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The evolution of the microstructural and mechanical properties of alloy 617B during long-term service on a key-component test platform at 700 °C was systematically investigated. The precipitation behavior and size changes of the M23C6 and γ′ phases were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that carbide M23C6 precipitated in the form of discontinuous particles, plates, or needles at grain boundaries and within grains, while the γ′ phase had a spherical shape and was distributed in a dispersed manner. With prolonged service time, both the M23C6 and γ′ phases gradually coarsened. After 24,000 h of service, the yield strength, tensile strength, and Brinell hardness of alloy 617B significantly increased; however, the impact toughness decreased, accompanied by intergranular embrittlement. The increase in precipitate volume fraction and its contribution to the strength of the alloy were evaluated by a precipitation strengthening model. The coarsening of M23C6 was identified as the main cause of embrittlement. The findings of this study provide important experimental data and theoretical support for the stability of 617B alloys under long-term high-temperature service conditions. [ABSTRACT FROM AUTHOR]

The precipitation behavior of a novel Ni-Fe-based superalloy developed for advanced ultra-supercritical (A-USC) coal-fired power plant applications during high-temperature aging treatment was investigated. The results showed that the major precipitates in the novel alloy were randomly distributed MC carbides, M23C6 carbides at grain boundaries, and the γ′-Ni3 (Al, Ti) phase in grain interiors after aging. MC remained relatively stable during both short-term and long-term aging. M23C6 quickly precipitated and exhibited a discrete distribution at grain boundaries during short-term aging, and partly developed into continuous films during long-term aging. After uniform precipitation, the shape of γ′ remained spherical, and the size kept increasing with aging time according to the Lifshitz–Slyozov–Wagner (LSW) model. The hardness of the novel alloy was mainly associated with the precipitation behavior of γ′; as γ′ gradually precipitated, the hardness steadily increased; after complete precipitation, as the size of γ′ increased, the hardness first increased and then decreased, reaching the peak hardness when the average radius of γ′ achieved the critical size. In addition, the novel alloy exhibited abnormal coarsening behavior at grain boundaries during both short-term and long-term aging. The coarsened grain boundaries were actually precipitate-free zones (PFZs) and the coarsened and elongated rod-like particles inside were identified as γ′ precipitates. The mechanism of strain-induced grain boundary migration and the discontinuous coarsening reaction is proposed for the formation of PFZs. Furthermore, PFZs were considered to be potential crack sources during the creep rupture test, leading to earlier failure of the material. [ABSTRACT FROM AUTHOR]

A novel Al-Mg-Si aluminum alloy with the addition of the micro-alloying element Er and Zr that was promptly quenched after extrusion has been studied. The solid solution and aging treatment of the novel alloy are studied by observing the microstructure, mechanical properties, and strengthening mechanism. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques are employed to examine the changes in the microstructure resulting from various solid solution treatments and aging treatments. The best strengthening effect can be achieved when the solubility of the MgSi phase and precipitate β″ (Mg2Si phase) is at their maximum. The addition of Er and Zr elements promotes the precipitation of the β″ phase and makes the b″ phase more finely dispersed. The aging strengthening of alloys is a comprehensive effect of the dislocation cutting mechanism and bypass mechanism, the joint effect of diffusion strengthening of Al3(Er,Zr) particles and the addition of Er and Zr elements promoting the precipitation strengthening of β″ phases. In this paper, by adding Er and Zr elements and exploring the optimal heat treatment system, the yield strength of the alloy reaches 437 MPa and the tensile strength reaches 453 MPa after solid solution treatment at 565 °C/30 min and aging at 175 °C/10 h. [ABSTRACT FROM AUTHOR]

