Isothermal Treatment Process Research Articles

The microstructure evolution of Al-5Ce alloy prepared by strain-induced melt activation during semi-solid isothermal treatment process

The coarse grains and rough eutectic microstructure of Al-Ce alloy seriously damage its performance. In this study, Al-5Ce alloys with fine eutectic fibers and bimodal grain distribution were fabricated by strain-induced melt activation (SIMA) process. The results demonstrated that with increasing isothermal temperature and soaking time, the Al11Ce3 phase changes from coarsening to remelting and the α-Al grains size decreases due to decomposition by liquid droplets during the semisolid isothermal treatment. And during the subsequent quenching process, the high cooling rate leads to the solidification of fine α-Al grains and the refinement of eutectic fibers and fiber spacing when the Al concentration exceeds the upper limit of eutectic composition, which forms bimodal grains with fine eutectic fibers.

Effect of Chemistry Modification, Hot Forging and Partitioning on Low Alloy Grade Tool Steel

Development of S6 tool steel has been discussed in this research by conducting a bit modification in the chemical composition using aluminum instead of molybdenum, and micro addition of boron to enhance the marteniste structure. Then, the hardenability and thermal stability have been detected in regarding to S6 tool steel. A novel isothermal treatment process has been suggested to enrich the retained austenite, and thereby, it has been tracked by using XRD, optical microscope, and SEM in conjugation with EDS. The effect of retained on the mechanical properties has been determined. The results ensure that aluminum has inhibited the graphite formation through the slow cooling regime. No change of hardenability or thermal stability of S6 tool steel triggered from adding of aluminum and microaddition of boron. In addition, the novel isothermal process leads to enrichment of retained austenite that has significantly affected on the combination of ductility with strength of the newly designed steel.

Microstructure evolution of a SIMA processed AZ91D magnesium alloy based on repetitive upsetting-extrusion (RUE) process

Repetitive upsetting-extrusion (RUE) was introduced into the strain induced melt activation (SIMA) process. The semi-solid feedstock of AZ91D magnesium alloy with superior thixotropic features was successfully prepared by the modified SIMA process. Prior to partial remelting, the process involving the application of RUE pre-deformation to as-cast alloy was conducted. Consequently, a severely strained structure consisting of fine α-Mg grains and highly fragmented β-Mg17Al12 particles was obtained. During the subsequent isothermal treatment between the solidus and the liquidus, the microstructure evolution of the RUE processed alloys subjected to various deformation cycles and temperatures was investigated at the temperature range of 550–580°C for various durations. The results indicated that the RUE-based SIMA process was a valid route for preparing high-quality thixotropic materials, and the isothermally treated microstructure consisted of fine solid globules surrounded by uniform liquid films. The microstructure evolution model for the modified SIMA process was proposed with reference to the micrograph examination. And the mechanisms responsible for the spheroidization and coarsening of the globular structure were discussed in detail. Further study on effective parameters clarified that the thixotropic features of semi-solid slurry were affected by the parameters of RUE process as well as the isothermal treatment process. And the desirable globular structure, featured by fine, homogeneous and well globularized solid grains and continuous liquid films, was obtained by three-cycle RUE process at 285°C followed by isothermal treatment at 560°C for 15min.

Microstructure Evolution of Semi-Solid AZ91D Alloy During Isothermal Heat Treatment Process

The effect of isothermal heat treatment on the microstructure evolution of semi- solid AZ91 D magnesium alloy was investigated. The results show that the roundness of the primary α- Mg phase increases with the increasing temperature and time. The microstructure evolution of the semi-solid AZ91 D experiences three stages during isothermal heat treatment process, i.e. the structure separation,spheroidization and final coarsening. The liquid phase of semi- solid alloy has three types, namely, the molten pool, the"entrapped liquid"pool and the liner liquid film which separates two neighbor particles.The solid phase has two types, namely, the primary α-Mg particles and the α-Mg phase formed in the second solidification.

