A study of thermal stability in electrodeposited nanocrystalline Fe-Ni invar alloy

The grain growth of an electrodeposited nanocrystalline Fe-Ni invar alloy was studied with differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The results showed very limited grain growth below the temperature of 0.36T_m, and the activation energy was 40 ± 3 kJ/mol in the low temperature range, which implied the possible mechanism of grain growth as re-ordering of grain boundaries; an abrupt grain growth happened above 0.36T_m with a DSC exothermic peak detected, of which the heat release was about 14 ± 2 J/g; at temperatures above 0.36T_m, the grain growth activation energy was obtained through Kissinger analysis and isothermal kinetics analysis, both of the results suggested the grain growth mechanism as grain boundary diffusion.     

Grain growth and stabilisation of nanostructured aluminium at high temperatures: review

Nanostructured (NS) materials have a large stored energy due to their large grain boundary area and thus tend to be unstable with respect to grain growth during high temperature annealing or deformation. This problem can limit the application of NS materials at high temperatures (>0·5T_m, absolute melting temperature), especially Al alloys owing to their low melting points. Restoration processes and grain growth in NS Al based materials are critically reviewed, with emphasis on nanostructure grain stabilisation at high temperatures. The mechanisms of normal and abnormal grain growth during isothermal annealing are presented, followed by consideration of thermal stabilisation by the addition of solute atoms/impurities and/or dispersion of second phase particles. Grain growth is significantly facilitated by applying deformation at elevated temperatures during preparation or further processing of semifinished NS materials. The dynamic restoration processes, dynamic grain growth and dynamic particle coarsening are addressed in NS Al. Finally, grain growth during consolidation of nanocrystalline powders (one of the principal methods to fabricate bulk NS Al) is presented, and the effects of processing parameters on grain size stabilisation are discussed.     

Evaluation of thermal stability of ultrafine grained aluminium matrix composites reinforced with carbon nanotubes

The effect of carbon nanotubes on the thermal stability of ultrafine grained aluminium alloy processed by the consolidation of nano-powders obtained by mechanical alloying was evaluated via measurements of grain size and mechanical property changes upon annealing at various temperatures. It was found that the grain size of the samples containing carbon nanotubes is stable up to high temperatures and even after annealing at 450 °C (0.7T_m) no evident grain growth was observed. The limited grain boundary migration was attributed to the presence of entangled networks of carbon nanotubes located at grain boundaries and to the formation of nanoscale particles of aluminium carbide Al₄C₃. It was also revealed that carbon nanotubes decompose at a relatively low temperature of 450 °C and form fine Al₄C₃ precipitates. This transformation does not significantly affect the mechanical properties due to the nanoscale size of the carbides.     

Stress relaxation following heating of nanocrystalline nickel

Stress relaxation of nanocrystalline nickel within the range of temperatures from 523 to 673K (0.17–0.27·T_m) in the regime of uniaxial compression is studied. The results obtained for nickel with more coarse grains are given for comparison. An average strain rate of nanocrystalline nickel within the investigated range of temperatures is 1.75·10⁻⁵–3.03· 10⁻⁵s⁻¹. The presence of two types of stress relaxation dependencies is shown. The most probable mechanism of plastic strain is grain boundary sliding controlled by grain boundary diffusion for 623–673K. At lower test temperatures, 523–573K, a plastic strain occurs by the powder law creep according to the Weertman model.     

Copper-based materials formed by high rate sputtering

This paper presents the results of microstructure and property determinations made on pure and alloyed copper sputter deposited at rates of 20–25 nm s^-1. For each material, the effects of substrate temperature and post-deposition annealing on the grain size and tensile properties were investigated. Fully dense coherent structures were obtained at all substrate temperatures. The absence of the porous poorly bonded structures obtained at temperatures in the range 0.1–0.2T_m by other workers is attributed to the low gas pressure, the normal incidence adatoms and the polished substrates used in the present study.All materials exhibited high tensile strenghts when deposited at 0.2T_m Values of 700 MPa were observed with pure copper, while precipitation-hardening and dispersion-hardening alloys produced values of 1000MPa. The principal difference between these materials was in the stability of their as-deposited structure and their strength during post-deposition annealing. For example, pure copper was unstable to recrystallization at room temperature (0.2T_m), while the dispersion- strengthened alloys exhibited only minor softening at 0.8T_m and very little grain growth at 0.95T_m. The mechanical properties are discussed in relation to the microstructure produced by high rate deposition.     