The hot deformation behavior of Al-Zn-Mg-Er-Zr alloy was investigated through an isothermal compression experiment at a strain rate ranging from 0.01 to 10 s−1 and temperature ranging from 350 to 500 °C. The constitutive equation of thermal deformation characteristics based on strain was established, and the microstructure (including grain, substructure and dynamic precipitation) under different deformation conditions was analyzed. It is shown that the steady-state flow stress can be described using the hyperbolic sinusoidal constitutive equation with a deformation activation energy of 160.03 kJ/mol. Two kinds of second phases exist in the deformed alloy; one is the η phase, whose size and quantity changes according to the deformation parameters, and the other is spherical Al3(Er, Zr) particles with good thermal stability. Both kinds of particles pin the dislocation. However, with a decrease in strain rate or increase in temperature, η phases coarsen and their density decreases, and their dislocation locking ability is weakened. However, the size of Al3(Er, Zr) particles does not change with the variation in deformation conditions. So, at higher deformation temperatures, Al3(Er, Zr) particles still pin dislocations and thus refine the subgrain and enhance the strength. Compared with the η phase, Al3(Er, Zr) particles are superior for dislocation locking during hot deformation. A strain rate ranging from 0.1 to 1 s−1 and a deformation temperature ranging from 450 to 500 °C form the safest hot working domain in the processing map. [ABSTRACT FROM AUTHOR]

An ultrafine-grained (UFG) Al–Cu–Mg alloy (AA2024) was produced by surface mechanical grinding treatment (SMGT) with a high strain rate, and the precipitation behavior inside the grain and at the grain boundary was investigated. During SMGT, element segregation at the boundary was rarely observed, since the solute atoms were impeded by dislocations produced during SMGT. During early aging, the atomic fraction of Cu at the grain boundary with SMGT alloys was approximately 2.4-fold larger than that without SMGT alloys, the diffusion rate of Cu atoms from the grain toward the grain boundaries was accelerated with SMGT alloys, because a higher local elastic stress and diffusion path were provided by high-density dislocations. The combined action, in terms of the composition of the alloy, the atomic radius, the diffusion path, and the diffusion driving force provided by high-density dislocations with SMGT alloys, led to a Cu/Mg atomic ratio of approximately 6.8 at the grain boundary. The average size of the precipitates inside the grain was approximately 2- and 10-fold larger than that formed after later aging with and without SMGT alloys, due to more nucleation sites at dislocation located inside the grain with SMGT alloys having attracted and captured numerous solute atoms during the aging process. [ABSTRACT FROM AUTHOR]

A novel Al-Cu-Zr alloy is designed in this paper, which provides a method for further improving the strength of Al-Cu alloys. In this paper, the addition of the micro-alloying element Zr in Al-Cu alloy was studied. The effect of aging treatment on the mechanical properties and precipitation behavior of the alloy was studied. With the addition of Zr, Al3Zr phases were formed in the alloy, which acts as obstacles to dislocation motion. In addition, Al3Zr phases can be used as the nucleation site of θ′ phases to promote precipitation. All this can improve the strength of Al-Cu alloys. After one-step aging, corresponding to the highest hardness, the largest amount of θ′ phases were observed in the alloy matrix. By contrast, after two-step aging, the θ′ phases were finer, and a large amount of Guinier–Preston (GP) zones formed during the pre-aging step, which were transformed into denser and finer θ′ phases in the secondary aging step. After the same solution treatment (540 °C/12 h), undergoing 120 °C/4 h + 175 °C/10 h two-step aging, the ultimate tensile strength, yield strength, and elongation of the Al-Cu-Zr alloy were 398.7 MPa, 313.3 MPa, and 7.9%, respectively. [ABSTRACT FROM AUTHOR]

In this work, the precipitates in Ti−Mo−V steel were systematically characterized by high-resolution transmission electron microscopy (HRTEM). The thermodynamics and kinetics of precipitates in Ti−Mo and Ti−Mo−V steels were theoretically analyzed, and the effect of vanadium on the precipitation behavior was clarified. The results showed that the precipitation volume fraction of the Ti−Mo−V steel was significantly higher than that of Ti−Mo steel. The randomly dispersed precipitation and interphase precipitation (Ti, Mo, V)C particles coexisted in the Ti−Mo−V steel. When the temperature was higher than 872 °C, the addition of vanadium could increase the driving force for (Ti, Mo, V)C precipitation in austenite, resulting in an increased nucleation rate and shortened incubation period, promoting the (Ti, Mo, V)C precipitation. When the temperature was lower than 872 °C, the driving force for (Ti, Mo, V)C precipitation in austenite was lower than that for (Ti, Mo)C precipitation, and the incubation period of (Ti, Mo, V)C precipitation was increased. Moreover, it was also found that the precipitated-time-temperature curve of (Ti, Mo, V)C precipitated in the ferrite region was "C" shaped, but that of (Ti, Mo)C was "ε" shaped, and the incubation period of (Ti, Mo, V)C was significantly shorter than that of (Ti, Mo)C. [ABSTRACT FROM AUTHOR]

