Quasicrystal–crystal structural transformation in Al–5 wt.% Mn alloy

The atomic structure of Al–5 wt.%Mn (Al–5Mn) alloy, prepared by rapid solidification, and pre-annealed at 623 and 773 K for 5 and 1 h, respectively, were characterized by X-ray powder diffraction (XRD) and extended X-ray absorption fine structure (XAFS) techniques. The sample in as-quenched stage was found crystalline, consisting of metastable α-Al (Al–Mn solid solution) and icosahedral quasicrystalline I-Al₆Mn phases. Five hours annealing at 623 K proved thermal stability of both the phases. Pre-annealing at 773 K/1 h on the other hand leads to α-Al phase decomposition and structural transformation of metastable I-Al₆Mn to stable orthorhombic Al₆Mn phase. The EXAFS results indicate that Mn atoms are located preferably on the outer shell of icosahedrons. During the I-Al₆Mn→o-Al₆Mn transformation the total Al atoms coordinating one Mn were found to be constant (∼10). Based on the results, only distance/symmetry changes in atomic arrangement around Mn atoms were suggested.     

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.     

Annealing Effect of Mn thin Films on GaAs

Thin Mn films were deposited on GaAs(100) surfaces at room temperature by a thermal evaporation system followed by annealing at 500 °C for times ranging from 2 to 8 h in nitrogen atmosphere. X-ray diffraction shows that for samples annealed at 500 °C, the interfacial reaction results in the formation of different phases such as Mn₂As, MnAs and Mn₃Ga. Rutherford Backscattering Spectrometry indicates a diffused layer of Mn–Ga–As system along with some minor oxygen content. Magnetization study done at 10 K shows M–H curve comprising a ferromagnetic and antiferromagnetic part and M–T measurement done from 10 to 300 K shows a transition around 45 K which may be related to the presence of GaMnAs alloy along with the presence of the above binary phases.     

Solidification behaviour of ceramic particle reinforced Al-alloy matrices

Novel metal matrix composites have been produced by cast production route. TiC and WC ceramic reinforcing particles have been successfully introduced into Al 6060, Al 319, Al 356, Al–7Si–5Mg, Al–20Cu and Al 2007 alloys. Refined grain structure and various intermetallic phase formation have been observed. Particle–melt and particle–solidification front interactions, solidification sequence and particle–matrix interfacial characteristics have been examined by means of metallography, SEM examination and EDX analysis. Particle distribution, intermetallic phase formation and location and grain structure are discussed in terms of ceramic-melt wetting characteristics, alloying element interfacial segregation and particle–solidification front thermal behaviour.     

Thermal stability of Al/nanocrystalline-Si bilayers investigated by in situ heating energy-filtered transmission electron microscopy

In situ heating energy-filtered transmission electron microscopy was employed to investigate the interfacial intermixing/reactions during thermal annealing of Al/nanocrystalline-Si (nc-Si) bilayers in the temperature range of 150–500 °C. In comparison with the Al/amorphous-Si (a-Si) bilayer, the Al/nc-Si bilayers were found to be much more stable against thermal annealing. Wetting and c-Si growth processes along Al grain boundaries, which take place during annealing of Al/a-Si bilayers, do not occur in Al/nc-Si bilayers, because of the lack of thermodynamic driving forces in the latter case. As a consequence, also in contrast with Al/a-Si bilayers, no layer exchange occurs in Al/nc-Si bilayers, not even after annealing at 500 °C. Instead, intermixing of Al/nc-Si is realized at the Al/nc-Si interface by the formation of Al spikes growing into the nc-Si sublayers at temperatures higher than 300 °C. The relatively low Al-spike formation temperature in Al/nc-Si systems, as compared with that for Al/single-crystalline Si systems, is ascribed to the higher Gibbs energy of nanocrystalline Si as compared to single-crystalline Si.     

