مریم مرکباتی

The aim of the current research is to achieve a fine grain structure (less than 200 micrometers) in Ti-48Al-2Cr-2Nb Intermetallic by a three-step heat treatment and to investigating the parameters affecting this process. For this purpose, after the homogenization at 1175 C for 24 hours for eliminating the cast structure, three-stage heat treatment including the first stage (single-phase annealing at 1400 C for one hour with cooling in furnace and air environments), the second stage (cyclic operations including repeated heating up to 1400 C for three, five and ten cycles and cooling in air and water environments) and the third stage (Two-phase annealing at temperatures of 1175, 1225 and 1275 C  for one hour) was performed. Finally, single-phase annealing at 1400°C for one hour and cooling in the furnace, then cyclic heat treatment with five cycles at the same temperature with cooling in air and annealing at 1225°C for one hour is suggested as the optimal process to create a semi-lamellar structure with the smallest colony size.

In This study, in order to evaluation of ductility and consequently the optimum workability region of novel Ti-3Al-8Mo-7V-3Cr titanium alloy (Ti-3873), the hot tensile tests were performed at a constant strain rate of 0.1 s-1 and the temperature range of 650-850 ℃ . To establish the relationship between microstructural evolution and ductility, the microstructure of the specimen was examined by optical microscopy (OM) and scanning electron microscopy (SEM) after and before hot deformation. The results showed that ductility at the temperatures of 650-750 ℃ , increased from 33% to 54% as a consequence of transformation of α  to β  phases and gradually eliminating the α  phase. The maximum ductility obtained at 850 ℃  with a 71% increase in ductility. Microstructural studies showed the elongated and serrated boundaries confirmed the occurrence of dynamic recovery. Recrystallized grains were also observed at 850 ℃ . Therefore, it can demonstrate that the restoration mechanism of the Ti-3873 alloy during hot tension is dynamic recovery and partial dynamic recrystallization. Finally, according to tension results, the appropriate range of deformation deformation of the Ti-3873 alloy in this study is 800-850 ℃ .

Grain refinement through dynamic recrystallization is one way to improve the mechanical properties of high nitrogen austenitic stainless-steel ingots during hot rolling. In this paper, by using simulation based on the finite element method and experimental results, the effect of conventional rolling and gradient temperature rolling on the microstructural evolution of Nitronic 50 austenitic stainless-steel ingot during hot rolling is investigated. The results of simulations showed that strain in the center of the ingot be increased under a gradient temperature, while the maximum strain value in the conventional rolling is on the surface. The microstructural results obtained from optical microscopy for gradient temperature rolling showed strain value in the center is more than the surface. The temperature difference between the center and the surface leads to a significant reduction in the strength center of the ingot against the surface. The difference in average grain size of dynamic recrystallized for gradient temperature in the surface and center was less than 3μm. The existence of primary grains (68μm average grain size) and bulged grain boundaries in the center of ingot and refinement and small grains in the surface area (8.5μm) in conventional rolling (1000℃) showed that dynamic recrystallization, due to low effective strain and temperature, has not occurred in the center of the ingot. The almost uniform distribution of austenite grains in the direction of ingot thickness for gradient temperature rolling showed that this method is an appropriate method for hot rolling of austenitic steel ingots compared to conventional conditions.

The purpose of this research is to investigate the effect of different heat treatment cycles on structural changes and mechanical properties of Ti-48Al-2Cr-2Nb intermetallic compound. Heat treatment at 1175°C for 24 hours resulted in the formation of a duplex structure including γ grains along with colonies of γ and α2 layers with an average grain size of 1160 micrometers. Heat treatment at 1400°C for 30 minutes created a near lamellar structure of γ and α2 phases with a grain size of 1300 micrometers. By increasing the time to 60 minutes, a fully lamellar structure with an average grain size of 1120 micrometers was created. The decrease in grain size with increasing the heat treatment time is mostly attributed to the completion of the recrystallization process with the creation of new colonies and the destruction of the remaining gamma grains. Heat treatment at a temperature of 1380C for 45 minutes resulted in the formation of a fully lamellar structure with a grain size of 950 micrometers, but the grain size distribution is more uniform than the lamellar structure formed at a temperature of 1400 degrees. By increasing the volume fraction of colonies, the hardness increased. The hardness of duplex structure was 282 Vickers and the near lamellar structure was 320 Vickers. The results of the stress rupture test showed that the creep rupture life of the lamellar structure was 115 hours, while the life of duplex structure was 22 hours. This difference is attributed to the small distance between the layers.

