Sharif Digital Repository

The microstructures of stainless steels consist of an austenitic matrix with almost continuous network of primery carbides.This class of steel had replaced the traditional super alloys with a reduction in costs and has similar properties under conditions of creep, which is one of the principal degradation mechanisms leading to failure in service at elevated temperaturs , therefore, their mechanical properties such as tensile strength and creep resistance are particularly important in hot deformation. Recently the effects of temperature and aging time on static strain aging in worked stainless steel have been investigated. In this study the deformation behavior of HP-series austenitic... 

Taking advantage of aluminum and its alloys in industry is increasing. A common method in metal forming is hot forming. In this condition a precise prediction of flow behaviour of the alloy is necessary to achieve a product of desirable mechanical and physical properties. In this situation, influence of parameters such as chemical composition, temperature, strain, strain rate as well as recovery and recrystallization phenomena should be considered and by using a suitable model, the metal behavior prediction is possible. Distribution of coherent Al3Sc dispersoids in high volume fraction accompany with Al6Mn result in strengthening to this alloy. Because of the high stacking fault energy,... 

The manufacture of ultra high strength materials has always been a target for aerospace and transportation industries. Currently, the limitation of energy resources even makes this goal more serious. Nowadays, more than 50% of total extrusion products are made from Al alloys and around 90% of them are the 6000 series alloys. Therefore, regarding to high strength, low weight, and hardening aluminum AA6000 alloys capabilities can play a major role in fulfilling this task. Over the last decade, a number of techniques collectively referred to as severe plastic deformation (SPD), have emerged as a promising approach for the production of bulk ultrafine-grained (UFG) nano-structured materials.... 

A new method for production of bimetallic rods, utilizing the equal channel angular extrusion (ECAE) process has been introduced before by previous researchers, but no attempt has been made to assess the effect of different temperatures and holding times in order to achieve a diffusional bond between the mating surfaces. In present research copper sheathed aluminum rods have been ECAEed at room temperature and subsequently held at a constant ECAE pressure, at different temperatures and holding times to produce a diffusional bond between the copper sheath and the aluminum core. The bonding quality of the joints was examined by shear strength test. The microstructure examinations were carried... 

In sliding wear, the process of making debris takes place with different mechanisms. Previous works estimating the wear rate have been carried out based on contact mechanics and material failure mechanisms. In present work, the possibility of existence of low-cycle fatigue as the dominant mechanism in the wear of an as-cast eutectoid steel is investigated. Moreover, wear coefficient and the the volume loss per distance of that steel was predicted. At first, wear tests were done in order to obtain the wear trend of the steel in different loads. Then, regarding the roughness of the disk surface, a finite element model was built to simulate the sliding of the pin surface over the asperities... 

The Austenitization, influence of ageing condition and the prediction kinetics of AISI 420 martensitic stainless steel were studied in this paper. The Austenitizing temperature of 9200C, 9500C and 10200C were achieved. The specimens were heated for 15, 30 and 60 minutes then quenched in 250C Oil. The cooled samples were tempered at 2000C for 1 hour. The maximum tensile strength obtained at 10200C and 60 minutes were about 1800 MPa. The effect and ageing temperature and time on the strengthening behavior of this steel was modeled and analyzed by shear punch test and means of artificial neural networks (ANNs). Johnson-Mehl-Avrami-Kolmogorov (JMAK) analyses were applied to characterize the... 

This study was conducted in order to investigate the effect of difference in electrical resistivity between low carbon steel and austenitic stainless steel on the physical and mechanical properties and fracture mode of dissimilar resistance spot welds of these alloys. For this purpose, plastic deformation using constrained groove pressing combined with rolling applied to low carbon steel in three levels of 40%, 156% and 272%. Microstructural characterization, micro-hardness measurements and shear-tension test were performed. It was shown that by increasing the pre-strain in low carbon steel its electrical resistivity increases from 13.08 μΩ.cm to 19.5 μΩ.cm and because of reduction of... 

Springs parts are widely used in many industries including the automative industries where the disability of the springs can cause damage. Spring forming processes induce residual stresses on the surface of spring. Residual stress is affected by various processes, such as heat treatment or shot peening. When a coiled spring is heat treated, depending on the temperature and time of heat treatment residual stresses are partially relieved, but they generally do not disappear.An important characteristic of spring steel is its resistance against stress relaxation at different temperatures and times. Stress relaxation in valve springs working at high temperatures causes some problems for... 

Sever plastic deformation can be used for decreasing the grain size and increasing the mechanical properties. In this method a square or circular cross section specimen is passed from two intersecting channel with equal cross section. Thereby, a large plastic strain without change of cross section is produced in the specimen. This large strain is the main reason for the interest of researchers in many fields. In the experimental research fields, researchers can study the response of material in the large plastic strain range, investigation of stage III of hardening and after it, change of microstructure to submicron and Nano, and increase of strength simultaneously with preserving of... 

In this study, the nano-scale deformation mechanism of cementite was investigated with aid of Molecular dynamics (MD) simulation using MEAM potential. For this purpose, flow localization was evaluated in various structures such as single crystal cementite, nanocrystalline (NC) cementite, and lamellar cementite at various temperatures and different stress conditions. In order to understand the deformation mechanism in the cementite single crystal, two cylindrical samples with 15 nm length and 5 nm diameter oriented along [001] and [011] were constructed. The result of tensile test simulation at 100 K, 300 K, 700 K and 1100 K revealed that the deformation mechanism in single crystal cementite... 

