Intermediate Annealing Step Research Articles

Mechanical properties and fracture surface of Al-Al2O3 nano/micro-composites produced by accumulative roll bonding

ABSTRACT The accumulative roll bonding (ARB) process has been utilised to produce layered materials and composites with high strength without intermediate annealing steps. This study focused on the size of reinforced particles (Al2O3) with an average size of 3 µm (m) and 80 nm (n) in aluminum matrix composites. Three different samples of monolithic Al, Al-1 vol.% (m)Al2O3, and Al-1 vol.% (n)Al2O3 particulate composites were produced by ARB. The SEM results revealed that the distribution of alumina particles in the fabricated composites was improved with increasing ARB cycles, while the Al-1 vol.% (m)Al2O3 composite presented the proper distribution of particles at higher cycles in contrast with the Al-1 vol.% (n)Al2O3 composite. The tensile strength of the produced composites was enhanced, and their ductility decreased with the increase in ARB cycles, while Al-1 vol.% (m)Al2O3 with a tensile strength of 253 MPa showed higher strength than Al-1 vol.% (n)Al2O3 with 229 MPa at the sixth cycle. By adding Al2O3 microparticles, the yield strength and tensile strength of the microcomposite sheet were increased to about 5 and 2.1 times, respectively, compared to the Al sheet.

Influence of Post-Weld Processing Techniques on Laser Beam-Welded Al–3Mg–0.25Sc Alloy Sheets

Fusion welding in aluminium alloys had always remained an issue for aero- and auto industries. An Al–3Mg–0.25Sc alloy is classified as weldable aluminium alloys. In this study, two welding techniques, keyhole and conduction welding, have been used for joining Al–3Mg–0.25Sc sheets of thickness 1.6 mm. Experiments were carried out using different beam diameter (0.16–2 mm dia), beam power (2–4.5 kW) and laser head speed (ranging from 0.5 to 6 m/min). The results for all autogenous keyhole welding clearly demonstrated a drop in ductility ( 50% of base metal). Residual stress-induced distortions affected the final shape of the product, and for that, a suitable stress relief annealing was also required. Hereby, we chose two heat treatment schedules: (1) solution treatment and annealing and (2) retrogression and re-ageing. Retrogression and re-ageing is a precipitation-controlled phenomenon and comprises of an intermediate annealing step between tempering and solutionizing temperature to dissolve the nano-clusters present in the matrix (retrogression) and re-precipitating and growing them by ageing (re-ageing). These two steps were repeated cyclically, and as a result, the strength (190 MPa) and ductility (~ 9%) of the material were improved substantially. The improvement of ductility and strength was explained in detail in the light of microstructural studies.

Facile fabrication of microlenses with controlled geometrical characteristics by inkjet printing on nanostructured surfaces prepared by combustion chemical vapour deposition

Precise positioning of microlenses with well-defined optical characteristics is key in the further development of CCD cameras, biosensors or optical fiber interconnects. Inkjet printing enables accurate microfabrication of microlenses however inks generally employed for this purpose contain solvents that need to be evaporated before the lens solidification process. Besides, the receiving substrate needs to be conditioned, sometimes using complex photolithographic steps to lead to large contact angles of the deposited ink drop that are needed to attain large numerical aperture microlenses. This paper describes the fabrication of microlenses with controlled geometrical characteristics by inkjet printing a solvent-free photocurable formulation. The employed photoacid catalyzed organic–inorganic hybrid ink can be cured just after deposition, without any intermediate evaporation or annealing step, enormously simplifying microlens fabrication process. Besides, a simple combustion chemical vapour deposition process, leading to a porous layer with nano-roughness, followed by a silanization step using a fluorosilane enables the generation of a surface that provides access to a large range of contact angles for the ink drops that are printed on this surface. Single droplet microball lenses with contact angles up to 115°, beyond the hemispherical microlenses, are demonstrated with this industrially viable, cost-effective and high-throughput method.