Microstructure,tensile properties,and creep behavior of high-pressure die-cast Mg–4Al–4RE–0.4Mn (RE = La,Ce) alloys

High-pressure die-cast (HPDC) Mg–4Al–4RE–0.4Mn (RE = La, Ce) magnesium alloys were prepared and their microstructures, tensile properties, and creep behavior have been investigated in detail. The results show that two binary Al–Ce phases, Al₁₁Ce₃ and Al₂Ce, are formed mainly along grain boundaries in Mg–4Al–4Ce–0.4Mn alloy, while the phase composition of Mg–4Al–4La–0.4Mn alloy contains only α-Mg and Al₁₁La₃. The Al₁₁La₃ phase comprises large coverage of the grain boundary region and complicated morphologies. Compared with Al₁₁Ce₃ phase, the higher volume fraction and better thermal stability of Al₁₁La₃ have resulted in better-fortified grain boundaries of the Mg–4Al–4La–0.4Mn alloy. Thus higher tensile strength and creep resistance could be obtained in Mg–4Al–4La–0.4Mn alloy in comparison with that of Mg–4Al–4Ce–0.4Mn. Results of the theoretical calculation that the stability of Al₁₁La₃ is the highest among four Al–RE intermetallic compounds supports the experimental results further.     

Dimensional effects observed for the electrical, dielectrical and optical properties of TiO2 DC magnetron thin films

In this paper, we present the dimensional effects observed for TiO₂ films deposited by DC circular magnetron sputtering. TiO₂ thin films are deposited in an Al–TiO₂–Al structure to investigate their non-linear characteristics, from which the carrier effective mass and the barrier potential at the Al–TiO₂ interface is calculated, and an important relationship between the effective carrier mass and film thickness is observed. The dielectric constant of TiO₂ thin films is also investigated, and is observed to vary with TiO₂ film thickness. Further, TiO₂ band gap is observed to vary with film thickness.     

Growth behavior of compounds due to solid-state reactive diffusion between Cu and Al

To examine experimentally the kinetics of the reactive diffusion between solid-Cu and solid-Al, sandwich Al/Cu/Al diffusion couples were prepared by a diffusion-bonding technique and then isothermally annealed in the temperature range of T = 693–753 K for various times up to 336 h. Owing to annealing, compound layers of the γ ₁, δ, ζ ₂, η ₂, and θ phases are formed between the Cu and Al specimens. The γ ₁, δ, ζ ₂, η ₂, and θ phases are the only stable compounds at T = 693–753 K in the binary Cu–Al system. At each annealing time, the thickness of the θ phase is much greater than those of the δ, ζ ₂, and η ₂ phases but smaller than that of the γ ₁ phase. Hence, the overall growth of the compound layers is governed by the γ ₁ and θ phases. The mean thickness of each compound layer is proportional to a power function of the annealing time. For the γ ₁ phase, the exponent m of the power function is 0.5 at T = 753 K. Such a relationship is called a parabolic relationship. As the annealing temperature T decreases, however, m gradually increases and then reaches to 0.66 at T = 693 K. On the other hand, for the θ phase, m is close to 0.5 at T = 723–753 K and becomes 0.42 at T = 693 K. In the γ ₁ and θ phases, grain growth occurs at T = 693–753 K. Thus, the layer growth of the θ phase is controlled by volume diffusion at T = 723–753 K but partially by boundary diffusion at T = 693 K. On the other hand, for the γ ₁ phase, volume diffusion is the rate-controlling process of the layer growth at T = 753 K, but interface reaction contributes to the rate-controlling process at T = 693–723 K. Consequently, the rate-controlling process varies depending on the annealing temperature in a different manner for each compound.     

