AW-7075-T6 sheet for shock heat treatment forming process

The forming behaviour of AW-7075-T6 sheet was studied across a range of shock heat treatment (SHT) temperatures of 200-480 °C. After SHT, formability of the samples was investigated by tension and deep drawing tests at room temperature. Differential scanning calorimetry (DSC) was used to study the precipitation states of the AW-7075 sheet in the as-received and shock heat treated conditions. Formability was started to improve with increasing shock heat treatment temperature from 300 °C onwards. Strain hardening resulted from the dissolution of η′ precipitates and the coarsening of remaining precipitates were found to contribute to the increase in formability at room temperature. Re-precipitation and coarsening of the precipitates were responsible for the post-paint baking strength of SHT samples.     

Microstructure and Mechanical Properties of Cu-Ag-Zr Alloy

The Cu-3Ag-0.5Zr alloy was produced by vacuum induction melting and subsequently processed through hot forging and rolling. Detailed microstructural characterization of solution-treated (ST) specimen shows three types of phases: Cu matrix, zirconium-rich phase, and Cu-Ag-Zr intermetallic phase. Transmission electron microscopy studies together with energy-dispersive x-ray spectroscopy analysis established the presence of Zr-rich large particles in the ST condition. Aging at 450 °C for 4.5 h after solution treatment resulted in the formation of uniformly distributed fine spherical silver precipitates with an average diameter of 9.0 ± 2.0 nm. Consequently, room temperature yield strength (YS) and ultimate tensile strength (UTS) of the aged specimen increased by 110% and 15%, respectively, compared to those of 120 and 290 MPa of the ST specimen. At elevated temperature, the YS decreased to 146 and 100 MPa at 540 and 640 °C, respectively, for the aged sample. On the contrary, the YS increased to 140 MPa at 540 °C, and thereafter a decrease was observed with a value of 105 MPa at 640 °C for the ST sample. This decrease in YS at higher temperatures is attributed to coarsening of precipitates and dissolution of the precipitates, whereas an increase in YS is attributed to in-situ aging of the samples.     

Effect of Thermal Aging on Coarsening Kinetics of γ′ in Alloy 617

The effect of thermal aging on coarsening kinetics of alloy 617, a candidate material for heat exchanger of the very high temperature reactor, was experimentally studied at 750 and 950 °C for up to 5300 h. Formation of various precipitates such as μ-phase, M₂₃C₆ and γ′ phases and significant coarsening of the γ′ phase have been observed in the microstructure of the aged samples. Experimental observation was compared to alloy thermodynamic calculation and γ′-phase precipitation kinetics simulation. Thermal aging effect on the microstructural evolution and mechanical behavior of alloy 617 was then discussed based on experimental and microstructural modeling results.     

Annealing and Test Temperature Dependence of Tensile Properties of UNS N04400 Alloy

Effects of annealing and test temperatures on the tensile behavior of UNS N04400 alloy have been examined. The specimens were annealed at 800, 1000, and 1200 °C for 4 h under vacuum in a muffle furnace. Stress-strain curves of the specimens were obtained in the temperature range 25-300 °C using a universal testing machine fitted with a thermostatic chamber. The results indicate that the yield strength (YS), ultimate tensile strength (UTS), and percentage elongation of the specimens decrease with increase of annealing temperature. By increasing the test temperature, the YS and UTS decrease, whereas the percentage elongation initially decreases with increase of test temperature from 25 to 100 °C and then increases with further increasing the temperature up to 300 °C. The changes in the tensile properties of the alloy are associated with the post-annealing microstructure and modes of fracture.     

Isothermal Treatment Effects on Precipitates and Tensile Properties of an HSLA Steel

The relationships between tensile properties and precipitates of a high-strength low-alloy steel depending on the isothermal conditions were investigated. While the isothermally treated steel at 300–500 °C for 1 and 24 h had no significant difference, the steel treated at 500 for 336 h, denoted as 500–336 h, not only showed a decrease in tensile stress but also exhibited a highly increased elongation. Transmission electron microscopy and atom probe tomography were utilized to evaluate the precipitates distribution. The results showed that, in the case of 500–336 h sample, the fraction of precipitates with a radius over 10 nm is the highest and that of a few nano-sized precipitates is the lowest among all samples. It can be explained that the coarsening of originally nano-sized precipitates, occurred by diffusion of dissolved carbon in 500–336 h, mainly affects the tensile behavior.     

