Papers by Keyword: Microstructural Evolution

Abstract: Post-weld heat treatment (PWHT) was investigated to evaluate its effects on dissimilar friction stir welded (FSWed) T-joints of AA6061 and low carbon steel. The non-PWHT joint was compared with four PWHT conditions involving solution treatment, quenching, natural aging, and subsequent artificial aging at 0-12 hours. Microstructural characterization revealed a largely continuous Al/steel interface in the non-PWHT joint, while PWHT promoted interfacial cracking and modified precipitation behavior in the stir zone and heat-affected zone of AA6061. Hardness increased monotonically with aging time, reaching ~95–100 HV after PWHT artificial aging at 12 hours. Tensile strength peaked at 212MPa after 4 hours of artificial aging, while maximum strain decreased from ~9% to ~5.3% after 12 h artificial aging, indicating ductility loss under prolonged aging. Fracture location after PWHT consistently occurred at SZ, highlighting a critical failure region governed by joint geometry and microstructural heterogeneity.

Abstract: The effects of minor additions of alloying elements Fe/Ni/Co (0.5wt.%), B (0.01–0.1wt.%), and Y (0.2wt.%) on the superplastic behavior, microstructural evolution and mechanical properties of Ti-4 wt.%Al-3wt.%Mo-1wt.%V alloys are investigated. By increasing the high-angle grain boundary fraction and related facilitation of the grain boundary sliding, these elements reduce the flow stress values at the initial deformation stage and improve flow stability at a steady state. The most pronounced effect is found at low deformation temperatures when acceleration of recrystallization and globularization of the microstructure is critical. As a result, minor additions of the studied elements provide good superplasticity at relatively low temperatures of 625–775 °C (m≈0.50 and elongation to failure ≈ 500–1000%) and post-forming room-temperature strength (YS≈830 MPa and UTS≈990 MPa).

Abstract: In this paper, the high temperature flow behaviors of 6061 Al alloy was studied by thermal compression experiments. The effects of temperature, strain rate and strain on the microstructure evolution and flow behavior of the alloy were investigated by experiments. The results show that the flow stress of the alloy increases with the increase of strain rate and it decreases with the increase of deformation temperature. The flow curve reaches the dynamic equilibrium under the interaction of work hardening and dynamic softening mechanism. The uprising deformation temperature promotes thermal excitation dynamic recrystallization of deformed microstructure. With the increase of strain, the microstructure of the alloy is transformed from equiaxed crystal morphology to fibrous structure and strain-induced dynamic recrystallization occurs. As strain rate increases, the action time of dynamic softening mechanism for the studied alloy is reduced, resulting in the fraction of dynamic recrystallized structure is reduced and the flow stress increases.

Abstract: Aluminum alloys have been attracting significant attention. Especially Al-Mg-Si alloys can exhibit an excellent balance between strength and ductility. Deformation mechanisms and microstructural evolution are still challenging issues. Accordingly, to describe how the type of phase influence mechanical behaviour of Al/Mg/Si alloys, in this paper atomic simulations are performed to investigate the uniaxial compressive behaviour of Al-Mg-Si ternary phases. The compression is at the same strain rate (3.10¹⁰ s−1); using Modified Embedded Atom Method (MEAM) potential to model the deformation behaviour. From these simulations, we get the total radial distribution function; the stress-strain responses to describe the elastic and plastic behaviors of GP-AlMg₄Si₆, U2-Al₄Mg₄Si₄ and β-Al₃Mg₂Si₆ phases. For a Detailed description of which phase influence hardness and ductility of these alloys; the mechanical properties are determined and presented. These stress-strain curves obtained show a rapid increase in stress up to a maximum followed by a gradual drop when the specimen fails by ductile fracture. From the results, it was found that GP-AlMg₄Si₆ & U2-Al₄Mg₄Si₄ phases are brittle under uniaxial compressive loading while β-Al₃Mg₂Si₆ phase is very ductile under the same compressive loading. The engineering stress-strain relationship suggests that β-Al₃Mg₂Si₆ phase have high elasticity limit, ability to resist deformation and have the advantage of being highly malleable. Molecular dynamics software LAMMPS was used to simulate and build the Al-Mg-Si ternary system.

Abstract: Aluminum alloys development always exit in the manufacturing process. Al/Mg alloys have been attracted significant attention because of their excellent mechanical properties. The microstructural evolution and deformation mechanisms are still challenging issues, and it is hard to observe directly by experimental methods. Accordingly, in this paper atomic simulations are performed to investigate the uniaxial compressive behavior of Al/Mg phases; with different ratio of Mg ranging from 31% to 56%. The compression is at the same strain rate (3.10¹⁰ s⁻¹), at the same temperature (300K) and pressure, using embedded atom method (EAM) potential to model the interactions and the deformation behavior between Al and Mg.From these simulations, we get the radial distribution function; the stress–strain responses to describe the elastic and plastic behaviors of β-Al₃Mg₂, ε-Al₃₀Mg₂₃, Al₁Mg₁ and γ-Al₁₂Mg₁₇ phases with 31, 41, 50 and 56% of Mg added to pure aluminum, respectively. The mechanical properties, such as Young’s modulus, elasticity limit and rupture pressure, are determined and presented. The engineering equation was used to plot the stress-strain curve for each phase.From the results obtained, the chemical composition has a significant effect on the properties of these phases. The stress-strain behavior comprised elastic, yield, strain softening and strain hardening regions that were qualitatively in agreement with previous simulations and experimental results. These stress-strain diagrams obtained show a rapid increase in stress up to a maximum followed by a gradual drop when the specimen fails by ductile fracture. Under compression, the deformation behavior of β-Al₃Mg₂ and γ-Al₁₂Mg₁₇ phases is slightly similar. From the results, it was found that ε-Al₃₀Mg₂₃ phase are brittle under uniaxial compressive loading and γ-Al₁₂Mg₁₇ phase is very ductile under the same compressive loading.The engineering stress-strain relationship suggests that β-Al₃Mg₂ and γ-Al₁₂Mg₁₇ phases have high elasticity limit, ability to resist deformation and also have the advantage of being highly malleable. From this simulation, we also find that the mechanical properties under compressive load of ε-Al₃₀Mg₂₃ phase are evidently less than other phases, which makes it the weakest phase. The obtained results were compared with the previous experimental studies, and generally, there is a good correlation.The Al-Mg system was built and simulated using molecular dynamics (MD) software LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator).