Wear characteristics of spray formed Al-alloys and their composites

In the present investigation, different Al based alloys such as Al–Si–Pb, Al–Si, Al–Si–Fe and 2014Al + SiC composites have been produced by spray forming process. The microstructural features of monolithic alloys and composite materials have been examined and their wear characteristics have been evaluated at different loads and sliding velocities. The microstructural features invariably showed a significant refinement of the primary phases and also modification of secondary phases in Al-alloys. The Pb particles in Al–Si–Pb alloy were observed to be uniformly distributed in the matrix phase besides decorating the grain boundaries. The spray formed composites showed uniform distribution of SiC particles in the matrix. It was observed that wear resistance of Al–Si alloy increases with increase in Pb content; however, there is not much improvement after addition of Pb more than 20%. The coefficient of friction reduced to 0.2 for the alloy containing 20%Pb. A sliding velocity of 1 ms⁻¹ was observed to be optimum for high wear resistance of these materials. Alloying elements such as Fe and Cu in Al–Si alloy lead to improved wear resistance compared to that of the base alloy. The addition of SiC in 2014Al alloy gave rise to considerable improvement in wear resistance but primarily in the low pressure regime. The wear rate seemed to decrease with increase in sliding velocity. The wear response of the materials has been discussed in light of their microstructural features and topographical observation of worn surfaces.     

The microstructure of Al-8 wt% Fe-based alloys prepared by rapid quenching from the liquid state

The results of a transmission electron microscope study of the microstructure of splat-quenched Al-8% Fe are described in detail. Two distinct structures, zone A and zone B, are examined in as-quenched samples, and each characterized in terms of the dispersions and types of phases present. The decomposition behaviours of zone A and zone B during annealing at temperatures between 573 and 823 K (300 and 550°C) are investigated and the associated phase transformations determined. The effect on the as-quenched, and annealed, microstructures of adding either 3% Mn or 1% Zr to the alloy is described. The observed microstructures and phase transformations are correlated with micro-hardness measurements.     

The Fe–Zn–Al–Cr system and its impact on the galvanizing process in chromium-added zinc baths

The zinc rich corner of the Fe–Zn–Al–Cr at 460 °C is of interest for galvanizing because Al is a usual addition element in zinc bath, whereas Cr is naturally present because it is supplied by the stainless steel roller dipping in the Zn bath during the process. Indeed, it is used to understand the formation and growth mechanisms of the solid phases during galvanizing in Al and Cr-added Zn bath. By using additional experimental results in the Al–Cr–Zn and Fe–Zn–Al–Cr systems, the zinc rich corner of the Fe–Zn–Al–Cr system at 460 °C was determined with more accuracy. Thus, new equilibria between the liquid and quaternary phases have been pointed out, namely Al₂Cr₃ stabilized by Zn and enriched with Fe and τ₁, the latter being isotypic with δ-FeZn₉. This quaternary system was assessed with the CALPHAD method using the PARROT module of the Thermo-Calc Software. The liquid and solid solutions are described by the Redlich-Kister-Muggianu equations. All the modeled phases are considered as stoichiometric in the binary systems.     

In situ production of Al–TiB<Subscript>2</Subscript> nanocomposite by double-step mechanical alloying

An in situ Al–TiB₂ nanocomposite was synthesized by mechanical alloying (MA) of pure Ti, B and Al powder mixture in a planetary ball mill. A double-step process was used to prevent the formation of undesirable phases like Al₃Ti intermetallic compound. In the first step, a powder mixture was tailored to obtain nominal Al–90 wt% TiB₂ composition and the second step involved the addition of Al to the mixture in order to achieve Al–20 wt% TiB₂. The structural and thermal characteristics of powder particles were studied by X-ray diffractometry (XRD), scanning electron microscopy (SEM), differential scanning calorimetery (DSC), and transmission electron microscopy (TEM). The results showed that the MA process leads to the in situ formation of nanosized TiB₂ particles in an Al matrix with a uniform distribution. It was also found that the double stage addition of aluminum can prevent the formation of undesirable compounds even after annealing at high temperatures_.     

Effects of Cu and Al on the crystal structure and composition of η (MgZn2) phase in over-aged Al–Zn–Mg–Cu alloys

Heat treatable Al–Zn–Mg alloys can be strengthened by the precipitation of second phase particles. In this paper, Al–6.57%Zn–2.83%Mg and Al–6.57%Zn–2.83%Mg–3.92%Cu alloys (in wt%) in T7 state (140 °C for 96 h) have been prepared. The effects of Cu and Al on the concentration and structure of equilibrium η (MgZn₂) phase have been investigated by high resolution transmission electron microscopy, aberration-corrected scanning transmission electron microscopy, selected election diffraction pattern simulations, and first-principles calculations. The effects of Cu and Al substitution on the diffraction characteristics of the η phase and the general rule of Cu and Al substitution in the η phase have been discussed.     