Coherency loss of Al3(Sc,Zr) precipitates by deformation of an Al–Zn–Mg alloy

Al alloys with additions of Sc and/or Zr exhibit a reasonably stable grain structure due to a uniform distribution of coherent Al₃(Sc,Zr) precipitates that forms at temperatures >300 °C. These precipitates are stable up to the solution treatment temperature and are able to pin subgrain and grain boundaries, inhibiting grain coarsening. The crystallographic structure of these precipitates presents a L1₂ superstructure coherent with the face-centred cubic Al matrix. Changes in the orientation relation between precipitates and the matrix are described in deformed, recovered and partially recrystallized samples of extrusions of AW7010 (AlZn6Mg2Cu2). The coherency of the intracrystalline Al₃(Sc,Zr) precipitates present in the extrusions is lost by severe deformation performed by an equal channel angular pressing process, which produced a fine-grained microstructure. The deformed sample recovers, forming a subgrain structure with restored coherency of the Al₃(Sc,Zr) precipitates. Rapid heating to 470 °C causes partial secondary recrystallization, which transforms the precipitates within the recrystallized grains into incoherent groups of particles that maintain their original orientation with each other.     

Tungsten Inert Gas and Friction Stir Welding Characteristics of 4-mm-Thick 2219-T87 Plates at Room Temperature and −196 °C

2219-T87 aluminum alloy is widely used for fabricating liquid rocket propellant storage tank, due to its admirable cryogenic property. Welding is the dominant joining method in the manufacturing process of aerospace components. In this study, the tungsten inert gas welding and friction stir welding (FSW) characteristics of 4-mm-thick 2219-T87 alloy plate at room temperature (25 °C) and deep cryogenic temperature (?196 °C) were investigated by property measurements and microscopy methods. The studied 2219 base alloy exhibits a low strength plane anisotropy and excellent room temperature and cryogenic mechanical properties. The ultimate tensile strength values of TIG and FSW welding joints can reach 265 and 353 MPa at room temperature, and 342 and 438 MPa at ?196 °C, respectively. The base metal consists of elongated deformed grains and many nano-scaled θ (Al₂Cu) aging precipitates. Fusion zone and heat-affected zone (HAZ) of the TIG joint are characterized by coarsening dendritic grains and equiaxed recrystallized grains, respectively. The FSW-welded joint consists of the weld nugget zone, thermo-mechanically affected zone (TMAZ), and HAZ. In the weld nugget zone, a micro-scaled sub-grain structure is the main microstructure characteristic. The TMAZ and HAZ are both characterized by coarsened aging precipitates and elongated deformed grains. The excellent FSW welding properties are attributed to the preservation of the working structures and homogenous chemical compositions.     

Static Recrystallization Behavior of Z12CN13 Martensite Stainless Steel

In order to increase the hot workability and provide proper hot forming parameters of forging Z12CN13 martensite stainless steel for the simulation and production, the static recrystallization behavior has been studied by double-pass hot compression tests. The effects of deformation temperature, strain rate and inter-pass time on the static recrystallization fraction by the 2% offset method are extensively studied. The results indicate that increasing the inter-pass time and the deformation temperature as well as strain rate appropriately can increase the fraction of static recrystallization. At the temperature of 1050-1150 °C, inter-pass time of 30-100 s and strain rate of 0.1-5 s^?1, the static recrystallization behavior is obvious. In addition, the kinetics of static recrystallization behavior of Z12CN13 steel has been established and the activation energy of static recrystallization is 173.030 kJ/mol. The substructure and precipitates have been studied by TEM. The results reveal that the nucleation mode is bulging at grain boundary. Undissolved precipitates such as MoNi₃ and Fe₃C have a retarding effect on the recrystallization kinetics. The effect is weaker than the accelerating effect of deformation temperature.     

Mechanical Behavior and Microstructure Evolution of Bearing Steel 52100 During Warm Compression

High-performance bearing steel requires a fine and homogeneous structure of carbide particles. Direct deformation spheroidizing of bearing steel in a dual-phase zone can contribute to achieving this important structure. In this work, warm compression testing of 52100 bearing steel was performed at temperatures in the range of 650–850°C and at strain rates of 0.1–10.0 s^?1. The effect of deformation temperatures on mechanical behavior and microstructure evolution was investigated to determine the warm deformation temperature window. The effect of deformation rates on microstructure evolution and metal flow softening behavior of the warm compression was analyzed and discussed. Experimental results showed that the temperature range from 750°C to 800°C should be regarded as the critical range separating warm and hot deformation. Warm deformation at temperatures in the range of 650–750°C promoted carbide spheroidization, and this was determined to be the warm deformation temperature window. Metal flow softening during the warm deformation was caused by carbide spheroidization.     

