Crystal structure of the orthorhombic U2-Al4Mg4Si4 precipitate in the Al–Mg–Si alloy system and its relation to the β′ and β″ phases

The atomic structure of a common precipitate in the Al–Mg–Si system has been determined. It is isotypic with TiNiSi (space group Pnma) and contains four units of MgAlSi in a unit cell of size a = 0.675 nm, b = 0.405 nm, c = 0.794 nm. EDS analyses support the composition. A model was based on the atomic structure of the β′ precipitate, electron diffraction and high-resolution transmission electron microscopy (HRTEM) images. A quantum mechanical refinement of the model removed discrepancies between simulated and experimental diffraction intensities. Finally, a multi-slice least square refinement confirmed the structure. The structural relation with β″ is investigated. A similar Mg–Si plane also existing in β″ and β′, can explain most coherency relations between the precipitate phases and with matrix.     

Spinodal decomposition existence of the β Ti–Cr binary alloy: computer simulation of the real alloy system and experimental investigations

The spinodal decomposition of β Ti–Cr binary alloys system still questionable, since there are rare experimental data moreover simulation results for the real alloy system have not been found. Transmission electron microscopy (TEM) and quantitative computer simulations results based on Khachaturyan's diffusion equation have been employed to study the microstructure evolution occurring in the β Ti–Cr alloys. Our study results reveal that the metastable β undergoes a phase separation reaction through a spinodal decomposition. The coherent two phase fields show extremely fine plate-like precipitates lying parallel to {1 0 0} plane. Those precipitates are high elastically-induced from the first step of phase separation.     

The Dislocation Structure of a Single-Crystal γ + γ′ Two-Phase Alloy After Tensile Deformation

The dislocation structures of an industrial single-crystal γ + γ′ two-phase alloy DD3 after tensile deformation from room temperature to 1273K were studied by transmission electron microscopy. The strength of this alloy decreased with an increase in the temperature, and showed a strength peak at 1033K. At room temperature, the dislocations shearing the γ′ particles were found to be 1/3<112> partial dislocations on the dodecahedral slip system <112>{111}. Some dislocation pairs on the cubic <110>{100} system that blocked the glide of dislocations were found at a medium temperature of 873K. As a result, dislocation bands were formed. Shearing of γ′ particles by 1/3<112> partial dislocations on the dodecahedral slip system <112>{111} was also found at this temperature. At the peak temperature of 1033K, because of the strong interaction between dislocations on the {111} and {100} planes, the extent of dislocation bands with high dislocation densities was extensive. The 1/3<112> partial dislocations on the dodecahedral slip system <112>{111} also existed. When the temperature reached the high temperature of 1133K, the range of dislocation bands was limited. The γ′ particles were sheared by <110> dislocation pairs on the octagonal <110>{111} system and the cubic <110>{100} system. At 1273K, the regular hexagonal dislocation networks were formed in the γ matrix and at the γ/γ′ interface. The Burgers vectors of the network were found to be b₁ = 1/2[110], b₂ = 1/2[1–10], b₃ = [100], and the last one was formed by the reaction of b₁ + b₂ → b₃. Dislocations shearing the γ′ particles were found to be <110> dislocation pairs on the octagonal system <110>{111} and cubic slip system <110>{100} at 1273K.     

Effect of γ′ precipitation during hot isostatic pressing on the mechanical property of a nickel-based superalloy

The effect of the precipitation of γ′ phase during hot isostatic pressing (HIPing) on the mechanical property of a nickel-based superalloy, GTD-111, was evaluated by conducting tensile and creep-rupture tests at 871 °C. In the 4-h two-step HIP process, the coupons were isostatically compressed (at 120 MPa) and heated to 1230 °C, well above the dissolution temperature of γ′ precipitates into the γ matrix, for the first 2 h, and cooled down to a temperature to induce the precipitation of γ′ phase and held for the last 2 h at 120 MPa or at ambient pressure. The precipitates were controlled in size by varying the temperature for the last half of the process. According to the result of the tensile test, the mechanical properties of the alloy were varied upon the microstructural evolution, and improved more than 40%, compared to those of the untreated ones. The precipitation of γ′ phase under high pressure further improved in the properties, suggesting that the precipitation of γ′ phase at high pressure provides an advantage for the rigidity of the structure. Based on these findings, a 6-h three-step HIP process was tried, and proved to be an effective substitute for the normal heat treatment, especially in terms of creep properties. This feature was mostly attributed to the homogenized microstructure of HIPed ones, as evidenced by the X-ray diffraction patterns.     