Effect of annealing on microstructure and mechanical properties of bulk nanocrystalline Fe3Al alloy with 5 wt.% Cu prepared by aluminothermic reaction

Microstructure and mechanical properties of bulk nanocrystalline Fe₃Al based alloy with 5 wt.% Cu prepared by aluminothermic reaction before and after annealed at 873, 1073 and 1273 K for 8 h were investigated. Microstructures of the alloy before and after the annealing consisted of a Fe-Al-Cu matrix, a little Al₂O₃ sphere and Fe₃AlC_x fiber phases. The matrix of the alloy before the annealing was composed a nanocrystalline phase with disordered bcc crystal structure and a little amorphous phase. The amorphous phase disappeared after the annealing and Fe₃Al phase with ordered DO₃ structure appeared in the alloy after annealed at 1073 and 1273 K in the matrix of the alloy. Size of the Fe₃AlC_x fiber phase increased with the annealing temperature. The alloy after the annealing had better plasticity, higher yield strength than that of the alloy before the annealing, and the alloy after annealed at 1273 K had the highest yield strength.     

Structure and properties of amorphous and nanocrystalline NiTi prepared by severe plastic deformation and annealing

The formation of homogeneous nanocrystalline structure by nanocrystallization of amorphous NiTi subjected to high pressure torsion is demonstrated. Structural evolution during annealing was investigated and homogeneous nanocrystalline structures with different grain sizes have been obtained by controlled annealing. Nanocrystallization results in the record value of room temperature strength for this material equal to 2650 MPa with an elongation to failure of about 5%. At elevated temperatures of (0.4…0.5)T_m nanocrystalline nitinol showed a high ultimate strength with sufficient elongation (up to 200%). The observation that the shape and the size of grains after deformation remain close to that of the initial state suggests that in nanocrystalline NiTi such mechanism as grain boundary sliding and grain rotation are active and the generation and motion of dislocations play the role of accommodation of stress concentration.     

Superplastic flow in nanocrystalline and sub-microcrystalline yttria-stabilized tetragonal zirconia

Tensile deformation of nanocrystalline ZrO₂ + 5 mol% Y₂O₃ at temperatures in the range of 1283–1403 K is described. It is demonstrated (a) that steady state flow is possible at temperatures of the order of 0.42 T _m, where T _m is the absolute melting point, (b) that 70% engineering strain could be obtained at 1403 K (0.46 T _m), and (c) that significant grain boundary sliding was present during deformation. Static and dynamic grain growth as also a decrease in the relative density of the specimen with deformation could be observed. The present results as well as those of Owen and Chokshi concerning superplastic flow in sub-microcrystalline materials taken from literature could be accounted for quantitatively using the grain boundary sliding controlled flow model of Padmanabhan and Schlipf, originally proposed for microcrystalline superplastic alloys.     

Kinetics of grain growth in 0·1 at.-%B doped nickel aluminide containing 23·2 at.-%Al

Abstract

Nickel aluminide containing 23·2 at.-%Al and 0·1 at.-%B was cold rolled and fully recrystallised at 825, 850, 875, 900, 925, 975, and 1000°C. Increase in grain size as a function of isothermal annealing time at these temperatures was measured. All the data showed excellent fit with the developed equations. The value of m, the exponent of the grain growth equation (D^m - D^m₀ = Kt), was 2·5. The activation energy for grain growth was 263 kJ g-atom^-1. Small changes in the boron content had an appreciable effect on the kinetic parameters. This was attributed to the solute drag effect of boron.     

