نشریه مهندسی متالورژی و مواد
سال بیست و سوم شماره 2 (پاییز و زمستان 1391)

Residual stresses resulting from rapid quenching of molten metal in melt spinning process can be removed by structural relaxation of samples at temperatures lower than the crystallization temperature in order to improve the magnetic properties. In this research, amorphous Fe78Si9B13 ribbons (Metglas 2605S2 used in transformer cores) with the thickness of 26 µm were prepared by chill block melt spinning. DTA measurements on the ribbons heated with the rate of 20 K/min showed that the starting crystallization temperature is 535 °C. The ribbons were then heat treated in a protective gas for 15 to 60 minutes at temperatures in the range of 300-560 °C. XRD results revealed an amorphous structure for all the heat treated samples except for the samples annealed at 500 and 560 °C in which the structure were consisted of three crystalline phases of α-Fe, Fe2B and Fe3B. The measurement of magnetic properties by Hysteresisgraph indicated that the saturation induction increases by an increase in the annealing time and/or annealing temperature. The best combination of magnetic properties was obtained after annealing at 400 °C for 15 minutes.

Magnesium alloys in liquid-phase condition form surface oxide films due to their high rate of oxidation. Oxide films enter the melt as a result of surface turbulence, and remain inside the workpiece after solidification. The incoherent nature of their interface causes decreases in mechanical properties and reliability of castings. The effect of critical velocity and greater velocities of entering the melt into the die cavity on fracture toughness has been studied in this research. Results indicated that an increase in the velocity decreases the fracture toughness, e.g. an increase of the velocity from 0.4 m/s (critical gate velocity of magnesium alloys) to 3.0 m/s decreases the fracture toughness for about 40%. Furthermore, increasing the velocity of melt entering the mold from 0.4 m/s to 1.0 and 3.0 m/s resulted in a decrease in the Weibull modulus from 13.34 to 7.28 and 4.93, respectively, and consequently, caused the results to be more scattered. SEM micrographs showed that increasing the velocity beyond the critical value increases significantly the quantity and size of the oxide films. Moreover, for the case of speeds greater than the critical value, double-layer films are observable due to considerable surface turbulence of the melt. Analysis of the fractured surfaces also showed that the failure mode of specimens produced using velocities greater than the critical value is of brittle type, whereas it was found to be ductile for the case of critical velocity.

Aluminum is very sensitive to oxidation reactions. If a fresh surface of melt is exposed to air, it oxidizes quickly and an oxide layer covers the melt surface. New oxide films formed in a very short time during casting are thin and folded. The metal surface can be folded and create a double-oxide film as a result of the melt turbulence. The entrance of this double-oxide film into the melt is one of the most important reasons for the formation of crack-like defects during casting. In this work, the characteristics of these new oxide films have been investigated based on the oxide-metal-oxide sandwich technique. To study the futures of new oxide film, the contact area between two adjacent and entrapped bubbles have been examined using scanning electron microscope (SEM). Results showed that the short time oxide films formed in pure aluminum have a thickness less than 100 nanometer.

Using the nanotechnology and decreasing the dimension of reinforcement particles to nanometer, outstanding mechanical properties are achieved in aluminum matrix nanocomposites. Fabrication of aluminum matrix nanocomposites by casting is of great importance because of its simplicity, low cost and ease of the control of matrix microstructure. In this study, the aging behavior of nanocomposite samples with different volume percents nano-particles, i.e. 1.5, 3 and 5%, have been investigated. For production of composite samples with Al2O3 nano-particles using the Al-4.5%wt.Cu alloy as its matrix, mixtures of Al powders (with the particle size of about 35 m) and suitable amounts of alumina powders (about 50 nm in size) were prepared by mechanical alloying process to be used in casting process. Microstructural studies showed a uniform distribution of reinforcements in Al-4.5wt.%Cu matrix in the samples with a considerable grain refining effect on the matrix. The effects of reinforcement particles on compressive properties, hardness, and density of composite samples as well as the kinetics of heat treatment were also investigated. Addition of Al2O3 particles as the reinforcement component to the matrix changed the aging behavior of the samples. The results showed that the addition of nano-sized alumina particles to the matrix accelerates the aging process of the matrix and results in higher peak hardness values.

In this study, an attempt has been made to study the effect of Bi2O3 addition as nucleating agent on the liquid-liquid phase separation, crystallization and dielectric properties of samples prepared by sintering and crystallization of powdered glass in PbO-TiO2-B2O3-SiO2 glass-ceramic system using DTA, XRD, SEM and LCR techniques. The results showed that the addition of Bi2O3 resulted in a quite distinct liquid-liquid phase separation, i.e. spinodal decomposition, in as-quenched glass samples with significant effects on the nucleation and crystallization process. Glass ribbons were heat treated at 630-557 C for different lengths of time. Based on the results obtained, it was shown that the addition of Bi2O3 promotes the crystallization of PbTiO3. The dielectric constant (εr) increased in all heat-treated samples with increasing time except for the sample heat treated for 5 hr for which εr, decreased. This behavior was attributed to the formation of PT-Bi2O3 solid solution. The dielectric constant and dissipation factors for the glass-ceramic samples were obtained in the ranges of 33-61 and 0.007-0.02, respectively.

In this study, attempts were made to recovery the used turbine blades made up of INCONEL738 by Electro Slag Remelting (ESR) method. Using an appropriate slag and addition of metallic aluminum, the original composition was maintained and the amount of oxygen, hydrogen and nitrogen remained in the acceptable level. In order to explore the effect of remelting with ESR on the microstructure, tensile properties and age hardening behavior of the investigated alloy, the as-cast and wrought samples were solution treated at 1120 °C and aged at 845 °C for different lengths of time. The results showed that the microstructure, tensile properties and age hardening behavior of ESR melted INCONEL738 are similar to corresponding standard values and hence, the ESR method was found appropriate to be used to recovery the INCONEL738 alloy from the used turbine blades.

Applying a Ni-P electroless coat on the surface of BN(h) particles has a beneficial effect on their wettability in vortex technique during composite manufacturing. In this investigation, different preliminary treatments consisting of different combinations of surface oxidation, sensitization and activation have been investigated. The results obtained from scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) showed that the preliminary treatment has an important role in deposition of Ni-P on the surface of BN(h) powder. A preliminary treatment consisting of oxidation in 200 ºC for 1 hour, sensitization in 10g/l SnCl2+30ml/l HCl solution for 15 minutes and then, activation in 0.25 g/l PdCl2 +30ml/l HCl solution resulted in the best results.