Metastable beta titanium alloys are suitable for use in the aerospace industry due to their high strength and good ductility, as well as their high strength-to-weight ratio. The aim of the current research is to investigate the microstructure and tensile properties of the alloy after single-step and two-step aging following thermal-mechanical cycles of single-phase β annealing and two-phase α+β annealing. For this purpose, on one strip of the alloy, solution annealing heat treatment in the single-phase β region, cold rolling and recrystallization and on the other strip, solution annealing heat treatment in the two-phase α+β region was performed. Afterwards, the specimens from the strips were subjected to single-step aging at 550⸰C. In addition, in order to perform two-step aging, specimens were subjected to heat treatment at 300 and 550⸰C for primary and secondary aging, respectively. Then the structural evolution of the alloy was investigated by SEM and X-ray diffraction pattern and the tensile properties of it by tensile test. It was found that the optimum heat treatment cycle of the Ti-3873 alloy was two-step aging after α+β solution treatment leading to 1190 MPa yield strength and 14.7% elongation. In this case, the obtained structure has no grain boundary alpha and the formed secondary alpha has a length of less than 0.5 µm and its average thickness is 0.15 µm.

The purpose of this research is to investigate the hot working behavior of the cobalt-nickel base superalloy with the chemical composition of Co-22.8Ni-3.4Al-8Cr-17.1W-1.5Ti-2.8Ta-1.5Nb-1.5Mo-0.06C-0.02B (%wt) by performing compression test, providing the constitutive equation and deformation procssing map and determining the safe and unsafe regions of deformation. In this regard, the hot compression test was performed in the temperature range of 1050-1200 degrees Celsius, with a step of 50 degrees Celsius and strain rates of 0.1, 0.01 and 0.001/s up to a strain of 0.7. The evaluation of the constitutive equations governing the hot deformation process of the superalloy showed that the presented model based on the hyperbolic sine equation predicts the experimental results with acceptable accuracy. Using the mentioned equation, the hot deformation activation energy of the investigated alloy in the present study was obtained as 497 kJ/mol. Based on the process in map drawn for the investigated alloy in the present study, at a strain of 0.4, an instability region was observed at a temperature of 1050 degrees Celsius and a strain rate of 0.01 1/s. the extent and intensity of instability region decreased with the increase in deformation temperature. According to the results of the processing map and the constitutive equations, the optimal conditions of deformation of the investigated alloy are in the temperature range of 1150  to 1200  and the strain rate of 0.1 1/s and the temperature range of 1100  to   1200 and the strain rate of 0.1 to 0.001 the peak efficiency of  45% energy consumption.

The medical cast CoCrMo ASTM F75 alloy is used in orthopedic implants such as artificial toggle and knee. In this alloy, due to the existence of defects such as chemical inhomogeneities and large grain size, heat treatment is performed. In this research the effect of solution annealing heat treatment on the microstructural evolution and mechanical properties of two ASTM F75 alloy (the free of carbon specimen and the specimen containing 0.21 wt% carbon) was investigated. The solution treatment done at 1175, 1225 and 1275oC for 0.5, 1, 2 and 4 hours. Subsequently the specimens quenched in water. The results show that in the specimen without carbon, the σ precipitates were formed. However, in the specimen containing carbon, the M23C6 carbides and σ were formed. Besides, by adding carbon the volume fraction of carbides increased and reached to 6.55 % leads to increase in strength. By increasing annealing temperature from 1175 to 1275 oC and the time from 0.5 to 4 hours, the volume fraction and the size of carbides have been decreased. In the specimen containing carbon, by annealing at 1225oC for 1 hours the distribution of precipitates became homogenous in the microstructure. Besides, most of carbides at the grain boundaries in the cast condition were disappeared by heat treatment. Finally, in the specimen containing carbon in the heat-treated condition, the mechanical properties such as the ultimate tensile strength raised up to 19% and the ductility increased twice in comparison to the one in the cast condition