Nanocomposite Al2O3–ZrO2 thin films grown by reactive dual radio-frequency magnetron sputtering

Crystalline alumina–zirconia nanocomposites have been synthesized at 450 °C and 750 °C with reactive magnetron sputtering using radio-frequency power supplies. The composition of the films ranged from pure alumina to pure zirconia as measured by ion beam techniques. Microstructural characterization showed the presence of monoclinic zirconia in the pure zirconia films and γ-alumina in the pure alumina films while the nanocomposites contained either an amorphous compound, γ-alumina, cubic zirconia or a mixture of these. The grain size was  5 nm for the nanocomposite compared to larger grains in the pure oxide films. Electron energy loss spectroscopy showed a clear progression from the pure alumina to the pure zirconia.     

Effects of heat treatments on the microstructures and mechanical properties of Mg–3Nd–0.2Zn–0.4Zr (wt.%) alloy

Microstructure and mechanical properties of as-cast and different heat treated Mg–3Nd–0.2Zn–0.4Zr (wt.%) (NZ30K) alloys were investigated. The as-cast alloy was comprised of magnesium matrix and Mg₁₂Nd eutectic compounds. After solution treatment at 540 °C for 6 h, the eutectic compounds dissolved into the matrix and small Zr-containing particles precipitated at grain interiors. Further aging at low temperatures led to plate-shaped metastable precipitates, which strengthened the alloy. Peak-aged at 200 °C for 10–16 h, fine β″ particles with DO₁₉ structure was the dominant strengthening phase. The alloy had ultimate tensile strength (UTS) and elongation of 300–305 MPa and 11%, respectively. Aged at 250 °C for 10 h, coarse β′ particles with fcc structure was the dominant strengthening phase. The alloy showed UTS and elongation of 265 MPa and 20%, respectively. Yield strengths (YS) of these two aged conditions were in the same level, about 140 MPa. Precipitation strengthening was the largest contributor (about 60%) to the strength in these two aged conditions. The hardness of aged NZ30K alloy seemed to correspond to UTS not YS.     

Structural and optical properties of (100 − x)(Li2B4O7) − x(SrO–Bi2O3–0.7Nb2O5–0.3V2O5) glasses and glass nanocrystal composites

Glasses of various compositions in the system (100 − x)(Li₂B₄O₇) − x(SrO–Bi₂O₃–0.7Nb₂O₅–0.3V₂O₅) (10  x  60, in molar ratio) were prepared by splat quenching technique. The glassy nature of the as-quenched samples was established by differential thermal analyses (DTA). The amorphous nature of the as-quenched glasses and crystallinity of glass nanocrystal composites were confirmed by X-ray powder diffraction studies. Glass composites comprising strontium bismuth niobate doped with vanadium (SrBi₂(Nb_0.7V_0.3)₂O_9−δ (SBVN)) nanocrystallites were obtained by controlled heat-treatment of the as-quenched glasses at 783 K for 6 h. High resolution transmission electron microscopy (HRTEM) of the glass nanocrystal composites (heat-treated at 783 K/6 h) confirm the presence of rod shaped crystallites of SBVN embedded in Li₂B₄O₇ glass matrix. The optical transmission spectra of these glasses and glass nanocrystal composites of various compositions were recorded in the wavelength range 190–900 nm. Various optical parameters such as optical band gap (E_opt), Urbach energy (ΔE), refractive index (n), optical dielectric constant and ratio of carrier concentration to the effective mass (N/m^*) were determined. The effects of composition of the glasses and glass nanocrystal composites on these parameters were studied.     

Charging kinetics in virgin and 1 MeV-electron irradiated yttria-stabilized zirconia in the 300–1000 K range

A study performed with a dedicated scanning electron microscope (SEM) on the surface electrical properties of (1 0 0)-oriented yttria-stabilized zirconia (YSZ) single crystals irradiated with 1 MeV electrons is presented. When compared with virgin YSZ, the 1 MeV-irradiated YSZ shows a decrease of the intrinsic total electron emission coefficient σ₀ and an increase of the time constant τ associated with the charging kinetics of the material at room temperature. These measurements performed with the SEM beam at 10 keV indicate that the defects induced by the 1 MeV-electron irradiation generate a positive electric field of the order of 0.5 × 10⁶ V/m at a depth of about 1 μm that prevents electrons to escape. When the SEM beam with a 1.1 keV energy is used, a smaller field (0.5 × 10³ V/m) is detected closer to the surface (20 nm). The fading of these fields during the thermal annealing in the 400–1000 K temperature range provides information on the nature of defects induced by the 1 MeV-electron irradiation.     

