Papers by Keyword: Precipitation Kinetics

Abstract: As aerospace engines advance to obtain higher thermal efficiencies, it is imperative to develop high temperature materials. Inconel 718 is a nickel-based superalloy that has been used for decades in aero-engine parts as it allows for use in high temperature applications. ATI 718Plus is a newer nickel-based superalloy that has been developed with a 55°C higher temperature capability over Inconel 718. ATI 718Plus components are manufactured by forging a wrought billet in stages to obtain the desired geometry and microstructure. Parts are heat treated to optimised proportions of γ’ and η phases. η phase is an acicular phase that precipitates on the grain boundaries, whereas γ’ is the primary strengthening phase. η phase is an important phase to understand as it is utilised in controlling the grain size during hot working processes at temperatures below its solvus temperature. When η phase is fully solutioned, the grain size is free to increase and hence the material mechanical properties can become detrimental. The short-term precipitation kinetics of η phase in strain-free ATI 718Plus is still not completely understood. In this study, the aims and objectives were to study the η precipitation kinetics in strain-free material as well as studying η phase precipitation in equilibrium conditions. TTT diagrams were produced for the η phase in strain-free material and compared to the limited data available in the open literature. In addition, the equilibrium η phase content, aspect ratio, length and width were determined and compared to the very little data that is currently published.

Abstract: Laser Beam Powder Bed Fusion (LBPBF) process has a unique feature termed as IntrinsicHeat Treatment (IHT), where solidified layers undergo series of heating and cooling (during thesubsequent building of a part). Thus, the LBPBF process offers the opportunity for the formation of microstructuralfeatures, which can have the potential to transform the mechanical properties of the part.In the case of AlMgSc alloy, L12 phase Al3Sc precipitates are thermodynamically favored to nucleatein the Al matrix due to coherency. After post-process analysis, it is evident that Al3Sc precipitatesformed during the LBPBF process, but it is unlikely to monitor (in-situ) the kinetics of precipitation.Therefore, based on inputs from the thermal model, the simulation of precipitation kinetics during theLBPBF process (IHT) is performed. The rapid heating and cooling cause the formation of new vacancies,where Al3Sc precipitates can nucleate and grow. The KWN model based on solid-state phasetransformation is used for modeling of precipitation kinetics. The thermal data at two locations in apart is collected and used to determine the average radius, number density, and volume fraction ofprecipitates. It is found that the IHT does not influence precipitation kinetics, and has no potential toalter the spatial properties of the part.

Abstract: The development of simulation tools for bridging different scales are essential for understanding complex joining processes. For precipitation hardening, the Kampmann-Wagner numerical model (KWN) is an important method to account for non-isothermal second phase precipitation. This model allows to describe nucleation, growth and coarsening of precipitation hardened aluminum alloys based on a size distribution for every phase which produces precipitations. In particular, this work investigates the performance of a KWN model by [1-3] for Al-Mg-Si-alloys. The model is compared against experimental data from isothermal heat treatments taken partially from [2]. Additionally, the model is used for investigation of the precipitation kinetics for a laser beam welding process, illustrating the time-dependent development of the different parameters related to the precipitation kinetics and the resulting yield strength.

Abstract: Nanosteels are used in automotive applications to accomplish resource-efficiency while providing high-tech properties. Quantitative data and further understanding on the precipitation kinetics in Nanosteels can contribute to fulfil this goal. Small-Angle Neutron Scattering measurements are performed on a Fe-C-Mn-V steel, previously heat-treated in a dilatometer at 650°C for several holding times from seconds to 10 hours. The evolution of the precipitate volume fraction, size distribution and number density is calculated by fitting the experimental Small-Angle Neutron Scattering curves. The effect of phase transformation on precipitation kinetics is also discussed. Complementary Transmission Electron Microscopy, Scanning Electron Microscopy and Inductively Coupled Plasma Optical Emission Spectroscopy measurements are performed to support the Small-Angle Neutron Scattering data analysis.

Abstract: ATI 718Plus components are manufactured by forging a wrought billet in stages to obtain the desired geometry and microstructure. Parts are then heat treated to optimized proportions of γ’ and η phases. η phase is a plate-like phase that precipitates on the grain boundaries of ATI 718Plus, similar to δ phase in Inconel 718. However, the complete kinetic behaviour of η phase precipitation during forging and heat treatment is still not fully understood. This paper investigates the effects of strain hardening on η phase precipitation kinetics in ATI 718Plus. This is achieved through the use of isothermal hot compression tests and heat treatment. Strain hardening was found to affect the η precipitation kinetics considerably. The results reported are a contribution to a fuller understanding of this important process