Abstract: Intermediate temperature creep properties are considered a key indicator of single crystal superalloys used for turbine blades of aircraft engines. The interrupted and ruptured creep tests were carried out in a second generation single crystal superalloy under the conditions of 760°C/785MPa. The creep rupture life as well as minimum creep rate were also in the same level of those in CMSX-4 and PWA1484. The microstructural evolution at different creep stages were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed the γ’ phases kept the cuboid morphology mostly until the creep rupture, and super lattice stacking faults (SSFs) extended along [-1 1 0] and [-1-1 0] orientations within the γ’ precipitate were the typical dislocation configuration.

Abstract: The overtemperature in a vane made of cast K465 superalloy is investigated in this paper. The vane was taken from a K465 guide ring after the service for specific period of time. The relationship between the temperature and content of γ´ precipitates was established for the K465 superalloy, which was then used to make the microstructural evolution analysis on failed turbine vane associated with overtemperature. The content of γ´ precipitates in dendritic region and the volume fraction of melting zone in interdendritic region in K465 superalloy were used as the microstructural parameters to check the overtemperature. It is found that there is a sound relationship between the parameters and the temperature. On the basis of this relationship, the microstructural evolution along with temperature variation of the overtemperature K465 turbine vane can be analyzed and the overall service temperature which the K465 guide ring experienced can be evaluated.

Abstract: In this paper, the microstructural evolution and mechanical properties of the as-cast Fe-25Mn-7Al-1.3C austenitic steel after different heat treatment were investigated. After solution treatment and subsequent aging treatment, the κ-carbides with perovskite structure were found to precipitate coherently within the austenite matrix, which improved the initial hardness and mechanical strength. The experimental steel exhibited an optimal comprehensive performance after being solution treated at 1050 °C for 1 h and then aged at 550 oC for 2 h. The tensile strength was 751 MPa, the yield strength was 581 MPa, the elongation was 48%, the hardness was 252 HB, and the Charpy V-notch impact toughness was 168 J, respectively. The impact wear test was carried out on MLD-10 abrasive wear testing machine, and the worn out surfaces under different heat treatment were characterized by scanning electron microscopy (SEM). The results indicated that the abrasion resistance of the steel under the additional aging treatment was better than that of the as-solutionized steel. The optimal abrasion resistance was obtained after being soluted at 1050 °C for 1 h and then aged at 550 oC for 2 h. However, with the aging time increasing, the coarse κ-carbides precipitating around the grain boundaries would deteriorate toughness, which lead to increase of the abrasive wear volume loss. Besides, obvious micro-cracking and relatively larger peeling pit were observed.

Abstract: In addition to precipitation strengthening effect as Cu precipitates in steel, Cu-containing steel is generally believed to have ability to resist corrosion. However, there are some issues, e.g. strengthening mechanism and appropriate copper content, remaining to be clarified. In this presentation, an offshore platform steel Fe-0.08C-1Mn-2.5Ni was used as experimental materials and the emphasis was put on the comparative study of Cu-addition in terms of microstructural evolution and mechanical properties by using microstructural characterization and mechanical properties measurement. The continuous cooling transformation behavior of the two Cu-containing steels with varying contents was first compared. The Cu-containing precipitates were then determined by transmission electron microscopy (TEM) in the hot rolled steel plate after quenching and tempering. Finally, the effect of Cu-addition on precipitation strengthening and low temperature toughness was discussed considering the interaction of Cu precipitates with dislocation and martensite microstructure.

Abstract: Titanium alloys are widely used in the aircraft industry. Under sheets form, they can be employed to the manufacturing of pylon or engine parts. With the aim of a cost reduction, this study proposes to act on the starting microstructure so as to improve the mechanical properties during the forming stages. In the present study, investigations are focused on Ti-6Al-2Sn-4Zr-2Mo (Ti6242) alloy specially used for the hot areas (e.g. parts close to the engine or the combustion chambe...). Presently, an important mechanical test campaign was performed on Ti6242 alloy, it examines, on the one hand, the microstructure qualified by the aircraft industry and, on the other hand, a new range of refined microstructures obtained by hot straining process. For each test, microstructural observations exhibited complex phenomena including simultaneously both grain growth and dynamic recrystallization. The occurrence, sequencing and coupling of the mechanisms, strongly depend on the starting microstructure and the test conditions (time-temperature and strain rate) investigated. They are not easy to understand and require further tests and observations. In such a framework, the implementation of mechanical models are efficient and relevant to promote a better knowledge of the microstructural evolution observed and their influence on the mechanical behavior.