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In this work, the microstructure evolution of the Nb-12Si-22Ti-14Cr-2Al-2Hf alloy (at.%) fabricated by liquidmetal-cooled directional solidification (LMC) was investigated and the solidification behaviors were discussed. The results revealed that the growth rate was of crucial in controlling the microstructure of this alloy and the higher growth rate was beneficial to the formation of metastable phase Nb3Si. The microstructure of non-melted region consisted of NbSS dendrites, NbSS/Nb5Si3 eutectics and Ti-rich NbSS/Cr2Nb eutectics while the microstructure of quasi-steady-state growth region of this alloy consisted of NbSS dendrites, Nb3Si laths and Ti-rich NbSS/Cr2Nb eutectic. Nb3Si had the advantage of nucleation at medium/high growth rate. Therefore, the microstructure evolution occurred in this study was owing to the increase of growth rate.

Directional solidification technique was successfully used to prepare highly reactive NiTiAl-based alloys using novel Y2O3-coated crucibles. Compared with the as-cast alloys fabricated by vacuum arc melting, the microstructure of the directionally solidified alloys was optimized and mechanical properties remarkably improved. After directional solidification, the microstructure of the alloys became much finer and preferentially grew along [001] orientation. The tensile strength and strain of the directionally solidified alloys at 1550°C were promoted to 1240 MPa and 2.1% respectively, which were 91.4% and 75% higher than that of the as-cast alloys. The strength increased up to 1328 MPa and 1419 MPa after homogenizing treating. The Y2O3-coated crucibles assured significant purification effect of the alloys. The maximum oxygen increase in the steady-state region of the ingot obtained at 1750°C was no more than 0.014 wt.%.

[Display omitted] • Y addition significantly increases the fatigue life of the IN713C alloy. • The decrease of porosity is attribute to the decrease of solidification temperature range. • Fatigue cracks in IN713C alloy mainly originate from pores and carbides. In order to improve the high temperature and low cycle fatigue performance of the IN713C alloy, trace amounts of yttrium (Y) element were added to such an alloy. The effect of Y on the microstructure and fatigue performance under push-pull loading condition of the IN713C alloy at 650 °C was investigated, and the fracture mechanisms of the IN713C alloy with different Y additions at strain amplitudes of 0.3%, 0.4% and 0.5% were also investigated. The results showed that addition of Y significantly increased the fatigue life of the IN713C alloy under different strain amplitude values. Compared with the traditional IN713C, the fatigue life of the alloy with 300 ppm Y was increased by 44.4%, 213.3%, and 61.4%, and that of the alloy with 500 ppm Y was increased by 184.6%, 66.1%, and 172.1%, respectively, at the strain amplitudes of 0.3%, 0.4%, and 0.5%. Shrinkage pores were found to be the main crack source at low strain amplitude (0.3%), while carbides and other interdendritic precipitates were the source of cracks at high strain amplitude (0.4% and 0.5%). The addition of Y increased the fatigue life of the IN713C alloy by reducing shrinkage and refining carbides and grain size. [ABSTRACT FROM AUTHOR]

This study investigated the effects of yttrium addition (0wt%–0.5wt%) on the purification, microsegregation and mechanical properties of K417 cast superalloy. The results showed that the addition of Y could significantly reduce the content of oxygen. Purified K417 superalloy with 4∼7ppm O content was obtained after adding a moderate amount of Y. With the increasing Y addition, the segregation degrees of alloying elements (Ti, V and Mo) dropped as a whole, γ′ phase was refined, and the size and volume fraction of γ/γ′ eutectic increased. The decrease in the degree of Ti segregation and the solid solution of Y in carbide periodicity changed the morphology of carbide. When the Y addition increased from 0wt% to 0.1 wt%, the decrease of O content, the refinement of γ′ phase and the formation of rod carbides increased the high temperature tensile strength of the alloy. When the Y addition increased from 0.2wt% to 0.5 wt%, the production of large-size Y-rich phase reduced the mechanical properties of the alloy. The plasticity of the alloy rised and then decreased with increasing Y addition. The appropriate amount of Y addition was about 0.01wt% to 0.05wt%. • The addition of Y hindered the segregation of alloying elements. • High purity K417 alloy with oxygen and nitrogen content of 5ppm was obtained. • Yttrium addition caused the carbide shape to exhibit periodic change in the shape of a block and a rod. • The formation of rod-shaped carbide promoted the improvement of high temperature tensile strength. [ABSTRACT FROM AUTHOR]