A yield strength model for the Al-Mg-Si-Cu alloy AA6111

A yield strength model is developed for the Al-Mg-Si-Cu alloy AA6111. The evolution of the strength of precipitates, as obstacles to dislocation motion, during various stages of aging is modeled according to the theories for strengthening mechanisms, as well as the microstructural and mechanical behavior of the alloy. The precipitation hardening component of yield strength is modeled for conditions where precipitates act as either strong or weak obstacles. The dislocation breaking angles for various stages of aging at 180 °C is estimated and the applicability of both strong and weak obstacle models examined. It is found that although the weak obstacle model could be a better choice for describing the very early aging stages and/or the low temperature processes, the entire aging period in the temperature range of 160–220 °C is well described by applying the strong obstacle model. The modeling results are related to the microstructural evolution in this alloy system.     

Structure and Properties of Ti-19.7Nb-5.8Ta Shape Memory Alloy Subjected to Thermomechanical Processing Including Aging

In this work, the ternary Ti-19.7Nb-5.8Ta (at.%) alloy for biomedical applications was studied. The ingot was manufactured by vacuum arc melting with a consumable electrode and then subjected to hot forging. Specimens were cut from the ingot and processed by cold rolling with e = 0.37 of logarithmic thickness reduction and post-deformation annealing (PDA) between 400 and 750 °C (1 h). Selected samples were subjected to aging at 300 °C (10 min to 3 h). The influence of the thermomechanical processing on the alloy’s structure, phase composition, and mechanical and functional properties was studied. It was shown that thermomechanical processing leads to the formation of a nanosubgrained structure (polygonized with subgrains below 100 nm) in the 500-600 °C PDA range, which transforms to a recrystallized structure of β-phase when PDA temperature increases. Simultaneously, the phase composition and the β → α″ transformation kinetics vary. It was found that after conventional cold rolling and PDA, Ti-Nb-Ta alloy manifests superelastic and shape memory behaviors. During aging at 300 °C (1 h), an important quantity of randomly scattered equiaxed ω-precipitates forms, which results in improved superelastic cyclic properties. On the other hand, aging at 300 °C (3 h) changes the ω-precipitates’ particle morphology from equiaxed to elongated and leads to their coarsening, which negatively affects the superelastic and shape memory functional properties of Ti-Nb-Ta alloy.     

Microstructure Stability During Creep of Friction Stir Welded AA2024-T3 Alloy

The poor weldability of the AA2024 aluminum alloy limits its use in industrial applications. Because friction stir welding (FSW) is a non-fusion welding process, it seems to be a promising solution for welding this alloy. In the current study, FSW was applied to butt weld AA2024-T3 aluminum alloy plates. Creep tests were conducted at 250 and at 315 °C on both the parent material and the friction stir welded specimens. The microstructures of the welded and non-welded AA2024-T3 specimens before and after the creep tests were studied and compared. A comprehensive transmission electron microscopy study together with a high-resolution scanning electron microscopy study and energy-dispersive x-ray spectroscopy analysis was conducted to investigate the microstructure stability. The parent material seems to contain two kinds of Cu-rich precipitates—coarse precipitates of a few microns each and uniformly dispersed fine nanosized precipitates. Unlike the parent material, the crept specimens were found to contain the two kinds of precipitates mentioned above together with platelet-like precipitates. In addition, extensive decoration of the grain boundaries with precipitates was clearly observed in the crept specimens. Controlled aging experiments for up to 280 h at the relevant temperatures were conducted on both the parent material and the welded specimens in order to isolate the contribution of exposure to high temperatures to the microstructure changes. TEM study showed the development of dislocation networks into a cellular dislocation structure in the case of the parent metal. Changes in the dislocation structure as a function of the creep strain and the FSW process were recorded. A detailed creep data analysis was conducted, taking into account the instability of the microstructure.     

