The effects of trace Sc and Zr on microstructure and internal friction of Zn–Al eutectoid alloy

The damping properties of Zn–22 wt.% Al alloys without and with Sc (0.55 wt.%) and Zr (0.26 wt.%) were investigated. The internal friction of the determined by the microstructure has been measured in terms of logarithmic decrement (δ) using a low frequency inverted torsion pendulum over the temperature region of 10–230 °C. An internal friction peak was separately observed at about 218 °C in the Zn–Al alloy and at about 195 °C in Zn–Al–Sc–Zr alloy. The shift of the δ peak was found to be directly attributed to the precipitation of Al₃(Sc, Zr) phases from the alloy matrix. We consider that the both internal friction peak in the alloy originates from grain boundary (GB) relaxation, but the grain boundary relaxation can also be affected by Al–Sc–Zr intermetallics at the grain boundaries, which will impede grain boundary sliding. In addition, Al–Sc–Zr intermetallics at the grain boundaries can pin grain boundaries, and inhibit the growth of grains in aging, which increases the damping stability of Zn–22 wt.% Al alloy.     

Mechanical behaviors of Al2O3 nanoparticles reinforced polyetheretherketone

The precipitation kinetics of Si in an Al–1.7 wt.%Si alloy after different thermal treatments has been studied by means of transmission electron microscopy (TEM), dilatometry and differential scanning calorimetry (DSC). The results obtained are explained by a model based on simple nucleation and growth/dissolution laws and are compared with measured precipitate size distributions. The evolution of precipitates in water-quenched samples during linear heating depicts the exothermic formation of platelets and globular Si precipitates (200–300 °C). The endothermal dissolution of Si platelets starts at lower temperatures than that of the globular precipitates. Coarsening and finally dissolution of globular precipitates is observed with increasing temperature. Samples slowly cooled from the solution treatment temperature present mostly globular precipitates, which are nucleated during cooling. Here, an exothermal effect related to the growth of Si precipitates increasing their volume fraction is observed at relatively high temperatures (350–460 °C) during linear heating. The formed precipitates are stable up to 460 °C, where the modelled critical radius becomes bigger than most of the Si precipitates formed so far.     

Aging behaviour of Al–Cu–Mg alloy–SiC composites

The age-hardening kinetics of powder metallurgy processed Al–Cu–Mg alloy and composites with 5, 15 or 25 vol.% SiC reinforcements, subjected to solution treatment at 495 °C for 0.5 h or at 504 °C for 4 h followed by aging at 191 °C, have been studied. The Al–SiC interfaces in composites show undissolved, coarse intermetallic precipitates rich in Cu, Fe, and Mg, with its extent varying with processing conditions. Examination of aging kinetics indicates that the peak-age hardness values are higher, and the time taken for peak aging is an hour longer on solutionizing at 504 °C for 4 h, due to greater solute dissolution. Contrary to the accepted view, the composites have taken longer time to peak-age than the alloy, probably due to lower vacancy concentration, large-scale interfacial segregation of alloying elements, and inadequate density of dislocations in matrix. The composite with 5 vol.% SiC with the lowest inter-particle spacing has shown the highest hardness.     

Experimental assessment of the Ru–Al–Ni ternary phase diagram at 1000 and 1100 °C

Knowledge of phase equilibria in the Ru–Al–Ni ternary system is relevant to the development of new single crystal Ni-based superalloys as well as to new high temperature protective coating systems for these alloys. A series of diffusion couple investigations have been performed across the Ru–Al–Ni ternary system in order to establish phase fields and possible diffusion paths. A continuous B2 phase has been shown to exist across the Ru–Al–Ni ternary between the RuAl and NiAl phases at temperatures of 1000 and 1100 °C. Ternary isothermal sections for Ru–Al–Ni at 1000 and 1100 °C are presented.     

Microstructure and mechanical properties of cold-rolled TiNbTaZr biomedical β titanium alloy

The influence of the plasma-sprayed coatings and of the atmosphere on creep of the Ti–6Al–4V alloy was investigated. Yttria partially stabilized zirconia (YSZ) with CoNiCrAlY bond coat was atmospherically plasma sprayed on Ti–6Al–4V substrates. Constant load creep tests were conducted on a standard creep machine in air and nitrogen atmospheres on uncoated samples and in air on coated samples, at stress levels of 520 MPa at 500 °C, 319 MPa at 600 °C and 56 MPa at 700 °C. Results indicated that the creep rates in nitrogen and of the coated alloy were lower than those of the uncoated in air.     