Abstract: Nanoscale cementite can be obtained in 0.17% carbon steel during isothermal treatment at 500^oC after ultra fast cooling (UFC) and thermo-mechanical treatment. The precipitation strengthening contribution to yield strength was more than 250 MPa, when the heat treatment time was less than 20 min. The carbon diffusion is impacted by Mn and Si, which are redistributed during the precipitation process. All the effects induced by substitutional elements can be manifested through the restricted carbon diffusion, which is equal to the carbon diffusion multiplied by adjustable parameters. Based on this, a kinetic model has been adapted to simulate the precipitation behaviors of cementite involving the evolution of the number density per unit volume, radius of cementite over time, and the evolution of carbon concentration in matrix. An excellent agreement in mean radius of particles between the predictions of the model and experimental observations was obtained. It was found that the nucleation period of cementite was very short and did not exceed 0.2 s, and there was an overlap between the nucleation period and the growth period, and the coarsening period began at about 1s. In the growth stage, the carbon concentration in the matrix dropped rapidly and the mean radius of particles increased quickly. In the coarsening stage, the carbon concentration remained unchanged and the number of particles per unit volume fell sharply.

Abstract: Based on the TTP curves of 7050 alloy, and the continuous cooling curves of 7B50 alloy at different positions of Φ70 mm improved Jominy specimen, the hardness distribution along the thickness direction of 7B50 alloy thick plates was analyzed and predicted by means of the isothermal precipitation kinetics and the quench factor analysis method. The results show that when 7050 alloy is isothermal treated at 200°C~400°C, the exponent n in its Johnson-Mehl-Avrami equation is close to 1, which indicates that the nucleation process of new precipitates is stable. In this equation the coefficient k is 7.420E-03 at 350°C, which indicates that the nucleation and growth rates of new precipitates are very fast. The hardness distribution along the axial direction of the improved Jominy specimen of 7B50 alloy is predicted by the quench factor analysis method. When the distance is no more than 65 mm from the spraying surface of the improved Jominy specimen, the deviation between the predicted and measured hardness of 7B50 alloy in T6 temper is less than 5%. The quench factor analysis method is feasible to predict the hardness distribution along the thickness direction of 7B50 alloy thick plates after quenching and aging. When the quench factor analysis method is extended to predict the actual water spray quenching process of 7B50 alloy thick plates, the average cooling rate is 21.6°C/s in the quench sensitive temperature range of this alloy, at 15 mm from the spraying surface of the plate. At the same position, the corresponding quench factor is equal to 6 and the predicted hardness is 187.4 HV which is equivalent 98.5% of the H_max (the maximum hardness) of 7B50 alloy in T6 temper.

Abstract: The precipitates and precipitation kinetics of super austenitic stainless steel with high Mo and N (HHSASS) were investigated by optical microscope (OM), scanning electron microscope (SEM) and quantitative metallography method. The results show that the TTP curves are C-shaped, the “nose” temperatures of precipitation are found to be 1000°C with the incubation periods of 120s and 600s, respectively. At 1000°C, some precipitates form as ellipsoidal-shaped and connect along the grain boundaries first. Then a few precipitates start forming as needle-shaped within austenite grains. Until aging for 300min, the field is filled with needle-shaped precipitates. The main precipitates in HHSASS are Sigma phase and Chi phase that are rich in Cr and Mo. The precipitates on the grain boundaries are ellipsoidal-shaped and those in the austenite grains are needle-shaped. About the structures of precipitates need to be further researched.

Abstract: Predicting yield strength of the cast is difficult, mainly due to inherent chemical inhomogeneity of the microstructure and metal matrix composite nature of the cast. In our approach to predict the yield strength of as cast material AlSi9Cu3(Fe), Scheil-Gulliver model has been used to calculate the phase fraction and chemical composition of each phase during solidification and at each temperature step. Inhomogeneity of the microstructure has been taken into account by considering the evolution of pre-eutectic and eutectic fractions separately. The solidification time-temperature data and cooling to room temperature are recorded using thermocouples and serve as input for the thermo-kinetic software “MatCalc”, that has been used for Scheil simulation and takes into account the evolution of microstructure after solidification and during any arbitrary cooling rate. The strengthening model takes into account the contributions of the intrinsic yield strength of the aluminum matrix, solid solution strengthening, precipitation hardening, effect of eutectic silicon portion and dendrite arm spacing size effect. The phases taken in to consideration include α-Al, Intermetallics, Si and Cu-rich precipitates. The predicted yield strength values are validated by comparing with the experimental values. This approach is extendable to calculate yield strength of the as-cast and heat-treated aluminum alloys.

Abstract: High formability is required to stamp aluminium into complex structural automotive components such as the A-pillar and B-pillar. Formability of an Al-Zn-Mg (AA7xxx) alloy sheet is characterised through hot stamping a prototype part and simulating paint baking procedures. The precipitation behaviour is assessed by differential scanning calorimetry and the tensile properties measured between 350°C and 475°C over a range of strain rates from 0.01s^-1 to 1s^-1. Natural ageing was found to increase the hardness of the hot stamped parts due to the formation of GP zone precipitates. A simulated three step paint baking procedure produced η precipitates and resulted in a yield strength and ultimate tensile strength of 480MPa and 512MPa, respectively.