The effects of cooling rate and Co addition (0–1 wt%) on the Fe-rich intermetallics in Al7Si0.3Mg alloy with 0.5%Fe were studied in this study. The evolution of microstructure and crystal structure have been characterized by metallographic and transmission electron microscopy. The results indicated that higher cooling rate and Co addition were beneficial to modify the morphology and decrease the size of Fe-rich intermetallics. At the particular cooling rate of 2 k/s, 0.75 wt% Co was proved to be sufficient to improving the morphology of intermetallics from harmful acicular β-Al5FeSi phase to Chinese script or skeleton α-Al15(Fe,Co)3Si2 phase. Above all, the Co/Fe ratio in intermetallics was found to be an importance parameter in determining the morphology and crystal structure of (Fe,Co)-rich intermetallics. With the ratio increased from 0 to 3, the morphology changed gradually: acicular shape→short rod-like shape→Chinese script/skeleton like shape→rod-like shape. And the crystal structure of (Fe,Co)-rich intermetallics evolved from Monoclinic→Hexagonal→Simple cubic→Body-centered structure correspondingly. The optimal ratio of Co to Fe in intermetallics, which has Chinese script with hexagonal structure, was in the range of 1–2. [ABSTRACT FROM AUTHOR]

Ultra-high density large-sized Al:ZnO sputtering targets were prepared by hot isostatic pressing(HIP). Studies were conducted on the characteristics of the microstructure of the targets and the photoelectric properties of the deposited film prepared by medium-frequency magnetron sputtering. Experiments showed that, compared to 98% density target, HIP process had no apparent influence on the change of grain dimension of the targets. However, a reduction of the quantity and size of the porosities in the target matrix was recognized and led to an improvement in the uniformity of resistance and thickness and IR transmittance of the deposited film. In this paper, the favorable machining allowance associated to the thickness of the HIPed targets, due to the oxygen-loss layer, was suggested by developing the empirical formula. [ABSTRACT FROM AUTHOR]

The solidification characteristics and high temperature tensile properties of IN713C superalloy were investigated using directional solidification methods at withdrawal rates of 10, 50 and 100 µm/s, respectively. The results show that carbide morphologies change from block and long strip to short rod and granular with increasing withdrawal rate. The increase of withdrawal rate promotes the microsegregation of Mo, Nb and Zr, and contributes to precipitation of the γ/γ' eutectic and boride. With the withdrawal rate increasing from 10 µm/s to 50 µm/s, the high temperature yield strength, ultimate strength and elongation of the IN713C alloy increased by 11.8%, 18.3% and 64% respectively. The increase of the γ/γ' eutectic and boride lead to slight decrease in strength when the withdrawal rate increased from 50 µm/s to 100 µm/s. The fractographic observation of tensile specimen indicates that the integrate carbide particles are an overwhelming majority at a higher withdrawal rate. When the withdrawal rate decreases, the particles become larger and more secondary cracks are generated. The morphology of carbides affects the tensile fracture mechanism of the alloy. With the decrease of carbide size, the crack source of the IN713C alloy changed from brittle carbides to the interface between carbide and matrix. • The continuous microstructures evolution of IN713C alloy during solidification were obtained. • Appropriate withdrawal rate can simultaneously enhance the strength and plasticity of IN713C alloy. • Strip carbides are the source of cracks, and granular carbides strengthen the superalloy. [ABSTRACT FROM AUTHOR]

Abstract: The effects of novel Y2O3-coated Al2O3 (Y2O3/Al2O3) crucibles on the microstructure and composition of directionally solidified TiAl alloys were investigated and compared with those of single layered Al2O3 and Y2O3 crucibles, based on which the corresponding alloy–crucible interaction mechanisms were discussed. The DS alloys exhibited a fully lamellar γ/α2 structure interspersed with some Al2O3 or Y2O3 particles. Differently from that in the case of using Al2O3 crucibles, no interfacial interaction layer was found in the ingots prepared using Y2O3/Al2O3 crucibles. Dissolution and erosion were the main mechanisms responsible for the alloy–crucible interactions which increased with the heating temperature and interaction time. Nevertheless, the interaction extents when using Y2O3/Al2O3 crucibles were much lower than using Al2O3 crucibles, making the former promising candidate crucibles for the high quality DS of highly reactive TiAl alloys. [Copyright &y& Elsevier]

