Σ3 CSL boundary distributions in an austenitic stainless steel subjected to multidirectional forging followed by annealing

The effect of processing and annealing temperatures on the grain boundary characters in the ultrafine-grained structure of a 304-type austenitic stainless steel was studied. An S304H steel was subjected to multidirectional forging (MDF) at 500–800°C to total strains of ~4, followed by annealing at 800–1,000°C for 30 min. The MDF resulted in the formation of ultrafine-grained microstructures with mean grain sizes of 0.28–0.85 μm depending on the processing temperature. The annealing behaviour of the ultrafine-grained steel was characterized by the development of continuous post-dynamic recrystallization including a rapid recovery followed by a gradual grain growth. The post-dynamically recrystallized grain size depended on both the deformation temperature and the annealing temperature. The recrystallization kinetics was reduced with an increase in the temperature of the preceding deformation. The grain growth during post-dynamic recrystallization was accompanied by an increase in the fraction of Σ3ⁿ CSL boundaries, which was defined by a relative change in the grain size, i.e. a ratio of the annealed grain size to that evolved by preceding warm working (D/D₀). The fraction of Σ3ⁿ CSL boundaries sharply rose to approximately 0.5 in the range of D/D₀ from 1 to 5, which can be considered as early stage of continuous post-dynamic recrystallization. Then, the rate of increase in the fraction of Σ3ⁿ CSL boundaries slowed down significantly in the range of D/D₀ > 5. A fivefold increase in the grain size by annealing is a necessary condition to obtain approximately 50% Σ3ⁿ CSL boundaries in the recrystallized microstructure.     

Temperature and orientation dependences of the grain-boundary diffusion coefficient of antimony in copper bicrystals

An investigation is made of the diffusion of antimony through the bulk and along grain boundaries in copper bicrystals containing a symmetric 〈100〉 misorientation boundary with misorientation angles from 20 to 37.2°. The bicrystals are grown by the method of horizontal zone recrystallization. The temperature range for these studies is 480–580 °C, where the solubility of Sb in Cu is about 6 atomic % and practically temperature-independent. The concentration profiles are obtained by x-ray spectral microanalysis, and the grain-boundary diffusion parameters are computed by the method of Whipple and Suzuoka. The orientation dependence of the triple product P=sδD _b (where s is the segregation coefficient, δ the width of the grain boundary, and D _b the grain-boundary diffusion coefficient) is nonmonotonic, with a maximum for the special ∑5 misorientation boundary (36.9°). The effective activation energy for grain-boundary diffusion ranges from ∼70 kJ/mol for ∑5 to140 kJ/mol for general boundaries. Fiz. Tverd. Tela (St. Petersburg) 39, 1153–1157 (July 1997)     

Recrystallization of pure and strontium-doped KCl crystals

The kinetics of microstructure transformations are studied during annealing of deformed single crystals of KCl and KCl:0.05wt %Sr²⁺ at temperatures of (0.35–0.55)T _m (where T _m is the melting temperature) and during storage at room temperature. The effect of deformation rates ranging from 0.01 mm/min to 0.1 mm/min at a deformation temperature T _d=0.5T _m on the crystal structure and on the recrystallization kinetics is noted. It is found experimentally that the incubation period for static recrystallization in single-crystal KCl:0.05wt%Sr²⁺ is shortened and recrystallization takes place at room temperature after deformation in this temperature range. Here, during the new recrystallization grains have a twinned orientation with respect to the initial single crystal during the first stage and to the subgrains of the deformed crystal. As the annealing temperature is raised, the stage in which twins grow in KCl:0.05wt%Sr²⁺ crystals is shortened and it is displaced by recrystallization through migration of high-angle grain boundaries of the common type. Deformation conditions which ensure prolonged (at least three months) stability of the post-deformation hardening of single crystals are found experimentally for Sr²⁺-doped deformed single crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 259–264 (February 1999)     

Direct bonding of silicon wafers with the concurrent formation of diffusion layers

An original technique for Si-Si direct bonding combined with impurity diffusion in a single process is suggested. A dopant (aluminum) source is located at the interface. The high-temperature treatment of the polished wafers in an oxidizing atmosphere results in the diffusion of Al atoms and the formation of a p-n junction in n-silicon. The presence of aluminum is shown to improve the continuity of the interface. Results obtained are explained within a model whereby the initial contact between the hydrophilic silicon surfaces in a water solution of aluminum nitrate Al(NO₃)₃ serves to increase the bonding area of the wafers at room temperature due to the interaction of Al-OH groups with water molecules adsorbed on the surfaces of the wafers.     