Interfacial microstructure and shear strength of Ti–6Al–4V/TiAl laminate composite sheet fabricated by hot packed rolling

A two layer Ti–6Al–4V(wt.%)/Ti–43Al–9V–Y(at.%) laminate composite sheet with a uniform interfacial microstructure and no discernible defects at the interfaces has been prepared by hot-pack rolling, and its interfacial microstructure and shear strength were characterized. Characterization of the interfacial microstructure shows that there was an interfacial region of uniform thickness of about 250 μm which consisted of two layers: Layer I on the TiAl side which was 80 μm thick and Layer II on the Ti–6Al–4V side which was 170 μm thick. The microstructure of Layer I consisted of massive γ phases, needlelike γ phases and B2 phase matrix, while the microstructure of Layer II consisted of α₂ phase. The microstructure of the interfacial region is the result of the interdiffusion of Ti element from Ti–6Al–4V alloy layer into the TiAl alloy layer and Al element from the TiAl alloy layer into the Ti–6Al–4V alloy layer. The shear strength measurement demonstrated that the bonding strength between the TiAl alloy and Ti–6Al–4V alloy layers in the laminate composite sheet was very high. This means that the quality of the interfacial bonding between the two layers achieved by the multi-path rolling is high, and the interface between the layers is very effective in transferring loading, causing significantly improved toughness and plasticity of the TiAl/Ti–6Al–4V laminate composite sheet.     

Influence of zirconium on grain refining efficiency of Al–Ti–C master alloys

The influence of Zirconium on the grain refinement performance of Al–Ti–C master alloys and the effect mechanism has been studied in this paper. The experimental results show that Zr not only results in poisoning the Al–Ti–B master alloy, but also poisons the Al–Ti–C master alloys. The poisoning effect is more obvious at higher melting temperature. When 0.12%Zr is added into the melt, the grain refinement performance of Al–5Ti–0.4C refiner with 0.2% addition level absolutely disappears at 800 °C. The experimental results also show that it is difficult to refine the commercial purity Al containing 0.15%Zr by Al–5Ti–0.4C master alloy. Further experiments show that the Zr element can interact with both TiAl₃ and TiC phases. If both of them are present, Zr preferentially reacts with TiAl₃ phase.     

Influence of a small amount of Al2O3 addition on the transformation of Y2O3-partially stabilized ZrO2 during annealing

Tetragonal-to-monoclinic phase transformations in 2 and 3 mol% Y2O3–ZrO2 ceramics and their composites with 5 vol% Al2O3 during annealing in water and in vacuum at 353–623 K were investigated to explore the effect of a small quantity of Al2O3 addition on the transformation. The dispersion of Al2O3 particles into the ZrO2(Y2O3) matrix was found to be effective to suppress the transformation directly induced by the attack of H2O during annealing in water, even though the amount was as small as 5 vol%. However, the transformation predominantly caused by thermal activation during annealing in vacuum was not affected by the limited amount of Al2O3 addition. The effect of suppression of Al2O3 on the water-induced phase transformation was considered to be realized through the hydroxydation of Al2O3 particles, by which the sample surface was effectively "protected" from further attack of H2O, which accelerated the low-temperature degradation transformation. This revised version was published online in November 2006 with corrections to the Cover Date.     

