Influence of semi-solid isothermal treatment on eutectic carbide in Cr12MoV steel

In-situ observation of eutectic solidification of Cr12MoV steel was conducted using high temperature confocal laser-scanning microscopy. The semi-solid isothermal treatment temperature of the steel was determined by thermodynamic calculation using Thermo-cal software. At the same time, the influences of isothermal treatment temperature and time on eutectic carbides in the steel were also studied. The results show that when the liquid metal cools at the rate of 47 ℃·min^-1, the eutectic reaction occurs rapidly at 1,214.7 ℃ in one second with the reticular liquid around austenite dendrites, transforming into a network of eutectic structure. After being held at 1,300 ℃ for 30 min, the carbide network is broken due to the impingement of refined primary austenite dendrites and secondary dendrites arms, and the thickness of eutectic structure is reduced.

Microstructure evolution of processed Mg-Al-Zn alloy by equal channel angular extrusion in semi-solid isothermal treatment

Microstructure evolution of processed Mg-Al-Zn alloy by equal channel angularextrusion(ECAE) in semi-solid isothermal treatment was investigated. The results show that withincreasing semi-solid isothermal treatment temperature, the a phase solid grain size of processedMg-Al-Zn alloy by ECAE increases firstly due to coarsening of a phase solid grains, then decreasesdue to melting of a phase solid grains. With the increase of extrusion passes during ECAE, the aphase solid grain size in the following semi-solid isothermal treatment decreases. The a phase solidgrain size of processed Mg-Al-Zn alloy by ECAE under route B_C is the smallest, while the a phasesolid grain size of processed material by ECAE under route A is the largest. The primary mechanismof spheroid formation depends on the melting of recrystallizing boundaries and diffusion of soluteatoms in the semi-solid state.

Microstructure and Gd-rich phase evolution of as-cast AZ31-xGd magnesium alloys during semi-solid isothermal heat treatment

The microstructure and Gd-rich phase evolution of as-cast AZ31-xGd(x=0,1.5 wt.%,2.0 wt.%and 2.5 wt.%)magnesium alloys during semi-solid isothermal heat treatment were investigated deeply in the present work.Results showed that the lamellar(Mg,Al)3Gd phases transformed into the particle-like Al2Gd phases in AZ31 magnesium alloys with Gd addition during semi-solid isothermal heat treatment,leading to yielding more sphericalα-Mg grains.When Gd content is 2.0 wt.%,the size of semi-solid spherical grains reaches the minimum.The main mechanism of grain refinement lies in the remelting of dendritic branches as well as the auxiliary effect of a small number of Al2Gd particles as grain refining inoculants.Meanwhile,Al2Gd particles enriched at the solid-liquid interfaces can remarkably retard the growth rate ofα-Mg grains.A reduction of deformation resistance has been successfully achieved in AZ31-2.0Gd magnesium alloy after semi-solid isothermal heat treatment,which shows a moderate compressive deformation resistance(230 MPa),comparing to the as-cast AZ31 magnesium alloy(280 MPa)and semi-solid AZ31 magnesium alloy(209 MPa).

Evolution and distribution of Al_2Sm phase in as-extruded AZ61-xSm magnesium alloys during semi-solid isothermal heat-treatment

The evolution and distribution of Al2Sm phase in as-extruded AZ61-xSm（x=0, 1.5, 2.0 and 2.5, mass fraction, %） magnesium alloys during semi-solid isothermal heat treatment were investigated. The results showed that when as-extruded AZ61 magnesium alloys were modified with Sm, the smaller and rounder grains were obtained during semi-solid isothermal heat treatment. When the Sm content is 2.0%（mass fraction）, the average size of the globular grains reached the smallest value of 90 μm. Although a few Al2Sm particles existed in the α-Mg grains, most of Al2Sm particles solidified at the edge of the globular grains with the width of 20 μm. These phenomena are mainly attributed to the forces acting on Al2Sm particles in front of the solid-liquid interface, leading to Al2Sm particles accumulating at the solid-liquid interface and then solidifying at the edge of the globular grains in the quenching process.