Thermal annealing behaviour of Al/Ni/Au multilayer on n-GaN Schottky contacts

Recently GaN-based high electron mobility transistors (HEMTs) have revealed the superior properties of a high breakdown field and high electron saturation velocity. Reduction of the gate leakage current is one of the key issues to be solved for their further improvement. This paper reports that an Al layer as thin as 3 nm was inserted between the conventional Ni/Au Schottky contact and n-GaN epilayers, and the Schottky behaviour of Al/Ni/Au contact was investigated under various annealing conditions by current--voltage (I--V) measurements. A non-linear fitting method was used to extract the contact parameters from the I--V characteristic curves. Experimental results indicate that reduction of the gate leakage current by as much as four orders of magnitude was successfully recorded by thermal annealing. And high quality Schottky contact with a barrier height of 0.875 eV and the lowest reverse-bias leakage current, respectively, can be obtained under 12 min annealing at 450°C in N₂ ambience.     

Correlation study between DKI and conventional DWI in brain and head and neck tumors

PurposeTo investigate the relationship between the Diffusion Kurtosis Imaging (DKI) parameters and conventional metrics provided by Diffusion-weighted imaging (DWI) in patients affected by Brain or Head and Neck (HN) cancer.MethodsTen patients affected by brain tumor and nine patients with HN tumor underwent a pre-treatment MR examination at 3 T. The largest tumor section was manually contoured by two expert neuroradiologists. The apparent diffusion coefficient (D_app) and apparent diffusional kurtosis (K_app) parameters were determined at the voxel level by using the DKI model, and compared to the apparent diffusion coefficient (ADC) and the tissue diffusion coefficient (D_mono) obtained from mono-exponential fitting methods. The Akaike Information Criteria (AIC) was calculated to assess the quality of the fitting methods. Cross-correlations between all the variables were assessed using the Spearman rank test.ResultsIncreased K_app values were found in each lesion. All parameters were strongly related, in particular an inverse relationship emerged between median values of K_app and D_app/D_mono/ADC in both patient groups, while D_app showed positive correlations with D_mono and ADC. From the analysis at the voxel level, significant inverse associations were found between K_app and D_mono within the lesions, while a weak or moderate association emerged between K_app and ADC or D_app.ConclusionsA significant association between the apparent diffusional kurtosis K_app and the tissue diffusion coefficient D_mono emerged for both brain and HN tumors at 3 T, suggesting that both variables may consistently reflect deeper insight into the microstructural characteristics of tumors.     

Solute-diffusion-controlled dislocation creep in pure aluminium containing 0.026 at.% Fe

Pure aluminium containing about 200?at.ppm Fe in solution is shown to creep about 10⁶ times slower at 200°C than the same aluminium containing a negligible amount of iron in solution. The high creep resistance of the Al–200?at.ppm?Fe alloy is attributed to the presence of subgrain boundaries containing iron solute atoms. It is proposed that the opposing stress fields from subgrain boundaries and from the piled-up dislocations during creep are cyclically relaxed, by iron solute diffusion, to allow climb of the lead dislocation in the pile-up. The mechanism is a form of mechanical ratcheting. The model is applied to Al–Fe alloys and correctly predicts that the creep rate is controlled by the rate of iron solute diffusion and by a temperature dependence equal to the activation energy for iron diffusion, namely Q _c?=?221?kJ?mol^?1. Basic creep studies on solid-solution alloying with solute atoms that diffuse slowly in the lattice of aluminium (e.g. manganese, chromium, titanium and vanadium) appear worthy of study as a way of enhancing creep strength and of understanding creep mechanisms involving solute-atom-containing subgrain boundaries.     

Mechanism and modelling of aluminium nanoparticle oxidation coupled with crystallisation of amorphous Al2O3 shell