Here we systematically investigate the crystallographic orientation relationship (OR) between MC-type precipitates (M, metal; C, carbon) and ferrite matrix in the Ti-Mo microalloyed steel with different processing. In the specimens without austenite deformation, the interphase precipitation can be obtained, and the precipitates obey Baker–Nutting (BN) OR with ferrite matrix. By contrast, in the specimens with austenite deformation, the supersaturated precipitates were formed in ferrite grains, which can obey BN, Nishiyama–Wasserman (NW), Kurdjumov–Sachs (KS) and Pitsch (P) ORs simultaneously. The cooling rate after austenite deformation can influence the OR between carbides and ferrite in the MC/ferrite system. At the cooling rate of 80 °C/s, carbides and ferrite can roughly satisfy these OR with the deviation ≥ 10°, while at the cooling rate of 20 °C/s, carbides and ferrite can strictly obey the specific OR. The energy accumulated in the deformation process and maintained in the fast-cooling process (80 °C/s) can offset the formation energy of the carbides. Thus, the carbides formed in the specimen with the cooling rate of 80 °C/s do not strictly satisfy the specific ORs to meet the rule of lowest energy, and then deviate by a small angle based on the specific ORs. [ABSTRACT FROM AUTHOR]

Ti-bearing microalloyed steel plates with a thickness of 40 mm were subjected to ultra-fast cooling (UFC) and traditional accelerate cooling after hot-rolling, aiming to investigate the effect of cooling rate on the microstructure and mechanical properties homogeneity, and thus obtain thick plates with superior and homogeneous mechanical properties. Yield strength, tensile strength, and elongation were 642 MPa, 740 MPa, 19.2% and 592 MPa, 720 MPa and 16.7%, respectively, in the surface and mid-thickness of the steel with ultra-fast cooling, while in the steel with traditional accelerate cooling, 535 MPa, 645 MPa, 23.4% and 485 MPa, 608 MPa, 16.2% were obtained in the surface and mid-thickness of the plate. The yield strength has been greatly improved after UFC, for the refinement of grain and precipitates produced by UFC. In addition, the equivalent grain size and precipitates size in the thick plate with UFC are homogeneous in the thickness direction, leading to uniform mechanical properties. The crystallographic characteristics of different precipitates have been studied. The precipitates formed in the austenite deformation stage obey Kurdjumov–Sachs orientation relationship with the ferrite matrix, while the fine precipitates formed in the ferrite obey [112]MC//[110]α and (1 ¯ 1 ¯ 1) MC // (1 ¯ 12) α orientation relationship with the ferrite matrix. [ABSTRACT FROM AUTHOR]

In this study, Al-1.00 Mg-0.65 Si-0.24 Cu alloy was solution heat-treated, water-quenched, and then pre-deformed for 5% before aging. The peak hardness and yield strength of the pre-deformed sample with subsequent artificial aging were similar to that of a T6 condition sample. It was also found that the pre-deformation treatment could inhibit the negative influence of natural aging to some degree. After seven days of natural aging, the pre-deformed sample obtained better peak hardness and yield strength upon artificial aging than the sample without pre-deformation. In addition, the pre-deformation treatment could reduce 50% of the artificial aging time to reach the peak aging condition compared with T6 treatment. For the peak aged condition in the pre-deformed sample, transmission electron microscopy (TEM) observation found two types of precipitates exhibited along the dislocations besides the β″ precipitates in the Al matrix. Both precipitates had disordered atomic arrangements on the ordered subcell (Si network). The disordered precipitates occupied a number of Mg and Si atoms, resulting in less β″ precipitates formed during artificial aging at 180 °C. [ABSTRACT FROM AUTHOR]