Magnetothermal Analysis of Nanocrystallization of Amorphous and Relaxations of Nanocrystalline Materials

For a few years it has been realized that nanocrystalline phases can be formed during crystallization of amorphous alloys annealed isothermally below the crystallization temperature of usual heating experiments. Data of this transformation monitored by the measurement of magnetic susceptibility are presented. A method using a magnetic balance with electronic stabilisation and combined computer facilities is applied. Constant heating and cooling rates as well as isothermal heat treatments are used. Magnetic measurements are able to detect the onset of the transformation of amorphous Ni-P alloys much earlier than was possible with differential scanning calorimetry. The transformation kinetics can be analyzed by means of the Avrami plot based on the Johnson-Mehl-Avrami equation.The kinetics of solid state reactions in the nanostructured material can be investigated similarly. Formation of a Ni-phase in a nanostructured Hf-Ni alloy could be detected in a very early stage, where calorimetric methods are not sensitive. Segregation phenomena could be detected from the experiments even after long time. The sensitivity of the applied method is not dependent on the heating rate as the sensitivity of scanning calorimetry is     

Effect of low-temperature annealing and deformation on the structure of metallic glasses by X-ray diffraction

The changes in the atomic structure of amorphous Pd-Si and Ni-P alloys due to low-temperature annealing, cold-rolling and isothermal creep have been studied by the conventional X-ray diffraction. The present results on the effect of low-temperature annealing were consistent with those of amorphous Fe₄₀Ni₄₀P₁₄B₆ alloy studied by the energy dispersive X-ray diffraction method. In addition, the present results have indicated that the effect of cold-rolling causes small changes in the structure of amorphous Pd₈₀Si₂₀ alloy which are qualitatively different from the structural relaxation, and the effect of annealing plays a significant contribution in the structural change during an isothermal creep test.     

The formation, structure and properties of nanocrystalline Ni-Mo-B alloys

The structure, structure evolution and microhardness of nanocrystalline Ni-Mo-B alloys were studied by X-ray diffraction, differential scanning calorimetry, transmission and high resolution electron microscopy and microhardness measurements. The nanocrystalline structure was produced by controlled crystallization of amorphous alloys. The annealed samples consist of the FCC nanocrystals with the amorphous regions between them. The grain size of the nanocrystals is about 20 nm and depends on the chemical composition of the alloy. The chemical composition of the amorphous phase between the nanocrystals changes at the annealing. A slight grain growth was observed when the annealing time increases. The diffusion of Mo and B from FCC to the amorphous phase occurs at the annealing. It results in the lattice parameter change. The microhardness of the alloys increases during the annealing. The microhardness values are the same in all alloys before the nanocrystalline structure decomposition. The microhardness is inconsistent with the Petch-Hall equation. The microhardness of the alloys is determined by the microhardness of the amorphous phase bands located between the nanocrystalline grains.     

A new challenge: grain boundary engineering for advanced materials by magnetic field application

This paper gives an overview of “Grain boundary engineering (GBE) for advanced materials by magnetic field application” based on recent experimental work performed on different kinds of structural and functional materials. It is shown that magnetic field application has a great potential and unique advantage as “non-contact processing” for microstructure control, irreplaceable by any other existing processing methods. The control of grain growth and texture by magnetic fields has been found to be generally applicable to many metallic materials, irrespective of whether they are ferromagnetic or not. Grain growth which is controlled by grain boundary migration was found to be strongly affected by magnetic field application. Recent attempts at the grain boundary engineering by magnetic field application through phase transformation have revealed that magnetic phase transformation can provide us a new approach to grain boundary engineering for iron alloys and steels, as well as a new nanocrystalline material produced by magnetic crystallization from the amorphous state. The possibility of engineering applications of enhanced densification using magnetic sintering and magnetic rejuvenation has been discussed for iron powder compacts and deformation-damaged iron alloys, respectively.     

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.     