Abstract: A series of Ni3Al single crystal specimens were prepared by directional solidification with seeding technique and the influence of processing parameters such as heating temperature and withdrawal rate on the microstructure and segregation tendency were investigated. In this work, dendrite structure was observed while the ranges of parameters were adopted. As the withdrawal rate or heating temperature increased, the primary dendrite arm spacing decreased gradually. The results strongly indicated that the average size of γ’ precipitates in dendrite core was larger than the one in inter-dendrite area in Ni3Al based alloys. With the increasing withdrawal rate or heating temperature, the size of γ’ precipitates in both dendrite core and inter-dendrite area decreased, while the morphology became regulated from spherality to cube. The tendency of segregation of Mo and Re in inter-dendrite area became more intense with the increasing withdrawal rate. No obvious variation of the segregation of elements was observed at different heating temperature. [Copyright &y& Elsevier]

To further investigate the microstructure characteristic and solidification mechanism, so as to provide knowledge for the microstructure control of a NiTi-Al based high-temperature structural material, the microstructure of Ni-43Ti-4Al-2Nb-2Hf (at.%) alloy ingots prepared by conventional casting (arc-melting) and directional solidification (DS) at various drawing velocities (2 mm.min-1, 18 mm.min-1, 30 mm.min-1 and 60 mm.min-1, respectively) was investigated by means of electron probe microanalyses. Experimental results reveal that the microstructures are composed of NiTi matrix phase, β-Nb phase and Ti2Ni phase for samples obtained by both conventional casting and DS. Conventional casting has an equiaxial structure, while DS has a slender and acicular cellular structure which grows along the [001] orientation preferentially. Small amounts of white β-Nb phase and black Ti2Ni phase co-exist at the grain boundaries or intercellular regions. With an increase in drawing velocity, the NiTi matrix phase is inclined to grow along (100) and (200) crystallographic planes, and the cellular arm spacing reduce gradually, but the directionality of the solidified structure weakens significantly. The homogeneous dispersion of β-Nb phase and the decrease of Ti2Ni phase in DS samples are beneficial to improving the mechanical properties. Solidification mechanism analysis indicates that the dark grey NiTi matrix phase initially precipitates from the liquid phase, and then the divorced eutectic reaction takes place, which produces the light gray matrix phase and β-Nb phase. Finally, the peritectic reaction happens, which generates the black Ti2Ni phase. The complete solidified path of the alloy is L→ NiTi+L → NiTi+ β-Nb+L → NiTi+β-Nb + Ti2Ni. [ABSTRACT FROM AUTHOR]

The effect of La addition on primary α-Al and the eutectic Si phase of Al-7Si alloy is investigated systematically in this work. The results indicate that La addition causes a multi-refining efficiency on the microstructure of Al-7Si alloy, including refinement of α-Al grains and secondary dendrite arm spacing as well as eutectic Si particles. The grain size, secondary dendrite arm spacing and area of eutectic Si particles are decreased by 26.8%, 7.7% and 26.7%, respectively, with the addition of 0.1 wt.% La. It is also found that La-rich phases of Al2Si2La form and distribute in the vicinity of the eutectic Si phase. The crystal structure and lattice parameter of Al2Si2La phase are determined to be hexagonal (a = b = 0.405 nm, c = 6.944 nm) based on the TEM analysis results. The multi-refinement effects are mainly attributed to the increased constitutional undercooling caused by the low solubility of La in Al alloy and the growth-restricting factor caused by the Al2Si2La phase. [ABSTRACT FROM AUTHOR]

In this paper, an artificial neural network (ANN) model with high accuracy and good generalization ability was developed to predict and optimize the mechanical properties of Al–7Si alloys. The quantitative correlation formulas of the mechanical properties with Mg content and heat treatment parameters were established based on the transfer function and weight values. The relative importance of the input variables, Mg content and heat treatment parameters, on the mechanical properties of Al–7Si alloys were identified through sensitivity analysis. The results indicated that the mechanical properties of Al–7Si alloys were sensitive to Mg content and aging temperature. Then the individual and the combined influences of these input variables on the properties of Al–7Si alloys were simulated and the process parameters were optimized using the artificial neural network model. Finally, the proposed model was validated to be a robust tool in predicting the mechanical properties of the Al–7Si alloy by conducting experiments. [ABSTRACT FROM AUTHOR]

The effect of yttrium on the microstructure and high-temperature oxidation performance of Nb-15Si-24Ti-4Cr-2Al-2Hf (at.%) alloys was investigated. Alloys containing different yttrium contents were designed, aiming at maximizing the beneficial effect of yttrium addition and avoiding the possible adverse effect of over-doped yttrium. Results showed that Y was primarily partitioned in γ-Nb 5 Si 3 , relative to Nb solid solution (Nbss), Nb 3 Si and α-Nb 5 Si 3 . Microstructural refinement and oxidation resistance enhancement were achieved by yttrium addition up to 0.3 at.%, accompanied by the transition towards parabolic oxidation kinetics. An adverse effect of over-doped yttrium (0.5 at.% addition) by coarsening the microstructure and deteriorating the oxidation resistance was confirmed. A relatively continuous glassy SiO 2 layer was generated in the oxide scale formed on the Y-containing alloys with enhanced oxidation resistance. [ABSTRACT FROM AUTHOR]