Effects of Sn content on the microstructure, phase constitution and shape memory effect of Ti–Nb–Sn alloys

The effect of Sn content on the microstructure, phase constitution and shape memory effect of Ti–16Nb–xSn (x = 4.0, 4.5, 5.0 at%) alloys were investigated by means of optical microscopy, X-ray diffraction, transmission electron microscopy and bending test. With the increase of Sn content, the β phase becomes stable. The solution-treated Ti–16Nb–4Sn alloy is composed of ″ and β phases at room temperature, whereas the solution-treated Ti–16Nb–5Sn alloy is only composed of β phase at room temperature. TEM observation shows that there is parallel lamellar ″ martensite with the substructure of () type I twin in the Ti–16Nb–4Sn alloy. There exists the dislocation wall inside the single β phase in the Ti–16Nb–5Sn alloy. The shape recovery ratio decreases with increasing the bending strain and the bending temperature, which is in correspondence with the different deformation mechanisms at different temperature ranges. The shape recovery ratio shows a decreasing trend with the increase of Sn content at the same bending strain and temperature. The maximum completely recovery strain is around 4%.     

Formation of nanocrystalline structure of Ti–6Al–4V alloy by cyclic hydrogenation–dehydrogenation treatment

Ti–6Al–4V (Ti64) sheet specimens were cathodically hydrogenated in sulfuric acid solution at ambient conditions. The hydrogenated specimens were then sent to go through the designed thermohydrogen processing (THP) twice to obtain a nano-sized grain structure. The average grain size of resulted microstructure was found to be 10–20 nm obtained by TEM. Qualitative and quantitative analyses performed by employing X-ray diffractometry (XRD) and elemental analysis (EA) showed that the addition of As₂O₃ as hydrogenation promoter in electrolyte significantly increased the hydrogen uptake. The high concentration of hydrogen arising from promoter action is the key factor in grain refinement. The optimal processing parameter found for grain-refining Ti64 was: (1) electrolytic hydrogenation at 100 mA cm⁻² for 3 h in 1 N H₂SO_4(aq) by adding 0.1 g L⁻¹ As₂O₃; (2) β transformation carried out at 850 °C for 1 h in air furnace, followed by a furnace cooling to 590 °C and held for 6 h; (3) oxide film removed and then dehydrogenated at 650 °C and 1.0 × 10⁻⁶ Torr for 10 h; (4) repeated the same processes once more.     

Fabrication of β-Si3N4 whiskers by combustion synthesis with MgSiN2 as additives

β-Si₃N₄ whiskers with diameter of 0.5–2 μm and aspect ratio of 10–15 have been successfully prepared by combustion synthesis under 30–50 atm nitrogen pressure. The addition of MgSiN₂ powder plays a significant role in the growth of β-Si₃N₄ whiskers. The as-prepared products were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM).     

Effect of Small γ Precipitates on the Two-way Shape Memory Effect in a Cu–Zn–Al Alloy

After the treatment for the Stabilization of Stress-Induced Martensite (SSIM) in Cu–Zn–Al alloys, it was found that the small γ precipitates in the β austenite are ellipsoidal with a large strain field oriented in the same direction, while in the martensite, the γ precipitates changed their shape from ellipsoidal to spheroidal, which relaxed the strain field. To check whether the strain field of the γ precipitates is available to produce thermoelastic martensitic transformation, in situ observations with a heating sample holder in TME were performed. It was found that after heating above the A_s temperature, the spherical γ precipitates in the martensite recovered their strain field and elliptical shape. During cooling, the strain field of the γ precipitates disappeared again. This means that the strain fields of the γ precipitates trained by the SSIM method play an important part in the thermoelastic martensitic transformation that presents the two-way shape memory effect.     