The aim of this study was to evaluate the effect of aging on the stress rupture and hot pressure of novel cobalt base superalloy of Co-7Al-7W-4Ti-2Ta composition. After melting the superalloy by VIM method and remelting by VAR method, the ingot was homogenized at 1250 °C for 10 hours. Then, aging treatment was performed on homogenized samples at 800 °C for 8, 16 and 24 hours. After microstructural and phase analysis by SEM and X-ray diffraction pattern testing, the high temperature mechanical properties including hot pressure and stress rupture tests were performed. The results showed that the alloy in the cast condition contains ɣ, ɣ‘, a and β phases in which the ɣ‘, a and β phases are dissolved in ɣ phase by homogenization treatment. By aging of the studied alloy, ɣ‘ precipitates nucleate and grow, where in by increasing aging time the volume fraction and size of these precipitates increases. The results of pressure tests in the temperature range of 500 °C to 1000 °C showed that the strength of the alloy increased at a temperature of 700 °C, which indicates the occurrence of the stress anomaly phenomenon as a consequence of ɣ‘precipitation. Besides, the highest resistance to stress rupture at 770 °C under 300 MPa for 22 hours was achieved for the specimen aged at 800 °C for 16 hours.

Metastable beta titanium alloys have the ability to achieve different microstructures as a result of various heat treatment cycles. The aim of the present study was to create a combination of fine spherical and needle-shaped alpha phase in a metastable beta Titanium alloy (Ti-3Al-8Mo-7V-3Cr) using two-phase solution annealing and aging to improve tensile properties. In this regard, one strip of the alloy was solution annealed in the two-phase region (α+β) at 750°C. Then, some of the solution treated specimens were aged in one step and the others in two steps. The microstructural observation and phase analysis were studied by scanning electron microscope (SEM) and X-ray diffraction (XRD), respectively followed by investigating tensile properties using tensile test. The results exhibited that the microstructure of the alloy after annealing in the two-phase region (α+β) consisted of a spherical primary alpha phase of 1 μm in the beta matrix. One-step aging at 600°C resulted in a microstructure without secondary alpha layers. This heat treatment cycle resulted a yield strength of 980 MPa and fracture strain of 13.9%. Two-step aging at 300°C and 600°C led to formation of the secondary alpha layers with 0.1 μm thickness and increased the yield strength and fracture strain to 1007 MPa and 15.8%, respectively.

The effect of temperatures and times of aging heat treatment on the characteristics and growth kinetic of γ̍ Precipitations were investigated in cold rolled Waspaloy superalloy, in this article. The 1 mm cold rolled strap were stabilized at 845 ˚C and then the samples were aged at 730, 460 and 800 ˚C until 23 hours. The microstructure of samples especially γ̍ phases were assessed with optical and scanning electron microscopy. The volume fraction and size of γ̍ particles were measured and analyzed. The microstructural and data analysis results showed that the growth kinetic of γ̍ Precipitations obey the LSW theory and the particle growth controlled via volume diffusion and coalescence of precipitates. The measured activation energy of γ̍ Precipitations that is thermally activated is 265 kJ/mol. This value is about equal to activation energy for diffusion of Al and Ti in FCC Ni that are 270 and 256 kJ/mol, respectively.

The aim of this study was to investigate the behavior of hot deformation and occurrence of restoration phenomena during the deformation of AISI H10 hot work tool steel. For this purpose, hot tensile test was performed on the steel in the temperature range of 900-1150 ºC with a temperature interval of 50 ºC and at a constant strain rate of 0.1s-1. The microstructures were examined and the curves of hot flow and ductility were drawn. According to the curves and microstructures, ductility was lower at temperatures of 900 ºC and 950 ºC due to inactivity of repair processes and the presence of carbides. Ductility increased in the temperature range of 1000-1100 ºC due to the occurrence of dynamic recrystallization. Finally, ductility decreased in the temperature of 1150 ºC due to the dissolution of carbide particles and grain growth. The results obtained from hot tensile test and microstructural studies at a constant strain rate of 0.1s-1 revealed that the appropriate temperature range for deformation of AISI H10 hot work tool steel was 1000-1100 ºC.