Isothermal nanocrystallization behavior of bulk metallic glass composites in supercooled liquid region

Bulk metallic glass composites (BMGCs) reinforced by micro- (spherical size of about 10 μm) and nano-sized (20–50 nm) Al3Zr crystals were synthesized by copper mold suction casting method. The isothermal crystallization of Cu40Zr44Ag8Al8 BMGCs in the supercooled liquid region was studied by differential scanning calorimeter (DSC), X-ray diffractometer (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The mechanisms for nucleation and growth of the crystalline phases precipitated in two-stage crystallization process and the effect of the isothermal treatment on the in situ crystalline phases were discussed. Results show that the BMGCs experience a primary crystallization process. Nano-sized Al3Zr and Cu10Zr7 phases precipitate from the amorphous matrix successively. Furthermore, isothermal annealing has no effect on the pre-existing particles in the as-cast sample, which creates a new method to synthesize BMGCs strengthened by duplex crystalline phases with a large size difference. And the reinforced phase could also be dominated with intention if the as-cast BMGCs preparation and isothermal treatment process can be controlled more accurately.

Resistivity Relaxation of Anisotropic Conductive Polymer Composites

The electrical properties of anisotropic carbon nanotubes (CNTs)/polycarbonate (PC)/ polyethylene (PE) (ACPC) strongly depended on the CNTs’ concentration. When the ACPC was subjected to isothermal treatment (IT), the resistivity variation in both the parallel and perpendicular directions had the characteristics of a relaxation as a function of temperature. During the IT the orientation of the PC microfibrils was gradually damaged and CNTs/PC microfibrils were deformed and changed to short fibers, leading to a transition from anisotropy to isotropy. The velocity of the conductive network reconstruction could be characterized by the relaxation time, and the resistivity of the composite during the IT process can be instantaneously predicted based on the relaxation equation. The relaxation time and the equilibrium resistivity of the composite during IT were determined by the IT temperature and CNT content.

Investigation of quench sensitivity and transformation kinetics during isothermal treatment in 6082 aluminum alloy

The quench sensitivity of 6082 aluminum alloy was investigated by time–temperature–property (TTP) curves. The sensitive temperature of quenching ranges from 250 °C to 440 °C in 6082 Al-alloy, and the nose temperature is about 360 °C. During isothermal treatment process, the Mg 2Si particles precipitate from the supersaturated solid solution, and the precipitation rate is the highest at the nose temperature. A number of rod-shaped β′ and β particles precipitate in the early stage of isothermal treatment at 360 °C. Prolonging the holding time leads to more and coarser β particles in the matrix. Both the precipitation of β′ and β particles results in loss of solute and decreasing of the subsequent age hardening effect. Also, the important coefficients k 2– k 5 and critical cooling rate for 6082 Al-alloy are identified, and the properties after different rates of cooling were predicted using quench factor analysis.

INTERFACE EFFECT PETWEEN DIFFERENT PHASES IN THE PROCESS OF PHASE SEPARATION AND CRYSTALLIZATION OF AN AMORPHOUS Li+ CONDUCTOR

We found that, in the isothermal treatment process of the amorphous Li+ conductor B2O3-0.7 Li2O-0.7 LiCl-0.1 Al2O3, the plot of the conductivity vesus time rises at first, then, after passing a maxium falls monotonically, and followed by two flat sections. It has been established that these phenomena are due to the phase sparation and crystallization of this amorphous material. In this work, we develop a view point of interface effect between different phases, i.e. we consider that there is a high conductivity layer at the interface between different phases and its contribution compensates or exceeds the decrease of conductivity caused by the bulk effect, of the crystallization. On the basis of this interface effect, an explanation about the above-mentioned experimental plot is given.

STUDY OF THE CONDUCTING BEHAVIOR OF AN AMORPHOUS Li+ CONDUCTOR IN THE ISO-THERMAL TREATMENT PROCESS

The con ducting behavior of an amorphous Li+ conductor B2O3-0.7Li2O-0.7LiCl-0.1Al2O3 has been measured continuously and automatically by using the a.c. Volt-Ampere method in the isothermal treatment process at a certain temperature below its transition temperature. It was found that the conductivities of specimens increased at the initial stage and decreased monotonically after passing a maximum, and then two flat steps appeared. Comparing these results with that obtained by DSC and XRD analysis, we confirmed that the presence of the maximum of the conductivities is principally due to the amorphous phase separation, and the flat steps on the diagram of the conductivities are due to the amorphous crystallization. These results were discussed preliminarily in the view of the interface effects between different phases.