مجله مواد نوین
سال یازدهم شماره 3 (پیاپی 41، پاییز 1399)

Both Ti-6Al-4V and 316L stainless steels are widely used as engineering alloys in many modern industries, especially aerospace, gas turbine engines and heat exchangers due to high mechanical properties, high creep, fatigue, and corrosion resistance. Fusion welding of these alloys is not easily possible due to their incomplete solubility in each other. Brazing is one of the best choices for joining dissimilar alloys. Infrared brazing is a novel process characterized with a high heating rate up to 500 °C/min. Accordingly, it is expected that both erosion of the substrate and excessive growth of intermetallic phases in the brazed joint are significantly decreased. This study is focused on the infrared brazing of Ti-6Al-4V and 316L stainless steel to investigate the influence of brazing parameters (brazing alloy, temperature and time) on microstructure and mechanical properties of base metals with CBS 34 (Ag-based) braze filler foils. Brazing was performed in a home-made infrared furnace at temperatures of 750, 780, 800, 850 and 900 °C for 3-5 min. Qualities of the brazed joints were evaluated by microstructure and phase constitution of the bonded joints with light optical microscope (LOM) and scanning electron microscope (SEM). Mechanical properties of the brazed joints were evaluated by hardness test. The optimum brazing parameters were at 850 °C-5 min for CBS 34 filler foils.

In this research, the microstructures and mechanical properties of DIN 16MnCr5 low alloy steel under ferrite-martensite dual-phase (with various martensite volume fraction) were compared to full martensitic condition. Dual-phase microstructures were developed by inter-critical annealing. For this purpose, the samples were heated at various inter-critical temperatures (from 740 to 840 °C) for 60 minutes and then water quenching to room temperature. Also, to develop full martensitic microstructure, steel samples were immediately quenched in water after austenitization at 900 ˚C for 60 minutes. The microstructure and mechanical properties of the specimens were analyzed by scanning electron microscope equipped with EDS analyzer, X-ray diffraction and tensile and microhardness tests, respectively. Metallographic observations showed that by increasing the inter-critical annealing temperature from 740 to 840 °C, the martensite volume fraction (Vm) increased from 23 to 87%. Comparison of mechanical properties of dual-phase samples showed that by increasing Vm from 23% to 87%, there was an abnormal tensile behavior at Vm=73%, so that the energy absorption capability (product of tensile strength and uniform elongation) for ferrite-martensite dual-phase samples is much higher than to full martensitic samples. This remarkable modification in the mechanical properties of the dual-phase samples is due to the different hardening responses of the ferrite and martensite microconstituents because of the interaction between them. By increasing the Vm from 23 to 87%, the martensite microhardness decreased continuously from 717 to 471 HV10g, while the ferrite microhardness showed a peak value at Vm=73%.

In this paper, 1.7225 steel subjected to severe plastic deformation by using a multi-directional forging method, and mechanical properties such as ultimate tensile strength, yield strength, hardness and, vibration properties were investigated. The use of SPD ( severe plastic deformation ) for changing the size of metals grain to enhance the mechanical properties of the metals were examined. The 1.7225 steel is used for machine tools with appropriate mechanical properties as a tool holder. First Evaluation of load needed for plastic deformation was conducted by using finite elements computer software. By using the results of the software first guess for applied load and stresses on the die for successful results were known. The results of this study showed that the yield and ultimate tensile strength increased 441Mpa and 540Mpa and hardness increased 18HRC. After 2 passes of severe plastic deformation, mechanical vibrations characteristics result no significance change in damping properties.

Due to superior properties, in recent years porous nano particles of hydroxyapatite are widely used in drug delivery and bone tissue engineering. In this study, porous nanoparticles of hydroxyapatite were synthesized via sol gel method by using organic polymers of gelatin, synthetic chitosan and the natural chitosan from shrimp shell. Structural and microstructural analyses were performed with X-ray diffraction (XRD) and scanning electron microscopy (SEM & FESEM). The surface area and pore size distribution were measured with BET method and the results were reported from the BJH and adsorption/desorption plots. The bioactivity of the samples was evaluated by immersing the samples in SBF solution. The FE-SEM results showed that the synthesized hydroxyapatite nano powders were agglomerates with the average particle size of 10-25 nm. BET results showed that hydroxyapatite nano particles were mainly mesoporous and gelatin addition resulted in finer particles and smaller pores with a narrow size distribution. Additionally, after immersing the samples in SBF solution, the weight differences and the SEM images showed that the hydroxyapatite samples which were synthesized by adding gelatin had higher bioactivity and more uniform apatite powders were deposited on this sample.