Zero resistance states in modified Y-Ba-Cu-O system with transition temperatures above 135 K

Superconducting transition of minor-dispersed phase in modified YBa₂Cu₃O_7?δ samples is found to occur at around 140K. The amount of this minor phase is enough to provide zero resistivity above 135 K. The measurements of the electrical resistivity indicated that the material is stable, thermally recyclable and reproducible. X-ray analysis of the sample with the highestT _c shows a major phase with perovskite-like structure witha=3·820(1) Å;b=3·873(1) Å andc=11·659(2) Å along with several unidentified weak peaks. Magnetic measurements confirmed the mixed-phase nature with diamagnetic transition temperatures at 137,91 and 86 K. The minor phase responsible for superconductivity with zero resistivity above 135 K is about 0·4% of the bulk and its nature is still unidentified. The details of the preparation and chemical modification process and the results are presented.     

Grain growth in nanocrystalline NbAl3 prepared by mechanical alloying

Grain growth and its kinetics were studied on an intermetallic compound, NbAl₃ powder prepared by mechanical alloying of elemental Nb and Al powders for 1.8 Ms in an argon atmosphere at ambient temperature. The initial and grown grain sizes were measured from the X-ray line broadening of as-alloyed and annealed powders. Isochronal annealing of mechanically alloyed powders from 573 to 1373 K indicated that substantial grain growth occurs only in a temperature range of 1048 to 1173 K and ceases at 1273 K regardless of anneal time. Accordingly isothermal annealing of 1.8 to 18 ks was carried out at 1048, 1073 and 1098 K to obtain the grain growth kinetic that is described by In (dD/dt) = In(r_o/3) –2.0 In D where D is the measured grain size and r _o a constant. This r _o depends on temperature according to r _o=r_o exp (– Q/kT) where Q is the activation energy for grain growth, k the Boltzmann constant and T the absolute temperature. Arrhenius plots of r _o against the reciprocal of temperature yield a straight line, from whose slope the activation energy for grain growth is deduced to be 162±2 kJ mol^–1. Of significance is the fact that the ultimate grain size at 1273 K is approximately 70 nm, which will not grow by further annealing even at 1373 K.On leave from Ibaraki University, Japan.     

Effect of thermal annealing on the microstructure,mechanical properties and residual stress relaxation of pure titanium after deep rolling treatment

The aim of this paper was to investigate the effect of thermal annealing on the microstructure, mechanical properties, and residual stress relaxation of deep rolled pure titanium. The microstructure and mechanical properties of the surface modified layer were analyzed by metallographic microscopy, transmission electron microscope and in-situ tensile testing. The results showed that the annealed near-surface layer with fine recrystallized grains had increased ductility but decreased strength after annealing below the recrystallization temperature, where the tensile strength was still higher than that of the substrate. After annealing at the recrystallization temperature, the recrystallized near-surface layer had smaller grain size, similar tensile strength, and higher proportional limit, comparable to those of the substrate. Moreover, the residual stress relaxation showed evidently different mechanisms at three different temperature regions: low temperature (T?≤?0.2?T_m), medium temperature (T?≈?(0.2?0.3)?T_m), and high temperature (T?≥?0.3?T_m). Furthermore, a prediction model was proposed in terms of modification of Zener-Wert-Avrami model, which showed promise in characterizing the residual stress relaxation in commercial pure Ti during deep rolling at elevated temperature.     