In this research, the hot deformation behavior of W360 tool steel was investigated using hot compression test at 1000-1200°C and strain rates of 0.001, 0.01, 0.1, and 1 s-1. According to the results, dynamic recrystallization was found the most important restoration factor of this alloy during hot deformation. Recrystallization was enhanced with an increase in temperature and strain rate. Also, the hot working process was optimized by drawing the processing map of this steel. Microstructural images obtained from the hot compression test showed that recrystallization started at 1000°C and the strain rate of 0.01 s-1 and developed with increasing temperature and strain rate due to an increase in the stored energy and suitable regions for nucleation. The results of drawing the processing map showed that the best hot deformation region was the temperature range of 1050-1150°C and strain rates of 0.1-1 s-1.

The aim of this study was to investigate the effect of temperature and time of aging on the microstructure and hardness of the novel Co-based superalloy with the composition of Co-7Al-7W-4Ti-2Ta. For this purpose, the new generation cobalt-based superalloy ingot cast by the VIM and VAR furnaces. Then aging treatment was performed at 700, 800 and 900°C for 8, 16 and 24 hours. Then, the specimens were cooled in air. Subsequently, the specimens were subjected to microstructural examinations by optical and electron microscopy and hardness test. The results showed that by increasing aging temperature from 700°C to 800°C, the γʹ precipitation and its growth occurred and its volume fraction and size increased. Therefore, the maximum hardness obtained for the specimen aged at 800°C. By increasing the temperature of aging to 900oC, the hardness decreased which was attributed to the precipitation of harmful β phase containing Co-Al-Ti and consumption of γʹ alloying elements results in reduction of the γʹ volume fraction.

The aim of this study was to investigate the suitable temperature range for hot deformation of three medium carbon Ni-Cr-Mo low alloy steels by hot tensile and hot torsion tests. Hot tensile tests were carried out in the te,prature range of 850-1150°C at a constant strain rate of 0.1 s-1 until fracture. Then, the tensile flow behavior, hot ductility and microstructural evolution of the steels were studied. Hot torsion tests were performed in the temperature range of 1200-780°C at strain of 0.1 with strain rate of 1s-1. The effect of titanium and niobium on the mean flow stress and the non-recrystallization temperature were investigated. The tensile test results showed that dynamic recrystallization was the dominant mechanism at temperatures above 950°C in the base steel and temperatures above 1050°C in the microalloyed steels. The results of hot torsion tests showed that the non-recrystallization temperatures of the base, Ti containing and Nb containing steels were 1070°C, 1069°C and 1116°C, respectively. Finally, the suitable hot deformation temperature range to achieve optimum mechanical properties in the base and Ti containing steels obtained as 950-1070°C and that of Nb containing steel obtained as 950-1100°C.

The aim of this study was to investigate the effect of temperature and time of homogenization treatment on the microstructure, distribution of alloying elements and hardness of the novel Co-based superalloy Co-7Al-7W-4Ti-2Ta. For this purpose, the specimens were first homogenized at 1250 and 1300 °C for 2, 4, 6 and 8 hours and then water-cooled. Subsequently, the specimens were subjected to hardness testing and microstructural examinations by optical and electron microscopy. The results showed that by increasing the homogenization temperature to 1300 °C, the porosity created by Ti oxidation and local melting of the Co-Al-Ti eutectic compounds lead to a decrease in hardness to 90 Vickers. This phenomenon is due to high segregation of alloying elements in the cast structure. The intensity and destructive effects of this segregation were reduced by remelting of alloy. However, by homogenization at 1250 °C, no local melting of eutectic zones or porosity were observed in the specimens and a more uniform structure was obtained with increasing time. Minimum and maximum hardness values after homogenization at 1250°C were 348 and 406 Vickers, respectively. Moreover, the microstructure became more homogenous by increasing the homogenization treatment time at this temperature.

