Iranian Journal of Materials Science and Engineering

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This study was launched to investigate the effects of heating rate and aging parameters on the kinetic of precipitation reactions in a high alloy high strength steel having Ni, Co, Mo and Ti. For this purpose, as quenched specimens were subjected to three types of aging methods with different heating rates. These methods consisted of aging in Pb bath, salt bath, and furnace at different aging cycles. The kinetic of precipitation in each method was studied by hardness measurements and was described adequately by the Johnson-mehl-Avrami equation. Remarkable increase in hardness and its rate is observed when the rate of heating increases. The substantial increase in hardness of the specimens aged rapidly in salt & Pb baths, compared with those aged normally in furnace, seemed to be due to the formation of thermo elastic stresses during sudden expansion of the substance subjected to rapid heating. According to the results obtained in this research, increase in the Avrami constants, n & k, and decrease in the start time of transformation, ts, are associated with heating rate increasing. Analysis of the observed and calculated data for hardness using Arrhenius equation, shows that for the same amount of volume fraction of precipitates, the activation energy of precipitates decreased for f=25 and 50%, while at f=90 % it increased by increasing heating rate.

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In this research the influence of Ti contents on the amounts of inclusions formation and mechanical properties of a high alloy high strength steel, C300, has been investigated. For this purpose several bars were casted under the same solidification conditions, but different amounts of Ti element. All the seven casted bars were homogenized at 1200°C for a period of 2 hours. Then, they were immediately hot rolled after homogenization so that the out rolling temperature was kept in the range of 1000-1200° C. The specimens were then solution annealed at 820°C for hour and finally they were aged for a period of 3 hours at 500°C. The samples were subjected to tensile, impact and hardness tests in order to relate the variation in volume percent of inclusions due to different amount of Ti, to mechanical properties. The results showed that by increasing the amount of Ti a serious decline in toughness properties of the alloy due to increase in inclusion population occurred. So this research provides a very useful information about the relation between volume fraction of inclusions and mechanical properties of a C300 high strength steel.

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Abstract: The investigation is carried out to characterize welding of AISI 310 austenitic stainless steel to Inconel 657 nickel-chromium superalloy. The welds were produced using four types of filler materials: the nickel-based corresponding to Inconel 82, Inconel A, Inconel 617 and austenitic stainless steel 310. This paper describes the effects of aging treatment on the joint. The comparative evaluation was based on microstructural features and estimation of mechanical properties. While Inconel A exhibited highest thermal stability and mechanical properties (hardness and ultimate strength), Inconel 82 weld metal also showed good thermal stability and mechanical properties. On the other hand, welds produced with Inconel 617 and 310 SS filler materials showed weak thermal stability and failed in the weld metals. It is therefore concluded that for the joint between Inconel 657 and 310 stainless steel, Inconel A and Inconel 82 filler materials offered the best compromises, respectively.

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Abstract: Guinier-Preston (GP) zone formation and precipitation behavior of T1 (Al2CuLi) phase during the ageingof an Al-Cu-Li-Zr alloy was studied by differential scanning calorimetry (DSC) technique and electrical resistancemeasurement of the samples. Results show that endothermic effects in the thermograms of the alloy between 180°Cand 240°C can be related to the enthalpy of GPzones dissolution. Formation of GPzones in the structure increasedhardness, tensile strength and electrical resistance of the Al-Cu-Li-Zr alloy. Furthermore, precipitation of T1 phaseoccurred in temperature range of 250ºC to 300ºC whereas its dissolution occurred within the temperature of 450-530ºC. Activation energies for precipitation and dissolution of T1 phase which were determined for the first time inthis research, were 122.1(kJ/mol) and 130.3(kJ/mol) respectively. Results of electrical resistance measurementsshowed that an increase in the aging time resulted in the reduction of electrical resistance of the aged samples.

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Nowadays, Ni-free austenitic stainless steels are being developed rapidly and high price of nickel is one of the most important motivations for this development. At present research a new FeCrMn steel was designed and produced based on Fe-Cr-Mn-C system. Comparative studies on microstructure and high temperature mechanical properties of  new steel and AISI 316 steel were done. The results showed that new FeCrMn developed steel has single austenite phase microstructure, and its tensile strength and toughness were higher than those of 316 steel at 25, 200,350 and 500°C. In contrast with 316 steel, the new FeCrMn steel did not show strain induced transformation and dynamic strain aging phenomena during tensile tests that represented higher austenite stability of new developed steel. Lower density and higher strength of the new steel caused higher specific strength in comparison with the 316 one that can be considered as an important advantage in structural applications but in less corrosive environment

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In this study, the microstructural variations, mechanical properties and forming limit diagrams (FLD) of Al 2024 aluminum alloy sheet with the thickness of 0.81mm are investigated during natural ageing (T4) treatment. The most formability in Al 2024 can be achieved just after solution treatment, and it is better to perform the forming process, on this aluminum alloy sheet, in this condition. However, in industrial applications, there is usually a postponement for some hours after solution treatment to begin the forming process that it means the forming process should be done at the natural ageing condition. This condition decreases the formability of Al 2024 sheets. To monitor the properties variations in natural ageing condition, FLDs are determined after specific times (e.g., 0.5, 1.5, 4 and 24 hours). The variations in micro-hardness, yield strength, ultimate tensile strength and elongation at break are observed with changing the ageing time. The scanning electron microscope (SEM) investigations illustrated that density and size of precipitates are changed with ageing time. Moreover, the Nakazima test is utilized to study the forming limits considering the natural ageing condition. Results show by increasing the ageing time, up to 4hr, the majority of properties variations could be seen, and from 4hr to 24hr, the variations are changed slower.