Multi-Layered Mesoporous TiO2 Thin Films: Photoelectrodes with Improved Activity and Stability

This work aims at the identification of porous titanium dioxide thin film (photo)electrodes that represent suitable host structures for a subsequent electrodeposition of plasmonic nanoparticles. Sufficient UV absorption and electrical conductivity were assured by adjusting film thickness and TiO 2 crystallinity. Films with up to 10 layers were prepared by an evaporation-induced self-assembly (EISA) method and layer-by-layer deposition. Activities were tested towards the photoelectrochemical oxidation of water under UV illumination. Enhanced activities with each additional layer were observed and explained with increased amounts of immobilized TiO 2 and access to more active sites as a combined effect of increased surface area, better crystallinity and improved transport properties. Furthermore, films display good electrochemical and mechanical stability, which was related to the controlled intermediate thermal annealing steps, making these materials a promising candidate for future electrochemical depositions of plasmonic noble metal nanoparticles that has been further demonstrated by incorporation of gold.

Impact of intermediate high temperature annealing on the properties of AlN/sapphire templates grown by metalorganic vapor phase epitaxy

In this work, we investigate AlN/sapphire templates grown by metalorganic vapor phase epitaxy (MOVPE) subjected to an intermediate high temperature annealing (HTA) step at different AlN film thicknesses. To avoid cracking of the MOVPE layers during HTA at 1700 °C these layers have to be grown under low incorporation of tensile strain. For each sample the growth is stopped at a certain thickness between 230 nm and 1.2 μm followed by HTA. X-ray rocking curve FWHM of 0002 and 10–12 reflections lead to an estimation of threading dislocation densities (TDD) as low as cm−2 for the thickest AlN layers of 880 nm and 1.2 μm after annealing. For all layer thicknesses MOVPE growth is subsequently continued to a total AlN layer thickness of 1.5 μm to reach for a smooth surface. The change of the AlN strain state after HTA leads to an increased wafer curvature at room temperature and compressive strain in the subsequently grown AlN layers increasing the wafer bow at room temperature. Hence, to serve as a base layer, e.g. for ultraviolet light emitting diodes, a trade-off between low TDD, surface smoothness and wafer bow has to be found.

One step towards MnAl-based permanent magnets - Differences in magnetic, and microstructural properties from an intermediate annealing step during synthesis

The influence of an additional annealing step during synthesis on the preparation of MnAl based permanent magnet alloys has been investigated. Bulk samples of Mn55Al45C2 alloys were synthesized using induction heating through drop synthesis from 1400 °C. Samples produced using cooling directly from 1400 °C (from the melt), and from 1400 °C to an intermediate annealing step at 1200 °C for ∼30 min before cooling were compared with respect to differences in phase purity, microstructure and magnetic properties. We found that the phase purity was significantly enhanced using the route with an intermediate annealing step at 1200 °C. From XRD the relative content of the τ-phase was improved from ∼91 wt% for the sample cooled directly from 1400 °C to ∼95.1–99.5 wt% for the sample exposed to an intermediate annealing step before cooling. Additionally, EBSD, and SEM with EDS indicates a clear difference in the phase composition and differences in the distribution of the magnetic τ-phase and the non-magnetic ε-, β-, and γ2-phases. Magnetic properties also indicate an improvement in saturation magnetization for the sample exposed to the extra annealing step during synthesis. Our results suggest that an intermediate annealing step in the production of MnAl based alloys will provide a simple way of achieving better phase purity and magnetic properties in the bulk alloy.

