austenitic Stainless Steel

Dynamic recrystallization (DRX) behavior in hot deformed (by uniaxial compression in a thermomechanical simulator in the temperatures range 1173 K to 1373 K [900 °C to 1100 °C]) Ti-modified austenitic stainless steel was studied using electron back scatter diffraction. Grain orientation spread with a “cut off” of 1 deg was a suitable criterion to partition dynamically recrystallized grains from the deformed matrix. The extent of DRX increased with strain and temperature, and a completely DRX microstructure with a fine grain size ~4 μm (considering twins as grain boundaries) was obtained in the sample deformed to a strain of 0.8 at 1373 K (1100 °C). The nucleation of new DRX grains occurred by the bulging of the parent grain boundary. The DRX grains were twinned, and a linear relationship was observed between the area fraction of DRX grains and the number fraction of Σ3 boundaries. The deviation from the ideal misorientation of Σ3 boundaries decreased with an increase in the fraction of Σ3 boundaries (as well as the area fraction of DRX) signifying that most Σ3 boundaries are newly nucleated during DRX. The generation of these Σ3 boundaries could account for the formation of annealing twins during DRX. The role of Σ3 twin boundaries on DRX is discussed.     

Stress corrosion cracking of copper bicrystals with 〈110〉-Tilt ∑3, ∑9, and ∑11 coincident site lattice boundaries

The stress corrosion cracking (SCC) behavior of copper bicrystals with 〈HO〉-tilt ∑3(111), ∑9(221), and ∑11(311) coincident site lattice (CSL) boundaries was investigated. Stress corrosion cracking tests were carried out in 1 N NaNO₂ aqueous solution at 303 ±2 K using a slow strain rate technique (SSRT). Transgranular SCC occurred along the primary slip traces on the top surface of the bicrystal having a ∑3(111) coincidence boundary. No cracks initiated on the grain boundary except for very small and shallow corrosion pits. In contrast, for the bicrystals with ∑9(221) or ∑11(311) coincidence boundaries, corrosion pits and cracks initiated on the grain boundary and propagated into the crystal interior along {110} traces, which are almost perpendicular to the tensile axis. The SCC behavior is closely related to the activated slip systems and the degrees of crystal rotation owing to deformation. Susceptibility to intergranular SCC is affected by the angle between the Burgers vector of the primary slip system and the grain boundary plane. The susceptibility of the ∑23(111) boundary to SCC is remarkably low in comparison with the other two types of grain boundaries. Y. Nakazawa, formerly Graduate Student, Department of Mechanical Engineering, Doshisha University This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

Inertia welding nickel-based superalloy: Part I. Metallurgical characterization

This article describes a quantitative study of the microstructure of nickel-based superalloy RR1000 tube structures joined by inertia welding. One as-welded and three post weld heat-treated (PWHT) conditions have been investigated. The samples were characterized mechanically by measuring the hardness profiles and microstructurally in terms of γ grain size, γ′ precipitate size and volume fraction, stored energy, and microtexture. Electron backscatter diffraction (EBSD) was used to characterize high-angle grain boundaries (HAGB) and the variation of microtexture across the weld line. The coherent γ′ precipitates were investigated over a range of scales on etched samples in a field emission gun scanning electron microscope (FEGSEM), using carbon replicas in a transmission electron microscope (TEM) and from thin slices by means of high-energy synchrotron X-rays. Dramatic changes in the microstructure were observed within 2 mm of the weld line. In this region, the hardness profile is influenced by changes in grain size, γ′ volume fraction, γ′ particle size, and the work stored in the material. Further away, the observed hardness variation is still significant although only minor microstructural changes could be observed. In this region, the correlation of microstructure and hardness is less straightforward. Here, a combination of small microstructural changes appears to give rise to a significant change in strength. No significant texture or grain distortion was observed in the extensively plastically deformed region due to recrystallization.     

New grain formation during warm deformation of ferritic stainless steel

Microstructural evolution accompanied by localization of plastic flow was studied in compression of a ferritic stainless steel with high stacking fault energy (SFE) at 873 K (≈0.5 Tm). The structure evolution is characterized by the formation of dense dislocation walls at low strains and subsequently of microbands and their clusters at moderate strains, followed by the evolution of fragmented structure inside the clusters of microbands at high strains. The misorientations of the fragmented boundaries and the fraction of high-angle grain boundaries increase substantially with increasing strain. Finally, further straining leads to the formation of new fine grains with high-angle boundaries, which become more equiaxed than the previous fragmented structure. The mechanisms operating during such structure changes are discussed in detail.     

