Effect of isothermal heat treatment on semi-solid microstructure of AZ91D magnesium alloy containing rare earth Gd

The AZ91 D magnesium alloy containing rare earth Gd was prepared in this study, and the effect of semi-solid isothermal heat treatment on the microstructure of the alloy was investigated to obtain an optimum semi-solid structure. Results show that Gd can refine the microstructure of AZ91 D magnesium alloy, and the optimum semi-solid AZ91 D microstructure can be achieved by adding 1.5wt.% Gd. After treated at 585 °C for 30 min, the well distributed rose-shaped and near-spherical semi-solid microstructures of AZ91D+1.5wt.%Gd alloy can be obtained. The liquid phase of the semi-solid alloy consists of three components, namely, the molten pool, the "entrapped liquid" pool and the liner liquid film which separates two neighbor particles. The solid phase is composed of two phases, the primary α-Mg particles and the α-Mg phase formed in the second stage of solidification. With the increase of holding time, melting which causes the decrease of the primary α-Mg particle size is the dominant mechanism in the initial stage while coalescence and Ostwald ripening tend to be the principles later.

Microstructure evolution of semi-solid Mg-10Gd-3Y-0.5Zr alloy during isothermal heat treatment

In this study,the microstructure evolution of semi-solid Mg-10Gd-3Y-0.5Zr alloy during isothermal heat treatment has been investigated.The results show that primary particles coarsen continuously during the holding.Coarsening rate decreases with the increase of isothermal temperature.When isothermal temperature increases from 600℃ to 620℃,the dominant mechanism for coarsening changes from particle coalescence to Ostwald ripening.Equiaxed as-cast microstructure is beneficial to the semi-solid microstructure after isothermal heat treatment,which brings about the refinement and spheroidization of primary particles,and shortening of holding time.Significant modification of second phases can also be achieved after isothermal heat treatment,due to its unique solidification process.The optimum processing parameters for Mg-10Gd-3Y-0.5Zr alloy in isothermal heat treatment are isothermal temperature of 610℃-620℃ and holding time of 20-40 min.

Investigation on microstructures and properties of semi-solid Al_(80)Mg_(5)Li_(5)Zn_(5)Cu_(5)light-weight high-entropy alloy during isothermal heat treatment process

The Al_(80)Mg_(5)Li_(5)Zn_(5)Cu_(5)light-weight high-entropy alloy with globular microstructure was fabricated by isothermal heat treatment.The effects of isothermal temperatures and holding times on the semi-solid microstructure evolution were investigated.The results indicate that,with increase of the isothermal temperature,the average grain size increases and the spheroidization time shortens.With prolongation of holding time,the shape factor increases firstly and then decreases,and the average grain size decreases at first and then increases when the isothermal temperature is below 520°C,however it increases gradually at 540℃.The optimal semi-solid microstructure is obtained at 520℃ for 30 min,whose shape factor and average grain size are 0.90 and 56.4μm,respectively.Compared with as-cast Al_(80)Mg_(5)Li_(5)Zn_(5)Cu_(5) light-weight high-entropy alloy,the compressive strength and plasticity of semi-solid Al_(80)Mg_(5)Li_(5)Zn_(5)Cu_(5) light-weight high-entropy alloy are increased by 36%and 108%,respectively.The formation of semi-solid microstructures includes three stages:melting separation,spheroidization,and coarsening growth.The sluggish diffusion effect of Al_(80)Mg_(5)Li_(5)Zn_(5)Cu_(5) light-weight high-entropy alloy leads to a low coarsening rate,resulting in slow grain growth.

Direct preparation of semi-solid billets by the semi-solid isothermal heat treatment for commercial cold-rolled ZL104 aluminum alloy

Semi-solid isothermal heat treatment was proposed to directly process cold-rolled ZL104 aluminum alloys and obtain semi-solid bil-lets.The effects of two process parameters,namely,temperature and processing time,on the microstructure and hardness of the resulting bil-lets were also experimentally examined.Average grain size(AGS)increased and the shape factor(SF)of the grain improved as the process temperature increased.The SF of the grain also increased with increasing processing time,and the AGS was augmented when the processing time was prolonged from 5 to 20 min at 570℃.The hardness of the aluminum alloy decreased because of the increase in AGS with increasing temperature and processing time.The optimal temperature and time for the preparation of semi-solid ZL104 aluminum alloys were 570℃and 5 min,respectively.Under optimal process parameters,the AGS,SF,and hardness of the resulting alloy were 35.88μm,0.81,and 55.24 MPa,respectively.The Lifshitz-Slyozov-Wagner relationship was analyzed to determine the coarsening rate constant at 570℃,and a rate constant of 1357.2μm3/s was obtained.

Grain growth of Al-4Cu-Mg alloy during isothermal heat treatment

The microstructure of an Al-4Cu-Mg alloy during isothermal heat treatment in the Strain Induced Melt Activation （SIMA） process was investigated and the kinetics of grain growth was analyzed, The grain growth during isothermal heat treatment of the Al-4Cu-Mg alloy coincided with the Ostwald ripening theory. During isothermal heat treatment, both grain shape and the high volume fraction of solid phase have significant effects on grain growth. Therefore, a new grain growth model based on the Ostwald ripening theory was proposed taking into consideration the grain shape and the volume fraction of solid phase. By comparing the calculated results with the experimental results, it was confirmed that the present model could be applied to grain growth during isothermal heat treatment of the Al-4Cu-Mg alloy in the SIMA process.