The oxidation of aluminium nanoparticles coupled with crystallisation of amorphous alumina shell is investigated through the thermogravimetric analyser and differential scanning calorimetry (TGA-DSC) and the transmission electron microscope (TEM). The thermogravimetric (TG) curves show stepwise shapes with temperature increase and could be divided into four stages. The reaction at the second stage is complex, including the simultaneous crystallisation of amorphous alumina (am-Al₂O₃) and Al oxidation. The crystallisation of am-Al₂O₃ promotes the reaction through generating fast diffusion channels, like micro-cracks and grain boundaries in the oxide shell to accelerate the ionic diffusion. An enhancement factor (f_react), which follows a power-law formula with the crystallisation rate, is introduced to quantify the impact of crystallisation on reaction. With heating rate increase, the second stage of TG curves shifts to the high temperature regime and the total weight gain at the second stage decreases slowly. A crystallisation-reaction model is constructed to fit and predict the weight gain after derivation of diffusivities and crystallisation kinetics. Modelling indicates that with heating rate rise, the mass increment at the second stage of TG curves decreases owing to the reduced reaction time, although the reaction is accelerated. The shift of TG curve to higher temperature is due to the polymorphic phase transition. Actually the derived kinetics of the crystallisation of amorphous alumina indicates that the polymorphic phase transformation mechanism works mainly below the heating rate of 3 K s^–1. At higher heating rate, the melting of Al takes place firstly and the crystallisation of am-Al₂O₃ follows to enhance the ionic diffusion. Therefore, when the heating rate is fast during ignition or combustion, the Al nanoparticles undergo both the melting of Al and the polymorphic phase transition of am-Al₂O₃ to accelerate the reaction.     

Low-energy sputtering events at free surfaces near anti-phase and grain boundaries in Ni3Al

Atomic recoil events on free surfaces orthogonal to two different anti-phase boundaries (APBs) and two grain boundaries (GBs) in Ni₃Al are simulated using molecular dynamics methods. The threshold energy for sputtering, E _sp, and adatom creation, E _ad, are determined as a function of recoil direction. The study is relevant to FEG STEM (a scanning transmission electron microscope fitted with a field emission gun) experiments on preferential Al sputtering and/or enhancement of the Ni–Al ratio near boundaries. Surfaces intersected by {110} and {111} APBs have minimum E _sp of 6.5?eV for an Al atom on the Ni–Al mixed (M) surface, which is close to the value of 6.0?eV for a perfect M surface. High values of E _sp of an Al atom generally occur at a large angle to the surface normal and depend strongly on the detailed atomic configuration of the surface. The mean E _sp, averaged over all recoil directions, reveals that APBs have a small effect on the threshold sputtering. However, the results for E _ad imply that an electron beam could create more Al adatoms on surfaces intersected by APBs than on those without. The equilibrium, minimum energy structures for a (001) surface intersected by either Σ5[001](210) or Σ25[001](340) symmetric tilt grain boundaries are computed. E _sp for surface Al atoms near these GBs increases monotonically with increasing recoil angle to the surface normal, with a minimum value, which is only about 1?eV different from that obtained for a perfect surface. Temperature up to 300?K has no effect on this result. It is concluded that the experimental observations of preferential sputtering are due to effects beyond those for E _sp studied here. Possible reasons for this are discussed.     

Free Surface and Interface Thermodynamics of Liquid Nickel in Contact with Alumina

Sessile drop experiments of Ni and Ni(2at.%Al) were conducted under controlled working conditions, at 1500°C, P(O₂) 10^–9 Torr. It is shown that Al and oxygen atoms engaged in the capillary driven mass transport at the interface have a significant impact on the surface/interface thermodynamics. The surface energy of liquid Ni determined from experiments in which Ni comes into contact with Al₂O₃ is significantly lower than that of high purity Ni, due to the segregation of Al. The free energy of segregation of Al to the free surface of Ni ( G _S) was found to range from –164 to –152 kJ/mol, indicating a relatively strong tendency for segregation of Al to the free surface of Ni(Al). It is proposed that an Al(O)-rich liquid layer forms adjacent to the Ni-Al₂O₃ interface, which improves interfacial adhesion. In the Ni(Al)-Al₂O₃ system, an increase in the Al content of the alloy leads to the improvement of both wetting and adhesion of the alloy on the ceramic, correlating with the improvement in the interface strength after solidification.     

HAADF-STEM study of β′-type precipitates in an over-aged Al–Mg–Si–Ag alloy

Coarse, rod-shaped precipitates growing along ?100?Al directions in an Al–1.0?wt% Mg₂Si alloy with 0.5?wt% Ag additions were investigated by high-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). All investigated precipitates had complex structures, being composed of domains separated by anti-phase resembling boundaries. The domains consist of a modified hexagonal β′-type structure that contains a considerable amount of Ag. Based on HAADF-STEM images, an average atomic model with space group P-62?m (189) and composition Al₃Mg₃Si₂Ag is proposed, having Al incorporation and Ag replacing certain Si atomic columns. Co-existence with the Ag-free β′-Mg₉Si₅ phase has been observed for some precipitates. The boundaries may be described as full or half units of the orthorhombic U2-AlMgSi precipitate phase. The HAADF-STEM images indicate partial replacements of Al atoms by Ag, in both the β′-type domains and the U2-type boundaries. Ag enrichment of the Al matrix near the precipitate/Al interface was observed for all the investigated precipitates     