Tracking Transformative Transitions: From CsPbBr3 Nanocrystals to Bulk Perovskite Films

The control of grain size and surface properties is an important parameter in controlling the optoelectronic and photovoltaic properties of metal halide perovskites. When CsPbBr3 nanocrystal (∼10 nm in diameter) films were annealed at 100–125 °C, they grow in size to produce ∼400 nm diameter crystallites while transforming into bulk perovskite films. Characteristic changes in the optical properties were noted when such transformation occurred from nanocrystals into bulk. By tracking absorbance and emission spectra and morphological changes of CsPbBr3 films at different annealing times and temperature, we were able to establish the mechanism of particle growth. The presence of nanocrystals and larger crystals during the intermediate annealing steps and narrowing size distribution confirmed the Ostwald ripening mechanism for the crystal growth. The energy of activation of crystal growth as determined from the temperature dependent optical properties was estimated to be 75 kcal/mol.

CZTSSe absorber layer formation and impact of annealing process on its properties

In the present work, the Copper Zinc Tin Sulfur Selenium (CZTSSe) absorber layer powder was prepared by solid state reaction process using ball mill machine and subsequently the paste was prepared. The paste was deposited using doctor blade method. The deposited films were annealed via two-step and three-step annealing method. For two-step annealing, in the first step the films were first ramped to 250 °C for 10 min and in the second step the temperature was ramped to 500 °C with three different hold times (5,10 and 20 min). Similarly, for the three-step process, samples were first ramped to temperature of 250 °C for 10 min, followed by an intermediate annealing step of 300 °C (10 min) and in the final step the temperature was ramped to 500 °C with two different hold times (5 and 10 min). The goal of the present work is to investigate the effect of annealing through two-step and three-step temperature profile on the CZTSSe absorber layer properties. The films were characterized using XRD (for phase analysis), SEM (for surface morphology), EDXRF (for composition analysis), and Hall Measurement (for electrical properties). It is found that the annealing hold time at higher temperature strongly influences the formation of CZTSSe grain growth, morphology and the electrical properties.

The effect of anisotropy on the intermediate and final form in deep drawing of SS304L, with high draw ratios: Experimentation and numerical simulation

High deep draw ratio of metal sheets is often required in industry to produce complex structural components and is considered to be problematic beyond a draw ratio of 1.7. In addition, anisotropy of material plays a great role in the final form of deep drawn products. In this research AISI SS304 L high draw ratio deep drawing (HDR) cups have been developed by using multiple intermediate annealing steps. The form of flange produced at each draw step is recorded, Numerical simulation of the process is carried out using ABAQUS StandardTM. Accurate modelling of the process requires anisotropic parameters. Tensile tests at 0o, 15o, 30o, 45o, 60o, 75o, 90o to the rolling direction are carried out to determine the Lankford coefficients and hardening coefficients in each direction. The detailed material data obtained is then applied to simulate the HDR process. The forms recorded experimentally are compared with simulation results and conclusion is drawn as to the accuracy of the simulation.

High strength and thermal stability of bulk Cu/Ta nanolamellar multilayers fabricated by cross accumulative roll bonding

Bulk Cu/Ta nanolamellar multilayers with an individual layer thickness from several micrometers down to 50 nm were successfully fabricated via a combination of cross accumulative roll bonding (CARB) and an intermediate annealing step. This fabrication technique allowed to effectively suppress the formation of plastic instabilities and edge cracks during the repeated rolling process. A transition of the layered morphology from non-planar interfaces at the submicron level to nearly planar interfaces at the nano-scale was observed with decreasing layer thickness. High resolution transmission electron microscopy, selected area electron diffraction and X-ray diffraction were performed, and the results indicate that the Cu/Ta nanolamellar multilayers with a layer thickness of 50 nm show a {100}Ta[110]∥{110}Cu[111] rolling texture relationship. Tensile tests revealed that the ultimate tensile strength of the composite was up to 950 MPa, which is approximately 5 times higher than that of the initial pure Cu and Ta. The hardness of the prepared multilayer maintained unchanged even after an annealing at 500 °C for 1 h. These unique properties are attributed to an atomically flat bimetal interface and the low amount of homophase grain boundaries resulted from the CARB process.