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.

Structural evolution of ZA27 alloy during semi-solid isothermal heat treatment

The structural evolution of ZA27 alloy modified by element Zr was studied during semi solid isothermal heat treatment, and its transformation mechanism was also discussed. The results indicate that the primary α phase changes from equiaxed grains to spherical grains gradually at semi solid temperature of 460 ℃. With increasing isothermal time, the eutectic between boundaries of α phase diffuses toward α phase, and the primary equiaxed grain arms merge and boundaries tend to disappear to form near particle grains. Further, the eutectic left on α boundaries melts to make the near particle grains separate, and form spherical structure at last. [

Microstructural evolution of equal channel angular pressed AZ91D magnesium alloy during semi-solid isothermal heat treatment

The microstructural evolution of AZ91D magnesium alloy processed by equal channel angular pressing during isothermal heat treatment at 570℃was investigated.The results indicated that the equal channel angular pressing followed by semi-solid isothermal heat treatment was an effective method to prepare semi-solid nondendritic slurry of AZ91D magnesium alloy.During this process,its microstructure change underwent four stages,the initial coarsening stage,the structure separation stage,the spheroidization stage and the final coarsening stage.The microstructural spheroidization effect was the best after being heated for 15 min for the alloy pressed for four passes,and the grain size was the smallest.With the further increase of heating time,the grain size and shape factor increased.When the heating time was kept constant,the grain size and shape factor decreased with the increase of pressing passes.

Effects of trace Cr on as-cast microstructure and microstructural evolution of semi-solid isothermal heat treatment ZC61 magnesium alloy

The content and kind of trace elements in magnesium alloys have important effects on their ascast and semi-solid microstructures. In this research work, effects of trace Cr on as-cast and semi-solid microstructures of ZC61 magnesium alloy were investigated by metal mold casting and semi-solid isothermal heat treatment. The results show that the addition of Cr can refine the α-Mg phase without generating a new phase, noticeably change the eutectic phase, and decrease the average size of solid particles at the same isothermal heat treatment conditions. Non-dendritic microstructures of all alloys are constituted of α_1-Mg phases, α_2-Mg phases and eutectic phases after water quenching. With isothermal temperature increased or holding time prolonged, the eutectic microstructure(α-Mg+MgZn_2+CuMgZn) at the grain boundaries in as-cast alloy is melted preferentially and then turned into semi-solid non-dendritic microstructure by processes of initial coarsening, microstructure separation, spheroidizing and final coarsening. Especially when the ZC61-0.1 Cr alloy was treated at 585 ℃ for 30 min, the ideal non-dendritic microstructure can be obtained, and the corresponding solid particle size and shape factor were 37.5 μm and 1.33, respectively. The coarsening process of solid α-Mg phase at higher temperature or longer time, which is affected by both combining growth and Ostwald ripening mechanism, is refrained when Cr is added to the ZC61 alloy.

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.

Microstructural evolution of Mg9AlZnY alloy with vibration in lost foam casting during semi-solid isothermal heat treatment

The nearly equiaxed grains of Mg9AlZnY alloy were obtained by vibrating solidification in lost foam casting(LFC) and the microstructure of Mg9AlZnY alloy was analyzed.On this basis,the morphology and size of α-Mg grains fabricated by semi-solid isothermal heat treatment(SSIT) at 530 ℃ and 570 ℃ holding different time were studied.The results show that the main constituent phases of Mg9AlZnY alloy are α-Mg,β-Mg17Al12 and Al2Y,and the Y can greatly refine α-Mg grains.The distribution of α-Mg grains equivalent diameters between 20 and 100 μm is up to 87%,and the average roundness of α-Mg grains reaches 1.37 in the specimen obtained at 570 ℃ and holding time 60 min.According to the analysis of solidification kinetics and thermodynamic,binary eutectic with low melting point melts firstly on SSIT process.As the liquid fraction increases with the solute diffusibility,both of the shape and size of α-Mg grains change ceaselessly.When the liquid fraction reaches equilibrium,the α-Mg grains are gradually spheroidized under the interfacial tension,and then the α-Mg grains begin to combine and grow.Evolution of α-Mg dendritic grains on SSIT process is obviously different from that of equiaxed grains.

