Phase Transformations in Rapidly Solidified Al-Cu-Li-Mg-Sc-Zr Alloy During Model Homogenization Studied by In Situ STEM

Low carbon steel plays an important role in many applications due to its high strength. Its high strength comes from the strengthening effect of nano-Cu-rich phase precipitates. In order to effectively adjust the microstructure of Cu-rich phase precipitates and obtain Fe-Cu-based steel with the best properties by adding different alloying elements (Mn, Al), it is necessary to understand the precipitation process of Cu particles. In this paper, based on the Ginzburg-Landau theory, the previous phase field model is modified, and the continuous phase field method is used to simulate the precipitation mechanism of nanometer Cu-rich precipitates and the inhibiting of the effect of Al content on Cu-rich precipitates of Fe-15%Cu-3%Mn-<i>x</i>Al (<i>x</i> = 1%, 3%, 5% mass fraction) alloy at 873 K isothermal aging. Combining with the free energy derived from thermodynamics database, the microstructure evolution corresponds to the real alloy system. By calculating the composition field variables and structural order parameters, the evolution of phase separation and precipitated phase morphology in aging process are simulated. Moreover, the influence law of morphology, quantity density, average particle radius, growth and coarsening of Cu-rich precipitated phase are discussed. The results show that in the early stage of aging process, the nano-Cu-rich phase precipitates through the spinodal decomposition mechanism, and is randomly distributed in the iron matrix. Furthermore, due to the difference in atomic diffusion rate, the core-shell structure with Cu-rich phase as a core is formed. With the aging time extending, the structure of Cu-rich phase precipitates changes from bcc to fcc. Because of the synergistic effect between Al and Cu, the diffusion of Cu is slowed down. Besides, with the Al and Mn atoms precipitating, Al/Mn clusters are segregated around the Cu-rich precipitates, forming the Al/Mn intermetallic core-shell structure, and gradually wrapping the Cu-rich phase uniformly. During the evolution of the precipitation stage, the Al/Mn clusters are isolated around the Cu-rich precipitation phase, forming a gradually uniform Al/Mn intermetallic phase core shell structure covering the Cu-rich phase, which is to hinder the buffer layer from forming in the precipitation stage of the reservoir. In addition, with the Al content increasing, the Al/Mn intermetallic phase promotes the growth of the buffer layer and hinders the Cu-rich precipitate phase from growing and coarsening.

The effects of cathodic hydrogen charging and subsequent aging on phase transitions and microstructures of rapidly solidified (RS) austenitic stainless steels (types 310 RS, 316 RS, and 316TiM RS) were investigated. The behavior of the martensitic phases,α′(bcc) ande(hcp), as well as the austenite phase,γ (fcc), of the RS steels during aging after charging was compared to the behavior of these phases of equivalent conventionally processed commercial solution-treated (ST) austenitic stainless steels (types 310 ST, 316L ST, and 316TiM ST) following identical cathodic-charging conditions by means of X-ray and electron diffraction techniques. The behavior of theα′ phase of both RS and ST steels (that formα′ phase) during aging was found to be very similar, while the behavior of bothγ ande phases during aging of all of the RS steels studied, as compared to the equivalent ST steels, was different. The development of lower internal stresses and minor lattice expansion of the RS steels, as compared to the ST steels, is probably due to a different distribution of hydrogen within the near-surface layer of the RS steels than that of the ST steels, which appears to be related to the markedly different microstructural characterizations of the RS steels from the ST steels. Scanning and transmission electron microscopy (SEM and TEM) observations indicated that the tendency toward cracking along the columnar-like structure is typical of all of the charged RS steels studied.