In the current study, the hot workability of W360 hot work tool steel was investigated by hot tension testing in the cast and wrought conditions in the temperature range of 900-1200°C at strain rate of 0.1s-1. The results showed that in both cast and wrought steels, ductility has increased with increasing temperature from 900 to 1000°C, due to dissolution of carbides and occurrence of dynamic recrystallization. The most recrystallization has occurred at 1050°C and the size of the grains has decreased. This reduction in wrought steel was more evident due to its smaller primary grain size. Wrought samples showed higher hot ductility and lower peak stress than cast samples. The ductility of cast steel depicted a significant decrease at 1200°C due to the presence of undissolved particles along grain boundaries and the stress concentration and thus formation of granular cracks surrounding them. It is while the breakdown of particles has prevented the stress concentration around them in the wrought steel. According to microscopic images of the samples after the hot tension test, in the wrought samples, the continuous alloyed carbide nets were broken during the rolling and occurrence of recrystallization and the carbides has become smaller and their distribution was more uniform. This issue reduces the stress concentration around the carbides in the wrought samples and thus leads to higher hot workability than the cast one. According to the results, the best hot deformation range of W360 steel was achieved in the temperature range of 1050 to 1150°C for both cast and wrought steels.

In present work effects of strain in hot working process and solution annealing, on microstructure and subsequent mechanical properties were investigated. The billet were hot rolled into final reduction in thickness 60 and 85% (R-60 and R-85) at 1150 ˚C. After that, solution annealing at 1140 ˚C to 1200 ˚C for 10 to 120 minutes were conducted to optimize of mechanical properties. The microstructures revealed grain size of R-85 sample was lower than another, but grain growth was observed for annealed sample above 1140 ˚C after 30 minutes. EDS analysis shown M6C carbide rich of tungsten is only precipitated carbide. It's also found that grain growth rate is much more for R-85 sample. SEM evaluations also confirmed faster solution rate of carbides for R-85 sample. Yield strength of R-85 is very higher relative to R-60 while after annealing at 1140 ˚C for 10 minutes it decreased drastically. Annealing in same condition for R-60 sample cause not only decreasing of strength, even improved ductility.

In order to investigation of dynamic and static restoration of SP-700 alloy, in this study continuous and interrupted hot torsion tests carried out at 850 and 1000ºC at different pass-strains and inter-pass times. The dominant mechanism in hot deformation at 1000°C is dynamic recrystallization (DRX) and consequently the entire microstructure comprises equiaxed grains, whereas at 850°C serration and tanglement of the grain boundaries were observed. Nevertheless, the microstructure of sample twisted at 850°C, indicates the occurrence of DRX and the formation of very fine grains. The mechanism of the formation of recrystallized grains in the vicinity of grain boundaries and triple points is bulging. With an increase in pass-strain (ε=0.5) at 1000°C, due to the increase in driving force for nucleation and growth of new grains, the kinetics of static restoration increases. In fact, at 850°C, in addition to static restoration there is another factor contributing in fractional softening which is β to α phase transformation.

In this paper, in order to study the flow behavior and elongation of as-cast ingots of SP-700 titanium alloy, hot tensile test was done in α/β dual phase and β single phase regions using strain rate of 0.1 s-1. Results showed that the hot tensile behavior of SP-700 in the α/β dual phase region (700-900 ºC) was different from the β single phase one (950-1100 ºC) due to the nature of alpha and beta phases and their crystallographic structure. This was since the number of slip systems and deformation mechanism in HCP structure were different from those in BCC structure. Beside, the intensive variation of elongation in microstructural studies showed that the dominant mechanism of hot tensile deformation of SP-700 alloy was dynamic recovery (DRV). Thus, serration of grain boundaries and occurrence of DRV were the reasons for the increase of elongation with the rise of temperature. However, beta grains growth and occurrence of grain boundary fracture made a slight decrease in elongation in the temperature range of 1000-1100 ºC.