Microstructure evolution of semi-solid Mg-14Al-0.5Mn alloys during isothermal heat treatment

A new Mg-14Al-0.5Mn alloy that exhibits a wide solidification range and sufficient fluidity for semi-solid forming was designed. And the rnicrostructure evolution of semi-solid Mg-14Al-0.5Mn alloy during isothermal heat treatment was investigated. The mechanism of the microstructure evolution and the processing conditions for isothermal heat treatment were also discussed. The results show that the microstructures of cast alloys consist of α-Mg,β-Mg17Al12 and a small amount of Al-Mn compounds. After holding at 520 ℃ for 3 min, the phases of β-Mg17Al12 and eutectic mixtures in the Mg-14Al-0.5Mn alloy melt and the microstructures of α-Mg change from developed dendrites to irregular solid particles. With increasing the isothermal time, the amount of liquid increases, and the solid particles grow large and become spherical. When the holding time lasts for 20 min or even longer, the solid and liquid phases achieve a state of dynamic equilibrium.

Effects of Sm addition on microstructural evolution of Mg-6Zn-0.4Zr alloy during semi-solid isothermal heat treatment

The application of segmental semi-solid thixoforming of magnesium alloys is confined due to the dimensional distinction existing in solid particles of the alloy billet from edge to center zones. In the present study, the effects of Sm addition on the microstructural evolution of Mg-6Zn-0.4Zr and Mg-6Zn-4Sm-0.4Zr alloys by semi-solid isothermal heat treatment were investigated, to obtain optimum semi-solid microstructures for the subsequently thixoforming. The results indicate that the grains of the Sm-bearing alloy are evidently refined and gradually evolve from dendritic to globular and elliptic particles. In addition, the distinctly dimensional effect of the Mg-6Zn-0.4Zr alloy is eliminated with 4% Sm addition; the particle sizes in all zones from center to the edge of the billet are almost identical. With the increment of isothermal heat treatment temperature, the dendritic microstructures completely disappear, and meanwhile, the irregular and globular particles gradually form. The size, morphology and the distribution of solid particles mainly depend on the formation and permeation of the liquid phase in the process of isothermal heat treatment. As the isothermal temperature increases from 570 °C to 610 °C, the average size and shape factor of solid particles of both the alloys with and without Sm addition gradually decrease while the liquid fraction gradually increases.

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.

Effect of Microstructures and Mechanical Properties of Large Deformation High Entropy Alloy CoCrCuFeNi in Semi-solid Isothermal Heat Treatment

The semi-solid slurries of the CoCrCuFeNi high entropy alloy(HEA)were fabricated through the recrystallization and partial melting(RAP)process by cold-rolling and partial remelting.The temperature range of the semi-solid region and the relationship between the liquid fraction and the temperature were determined by the differential scanning calorimetry(DSC)curve.The effect of isothermal temperature and holding time on the evolution of the microstructure and mechanical properties of the rolled samples was analyzed.The results show that the microstructure was significantly deformed,and the tensile strength has been increased by 107%after 63%rolling deformation of the CoCrCuFeNi high entropy alloy(HEA).The high-entropy alloy after cold rolling was maintained at 1150 and 1300℃for 20,30,60,and 120 minutes respectively,the plasticity has been improved compared with the rolled high entropy alloy.The optimal plasticity was reached 13.7%and 7.9%at 1150℃and 1300℃for 30 minutes,respectively.After semi-solid isothermal heat treatment,the grain morphology changed from dendritic of as-cast or rolled to spherulite and the grain size increased significantly with time and the holding temperature increased.