Structures and magnetic properties of overaged Fe-AI-Mn-C alloys

Abstract

Microstructures and magnetic properties in the aged hardened Fe-9AI-30Mn-x(C,Si) alloy, during overaging at 823 Kfor 48 h to 313 days, have been investigated by transmission electron microscopy, X-ray diffraction line profiles, and vibrating sample magnetometry. The results reveal that the precipitate k phase ((Fe,Mn)₃AIC) decomposition in this alloy, overaged at 823 k for one week, proceeded by two separate mechanisms; (a) wetting of the½α_o′<100> antiphase boundary segment of D0₃ ((Fe/Mn)₃Al) domains by B2 ((Fe/Mn)Al) phase, and (b) precipitation of B2 ((Fe/Mn)Al) structure within the domain. A similar superparamagnetic behaviour was discovered when the alloy was overaged at 823 K for 120-313 days. The super soft magnetic properties were mainly attributed toferromagnetic B2 ((Fe/Mn)Al) domains, D0₃, and ;α′-Mn phases.     

From Ti–Al- to Ti–Al–N-sputtered 2D materials

This paper reviews thin films constituted by elements based on the Ti–Al–N system, bearing in mind the role of the condensed phases in the development of structural components and functional devices. In recent decades, the Ti–Al, Ti–N and Al–N nanocrystalline binary systems have rapidly attracted research and industry interest. These systems have revealed a great performance via atomic-level structural control, making it possible to tailor new atomic structures and morphologies suitable in different applications as protective and hard coatings and as thermal/diffusion barriers. The binary phases based on nitrogen were the first to exhibit a wealth of interesting mechanical and electrochemical behaviours. However, more recently the Ti-Al and, particularly, the Ti_{1 − x} Al _x N thin films have been applied with success in the industry. The purpose of this paper is to compile the master results concerning the production and characterisation of binary and ternary thin films of the Ti–Al–N system using similar deposition strategies. These materials form a good base to analyse the correlation between the chemical composition and the atomic structure, the preferred orientations and the morphology of 2D monolithic materials. The deposition strategies adopted and the thin films’ chemical compositions determine the as-deposited structures and, consequently, the mechanical behaviour of the thin films produced, particularly the hardness. In general, an intermediary amorphous stage is observed, i.e., the thin films exhibit a loss of crystallinity in the transition from a saturated solid solution to a new compound.     

Orientation study of iron phthalocyanine (FePc) thin films deposited on silicon substrate investigated by atomic force microscopy and micro-Raman spectroscopy

In this article, we present temperature and orientation study of iron phthalocyanine (FePc) thin films with different thickness deposited on silicon substrate. The organic thin films were obtained by the quasi-molecular beam evaporation. The micro-Raman scattering spectra of FePc thin layers were investigated in the spectral range 550–1,800 cm⁻¹ using 488-nm excitation wavelength. The Raman scattering and atomic force microscopy studies were performed at room temperature before and after annealing process. Annealing process of thin layers was carried out at 453 K for 6 h. From polarized Raman spectra using surface Raman mapping procedure the information on distribution of polymorphic phases of FePc layers has been carried out. Moreover, the obtained results showed the influence of the annealing process on the ordering of the molecular structure of thin films deposited on silicon substrate. For the very thin layers we did not observe the change of the polymorphic phase but only reordering of the thin layers and change of molecular structure to intermediate phase. Using atomic force microscopy method, we observed arrangement of the thin layers structure connected with the change of roughness of the thin layers after annealing process. The obtained results indicate that the structure of thin layer deposited on silicon substrate is strongly affected by the annealing process.     

Behavior of magnesium and silicon in the formation of MgO/AlN composite

MgO/AlN composites have been fabricated by directed metal nitridation of Al–Si alloy in flowing N₂ at 1473 K. A mixture of magnesia particles and chemically pure magnesium powder was placed on the surface of Al–Si alloy block as reinforcement materials. Mg powder initiates the infiltration and nitridation of Al alloy melt by eliminating protective Al₂O₃ film at the reaction frontier. New Mg vapor from the interface reaction between Al and MgO particles, keeps as continuous deoxidization agent as the added Mg powder. The spinel layer thickness due to the reaction of Al melt with MgO particles is controlled by Mg content. Si not only reduces the surface tension and viscosity of Al alloy melt, but also leads to increase in N₂ content.