Effects of carbon content of carbon steel on its dissolution into a molten aluminum alloy

Hot dip aluminizing of carbon steels with different carbon concentration ranging 0.2–1.1 wt.% was carried out in a molten Al–9.08 wt.% Si–0.98 wt.% Fe alloy at 660 °C. The steel specimens lost weight as a result of dissolution into the melt, and an intermetallic layer was formed on the surface of them. The specimens showed varied dissolution rates depending on carbon concentration. The specimen with the highest carbon content exhibited the slowest dissolution rate. The thickness of the intermetallic layer increased with dipping time following a parabolic relationship. The growth rate of the layer decreased with increase of the carbon content. A diffusion mechanism to control the dissolution of the carbon steel into the molten aluminum alloy was suggested, and the effect of carbon content on the dissolution of the steel substrate into the melt was discussed in connection with the proposed diffusion mechanism and microstructural observations.     

Investigation into effects of re-shot-peening on fretting fatigue behavior of Ti–6Al–4V

The effects of re-shot-peening treatment on fretting fatigue life/strength and the recovery of residual stress of the initially shot-peened Ti–6Al–4V were investigated at room and elevated temperatures. After subjecting to fretting fatigue up to about 40% of the total expected life of the initially shot-peened Ti–6Al–4V or to thermal exposure to 370 °C only, residual stress relaxed in the range of 20–50% of its value before fretting fatigue. The magnitude of stress relaxation depended upon the applied load level and test temperature. Re-shot-peening successfully recovered the relaxed residual stress up to the same level as obtained after the initial shot-peening. Further, fretting fatigue life after re-shot-peening, excluding pre-re-shot-peening fatigue life, was very close to that of the initially shot-peened specimen at a given stress level and test temperature. It thus appears that re-shot-peening nullified the effect of fretting fatigue damage after the initial shot-peening.     

High temperature structural evolution of a CuAlNi alloy studied by in situ X-ray diffraction and isothermal mechanical spectroscopy

The purpose of this work is to study the microstructural mechanisms associated with the eutectoid transition in a ternary Cu–12 wt.% Al–3 wt.% Ni alloy. The samples have been initially annealed at 850 °C, then slowly cooled down to room temperature. The experiments have been carried out both on cooling and on heating above 500 °C using isothermal mechanical spectroscopy and X-ray diffraction (fitted with a temperature camera). On heating, a relaxation peak with a high intensity rises up above 600 °C, then on cooling, the peak totally disappears below 580 °C, the effect being reproducible. The structural analysis, undertaken in the same temperature domain, has clearly evidenced each step of the evolution, particularly the eutectoid transformation. Consequently, the damping effect seems to be associated to the presence of the high temperature β phase.     

Morphology of sputter deposited Al alloy films

Morphology of Al–2.0at%Ta and Al–2.0 at.% Nd alloy films before and after annealing was investigated for applications of interconnections for liquid crystal displays. It was found that the morphology and the microstructure of Al–2.0 at.% Nd alloy films changed markedly by annealing at the temperature region from 200°C to 300°C, while the morphology of Al–2.0 at.% Ta alloy films did not change by annealing up to 400°C. For the case of Al–2.0 at.% Nd alloy films, the incline of the <111> fiber texture to the substrate normal was observed during annealing. Structural characteristics of the Al films were investigated by TEM, SAD and XRD to determine the influence of alloying elements on the morphology and the fiber texture. From these results, it was concluded that the microstructures strongly influence the morphology and the grain orientation of Al alloy films.     

