Volume 109, Issue 1

Two-phase PtAl 2 +β-(Ni,Pt)Al and single-phase β-(Ni,Pt)Al coatings were fabricated on DD5 Ni-based superalloy and pure Ni substrates. A coating/pure Ni substrate system was studied to determine the diffusion behaviour of Pt in the Ni–Al–Pt ternary system. The evolutions of morphology, phases and component concentration during thermal cycles were analysed. It was found that there is a phase shift from PtAl 2 to β* in the two-phase coating, and Kirkendall porosity was observed in the single-phase coating. Our results further indicated that there are two different diffusion paths of Pt in the Ni–Al–Pt system.

The phase equilibria in the Al–Mo–Y system have been experimentally investigated at 873 K. The existence of 10 binary compounds and 1 ternary phase Al 43 Mo 4 Y 6 has been confirmed by using X-ray powder diffraction and scanning electron microscopy equipped with energy dispersive X-ray analysis. A new ternary phase Al 4 Mo 2 Y with space group I4/mmm (No. 139) has been found at 873 K. The structure, phase stability, elastic properties and densities of state of the Al 4 Mo 2 Y compound have been theoretically investigated by using first-principles calculations. The result of cohesive energy proves that Al 4 Mo 2 Y is stable. The elastic constants, bulk modulus, shear modulus, Young's modulus, the ratio of bulk modulus/shear modulus and Poisson's ratio of Al 4 Mo 2 Y are presented. According to the phase-disappearing method, the solubility of Al in Mo, AlMo 3 and Al 8 Mo 3 phase is determined to be about 16 at.%, 7.5 at.% and 1 at.%, respectively.

The objective of this study was to evaluate the thixoformability of 318.1 aluminum alloy in terms of its thermodynamic, metallurgical and rheological characteristics. The influence of processing parameters on the microstructural evolution of the alloy in the semisolid state and, consequently, its rheological behavior and thixoformability, were studied. The processing parameters investigated were temperature, heating rate and duration of heat treatment. The study was motivated by the dearth of articles in the literature on the thixoformability of this material in industrial environments. The alloy was first subjected to heat treatment to spheroidize its microstructure and observe the microstructural evolution. Hot-pressing tests were then performed to observe the rheological behavior. Micrographs were used to determine average primary-phase grain size, globule size and shape factor. The results confirmed the material to be suitable for semisolid processing from a metallurgical perspective because of its good morphological consistency. Heat treatment successfully spheroidized the microstructure, giving shape factors as high as 0.62 and primary-phase globule and grain sizes as high as 70 μm and 110 μm, respectively. The apparent viscosity was of the order of 10 5 Pa · s, which is similar to that of liquid glass according to the literature, indicating very good formability. Little variation in the results was observed, showing the suitability of the material for thixoforming from a rheological perspective.

Effects of 2 and 6 wt.% Al additions on the high temperature tensile behavior of Mg were investigated. Elongation-to-failure tensile tests were conducted at four temperatures (300, 350, 400, and 450 °C) and two strain rates (10 − 2 and 10 −3 s −1 ) to study tensile ductility. Strain-rate-change tensile tests were conducted at the same temperatures over a strain-rate range of 5 × 10 −5 –2 × 10 −2 s −1 . The experimental data show that the coarse-grained alloys exhibit elongations of about 100% at 450 °C and 10 −3 s −1 , and the Mg-6Al alloy presents a significant elongation improvement at 400 and 450 °C. Compared with pure Mg, Al additions of 2 and 6 wt.% effectively reduce the stress exponent, which decreases with increasing Al content. The reduced stress exponents may result from solute-drag creep created by Al atoms released from dissolution of Mg 17 Al 12 phase at high temperatures.

Hot rolled aluminum alloy 6082 was subjected to tensile tests and coefficient of thermal expansion measurements. These tests were realized in order to investigate the effect of direction on its anisotropic behavior. Specimens for both tests were cut in three directions (long transverse, longitudinal rolling and short transverse) with respect to rolling direction. Results showed that the longitudinal rolling and long transverse directions had limited effect on thermomechanical properties as compared to the short transverse direction. It was found that the short transverse direction exhibited higher values of stress hardening capacity and thermal expansion coefficient, lower ductility and dilatation, than the longitudinal rolling and long transverse directions. Thermomechanical anisotropy was attributed to the microstructure and precipitation behavior. It was assessed by means of tensile tests, coefficient of thermal expansion measurements, optical microscopy, X-ray diffraction and differential scanning calorimetry techniques.