Grain size effects on the tensile properties and deformation mechanisms of a magnesium alloy, AZ31B, sheet

The grain size dependence of the tensile properties and the deformation mechanisms responsible for those properties are examined for Mg alloy, AZ31B, sheet. Specifically, the Hall–Petch effect and strain anisotropy (r-value) are characterized experimentally, and interpreted using polycrystal plasticity modeling. {1 0 . 2} extension twins, {1 0 . 1} contraction twins, and so-called “double-twins” are observed via microscopy and diffraction-based techniques, and the amount of twinning is found to increase with increasing grain size. For the sheet texture and tensile loading condition examined, {1 0 . 2} extension twinning is not expected, yet the polycrystal plasticity model predicts the observed behavior, including this ‘anomalous’ tensile twinning. The analysis shows that the Hall–Petch strength dependence, of the polycrystal as a whole, is primarily determined by the grain size dependence of the strength of the prismatic slip systems.     

Lattice rotation and stability of 22.5° ND rotated cube orientation in cold rolled polycrystalline AA 5182 aluminum alloy

AA 5182 aluminum alloy with a strong cube texture was cold rolled to different reductions at an angle of 22.5° to the prior rolling direction. The texture evolution at this new rolling direction was investigated by X-ray diffraction. The rotation paths and stability of the 22.5° ND rotated cube orientation were determined based on the variation in the three-dimensional orientation distribution function (ODF) with rolling reduction. The results show that most of the grains with the 22.5° ND rotated cube orientation are directly rotated to the β fiber along different rotation paths, but there are a few grains moving through the cube orientation to the β fiber. The {0 0 1}<1 1 0> oriented grains possess the lowest stability during rolling, and the stability increases as the initial orientation changes from the {0 0 1}<1 1 0> orientation to the {0 0 1}<1 0 0> orientation along the ₁ axis.     

Microstructures and properties of biomedical TiNbZrFe β-titanium alloy under aging conditions

A metastable β-titanium alloy Ti–28Nb–13Zr–0.5Fe (TNZF alloy for short) was designed for implant biomedical application. The forged specimens were solute-treated at 850 °C followed by water quenching and then aged at 350 °C, 450 °C, and 550 °C for 2–6 h in order to evaluate the effect of phase transformation during ageing on the biomechanical compatibility of the alloy. The quenched microstructure consists of lath α″ martensite and β phase. A large quantities of shuttle-like ω phase precipitate at 350 °C, leading to the drastic increase of strength and elastic modulus and the decrease of plasticity. Ageing at 450 °C for 4 h, small amount of elliptic ω phase and dot α phase precipitate from β matrix. With increasing ageing time α precipitations begin to coarsen and precipitation free zones (PFZs) form around prior β grain boundaries. Needle-like α phase precipitates on grain boundaries and intra-grains when aged at 550 °C. Both PFZs and grain boundary α precipitates are prone to bring about the intergranular fracture and thus have adverse effects on the tensile strength and fracture plasticity. The quenched microstructure has good combination properties of high strength, high plasticity and low elastic modulus.     

Electron microscopy and hardness study of a semi-solid processed 27 wt%Cr cast iron

A semi-solid processed 27 wt%Cr cast iron was studied by electron microscopy and its microstructure was related to the hardness. In the as-cast condition, the primary proeutectic austenite was round in shape while the eutectic M₇C₃ carbide was found as radiating clusters mixed with directional clusters. Growth in the [0 0 1]M₇C₃ with planar faces of {0 2 0}M₇C₃ and was usually observed with an encapsulated core of austenite. Destabilisation heat treatment followed by air cooling led to a precipitation of secondary M₂₃C₆ carbide and a transformation of the primary austenite to martensite in the semi-solid processed iron. Precipitation behaviour is comparable to that observed in the destabilisation of conventional cast iron. However, the nucleation of secondary M₂₃C₆ carbide on the eutectic M₇C₃ carbide was observed for the first time. Tempering after destabilisation led to further precipitation of carbide within the tempered martensite in the eutectic structure. The maximum hardness was obtained after destabilisation and tempering heat treatment due to the precipitation of secondary carbides within the martensite matrix and a possible reduction in the retained austenite.     