A Coiled Carbon Nanotube (CCNT) can be geometrically viewed as a graphene sheet whose rolled and twisted to form a helically coiled nanotube. It is the strongest material ever measured with strengthexceeding more than hundred times of steel.The strength of a CCNT iscritically influenced by temperature, tube diameter, strain rate and pitch angle. In this study,a Molecular Dynamics (MD) simulationis performed to understand the effect of temperature, strain rate, tube diameter and pitch angle on the mechanical properties of CCNTs. Results indicate thatexisting the stone-wales defects is necessaryfor thermodynamic stability of CCNTs.According to the stress-strain curves it was observed thatyield... 

The specific properties of some of magnesium alloys have caused these alloys to be used in medical and industrial applications. Adding new elements and applying severe plastic deformation to these alloys have been considered as a clue for improving their mechanical properties. The aim of this project is to improve the mechanical properties of a magnesium alloy by using new alloying elements and applying the ECAP process.
In this project, a new magnesium alloy by composition of Mg-1.78Zn-0.72Si-0.41Ca was cast. After casting, solution treating and annealing of the alloy were studied. Then, the effect of ECAP process on microstructure and mechanical properties of the annealed and solution... 

The Al-Mg-Si alloys are an important group of industrial materials used extensively in the automotive and aerospace industry, due to their high strength to weight ratio, good corrosion resistance and the recycle ability. In order to achieve a combination of optimized microstructure and properties during the production processes, the study of deformation behavior and heat treatment of these alloys is necessary. Therefore, the prediction of microstructural evolution and mechanical properties as a function of alloy chemical composition and thermomechanical treatment is important for these alloys, from both the scientific and industrial aspects. In the present research, the flow behavior of this... 

The austenitic heat resistant stainless steels have been widely used in petrochemical industry as furnace tubes which are exposed to extremely high temperatures. In this study, the quench aging process of a HP modified heat resistant stainless steel is assessed and the kinetics of aging is evaluated by the JMAK model. The microstructure of steel in as cast and aged conditions was characterized by optical microscope and scanning electron microscope equipped with EDS. The effects of aging time and temperature on the microstructure, tensile propertices, shear strength, hardness, and ductility of the aged steel were determinded. The results of tensile and shear punch tests show a linear... 

This study was conducted in order to investigate on production and propeties of lead free solder alloy. For this purpose, Sn-Ag alloy with 3.5 weight percent of silver and sn-3.5Ag-0.7Cu at eutectic composition were produced with casting procedure.Single-lap Specimens were used to simulate real solder joints with the solder reflowed between two pure Cu substrate.Microstructural characterization, electrical resistance measurements and shear-tension test were performed.A deacrease of electrical resistance was observed in produced alloys in comparison with Sn-Pb . Experimental results showed that mechanical properties of produced alloys was better than common pb content alloy.The effect of... 

Production of nanocrystalline (under 100nm) or submicron materials (100-1000nm), has been created to one of the most important problems in engineering. A well-known method to create Ultra-Fined Grains (UFG) is imposing high strains to metals in order to break the structure. Among lots of ways forproducing UFG metals, Equal Channel Angular Pressing (ECAP) is one of the most famous and common ways. On the other hand, Shot Peening has been being used for a long time to improve mechanical properties of metals by introducing nanocrystalline layer at the surface of specimen. The aim of the work, is investigation of the effect of combination of ECAP and Shot Peening on High Cycle Fatigue of 6082... 

In this project, in order to model the flat rolling problems, an analytical method based on Bezier functions has been presented. To this goal, using the Bezier functions, the velocity field of the flat rolling has been stablished and the volume constancy condition and velocity boundary conditions have been considered in calculations. Using the achieved velocity field, the strain rate and strain fields has been determined. Then, with derived fields, power, pressure, and force of the deformation in flat-rolled billets from different materials such as copper, aluminium and steel have been calculated. To verify the accuracy of the results, the rolling process has been carried out on the... 

One of the methods of forming the metal parts is to apply a localized strain field to the workpiece. This field can be generated by creating a deformation in an actuator connected to the workpiece, or directly by a localized imposing heat (thermal localized strain) to the workpiece. For example in laser forming process, the workpieces in the forms of sheet and tube can be deformed by creating localized strain field induced from thermal field caused by laser irradiation. The main mechanisms in laser forming are temperature gradient mechanism, upsetting mechanism and buckling mechanism. It is worth mentioning that until now, the localized strain field induced by laser irrediation is... 

In this project the effect of rolling temperature on mechanical properties and microstructure evolution of a low carbon steel was studied. The steel was rolled up to a nominal strain of 67% at three different temperatures consisted of room, blue brittleness, and subzero temperature. Micreohardnessand tensile tests were used to characterize the recrystallization kinetics. Optical and scanning electron microscopic examinations were carried out to evaluate the mechanical, grain size, and the kinetics of static recrystallization by the Avrami equation. The results indicated that the volume fraction of static recrystallization increases with the increase in annealing period. The Avrami exponent... 

Special features of magnesium such as low density, perfect bio-compatibility in the human body, and close elastic modulus to the human bone have increased the use of Mg alloys in the bio-medical applications. Nevertheless, low mechanical properties and poor corrosion resistance of Mg alloys have limited their bio-medical applications. The addition of alloying elements together with the use of precipitation hardening and plastic deformation are the most effective methods to improve the mechanical properties and increase the corrosion resistance of Mg alloys. Thus, the present research with the aim of improving the mechanical properties of an Mg-Zn alloy by Ca and Gd additions together with...