Nanostructure in an Al-Mg-Sc alloy processed by low-energy ball milling at cryogenic temperature

Spray-atomized Al-7.5Mg-0.3Sc (in wt pct) alloy powders were mechanically milled at a low-energy level and at cryogenic temperature (cryomilling). The low-energy milling effectively generated a nanoscale microstructure of a supersaturated face-centered cubic (fcc) solid solution with an average grain size of ∼26 nm. The nanoscale microstructure was fully characterized and the associated formation mechanisms were investigated. Two distinct nanostructures were identified by transmission electron microscopy (TEM) observations. Most frequently, the structure was comprised of randomly oriented equiaxed grains, typically 10 to 30 nm in diameter. Occasionally, a lamellar structure was observed in which the lamellas were 100 to 200 nm in length and ∼24 nm wide. The morphology of the mixed nanostructures in the cryomilled samples indicated that high-angle grain boundaries (HAGBs) formed by a grain subdivision mechanism, a process similar to which occurs in heavily cold-rolled materials. The microstructural evidence suggests that the subdivision mechanism observed here governs the development of fine-grain microstructures during low-energy milling.     

On the Microstructural Stability of Ultrafine-Grained Interstitial-Free Steel under Cyclic Loading

The microstructural stability of ultrafine-grained (UFG) interstitial-free (IF) steel under cyclic loading was investigated. The samples were extracted from material processed along two different equal channel angular extrusion (ECAE) routes (4C and 4E) at room temperature. Low-cycle fatigue tests were carried out in addition to electron and optical microscopy in order to characterize the microstructural evolution induced by cyclic deformation. The results revealed substantial differences in microstructure resulting from different processing routes. Specifically, the volume fraction of high-angle grain boundaries (HAGBs) and low-angle grain boundaries (LAGBs) varied significantly depending on the processing route. The different microstructural characteristics stemming from different ECAE routes expressively influence the fatigue response. Route-4C-processed material displays cyclic softening, while processing along route 4E leads to microstructural stability under cyclic loading. This highly route-dependent trend in the cyclic stress-strain response is attributed to the instability of the LAGBs and stability of HAGBs during cyclic deformation, which is further supported by electron backscattering diffraction results. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application,” which occurred September 25–27, 2006 in Kloster Irsee, Germany.     

Effect of grain boundaries on isothermal solidification during transient liquid phase brazing

The influence of liquid penetration at grain boundary regions on the rate of advance of the solid-liquid interface during isothermal solidification of transient liquid phase (TLP) brazed nickel joints has been examined. The test samples used in this study were Ohno-cast nickel with a grain size of >4 mm and a fine-grained nickel with a grain size of around 40 μm. Both Ni-base materials had the same chemical composition. The rate of isothermal solidification was greater when fine-grained nickel was employed during TLP brazing using Ni-11 wt pct P filler metal at 1200 °C. Liquid penetration at grain boundaries accelerates the isothermal solidification process by increasing the effective solid-liquid interfacial area and increasing the rate of solute diffusion into the base material. An analysis of electron channeling patterns has confirmed that random high-angle boundaries have a greater influence on the rate of isothermal solidification than ordered boundaries including small-angle or twin boundaries. Formerly Visiting Scientist, Department of Metallurgy and Materials Science, University of Toronto. Formerly Postdoctoral Fellow, Department of Metallurgy and Materials Science, University of Toronto     

gS = 99 and Σ = 41 Grain boundaries

The structure and crystallography of two special high-angle grain boundaries which have been observed in spinel are discussed in terms of coincident-site lattice concepts. The two grain boundaries correspond to relatively high-Σ values; the value of Σ = 99 is higher than that which is usually considered to be significant for grain boundaries. It is proposed that it is not the actual value of Σ which is important for these interfaces but rather the fact that the misorientations which are responsible for producing the grain boundaries cause several pairs of low-index crystal lattice planes to be nearly parallel to one another at the interface. The relationship between this interpretation and the frequent observation of both low-Σ interfaces and asymmetric grain boundaries in this and other materials is emphasized.     

SIMS study of hydrogen at the surface and grain boundaries of nickel bicrystals

The distribution of deuterium at the surface and at grain boundaries of cathodically charged nickel bicrystals was studied using Secondary Ion Mass Spectrometry. Both high energy and low energy boundaries were studied under conditions where deuteride formation occurred during charging. The formation of the deuteride along the grain boundaries was compared to that in the adjacent grains and interpreted in terms of the deuterium diffusion in the lattice and along the grain boundaries. Decomposition of the deuteride was also studied. The results are interpreted in terms of the dependence of diffusion along grain boundaries on the structure of the boundary.     

Grain boundary sliding and stress concentration during creep

Importance of grain boundary sliding to creep intergranular fracture is focussed. Previous metallographic and fractographic studies of creep intergranular fracture on metal bicrystals and polycrystals are briefly reviewed in order to show the close relationship between grain boundary sliding and fracture. Deformation ledge and migration irregularity are shown to be potential sites of stress concentration and crack nucleation on sliding grain boundaries without particles. The effect of grain boundary structure on creep intergranular fracture is discussed on the basis of the effect of grain boundary structure on sliding, the contribution of sliding to the overall creep deformation, and a sliding-fracture diagram. Recent observations of the effect of grain boundary structure on creep intergranular fracture on alpha iron-tin alloy polycrystals are shown. This paper is based on a presentation made at the symposium “The Role of Trace Elements and Interfaces in Creep Failure” held at the annual meeting of The Metallurgical Society of AIME, Dallas, Texas, February 14-18, 1982, under the sponsorship of The Mechanical Metallurgy Committee of TMS-AIME.