Nb-doped TiO2 sol–gel films for CO sensing applications

Nb doped titania (TiO2:Nb) multilayered films (1–10 layers) with anatase structure were obtained by the low-cost sol–gel and dipping method on microscope glass substrates, followed by thermal treatment at 450°C for 1h. After each layer deposition, an intermediate annealing step was performed at 300°C for 30min. Doping TiO2 sol–gel films with a low amount of Nb (0.8at%) allows obtaining an improved CO sensor able to operate under environmental atmosphere (air). It was found that the sensor sensitivity is less dependent on the film thickness but is significantly influenced by Nb doping at the optimal working temperature of 400°C. Good recovery characteristics were obtained for a wide CO detection range, between 0 and 2000ppm. The gas-sensing behavior of the films was correlated with the structural, chemical and morphological properties of the multi-layered structures.

Annealing and sintering effects in thermoelectric skutterudites synthesized by gas atomization

Skutterudites on the basis of CoSb3 are among the best thermoelectric materials for generator applications in the intermediate temperature range. Still, to be suitable for a wide range of applications methods for low cost fabrication of large amounts of this material are needed. A process potentially fulfilling these requirements is gas atomization plus consecutive short‐term sintering. As the powder produced by atomization is usually not single phase skutterudite material, the subsequent processes of sintering and eventually annealing prior to sintering play an important role for the resulting material's thermoelectric properties. In this work, we investigate the influence of powder annealing and sintering on the properties of indium‐filled skutterudites synthesized by gas atomization. The phase composition and thermoelectric properties are investigated as functions of the sintering temperature. Furthermore, the influence of an intermediate annealing step prior to sintering is investigated. It is shown short sintering times at temperatures around 450 °C are sufficient to achieve dense single phase skutterudite material from atomized powder whereas for annealed skutterudite powder sintering temperatures higher than 550 °C are needed for dense samples. Furthermore, sintering temperature and annealing are shown to have strong influences on the microstructure and thermoelectric properties of the material.

Forming limits of Mg alloy ZEK100 sheet in preform annealing process

Preform annealing (PFA) is a new two-stage forming technology with an intermediate annealing step, which extends the formability of light-weight (Al and Mg) alloys that comparable to deep drawing steels. ZEK100 is the only rare-earth containing magnesium sheet alloy that available commercially. The objective of this research is to understand the effect of preform and anneal on the forming limits of ZEK100. Various pre-strain paths and levels were introduced in the preform step followed by an optimum annealing treatment. The total strain-based forming limits of the PFA material are strongly dependent on the pre-strain level and annealing conditions. Based on the strain energy principle, an equivalent strain is employed to evaluate the effects of the pre-strain and anneal on the total limit equivalent strain of the PFA material, which is fitted as a function of the equivalent pre-strain and strain ratio of major strain to minor strain; and there is a critical equivalent pre-strain that leads to the lowest total limit equivalent strain at all strain ratios. Based on this concept, a new forming limit diagram (FLD) is constructed with the equivalent strain and the strain ratio as the horizontal and vertical axes, respectively, to evaluate the formability of Mg sheet in this two-stage forming process with intermediate annealing.

Atomic layer deposition for high aspect ratio through silicon vias

The paper presents atomic layer deposition (ALD) processes and process integration for the deposition of insulating liners, copper diffusion barriers, and seed layers for direct copper plating in high aspect ratio (>20:1) through silicon vias. A TaN-based copper diffusion barrier was deployed on an aluminum oxide insulating liner. The latter has the potential to act also as a dielectric barrier against copper diffusion according to BTS and TVS measurements. Furthermore, ruthenium ALD films applied as seed layers for direct copper plating were deposited with an intermediate annealing step to improve film adhesion and ensure the deposition of thick films without any delamination. The step coverage of the presented ALD processes was confirmed by SEM measurements on cross-sections of coated TSV structures. Finally a subsequent electrochemical deposition (ECD) of copper was conducted revealing the satisfying functionality of the Ru seed layer.