Effect of Semi-Solid Isothermal Heat Treatment on Microstructure of ZL104 Aluminum Alloy

The feasibility of fabricating ZL104 aluminum alloy with non-dendritic microstructure by semi-solid isothermal heat treatment process and the effects of holding temperature and time on the semi-solid isothermal heat-treated microstructure of the alloy, are investigated. The research results indicate that it is possible to produce ZL104 alloy with non-dendritic microstructure by a suitable semi-solid isothermal heat treatment. After treated at 580 ℃ for 120 min, the ZL104 alloy can obtain a non-dendritic microstructure with an 49% liquid content and an average size range of 115 μm of the unmelted primary solid particles. In addition, an increase in the holding time from 30 to 120 min for a given holding temperature of 580 ℃ or in the holding temperature from 570 to 580 ℃ for a given holding time of 120 min causes the average size and globular tendency of unmelted primary solid particles in the semi-solid ZL104 alloy to decrease and become more obvious, respectively.

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.

Influence of Semi-Solid Isothermal Heat Treatment on Microstructure of Gray Cast Iron

Materials serve as an enabling technology contributing to solutions in problems of concern to society. In this research, the effect of semi-solid isothermal heat treatment on graphite morphologies and matrix structure for gray cast iron were studied. Microstructure observations and measurements of flaky graphite morphology are reported as a function of isothermal heating time range of 5 to 25 minutes after isothermal heating temperature at 1163℃. The effect heating time, on the semisolid microstructures during partial re-melting was investigated. Flack graphite morphology was changed significantly by isothermal heating of gray cast iron at 1163℃ for heating time above 15 min resulting fine graphite morphology in matrix structure. Hardness increases with increasing heating time due to the amount of cementite and fine pearlite matrix for air cooled gray cast iron. The optimum heating treatment condition was achieved at the temperature of 1163℃ for the range of 15 to 20 min.

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.

Influence of Semi-Solid Isothermal Heat Treatment on Microstructure and Mechanical Properties of Ductile Cast Iron

In this research, the effect of semi-solid isothermal heat treatment on microstructure, hardness and impact toughness of ductile iron (DI) is studied. Heat-treated air-cooled ductile iron shows spheroid graphite, cementite and fine pearlite matrix structure. At the early stages of heating time (up to 20 min), the DIs show significant decrease in amounts of graphite and significant increase in amounts of cementite. By increasing heating time, above 20 min, the DIs show slightly decrease in amounts of graphite and slightly increase in amounts of cementite. Hardness values increase by increasing heating time. Meanwhile, the impact toughness decrease with increasing heating time. The optimum heating treatment condition for reasonable structure and mechanical properties could be achieved at the temperature of 1165℃ for the heating time range of 10 to 15 min.

Microstructural evolution of AZ61 magnesium alloy predeformed by ECAE during semisolid isothermal treatment

The microstructural evolution of AZ61 magnesium alloy predeformed by equal channel angular extrusion（ECAE） during semisolid isothermal treatment（SSIT） was investigated by means of optical metalloscopy and image analysis equipment.The process involved application of ECAE to as-cast alloy at 310 ℃ to induce strain prior to heating in the semisolid region for different time lengths.The results show that extrusion pass,isothermal temperature and processing route have an influence on microstructural evolution of predeformed AZ61 magnesium alloy during SSIT.With the increase of extrusion pass,the solid particle size is reduced gradually.When isothermal temperature increases from 530 ℃ to 560 ℃,the average particle size increases from 22 μm to 35 μm.When isothermal temperature is 575 ℃,the average particle size decreases.The particle size of microstructure of AZ61 magnesium alloy predeformed by ECAE at BC during SSIT is the finest.