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In this paper, the effect of two-step precipitation hardening on the mechanical properties of Al-3.7Cu-1Mg was investigated. For this meaning, some specimens were subjected to the first step aging at 175, 190 and 205°C for 2 h, once the samples solution treated at 500°C. To have stable precipitates uniformly distributed in the microstructure and to reduce the heat treatment time, the second step was implied at 65°C. The tensile and hardness tests were performed at ambient temperature immediately after aging. The results indicated that depending on the first step temperature, the second aging time affects the alloy mechanical behavior in different aspects. A factor named SNMP introduced to determine the cycle giving the best mechanical properties. The strength and elongation increase 1.5 and 2 times respectively; compared to the values reported in the DIN EN 755-2 standard by performing the two-step aging cycle, consisting of the first-stage at 175°C and the second step at 65°C for 10 hours. Moreover, using the proposed two-step aging, the heat treatment time was reduced considerably compared to the conventional precipitation hardening process.

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The hot deformation behavior of the heat-treated AA6061 and AA 6063 aluminum alloys by T6-1, T6-2 artificial aging treatment, and O annealing treatment were studied by compression testing over a temperature range of 350–550  and strain rates of 0.005-0.1 s^-1. It was observed that the flow stresses of the studied aluminum alloys treated by the T6-1 and T6-2 heat treatments were significantly higher than those of the O annealing treatment. Moreover, the stress-strain curves of the heat-treated alloys by the T6-1, T6-2, and O heat treatments demonstrated significant softening during deformation at the lowest strain rate under any of the deformation conditions. For several strains, the activation energy of hot deformation was specified and obtained to vary significantly with strain for the heat-treated alloys by the T6-1 and T6-2 treatments. The stress-strain data calculated from a linear equation, with strain-dependent parameters, shows a great fit with the experimental data for the heat-treated aluminum alloys.

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The effect of the milling time & ageing on the hardness, density, and wear characteristics of Al 7150 alloy specimens made via powder metallurgy has been studied. The different constituents of Al 7150 alloy were processed in a planetary ball milling set up with a BPR of 10:1 for 5 hours, 10 hours, and 20 hours. At 400 °C, the milled powders were subsequently hot compacted in a punch die setup. The hot-pressed specimens were solutionized initially, then aged artificially at 115 °C for 3, 6, 12, 24, 30, 45, 60, and 96 hours. The relative density was inversely proportional to the milling time. Microhardness tests showed a maximum VHN of 255 was measured for the 24 h aged T6 specimens produced from 20 h milled powders whereas the non-aged specimens, made from unmilled 7150 alloy powders showed a VHN of 40. However the samples showed a decline in microhardness beyond 24 h of ageing. Under various conditions of sliding distance and loading conditions, the samples subjected to T6 aging showed a reduced volumetric wear rate indicating the beneficial effect of artificial aging up to 24 hours. The volumetric wear rate gradually declined for the samples aged beyond 24 hours of aging. The HRTEM studies revealed a high density of uniformly scattered (MgZn₂) precipitates in the base matrix, as well as (MgZn₂) phases precipitating along grain boundaries. The presence of such second phase precipitates in the matrix improved the wear characteristics of the alloy matrix. The results showed that optimization of process parameters such as milling time, ageing as well as reducing the particle size of the base powders, the hardness and wear behavior of Al 7150 alloy may be improved.

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Microstructure evolution and mechanical properties of Zn-22Al alloy after post-ECAP natural/artificial aging were investigated. A homogenization treatment was applied to the casting samples. In addition, after preparing the samples for the ECAP, secondary homogenization treatment was done and then the samples quenched in the water to form a fine grain structure. After 8 passes of ECAP, some ECAPed samples were naturally aged and some ECAPed samples were artificially aged. Natural aging after 8 passes of ECAP showed that Zn-22Al alloy has a quasi-stable microstructure because limited grain growth occurred. Two-phase structure of Zn-22Al alloy prevented excessive grain growth after natural aging. On the other hand, artificial aging after 8 passes of ECAP caused a relatively much grain growth took place. In shorter times of artificial aging, the grain growth rate is faster due to the high surface energy of grain boundaries. On the contrary, as the time of artificial aging increased, the surface energy of grain boundaries decreased, which leads to a decrease in the grain growth rate. In addition, texture evolution was studied after aging artificial. Therefore, the main texture of α and η phases was determined.
 

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Phase transformations and the evolution of hardness during elevated-temperature annealing of Inconel 718 superalloy manufactured by the laser powder bed fusion (L-PBF) were investigated. The microstructural evolution, elemental analysis, phase formation, and hardening were characterized by scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and Vickers indentation test, respectively. It was observed that the effect of annealing treatments is directly governed by the annealing parameters (i.e. time and temperature), for which the hardness measurement as a fruitful and convenient tool can reveal this effect. The increase of the hardness, which was obtained by the annealing (aging) treatments at the temperature range of 800-900 °C, indicated precipitation of the Ni₃Nb γ˝ strengthening phase; while owing to the coarsening of precipitates as a results of overaging at this temperature range, the hardness decreased. For instance the length and aspect ratio of precipitates in the aged sample at 800 °C for 1 h is 67.14 nm and 0.32, respectively; while these values in the aged sample at 800 °C for 8 h is 78.34 nm and 0.44, respectively. On the other hand, the decrease of the hardness at temperatures of 950 and 1000 °C was attributed to the decrease of dislocation density in conjunction with the Ni₂Nb Laves phase dissolution. Hence, it is crucial to determine the annealing parameters according to the required microstructure and properties.