Discontinuous reactions in melt-spun Cu–10 at. %Co alloys and their effect on magnetic anisotropy

The occurrence of discontinuous reactions under isothermal annealing of melt-spun Cu–10 at. % Co alloys, consisting of ribbons (20 µm thick) with columnar grains in the as-solidified state, has been investigated. The microstructure of the ribbons for different annealing temperatures (723–923 K) and annealing times (5–60 min) was determined by transmission electron microscopy, including analysis by energy-dispersive X-ray spectroscopy. Magnetic properties at room temperature were measured by means of hysteresis curve measurements in a vibrating sample magnetometer. Different types of microstructure were observed within grains and at grain boundaries. The spinodal decomposition microstructure was observed during the early stages of annealing for all annealing temperatures. Spherical precipitates grew from the modulated structure at a later stage, forming homogenous distributions throughout the grains. Heterogeneous distributions of incoherent precipitates formed at T > 873 K. As result of discontinuous precipitation, all grain boundaries exhibited arrays of rod-like Co precipitates with diameters and inter-rod spacing of few nanometers. The coarsening of discontinuous precipitates is attributed to a grain boundary-controlled phenomenon, called discontinuous coarsening (DC). The columnar morphology of the grains in the as-solidified alloy was connected with Co rods that were primarily oriented along the ribbon plane. This structure is connected with magnetic anisotropy, which is later weakened by DC. These results elucidate the unusual magnetic behavior of melt-spun Cu–Co alloys and provide a key to understanding their higher magnetoresistance in comparison with other heterogeneous systems.     

High-Field Galvanomagnetic Properties and Structure of Ni-Mn-Ga Nanocrystalline Alloys with Shape Memory

Ni-Mn-Ga based compounds are of great interest due to their magneto- and temperature operated shape memory. To obtain new results on their electronic structure, we studied the low-temperature kinetic properties and the structure of Ni-Mn-Ga alloys at temperatures T?T _M , T _C (T _M is the temperature of the martensitic transition and T _C the temperature of the magnetic (Curie) transition, which are both close to room temperature). Ordered (cast) and disordered samples, having a nanocrystalline substructure, were investigated. The galvanomagnetic and electrical properties were measured in the temperature interval from 2 to 80 K and in magnetic fields of up to 15 T. We find that the electrical and the high-field properties of these alloys strongly change due to the transition into the nanocrystalline state.     

Recovery and recrystallization processes in binary Fe3Al based alloys

Three binary alloys of composition Fe-24Al, Fe-28Al and Fe-34Al were prepared to study the recovery, recrystallization and grain growth processes in Fe₃Al based alloys. These alloys were rolled and annealed at temperatures in the range 873 K to 1273 K for two hours. Grain size measurements were performed as a function of composition, annealing temperature and time. Transmission electron microscopy showed recovered and recrystallized grains after annealing at 873 K. The mechanisms of recovery processes was found to be by the migration of single dislocations towards each other to form linear arrays which can subsequently form square or hexagonal dislocation networks. Recrystallization can take place either by the enclosure of dislocation free regions by dislocation networks or by the preferential growth of subgrains. The composition dependence of the recovery and recrystallization processes is weak.     

The Morin transition in nanostructured pseudocubic hematite: Effect of the intercrystallite magnetic exchange

The Morin transition in α-Fe₂O₃ particles with homogeneous pseudocubic morphology of about 1.8 μm side has been studied by different techniques. The particles are made up of nanometric crystallites with a narrow size distribution. Samples annealed at 673, 773 and 873 K, yielded Morin transition temperatures, T_M, of about 230, 241, and 245 K, respectively, which are lower than the bulk value, T_M ≈ 263 K. In addition, the temperature range ΔT_M through which these samples undergo the transition is very narrow with respect to the values reported in the literature. Ranges of 20, 10 and 8 K, were obtained for the samples annealed at 673, 773 and 873 K. In this work, through the estimation of the exchange magnetic energy, we demonstrate that the existence of the Morin transition, and its shift and breadth are related to the presence of intercrystallite interactions within the pseudocubic particles. These intercrystallite interactions are modified by the annealing treatments, which help to remove non-stoichiometric hydroxyl groups and incorporated water molecules.     