Nanoscale Grain Growth Behaviour of CoAl Intermetallic Synthesized by Mechanical Alloying

Grain growth behaviour of the nanocrystalline CoAl intermetallic compound synthesized by mechanical alloying has been studied by isothermal annealing at different temperatures and durations. X-ray diffraction method was employed to investigate structural evolutions during mechanical alloying and annealing processes. The disordered CoAl phase with the grain size of about 6 nm was formed via a gradual reaction during mechanical alloying. The results of isothermal annealing showed that the grain growth behaviour can be explained by the parabolic grain growth law. The grains were at nanometric scale after isothermal annealing up to 0·7 T _m. The grain growth exponent remained constant above 873 K indicating that grain growth mechanism does not change at high temperatures. The calculated activation energy indicated that the grain growth mechanism in the disordered CoAl phase at high temperatures was diffusing Co and Al atoms in two separate sublattices. Furthermore, an equation has been suggested to describe the grain growth kinetics of nanocrystalline CoAl under isothermal annealing at temperatures above 873 K (T/T _m ≥ 0·5).     

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.     

Grain growth behavior of nanocrystalline Inconel 718 and Ni powders and coatings

Nanocrystalline Inconel 718 and Ni powders were prepared using two approaches: methanol and cryogenic attritor milling. High velocity oxy-fuel (HVOF) spraying of milled Inconel 718 powders was then utilized to produce coatings with a nanocrystalline grain size. Isothermal heat treatments were carried out to study the thermal stability of the methanol milled and cryomilled powders, as well as the HVOF-derived coatings. All nanocrystalline Inconel 718 powders and coatings studied herein exhibited significant thermal stability against grain growth by maintaining a grain size around 100 nm following annealing at 1273 K for 60 min. In the case of the cryomilled nanocrystalline Ni powders, isothermal grain growth behavior was studied, from which the parameters required for the prediction of the microstructural evolution during a non-isothermal annealing were acquired. The theoretical simulation of grain growth behavior of nanocrystalline Ni during non-isothermal annealing conditions yields results that are in good agreement with the experimental results.     

Structural evolution during mechanical milling and subsequent annealing of Cu–Ni–Al–Co–Cr–Fe–Ti alloys

This study reports the structural evolution of high-entropy alloys from elemental materials to amorphous phases during mechanical alloying, and further, to equilibrium phases during subsequent thermal annealing. Four alloys from quaternary Cu_0.5NiAlCo to septenary Cu_0.5NiAlCoCrFeTi were analyzed. Microstructure examinations reveal that during mechanical alloying, Cu and Ni first formed a solid solution, and then other elements gradually dissolved into the solid solution which was finally transformed into amorphous structures after prolonged milling. During thermal annealing, recovery of the amorphous powders begins at 100 °C, crystallization occurs at 250–280 °C, and precipitation and grain growth of equilibrium phases occur at higher temperatures. The glass transition temperature usually observed in bulk amorphous alloys was not observed in the present amorphous phases. These structural evolution reveal three physical significances for high-entropy alloys: (1) the annealed state of amorphous powders produces simple equilibrium solid solution phases instead of complex phases, confirming the high-entropy effect; (2) amorphization caused by mechanical milling still meets the minimum criterion for amorphization based on topological instability proposed by Egami; and (3) the nonexistence of a glass transition temperature suggests that Inoue's rules for bulk amorphous alloys are still crucial for the existence of glass transition for a high-entropy amorphous alloy.     

Modelling recovery and recrystallisation during annealing of AA 5754 aluminium alloy

Abstract

A microstructure model taking into account recovery and recrystallisation has been developed to predict the yield stress and the recrystallised grain size during continuous annealing of cold rolled AA 5754 sheet alloy. Using isothermal annealing tests, recovery and recrystallisation kinetics were quantified as a function of temperature and cold reduction. The model was formulated employing the internal state variable approach with the following three state variables: dislocation density, volume fraction recrystallised, and grain size. A rule of mixtures is adopted to separate the effect of recovery and recrystallisation in the overall softening. Model validation has been carried out by comparing the predicted softening curves with those obtained in continuous heating tests replicating heating rates of industrial continuous annealing lines. The model can be applied to non-isothermal processing routes of industrial cold rolled AA 5754 with thickness reduction in the range 40-80%.     