Investigations on warm forming of AW-7020-T6 alloy sheet

7000 series aluminium alloys have greater strength than conventional aluminium alloys used in the automotive industry, but little has been reported on their formability. In this paper the strength and formability of age-hardenable AW-7020 alloy sheet in the T6 temper condition was investigated at temperatures between 150 and 250 °C by warm tensile, Swift-cupping and cross-die deep-drawing tests. Differential scanning calorimetry (DSC) investigations were carried out to study the precipitation state of AW-7020 sheet in as-received, warm cross-die deep-drawn and post-paint-baked conditions. Formability was found to improve at temperatures above 150 °C and was sensitive to temperature and strain rate. There was also an onset of dynamic recovery from 150 °C. DSC results showed the presence of η′ precipitates in T6 temper and that these coarsen during the warm cross-die deep-drawing and paint baking processes with ∼30% drop in ultimate tensile and yield strengths. Dynamic recovery and coarsening of η′ precipitates were found to contribute to the increase in formability at elevated temperatures.     

Coarsening of γ′ in Ni–Al alloys aged under uniaxial compression: I. Early-stage kinetics

The kinetics of coarsening of γ′ precipitates under applied compressive stress were investigated in monocrystalline Ni–Al alloys (nominal composition 13.36 at.% Al) aged at 640 °C. The specimens tested were doubly tapered cylinders or right circular cylinders. The maximum stress used was ~150 MPa; plastic deformation was less than 4% at the longest aging time (1021 h). The scatter in the data was large and is attributed to experimental factors discussed in the text. We find that compressive stress retards the kinetics of coarsening by 20–25% at 150 MPa. The particle size distributions become broader as the stress increases. We attribute the slower kinetics under compressive stress to its effect on the coefficient of diffusion of Al. A semiquantitative model in support of this idea is the subject of a companion paper.     

Multiscale simulations on the coarsening of Cu-rich precipitates in α-Fe using kinetic Monte Carlo,molecular dynamics and phase-field simulations

The coarsening kinetics of Cu-rich precipitates in an α-Fe matrix for thermally aged Fe–Cu alloys at temperatures above 700 °C is studied using a kinetic Monte Carlo (KMC) simulation and a phase-field method (PFM). In this work, the KMC approach adequately captures the early stage of the system evolution which involves nucleation, growth and coarsening, while the PFM provides a suitable framework for studying late-stage coarsening at large precipitate volume fraction regimes. Hence, both models complement each other by transferring the results of KMC along with precipitate–matrix interface energies from a broken-bond model to a quantitative PFM based on a grand chemical potential formulation and the CALPHAD database. Furthermore, molecular dynamics simulations provide information on the structural coherency of the precipitates and hence justify the sequential parameter transfer. We show that our PFM can be validated quantitatively for the Gibbs–Thomson effect and that it also predicts the coarsening kinetics correctly. It is found that the kinetics closely follow the LSW (Lifshitz–Slyozov–Wagner) law, whereas the coarsening rate constant increases with an increase in volume fraction of precipitates.     

Effect of Thermomechanical Processing on the Microstructure and Mechanical Properties of Nb-Ti-V Microalloyed Steel

The paper presents the results of thermomechanical treatment via forging on the microstructure and mechanical properties of newly obtained microalloyed steel containing 0.28% C, 1.41% Mn, 0.027% Nb, 0.028% Ti, and 0.019% V. The investigated steel is assigned to the production of forged elements for the automotive industry. Conditions of forging using the thermomechanical processing method were developed based on plastometric tests. Continuous and double-hit compression tests were conducted using the Gleeble 3800 thermomechanical simulator. The samples were investigated in a temperature range from 900 to 1100 °C and a strain rate of 1 and 10 s^?1. To determine the recrystallization kinetics of plastically deformed austenite, discontinuous compression tests of samples using the applied deformation were conducted in a temperature range from 900 to 1100 °C with isothermal holding of the specimens between successive deformations for 2-100 s. Observations of the microstructures of thin foils were conducted using a TITAN80-300 FEI transmission electron microscope. The applied thermomechanical treatment allows to obtain a fine-grained microstructure of the austenite during hot-working and production of forged parts. These acquire advantageous mechanical properties and guaranteed crack resistance after controlled cooling from the end plastic deformation temperature and successive tempering. Forgings produced using the thermomechanical treatment method, consecutively subjected to tempering in a temperature range from 550 to 650 °C, reveal values of YS_0.2 which equal from 994 to 892 MPa, UTS from 1084 to 958 MPa, KV from 69 to 109 J, KV^?40 from 55 to 83 J, and a hardness ranging from 360 to 300 HBW.     