Forming limit analysis for two-stage forming of 5182-O aluminum sheet with intermediate annealing

Stress-based forming limit diagrams for a two-stage forming technique with an intermediate annealing step were generated for aluminum alloy 5182-O with a new experimental/theoretical methodology. It was demonstrated that unlike strain-based forming limits, stress-based forming limits are independent of pre-strain levels, pre-strain paths and annealing conditions, and converge to a single forming limit curve that is close to the stress forming limit of the as-received material. For this purpose, AA5182-O specimens were pre-strained to various levels in uniaxial, plane strain, and equibiaxial tension; smaller shapes were extracted, annealed, and tested in limiting dome height (LDH) tests. Strain fields for the forming limit diagrams were measured with stereo digital image correlation during each LDH test, where the onset of localized necking was identified to construct the forming limits. In the calculation of stress-based forming limit curves, only the as-received material properties were required, while the effects of pre-strain and annealing were accounted for with a calculated constant “effective plastic strain.” An analytical method is described to develop a new experimental/theoretical methodology effectively predicting the pre-strain and annealing effects. This enables reliable prediction of formability in multi-stage forming processes interrupted by annealing treatment employed to recover additional stretchability in critical areas of a formed part.

On the influence of small quantities of Bi and Sb on the evolution of microstructure during swaging and heat treatments in copper

In the present work, the influence of small amounts of Bi and Sb on the microstructural evolution of Cu during an ingot metallurgy processing route is investigated. Both elements are known to segregate to grain boundaries in Cu. Cu ingots with an outer diameter of 40 mm containing 0.008 wt.% Bi and 0.92 wt.% Sb, respectively, were vacuum induction melted, cast, and gradually swaged down to a final diameter of 11.7 mm with several intermediate annealing steps. Subsequent annealing treatments were conducted to investigate the microstructural evolution of the swaged bars. Optical microscopy, hardness testing and orientation imaging microscopy were used to characterize the deformation and recrystallization behavior, as well as the evolution of texture in the alloys. The results are then compared to those obtained for pure Cu. It is shown that even small amounts of alloying elements significantly alter the hardening behavior and suppress recrystallization at low temperatures. At higher temperatures, recrystallization in Cu, Cu–Bi and Cu–Sb leads to different textures.

Influence of intermediate annealing on the microstructure of in situ MgB 2/Fe wire

A two-step process with an intermediate annealing was used in an attempt to improve the critical current density ( J c) through a core densification of in situ MgB 2/Fe wires. MgB 2/Fe wires were first drawn to an outer diameter of 1.9 mm, and heat-treated at 600 °C for 0.5, 1, 2 and 4 h in flowing Ar gas as an intermediate annealing (IA) step. In the cases of the wires heat-treated at 600 °C for 0.5–2 h, un-reacted Mg and MgB 2 phases coexisted. The sample heat-treated for 4 h showed a full MgB 2 formation through a solid state reaction. The intermediate-annealed wires were then further drawn to an outer diameter of 1.42 mm as an attempt to reduce the porosity. This second mechanical deformation crushed the superconducting grains into smaller pieces and mixed preformed MgB 2 phase with un-reacted Mg and B powders. And then, the second heat-treatment was performed at 650 °C for 0.5 h. The J c values of the MgB 2 wires prepared with an intermediate annealing step were notably lower, over a wide range of magnetic fields, when compared to the reference sample heat-treated once at 650 °C for 0.5 h. The decrease in J c is probably due to the introduction of a large amount of weak links between the grains.