Modelling of continuous recrystallization in aluminium alloys

Microstructure and microtexture analyses have been made of three aluminium alloys after annealing alone and after concurrent straining and annealing, and simulative models of microstructure/microtexture evolution processes have been formulated. Both experimental and modelling results are presented as boundary misorientation distributions. For each alloy, the results show that annealing alone does not significantly alter the boundary misorientation distribution, while concurrent straining and annealing (up to a strain of 0.5) decreases the fraction of low-angle boundaries. To understand the mechanisms by which concurrent straining and annealing alter the boundary misorientation distribution, three simulative models of microstructure/microtexture evolution during concurrent straining and annealing have been formulated. Application of the models to experimentally determined initial microstructure/microtexture states shows that the boundary sliding (sub)grain rotation model decreases the fraction of low-angle boundaries, the dislocation glide (sub)grain rotation model increases the fraction of low-angle boundaries, and the (sub)grain neighbour switching model has a modest effect on the boundary misorientation distribution. A combination of the boundary sliding (sub)grain rotation model and the (sub)grain neighbour switching model most closely reproduces the boundary misorientation distributions found experimentally.     

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.     

Wetting transition of grain boundaries in the Sn-rich part of the Sn–Bi phase diagram

The microstructural evolution of tin-rich Sn–Bi alloys after the grain boundary wetting phase transition in the (liquid + β-Sn) two-phase region of the Sn–Bi phase diagram was investigated. Three Sn–Bi alloys with 30.6, 23, and 10 wt% Bi were annealed between 139 and 215 °C for 24 h. The micrographs of Sn–Bi alloys reveal that the small amount of liquid phase prevented the grain boundary wetting transition to occur during annealing close to the solidus line. The melted area of the grain boundary triple junctions and grain boundaries increased with increasing the annealing temperature. When the amount of liquid phase exceeded 34 wt% during annealing, increasing temperature has not affected the wetting behavior of grain boundaries noticeably and led only to the increase of the amount of liquid phase among solid grains in the microstructure. The XRD results show that the phase structure and crystallinity remained unchanged after quenching from various annealing temperatures.     

化学镀Ni-P合金镀层的微观结构

用X射线衍射仪和透射电镜研究了化学镀Ni-P合金镀层的微观结构,提出了镀层结构模型,分析了镀层生成的结晶机理,比较了低磷与高磷Ni-P合金的晶体结构区别;前者为致密的镍基相中弥散分布粒状Ni3P相的晶体结构,后者为几何形状不规则的非晶态结构。对非晶态组织在300℃和400℃进行热处理,合金镀层发生晶化转变,且生成的Ni3P微晶显示出调幅结构。     

Effect of the microstructure on the anti-fouling property of the electroless Ni-P coating

The Ni-P coatings with different contents of nanocrystalline phase were prepared by electroless plating. Crystallization fouling adhering experiments indicated that these electroless Ni-P coatings have better anti-fouling property than that of un-coated sample. The effect of microstructure on anti-fouling property of Ni-P coatings is that the adhering amount of crystallization fouling increased with the increasing of nanocrystalline phase. It is considered that the degree of crystallization fouling adhesion is related to the corrosion resistance of the sample. The amorphous Ni-P coating with excellent corrosion resistance is not easy to form “transitional interface” connecting fouling and matrix.     

In-situ X-ray diffraction studies of time and thickness dependence of crystallization of amorphous TiO2 thin films and stress evolution

Remarkable properties of titanium dioxide films such as hydrophilicity or photocatalytic activity depend largely on their phase composition, microstructure and in particular on the crystallinity. By in-situ X-ray diffraction studies of isochronal and isothermal annealing of amorphous films with different thickness at different temperatures it was found that the crystallization process can be quite well described by the Johnson-Mehl-Avrami-Kolmogorov formula modified by the introduction of crystallization onset. This and other parameters of the formula strongly depend on the film thickness. For thickness below about 500 nm the crystallization is very slow. Simultaneously, the appearance and increase of tensile stresses with the annealing time were observed and these stresses were confirmed by detailed studies by both total pattern fitting and sin²ψ method on post-annealed samples. The stresses rapidly increase with decreasing thickness of the films. It seems that there is a strong correlation between the stresses and crystallization onset and/or crystallization rate. Tensile stresses that are generated during crystallization further inhibit crystallization and cause significant thickness dependence of the crystallization. The temperature and time dependence of microstructure of crystallized amorphous films differ significantly from those obtained for as-deposited nanocrystalline films or nanocrystalline powders. During annealing, quite large crystallites are formed quickly with the preferred orientation (001) that is suppressed with the proceeding time.     

