Sharif Digital Repository

Transient-liquid-phase (TLP) bonding of the IN-738LC base alloy with BNi-3 interlayer was investigated in this article. Effects of the TLP bonding thermal cycle on the base alloy were studied microstructurally and macrostructurally. Microscopic investigation revealed that the microstructural evolution in the TLP bonding area was affected substantially by the bonding temperature. Therefore, a critical bonding temperature (Tcr) was defined for TLP bonding of IN-738LC/ BNi-3/IN-738LC samples. Unlike bonding below Tcr, TLP bonding at temperatures higher than Tcr resulted in formation of individual γ-γ' colonies in the adjacent base alloy to TLP bonding zone and caused significant reduction in... 

The densification and microstructural evolution during co-sintering of M2 tool steel/316L stainless steel composite layers with and without boron addition was studied. A pressureless sintering method in conjunction with a powder layering technique was used to fabricate the stepwise graded composite layers. Isothermal and non-isothermal sintering response of the individual and composite layers was examined and the microstructural features of the bonding zone were studied. Shear strength and microhardness of the bonding zone were also measured. It was shown that an enhanced densification is obtained in the composite layers due to (i) sintering shrinkage incompatibility between two steels, (ii)... 

The influence of different amounts of cold work on the evolution of microstructure during subsequent aging of AEREX™350 superalloy was investigated. Simple compression testing was used to impose desired plastic strain while microhardness testing, XRD, SEM and TEM techniques were used to characterize the response of the alloy following aging treatment. It was found that high levels of cold work followed by aging resulted in a drastic increase in the formation of Widmanstättan η phase and incidence of shear bands in the microstructure. Based on microstructural data, the effect of plastic strain on η phase formation and the physical origin of the localized shear bands are discussed. © 2007... 

In this paper, a mathematical model is employed to evaluate the kinetics of recrystallisation in the hot rod rolling process. To reach this goal it is required to accurately assess the temperature and the velocity fields within the metal being deformed. These parameters are evaluated by solving the governing equations of heat conduction and plastic deformation. In addition, a neural network model is coupled with finite element analysis to calculate the flow stress of deforming metal as a function of deformation parameters. To verify the validity of the employed model, the predictions and the measured surface temperatures after each pass are compared and a good agreement is observed. © 2008... 

An approach composed of the thermodynamics-based dislocation model and the Taylor theory is used to investigate the evolution of microstructure and flow stress during high-pressure torsion (HPT). The incremental temperature rise is considered through the modelling of HPT. The temperature can affect the annihilation of dislocations and thus the dislocation density. The model predicts the dislocation density, sub-grain size and flow stress during HPT. The modelling results are compared with the experimental data and the modelling results without considering the incremental temperature rise. A remarkable agreement is observed between the modelling results with considering the temperature rise... 

Results of an investigation on the evolution of microstructure during simple compression testing of two face-centered-cubic (fcc) polycrystals, IN625 superalloy and 316L stainless steel, are reported and a review on the existing data related to the uniaxial deformation of fcc polycrystals is presented. It is found that while a number of fcc polycrystals show linear hardening behavior, the evolution of the underlying microstructure may be quite different. It is argued that, in contrast to the current belief, deformation twinning may not be the sole cause of linear hardening in low stacking fault energy (SFE) fcc polycrystals. It is suggested that, due to the requirement of slip system change... 

In this work, effective parameters of SIMA process to obtain non dendritic microstructure in A356 alloy were investigated. In addition, the effect of SIMA process on the evolution of morphology of silicon and intermetallic phases in this alloy was studied. Microstructure images obtained from optical microscopy and SEM observation showed that increase in plastic work up to 40% and then holding of samples in the semi solid state at temperature of 580°C, causes that primary dendritic structure changes to non dendritic, fine and globular structure, but optimum reheating time completely depended on initial thickness of samples. If all parameters of SIMA process are the same, the grain boundaries... 

This work presents the effect of co-sintering on the densification and microstructural evolution in the two-layer stepwise graded composite of INCONEL 718 and INCONEL 625 superalloys. A pressureless co-sintering method in conjunction with a powder layering technique was used. The sintering was carried out in solid state and liquid phase in temperature ranging from 1260 °C through 1300 °C for 60 minutes in a low pressure of an argon atmosphere. Nonisothermal sintering behavior was also examined by dilatometric analysis. Similarly, the sintering response of the individual layers was characterized. The results reveal an enhanced densification rate during co-sintering of the composite layers.... 