Fe-Si-B systems amorphous alloys were rapidly solidified from the melt by the single roller method. In this process the alloys were melted in a crucible and the molten alloy is jetted through a small orifice by gas pressure or gravity. The effects of the jetting temperature of the melting alloy on the ribbon thickness of rapidly solidified Fe75Si10B15 and Fe79:5Si8:5B12 alloys were examined. The rapidly solidified Fe75Si10B15 and Fe79:5Si8:5B12 alloy ribbons’ mean thickness decreased continuously with the increased jetting temperature. However, the temperature dependence of the ribbon thickness in the Fe79:5Si8:5B12 alloy was greater than that of the Fe75Si10B15 alloy. The results demonstrate that the viscosity of liquid Fe-Si-B systems alloys influences the ribbon thickness. Moreover, when the contact conditions between the rotating roll comes and puddle are good, the ribbon thickness decreases. [doi:10.2320/matertrans.M2010321]

The Zn-Mg system has potential glass-forming ability, and therefore studies were made of rapidly solidified zinc-based Zn-Mg alloys containing up to 6 wt% Mg. These alloys exhibited interesting eutectic phase selections and structural transitions across the ribbon thickness which are represented on a microstructure selection diagram for rapid solidification conditions. Although rapid solidification is known in many cases to produce metastable phases, in this case the equilibrium eutectic mixtures of Zn-Mg2Zn11 are observed after rapid solidification, whereas the metastable eutectic mixture Zn-MgZn2 forms under normal solidification conditions. However, in the melt-spun Zn-Mg alloy which is exactly at eutectic composition, three different structures are observed across the ribbon thickness. These three structures do not exist simultaneously in the same region, but structural transitions occur as the thickness increases from the wheel side to the free side. Eutectic and hypereutectic alloys show a tendency to form a metallic glass. In these alloys a critical growth velocity exists beyond which eutectic solidification is not possible, suggesting a possible transition from eutectic solidification to amorphous phase formation. The eutectic phase selection and the extent to which a specific microstructure is present depends on the variation in growth rate and solid-liquid interface stability during rapid solidification.[/p]

On the microstructure and solution treatment of hot tearing resistant semi-solid 206 alloy

Primary phase transformation mechanism in a hypereutectic Al-Ce alloy during rapid solidification

Coupling magnetic-thermal field effect on precipitation kinetics of Al–Cu–Mg alloy investigated by SAXS and WAXS

<p indent="0mm">The solidification kinetics and physical properties of Mo-48%Ni alloy were studied using the free fall technique. The cooling rate and undercooling of this alloy both increased exponentially as the droplet diameter decreased, and they could reach <sc>2.41×10⁴ K s⁻¹</sc> and <sc>322 K</sc> (0.19 <italic>T</italic>_L), respectively. The high cooling rate significantly suppressed the subsequent solid state transformation process. The primary NiMo phase exhibited a morphological transition from coarse dendrite to a remarkably refined structure, and the regular lamellar (Ni+NiMo) eutectics changed into anomalous eutectic. The maximum length of the primary NiMo phase fell from 142.2 to <sc>28.4 μm</sc>, whereas its volume fraction initially rose and then fell, with a maximum of 60.1%. Moreover, the variation in the Mo content of the primary phase coincided well with that of the eutectic structure. As regards mechanical properties, the nanoindentation measurements indicated that the elastic moduli of the primary phase and the eutectic structure both showed a downward trend along with the microstructure evolution. The eutectic hardness decreased monotonously from 14.1 to <sc>8.7 GPa</sc>, whereas the primary phase hardness increased slightly from 8.0 to <sc>8.5 GPa</sc> and finally fell to <sc>5.4 GPa</sc>. In addition, the growth morphology of the (Ni) solid solution within the eutectic structure was shown to have an obvious effect on the magnetic properties of the Mo-48%Ni alloy. As the rate of cooling increased, the saturation magnetization and residual magnetization diminished. The alloy’s coercivity increased from 11.8 to <sc>25.7 A m⁻¹</sc>, indicating enhancement of the temperature resistance induced by microstructure refinement. The rapid solidification under space simulation conditions affected the mechanical and magnetic properties of the Mo-48%Ni alloy to a certain degree.