Critical current of an inhomogeneous Josephson junction at an intergrain boundary with a random distribution of dislocations

The critical current J _c(θ) of an intergrain boundary is calculated as a function of the contact misorientation angle θ of the granules. It is assumed that the ordering parameter is suppressed in regions near boundaries with an enhanced mechanical stress induced by randomly distributed surface dislocations. The stress distribution function is determined using a probabilistic approach. Assuming that the weak coupling at the boundary is Josephson coupling, an analytic expression is found for the angular dependence J _c(θ) (for tilt and twist boundaries). The magnitude of the residual critical current of a boundary in a strong magnetic field is estimated. Fiz. Tverd. Tela (St. Petersburg) 40, 393–402 (March 1998)     

Grain boundary wetting in the Al-Mg system and synthesis of magnesium diboride in contact with melt

The interaction between a Mg-containing melt and B under conditions of partial and complete wetting of Al/Al grain boundaries by Al-Mg melt has been investigated. The study was performed on Al polycrystals with Mg contents of 5, 10, 15, 18, and 25 wt %. Correspondingly, the Mg content in the melt was determined by the liquidus line and was in the range from 5 to 30 wt %. The obtained metal-matrix composites were investigated by light and scanning electron microscopy, electron-probe microanalysis, and X-ray diffraction. The possibility of synthesizing MgB₂ in the contact with a melt having a relatively low Mg content (from 15 to 30 wt %) has been demonstrated.     

On the kinetics of a two-level system with a diffusion-modulated interaction

For a two-level system corresponding to a particle of spin ½ in a random field in the Z direction, the relaxation function

has been estimated, the magnitude Ω(t) being the sum of the isotropic interactions of the particle in question with particles j executing diffusional motion. Specifically Ω(t)=ω₀ + Σ ω(Mj, rj), where ω₀ = constant, M_j is a random time-independent parameter, ω(M, r) decreases with r faster than r ^-3 and r _j = r _j (t) is a diffusional, random function. From the expression for <σ⁺(t)>, we establish general features of the relaxation phenomenon for diffusional processes, and calculate the relaxation rate 1/T ₂ and relaxation shift Δω to be 1/T ₂-iΔω = 4π CAv_M λ _M , where C is the concentration of particles and λ _M is the scattering length for an equation of the Schrödinger type with an imaginary potential -iω(M, r) instead of U/?, and diffusion coefficient instead of ?/2m. We also found that for the case of ‘external’ relaxation, the Redfield approach proved valid only under the simultaneous restrictions of low concentration and weak interaction.     

Investigation on bainite transformation and metastable omega/orthorhombic martensite phases in as-quenched Cu-bearing ferrite steel

Abstract

The as-quenched microstructures of low-carbon Cu-bearing ferrite steel are investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is found that the granular bainite formation in Cu-bearing ferrite steel is attributed to a mixed mode of diffusional and displacive mechanisms. Besides, the particle-like or plate-shaped ω phases, along with thin-plate martensites consist of twin, are observed in the Mn1.4Cu-bearing steel, and the orientation relationship is determined as: [2 ?1 ?10]_ω // [0 1 1]_α and (0 1 ?1 0)_ω // (2 ?1 1)_α. The ultra-fine or ellipsoidal shaped nanoscale orthorhombic martensite phases, along with laths martensites consist of a high density of dislocations, are observed in the Mn0.66Cu-bearing steel, and the orientation relationship is determined as: [0 1 ?1]o//[?1 1 1]α and (?1 1 1)o//(1 0 1) α. A model is proposed to describe the overall martensite transformation kinetics by taking into account the Mn content, heterogeneity of C content and dislocation reactions play a major role in the subsequent martensite transformation.     

Effect of surface roughness on the development of protective Al2O3 on Fe-10Al (at.%) alloys containing 0-10 at.% Cr

The effect of alloy surface roughness, achieved by different degrees of surface polishing, on the development of protective alumina layer on Fe-10 at.% Al alloys containing 0, 5, and 10 at.% Cr was investigated during oxidation at 1000 °C in 0.1 MPa oxygen. For alloys that are not strong Al₂O₃ formers (Fe-10Al and Fe-5Cr-10Al), the rougher surfaces increased Fe incorporation into the overall surface layer. On the Fe-10Al, more iron oxides were formed in a uniform layer of mixed aluminum- and iron-oxides since the layer was thicker. On the Fe-5Cr-10Al, more iron-rich nodules developed on an otherwise thin Al₂O₃ surface layer. These nodules nucleated preferentially along surface scratch marks but not on alloy grain boundaries. For the strong Al₂O₃-forming Fe-10Cr-10Al alloy, protective alumina surface layers were observed regardless of the surface roughness. These results indicate that the formation of a protective Al₂O₃ layer on Fe-Cr-Al surfaces is not dictated by Al diffusion to the surface. More cold-worked surfaces caused an enhanced Fe diffusion, hence produced more Fe-rich oxides during the early stage of oxidation.     