Effects of testing temperature and thermal treatments on some mechanical properties of a Si3N4–TiN composite

Strength and fracture toughness of an electroconductive hot-pressed Si₃N₄–35vol.% TiN ceramic composite were evaluated in air as a function of testing temperature up to 1200 °C. The toughness already shows a clear decrease at 800 °C and then remains almost constant, and the flexural strength steadily decreases with increasing testing temperature. At 1200 °C, the strength value is about 40% of that measured at room temperature. After thermal treatments in air (800, 1000 and 1200 °C) and argon (1200 °C) for 100 h, the Young's modulus, hardness, fracture toughness and flexural strength were measured at room temperature and compared to the baseline material. Young's modulus and hardness remain unchanged. The fracture toughness does not show any clear trend with the treatment temperature, while the strength, which is unaffected by the thermal treatment in argon, decreases with increasing treatment temperature in air. The long-term oxidation involves microstructural changes at the surface and in the bulk, such as the formation of a surface oxide layer and a porous sub-layer. In the bulk, the main modification is the partial crystallization of the grain boundary phase.     

Vacuum heat treatment of LiAlO2 densified silicon nitride ceramics

The aim of the present work has been to produce high-dense Si₃N₄ ceramics by a cheaper pressureless sintering method and then to attain vacuum heat treatment to remove residual grain boundary glass in gaseous form. LiAlO₂ was used as a sintering additive rather than using Li₂O, since its grain boundary glass is not stable above 1200 °C. LiAlO₂ was synthesised from 42% Li₂CO₃ and 58% Al₂O₃ powder mix reacting together at 1450 °C for 3 h in a muffle furnace. X-ray analysis showed that 95% LiAlO₂ was obtained. LiAlO₂ was milled and added to silicon nitride powder as a sintering additive. Hot-pressing and pressureless sintering of LiAlO₂ containing Si₃N₄ compacts were carried out at temperatures between 1450–1750 °C. The sintered samples were vacuum heat-treated at elevated temperatures under high vacuum to remove intergranular glass and to increase refractoriness of Si₃N₄ ceramics. Scanning electron microscope images and weight loss results showed that Li in grain boundary glass (Li–Al–Si–O–N) was successfully volatilised, and oxidation resistance of the sintered samples was increased.     

The effect of chemical grain refinement and low superheat pouring on the structure of NRC castings of aluminium alloy Al–7Si–0.4Mg

The effect of the addition of Al–5Ti–1B (wt.%) chemical grain refiners on the nuclei generation for a range of superheats during pouring in new rheocasting (NRC) of aluminium alloy Al–7Si–0.4Mg (wt.%) has been investigated. The contributions to the grain density by the grain refiner additions and impurity particles were quantified and it was found that the addition of grain refiner provides increasing number of nucleation sites as the superheat is decreased from 105 to 35 °C. However, at superheats of 15 °C, which are more typical of NRC, the grain density is similar in the alloy both with and without grain refiner additions. At this superheat, the equiaxed grain morphology is globular rather than dendritic and it is postulated that the grain size is dependant upon grain coarsening mechanisms rather than the number of heterogeneous nucleation events. In agreement with previous studies on semi-solid processing, it was found that the achievement of a fine globular grain structure led to a more homogeneous casting being produced. The mechanism of the macrosegregation observed in these castings is discussed and explained by the ‘sponge effect’.     

Effect of Mg and Sr-modification on the mechanical properties of 319-type aluminum cast alloys subjected to artificial aging

Aluminum-based 319-type cast alloys are commonly used in the automotive industry to manufacture cylinder heads and engine blocks. These applications require good mechanical properties and in order to achieve them through precipitation hardening, artificial aging treatments are applied to the products. The standard artificial aging treatment for alloy 319, as defined by the T6 heat treatment temper, consists in solution heat-treating the product for 8 h at 495 °C, water quenching at 60 °C, and then artificially aging at 155 °C for 2–5 h.

The present paper reports on aging heat treatments that were performed on four Al–Si–Cu–Mg 319-type alloys: 319 base alloy, Sr-modified 319 alloy, 319 alloy containing 0.4 wt% Mg, and the Sr-modified 319 + 0.4 wt% Mg alloy. This investigation was carried out in order to examine the effect of Sr-modification and additions of Mg on the microhardness, tensile strength and impact properties of 319-type alloys over a range of aging temperatures and times (150–240 °C, for periods of 2–8 h).

The results show that the best combination of properties is found in the Sr-modified alloy containing 0.4 wt% Mg (i.e. alloy 319 + Mg + Sr). Also, the optimum artificial aging temperature changes when Mg is present in the alloy.     

