مجله مواد نوین
سال سوم شماره 2 (پیاپی 10، زمستان 1391)

Combustion synthesis (CS) as a well-known economical production method for advanced ceramic and composite materials production has some drawbacks. Incomplete exothermic reactions due to non-ideal mixing of starting powders and thermodynamicaly unreacted، and fabrication of porous products with high distortion are the main problems of this method. In this study، Al-TiO2-H3BO3 thermite system was used for in-situ fabrication of Al2O3-TiB2 ceramic composite using CS. Three factors of mixing process، using diluent materials، and using a steel chamber are considered to investigate the possibility fabrication of non-distorted samples with controlled porosity. The results indicated that 2 h mixing of starting powders with a 5 mm diameter ball before mechanical activation and combustion synthesis has a considerable effect on increasing the purity of final products. In addition، it was shown that the addition of diluent materials (73 vol. % Al2O3-23 vol. % TiB2) to the starting powders would lead to formation of samples with lower porosity. Finally، in was indicated that using a steel chamber around the sample prevented the expansion of sample during CS، leading to formation of a specimen with the least distortion.

The usage of reinforced metal matrix composite (MMC) is steadily increasing due to its properties such as high specific strength، high hardness and good wear resistance. A novel surface modifying technique، friction stir processing (FSP)، has been developed for fabrication of surface composites. This process uses a non-consumable tool to generate frictional heat and stirring action. The feasibility to make bulk dispersal CNT reinforced Al metal matrix composite were studied successfully in this paper. Samples were subjected to various number of FSP passes from one to four. Microstructural observations were carried out by employing scanning electron microscopy of the modified surfaces. Mechanical properties include Microhardness and wear resistance، were evaluated in detail. The wear characteristics of the resulted surface composites were evaluated using a pin-on-disk wear tester at room temperature. The results showed that the hardness of 4 passes produced composite surface was 195 HV which is twice harder as compared to the base aluminium alloy. Also، FSP reduced friction coefficient by approximately 50% in the case of 4 pass samples

Metallic sandwich structures with aluminum foam core are good energy absorbers. In this paper، perforation test were carried out on different samples to study energy absorption. In the experiments، effect of several parameters، i. e. skin thickness and thickness of foam core، on the energy absorption، delamination zone of back faces، plateau force (Fp) and back face deformation strain (φ) are discussed. Results showed that increasing skin thickness will results in more absorbed energy، plateau force، back face delamination and deformation strain. Moreover foam core thickening has the same effect on absorbed energy and plateau force

The aim is investigation on influence of effective forces in Supersolidus Liquid Phase Sintering of Cu-28Zn alloy which causes heterogeneity in different sections of microstructure. For this purpose prealloyed Cu-28Zn powder was sintered at various temperatures، 840-890 ˚C، under nitrogen atmosphere for 60 minutes. Metallography and fractography studies revealed that small increase in sintering temperature as a consequence of creating different amount of liquid phase results in considerable particle rearrangement، and varieties in obtained grain size، zinc accumulation in grain boundaries، pores’ shape and size. As well as the effect of produced liquid phase amount، which is a function of sintering temperature، would be clear by comparing fracture morphology of diverse parts of sections. Finally، according to attained physical and mechanical properties، which were in a good agreement with achieved microstructure، optimum sintering temperature equal of 860 ˚C was determined.

Layering of microstructure parallel to the direction of flow of steel during hot working process is called banding. In the present paper، the severity of pearlitic–ferritic banding in API GRB microalloyed steel and its effects on impact energy is studied. Specifically، Impact properties are compared between banded and non-banded samples along two directions perpendicular and parallel to the rolling direction. Metallographic examinations together with impact charpy tests at zero and 20 °C were done both parallel and perpendicular to the rolling direction. The results showed the dependence of impact energy to the sample direction relative to the rolling i. e decreasing energy in rolling direction and increasing it in per pendicular direction to the rolling

Cladding of steel with titanium alloys to improve corrosion behavior and to reduce production costs، is used widely in aerospace، chemical، nuclear and biomaterials industries. Roll bonding is one of the solid state processes to produce this combination. In this research the high quality bonding of hot roll bonding of titanium on steel substrate was investigated in terms of physical and mechanical properties، and the effect of nickel interlayer to prevent interdiffusion was evaluated. The microstructure at interface was observed by an optical and scanning electron microscope and the presence of intermetallic phases in the reaction zone was examined by X-ray diffraction on surfaces of the joints. The experimental results showed that the no reaction layer will be formed at the interface of steel with nickel but The interface between titanium alloy and nickel consists of the several diffusion layers include the intermetallic compounds of TiNi3، TiNi، Ti2Ni and their mixture so that the total thickness of intermetallic layers at the interface between titanium and nickel and the hardness of roll bonded joints increases with the bonding temperature

In this study، the microstructure and mechanical properties of dissimilar welding of Super duplex stainless steel UNS 32750 to austenitic stainless steel AISI 304L by gas tungsten arc welding process with direct current electrode negative polarity was investigated. For this purpose، two filler metals including ER25104L and ER309LMo were used. The microstructure of the different zone in each joint، including weld metals، heat affected zone، interface were evaluated using optical microscopy and the relationship between mechanical properties and microstructure of welded joints is evaluated. Results indicate that austenitic and ferritic structures such as dendrite in the weld metal 25104L، 309LMo weld metal was observed as the primary ferrite with austenitic matrix. In addition microstructure was seen as skeleton ferrite morphology. In addition، the mechanical properties including the bend test، ultimate strength، impact resistance and hardness were investigated. All the specimen underwent ductile fracture in HAZ in the tension test. The maximum fracture energy related to ER25104L. The welded specimen with ER25104L filler metal showed the maximum elongation. The maximum and minimum hardness corresponded to the ER25104L and ER309LMo، respectively. Mechanical properties of joints welded by the two kinds of filler metals are satisfactory، althought the mechanical properties of 25104L filler metal is superior to that by 309LMo filler metal. Based on the present work، it is concluded that 25104L provides the optimum qualities for joining dissimilar metals between 32750 super duplex stainless steel and 304L austenitic stainless steel

The aim of this research was fabrication and characterization of bioactive glass/zirconia composite coating via the sol-gel method for corrosion behavior improvement of metallic substrate. Coating was produced on AISI 316L stainless steel (SS) substrate by spin-coating technique. Structural characterization techniques including XRD، SEM and EDS were used to investigate the microstructure، morphology and the chemical composition of the coating. The hardness of coating was evaluated using knoop microhardness test method. Electrochemical potentiodynamic polarization tests were performed in two different types of physiological solutions at 37 ºC in order to determine and compare the corrosion behavior of the coated and uncoated SS specimens. Crack-free and homogeneous bioactive glass/zirconia composite coatings were obtained without major defects. The composite coating also improved corrosion resistance of the AISI 316L SS substrates as the corrosion current density of coated samples in comparison with pristine samples was decreased. It was concluded that the sol-gel bioactive glass/zirconia composite coating can be used for improvement of corrosion behavior and thus biocompatibility of the 316L SS substrates