Atomistic studies of the structure and composition of grain boundaries in Cu3Au and Ni3Al

In this paper we investigate the atomic structure and composition of grain boundaries in Cu₃Au (weakly ordered compound) and Ni₃Al (strongly ordered compound). Computer simulations employing both the molecular statics and Monte Carlo methods were performed and the Finnis-Sinclair type many-body central force potentials used. First, grain boundaries in stoichiometric alloys are studied with the goal to investigate the impact of ordering strength on the grain boundary structure and composition. In Cu₃Au grain boundaries may become compositionally disordered even at room temperature and the compositional disordering is associated with segregation of gold. In contrast, in Ni₃Al grain boundaries remain compositionally ordered up to very high temperatures. Secondly, the structures of grain boundaries and the effect of Ni and Al segregation in non-stoichiometric Ni₃Al are investigated. Nickel segregation leads to compositional disordering at grain boundaries, while aluminum segregation, which is strongly selective, leads to an ordered grain boundary structure with high Al content. The possible relationship between structural and compositional characteristics of grain boundaries and their mechanical properties, in particular the grain boundary brittleness and its alleviation by additional alloying, are then discussed in the light of the results of this study.     

合金元素Zr韧化不同计量比Ni3Al合金的微观机制

利用正电子湮没技术（PAT）测量了不同化学计量比二元Ni3₃Al合金及不同Zr含 量Ni3₃Al合金的正电子寿命谱，并估算了合金基体和晶界缺陷处的自由电子密度.结果表明，二元Ni77₇₇Al23₂₃合金的基体和缺陷态的自由电子密度都比二元 Ni74₇₄Al26₂₆合金的高. Ni3₃Al合金晶界缺陷处开空间大于Ni空位或Al空位的开空间，晶界缺 陷处的自 关键词： 3Al合金')" href="#">Ni3₃Al合金 微观机制 自由电子密度 韧化     

Yttrium influence on the alumina growth mechanism on an FeCr23Al5 alloy

The mechanism by which yttrium modifies alumina growth was studied by comparing the behaviour of a high purity FeCr₂₃Al₅ alloy, either undoped or Y doped by implantation. By combining several techniques, in particular XPS, nuclear reaction analyses and electrochemical measurements, it is shown that the growth of Al₂O₃ scales on pure samples is mainly ensured by aluminum short-circuit diffusion. The presence of yttrium promotes the oxygen diffusion along grain boundaries, while retarding Al short-circuit diffusion and increasing Al lattice diffusion. From this growth mechanism with both cationic amd anionic diffusion along different paths, suggestions for the improvement of scale adherence due to yttrium are proposed. The simultaneous study of C- and Y-doped samples indicates that synergetic effects occur.     

Investigation of the influence of the conditions during recrystallization annealing on the plasticity of molybdenum-base alloys with low concentrations of additives

An investigation was made of the conditions during recrystallization annealing on the plasticity and the nature of fracture of Mo-base alloys containing zirconium or titanium (TSM-4, TsM-6, Mo-Ti-C) and alloys free of active oxide-forming elements (TSM-3). Annealing at T > T_recr took place under traditional conditions employed in heat treatments (in 10^–4–10^–6 torr vacuum, in hydrogen-filled furnaces) or under conditions ensuring minimal saturation of the alloys with oxygen during heat treatments (this involved preliminary outgassing vacuum furnaces and wrapping of the samples in molybdenum foil) or under conditions of controlled partial oxygen pressure (in the presence of the Al₂O₃ powder in the annealing zone). The degree of saturation of the alloys with oxygen was deduced from the appearance of zirconium or titanium oxide in the surface layers of the alloys containing these elements. The results indicated that the conditions during heat treatment and alloying of molybdenum with zirconium and titanium had an important influence on the plasticity and fracture of the Mo-base alloys. The results obtained were analyzed on the basis of the ideas on intergranular brittleness in bcc materials because of segregation of oxygen at grain boundaries and a refining influence of zirconium and titanium on the Mo alloys saturated with oxygen, involving binding of the oxygen in oxides and thus preventing its segregation at grain boundaries.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 93–99, August, 1980.