Primary Alpha Grain Coarsening Behavior of Ti-6Al-2Zr-1Mo-1V Alloy in the Alpha + Beta Two-Phase Field

Grain coarsening of titanium alloys takes place easily at high temperatures, which significantly affects the mechanical properties of the material. In this study, the coarsening mechanisms and kinetics of the Ti-6Al-2Zr-1Mo-1V (TA15) alloy in the (α + β) two-phase field were investigated by heat treatment experiments. The experimental results showed that the microstructural morphology evolved from the cuboidal, rod-like, or plate-like at the initial stages to the equiaxed or spherical. The coarsening mechanism was different at different temperature. At lower temperature (900 °C), the coarsening exponent (n) was close to 4.1. This indicated that the coarsening process was controlled by diffusion along grain boundaries. At higher temperatures (940 and 970 °C), the values of n were equal to 3.2 and 3.3, respectively. It could be concluded that the coarsening processes were mainly controlled by diffusion through the matrix. Furthermore, the activation energy of diffusion (Q _act) and the coarsening rate constant (K) were calculated and compared with the theoretic values. That confirmed the coarsening mechanism mentioned above.     

Effect of Tempering Temperature on the Microstructure and Properties of Fe-2Cr-Mo-0.12C Pressure Vessel Steel

To obtain the high-temperature strength and toughness of the medium–high-temperature–pressure steel, the microstructure evolution and mechanical properties of Fe-2Cr-Mo-0.12C steel subjected to three different tempering temperatures after being normalized were investigated. The results show that the microstructure of the sample, tempered in the range 675-725 °C for 50 min, did not change dramatically, yet the martensite/austenite constituents decomposed, and the bainite lath merged together and transformed into polygonal ferrite. At the same time, the precipitate size increased with an increase in tempering temperature. With the increase in the tempering temperature from 675 to 725 °C, the impact absorbed energy of the Fe-2Cr-Mo-0.12C steel at ?40 °C increased from 257 to 325 J, and the high-temperature yield strength decreased; however, the high-temperature ultimate tensile strength tempered at 700 °C was outstanding (422-571 MPa) at different tested temperatures. The variations of the properties were attributed to the decomposition of M/A constituents and the coarsening of the precipitates. Fe-2Cr-Mo-0.12C steel normalized at 930 °C and tempered at 700 °C was found to have the best combination of ductility and strength.     

Effect of boron and carbon addition on microstructure and mechanical properties of the aged gamma-prime strengthened alumina-forming austenitic alloys

The goal of this work was to understand the effects of aging at 800 °C on the microstructures and mechanical properties of two recently-developed AFA stainless steels based on Fe-14Cr-32Ni-3Nb-3Al-2Ti (wt.%), one of which contained small additions of boron and carbon. To that end both the size distributions and growth kinetics of the B2, Laves phase, L1₂ precipitates present were quantified. While the lattice parameter, morphology, size and coarsening behavior of the L1₂ precipitates was the same in both AFA alloys, the B and C enhanced the grain boundary coverage by both Laves phase and B2-NiAl precipitates, but suppressed their coarsening. These interstitial additions also suppressed the formation of twins and discontinuous precipitation, which were observed in the B and C-free material. It is shown that the yield strength at 700 °C is largely controlled by the size of the L1₂ precipitates, with the largest strengthening effect obtained after aging for 2.4 h for both AFA alloys. Longer aging time led to a loss of strength mainly due to the coarsening of the L1₂ precipitates.     

Nanoscale structural evolution of Al3Sc precipitates in Al(Sc) alloys

Precipitation of the Al₃Sc (L1₂) phase in aluminum alloys, containing 0.1, 0.2 or 0.3 wt% Sc, is studied with conventional transmission and high-resolution (HREM) electron microscopies. The exact morphologies of the Al₃Sc precipitates were determined for the first time by HREM, in Al–0.1 wt% Sc and Al–0.3 wt% Sc alloys. The experimentally determined equilibrium shape of the Al₃Sc precipitates, at 300°C and 0.3 wt% Sc, has 26 facets, which are the 6 {100} (cube), 12 {110} (rhombic dodecahedron), and 8 {111} (octahedron) planes, a Great Rhombicuboctahedron. This equilibrium morphology had been predicted by first principles calculations of the pertinent interfacial energies. The coarsening kinetics obey the (time)^1/3 kinetic law of Lifshitz–Slyozov–Wagner theory and they yield an activation energy for diffusion, 164±9 kJ/mol, that is in agreement with the values obtained from tracer diffusion measurements of Sc in Al and first principles calculations, which implies diffusion-controlled coarsening.