Effect of Semi-solid Isothermal Heat Treatment on Microstructure of VW63Z Alloy

The effects of isothermal heat treatment on the semi-solid microstructure evolution of VW63Z(Mg-6Gd-3Y-0.4Zr,wt.%)alloy are studied.It shows that the microstructure of VW63Z alloy could transform from equiaxed crystal to semi-solid spherical crystal after isothermal heat treatment above 620℃.With the heating temperature elevating from 620℃ to 635℃ and the holding time prolonging from 10 min to 35 min,the liquid fraction increases gradually.The semi-solid microstructure evolution of VW63Z alloy can be divided into three stages,i.e.,particle coarsening and spheroidization;particle necking,coalescence,and Ostwald ripening;and dynamic equilibrium.The semi-solid process window of VW63Z alloy ranges from 620℃ to 635℃,where the best process parameters are holding at 635℃ for 20 min-30 min.The solid fraction,the average particle size,and the shape factor are 41.1%-53.8%,81.5μm-83.2μm,and 0.70-0.75,respectively.The maximum relative deviations of the solid fraction,the particle size,and the shape factor at different heights of the same billet are 44.6%,17.4%,and 16.6%,respectively,which means that it should pay attention to the uniformity of edge and core of VW63Z alloy during isothermal heat treatment.The driving force of microstructure is supposed to be the reduction of solid-liquid interface free energy.

Effects of Semi-solid Isothermal Heat Treatment on Microstructures and Damping Capacities of Fly Ash Cenosphere/AZ91D Composites

The fly ash cenosphere/AZ91D composites were successfully prepared and isothermally heat-treated at different tem- peratures for different time. The effects of semi-solid isothermal heat treatment on the microstructures and damping capacities of fly ash cenosphere/AZ91D composites were investigated. With the increase in isothermal temperature or holding time, the small liquid droplets within grains increased in size but decreased in quantity. The average size and shape factor of Mg2Si particles increased with the rise of isothermal temperature. The damping capacities of the composites were improved by isothermal heat treatment. At room temperature, the composites after heat treatment at 520 and 550 ℃ had a higher damping capacity due to interface damping when the strain amplitude was lower than about 8.8 × 10^-5, and the composite after heat treatment at 580 ℃ had a better damping capacity because of the dislocation damping under the condition of high strain amplitude. The damping capacities of the composites increased with the rise of the test temper- ature, and the damping mechanisms varied depending on different test temperatures. The interface damping played an important role when the test temperature was below about 100 ℃, and the dislocation damping and grain boundary damping took effect with the rise of test temperature.

Solid-state cold spraying of FeCoCrNiMn high-entropy alloy:an insight into microstructure evolution and oxidation behavior at 700-900℃

About 3 mm thick five-element equimolar high-entropy alloy(HEA) FeCoCrNiMn was successfully deposited by solid-state cold spraying(CS).The high-temperature oxidation behavior of the CSed HEA was investigated at 700-900℃.Heat treatment was performed on the CSed HEA before oxidation to heal the incomplete interfaces between the deposited particles.Results show that the microstructure of the CSed HEA is characterized by grain refinement and abundant interparticle incomplete interfaces.Post-spray heat treatment promotes recrystallization and grain growth in the CSed HEA.After oxidation testing,the oxide scales are composed of multi-layers:a Mn_(2)O_(3)(or Mn_(3)O_(4)) outer layer,a Mn-Cr spinel intermediate layer and a Cr_(2)O_(3) inner layer.The CSed HEA exhibits higher parabolic rate constants and more favorable internal oxidation than the bulk HEAs that have similar compositions in the literature.Such a discrepancy becomes pronounced at higher temperatures.The grain refinement and numerous particle boundaries are responsible for such a distinctive performance of the CSed HEA.

σ-Phase in Lean Duplex Stainless Steel Sheets

The precipitation kinetics of secondary phases in two austeno-ferritic lean duplex stainless steels（lean DSS）were examined after aging the materials at 800 ℃.Owing to the instability of ferrite,all DSS are known to be sensitive to solid-state phase transformations in the critical temperature range 600-1,000 ℃ and different secondary phases may form,depending on composition and microstructure.The performed thermodynamic simulations revealed the proneness to the precipitation of such phases also have been done in lean DSS,but only information on the equilibrium microstructures were achieved.Therefore,the materials were aged at various times,in order to verify the simulations and determine the precipitation kinetics.The occurred structural modifications were observed and quantified by scanning electron microscope and X-ray diffraction measurements,determining phase type,composition and volumetric fraction.At 800 ℃,grade 2101 was found to be only affected by Cr_2N nitrides precipitation,whereas a significant amount of σ-phase was found to form in LDX 2404 for treatment longer than 1 h,almost totally replacing ferrite after 50 h.Up to now,the intermetallic σ-phase has been observed only in the high alloyed DSS,and the unexpected precipitation in grade 2404 highlighted that the increased content of molybdenum in this steel might be considered as determinant for the formation.