Rapid solidification of undercooled FeCoNiCu multi-principal element alloy: Mechanical and tribological properties

Study of aluminium–copper–iron alloys: application for laser cladding

Solution and aging heat treatment are effective means to improve the mechanical properties of Al-Li alloys. The solution treatment and aging treatment tests of 2060 Al-Li alloy were carried out. The yield strength, tensile strength, elongation, Vickers hardness, and microstructure of specimens after heat treatment were obtained by the tensile test at room temperature, Vickers hardness test, SEM analysis, TEM analysis, and EDS analysis. The effects of solution and aging heat treatment parameters on mechanical properties of 2060 Al-Li alloy were analyzed by response surface model and test results. The results show that sufficient solution can make the Cu-rich second phase of the alloy continuously dissolves into the aluminum matrix, and consequently obtain the supersaturated solid solution, the insoluble second phase is mainly θ’ (Al2Cu) phase, T(Al2Cu2Mg3) phase, and S’ (Al2CuMg) phase. The strength and hardness of the alloy are improved, but the ductility worsens with the degree of solution treatment enhances. With the increase of aging temperature and aging time, the strength and hardness of the alloy increase, but the elongation decreases. Taking tensile strength, elongation, and Vickers hardness of the alloy as the optimization objectives, the NSGA-II multi-objective optimization algorithm was used to optimize the process parameters of solution and aging heat treatment, and the heat treatment experiment was carried out. The optimization results show that the best mechanical properties of the alloy with matching strength and toughness can be obtained under the solution treatment at 466 °C/60 min and aging treatment at 185 °C/13 h.

Nanoparticle promoted solution treatment by reducing segregation in AA7034

The aluminium-based hybrid metal matrix composites have noteworthy applications in sub-sea installations, structures of deep-sea crawlers, submarine parts, engine cylinders, drum brakes etc., as they possess high strength, corrosion resistance, chemical, and dimensional stability. In this investigation, the pitting corrosion behaviour of friction welded and post-weld heat-treated AA6061/SiC/graphite hybrid composites were analysed. The corrosion rates of AW (as welded), ST (Solution treated), STA (Solution treated and Aged), and AA (Artificially Aged) weld joints were experimentally determined. The corrosion behaviour has been discussed in light of microstructure. The experimental results revealed that the STA joints exhibited better corrosion resistance characteristics as compared to AW, AA, and ST joints. The corrosion rate was high for AW joints, followed by AA and ST joints, respectively. Taking into account the corrosion rates of AW and STA joints, the STA joints have a corrosion rate 34.6% lesser than that of AW joints. A comparison of AA and ST with STA joints reveals that the rate of corrosion for STA joints was 31.1% lesser than that of AA joints and 28.8% lesser than that of ST joints. A lower corrosion rate was observed for STA joints as compared to AA, AW, and ST joints.

Studying on the morphology of primary phase by 3D-CT technology and controlling eutectic growth by tailoring the primary phase

Phase transformations during artificial and isothermal aging of Ti-6.8Mo-4.5Fe-1.5Al have been investigated over the temperature range from 300 °C to 750 °C utilizing hardness measurements, X-ray diffraction, optical microscopy, and electron microscopy. Artificial aging following solution treatment and water quenching initially involved growth of the athermal ω phase. This was followed by formation of the α phase, either in association with the ω phase, through homogeneous precipitation within the matrix, or through heterogeneous grain-boundary nucleation. Similarly, isothermal decomposition of the metastable β phase resulted in the precipitation of ω phase exhibiting an ellipsoidal morphology. While precipitation of ω was immediate at 345 °C, an incubation period was observed upon aging at 390 °C. Isothermal aging above this temperature involved direct precipitation of the α phase, either homogeneously within the β matrix or heterogeneously at β grain boundaries. The extent of homogeneous vs heterogeneous α nucleation during isothermal aging depended upon aging temperature; low aging temperatures promote homogeneous nucleation and higher aging temperatures promote α heterogeneous nucleation. Finally, continued aging resulted, independent of aging path, in coarsening and spheroidization of the α phase.