Materials Science Forum Vol. 1016

Abstract: Cold spray (CS) is a solid-state deposition technique of micron-sized metallic powder in an ultra-high velocity gas using a de Laval nozzle. CS is a unique deposition technique due to its use of relatively lower gas temperatures in comparison to other thermal processes. Consequently, high-temperature oxidation and phase transformations of deposited powders are largely restricted while the operating cost of CS is much lower than that of other thermal processes. Generally, the low pressure cold spray (LPCS) technique is used for the deposition of metallic powders on metallic substrates, while only a few studies of metallic particle deposition on ceramic substrates have been conducted, and it was found that the deposition of metallic powders on ceramic substrates was quite difficult. In this study, improved LPCS deposition of copper coatings on zirconia substrates was investigated. It is known that deposition of a metallic powder on a ceramic substrate is difficult due to the differences in material bonding and several properties of the two materials. These difficulties in LPCS deposition were solved using three different approaches, namely 1) use of copper and aluminum composite powders and 2) laser pre-treatment and 3) laser texturing of zirconia substrates. It was found that pure copper powder coatings on the as-received and various treated substrates were delaminated in the interface as expected. However, the deposition was improved for all substrates by using the copper and aluminum composite powder. While the laser pre-treated substrate was not effective for the deposition of the copper and aluminum composite powder, thick coatings were obtained for the deposition on the laser pre-treated with heat treatment substrate and the laser-textured substrate.

Abstract: A series of accelerated degradation experiments at high temperatures have been performed for Pd-coated V-10 mol% Fe alloy membranes in order to investigate the degradation behavior of hydrogen permeability. The degradation of the membrane becomes severer with increasing testing temperature. The temperature dependence of the 20% degradation rate almost obeys the Arrhenius relationship, suggesting that the degradation phenomenon occurs by a kind of thermal activation process. It is found that the addition of a small amount of W into Pd overlayer improves the durability of the membrane significantly.

Abstract: Three types of methods, including Laser flash, Hot-Disk, and Wiedemann-Franz law, have been applied for thermal conductivity measurements of Al-Si casting alloys. The first two methods can obtain the thermal parameter directly for specific samples, while the third one calculates the target value by formula containing the electrical conductivity. Thus, the latter is widely used in the foundries because of its convenient and rapid characteristics. The purpose of this paper was to make a polite comparison among them and optimize the key constant C in the Wiedemann-Franz law to improve the calculation accuracy for Al-Si alloys. Measurements were conducted on the same set of specimens of Al-Si-xCu alloys (x ranges from 0.1 wt.% to 2.0 wt.%) at room temperature. The results showed that the measured value of Laser Flash method was well consistent with Hot Disk. While that of the Wiedemann-Franz law was different with them, the average deviation percentages were 2.17% and 2.36% when using empirical constant C (12.6 W/m·K) in the formula. Then, the constant C was modified to 8.4 W/ m·K and the average deviation percentage were decreased to 0.4% and 0.2% respectively. The reason for the differences was analyzed and a thermal conductivity evaluation model was proposed.

Abstract: Microstructure and dislocations were observed for Ti-600 alloy crept at the temperature of 600°C with the stress of 200MPa and 300 MPa. The results indicate that more precipitation phases could be found both in β phases and at phase boundaries for the alloy after creep tests, and the width of the phase boundary become broader obviously. Also more dislocations could be seen at or around the precipitation phases for the alloy crept at 600°Cwith the stress of 200MPa. Dislocation density is rather big in some regions, dislocations aggregate in precipitation phases, phase boundaries and sub-grain boundaries. Some tangled dislocations or regular arranged dislocations could also be found around precipitation phases. Condensed three-dimensional dislocation meshes could be found in the alloy crept for the alloy crept at 600°Cwith two stresses. For the alloy crept at 600°Cwith the stress of 200MPa, faults originated from boundaries of α lamellar could be found in α lamellar. Some of these faults extend to boundaries of α lamellar, and some stop or end off in α lamellar. While for the alloy crept at 600°Cwith the stress of 300MPa, dislocation tend to arrange in dislocation walls, form bamboo-like structure along the direction of α lamellar, which would expand and penetrate the whole grain of elongated dislocation walls, and at last stop at grain boundaries.