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 fiber-reinforced functionally graded materials by a centrifugal in situ method from Al–Cu–Fe ternary alloy

Ternary Al–13.8at%Cu–1.6at%Fe alloy was prepared from Al–Cu and Al–Fe alloys at 1000 °C. The ternary Al–Cu–Fe alloy was centrifugally cast to fabricate a new type of functionally graded material (FGM) by a centrifugal in situ method. The structure is expected to differ from that of binary alloys. It was found that the fabricated FGM rings consist of four different phases, namely, Al, Al₂Cu, Al₇Cu₂Fe(ω) and Al₁₃Fe₄ phases. The shape of ω phase was fiber (needle) judging from the observation by a scanning acoustic microscope (SAM). The position dependence of the microstructure was examined on the fabricated FGM rings, and the volume fraction of ω phase was found to increase toward the outer region of the ring. Moreover, orientation and aspect ratio of the ω phase varied in the rings in a gradually graded manner. Therefore, the present study explores a method to produce fiber-dispersed FGMs by applying a centrifugal in situ method to ternary alloys.     

Diffusion barrier properties of sputtered TaNx between Cu and Si using TaN as the target

TaN_x films sputtered from a TaN target were used as diffusion barriers between Cu thin films and Si substrates. Material characteristics of TaN_x films and metallurgical reactions of Cu/TaN_x/Si systems annealed in the temperature range 400–900 °C for 60 min were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, cross-sectional transmission electron microscopy, and sheet resistance measurements. We found that the deposition rate decreased with increasing bias. TaN, β-Ta, and Ta₂N phases appeared and/or coexisted in the films at specific biases. A step change in N/Ta ratio was observed whenever a bias was applied to the substrate. After depositing a copper overlayer, we observed that the variation percentage of sheet resistance for Cu (70 nm)/TaN_x (25 nm, x=0.37 and 0.81)/Si systems stayed at a constant value after annealing up to 700 °C for 60 min; however, the sheet resistance increased dramatically after annealing above 700 and 800 °C for Cu/TaN_0.37/Si and Cu/TaN_0.81/Si systems, respectively. At that point, the interface was seriously deteriorated and formation of Cu₃Si was also observed.     

The effects of mischmetal, cooling rate and heat treatment on the hardness of A319.1, A356.2 and A413.1 Al–Si casting alloys

The present study investigated the effect of mischmetal as a modifier, as well as the effects of cooling rate and heat treatment on the hardness of non-modified and Sr-modified A319.1, A356.2 and A413.1 Al–Si casting alloys. The main aim of the study was to determine the effect of mischmetal in terms of mischmetal-containing intermetallic phases, as well as the effects of the chemical composition of the alloys, cooling rate and heat treatment on the corresponding hardness values obtained for the alloys in question. Two cooling rates were employed to provide estimated hardness levels of 85 and 110–115 BHN, levels conforming to levels most commonly observed in commercial applications of these alloys.

The hardness measurements revealed that the hardness values of the as-cast alloys were higher at high cooling rates than at low cooling rates. Non-modified alloys (i.e. those with no Sr addition) displayed slightly higher hardness levels compared to the Sr-modified alloys. Also, the hardness decreased with the addition of mischmetal at both cooling rates.

Two peak hardness values were observed at 200 °C/5 h and 240 °C/5 h at high cooling rates in the non-modified A319.1 alloy after aging at different temperatures between 155 °C/5 h and 240 °C/5 h, while the Sr-modified alloy showed only one peak at 200 °C/5 h. Two maximum hardness values were observed at 155 °C/5 h and 180 °C/5 h in both non-modified and Sr-modified alloys at low cooling rates. The alloys containing 0 and 2 wt% mischmetal additions exhibited the highest hardness values at both cooling rates; the hardness decreased with further mischmetal additions.

Peak hardness was observed at 180 °C/5 h in the non-modified and Sr-modified A356.2 alloys under both cooling rate conditions after aging at different temperatures between 155 °C/5 h and 240 °C/5 h. The alloys free of mischmetal exhibited relatively higher levels of hardness than those containing mischmetal. The hardness decreased with increasing mischmetal addition. At the high cooling rates, the non-modified alloys displayed higher hardness values than the Sr-modified alloys, while an opposite trend was observed at the low cooling rate.