In this research, Cu-Al2O3 composite tubes were fabricated by electroforming method using a rotating stainless steel rod as cathode and Cu-P alloy as anode. The microstructure, surface morphology, and the effect of alumina second phases on corrosion resistance of the fabricated tubes were investigated by the means of SEM images, X-ray diffraction, and corrosion tests, respectively. X-ray diffraction pattern proved the existence of alumina particles in the copper matrix of the fabricated tubes and also proved that no chemical reaction occurred between these two phases. SEM images indicated that the amount of alumina particles in the copper matrix is a function of its concentration in the electroforming bath, so that it would be maximum when 15 g alumina per liter was used, in the condition of the present study. The results of potentiodynamic polarization test carried out in 3.5 wt. % NaCl solution illustrated that corrosion potential and corrosion current density changed to positive and negative values, respectively, and corrosion rate decrease at all by increasing the amount of alumina particles in copper matrix of the tubes. Electrochemical impedance analysis also proved that corrosion resistance increase due to the presence of alumina in copper matrix.

In the present work, API X70 steel, a type of high strength low alloy (HSLA) steel, hot-rolled at different conditions and the effect of various hot rolling parameters on microstructure and mechanical properties investigated through thermomechanical control processing (TMCP). Optical and scanning electron microscopy were used for microstructural investigations and the tensile and hardness test utilized for mechanical characterization. The steel preheated at 1250 for 30 min and hot rolled with the speed of 10 rpm in the temperature range of 1100 to 730 through 3 (sample A), 4 (sample B), and 5 (sample C) passes and cooled in air. A total strain of 0.62 was imposed on all samples. In the 5 pass condition, a sample rolled with a finishing rolling temperature of 900 (sample D), another with a speed of 20 rpm, and the rest (sample E) rolled and quenched in water (sample F). Results showed that, except for sample E, all samples' microstructure consists of ferrite and pearlite but with different morphology. Sample E showed the least grain size. Due to the formation of the quasi-stable phases, sample F exhibited the highest strength/hardness and the lowest ductility. The best combination of strength and ductility was achieved in sample E.

In the present work, the effect of cobalt concentration on the magnetic properties of the Fe-Co alloy thin film with the composition of CoxFe1−x (x= 0–0.3) was investigated in detail. The films deposited at the incident angle of 42° using sputtering technique. The crystalline structure of the deposited films was evaluated via the X-ray diffraction method. The static and dynamic magnetic characteristics were determined from magnetization loops and permeability spectra, respectively. With regard to the phase analysis results, similar diffraction patterns have been found representing the BCC phase with (110) preferred orientation. Nevertheless, the lattice parameter decreases with increasing Co content. M-H loops indicate different behaviour along with the easy and hard direction. Based on that, the magnetic anisotropy field rises from 60 Oe at Fe film to 333 Oe at Fe0.7Co0.3 film which in turn leads to an increase in the ferromagnetic resonance frequency from 2.44 GHz at Fe film to 7.66 GHz at Fe0.7Co0.3 film. In addition, the resonance frequency of Fe0.7Co0.3 film exhibits a slight decrease from 7.66 GHz at room temperature to 6.98 GHz at 420 K indicating good thermal stability of the obtained film.

In this study, the possibility of establishing steel/tungsten carbide joints via a novel method based on combustion synthesis process was studied. An experimental setup was designed for rapid heating and ignition of a cylindrical sample composed of compressed, exothermic mixture of Ni and Ti powders. The sample was placed as the bonding intermediate between steel-WC parts. The combustion reaction resulted in the formation of NiTi intermetallic compound, which was melted down and attached to both steel and WC parts. To prepare the green samples, Ni and Ti powders were weighed out to a molar ratio of Ni:Ti=1:1, and cold-pressed. The heating and ignition of the samples were carried out using inductive heating. After the joining was accomplished, the samples were cut in halves and characterized using XRD and SEM-EDS techniques. The analyses showed the formation of NiTi as the main phase as the product of synthesis. The elemental composition profile across the interfacial regions proved the diffusion of tungsten and cobalt elements in NiTi bonding layer and interdiffusion of titanium and nickel. Examination of X-ray maps within the joint confirmed the accumulation of nickel and tungsten in the carbide/NiTi joint and titanium and iron elements in the steel/NiTi joint due to high process temperature, accelerated diffusion, and bond formation at the joint. The results showed that by using the combustion synthesis reactions for the formation of NiTi compound, it is possible to establish acceptable joints between tungsten carbide and steel via an intermediate layer of NiTi.