Superconducting Properties and Microstructure of Nanocrystalline MgB2 Bulk Prepared by Sintering of Ball-Milled Powders

Ultrafine nanostructured MgB₂ bulks with an average grain size less than 10 nm have been fabricated by high-energy ball milling and subsequent high pressure sintering. Microstructural evolution in MgB₂ subjected to high-energy ball milling has been investigated by means of X-ray diffraction (XRD). The finer grain size of MgB₂ powders of about 7 nm has been estimated from Rietveld refinement analysis of XRD data, which is confirmed by a transmission electron microscope (TEM) observation. There is almost no grain growth in the subsequent sintering at low temperature of 600?°C under pressure of 3?C5 GPa for 10?C30 min. The nanocrystalline MgB₂ bulks exhibit the lower onset critical transition temperatures (T _{c onset}) of 32?C33?K. The relative wider width of the magnetic hysteresis loops at high external magnetic field and the higher critical current density (J _c ) are obtained in nanocrystalline bulks. J _c is as high as 10⁵?A/cm² in 8?T at 10?K and 2.7×10³?A/cm² in 4?T at 20?K.     

Microstructure and radiation damage of commercially produced Nb3Sn tapes

The microstructure and fast neutron irradiation damage of Nb₃Sn tapes produced by a liquid tin diffusion method have been studied using electron microscopy, X-ray diffraction and secondary ion mass spectrometry.The Nb₃Sn layer consists of an outer region of clusters of Nb₃Sn grains, which have a high oxygen content, and an inner region of smaller equiaxed grains. The rate of growth of the Nb₃Sn layer and the kinetics of grain growth in these commercial tapes are compared with published results for a laboratory system and Nb₃Sn formed by the ‘bronze route’.In Nb₃Sn irradiated at 70°C to doses up to 5.4 10²³ neutrons m^?2 disordered regions and dislocation loops are observed; the latter dissappeared on annealing for short times at temperatures from 300 to 750°C Nb₃Sn tapes irradiated at higher temperatures only show dislocation loops which form pairs on annealing. These results are correlated with previously determined T_c measurements.     

Annealing effect on the characteristics of La0.67Sr0.33MnO3 polycrystalline thin films produced by the sol–gel dip-coating process

The effect of annealing temperature on selected characteristics of polycrystalline La_0.67Sr_0.33MnO₃ films, which have been produced on quartz substrates, was investigated. X-Ray powder diffraction patterns showed that the phase formation started at 873 K and all the films had perovskite structure. By increasing the annealing temperature, the lattice parameters were decreased. Scanning electron microscope indicated that the film thicknesses were approximately 3 μm and the average grain size of the samples varied between 30–100, 50–110, 70–120, and 100–150 nm for films annealed at 873, 973, 1,073, and 1,173 K, respectively. All the films showed a paramagnetic–ferromagnetic (T_C) and metal–insulator (T_IM) phase transition. The T_C indicated a small variation [from 131 K (S4) to 124 K (S1)] as a function of annealing temperature, whereas the T_IM went down from 212 K (S4) to 110 K (S1), a strong decrease of 102 K. A colossal magneto resistance with magneto resistance ratios of 130, 139, 156, and 163% were observed near T_C and at 6 T magnetic field.     

Indice de réfraction du silicium amorphe hydrogéné préparé par glow discharge entre 95 et 723 K

Optical properties of hydrogenated amorphous silicon films prepared by glow discharge on fused quartz substrates held at temperatures T_s of between 50 and 350°C were determined from near-normal specular reflectance and transmittance measurements in the energy range 0.5–2.5 eV. The measurements were made at various temperatures T_m in the range 95–723 K. The refractive index dispersion data below the interband absorption edge were analysed using the Wemple and Didomenico model. The dispersion energy E_d of samples deposited at 280 and 350°C was found to be independent of T_m but a function of the hydrogen content for T_m < 573 K; the variation in the average gap E_m with temperature T_m was compared with the theoretical models of Varshni and Ravindra. For T_m > 573 K reversible or irreversible variations in the static refractive index n₀ lead to an increase in the dispersion energy owing to a rearrangement of the matrix with or without alteration of the hydrogen content. An interpretation in terms of rearrangement of the matrix under annealing without a decrease in the hydrogen content is proposed for the observed irreversible variations in the static refractive index n₀ of the film deposited at 50°C.