A Monte Carlo model is modified by using Turnbull-Fisher nucleation rate model to simulate the ultra-fine microstructure evolution during annealing of pure copper processed by equal channel angular pressing (ECAP). The simulation is utilized to predict the grain size of pure copper samples at different annealing temperatures and pass numbers of ECAP. Also, in the simulation the constant nucleation rate model is assumed and a good agreement is achieved between the simulation results and experimental data. Thus, it can be concluded that the constant nucleation rate is prevailed during the microstructure evolution. © 2007 Elsevier B.V. All rights reserved  

In this paper dynamic strain ageing behavior in an Al-Mg-Si alloy related to equal channel angular pressing (ECAP) was investigated. In order to examine the combined plastic deformation and ageing effects on microstructure evolutions and strengthening characteristics, the Al6061 alloy were subjected to φ=90° ECAP die for up to 4 passes via route Bc at high temperatures. For investigating the effects of ageing temperature and strain rate in ECAP, Vickers hardness tests were performed. The combination of the ECAP process with dynamic ageing at higher temperatures resulted in a significant increase in hardness. The microstructural evolution of the samples was studied using electron... 

In this study, we attempt to characterize the microstructural evolution during friction stir butt welding of Al-Cu-laminated composites and its effect on the fracture behavior of the joint. Emphasis is on the material flow and particle distribution in the stir zone. For this purpose, optical microscopy and scanning electron microscopy (SEM) images, energy-dispersive spectroscopy EDS and XRD analyses, hardness measurements, and tensile tests are carried out on the joints. It is shown that intermetallic compounds exist in lamellas of banding structure formed in the advancing side of the welds. In samples welded from the Cu side, the banding structure in the advancing side and the hook... 

A new integrated physically based constitutive model was developed for an age hardenable Al- Mg-Si alloy. The kinetics of precipitation during various stages of aging was modelled. The precipitate features consisted of particle radius and volume fraction obtained from the kinetics model, which was used to compute the alloy yield strength/hardness. A published multiinternal variable workhardening model was improved to take into account the effects of solute solution and precipitates on the alloy hardening capacity after performing different cycles of aging treatment. The flow curves and hardness predicted by the model were in good agreement with the experimental results. The model is able to... 

Two-internal variable thermodynamics model is presented to investigate the evolution of microstructure and flow stress during severe plastic deformation. Previous studies have shown that due to heterogeneous distribution of dislocations during severe plastic deformation, the use of multivariable models is needed. In this regard, a two-internal variable model is presented. In the present paper, the dislocation densities in the subgrain boundaries and interiors are considered as internal variables. The model uses general laws of thermodynamics and describes the evolution of the dislocation densities on the basis of parameters such as the self-diffusion activation energy and the stacking fault... 

Co-based FSX-414 superalloy is especially used for first-stage nozzles of gas turbines. Transient liquid phase (TLP) bonding has potential as repair process for these nozzles. In this study, homogenization of TLP-bonded FSX-414 superalloy at optimum bonding condition (1150 °C/5 min) was conducted at 1175, 1200, and 1225 °C for 1, 3, and 6 h. Homogenization at 1175 °C/1 h had no effect on removing the diffusion-affected zone (DAZ) phases. Increasing the time to 6 h was effective in removing DAZ phases and boride phases formed due to liquefaction, but compositional homogenization was not complete. Homogenization at 1200 °C for 1 h caused boride phases to form adjacent to the joint and in the... 

An analytical model is developed to study the effect of cooling rate on the final grain size of stirred zone of the Al5083 subjected to friction stir processing. The effect of cooling rate on the grain size of the stirred zone was investigated experimentally and numerically. A new microstructural evolution model was also suggested illustrating the mechanisms contributed in refining the microstructure. A new mechanism termed meta-dynamic recovery (MDRV) is introduced here in this regard. The simulation results also show that the rapid cooling rate resulted in superior mechanical properties through refining the microstructure of the stirred zone. However, decreasing the rotational speed and...