Abstract: Copper Ferro Alloys (CFAs) have an excellent shielding effect in the electromagnetic field, as well as the similar good conductivity and ductility with copper, and strong magnetism and toughness as analogous to iron. Consequently, it is considered to be novel structural and functional materials with huge development potential and wide application foreground. The influence of the content, size and distribution of Fe phase in the Cu matrix on the electromagnetic shielding property of CFAs is crucial. In the present study, CFAs with various Fe content were fabricated via powder metallurgy (P/M) combining with deformation processing. The microstructure, electrical conductivity, magnetic and mechanical properties of CFAs were investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-Ray diffraction (XRD), and tensile test. The results indicated that P/M CFAs with the homogenous and fine in-situ Fe particles showed better comprehensive performance compared to those prepared by conventional casting. Based on the microstructure observation, mechanical properties were discussed.

Abstract: In this paper, the microstructure evolution during continuous hot rolling process of GCr15 steel rod was investigated. A series of multi-field coupled finite element models were established based on commercial finite element software MSC.Marc. The kinetics equations of austenite grain size evolution of GCr15 steel were coupled to these models by a designed MSC.Marc subprogram. The field variables, including temperature, equivalent stress, equivalent strain, and equivalent strain rate, were calculated. The distributions of dynamic recrystallization, metadynamic recrystallization, and static recrystallization fractions were investigated. The distribution and evolution of austenite grain size at different stages in the continuous hot rolling process were analyzed. To verify the models, the temperatures of GCr15 steel rod at different stages in the continuous hot rolling process were measured. And the austenite grain sizes at cross section of the rod after the continuous hot rolling process were measured. The simulation results show a good agreement with the experimental results.

Abstract: The effect of tempering time on the microstructure and mechanical properties of SA738 Gr.B nuclear power steel was studied using SEM, TEM and thermodynamic software, and its precipitation and microstructure evolution during tempering were clarified. The results showed that SA738 Gr.B nuclear power steel has better comprehensive mechanical properties after tempering at 650 °C for 1h. With the extension of the tempering time, M₃C transformed into M₂₃C₆ with increasing size, which affected the yield strength and impact energy. When the tempering time is 8h ~ 10h, due to the transformation of M₃C to M₂₃C₆, the composition of matrix around the carbide changed, causing the temperature of A_c1 dropped, forming twin-martensite which deteriorated the impact toughness of the steel.

Abstract: The low carbon content powder metallurgy (PM) 625 alloy were manufactured by vacuum induction gas atomization (VIGA) and hot isostatically pressing (HIP) for marine-based application such as parts in the subsea Xmas tree. Corrosion experiment was performed in simulated deep seawater and subsea oil & gas service environment. The microstructures and properties of low carbon 625 alloy were comparably investigated with that of the as-cast alloy. The results indicated that the dendritic arm spacing (DAS) of the as-cast 625 alloy is 2 orders of magnitude higher than that of the powders, whereas the HIPed alloys possess a fine equiaxed grain structures without dendritic segregation and an average grain size of 14.5μm. No minor phase has been found beside the γ matrix in the original powders with different particle size. The tensile strength of low carbon PM 625 alloy is 26% higher than that of as-cast 625 alloy. PM 625 alloy possesses an excellent corrosion resistant in simulated deep seawater and oil & gas service environment for 30 days.

Abstract: The influence of various factors on the efficiency of microstructure refinement in two-phase titanium alloys with respect to a well-known Ti-6Al-4V alloy was discussed. The kinetics of microstructure evolution in titanium alloys with a lamellar type α/β microstructure during large plastic deformation depends mainly on temperature and strain rate, type of the initial microstructure, thickness of the α lamellae, path of deformation and chemical composition. Each parameter should be controlled to provide the most efficient microstructure refinement during conventional metalforming methods.

Abstract: The main point of successful manufacture of metallic composites by direct bonding of dissimilar materials is achieving a homogeneous interface bonding. Two different types of deformation techniques for fabrication of metal composites were investigated. The first one was developed on the basis of high pressure torsion associated with a high energy impact on the material where part of energy involved can be dissipated via non equilibrium phase transition realization. This deformation due to high shear deformations allows not only to form a nanostructure, but also to bond dissimilar metals. Moreover, this method allows for a relatively short time and in a number of compounds to receive in one step at room temperature monolithic composites of sufficient size to certify the structure and properties. The second technique is diffusion bonding which integrate one material with the other by pressure under high temperature. In order to clarify the bonding mechanism by plastic deformation of dissimilar materials, the microstructural and some mechanical properties were studied in the processed samples.