Sharif Digital Repository

The present study Is carried out to investigate fatigue crack initiation and kinking behaviours of spot welded coach peel (CP) specimens of low carbon steel sheets subjected to cyclic loading by experimental and finite element analysis methods. Evaluations of fatigue crack growth stages were performed by crack tip plastic strains and J integral analyses and also by microhardness measurements on process zone. According to the experimental and analytical results, fatigue crack initiation and growing stages in the spot welded CP specimens can be divided to three stages. Stage I corresponds to 'gap sharpening stage' observed at the beginning steps of cyclic loading with crack growing on the... 

IN738LC is one of the superior nickel base superalloys utilized at high temperatures in aggressive environments. However, experiencing high temperatures and stresses during service causes microstructure deterioration and degradation of mechanical properties in this alloy. To restore the microstructure and mechanical properties of the degraded alloy, rejuvenation heat treatments can be considered. In this study, the evolution of microstructural features in a creep damaged IN738LC superalloy during different stages of rejuvenation heat treatment cycles was investigated. During solution treatment stage, dissolution of coarsened γ′ precipitates, grain boundary films and transition zone around... 

The present study investigates the evolution of γ′ precipitate microstructure in a creep damaged IN738LC superalloy during different stages of rejuvenation heat treatment cycles. Dissolution of coarsened γ′ precipitates in the creep damaged alloy occurred during different solution treatments. The precipitates that did not dissolve completely during solution treatment and those formed during subsequent cooling were considered to be as remnant and cooling precipitates respectively. It was found that higher solution treatment temperatures result in lower volume fraction of remnant precipitates and further formation of cooling precipitates. Slower cooling rates after solution treatment gave the... 

Simultaneous effects of notch and texture on strengthening mechanisms of rolled thin sheets of commercially pure titanium were investigated. The presence of notch led to the restriction of deformation systems and different fracture behaviors compared to un-notched specimens. The loss of material's ability to accommodate plastic deformation at the notch tip with increase in rolling reductions changed the notch strengthening phenomenon to the notch weakening one. At medium levels of deformation, due to the simultaneous development of a triaxial stress state and strong basal texture at the notch tip, a new strengthening mechanism which is called "notch-texture strengthening mechanism" led to a... 

High-bainite dual phase (HBDP) steels with 34. vol.% ferrite were subjected to tensile strains of 1%, 3%, and 5% at intermediate temperatures (150-450. °C). Mechanical behavior of pre-deformed steels was then investigated at room temperature. A maximum value for both yield stress and ultimate tensile strength was observed for pre-deformed steels. A minimum elongation was also found in the same pre-deformed specimens at the range of 250-350. °C. It was found that dynamic strain aging (DSA) plays the major role in mechanical properties of pre-strained HBDP steels. The DSA indicator parameter (DSAP) was introduced to evaluate DSA mechanism. Maxima of DSAP were likewise observed at the range of... 

A nickel base superalloy Rene 80 was bonded in the as received cast condition using transient liquid phase diffusion bonding with a Ni-base interlayer. The bonding process was carried out at 1100 °C under argon and vacuum atmospheres for various hold times. The bonds then homogenized at 1206 °C for 1 and 2 h. Microstructures of bonds were studied with optical and scanning electron microscopy. Microstructure studies showed intermetallic compounds within the joint region. However, these intermetallic compounds were removed by a post-bond heat treatment at 1206 °C for 2 h. Shear and hardness tests were undertaken to determine the mechanical properties of bonded samples. Results showed that the... 

In the present study, 30MSV6 microalloyed steel was heat treated under different conditions, and the relation between its microstructure and mechanical properties was investigated. Scanning electron microscopy and transmission electron microscopy were used to characterize the microstructure of the heat-treated steel, and the effect of microstructure on tensile strength and fatigue behavior was determined. Microstructural analysis indicated that precipitates were formed at different sites such as grain boundaries and sub-grain boundaries. Furthermore, microstructural studies accompanied by the evaluation of mechanical properties revealed that the optimal heat treatment cycle of 30MSV6... 

The microstructural evolutions and mechanical properties of vanadium microalloyed steel (30MSV6) during precipitation hardening were studied. The effects of aging temperature and cooling rate on mechanical strength (yield strength and ultimate tensile strength) were similar. Increasing aging temperature or cooling rate firstly increased the mechanical strength of specimens up to their maximum values, which then decreased with further increase in aging temperature or cooling rate. Microstructural evolutions revealed that cooling rate had significant effects on the pearlite interlamellar spacing and size of pre-eutectoid ferrite. Unlike the effect of austenitizing temperature, the pearlite... 

Nickel titanium shape memory alloys (NiTi-SMAs) were successfully produced from elemental Ni/Ti powders by powder metallurgical method and then subjected to age treatment. Microstructure was examined by SEM and XRD and phase transformation temperatures were measured by dilatometric method. The phase transformation temperatures increased with both duration and temperature of the age treatment. The porous product exhibited desirable shape memory effect. © 2009 Elsevier B.V. All rights reserved  

In this investigation, a neural network model was used to predict mechanical properties of dual phase (DP) steels and sensivity analysis was performed to investigate the importance of the effects of pre-strain, deformation temperature, volume fraction and morphology of martensite on room temperature mechanical behavior of these steels. In order to train the neural network, dual-phase (DP) steels with different morphology and volume fractions of martensite were deformed between 2 and 8%, at high temperature range of 150-450 °C. The results of this investigation show that there is a good agreement between experimental and predicted values and the well-trained neural network has a great... 

Dual phase steels with different martensite volume fraction and morphology were tensile tested at temperature range of 25-550 °C. Stress-strain curves of all steels showed serration flow at temperature range of 250-450 °C and smooth flow at the other temperatures. Both yield and ultimate tensile strengths increased with increasing testing temperature up to about 450 °C and then decreased at higher temperatures. At a given temperature, yield stress, tensile strength, and work hardening increased with increasing volume fraction of martensite. Similar behavior was observed by changing martensite morphology from network to fibrous martensite. The change in mechanical properties was related to... 

AISI 4340 steel bars were heated at 900 °C for one hour, annealed at 738 °C for different durations and oil-quenched in order to obtain dual phase steels with different ferrite volume fractions. A 3% prestrain at the temperature range of 150-450 °C was then imposed to the samples, and room temperature tensile tests were carried out, afterwards. Results indicate that the maximum values for both yield and ultimate tensile strength would exist for the samples pre-strained at the temperature range of 250-300 °C. Also, a sudden drop of the ductility was observed at the mentioned temperature range. The observed behavior might be attributed to the occurrence of dynamic strain aging taken place at...