In this study, the effects of cold rolling and intermediate and final annealing on the microstructure and tensile properties of Hastelloy X alloy was investigated. Three-stage cold rolling followed by intermediate annealing at 1175 ºC for 0.5 hr were performed on 10 mm thick strip at hot rolled and solution annealed condition to obtain the final thickness of 1.25 mm. The specimens prepared for tensile test within temperature range 660- 860 ºC, annealed at 1100-1200ºC for 1 hr. Microstructural investigation of the specimen’s solution annealed at temperature 1175 ºC indicated the matrix is austenite and dispersed particles are M6C carbides which are molybdenum rich precipitates. Hot tensile test results for specimens annealed at 1100, 1150 and 1200 ºC indicated a minimum ductility, at 760, 760 and 860 ºC, respectively. Tensile tests showed that the yield strength of the alloy had a weak temperature-dependence for temperature range 660- 860ºC, due to precipitation of molybdenum rich M6C carbides.

Microstructure and low cycle-high temperature fatigue properties of Nimonic 105 superalloys with and without B and Zr is investigated in this article. Fully reversed strain-controlled tests were performed at 750°C, R=0 and strain rate of 3×10-3 s-1. The results show that Zr cause to ZrC formation at the grain boundaries and grain interior. Also Zr is reduced the grain size of the alloys. The carbides at the absence of Zr is of the type of (Cr,Mo)23C6 at the grain boundaries. The ᵧ’ size decreased by B additions to the alloys and the number of twins increased. 0.013wt% B improves low cycle-high temperature fatigue of the alloy. At the presence of B, fracture is of the types of intragranular and intergranular but by addition of 0.16wt% Zr the only fracture type is intragranular. Hard and none coherent MC type precipitates by Zr addition are the initiation place for micrcraking and the cause of fatigue life reduction .

The aim of this research is to determine the beta transus temperature of two Ti-6242 alloys and to study the effect of different amount of alloying elements on alpha and beta phase stabilization by microstructural observation and hot torsion test. Determination of beta transus temperature has an important influence on designing thermomechanical and heat treatment cycles of titanium alloys. Hence, this is an effective parameter to control mechanical properties of two phase titanium alloys. In this regard, hot torsion tests were performed at temperature range of 960 °C to 1090 °C using the strain rate of 0.001 s-1 by cooling with the rate of 0.5 °C/s. besides, in order to deduce the temperature of grain boundary alpha phase nucleation and microstructural evolution, the specimens were heat treated at temperature range of 980 °C to 1020 °C for 40 minute and water quenched. It was found that, 10 % increase in Aleq/Moeq ratio, makes a 5 °C increase in beta transus temperature of the Ti-6242 alloy. Accordingly, this temperature was estimated in the temperature range of 1000 °C to 1010 °C for these alloys. Furthermore, there is a 10 °C deviation in the hot torsion test results and microstructural analysis, which is attributed to dynamic strain-induced transformation phenomenon.

The age-hardening curves of hardness measurements obtained for Ni-Span C 902 superalloy under different amounts of cold work, aging temperatures and times showed leveling and pronounced oscillations, indicating instability and reflecting a competition between the effect of sub-structure coarsening and the effect of solute drag and precipitation hardening. An artificial neural network (ANN) was used to model the nonlinear relationship between the parameters of the aging process and the corresponding hardness measurements. The predicted values of the ANN are in accordance with the experimental data. Results showed that the non-deformed and 50 pct cold rolled alloy exhibited a maximum hardness at a tempering parameter of 22 and 21, respectively.

Pzyrygrm flexible base alloy Fe-36Ni in the cast condition and the presence 05/0 and 04/0 wt% wt% aluminum, titanium in the temperature range ° C1150-850 with strain rate s-101/0 were studied. Microstructural studies showed that the addition of aluminum, aluminum-containing deposits such as aluminum oxide structure significantly increases. This strain has been focusing deposits suitable locations, as well as Qflshdn grain boundaries that are leading to the loss of flexibility. Titanic also formed deposits of titanium dioxide and titanium nitride grain size reduction and enhanced flexibility Pzyrygrm.

The hot ductility of IMI834 alloy in two conditions of first hot rolling and second hot rolling has been studied by conducting tensile tests with a strain rate of 0.1 s−1 and temperature range of 800–1000 °C. In first hot rolling condition, IMI834 alloy showed little hot ductility in the temperature range 800–900 °C and ductility loss occurred at 850 °C. In second hot rolling condition, hot ductility increased at all temperatures and ductility loss is not occurred. Especially at 900 °C, hot ductility increased from 31% in first hot rolling to 95% in second hot rolling.