Optimized Al:ZnO sputtering target was prepared by cold isostatic pressing (CIP) using nanostructured zinc oxide powder and aluminum oxide powder as raw material. Compared with the target prepared by conventional raw materials, the performance of the optimized Al:ZnO sputtering target is greatly improved. The microstructure of the optimized Al:ZnO sputtering target is refined and its average grain size is less than 5 μm with 99.7% theoretical density. Al:ZnO thin films of both optimized and conventional targets were prepared by RF magnetron sputter and their properties were characterized, respectively. The Al:ZnO thin films obtained by optimized target feature better uniformity and compactness, and the internal stress is −378.8 MPa, which is nearly 2/3 lower than that of the conventional target. The film obtained by optimized targets also features a 97% IR transmittance, 1.71 nm Rq surface roughness and non-offset (002) XRD peak. It can be speculated that the optimized Al:ZnO target has great potential to prepare micrometer scale Al:ZnO films and employed in thin-film ZnO device industry. [ABSTRACT FROM AUTHOR]

The microstructural characterization, fracture toughness and oxidation resistance of Nb-15Si-24Ti-4Cr-2Al-2Hf (at%, V-free) alloys and Nb-15Si-24Ti-4Cr-2Al-2Hf-1V (at%, 1V) alloys were investigated. Results showed that the V-free alloys and 1V alloys both consisted of Nbss, αNb 5 Si 3 and γNb 5 Si 3 phases, and V was primarily partitioned in Nbss phases. Compared with the V-free alloys, the 1V alloys were featured by a better continuity of Nbss, finer average diameter of silicides and lower volume fraction of γNb 5 Si 3 . The 1 at% addition of V enhanced the fracture toughness of Nb-Si based alloys from 9.87 to 12.98 MPa m 1/2 at room temperature. The fracture surfaces of 1V alloys were more undulated, compared with those of the V-free alloys. The 1 at% addition of V significantly improved the oxidation resistance of Nb-Si based alloys, by reducing the weight gain from 264.7 to 148.5 mg/cm 2 after oxidation at 1300 °C for 100 h. The transition from a paralinear oxidation kinetic to a parabolic oxidation kinetic was triggered by the addition of V. [ABSTRACT FROM AUTHOR]

Abstract: The Nb–12Si–22Ti–14Cr–2Al–2Hf alloy (at%) was directionally solidified at 1750°C with withdrawal rates of 0.24mm/min, 1.2mm/min, 6mm/min, 18mm/min, 30mm/min, 50mm/min, 70mm/min and 100mm/min, and subsequent heat treatment was carried out at 1375°C for 10h. The microstructures and room-temperature fracture toughness of the directionally solidified and heat treated samples were investigated. The results showed that instead of the microstructure consisting of NbSS dendrites, NbSS+Nb5Si3 eutectics and Cr2Nb+Ti-rich NbSS eutectics, the microstructure composed of NbSS dendrites, Nb3Si laths and Cr2Nb+Ti-rich NbSS eutectics was observed when the withdrawal rate was higher than 30mm/min. After the heat treatment, the Nb3Si transformed into extremely fine NbSS and Nb5Si3, and the NbSS dendrites changed to be remarkably continuous. The room-temperature fracture toughness could reach a maximum of 15.7MPa·m1/2 for the alloy withdrawn at 50mm/min plus heat treatment. It is suggested that the microstructure with fine intermetallic phases (Nb5Si3 and Cr2Nb) and continuous NbSS would be of great benefit to the room-temperature fracture toughness. [Copyright &y& Elsevier]

Abstract: Y2O3 crucibles with different porosities were manufactured to investigate the physical erosion during the casting of Ti–54Al (at%) alloys. The effects of multiple superheating times on the chemical composition of the casting, the introduction of inclusions into the alloy and on the metal–crucible interface were studied. A large number of ceramic inclusions were introduced by the physical erosion of the crucible walls in contact with the molten alloy. The mechanisms by which the physical erosion was reduced are discussed in relation to the crucible wall permeability coefficient A′. By reducing the porosity content of the crucible, which led to a decrease of A′, the net increments of oxygen of the remelted Ti–Al as-cast samples were limited to a minimum level at 1873K. [Copyright &y& Elsevier]