Ultraviolet emitting (Y1−xGdx)2O3−δ thin films deposited by radio frequency magnetron sputtering: Combinatorial modeling, synthesis, and rapid characterization

Gadolinium-doped, yttrium oxide thin films have been deposited on silicon (001) substrates by radio-frequency (RF) magnetron reactive sputtering that exhibit cathodoluminescence (CL) at ultraviolet frequencies. The maximum CL brightness occurred at λ314–315 nm characteristic of the ⁶P_3 / 2 → ⁸S (λ = 314 nm) transition observed in Gd-doped, yttrium oxide powders. The radiative recombination takes place at the rare earth activator Gd³⁺ site embedded in the Y₂O_3−δ host; the optical transition resides within the band gap of the Y₂O_3−δ host and the transition observed is characteristic of atomic gadolinium. A combinatorial approach to sputtering was used to deposit a film of variable composition from 1 to 23 at.% Gd in Y₂O_3−δ in order to rapidly discern the composition node of optimal CL brightness. A simulation was created for the purpose of predicting the film combinatorial composition for binary and ternary alloys prior to sputtering experiments in order to facilitate our combinatorial thin film synthesis technique. The model prediction varied from the real experimental composition profile by only 2.2 at.% Gd ± 1.6 at.% proving the predictor as a useful aide to complement combinatorial thin film experiments. A film of composition Y_1.56Gd_0.44O_3.25 (8.3 at.% Gd) yielded the maximum CL brightness. CL brightness increased continuously up to the 8.3 at.% Gd composition due to the increased number of activators present in the host. Beyond this composition the brightness drastically decreased. The oxygen composition in the combinatorial film was strongly dependent on the Gd composition; films were sub-stoichiometric δ > 0 below 6 at.% Gd and was over-stoichiometric δ < 0 beyond this composition.     

Thermal stability and mechanical properties of nanostructured Ti–24Nb–4Zr–7.9Sn alloy

Thermal stability of the nanostructured grains of cold-rolled Ti–24Nb–4Zr–7.9Sn alloy and corresponding variations in mechanical properties were investigated. The activation energy for grain growth was found distinct below and above the ( + β)/β transus of 950 K, with values of 47 and 206 kJ/mol, respectively. Due to the pinning effect of the precipitates at β grain boundaries, grains sizes can be maintained at less than 100 nm during prolonged annealing at temperatures up to 773 K, and are less than 1 μm for annealing temperature up to 923 K and time up to 2 h. Annealing above the β transus resulted in coarse grains with sizes of tens of micrometers in less than 2 h. Tensile and hardness tests showed rapid strengthening with the increase of annealing time below 773 K, which was attributed to both the rapid formation of nano-sized precipitates and the slow growth rate of β grains. By adjusting the grain size of the cold-rolled material the high strength/low Young's modulus match desirable for implant applications can be improved over the hot-rolled bars with coarse grains.     

Variation in the reaction zone and its effects on the strength of diffusion bonded titanium–stainless steel couple

Solid state bonding was carried out between commercially pure titanium and 304 stainless steel at 850 °C temperature for 30–150 min under uniaxial load in vacuum. The transition joints were characterized using optical and scanning electron microscopes and revealed the presence of reaction layers in the diffusion zone. The chemical composition of these reaction products was determined by energy dispersive spectroscopy and the presence of phase/phase mixtures σ, -Fe + χ, χ + λ, λ + FeTi + β-Ti and β-Ti were predicted. Their existence was also confirmed by X-ray diffraction technique. The concentration–penetration plots for Ti, Fe, Cr and Ni are obtained from electron probe microanalysis, which also exhibit concentration gaps in the profile indicating formation of intermetallics in the diffusion zone. The width of intermetallic phases attains its maximum for 90 min joining time, then drops and again rises for 150 min bonding time. The maximum bond strength of 76% of that of titanium was achieved for the assemblies processed for 90 min owing to better coalescence of the mating surfaces, though the volume fraction of intermetallics is high.     

The effect of copper addition on the structure and strength of an Al–Li alloy

In this study the effect of copper addition on the structure, precipitation kinetics and hardness in the Al–Li and Al–Li–Cu alloys aged at 200°C was investigated. The structures of precipitates were studied using X-ray-small-angle-scattering (XSAS) and transmission electron microscopy (TEM) methods. The changes in the structure parameter (Rg) of both alloys was calculated using two methods, the Guinier approximation and correlation function γ(r). By use of a plot of r γ(r) the distribution law of the T₁ disc thickness was obtained and the coexisting spherical particles of δ′ were estimated. Two types of δ′ precipitates of approximately 2 nm size and above 8 nm and the T₁ precipitates of thickness between 3 and 4 nm were observed.