Composite materials and layered alloys have become an interesting topic in materials development. The roll bonding (RB) process as a solid phase method of bonding has been widely used in the manufacturing of layered strips. The poor bond strength is one of the major drawbacks of the RB process. In the present study, Pb particles were used to enhance the bond strength between aluminum strips as filler material. AA1050 particle-reinforced sheets with different contents of interlayer Pb particles were fabricated at various rolling temperatures and thickness reduction ratios. Effects of roll bonding parameters that create successful bonds such as amount of Pb particle content, plastic strain from rolling process and rolling temperature were investigated. The bond strength was evaluated by the peeling test. It was found that by increasing the rolling temperature, thickness reduction ratio and Pb content, successful bonds with higher strength were created. Scanning electron microscopy was utilized to characterize the peeled surface of AA1050/Pb samples.

Spark plasma sintering has been recognized in the recent past as a very useful tool capable of producing materials with high strength and low porosity when compared to the traditional powder metallurgy technologies. In addition, the possibility of producing metal-matrix composites with enhanced mechanical and wear properties has been demonstrated. Obviously, the final properties of spark plasma sintered composites depend on the reinforcement type, size and percentage. The present paper analyzes the possibility of producing spark plasma sintered aluminum-based composites with various types and sizes of reinforcement (Al 2 O 3 nanosized and microsized particles blended with aluminum in different percentages). A strong variation in the microstructural behavior, in mechanical properties and in deformation mode has been observed by varying the type, percentage and combination of reinforcements in the aluminum matrix. The material evolution was deeply analyzed through nanoindentation, X-ray diffraction and scanning electron microscopy.

NiAl intermetallic coating thickness of about 50 μm was fabricated on hypo-eutectoid steel by mechanical alloying using pre-activated Ni–Al composite granules as coating material. First, Ni and Al powders were mixed with the composition of Ni-50 at.% Al and mechanically activated in a planetary ball mill, until the composite granules of this powder mixture, having maximum activity (9 cm sec −1 ), were formed after 120 min of milling at 200 rpm. The composite granules were then taken out from the planetary ball mill just before the critical time, i. e. the time at which these granules synthesize and convert to an intermetallic NiAl compound. The highly activated composite granules of Ni–Al were then put into the vial of a vibratory ball mill with the substrate on top of the chamber. After mechanical alloying for 60 min in the vibratory ball mill, the composite granules were synthesized fully and heat was produced during the synthesis which helped producing a thick and strong adhesive coating of NiAl intermetallic on the steel substrate. The main advantage of this technique is that not only is time saved but also there is no need for any post mechanical alloying process such as annealing or laser treatment etc. to get homogeneous, strongly bonded intermetallic coatings. X-ray diffraction analysis clearly indicates the formation of NiAl phase. Micro-hardness of the coating and substrate was also measured. The cross-sectional microstructure of the composite granules and the final coating were studied by scanning electron microscopy.

A hydroxyapatite and tricalcium phosphate nanocomposite containing 5% silica was developed for dental applications. The biomaterial was prepared by one-step synthesis via the wet route. The resulting dry material consisted of hydrated calcium phosphate agglomerates with sizes of up to 200 μm. The presence of silica was found to lower the phase transformation temperature of the calcium phosphates and increase the open porosity of the biomaterial compared to that of hydroxyapatite. The hydrated calcium phosphate transformed into hydroxyapatite (HA) and beta tricalcium phosphate (TCP) at approximately 682 °C. After 2 h of calcination at 900 °C, the volume ratios of HA and TCP in the nanocomposite were 84 and 16%, respectively. The open porosity in the triphasic nanocomposite and in the HA was 46.35% and 41.52%, respectively, after 3 h of sintering at 1 100 °C. Samples of grade 2 titanium were sandpapered and etched with an acid solution of HCl/H 2 SO 4 prior to deposition of the calcined nanocomposite. The particles were deposited homogeneously and reduced the contact angle of the titanium surface.

An Sn–Ag–Cu based solder alloy, including Ni, Bi and Sb additives, is investigated in this study under isothermal aging conditions. Shear creep tests are conducted on different aged solder joints with copper substrate, giving the creep resistance evolution with aging time. Microscopy analyses reveal the limited growth of the Cu-rich intermetallic layer due to the Ni content and allow for determination of the time-dependent growth of the Sn-rich intermetallic layer. The aged specimens also exhibit a partial dynamically recrystallized microstructure after creep deformation and a brittle-to-ductile fracture transition is found with occurrence of intergranular cracking in the joint.

A new method for the synthesis of ultrafine powder of chromium carbide is proposed, which combines liquid-phase precipitation on a carbon support and low-temperature microwave heat treatment in argon flow. The whole spectrum of the intermediate compounds obtained during thermolysis, reduction and carbonization of chromium precursors to the final product Cr 3 C 2 is presented. The structure, morphology, particle size distribution, as well as the phase composition and the specific surface area values of the produced chromium compounds are shown.