The decrease in the hardness values may be attributed to the interaction of the mischmetal with the alloying elements Cu and Mg to form the various intermetallic phases observed. In tying up these elements, the volume fraction of the precipitation-hardening phases formed in the A319.1 and A356.2 alloys (i.e. the Al₂Cu and Mg₂Si phases) is significantly reduced, thereby decreasing the hardness. The addition of mischmetal was also reported to change the precipitation sequence of the Mg₂Si phase in the A356.2 alloy. In the case of the A413.1 alloy, the low content of alloying elements resulted in a weak response of the alloy to the age-hardening process at all aging temperature/time conditions (155 °C/5 h–240 °C/5 h), and at both cooling rates. Thus, no peak hardness was observable in these alloys.     

Deep levels in GaInAs grown by molecular beam epitaxy and their concentration reduction with annealing treatment

X-ray diffraction (XRD), current–voltage (IV), capacitance–voltage (CV), deep-level transient Fourier spectroscopy (DLTFS) and isothermal transient spectroscopy (ITS) techniques are used to investigate the thermal annealing behaviour of three deep levels in Ga_0.986In_0.014As heavily doped with Si (6.8 × 10¹⁷ cm⁻³) grown by molecular beam epitaxy (MBE). The thermal annealing was performed at 625 °C, 650 °C, 675 °C, 700 °C and 750 °C for 5 min. XRD study shows good structural quality of the samples and yields an In composition of 1.4%. Two main electron traps are detected by DLTFS and ITS around 280 K, with activation energies of 0.58 eV and 0.57 eV, capture cross sections of 9 × 10⁻¹⁵ cm² and 8.6 × 10⁻¹⁴ cm² and densities of 2.8 × 10¹⁶ cm⁻³ and 9.6 × 10¹⁵ cm⁻³, respectively. They appear overlapped and as a single peak, which divides into two smaller peaks after annealing at 625 °C for 5 min.

Annealing at higher temperatures further reduces the trap concentrations. A secondary electron trap is found at 150 K with an activation energy of 0.274 eV, a capture cross section of 8.64 × 10⁻¹⁵ cm² and a density of 1.38 × 10¹⁵ cm⁻³. The concentration of this trap level is also decreased by thermal annealing.     

Interfacial microstructure and strength of partial transient liquid-phase bonding of silicon nitride with Ti/Ni multi-interlayer

Partial transient liquid-phase bonding (PTLP bonding) of silicon nitride (Si₃N₄) ceramic has been performed using Ti/Ni multi-interlayer in vacuum at 1273–1423 K. Interfacial microstructures were examined by scanning electron microscope, electron probe micro-analysis, and X-ray diffraction. The joint strength has been measured by four-point bending tests from room temperature up to 1000 °C. Interfacial structure of Si₃N₄/TiN/Ti₅Si₃ + Ti₅Si₄ + Ni₃Si/(NiTi)/Ni₃Ti/Ni is formed after bonding process. The NiTi layer is gradually consumed with simultaneous growth of the reaction layer and the Ni₃Ti layer. The room temperature joint strength is significantly affected by the reaction layer thickness, whereas the elevated temperature joint strength significantly depends on whether the low melting point NiTi layer exists in the joint. The joint strength of more than 100 MPa is retained up to 800 °C as the NiTi layer is completely consumed. A model is proposed to optimize the PTLP bonding parameters for optimizing joint strength at both room temperature and elevated temperature.     

Preferred orientations of primary Al4Sr dendrites in a rapidly solidified Al–Sr alloy

Preferred crystallographic orientations of primary Al₄Sr dendrites in a rapidly solidified Al–23 Sr (wt.%) alloy have been investigated using transmission electron microscopy (TEM). The Al₄Sr dendrites with 90° branches are dominant in the Al–23 Sr alloy melt-spun at 500 rpm and the dendrite orientation is the 110 direction. Wheel speed has a significant effect on the morphology and preferred orientation of the Al₄Sr dendrites in the melt-spun Al–23 Sr alloy.