Impact of Annealing Temperature Ramps on the Electrical Activation of N<sup>+</sup> and P<sup>+</sup> Co-Implanted SiC Layers

We have performed nitrogen and phosphorus co-implants at room temperature to obtain high n-type carrier concentration layers in SiC. An inductive heating RTA furnace has been used for the activation annealing. The influence of the temperature ramp parameters such as rise/decrease temperature speed and intermediate annealing steps on the dopant activation rate and surface morphology have been investigated. A reduction of the temperature ramp slope reduces the surface roughness by 50%. Inclusion of a pre-activation annealing step at low temperatures (1300°C) further reduces the surface roughness. However, the use of slower ramps or an intermediate annealing step during ramp up reduces the free carrier concentration. The faster the ramp up, the higher the activation rate and the resulting doping. We also demonstrate that the inclusion of a postactivation annealing at intermediate temperatures (1150°C) reduces significantly the surface roughness. In addition, the use of this post-annealing treatment does not degrade the activation rate nor the carrier Hall mobility, and activation rates close to 100% have been obtained.

Deposition of NiAl coating for improvement of oxidation resistance of cold-rolled Ni 3Al foils

β-NiAl, an important intermetallic compound in Ni–Al systems, together with γ′-Ni 3Al, has been used as a coating material for Ni-base superalloys, because of its excellent oxidation resistance. In this study, NiAl-based coating through RF magnetron sputtering was selected for deposition on Ni 3Al thin foils which were cold-rolled up to 96% without any intermediate annealing steps. The thickness of the coating on Ni 3Al foils is restricted below 10 μm, which is much thinner than general coatings on bulk alloys. Insufficient coating thickness can induce rapid Al depletion in the NiAl coating layer. However, the addition of 3 at.% Ti was very effective in retarding atomic diffusion and Al depletion in the coating layer. The binary Ni–50Al coating did not form a stable α-Al 2O 3 oxide during cyclic oxidation tests at 1000 °C. Moreover, rapid Al diffusion between the coating layer and substrate was observed. Ni–47Al–3Ti–0.1Y was then selected as a modified coating composition. The Ni–47Al–3Ti–0.1Y coating showed superior oxidation resistance due to stable α-Al 2O 3 formation and effective retardation of Al diffusion from the coating layer. In addition, Ni-oxide and α-Al 2O 3 coexisted in the bare Ni 3Al foil without any coating, and the bare Ni 3Al foil exhibited more stable oxidation behavior than the bulk Ni 3Al alloy.

Studies of co-implanted helium and hydrogen with an intermediate annealing step for thermal splitting of bonded silicon to oxide-coated wafers

Layer transfer by thermal or mechanical splitting of implanted silicon wafers can often require processing at elevated temperatures before bonding and annealing. The possibility has been investigated here of making two implants, the first defining a damage plane for splitting, the second being made after high-temperature processing. Helium and hydrogen implanted wafers were annealed at various temperatures, Ta, this being followed by a second implantation of either gas. Thermal splitting of bonded wafer pairs was studied for both implants to the same projected range, Rp, and with hydrogen implanted to Rp=25% less than helium. In the former case, complete thermal splitting occurred for Ta⩽550°C; for Ta>550°C, wafers failed to split or delaminated at the bond interface. When hydrogen was implanted to a shallower depth complete thermal splitting occurred for Ta⩽500°C. The split occurred at the hydrogen Rp for Ta<400°C and at the helium Rp for Ta=or>400°C. Above Ta=500°C a decreasing percentage of the total silicon area was transferred. At 400°C measurements suggested that splitting occurred at both helium and hydrogen Rp planes and it is believed that helium platelet formation at this temperature is an important factor. For Ta<400°C, helium, a proportion of which may have diffused towards the peak of the hydrogen distribution, is believed to react and produce splitting there during bond-annealing. For Ta>400°C, sufficient hydrogen is thought to diffuse during bond-annealing towards the peak of the helium distribution and react there to produce the weakest splitting plane. For Ta>500°C, loss of helium, annealing out of strain and break-up of platelets into bubble-cavities reduces the probability of thermal splitting. When hydrogen was implanted first, complete thermal splitting occurred for Ta⩽300°C but was inconsistent at 400°C and above.