Microstructure of Differently Annealed NanocrystallineFe_(72.7)Cu_1Nb_(1.8)Mo_2Si_(13)B_(9.5) Alloy

The microstructure of differently annealed nanocrystalline Fe72.7Cu1Nb1.8Mo2Si13B9.5 alloy was investigated by using Mssbauer spectroscopy and transmission electron microscope. The specimens were isochronally annealed at temperatures between 480℃ and 600℃ for 0.5 h. The experimental results show that the microstructure mainly consists of the nanoscale bcc α-Fe(Si) grains and the residual amorphous matrix phase. A trace paramagnetic phase was found for annealing about above 500℃. The volume fraction of cr-Fe(Si) grain increases with increasing annealing temperature, whereas the average size of grain is almost unchanged above 480℃ up to 580℃. The calculated thickness of the intergranular layer of the residual amorphous matrix clearly decreases with increasing annealing temperature.     

A new phenomenon: the transient metastable graphitization of alloyed white iron

In previous papers the case of stable-mottled irons has been analyzed. Hereunder a different case is looked into, when the mottled structure is a transient metastable state. Then the sequence of transformations during isothermal graphitizing annealing of some alloyed white irons can be summed up as follows: {ie99-1} In the transient mottled state the alloys have a much lower hardness and can be machined before the final heat treatment. A simplified theoretical approach to this phenomenon is being offered.     

Correlation of microstructure and magnetic properties in Cu–Co–Ni alloys

The present study concerns correlation of microstructure and magnetic properties of nanocrystalline binary 50Cu–50Co and ternary 50Cu–25Co–25Ni (wt%) alloys prepared by ball milling and subsequent isothermal annealing of the ball milled alloys. High resolution transmission electron microscopic (HR-TEM) investigation has shown deformation-induced microstructural features. Field emission scanning electron microscopy (FE-SEM) has revealed a distinct change in morphology of as-milled CuCoNi alloys after annealing. Differential scanning calorimetric (DSC) and X-ray diffraction (XRD) analysis have revealed that annealing of the CuCoNi alloy above 350 °C results into precipitation of nanocrystalline Co (fcc) in the CuNi matrix by spinodal decomposition. It is also demonstrated that isothermal annealing of the ball milled alloys in the temperature range between 350 and 650 °C significantly influence the magnetic properties, e.g. coercivity (H_c), remanence (M_r) and magnetic saturation (M_s) due to annihilation of defects such as stacking and twin fault along with dissolution and/or precipitation of magnetic phases in the Cu-rich matrix.     

TEM Investigations of Thermal Stability of Nanocrystamne Co_(81)Cr_(19)

In situ transrnission electron microscopic observations were carried out to study the thermal stability of nanocrystalline Co81 Cr19 (n-Co81 Cr19) alloy prepared by d.c. sputtering, The TEM results show that the originally existed nanocrystalline phase hcp-CoCr is thermally stable, no apparent grain growth was observed until annealing to the final stage at.800℃. But the evolution of .microstructure of amorphous oxide Co2Cro4. which is formed duting in situ annealing. is drastically affected by the electron irradiation. In the electron irradiated area. the newly formed amorphous oxide Co2CrO4crystallizes at about 550℃. and grows into fine crystals finally. However, in the electron nonirradiated area, the amorphous oxide Co2CrO4 crystallizes at about 630℃. and grows into large strip shaped crystals during afterward annealing. the space between strip shaped crystals are also fine grained hcp-CoCr alloy. XRD analysis result of the thick film Co81 Cr19 is in good agreement with that of in situ TEM for the excellent stability of hcp-CoCr